KR100491518B1 - Method and apparatus for hard handoff in a cdma system - Google Patents

Abstract

In a communication network, a network user communicates with another user via remote unit 30 via at least one base station 100. The communication network includes a first mobile switching center (MAC-1) that controls communication through a first set of base stations that includes a first base station 100. The remote unit 30 stores an active base station list having an entry corresponding to each base station through which active communication is made. The first base station 100 has an entry on the active base station list. The first base station 100 measures the round trip delay of the active communication signal between the first base station 100 and the remote unit 30. If the round trip delay of the active communication exceeds the threshold and the first base station 100 is designated as the reference base station, the handoff of the active communication signal begins. Optionally, the remote unit 30 also stores a list of candidate base stations including an entry against each base station for which active communication is possible but not currently made. If the candidate base station list includes an entry corresponding to the triggering pilot signal, handoff of the active communication signal is performed.

Description

METHOD AND APPARATUS FOR HARD HANDOFF IN A CDMA SYSTEM}

The present invention relates to a cellular communication system in which a plurality of base stations are arranged. More specifically, the present invention relates to a new and improved technique for handing off communications between base stations of different cellular systems.

The use of code division multiple access (CDMA) modulation technology is only one of a variety of techniques to facilitate communication where a large number of system users exist. Although time division multiple access (TDMA) and frequency division multiple access (TDMA) are known, CDMA has significant utility over such other modulation techniques. The use of CDMA technology in a multiple access communication system is disclosed in US Pat. No. 4,901,307, which is incorporated herein by reference "spread spectrum multiple access communication system using satellite or terrestrial repeater".

In the above-mentioned patents, the multiple access technology allows a plurality of mobile telephone system users with transceivers (or known as remote units) to use satellite CDMA spread spectrum communication signals or satellite repeaters or (known as base stations or cell-sites). Used when communicating via terrestrial base station. Using CDMA communication, the frequency spectrum can be reused many times. The use of CDMA technology results in much higher spectral efficiency than can be achieved using other multiple access technologies, thus allowing for increased system user capacity.

Conventional FM cellular telephone systems used in the United States are commonly referred to as AMPS (Advanced Mobile Phone Service) and are described in the Electronic Industry Association Standard (EIA / TLA-553) "Mobile Station-Territorial Compatibility Statement". Described. In such conventional FM cellular telephone systems, the available frequency band is divided into channels, which typically have a bandwidth of 30 kHz. The service area of the system is geographically divided into the area of coverage of the base station, which varies in size. Available frequency channels are divided into sets. Frequency sets are assigned to areas of service coverage in a manner to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the area of coverage is uniformly sized hexagons. Frequencies used within a region of one service range are not used in the region of the six nearest adjacent service ranges.

In conventional cellular systems, the handoff regime allows the communication connection to continue even when the remote unit crosses a boundary between the regions of the service range of two different base stations. In an AMPS system, handoff from one base station to another begins when the receiver in the active base station processes a call notification that the received signal strength from the remote unit falls below a predetermined threshold. The low signal strength indication means that the remote unit should be adjacent to the regional boundary of the base station's service range. When the signal level falls below a predetermined threshold, the active base station requests the system controller to determine whether the adjacent base station receives a remote unit signal having a better signal strength than the current base station.

In response to the query of the active base station, the system controller sends a message to the adjacent base station with the handoff request. Each base station adjacent to the active base station uses a specific scanning receiver to find a signal from the remote unit on the channel it is operating on. If one of the adjacent base stations reports the appropriate signal level to the system controller, a handoff is attempted for the neighbor base station currently labeled as the target base station. The handoff is then initiated by selecting an idle channel in the channel set used at the target base station. The control message is sent to the remote unit and requires switching from the current channel to a new channel supported by the target base station. At the same time, the system controller switches the call connection from the active base station to the target base station. This procedure is called hard handoff. The term "hard" is used to indicate the "connect after block" characteristic of the handoff.

In conventional systems, the call connection is suspended (ie disconnected) if the handoff to the target base station is not successful. There are many reasons for the failure of hard handoff. Handoff may fail if there is no idle channel available in the target base station. In addition, handoff may fail if one of the adjacent base stations notifies the reception of a signal from a remote unit when the base station is actually receiving another remote unit signal using the same channel for communication with a remote base station. This notification error causes the call connection to transfer to the wrong base station. In general, at a wrong base station, the signal strength from the actual remote unit is not sufficient to maintain communication. In addition, the handoff fails if the remote base station fails to receive a command to switch channels. Actual operational experiments indicate that handoff failures often occur, which significantly lowers the stability of the system.

Another common problem with conventional AMPS telephone systems occurs when the remote unit is adjacent to the boundary between two coverage areas for an extended time period. In this situation, the signal level tends to fluctuate for each base station as the remote unit changes location or as other reflective or attenuating objects change location within the area of service coverage. Fluctuations in signal levels result in a "ping-pong" situation, where a repeated request is made to handoff a call back and forth between two base stations. This additional unnecessary handoff increases the chance of a call being dropped by chance. In addition, repeated handoffs adversely affect the signal quality, even if the handoff is successful.

In US Pat. No. 5,101,501, filed March 31, 1992, incorporated herein by reference, and entitled "Methods and Systems for Providing Soft Handoff in CDMA Cellular Communication Systems Communications," Handoff of CDMA Calls A method and system for providing communication with a remote unit via one or more base stations is disclosed. Using this type of handoff communication within the cellular system, the cellular system is not interrupted by handoff from the active base station to the target base station. This type of handoff is considered a "soft" handoff and simultaneous communication is established using the target base station, which becomes the second active base station, before the communication with the first active base station is interrupted in the soft handoff.

 Enhanced soft handoff technology is disclosed in US Pat. No. 5,267,261, filed November 30, 1993, registered as "Mobile Station Support Soft Handoff in CDMA Cellular Communication Systems" and referred to herein as the following 261 patent. In the system of the 261 patent, the soft handoff procedure is controlled based on measuring at the remote unit the strength of the "pilot" signal transmitted at each base station in the system. This pilot strength measurement facilitates soft handoff by facilitating the identification of survivable base station handoff candidates.

More particularly, in the system of the 261 patent, the remote unit monitors the signal strength of the pilot signal from an adjacent base station. It is not required that the coverage area of the adjacent base station be substantially in contact with the coverage area of the base station establishing active communication. When the measured signal strength of the pilot signal from one of the adjacent base stations exceeds a predetermined threshold, the remote unit sends a signal strength message via the active base station to the system controller. The system controller instructs the target base station to establish communication with the remote unit and requires the remote unit to establish simultaneous communication with the target base station while maintaining communication with the active base station through the active base station. This process continues for additional base stations.

When the remote unit detects that the signal strength of the pilot corresponding to one of the base stations with which the remote unit is communicating falls below a predetermined level, the remote unit sends the measured signal strength of the base station to the system controller via the active base station. Notify. The system controller sends a message to the identified base station and the remote unit in order to interrupt communication through the identified base station while maintaining communication through other active base stations or base stations.

While the technique described above is well suited for call switching between base stations in the same cellular system controlled by the same system controller, more different situations arise by moving the remote unit to an area of service coverage serviced by the base stations of other cellular systems. Is provided. One complication in this "system to system" handoff is that each system is controlled by a different system controller, and there is generally no direct link between the base stations of the first system and the system controller of the second system. will be. The two systems are thus excluded from performing simultaneous remote unit communication via one or more base stations during handoff processing. Even when the presence of a link between systems between two systems is available to facilitate soft handoff between systems, often the dissimilar characteristics of the two systems further complicate the soft handoff process.

When resources are not available to handle soft handoff between systems, performing a "hard" handoff of a call connection from one system to another is important in order to ensure that services remain uninterrupted. do. Handoff between systems should be performed at a time and place that can easily result in a successful switchover of a call connection between systems. E.g;

(i) when an idle channel is available at the target base station,

(ii) when the remote unit is within the target base station and active base station area, and

(iii) Handoff is performed only when the remote unit is in a position that is sure to receive a command to switch channels.

Ideally, such hard handoffs between each intersystem should be handled in a manner that minimizes the possibility of "ping-pong" handoff requests between base stations of other systems.

These and additional drawbacks to existing handoff techniques between systems impair the quality of cellular communication and are expected to further reduce performance as cellular systems continue to increase. Accordingly, there is a need for an intersystem handoff technique that can reliably perform call handoff between base stations of different systems.

1 illustrates an embodiment for a cellular WLL, PCS, or wireless PBX system.

2 shows a cellular communication network consisting of first and second cellular systems controlled by a first (MSC-I) and a second (MSC-II) mobile switching center, respectively.

3 illustrates a cellular communication system arranged side by side with a point-to-point microwave link between two directional microantennas.

4A shows a hard handoff area of a highly idealized, FM system.

4B shows the hard and soft handoff regions of a highly idealized CDMA system.

4C shows a handoff region that corresponds to a very idealized, different frequency handoff of CDMA to CDMA.

Figure 5 illustrates a second set of system base stations of internal, translation, and is used to illustrate the functionality of the remote unit measurement oriented hard handoff table.

6 shows an antenna pattern for a three sector divided base station.

7 illustrates the use of detection rules in the same frequency handoff of CDMA to CDMA.

8 illustrates the use of detection rules in different frequency handoffs of CDMA to CDMA.

9 shows two base stations arranged side by side in a configuration providing different frequency handoffs of CDMA to CDMA.

10 shows a handoff from a CDMA system to a system that provides services using different technologies.

11 illustrates an alternative configuration for providing different frequency handoffs of CDMA to CDMA using a single multi-sector partitioned base station.

12 shows a block diagram for a base station of the prior art including receive diversity.

FIG. 13 shows a block diagram of a border base station with transmit diversity to provide path diversity. FIG.

14 illustrates the use of base stations arranged side by side to perform hard handoff.

FIG. 15 illustrates the use of a densely located base station with overlap of a substantial portion of the service coverage area to perform hard handoff.

FIG. 16 illustrates the use of a "conical silence region" of a CDMA system crossed by a point-to-point microwave link.

FIG. 17 illustrates the use of a "conical silence region" in a CDMA system in which the conical silence region of the service range and the service range region of the microwave link are crossed by substantially the same point-to-point microwave link.

The present invention uses two different techniques to facilitate hard handoff from the first base station controlled by the first system controller to the second base station controlled by the second system controller. When a remote unit located within the coverage area of a designated base station reports detection of a triggering pilot signal, the detection rule triggers a handoff. This operation depends on the coverage area where the remote unit is deployed and the triggering pilot signal detected. When the active set of the remote unit contains only one base station and the base station is designated as the reference base station, and the round trip delay between the remote unit and the reference base station exceeds a predetermined threshold, the hand-down rule is handed off Trigger

The detection rule and the handdown rule can be used in connection with the formation of a physical service coverage area that provides both intrasystem and intersystem spatial hysteresis. The rules may be combined with other network configurations to provide maximum benefits, such as the use of CDMA to CDMA handoffs of different frequencies.

The above techniques of the present invention can be easily understood in view of the following detailed description with reference to the drawings.

An example of a cellular telephone system, a private branch exchange (PBX) system, a wireless local loop (WLL), a personal communication system (PCS) or another analog wireless communication system is shown in FIG. Has been shown. The system shown in FIG. 1 uses various multiple access modulation techniques to facilitate communication between a large number of remote units and a plurality of base stations. Many multi-access communication system technologies, including time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), and amplitude modulation (AM) schemes such as amplitude companded in a single sideband This is known. However, CDMA spread spectrum modulation techniques have significant utility over such modulation techniques for multiple access communication systems. The use of CDMA technology in a multiple access communication system is described in US Pat. No. 4,901,307, filed February 13, 1990, which is incorporated herein by reference and registered as "spread spectrum multiple access communication system using satellite or terrestrial relay station." Disclosed in the call. Many of the ideas described in this specification can be used in various communication techniques, although the preferred embodiments described are described with reference to CDMA systems.

In the above-mentioned US Pat. No. 4,901,307, a plurality of mobile telephone system users each having a transceiver communicates via a satellite or terrestrial relay station using CDMA spread spectrum communication signals. The use of CDMA results in much higher spectral efficiency than can be obtained using other multiple access techniques.

In a typical CDMA system, each base station transmits a unique pilot signal. In a preferred embodiment, the pilot signal is an unmodulated direct sequence spread spectrum signal that is continuously transmitted by each base station using a common pseudo noise (PN) spreading code. Each base station or base station sector transmits a common pilot sequence offset spaced apart with respect to other base stations. The remote unit identifies the base station based on the code phase offset of the pilot signal received from the base station. The pilot signal also provides the signal strength measurement basis and phase reference for coherence demodulation used in handoff decisions.

Referring to FIG. 1, a system controller and switching center 10, referred to as a mobile switching center (MSC), generally includes an interface and processing circuitry for providing system control to a base station. The controller 10 also controls the routing of telephone calls from a Public Switched Telephone Network (PSTN) to a suitable base station for transmission to a suitable remote unit. The controller 10 also controls routing from the remote unit to the PSTN via at least one base station. The controller 10 directs calls between remote units via a suitable base station.

A typical wireless communication system includes a predetermined base station having a plurality of sectors. A multi-sectored base station includes a plurality of independent transmit and receive antennas as well as some independent processing circuitry. The present invention is equally suitable for each sector of a sector-divided base station and independent base stations of a single sector. The term base station is assumed to refer to a sector of a base station or a base station of a single sector.

The controller 10 may be coupled to the base station by various means such as a dedicated telephone line, a fiber optic link or a microwave communication link. 1 shows a typical base station 12, 14, 16 and a typical remote unit 18. The remote unit 18 may be a vehicle telephone, a palm-sized portable unit, a PCS unit, or a fixed location wireless subscriber line unit, or any other compliant voice or data communication device. Arrows 20A-20B show possible communication links between base station 12 and remote unit 18. Arrows 22A- 22B show a possible communication link between base station 14 and remote unit 18. Similarly, arrows 24A- 24B show a possible communication link between base station 16 and remote unit 18.

The location of the base stations is designed to provide services to remote units located within the area of their coverage. When the remote unit is not in use, i.e. no call is in progress, the remote unit continuously monitors the pilot signal from each nearby base station. As shown in FIG. 1, the pilot signal is transmitted by the base stations 12, 14, 16 to the remote unit 18 via the communication links 20B, 22B, 24B, respectively. In general, the term "forward link" refers to a connection from a base station to a remote unit. In general, the term "reverse link" refers to a connection from a remote unit to a base station.

In the embodiment shown in FIG. 1, the remote unit 18 is considered to be within the area of service coverage of the base station 16. There is a tendency to receive pilot signals from base station 16 at a higher level than other pilot signals monitored by remote unit 18. When the remote unit 18 initiates traffic channel communication (ie, a phone call), a control message is sent to the base station 16. Receiving the call request message, the base station 16 signals the controller 10 and transmits the called telephone number. The controller 10 connects the call to the intended recipient via the PSTN.

If the call originated from the PSTN, the controller 10 sends the call information to the set of base stations located close to the location where the remote unit most recently registered its presence. The base station broadcasts the paging message in response. When the intended remote unit receives its paging message, the unit responds with a control message sent to the nearest base station. The control message informs the controller 10 that a particular base station is used for communication with the remote unit. The controller 10 initially sends a call to this remote unit via this base station.

If the remote unit 18 moves out of the area of service of the initial base station, for example base station 16, the communication is switched to another base station. The procedure for switching communication to another base station is referred to as handoff. In the preferred embodiment, the remote unit initiates and assists in the handoff process.

"Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System", commonly referred to simply as IS-95, TIA / EIA / IS According to -95, a "remote unit-assisted" handoff is initiated by the remote unit itself, which is equipped with a search receiver, which in addition to carrying out other functions, the receiver of the adjacent base station. Is used to scan the pilot signal, if it is determined that the pilot signal from one of the neighboring base stations, for example base station 12, is stronger than a predetermined threshold, then the remote unit 18 may be the current base station, i.e. base station 16; Message is transmitted to the controller 10 via the base station 16. Upon receiving the information, the controller 10 receives the remote unit 18 and the base station ( Initiates a connection between 12. The controller 10 requires the base station 12 to allocate a resource to the call, in the preferred embodiment, the base station 12 assigns a channel element to handle the call, This assignment is reported back to the controller 10. The controller 10 informs the remote unit to search for signals from the base station 12 through the base station 16, and informs the base station 12 of the remote unit traffic channel parameters. Remote unit 18 communicates through base stations 12 and 16. During this process, the remote unit continues to identify and measure the signal strength of its received pilot signal. Handoff is achieved.

The above-described procedure may be considered a "soft" handoff in which a remote unit communicates through one or more base stations at the same time. During soft handoff, the MSC combines or selects the signals received from each base station with which the remote unit communicates. The MSC relays the signal from the PSTN to each base station with which the remote unit communicates. The remote unit combines the signals received from each base station to produce an overall result.

Observing the procedure of soft handoff, it is clear that the MSC provides centralized control of the process. Remote unit-assisted handoff is more complicated if the remote unit is by chance not in the same cellular system, ie within the coverage area of two or more base stations not controlled by the same MSC.

2 shows a cellular communication network 30 comprising first and second cellular systems under control of a first and second mobile switching center (MSC-I-MSC-II), respectively. MSC-I and MSC-II are each coupled to base stations of the first and second cellular systems by various means such as dedicated telephone lines, fiber optic links or microwave communication links. In FIG. 2, five exemplary base stations B _1A- B _1E that respectively define the coverage area areas C _1A -C _1E of the first system and the coverage area areas C _2A -C _2E of the second cellular system. Five exemplary base stations B _2A- B _2E are respectively defined.

For illustrative purposes, the areas of service coverage (C _1A -C _1E , C _2A -C _2E ) and the areas of service coverage shown in FIG. 3, which are continuously derived, are shown in circular or hexagonal shapes and are highly idealized. In a real communication environment, the coverage area of a base station varies in size and shape. The coverage area of the base station is different from the idealized circular or hexagonal shape and tends to overlap with the coverage area defining the coverage area. In addition, the base station may also be sector divided into three sectors as is known in the art.

Hereinafter, the areas of service coverage (C _1C -C _1E , C _2C -C _2E ) are referred to as border or transition service coverage areas because they are close to the boundary between the first and second cellular systems. do. The area of the remaining service coverage in each system is referred to as an internal or interior service coverage area.

2, MSC-II does not have direct access for communication to base stations B _1A- B _1E , and MSC-I does not have direct access for communication with base stations B _2A- B _2E . Able to know. As shown in FIG. 2, MSC-I and MSC-II may communicate with each other. ELA / TIA / IS-41, for example, named "Cellular Radio Telecommunication Intersystem Operation" between switching of different operating regions as shown by the intersystem data link of FIG. Defines the standard for communication. In order to provide soft handoff between one of the base stations B _1C -B _1E and one of the base stations B _2C -B _2E , a large amount of call signal and power control information is transmitted between the MSC-I and MSC-II. Should be. The extended nature of the switch-to-switch exchange and the large amounts of call signals and power control information cause excessive delays and excessive resource sacrifices. Another difficulty in providing soft handoff is that the structure of the system controlled by MSC-I and the structure of the system controlled by MSC-II are very different. Accordingly, the present invention provides a mechanism of hard handoff between two systems to avoid the complexity and cost of soft handoff between systems.

The mechanism for hard handoff can be used in many situations. For example, a system controlled by MSC-II may not use CDMA to communicate signals and may instead use FM, TDMA or other methods. In such a case, since soft handoff is possible only when both systems operate using CDMA, hard handoff is required even if an intersystem soft handoff mechanism is provided in a system controlled by MSC-1. Accordingly, the present invention is used to hand off a remote unit between two systems using different air interfaces. The second system needs to be modified to transmit a pilot signal or other CDMA beacons to help initiate the hard handoff process. A system using pilot beacons is described in US patent application Ser. No. 08 / 413,306, filed March 30, 1995, named co-pending "METHODE AND APPARATUS FOR MOBILE UNIT ASSISTED CDMA TO ALTERNATIVE SYSTEM HARD HANDOFF." An alternative system is described in US patent application Ser. No. 08 / 522,469, filed August 31, 1995, named co-pending "SAME FREQUENCY, TIME-DIVISION-DUPLEX REPEATER." The system uses a pilot beacon, which is described in US patent application Ser. No. 08 / 322,817, filed Oct. 13, 1995, named co-pending "METHOD AND APPARATUS FOR HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATION SYSTEM."

Another situation where hard handoff is available is when the remote unit changes the frequency at which it operates. For example, a point-to-point microwave link can operate in coexistence with a CDMA communication system within the PCS band. In FIG. 3, a point-to-point microwave link 140 is shown between the directional microwave antenna 130 and the directional microwave antenna 135. Base stations 40, 100, and 110 may be required to avoid the use of frequency bands used by point-to-point microwave links 140, thereby avoiding interference between the two systems. Because the directional microwave antenna 130 and the directional microwave antenna 135 are quite directional, the point to point microwave link 140 has a very narrow field. As such, other base stations and sectors 50 and 70 of the system, such as base stations 115 and 120, may operate without interference with the point-to-point microwave link 140. Thus, the remote unit 125 can be operated on a CDMA channel in the same frequency band as the point-to-point microwave link 140. When the remote unit 125 moves toward a base station 110 that does not support communication on the frequency at which it is currently operating, it is not possible to perform a soft handoff from the base station 115 to the base station 110. Instead, base station 115 instructs remote unit 125 to perform a hard handoff to another frequency band supported by base station 110.

Another situation in which hard handoff can be used is when the remote unit needs to change the frequency at which it operates to distribute the load more evenly. For example, within the PCS band, CDMA communicates using traffic channel signals of several frequency bands, such as frequency band f ₁ and frequency band f ₂ . When the frequency band (f ₂₎ is a heavily load than using the active communication signal frequency band (f ₁₎ takes, it is useful to move the active communication sinhojung part of the frequency band (f ₂₎ to the frequency band (f ₁₎ Do. To achieve load sharing, one or more remote units operating in frequency band f ₂ are required to begin operation in frequency band f ₁ by performing hard handoff inside the system.

The most stable way to perform a hard handoff is to have the base station 115 perform a hard handoff to an alternative frequency within itself. Thus, when the remote unit 125 receives more and more stable signals from the base station 115 at a given point, the base station 115 causes the remote unit 125 to operate on another frequency supported by the base station 115. Require. Base station 115 initiates transmission and attempts to receive a remote unit transmission signal at a new frequency. Alternatively, hard handoff occurs between the first frequency of base station 115 and the second frequency of base station 110. None of the two types of hard handoff require communication between systems.

Referring back to FIG. 2, for transmission to the designated remote unit, the first mobile switching center (MSC-I) controls the route setup for telephone calls from the PSTN to the appropriate base stations B _1A- B _1E . The MSC-I also controls routing for calls to the PSTN from at least one base station from a remote unit within the coverage area. MSC-II operates in the same way to control the operation of the base station (B _2A -B _2E) in order to route the call setup between the PSTN and base station (B _2A -B _2E). Control messages and similar messages are communicated between MSC-I and MSC-II via data link 34 between systems using industry standards or subsequent correction standards such as IS-41.

When the remote unit is located within the coverage area of the internal base station, the remote unit is programmed to monitor pilot signal transmission from a set of adjacent base stations. Consider the case where the remote unit is located within the service coverage area (C _1D ) but accesses the service coverage area (C _2D ). In this example, the remote unit begins to receive a useful signal level from base station B _2D , which is then reported to base station B _1D and any other base station (s) with which the remote unit is currently communicating. When a useful signal level is received by the remote unit, it may be determined by measuring one or more measurable parameters of the received signal (e.g., signal strength, signal to noise ratio, frame error rate, frame cancellation rate, bit error rate, and / or relative time delay). Can be. In a preferred embodiment, the measurement is based on the strength of the pilot signal received by the remote unit. The remote unit of the same frequency is detected from the base station B _1D to the base station B _2D after detecting a useful received signal level at such a remote unit and reporting this detection to the base station B _1D using signal strength or both messages. Support hard handoff can proceed as follows;

(i) The base station B _1D relays the report signal level of the remote unit received from the base station B _2D to the MSC-I, and the MSC-1 recognizes that the base station B _2D is controlled by the MSC-II. .

(ii) The MSC-I requires channel resources of the base station B _2D from the MSC-II and inter-system trunk facilities between the two systems via the data link 34 between the systems.

(iii) The MSC-II responds to this request by providing information to the MSC-I via a data link 34 between systems, which information identifies other information as well as the channel over which communication is established. In addition, the controller reserves a designated channel for communication with remote units and trunk resources within the base station B _2D .

(iv) MSC-1 provides new channel information to the remote unit via base station B _1D and defines the time at which the remote unit starts communication with base station B _2D .

(v) Communication is established through hard handoff between the remote unit and the base station B _2D at the specified time.

(vi) MSC-II informs MSC-I of the successful conversion of the remote unit to the system.

One difficulty with this method is that it does not know whether the signal from the remote unit is received by the base station B _{2D at a} level sufficient to support communication at that time. The MSC-I requires the remote unit to establish communication with the base station B _2D . Similarly, base station B _2D may not receive a useful received signal level from a remote unit. As a result, the call connection may be interrupted during the transmission control procedure to the MSC-II. If the call connection is interrupted, an error message is sent from MSC-II to MSC-I rather than an acknowledgment signal.

Another difficulty in providing hard handoff is the nature of the boundary of the coverage area of a CDMA system. In an FM system such as AMPS, the overlap area of service coverage area is rather wide. The overlap area of the service coverage area is the area where communication is supported between the remote unit and either one of the two different base stations. In the FM system, since the hard handoff occurs successfully only when the remote unit is located within the overlap area of the service area area, the overlap area of this service area should be wide. For example, FIG. 4A shows a very ideal FM system. Base station 150 and base station 165 may provide forward and reverse link FM communication to remote unit 155. (The forward link refers to the connection from the base station to the remote unit. The reverse link refers to the connection from the remote unit to the base station.) Within zone 160, the signal strength from base station 150 and base station 165 is remote. The level is sufficient to support communication with the unit 155. Due to the nature of the FM system, the base stations 150 and 165 cannot communicate with the remote unit 155 at the same time. When a hard handoff from base station 150 to base station 165 occurs within area 160, the new frequency is not used for communication between base station 150 and remote unit 155, rather than base station 165. And for communication between the remote unit 155. Base station 165 never transmits on a given frequency used by base station 150, so base station 165 nominally does not communicate any communication between base station 150 and any remote unit with which it is communicating. It does not provide interference. The boundary 182 represents the location beyond which communication from the base station 165 to the remote unit 155 is not possible. Similarly, boundary 188 indicates a location beyond which communication from base station 150 to remote unit 155 is not possible. Obviously, not only FIG. 4A but also FIG. 4B and FIG. 4C are not shown according to size, and in fact the overlapping area of the service range is relatively narrow compared to the total service coverage area of each base station.

Using CDMA soft handoff, it is not important that there is an overlapping area of coverage area where communication can be fully supported by only one of the two base stations. In the area where soft handoff occurs, it is sufficient that stable communication is maintained when the communication is established simultaneously with two or more base stations. In a CDMA system, active base stations and adjacent base stations generally operate at the same frequency. Thus, as the remote unit approaches the coverage area of an adjacent base station, the signal level from the active base station drops and the interference level from the adjacent base station increases. Since interference from adjacent base stations increases, the connection between the active base station and the remote unit is compromised if no soft handoff is established. The connection is particularly at risk if the signal is faint for the active base station and not for the adjacent base station.

4B shows a fairly ideal CDMA system. CDMA base station 200 and CDMA base station 205 may provide CDMA communication for the forward and reverse links to remote unit 155. Within the dark area 170, even when communication is established with either base station 200 or base station 205, the signal strengths from both base stations 200 and 205 support communication with remote unit 155. Is enough level. Beyond the boundary 184, communication through the base station 205 alone is not stable. Similarly, communication through base station 200 only beyond boundary 186 is not stable.

Regions 175A, 170, and 175B show regions where the remote unit tends to be soft handed off between base stations 200,205. Establishing communication through the two base stations 200, 205 improves overall stability even if the communication link with the remote unit in the area 175A for the base station 205 is not alone stable to support communication. Beyond the boundary 180, the signal level from the base station 205 is not sufficient to support communication with the remote unit 155 even in soft handoff. Beyond the boundary 190, the signal level from the base station 200 is not sufficient to support communication with the remote unit 155 even in soft handoff.

It should be noted that FIGS. 4A and 4B are shown in relation to each other. Codes 180, 182, 184, 186, 188, and 190 used to indicate boundaries increase in value as the distance from the base station 150 and the base station 200 increases. As such, the soft handoff area between boundaries 180 and 190 is the widest. The overlapping area of the FM service range between the boundaries 182 and 188 is located within the CDMA soft handoff area. The CDMA hard handoff area is the narrowest area between boundaries 184 and 186.

Note that if base station 200 belongs to the first system and base station 205 belongs to the second system, then base station 200 and base station 205 cannot communicate with remote unit 155 at the same time. Thus, it should be noted that when communication requires switching from base station 200 to base station 205, hard handoff is required from base station 200 to base station 205. It should be noted that the remote unit must be located in the region 170, in the CDMA hard handoff region between the boundaries 184, 186 so that the hard handoff has a high probability of success. The hard handoff area 170 is very narrow and it is difficult that the remote unit 155 takes very little time to move into and out of the hard handoff area 170. In addition, it is difficult to identify whether the remote unit 155 is in the hard handoff area 170. When determining that the remote unit 155 is in the hard handoff area 170, it must be determined to which base station and when hard handoff should occur. The present invention solves this problem.

A first aspect of the present invention is directed to a system and method for determining the area of a coverage area and determining the base station to which a hard handoff should be attempted if a hard handoff is needed so that it can be successfully performed. The hexagons arranged in the form of tiles shown in FIG. 3 are very ideal. When the system is used in practice, the resulting coverage areas have a wide variety of shapes. 5 shows a more realistic representation of a base station set. Base stations T ₁ -T ₃ and base stations I ₁ -I ₃ are part of a first communication system controlled by system 1 controller 212. Base stations I ₁ -I ₃ are internal base stations that contact other base stations of the same system. Base stations T ₁ -T ₃ are transitional or boundary base stations with service coverage areas that border service coverage areas of base stations belonging to different operating systems. Base stations S ₁ -S ₃ are part of a second system controlled by a system 2 controller. The outermost thick concentric circles surrounding base station S ₃ , base stations I ₁ -I ₃ and base stations T ₂ -T ₃ represent an idealized service coverage area of the base station, in which communication with the base station is established. Makes it possible to set The outermost thick wavy lines surrounding the base stations S ₁ -S ₂ , the base stations T ₁ , represent a more realistic coverage area of the base station. For example, wavy line 228 represents the coverage area of base station S ₁ . The shape of the coverage area is significantly influenced by the terrain in which the base stations are located, such as the height, number, reflectivity, and height of tall buildings in the coverage area, as well as hills and other obstacles within the coverage area. The realistic coverage area is not shown for each base station to simplify the figure.

In a practical system, a given base station may be sector divided like three sectors. 6 shows a pattern of antennas for three sector-divided base stations. The three sector-divided base station is not shown in FIG. 5 to simplify the figure. The concept of the present invention can be applied directly to a sector partitioned base station.

In FIG. 6, the service coverage area 300A is represented by the narrowest width line. Service coverage area 300B is represented by a medium width line. Service coverage area 300C is represented by the thickest line. The three coverage areas shown in FIG. 6 are shaped using standard directional dipole antennas. The edge of the coverage area can be thought of as the location where the remote unit receives the minimum signal level required to support communication over the sector. As the remote unit moves into the sector, the strength of the signal received from the base station, as recognized by the remote unit, increases. At point 302 the remote unit communicates via sector 300A. At point 303 the remote unit communicates through sector 300A and sector 300B. As the remote unit moves past the edge of the sector, communication over that sector is attenuated. The remote unit operating in soft handoff mode between the base station of FIG. 6 and an adjacent base station not shown is likely to be located adjacent the edge of one of the sectors.

Base station 60 of FIG. 3 illustrates a more ideal three sector partitioned base station. Base station 60 includes three sectors, each of which covers more than 120 degrees of coverage area of the base station. Sector 50 with service coverage area shown by solid line 55 overlaps with service coverage area of sector 70 with service coverage area shown by rough dotted line 75. Sector 50 also overlaps sector 80 with the service coverage area indicated by fine dotted line 85. Location 90 as shown, for example, as X, is located within both sector 50 and service coverage area of sector 70.

In general, the base station is sector-divided to increase the number of remote units that can communicate through the base station, while at the same time reducing the total interference power to the remote units located within the coverage area of the base station. For example, sector 80 does not transmit a signal intended for a remote unit at location 90, so that no remote unit located within sector 80 can be remotely located at location 90 through base station 60. It is not interfered by the communication of the unit.

For remote units located at location 90, sectors 50 and 70 and base stations 115 and 120 contribute to the total interference. The remote unit at location 90 may be soft handed off with sectors 50 and 70. The remote unit at location 90 may be soft handed off using both base stations 115 and 120 at the same time.

Remote unit-assisted soft handoff operates based on the pilot signal strength of several sets of base stations measured by the remote unit. The active set is a set of base stations for which active communication is established. The contiguous set is a set of base stations that surround an active base station, including base stations that are likely to have sufficient signal strength to establish communication. The candidate set is a base station set having a pilot signal strength at a level sufficient to establish communication.

When communication is initially established, the remote unit communicates via the first base station, and the active set includes only the first base station. The remote unit monitors the pilot signal strength of the base station in the active set, the candidate set and the adjacent set. When the pilot signal of base stations in an adjacent set exceeds a predetermined threshold level, the base station is added to the candidate set and removed from the adjacent set of remote units. The remote unit sends a message identifying the new base station to the first base station. The system controller determines whether to establish communication between the new base station and the remote unit. When the system controller determines to establish communication, the system controller sends a message to the new base station while simultaneously identifying the information related to the remote unit and sending a command to establish communication with it. The message is also sent to the remote unit via the first base station. The message also identifies a new active set that includes the first base station and the new base station. The remote unit searches for the information signal transmitted from the new base station and communication is established with the new base station without terminating the communication through the first base station. This procedure may continue with additional base stations.

As the remote unit communicates through multiple base stations, it continuously monitors the signal strengths of the active set, the candidate set, and the adjacent set of base stations. If the signal strength corresponding to the base station of the active set falls below a predetermined threshold for a predetermined time period, the remote unit generates and sends a message to report such an event. The system controller receives this signal through at least one base station in communication with the remote unit. The system controller decides to stop the communication through the base station with weak pilot signal strength.

Upon determining to stop communication through the base station, the system controller generates a message identifying the base station of the new active set. The new active set does not include the base station from which communication will be interrupted. The base station to which communication is to be established sends a message to the remote unit. The system controller also communicates the information to the base station to interrupt communication with the remote unit. Thus, remote unit communication is routed only through the base stations identified in the new active set.

When the remote unit is soft handed off, the system controller receives the decoded packet from each base station, which base station is a member of the active set. From the signal set, the system controller generates a single signal for transmission to the PSTN. The signals received from the common remote unit within each base station are combined before they are decoded, thus making full use of the received multiple signals. The decoded result from each base station is provided to the system controller. Once a signal is decoded, it cannot be easily and usefully combined with other signals. In a preferred embodiment, the system controller should select a one-to-one corresponding base station with which communication is established between the plurality of decoded signals. The most useful decoded signal is selected from the signal set from the base station and the rest of the signal is discarded.

In addition to soft handoff, the system may also use "softer" handoff. Softer handoff is generally referred to as handoff between sectors of a common base station. Since the sectors of the common base station are very closely connected, the handoff between the sectors of the common base station is performed by combining the undecoded data rather than the selection of the decoded data. The present invention applies to softer handoffs used in systems or otherwise or evenly. The procedure for softer handoff is the continuation of US Patent Application No. 08 / 144,903, filed Oct. 10, 1993, filed Mar. 13, 1995, and entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE." US Patent Application No. 08 / 405,611, entitled "STATION".

In a preferred embodiment, the selection process is performed by a system controller in the selector bank subsystem (SBS). SBS consists of a selector set. Each selector handles active communication for one remote unit. In interrupting the call connection, the selector can be assigned to another active remote unit. The selector provides all manner of control to both the remote unit and the base station. The selector receives and transmits base station messages. One example of such a message is a signal transmitted by a base station whenever the round trip delay varies by a threshold amount between the base station and the remote unit. The selector also instructs the base station to send a message to the remote unit. One example of such a message is a message sent to a base station that requires the remote unit to instruct to provide a pilot strength measurement message (PSMM). The use of both of these signals is described more fully below. In most general embodiments, it is not necessary to be a selector for controlling the handoff process, and a predetermined type of communication control unit may perform a function assigned to the selector.

When the remote unit is established to communicate with the base station, the base station can measure a round trip delay (RTD) associated with the remote unit. The base station aligns its transmission to the remote unit over time based on universal time. The signal is transmitted from the base station to the remote unit via a wireless air link. The transmitted signal requires some time to move from the base station to the remote unit. The remote unit uses the signal received from the base station to align the transmissions sent back to the base station. By comparing the alignment time of the signal received by the base station from the remote unit with the alignment time of the signal transmitted by the base station to the remote base station, the base station can determine the round trip delay. Round trip delay is used to estimate the distance between the base station and the remote unit. According to a preferred embodiment, the base station reports the round trip delay to the selector whenever the round trip delay changes by more than a predetermined amount.

One feature of the present invention is to use a round trip delay between a remote unit and a base station that is a member of an active and candidate set to identify the location of the remote unit. Obtaining a round trip delay between the remote unit and the base station that is a member of the candidate set is somewhat more complicated than determining the round trip delay for a member of the active set. Since the base station that is part of the candidate set does not demodulate the signal from the remote unit, the round trip delay cannot be measured directly by the active base station.

The messages sent from the remote unit to the base station, including pilot signal information for the candidates and members of the active set, are referred to as pilot strength measurement messages (PSMMs). The PSMM is transmitted by a remote unit in response to a request from a base station, or the signal strength for an adjacent set of base stations exceeds a threshold, or the signal strength for a candidate set of base stations exceeds the strength of one of the base stations of the active set or handoffs. It is sent by the remote unit due to the elapse of the pause timer.

Four parameters control the soft handoff process. First, the pilot detection threshold T_ADD specifies the level at which a pilot signal of a base station that is a member of an adjacent set must be exceeded in order to be registered as a member of a candidate set. The pilot drop threshold T_DROP specifies the level at which the pilot signal strength of a base station that is a member of an active or candidate set must fall below to trigger a timer. The duration of the triggered timer is defined by T_TDROP. After the time defined by T_TDROP has elapsed, if the strength of the pilot signal is still below the T_DROP level, the remote unit starts to remove the base station from the currently belonging set. The comparison threshold T_COMP of the candidate set to the active set sets the amount by which the pilot signal strength for a member of the candidate set must exceed the pilot signal strength for a member of the active set to trigger the PSMM. These four parameters are stored in the remote unit. Each of these four parameters can be reprogrammed to a new value by a message sent from the base station.

The PSMM contains two pieces of information related to the present invention. The PSMM includes a record for each pilot signal corresponding to a base station that is a member of an active or candidate set. First the PSMM includes a measurement of signal strength. Secondly, the PSMM includes a measurement of the pilot signal phase. The remote unit measures the phase of the pilot signal for each pilot signal in the candidate set. The pilot signal phase is measured at the remote unit by comparing the phase of the useful multipath component that arrives fastest among the candidate pilot signals with the phase of the useful multipath component that arrives fastest among the members of the active set. The pilot signal phase can be measured with the associated PN chip. The pilot signal for the base station of the active set that provides the fastest arriving signal is referred to as the reference pilot signal.

The system controller converts the pilot signal phase into an estimate for round trip delay using the following equation.

RTD_can1 = RTD_ref + 2 * (P {i} lotPhase_can1-ChannelOffeset_can1 * P {i} lotInc)

Where RTD _can1 = calculated estimate of the round trip delay of the base station having an entry in the candidate set;

RTD _ref = round trip delay reported for reference pilot signal;

PilotPhase _can1 = phase with respect to the detected universal time of the remote unit, reported to the PSMM in units of PN chips;

ChannelOffset _can1 = channel offset of the active base station being a _unitless number;

PilotInc = offset index increment of the pilot sequence in units of PN chips per channel across the system wide. The reported round trip delay (RTD _ref ) for the reference pilot signal is provided by the base station to the selector. The round trip delay for the reference pilot signal serves as the basis for estimating the round trip delay between the remote unit and the base station that is a member of the candidate set. In the preferred embodiment, each base station should remember that the remote unit transmits the same pilot sequence offset to identify the base station based on the code phase offset of the pilot signal. The offset index increment PilotInc of the pilot sequence is a code phase offset increment that offsets the base station pilot signal. The channel offset ChannelOffset _can1 of the candidate base station specifies which code phase is assigned to the candidate base station. The relative phase of the candidate base station PilotPhase _can1 is the code phase offset of the candidate base station measured by the remote unit in units of PN chips for comparison with a reference pilot signal. PilotPhase _can1 is reported to the base station as a PSMM. ChannelOffest _can1 And PilotInc are known selectors.

If there is no delay in the transmission in the system, the phase of the candidate base station is the product of the channel offset (ChannelOffest _can1 ) and the offset index increment (PilotInc) of the entire system pilot sequence. Since there is a transmission delay in the system, the remote unit recognizes the reference pilot signal and the pilot signal of the active base station through different and variable delays. Subtracting the system derived PN offset (the product of ChannelOffset _can1 and PilotInc) from the perceived PN offset creates a relative offset between the reference pilot signal and the pilot signal of the candidate base station. If the difference is negative, the RTD between the reference base station and the remote unit is greater than the RTD between the active base station and the remote unit. The difference perceived by the remote unit only reflects the forward link related delay. The forward link related delay is doubled to calculate the full round trip delay.

For example, assume that the offset index increment of the entire system pilot sequence is 64 PN chips and the following information is used as the basis for the round trip delay measurement.

PilotPhase _ref = 0 RTD = 137 (base station Id = 12)

PilotPhase _ref = 948 RTD = 244 (base station Id = 14, relative offset 52 PN)

PilotPhase _ref = 1009 (base station Id = 16, relative offset -15 PN)

In a preferred embodiment, because each base station or base station selector transmits the same pilot sequence offset in time, base station identification may be considered the channel PN offset used by the base station to transmit the pilot signal. Furthermore, base stations 12 and 14 (which may be assumed to be referred to as the base stations shown in FIG. 1) are assumed to be members of the active set, and RTD measurements measured by base stations 12 and 14 are 137 PN chips and 244. It should be assumed that each is reported as a PN chip.

Note that the right side of the pilot phase and round trip delay data for base station 14 is the calculated relative offset. The measured pilot phase of base station 14 is a 948 PN chip. The fixed offset of base station 14 is the product of base station ID 14 and the offset increment 64 of the pilot sequence, which is equivalent to a 896 PN chip. The difference between the measured pilot phase and the pilot phase offset of the base station is the relative offset between the base station and the remote unit, which in this case is 52 PN chips 948-896. Since base station 14 directly measures round trips as a member of the active set, it is unnecessary to use these values to calculate the round trip delay between base station 14 and the remote unit.

However, since base station 16 is part of the candidate set, no round trip delay measurement is performed by base station 16, and Equation 1 above should be used to determine the round trip delay. For the base station 16, the parameters are as follows.

RTD _ref = 137 PN chip;

PilotPhase _can1 = 1009 PN chip;

ChannelOffset _can1 = 16; And

PilotInc = 64 PN chips per channel.

Substituting these numbers directly into Equation 1, a round trip of 107 PN between the remote unit and the base station 16 is calculated. As described above, to find the absolute offset of the candidate base station, PilotPhase _can1 is subtracted from the product of the ChannelOffset _can1 and PilotInc In this case, it generates a -15PN chip. Note that the round trip delay between the base station 16 and the remote unit is less than the round trip delay between the base station 12 and the remote unit.

The first method of identifying the location of the remote unit relies on the use of a Measurement Directed Hard Handoff (MDHO) status of the remote unit. To minimize processing impact, the system enters the MDHO state only when certain members of the active set are marked as transition base stations. In another embodiment, the system enters the MDHO state only when all members of the base station of the active set are converted to transition base stations. In the third embodiment, the system enters the MDHO state only when there is a single base station in the active set and the base station is a transition base station. In the fourth embodiment, sufficient processing resources exist so that the MDHO state is always active. In the MDHO state, the selector monitors a round trip delay for one member of the active set and calculates a round trip delay for a base station that is a member of the candidate set. After the condition that triggers the MDHO state is changed, the MDHO state is exited.

MDHO status is based on the use of MDHO tables. In the MDHO table, each row represents a section for an area of the service area area where the service area area becomes an overlapping area. As mentioned above, the overlap area of the service coverage area is an area where communication can be supported between the remote unit and one of two different base stations. Each row contains a list of base station identification number pairs and round trip delay ranges. The round trip delay range is defined for a period of maximum and minimum round trip delays.

To use the MDHO table, either the network planning tool or experimental data is used to identify the set of regions and the appropriate corresponding action for each region. Alternatively, rule-based or experimental systems can be used to generate the MDHO table. As discussed above, FIG. 5 shows a set of interior, transition, and second system base stations and is used to illustrate the functionality of the remote unit MDHO table. The blurry lines surrounding the base station represent a round trip delay measurement threshold. For example, base station location representing (S ₂₎ for surrounding the cloudy line 222 is the base station (S ₂₎ the round trip delay of the direct path are 200 PN chips in a remote unit located on a cloudy line 222 from Indicates. BS cloudy line surrounding the (S ₂₎ (220) indicates the position of a direct path to a remote unit located on the blurred line 222 at the base station (S ₂₎ representing the round trip delay of 220 PN chips. Thus any remote unit located between the dim line 200 and the dim line 222 exhibits a round trip delay between the 200 and 220 PN chips.

Similarly, the lines are blurred (226) surrounding the base station (T ₁₎ represents a position of a direct path to a remote unit located on a cloudy line 226 at the base station (T ₁₎ representing the 160PN chip. BS cloudy line surrounding the (T ₁₎ 224 has a direct path to a remote unit located on a cloudy line 224 at the base station (T ₁₎ represents the position represent the 180PN chip. Thus any remote unit located between the dim line 224 and the dim line 226 exhibits a round trip delay between the 160 and 180 PN chips.

Further cloudy line surrounding the base station (S ₁₎ 232 represents the location of the direct path to a remote unit located on a cloudy line 232 at the base station (S ₁₎ representing a 170PN chip. BS cloudy line surrounding the (S ₁₎ 230 has a direct path to a remote unit located on a cloudy line 230 at the base station (S ₁₎ represents the position represent the 180PN chip. Thus any remote unit located between the dimmed line 230 and the dimmed line 232 exhibits a round trip delay between 170 and 180 PN chips for base station S ₁ .

As mentioned above, multipath signals that do not take a direct path between a remote unit and a base station are generated by reflective elements in the environment. If the signal does not take a direct path, the round trip delay is increased. The fastest arriving signal is the one that has taken the shortest path between the remote unit and the base station. The fastest arriving signal is the signal that is measured to estimate the round trip delay in the context of the present invention.

It should be noted that a particular area may be identified by the round trip delay between the various base stations. For example, areas 240 and 242 of the service coverage have a round trip delay between the remote unit and the base station T ₁ between 160 and 180 PN chips and a round trip delay between the remote unit and the base station S ₂ between 200 and 220 PN. It can be identified by the fact that it is a chip company. The service coverage area 242 is further defined by the fact that the pilot signal from base station S ₁ can be recognized at all round trip delays. Suitable operation for the remote unit located in area 240 and in communication with base station T ₁ should assume that a hard handoff of the same frequency to CDMA base station S ₂ is performed. In addition, the total interference in region 242 is so high that an alternative method is to perform a hard handoff to the AMPS system supported by the base station S ₁ .

Table 1 shows part of an example MDHO table. The first column indicates which service area overlap regions correspond to a row in the MDHO table. For example, the area 242 of the service range corresponds to the area N of the service range of Table 1, and the area 240 of the service range corresponds to the area N + 1 of the service range of Table 1. The remote unit located within the coverage area 242 matches the parameters given for the coverage area 240. In an exemplary embodiment, the MDHO table is arranged in numerical order, and the first region that matches a given parameter is the only way if the region N is removed from the possible position given the set of parameters N + 1. It is chosen to be compared with. The second column contains the first base station ID. The third column contains the range of round trip delays that correspond to the area of service coverage specified by the row. The fourth and fifth columns contain the second base station ID, and the sixth and seventh columns contain round trip delay pairs. More columns may be added as required to specify the base station ID and round trip delay.

In a preferred embodiment, the MDHO table is stored in the selector bank subsystem controller (SBSC). The SBSC stores a pilot database that provides contiguous lists and pilot offsets and other data required for standard operation. In the preferred embodiment, the selector requires the SBSC to access the MDHO table each time a new PSMM is received and whenever the RTD measurement for any active base station changes by a significant amount.

(coverage region: service coverage area, BSid: base station ID, RTD range: RTD range, Action: operation, System ID: system ID, Target BSid: target base station ID)

The column labeled Actions indicates the action taken when the location of the remote unit is mapped to one of the service coverage areas.

Several preferred types of operation exist, as follows.

CDMA to AMPS hard handoff of intersystem base stations;

CDMA to AMPS hard handoff of system internal base stations;

CDMA to CDMA hard handoff of an internal base station;

Hard handoff of different frequencies of inter-system CDMA to CDMA; And

Hard handoff at the same frequency between systems CDMA to CDMA

If more round trip delay information is required to identify the location of the remote unit, the T_ADD and T_DROP thresholds can be modified when the remote unit is in the MDHO state. By reducing both the T_DROP and T_ADD thresholds, the low pilot signal strength entitles the base station to candidates and members of the active set, and the low pilot signal strength stays long in the candidate and active set before being removed. The increased number of base stations listed in the candidate set and active set increases the number of round trip delay data points that can be used to locate the remote unit. Reducing T_ADD and T_DROP has the negative effect that the entire system in each handoff state uses system resources from two base stations. It is desirable to minimize the number of remote units in the handoff state to conserve resources at each base station and maximize performance. Thus, in the preferred embodiment, the values of T_ADD and T_DROP only decrease within the transition base station. Also, after falling below T_DROP, the length of time specified by T_TDROP may be increased to increase the amount of time the base station stays in the active set.

In a preferred embodiment, if the second system does not transmit a CDMA pilot signal from the border base station to the frequency used in the first system, the second system sends a pilot signal or other CDMA beacons to assist in initiating hard handoff processing. To be modified. The hard handoff process is described in detail in US Patent Application No. 08 / 413,306 and US Patent Application No. 08 / 522,469, above. In an alternative embodiment, even if the system does not transmit a CDMA pilot signal from the border base station, the border base station in the second system does not generate a pilot signal, but the base station of the MDHO table corresponding to the base stations S ₁ -S ₃ . There is no entry in the ID column. The pilot beacon unit may also be used on an internal base station to help identify the area affected by the point-to-point microwave link.

In certain circumstances, it is also possible to eliminate the use of the candidate base station as a means for identifying the location of the remote unit, thus leaving only active base station information for determining the remote unit location. For example, using a smart network plan can effectively identify overlapping areas of service coverage areas using only round trip delays for members of the active set.

As described above, for the sake of simplicity, the sector-divided base station is not shown in FIG. In fact, the presence of sector division helps in the positioning process by narrowing the area where the remote unit is located. For example, the shape of the base station of FIG. 3 should be noted. Before the round trip delay is considered, the service coverage area of the base station 60 includes six different areas; (i) an area covered by the service range only by sector 50, (ii) an area covered by the service range by sector 50 and sector 70, and (iii) an area covered by service sector only by sector 70 Area (iv) area covered by service area by sector 70 and sector 80, (v) area covered by service area only by sector 80, and (vi) sector 80 and sector ( 50) is divided into areas that apply to the service scope. If a network scheme is used to align three sector-divided base stations along a boundary between two systems, it is possible to eliminate the use of pilot beacons and the use of round trip delay decisions of candidate base stations within the boundary base station of system 2. .

Each base station in the system is initially calibrated so that the sum of unloaded receiver path noise measured in decibels and the desired pilot power measured in decibels is equal to any constant. The calibration constant is constant for the entire system of the base station. As the system becomes under load (ie, as the remote unit begins communicating with the base station), the reverse link handoff boundary moves efficiently towards the base station. Thus, in order to simulate the same effect on the forward link, the compensation network reduces the pilot power as the load increases, thereby maintaining a constant relationship between the reverse link power received at the base station and the pilot power transmitted from the base station. do. The process for balancing the forward link handoff boundary with the reverse link handoff boundary is referred to as "base station bridging". The base station briefing was disclosed in U.S. Patent No. 5,548,812 filed on August 20, 1996, entitled "METHODE AND APPARATUS FOR BALANCING THE FORWARD LINK HANDOFF BOUNDARY TO THE REVERSE LINK HANDOFF BOUNDARY IN A CELLULAR COMMUNICATION SYSTEM."

The breathing process may adversely affect the operation of the MDHO state. Referring to FIG. 4B, when the power transmitted by the base station 200 decreases in comparison with the power transmitted by the base station 205, the overlap boundary of the service coverage area moves adjacent to the base station 200 and further the base station ( 205). The signal level does not affect the round trip delay between the remote unit and the base station at any one location. Accordingly, when the actual boundary changes, the MDHO table continues to identify the same location used for handoff.

There are various ways to deal with the breeding problem. One way is to sufficiently narrow the overlapping areas of the service coverage areas stored in the MDHO table so that the overlapping areas of the service coverage areas remain in the breeding state of a valid independent entity.

A second way to address the problem of base station briefing is to block or limit the briefing at the border base station. The breeding mechanism affects the forward link signal to allow forward link performance to simulate the original reaction of the reverse link to the level of load. Thus, elimination of bleeding does not remove the risk of maintaining the factor even if the boundary changes with the load on the reverse link and the load system does not use the bleeding.

A third way to address the problem of base station breathing is through network planning. The bridging effect is minimized if the boundary base station of the second system does not transmit a traffic channel signal (eg a specific signal of the active remote unit) at the frequency used by the boundary base station of the first system. If the border base station transmits a pilot signal from the pilot beacon, the breeding effect is also minimized since no traffic channel signal is generated when using the pilot beacon unit. The power output by the pilot beacon unit remains constant over time.

A fourth method for dealing with base station breeding problems is through the use of rule based systems. If the border base station is breeding, the breeding parameter is sent from each base station to the system controller. The system controller updates the MDHO table based on the current value of the breeding. In general, the system controller increases the round trip delay value in the MDHO table to reflect the breeding effect.

The breathing effect rarely occurs in most situations. Because these border zones are traditionally a source of technical and economic problems, network planning generally seeks to place the border between two systems within a low traffic zone. The lower the traffic volume, the smaller the breeding effect.

In some cases, it is desirable to avoid storing and accessing the MDHO table. In this case, other methods can be used to achieve handoff. For example, in an alternative embodiment, two means are used to trigger a handoff. The first method is called the detection rule. Any base station (or base station sector) is designated as the reference base station (R). If the remote unit is located within the coverage area of the reference base station and it reports detection of the triggering of the pilot signal P _B , the selector triggers a handoff with the target base station determined by the data set R, P _B. do. Detection rules are generally not always used with pilot beacon units.

The second method is called the hand-down rule. Any base station is indicated as a border base station. If the active set of the remote unit contains only one base station, and the base station is a border base station and the round trip delay of the reference pilot signal exceeds a threshold, the selector triggers a handoff. Alternatively, if the active set of the remote unit contains only one base station that is the reference base station and the round trip delay of the reference pilot signal exceeds a threshold, the selector triggers a handoff. In general, the threshold varies between base stations and is independent of the rest of the active set. Hand-down behavior is determined by the current reference pilot. The handdown rule is the first method of the rule set for measurement oriented handoff (MDHO). Note that it is not inevitable that a base station designated as a border base station has a service coverage area adjacent to the coverage area of a base station of another system. Hand down rules can be used for both system handoffs and handoffs within the system.

Detection rules and hand down rules depend on the physical characteristics of the system. Using these two rules requires the design of the network, such as the placement of base stations, the orientation of sectors within multi-sector partitioned base stations, and the physical placement of antennas.

If the remote unit or base station attempts to initiate a call in the border base station, the remote unit and base station exchange messages on the access channel. In a preferred embodiment, the overhead channel manager is located at the base station and controls the access channel. The overhead channel manager examines the round trip delay estimate calculated from the outgoing message. If the round trip delay exceeds the threshold, the overhead channel manager notifies the mobile station of instructing the base station to send a service redirection message to the remote unit. The service redirect message causes the AMPS-enabled remote unit to change to an AMPS system or other CDMA frequency or system. Redirection messages also depend on the type of service required by the remote unit. If a data connection is required rather than a voice connection, the AMPS system cannot support the connection. For this reason, operation should generally depend on the performance and condition of the remote unit. In general, each remote unit in the system has a rating designation that specifies its performance. The current state of this remote unit is queried by the base station and is determined depending on the information returned.

7 illustrates the use of the detection rule of the same frequency handoff of CDMA to CDMA. It is assumed that the remote unit is moving from the area C _1A / C _2, the system (S ₂₎ from the system (S _1). As the remote unit approaches C ₂ , it begins to recognize the pilot signal transmitted nearby. Using the detection rule, if C _1A is the reference base station, the sector requests a handoff to an AMPS base station that is side by side with the coverage area C _1A . As noted above, hard handoff from an FM AMPS system to another FM AMPS system is performed over a much larger physical area than a hard handoff from a CDMA system to another CDMA system operating at the same frequency. It should be noted that there must be at least substantial overlap between the coverage area of the CDMA base station and the coverage area of the AMPS base station within a one-to-one mapping or border base station. When switched to FM AMPS operation, the likelihood of successful hand-off between systems between FM systems is very high.

8 illustrates the use of detection rules for different frequency handoffs of CDMA to CDMA. In FIG. 8, the area that corresponds to the system (S ₂₎ is a system (S ₂₎ the frequency (f ₂₎ communicating with traffic channel signals to the frequency, but (f ₁₎ is not communicating with traffic channel signals to. In FIG. 8, the area corresponding to system S ₁ communicates with the traffic channel signal at frequency f ₁ but not with the traffic channel signal at frequency f ₂ . There may or may not be a pilot beacon unit operating at the border base station of either or both of the systems S ₁ or S ₂ . If a pilot beacon unit is present, a detection rule is used. Alternatively, if C _1A and C _1B are the only base stations in the active set, the hand down rule can be applied if the round trip delay measurement exceeds the threshold. In both cases, handoffs can be made within C _1A or C _1B to AMPS base stations arranged side by side.

The structure of FIG. 8 is very useful than the structure of FIG. 4C shows the usefulness of handoff using two different CDMA frequencies. 4C is a highly idealized representation of a handoff region using two different CDMA frequencies following the same format as in FIGS. 4A and 4B. In FIG. 4C, the base station 205 does not transmit a traffic channel signal on the same frequency as the base station 200, as represented by the dotted transfer arrows emanating from the base station 205 and the remote unit 155. The boundary 189 represents a point at which stable communication can be established at a frequency f ₁ between the remote unit 155 and the base station 200. The area 176 between the boundary 180 and the boundary 189 is located at the same time that the remote unit 155 can detect the pilot signal from the base station 205 when the base station 205 is equipped with a pilot beacon unit. Represents an area that can communicate via the base station 200.

The comparison of Figures 4B and 4C provides the advantage of different frequency handoffs. If base station 205 does not transmit a pilot signal, there is no interference from base station 205 with respect to the signal between base station 200 and remote unit 155. If the base station 205 transmits a pilot signal, the amount of interference with respect to the signal between the base station and the remote unit 155, due to the pilot signal from the base station 205, may occur when the base station 205 transmits a traffic channel signal. Significantly smaller than the interference generated. As such, boundary 189 may be closer to base station 205 than boundary 186.

The boundary 181 represents a point at which stable communication can be established at a frequency f ₂ between the remote unit 155 and the base station 205. The area between the boundary 181 and the boundary 190 is the base station 205 while the remote unit 155 can detect the pilot signal from the base station 200 when the base station 200 is equipped with a pilot beacon unit. ) Indicates the area to communicate with. Again, it should be appreciated how close the boundary 181 can be to the base station 200 than the boundary 184. Boundary region between 181 and boundary 189, 174 is a frequency (f ₁₎ of the base station 200, frequency (f ₂₎ of the base station 205 or geuyeok is a handoff of the communication can be achieved that in the Indicates the area in which 4b shows how wider the area 174 is than the area 170. Large area 174 is more useful for hard handoff processing. The fact that two different frequencies are used does not significantly affect the hard handoff process, since the switching of communication in the case of the same frequency or different frequencies has a "connect after block" hard handoff characteristic. A slight problem for the case of different frequencies is that the remote unit requires a certain amount of time for the switching operation from the second frequency to the second frequency.

In the preferred embodiment, both the base station and the remote unit use different frequencies for transmission than for reception. In the example illustrating handoff between two different CDMA operating frequencies, in Figure 4C and other figures and characters, a single frequency (e.g., characterization) designates the use of a set of transmit and receive frequencies for simplicity purposes. Note that even though the frequency f ₁ ) is mentioned, both the transmit and receive frequencies are different after the handoff is performed.

Referring to FIG. 8, it is not necessary to refrain from operating all base stations of system S2 at frequency f ₁ . It is only necessary to refrain from operating at the frequency f ₁ only the boundary base station and the internal base station in the system S ₂ . The internal base station of system S ₂ may use frequency f ₁ for CDMA or FM or TDMA or point-to-point microwave link or any other function.

9 shows another embodiment of a transition region between two systems. The structure of FIG. 9 requires cooperation between service providers of the first and second systems and can be best applied when both systems belong to the same service provider. 9 shows base stations B ₁ and B ₂ side by side or substantially collocate and provide different frequency handoffs from CDMA to CDMA. Both base station B ₁ and base station B ₂ are two sector-divided base stations that provide coverage for the coverage area 310. Base station B ₁ of system S ₁ provides CDMA service at frequency f ₁ in two sectors α and β, and base station B ₂ of system S ₂ is frequency f ₂ . Therefore, CDMA services are provided in two sectors α and β.

Highway 312 crosses service coverage area 310. As the remote unit moves from the system S ₁ using the frequency f ₁ to the coverage area 310, a standard internal system soft handoff is required to convert the call control to the base station B ₁ , sector β. Used. As the remote unit moves down to continue along highway 312, soft or softer handoff to switch a communication with base station (B _1), the sector base station (B ₁₎ a sector (α) from (β) Used. When the sector α of base station B ₁ becomes the only sector in the active set, the handdown rule triggers handoff triggering to the system S ₂ sector β of base station B ₂ at frequency f ₂ . to provide.

Handoff for the remote unit moving from system S2 to system S1 occurs in a similar manner between sector α of base station B2 to sector β of base station B ₁ . The base station (B ₁₎ of a sector (α) the base station (B ₂₎ of a sector (β) and are arranged side by side, the base station (B ₂₎ of the sector (α) the base station (B ₁₎ arranged side by side with a sector (β) of In each case, the hard handoff can be performed successfully without the risk that the remote unit is not within the coverage area of the target base station.

The structure of FIG. 9 has several advantages. Because the system (S ₂₎ not in the not the same as the area of a region that a handoff is performed to the system (S ₁₎ the system (S ₂₎ a hand-off to the system (S ₁₎ performed on, for a "ping-pong" condition The possibilities are minimal. For example, if the area which the system (S ₁₎ a hand-off to the system (S ₂₎ carried out in the same as the area which the system (S ₂₎ the system (S ₁₎ by performing hand-off from, entering the handover region Remote units moving within the area can be continuously handed off to one system and then back off to another system. The structure of FIG. 9 introduces spatial hysteresis. If the remote unit transfers control from system S ₁ to system S ₂ within the lower half of the coverage area 310, then the sector α of base station B ₂ does not change direction and the remote unit The remote unit does not transfer control back to the system S ₁ unless fully reenter the upper half of the coverage area 310 to be part of the remote unit's active set.

As with the structure of FIG. 8, all base stations in system S ₂ in the structure of FIG. 9 do not need to refrain from using frequency f ₁ . It is necessary to refrain from using the frequency f ₁ only at the edge base station and the next layer of the internal base station in the system S ₂ . The internal base station of system S ₂ may use the frequency f ₁ to transmit on CDMA or FM or TDMA or point-to-point micro link or any other function. Also, the base station need not include exactly two sectors, and a very large number of sectors can be used.

10 illustrates a situation where a CDMA system is adjacent to a system that provides services using different technologies. This situation can be handled in a similar manner to FIG. 8. 10 shows the spatial topology of Detroit, Michigan, USA. Detroit borders Canada on one side. The river defines the border between Detroit and Canada. Several bridges cross the river to connect the two countries.

On the US side of the river is a CDMA system S ₁ . On the Canadian side of the river is a TDMA system S ₂ . Both the US and Canada operate AMPS systems in addition to selected digital technologies. The remote unit moving on the Detroit side system is still in the CDMA service range and, if possible, in soft and softer handoff, but the remote unit is in sector (α) or service range of the service area area (C _A ). If the round trip delay exceeds a predetermined threshold when it is found to be exclusively within the coverage area of sector α of region C _C , then it is handed to the AMPS base stations arranged side by side using hand down rules. Handoff is triggered. The remote unit on the water may or may not stay within the CDMA coverage area depending on the selected RTD threshold. The network plan should ensure that the antennas are properly oriented and that the base stations are located so that the AMPS base station is uniquely determined depending on the transition sector and the call is not broken when this sector becomes the only sector in the active set.

14 shows an embodiment of the invention in which a carrier operating two systems may arrange two base stations side by side. 14 is a graphical representation. The service coverage area C _1A corresponds to an internal base station in system S ₁ operating at frequency f ₁ . The service coverage area C _1B corresponds to a transition base station in system S ₁ operating at frequency f ₁ . The pilot signal P ₁ is a pilot beacon operating at a frequency f ₁ arranged side by side with the service coverage area C _2A . The coverage area C _2A corresponds to an internal base station in system S ₂ operating at frequency f ₂ . The coverage area C _2B corresponds to the transition base station in system S ₂ operating at frequency f ₂ . The pilot signal P ₂ is a pilot beacon operating at a frequency f ₂ arranged side by side with the service coverage area C _1A .

In the structure of FIG. 14, as the remote unit moves between system S ₁ and system S ₂ , hard handoff must be performed between the base station of C _{1B and} the base station of C _2B . Since the internal base station does not transmit the traffic channel signal at the frequency at which hard handoff is formed, C _1B of frequency f ₁ The communication stability between the base station and the remote unit located in the coverage area C _1B and C _2B is high. Similarly, the communication stability between the base station of C _2B at frequency f ₂ and the remote units located in the coverage area C _1B and C _2B is also high.

One problem with the structure of FIG. 14 is the parallel collocation of service coverage areas C _1B and C _2B . Parallel deployment of base stations generally requires some degree of coordination between two system operators. If both systems operate using different carriers, the carriers do not want to share a physical facility. Parallel deployment also creates regulatory problems. FIG. 15 is similar to FIG. 14 except that the service coverage area C _1B and service coverage area C _2B are not completely parallel. The principle of this embodiment applies when the service coverage areas of the two base stations substantially overlap. The spatial hysteresis area is reduced by approximately the amount by which two service coverage areas are offset from each other.

In either Figure 14 or Figure 15, the operation is the same and very simple. The remote unit moving from system S ₁ towards system S ₂ initially communicates with service coverage area C _1A using frequency f ₁ . As the remote unit is adjacent to two co-located service coverage areas, a soft handoff of frequency f ₁ is used to divert communication to the service coverage area C _1B . If the remote unit continues to face the system S ₂ , the remote unit starts to detect the pilot signal from the pilot beacon P ₁ . When the active set includes only base stations corresponding to the service range C _1B , or when the pilot signal strength of the pilot signal P ₁ exceeds a predetermined threshold, the service range from the base station corresponding to the service coverage area C _1B . Hard handoff to a base station corresponding to region C _2B is performed. As the remote unit continues towards system S ₂ , soft handoff is used to switch communications between the base station corresponding to the coverage area C _2B and the base station corresponding to the coverage area C _2A . . Interaction is used to achieve a handoff from system S ₂ to system S ₁ .

The structure of FIGS. 14 and 15 is similar to that of FIG. 9 in that it introduces a measurement of spatial hysteresis. For example, the connection of a remote unit moving from system S ₁ to system S _{2 is} shown by dashed line 356. It should be noted that the remote unit remains serviced by the system S ₁ at a frequency f ₁ by the base station corresponding to the coverage area C _1B until the position indicated by the arrow 350 is reached. . Similarly, the connection of the remote unit moving from system S ₂ to system S _{1 is} shown by dashed line 354. Note that the remote unit remains serviced by the base station corresponding to the coverage area C _2B until it reaches the location indicated by arrow 352. Accordingly, the service providing communication to the remote unit between arrows 350 and 352 depends on which system provides communication when the remote unit enters the area. The remote unit moves in the area between arrows 352 and 350 without handoff between the two systems.

Referring to FIG. 4B, another solution to the dilemma of hard handoff is to increase the size of the hard handoff region 170. One reason for the very narrow area is due to the fading effect. Since the remote unit located within the hard handoff area 170 may establish communication with the base station 200 or the base station 205, if the signal does not fade for a base station that is not active but fades for an active base station, However, interference from base stations that are not active is severe. One way to increase the size of the area and increase the communication stability within the area is to minimize the amount of fading experienced by the remote unit in this area. Diversity is a way to reduce the harmful effects of fading. Three important types of diversity; There is time diversity, frequency diversity and spatial diversity. Time and frequency diversity naturally exist in spread spectrum CDMA systems.

Spatial diversity, also called path diversity, is created by multiple signal paths of a common signal. Path diversity can be usefully used through spread spectrum processing by separately receiving and processing signals arriving with different propagation delays. Examples of path diversity usage are U.S. Patent Nos. 5,101,501, filed March 31, 1992 and registered as "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," and "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM, filed April 28, 1992." US Patent No. 5,109,390, which is incorporated herein by reference.

The presence of a multipath environment provides path diversity for wideband CDMA systems. If two or more signal paths are created with differentials of path delays greater than a single chip interval, two or more receivers may be used to receive signals individually at a single base station or remote unit receiver. (The hysteresis of the required one chip path is a function of the means of performing tracking of time in the receiver.) After signals are received individually, they can be diversity combined prior to decoding processing. The total combined energy from the multiple paths is thus used in the decoding process, thus increasing the energy and accuracy of the decoding process. Signals of multipath generally exhibit independence in fading, i.e. signals of different multipaths generally do not fade together. Thus, if the outputs of the two receivers are diversity combined, significant loss in performance occurs only when the multipath signal fades at the same time.

Referring to FIG. 4B, it is assumed that the base station 200 is an active base station. If there are two separate signal components from base station 200 received by remote unit 155, the two separate signals are faded independently or almost independently. As a result, the entire signal from the base station 200 does not suffer severe fading that occurs when receiving only a single separate signal. As a result, it is unlikely that a signal from base station 205 will influence the signal from base station 200 to remote unit 155.

Rather than rely on natural and statistically developed multipath signals, multipaths can be artificially derived. A typical base station has two receive antennas and one transmit antenna. Often the transmit antenna is the same as one of the receive antennas. Such a base station structure is shown in FIG.

In FIG. 12, transmitter 330 provides a transmit signal to diplexer 332 which in turn provides a signal to antenna 334. Antenna 334 provides a first received signal to port 1 of receiver 338, and antenna 336 provides a second received signal to port 2 of receiver 338. In receiver 338 the received signals of port 1 and port 2 are received separately and combined prior to decoding to maximize usability. Antenna 334 and antenna 336 are configured in such a way that the signal received from each antenna fades independently of the signal received from the other. Since the received signals from the antennas 334 and 336 are provided to different receivers and the signals are not combined until they are demodulated in the receiver 338, the signals received at the antenna 334 are received by the antenna 336. Offset by at least one PN chip direction from the side is not serious.

To introduce diversity into the system of FIG. 12, a second diplexer 332 can be used to couple the transmit signal to the first receiving antenna via a delay line. Such a structure is shown in FIG.

In FIG. 13, the transmitter 330 supplies a transmission signal to the diplexer 332 which in turn supplies the signal to the antenna 334. In addition, the transmitter 330 provides a transmission signal (including the same signal as the original transmission signal in the most basic embodiment) to the delay line 340, the diplexer 342 and the antenna 336. As in FIG. 12, antenna 334 and antenna 336 are configured such that signals received from each antenna of the remote unit are independently faded. Since the signal is received via a single antenna of the remote unit and is also independent of fading, the two signals must be separated enough in time so that the remote unit can separate the signals individually. Using a delay greater than a single chip on the signal from the antenna 334 so that the remote unit can distinguish the signals and receive and demodulate them separately, the delay line is used to remotely control the signal emitted by the antenna 336. Add enough delay to reach the unit. In the preferred embodiment, the diversity base station configuration of FIG. 13 is used only within the border base station.

In an alternate embodiment, delay line 340 includes a gain adjustment element. The gain adjustment element is used to adjust the level of the signal transmitted by the antenna 336 with respect to the signal transmitted by the antenna 334. The advantage of this configuration is that the signal from antenna 336 does not interfere significantly with other signals in the system. However, the signal level from antenna 336 relative to the signal level from antenna 334 is important when the signal from antenna 334 fades. Thus, in the preferred embodiment, if the signal from the antenna 334 is subjected to strong fading for the remote unit, the signal from the antenna 336 will be quite large to provide stable communication during the fading period.

It is desirable to provide a signal from antenna 336 only when at least one remote unit is located within the hard handoff area. This technique can also be applied to the following alternative embodiments.

Different embodiments create separate signal paths carrying different sets of signals for transmission over antenna 336. In this embodiment, the base station determines which remote units require diversity (i.e. which units are located within the hard handoff area). The signal set transmitted via the antenna 336 only includes traffic channel signals and pilot signals for remote units in the hard handoff region. Alternatively paging and synchronous channel transmissions are also included. As just described above, when at least one remote unit is located in the hard handoff area, it is desirable to provide another signal from the pilot and antenna 336. Remote units that require diversity are identified, for example, by detecting remote units that require more transmit power than a predetermined threshold, or based on a round trip delay. The use of two transmitters reduces the net amount of transmitted power, thus reducing interference in a system including interference with remote units in hard handoff area 170 in communication with base station 205. In FIG. 13, dashed line 348 shows a second embodiment in which two separate signal paths carrying different sets of signals are used. It is assumed that a certain delay between the two necessary signals is induced in the transmitter 330.

It should also be noted that the second radiator need not be arranged side by side with the base station. It may be located far away or adjacent to the hard handoff boundary. Alternatively, instead of using the first receiving antenna to transmit the diversity signal, the signal may be transmitted from a separate antenna. The separate antenna may be a high directional spot antenna that concentrates energy on the hard handoff area.

Particularly useful structures can be achieved by using separate signal paths with separate antennas. In this case, much diversity can be obtained by assigning a signal to be transmitted by a separate antenna to a PN offset different from the PN offset nominally assigned to the transmitter 330. In this way, the base station performs a softer handoff when the remote unit enters the coverage area of the separate antenna. Separate PN offsets are useful for identification when the remote unit is located within the hard handoff area. This embodiment may be performed to provide the same results with respect to the variety of different topologies.

It should be noted that there are various ways of introducing diversity into the system. For example, the effect of fading can be minimized by fluctuations in the phase of the signal from the diversity antenna. The fluctuation of phase disrupts the alignment of the amplitude and phase of the multipath signal, which can produce strong fading in the channel. An example of such a system is described in detail in US Pat. No. 5,437,055, filed Jul. 25, 1996 and registered as "ANTENNA SYSTEM FOR MULTIPATH DIVERSITY IN AN INDOOR MICROCELLULAR COMMUNICATION SYSTEM."

The adverse effects of fading can be further controlled to a predetermined range within the CDMA system by controlling the transmit power. Fading reducing the power received by the remote unit from the base station may be compensated for by increasing the power transmitted by the base station. The power control function operates in conjunction with the time constant. Depending on the time constant of the power control loop and the length of time of fading, the system can compensate for fading by increasing the transmit power of the base station. The nominal power level sent from the base station to the remote unit is

It can be incremented when the remote unit is located in the area where hard handoff is performed. Again, remote units requiring increased power levels can be identified, for example, based on round trip delays or by reporting pilot signals that exceed a threshold. By only increasing the transmit power for this demanding remote unit, the net transmit power amount is reduced and thus the overall interference in the system.

As described above with respect to FIG. 3, one example of a situation requiring hard handoff to be performed is a situation where a remote unit must change the frequency at which it operates within a single system. For example, such handoffs prevent interference with point-to-point microwave links operating in common with a CDMA communication system, or convert all traffic channel signals into a single frequency so that different frequency handoffs of CDMA to CDMA can be achieved. It is formed to occur at the boundary. In FIG. 3, point to point microwave link 140 is shown between directional microwave antenna 130 and directional microwave antenna 135. Because the directional microwave antenna 130 and the directional microwave antenna 135 are very directional, the point to point microwave link 140 has a very narrow field. As such, other base stations in the system, such as base station 115 120 and sectors 50, 70, 80 may operate without interference with the microwave link 140 of point to point.

In a preferred embodiment, the CDMA signal is transmitted at a microwave frequency, such that point-to-point links across the system cause interference only if the link is also operating at the microwave frequency. The point-to-point link in most common embodiments may operate at frequencies higher or lower than those generally designated as microwave frequencies.

Although the technique described above can be applied to such a hard handoff, in general, a hard handoff inside the system has the advantage that the two base stations where the handoff is achieved outweighs the hard handoff between systems controlled by the same controller. . FIG. 11 illustrates another structure for providing different frequency handoffs of CDMA to CDMA using a single multi-divided base station. Both base station B _1A and base station B _1B have two directional sectors labeled with sectors α and β. In the base station B _1A , the sectors α and β operate at a frequency f ₁ . In the base station B _1B , the sectors α and β operate at a frequency f ₂ . Both base station B _1A and base station B _1B have one omni-directional sector γ that operates at a different frequency than the directional sector of the base station. For example, at base station B _1A , sector γ operates at frequency f ₂ and at base station B _1B operates at frequency f ₁ .

11 uses a hand down rule. The omni-directional sector γ is indicated as a border sector with a round trip delay threshold of zero, meaning that if the γ sectors are the only base stations in the active set, the handoff is triggered immediately whatever the round trip delay is. It should be noted that although the γ sector is not actually the boundary base station between the two systems, the operation taken in terms of the remote unit is the same. As the remote unit moves from an adjacent coverage area within system S ₁ of frequency f ₁ to base station B _1A , soft handoff is established to establish communication with sector α of base station B _1A . Softer handoff and soft handoff are used to switch the connection to sector β of base station B _1A . Soft handoff is then used to switch the connection to the sector γ of the base station B _1B , denoted by the border base station. Of base station (B _1B ) Sector (γ) is as soon as it is the only member of the Active Set, a hard handoff to a sector (β) of the base station (B _1B) is performed from the sector (γ) of the base station (B _1B).

This structure also means that when the operation is switched to frequency f ₂ , the remote unit does not enter sector γ of base station B _1A such that sector γ of base station B _1B becomes the only member of the active set. It should be noted that if not, introduces spatial hysteresis that the operation does not switch back to frequency f ₁ . Also, the choice of using three different sectors is that most multi-sector partitioned base stations are made up of three sectors and can be used accordingly. Note that base station equipment relies on the general support of three sectors. do. As such, a design using three sectors is practical. Of course, more or fewer sectors may be used.

There are two different types of situations where such a structure is used. The structure of FIG. 11 can be used where all traffic has to convert frequency. In such a case, the base station does not use the frequency f ₂ for the left side of the base station B _1A and the base station does not use the frequency f ₁ for the right side of the base station B _1B . In this case, all remote units entering one side and leaving the other side must convert frequency. In an alternative situation, the base station for the right side of base station B _1B uses only frequency f ₂ , for example, because the microwave link prohibits the use of frequency f ₁ in the region. However, the base station for the left side of the base station B _1A may operate on frequency f ₁ or frequency f ₂ . In such a case, all, some or some remote units moving from base station B _1B to base station B _1A must convert from frequency f ₂ to frequency f ₁ .

A second, very different way of dealing with a point-to-point microwave link or other area that requires some spectrum to be clarified is shown in FIG. 16. In FIG. 16, a "conical silence region" is formed around the point-to-point microwave link 140, shown by beams 364 and 366. The conical silence region is a pilot signal that acts as a reference signal to the remote unit that detects it. When the remote unit reports a pilot signal detection corresponding to the conical silence region, the system controller knows that the pilot signal is an indication of the conical silence region than the pilot signal in which the candidate pilot signal may exist. The system controller uses the reception of a pilot signal corresponding to the conical silence region as a stimulus to initiate hard handoff. In general, the handoff performed is a different frequency handoff of CDMA to CDMA inside the system even if other types of handoffs are performed.

In particular, it is an interesting feature of the conical silence region that the pilot signal of the conical silence region is not related to any base station. In general, the conical silence region pilot signal is generated by a pilot beacon unit arranged side by side with the directional microwave antennas 130,135. There are two different conical silence region topologies that can be used. In the first topology shown in FIG. 16, the beams 364 and 366 are actually narrow transmission bands that guide one side of the microwave link 140 at point-to-point. In the second topology shown in FIG. 17, beams 360 and 362 define the edge of the service coverage area of pilot signal transmission. In FIG. 17, the service coverage area of the pilot signal and the service coverage area of the point-to-point microwave link 140 actually overlap in the same area. In general, beams 364 and 366 are generated by two separate antennas separate from the microantenna. Beams 360 and 362 can be generated by the same antenna as the microwave signal, by different but homogeneous antennas, or by an antenna that defines a slightly wider coverage area than the microwave antenna.

The first topology of FIG. 16 has the advantage that the pilot signal of the conical silence region does not interfere with the point to point microwave link even if the point-to-point microwave link operates at the same frequency as the pilot signal of the conical silence region. The first topology is that if the remote unit passes the pilot signal beam in the conical silence region without detecting the signal and does not change the frequency, the connection may be interrupted or the connection may continue to form interference on the point-to-point microwave link. Has the disadvantage of being. Also, if power is located within beams 364 and 366 while power is applied to the remote unit, the remote unit may not detect the pilot signal and may cause interference to the microwave link.

The microwave link is bidirectional, and the operation of such a link requires two CDMA frequency channels. In one embodiment, two CDMA reverse link channels are removed to provide a point-to-point microwave link. Conical silence region pilot signals of two different forward links are transmitted in a conical silence region corresponding to each of the two reverse link channels removed for the point-to-point microwave link. In this way, the two pilot signals overlap the point-to-point microwave link coverage area without interference from the actual communication between the two directional antennas due to frequency diversity.

In a third embodiment, the pilot signal can coexist within the same frequency as the point-to-point microwave link without causing a significant amount of interference with the point-to-point microwave link. The CDMA pilot signal is a wideband, low power spread spectrum signal. This type of signal is perceived as simple Gaussian noise for other types of communication systems. Inherent CDMA signal characteristics do not induce significant interference and allow it to coexist uniquely with other communication systems.

The distance between point-to-point microwave link antennas is greater than the distance between typical base stations and the edges of the coverage area defined by the base station. Accordingly, the delay at which the remote unit recognizes the pilot signal in the conical silence zone is considerably longer than the delay typically associated with cellular systems. As such, the pilot signal of the conical silence region needs to be recognized as one of a set of consecutive pilot signal offsets. For example, the delay induced within the pilot signal offset of the conical silence region is greater than the normal offset between pilot signals that causes the perceived pilot signal offset to be mapped to the next consecutive pilot signal offset. This type of operation is generally not a problem because a particular system only uses all seventh or eighth PN offsets. The set of offsets for which the pilot signal of the conical silence region is expected can be added to the adjacent set such that the remote unit searches for this signal in the same way it searches for entries in another adjacent list.

Upon detecting the conical silence zone, the action taken depends on the base station with which active communication is established. Since the same conical silence area passes through the coverage area of many base stations, the pilot signal itself provides very little information about the location of the remote unit or the action required to be taken. The base station and frequency at which handoff is performed depends on the base station that is part of the active set at the moment the pilot signal recognizes. The action taken is also determined by members of the active set and the candidate set. Additionally, the action to be taken is based on the perceived PN offset for the conical silence region pilot signal. It is also desirable to postpone the action to be taken until the signal strength of the conical silence region pilot signal exceeds a second high threshold. Since the conical silence region pilot signal provides a small amount of information, the same pilot signal offset can be used throughout the system to protect a number of different point-to-point micro links. In FIG. 16, all beams 364 and 366 may operate at four different PN offsets or the same offset.

If the distance between two point-to-point microwave link antennas is long, it is necessary to use a repeater to extend the service range of the pilot signal. A method and apparatus for providing a repeater for a CDMA system is disclosed in co-pending US patent application Ser. No. 08 / 522,469, entitled "SAME FREQUENCY, TIME-DIVISION DUPLEX REPEATER," filed August 31, 1995.

Alternatively, a series of antennas that provide pilot sequences of the same or different offsets can be installed along the path of narrower and more precise microwave lengths, stably defining the conical silence region.

Many concepts of the invention can be combined. For example, detection and handdown rules can be used in connection with configuring a physical coverage area that provides both intrasystem and intersystem spatial hysteresis. The rule may also be combined with other network planning structures to provide maximum gains, such as the use of different frequency handoffs of CDMA to CDMA. The parameters controlling the soft handoff process can be adjusted to increase the number of base stations in the candidate and active set. The breeding of the base station can also be adjusted. The MDHO concept of a remote unit can be combined with the structure of a physical coverage area that provides both intra-system and inter-system spatial hysteresis. It may also be combined with other network planning structures to provide maximum gains such as the use of different frequency handoffs of CDMA to CDMA.

The foregoing description of the preferred embodiment has been disclosed to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments can be readily made by those skilled in the art, and the basic principles defined herein can be applied to other embodiments without using the originality. Thus, the present invention is not limited to the embodiments shown in the specification, but is consistent with the maximum range consistent with the principles and the novel features disclosed.

Claims (22)

In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit In a method for instructing communication between the first base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station in which active communication is made, wherein the first base station has an entry on the active base station list; Measuring, at the first base station, a round trip delay of a first active communication signal between the first base station and the remote unit; If the first base station is designated as a border base station, initiating a handoff of the first active communication if the round trip delay of the first active communication exceeds a threshold,        The system further includes a second mobile switching center for controlling a second set of base stations comprising a second base station, the method further comprising: Transmitting communication signals from an alpha sector of a first base station defining a first base station alpha service area at a first frequency, wherein the alpha sector of the first base station is designated as a border base station; Transmitting a communication signal from a beta sector of a first base station defining a first base station beta service area at the first frequency; Transmitting communication signals from an alpha sector of a second base station defining a second base station alpha service area at a second frequency, wherein the second base station alpha service area overlaps with a beta service area of the first base station; The alpha sector of the 2 base station is designated as the boundary base station; And Transmitting communication signals from a beta sector of a second base station defining a second base station beta service area at the second frequency, wherein the second base station beta service area substantially overlaps with the first base station alpha service area. Method further comprising a.       In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit In a method for instructing communication between the first base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station in which active communication is made, wherein the first base station has an entry on the active base station list; Measuring, at the first base station, a round trip delay of a first active communication signal between the first base station and the remote unit; If the first base station is designated as a border base station, initiating a handoff of the first active communication if the round trip delay of the first active communication exceeds a threshold:        Transmitting communication signals from an alpha sector of the first base station defining a first base station alpha service area at a first frequency; Transmitting communication signals from a beta sector of the first base station defining a first base station beta service area at the first frequency; Transmitting communication signals from a gamma sector of the first base station defining a first base station gamma service area at a second frequency, wherein the first base station gamma service area is the first base station alpha service area and the first base station beta; Substantially overlapping a service area, wherein the first base station gamma sector is designated as a border base station;        Transmitting communication signals from an alpha sector of the second base station defining a second base station alpha service area at a second frequency; Transmitting communication signals from a beta sector of the second base station defining a second base station beta service area at the second frequency, wherein the second base station beta service area is adjacent to the first base station service area; Transmitting communication signals from a gamma sector of the second base station defining a second base station gamma service area at the first frequency, wherein the second base station gamma service area is the second base station alpha service area and the second base station. Substantially overlapping the beta service area, wherein the second base station gamma sector is designated as a border base station. 3. The method according to claim 1 or 2, wherein the handoff initiation step is performed when the active base station list includes a single entry, the single entry corresponding to one of a set of border base stations, And a base station has a service area adjacent to the service area corresponding to the base station controlled by the first mobile switching center and controlled by the second mobile switching center. 3. The method according to claim 1 or 2, wherein the handoff start step is performed when each entry in the active base station list corresponds to a border base station set, wherein each base station in the border base station set is selected by the first mobile switching center. And a service area controlled and adjacent to the service area corresponding to the base station controlled by the second mobile switching center.      3. A method according to claim 1 or 2, further comprising the step of determining by the active communication control unit the type of handoff to be attempted in the handoff start step.      3. The method of claim 1 or 2, further comprising determining a type of handoff to be performed in the handoff start step based on the active base station list.      3. The method of claim 1 or 2, further comprising determining a type of handoff to be performed in the handoff start step based on the active base station list and the candidate base station list. In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit In a method for instructing communication between the first base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station in which active communication is made, wherein the first base station has an entry on the active base station list; Measuring, at the first base station, a round trip delay of a first active communication signal between the first base station and the remote unit; If the first base station is designated as a border base station, initiating a handoff of the first active communication if the round trip delay of the first active communication exceeds a threshold; And        Determining, by the active communication control unit, the handoff type to be attempted at the handoff start step;         The type of handoff to be attempted is a handoff from the first base station in communication with the remote unit using code division multiple access (CDMA) to the first base station operating using an alternative modulation technique. How to. 9. The method of claim 8, wherein said alternative modulation technique is frequency modulation (FM). 9. The method of claim 8, wherein said alternative modulation technique is time division multiple access (TDMA). In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit In a method for instructing communication between the first base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station in which active communication is made, wherein the first base station has an entry on the active base station list; Measuring, at the first base station, a round trip delay of a first active communication signal between the first base station and the remote unit; If the first base station is designated as a border base station, initiating a handoff of the first active communication if the round trip delay of the first active communication exceeds a threshold; And        Determining, by the active communication control unit, a handoff type to be attempted in the handoff start step,        The type of handoff to be attempted is from the first base station communicating at a first frequency with the remote unit using code division multiple access (CDMA) and to the first base station communicating at a second frequency using CDMA. Handoff. In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit And a method for instructing communication between the first base station and the second base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station to which active communication is made, wherein the first base station has an entry on the active base station list as a reference base station; Storing a candidate base station list in the remote unit, the candidate base station list including an entry corresponding to each base station with which communication is possible but not currently made; And Initiating a handoff of the active communication signal if the candidate base station list includes an entry corresponding to a triggering pilot signal,        The system further includes a second mobile switching center for controlling a second set of base stations comprising a second base station, the method further comprising: Transmitting communication signals from an alpha sector of a first base station defining a first base station alpha service area at a first frequency, wherein the alpha sector of the first base station is designated as a border base station; Transmitting a communication signal from a beta sector of a first base station defining a first base station beta service area at the first frequency; Transmitting communication signals from an alpha sector of a second base station defining a second base station alpha service area at a second frequency, wherein the second base station alpha service area overlaps with a beta service area of the first base station; The alpha sector of the 2 base station is designated as the boundary base station; And Transmitting communication signals from a beta sector of a second base station defining a second base station beta service area at the second frequency, wherein the second base station beta service area substantially overlaps with the first base station alpha service area. Method further comprising a.       In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit And a method for instructing communication between the first base station and the second base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station to which active communication is made, wherein the first base station has an entry on the active base station list as a reference base station; Storing a candidate base station list in the remote unit, the candidate base station list including an entry corresponding to each base station with which communication is possible but not currently made; Initiating a handoff of the active communication signal if the candidate base station list includes an entry corresponding to a triggering pilot signal;        Transmitting communication signals from an alpha sector of the first base station defining a first base station alpha service area at a first frequency; Transmitting communication signals from a beta sector of the first base station defining a first base station beta service area at the first frequency; Transmitting communication signals from a gamma sector of the first base station defining a first base station gamma service area at a second frequency, wherein the first base station gamma service area is the first base station alpha service area and the first base station beta; Substantially overlapping a service area, wherein the first base station gamma sector is designated as a border base station;        Transmitting communication signals from an alpha sector of the second base station defining a second base station alpha service area at a second frequency; Transmitting communication signals from a beta sector of the second base station defining a second base station beta service area at the second frequency, wherein the second base station beta service area is adjacent to the first base station service area; Transmitting communication signals from a gamma sector of the second base station defining a second base station gamma service area at the first frequency, wherein the second base station gamma service area is the second base station alpha service area and the second base station. Substantially overlapping the beta service area, wherein the second base station gamma sector is designated as a border base station. 14. The method of claim 12 or 13, wherein the handoff start step is performed when the active base station list includes a single entry corresponding to a reference base station. 14. The method of claim 12 or 13, wherein the handoff start step is performed when each entry in the active base station list corresponds to a reference base station. 14. A method according to claim 12 or 13, further comprising the step of determining at the active communication control unit the type of handoff to be attempted at the handoff start step.       14. The method of claim 12 or 13, further comprising determining a type of handoff to be performed in the handoff start step based on the active base station list.       14. The method of claim 12 or 13, further comprising determining a type of handoff to be performed in the handoff start step based on the active base station list and the triggering pilot signal.       In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit And a method for instructing communication between the first base station and the second base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station to which active communication is made, wherein the first base station has an entry on the active base station list as a reference base station; Storing a candidate base station list in the remote unit, the candidate base station list including an entry corresponding to each base station with which communication is possible but not currently made; Initiating a handoff of the active communication signal if the candidate base station list includes an entry corresponding to a triggering pilot signal; And        Determining, at an active communication control unit, a handoff type to be attempted at the handoff start step, The type of handoff to be attempted is a handoff from the first base station in communication with the remote unit using code division multiple access (CDMA) to the first base station operating using an alternative modulation technique. How to. 20. The method of claim 19, wherein said alternative modulation technique is frequency modulation (FM). 20. The method of claim 19, wherein said alternative modulation technique is time division multiple access (TDMA). In a communication network comprising a first mobile switching center where a network user communicates via remote unit with another user via at least one base station and controls communication via a first set of base stations comprising a first base station, the remote unit And a method for instructing communication between the first base station and the second base station, Storing an active base station list in the remote unit, the active base station list including an entry corresponding to each base station to which active communication is made, wherein the first base station has an entry on the active base station list as a reference base station; Storing a candidate base station list in the remote unit, the candidate base station list including an entry corresponding to each base station with which communication is possible but not currently made; Initiating a handoff of the active communication signal if the candidate base station list includes an entry corresponding to a triggering pilot signal; And        Determining, at an active communication control unit, a handoff type to be attempted at the handoff start step,        The type of handoff to be attempted is from the first base station communicating at a first frequency with the remote unit using code division multiple access (CDMA) and to the first base station communicating at a second frequency using CDMA. Handoff.