KR100573548B1 - Method and apparatus for performing idle handoff in a multiple access communication system - Google Patents

Abstract

In a mobile communication system 14 having a first communication device in communication with a plurality of second communication devices, a method of performing a handoff within the mobile communication system 14 includes a first method from one or more of the second communication devices. Transmitting to the communication device 2 a handoff list 60 of a second communication device which is allowed to handoff. The channel assignment message is sent from each of the second communication devices in the handoff list. Handoff list 60 includes a base station 26 a-o controlled by one controller 32 that determines which of the second communication devices sends a channel assignment message. Base stations 26a-o in handoff list 60 have pilot signals and the energy levels of the pilot signals are measured by the first communication device. The measured energy levels are sent to the controller 32 and compared by the controller 32 to provide a list of energy levels of the base stations 26a-o according to the comparison. The energy level list is sent to the first communication device and a handoff is performed according to the energy level list.

Description

METHOD AND APPARATUS FOR PERFORMING IDLE HANDOFF IN A MULTIPLE ACCESS COMMUNICATION SYSTEM}

Technical Field

The present invention relates to a communication system. More specifically, the present invention relates to a novel and improved method and apparatus for performing idle handoff in a multiple access communication system. The present invention also relates to an improved method of specifying traffic channels in a multiple access communication system. The present invention also relates to a method for reducing the number of handoffs required, which occur while a mobile station is queued and waiting for a traffic channel in a multiple access communication system.

Prior art

Typically, a communication system prohibits handoff while the mobile station is in system access. The system access state is a state in which communication is initiated by a mobile station in transmission on an access channel or by a base station in transmission on a paging channel. In an embodiment, the message is transmitted according to a code division multiple access (CDMA) communication format, which is referred to herein and assigned to the assignee of the present invention, called "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters". US Patent No. 4,901,307, and US Patent No. 5,103,459, entitled "System and Method for Generating Waveforms in a CDMA Cellular Telephone System." The use of paging and access channels for call origination is well known in the art and is described in detail in the TIA / EIA interim standard IS-95-A entitled "Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System". have. One of the characteristics of the CDMA system is that the same frequency is reused in all cells. Diversity combining is a method by which receivers receiving signals carrying the same information combine signals propagating through different paths to provide an improved estimate of the transmitted signal. Receiver designs that take advantage of diversity signals that are transmitted by different transmitters or travel through different propagation paths and carry the same information are referred to as "Diversity Receiver in a CDMA Cellular Telephone System", assigned to the assignee of the present invention. It is described in US Pat. No. 5,109,390.

Soft handoff is a method where a mobile station moving from one cell to another cell receives information from a base station serving two or more cells in the boundary area while the mobile station is located near the boundary. The signal transmitted by the base station is coupled to the receiver of the mobile station by the diversity combining method described above. A method and apparatus for providing soft handoff in a CDMA communication system in which a plurality of base stations communicate at or near a cell boundary is assigned to the assignee of the present invention and referred to herein as "Method and System for Providing a Soft." US Patent No. 5,101,501 entitled "Handoff in Communications in a CDMA Cellular Telephone System" and US Patent No. 5,267,261 entitled "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications System". Hard handoff, in contrast to soft handoff, is that a mobile station moving from one cell to another is dropped by the escaping cell before it is picked up by the entering cell.

Using the same frequency and soft handoff in all cells results in high CDMA system capacity. By reusing the same frequency in neighboring cells, a drastic change in the forward link signal-to-noise ratio occurs near the cell boundary. This is because the cell received by the mobile station may fade and the force of neighboring cells may increase (anti-fade). In general, when a mobile station receives two cells, the received traffic channel energy per spreading chip versus the total spectral noise density for the signal transmitted by the cell 1 is given by:

Equation 1

The received traffic channel energy per spread chip versus the full spectral noise density for the signal transmitted by the cell 2 is given by:

Equation 2

In equations (1) and (2),

Ioc is the total thermal noise,

Ec / Ior ₁ , Ec / Ior ₂ are the fractions of the traffic channel power transmitted by cells 1 and 2.

,

는 각각 셀 (1) 및 (2) 로부터 이동국에서 수신되는 트래픽 채널 파워의 분수이다.

,

Is the fraction of the traffic channel power received at the mobile station from cells 1 and 2, respectively.

, 및

에 비해 작은 것으로 간주한다. 셀 (1) 이 셀 (2) 이 비해 페이드할 때,

는

에 비해 작게 되고 따라서 비율 (

) 은 커진다. 따라서, Ec/Io₁ 는 작게 된다. 이동국이 소프트 핸드오프에 있지 않다면, 이러한 신호대 잡음비 변화는 문제를 발생시킨다. 그러나, 이동국이 이웃 셀과 소프트 핸드오프에 있다면, 이동국이 양쪽 셀로부터 순방향 트래픽 채널의 다이버시티 결합을 수행하기 때문에 신호 대 잡음비 변화는 문제가 되지 않는다. Ec/Io₁ 로 주어진 제 1 경로가 작게 되는 반면, Ec/Io₂ 로 주어진 제 2 경로는 커진다. 따라서, 한 셀에 의한 페이딩은 나머지 한 셀로부터의 신호 대 잡음비를 증가시킨다.Ioc is

, And

It is regarded as small. When cell (1) fades compared to cell (2),

Is

Becomes smaller compared to

) Becomes large. Therefore, Ec / Io ₁ becomes small. If the mobile station is not in soft handoff, this change in signal-to-noise ratio creates a problem. However, if the mobile station is in soft handoff with a neighboring cell, the signal-to-noise ratio change is not a problem since the mobile station performs diversity combining of the forward traffic channel from both cells. The first path given by Ec / Io ₁ is made small while the second path given by Ec / Io ₂ is made large. Thus, fading by one cell increases the signal-to-noise ratio from the other cell.

Paging is a method of transmitting information indicating the start of a mobile end service or information instructing a mobile station to receive new overhead information to the mobile station. The method of initiating a base station-initiated call is a continuation of US Pat. No. 5,392,287 and US Pat. No. 5,392,287, filed March 7, 1994, entitled "Apparatus and Method for Reducing Power Consumption in a Mobile Communications Receiver." And are described in detail in co-pending US patent application Ser. No. 08 / 206,701, both of which are incorporated by reference. The present invention is equally applicable to the mobile station-initiated call, which method is assigned to the assignee of the present invention and is described in detail in co-pending US patent application Ser. No. 08 / 219,867, filed January 18, 1996.

In a slot paging system, the mobile station monitors the paging channel for a predetermined short time interval and then does not monitor the paging channel until the next predetermined time interval. In IS-95, this method of periodically monitoring the paging channel is called slotted mode, and the mobile station may monitor the paging channel for 80 ms every 1.28 seconds. The period between monitoring intervals can be further extended as desired by the user. Before each predetermined time slot in which the mobile station can be paged, the mobile station is active to resynchronize or improve synchronization with the base station. The mobile station then monitors for paging or other messages in the slot. After one interval, the mobile station may be inactive and not monitor the paging channel until immediately before the next designated slot. Before the time when the mobile station is active to communicate traffic information with the mobile communication system and after the time when the mobile station is in timing synchronization with the communication system, the mobile station is in an idle state. In the idle state, the mobile station can receive a message, receive an incoming call, initiate a call, or initiate a message transmission. When the mobile station is in the idle state, IS-95-A causes the mobile station to perform idle handoff at any time other than the interval at which the mobile station is required to monitor its designated slot.

However, when the mobile station places a call or receives a paging, the mobile station is in system access to send an outgoing message or page response message. While in the system access state, the IS-95 mobile station does not operate in slot mode. This is referred to as nonslot operation. In particular, the mobile station continuously monitors the paging channel until it is assigned a different state by the base station or until an error condition occurs that causes the mobile station to leave the system access state. The embodiment is described in the context of an origination operation and an origination message, but this concept also applies directly to a mobile terminated call process and page response message. When the mobile sends an outgoing message and receives an acknowledgment, the mobile waits for a channel assignment message indicating on which traffic channel the communication from the base station to the mobile station will be performed.

After receiving the channel assignment message, the mobile station tunes to the assigned traffic channel, receives information on the forward traffic channel, and begins to transmit on the reverse traffic channel. The forward traffic channel is a channel through which information is transmitted from the base station to the mobile station, and the reverse traffic channel is a channel through which information is transmitted from the mobile station to the base station.

The interval between the time that a mobile station sends an outgoing message and the time that the mobile receives a channel assignment message depends on the realization of the individual infrastructure vendor. This can range from less than 1/2 second up to several seconds. The mobile is in system access until the time the mobile receives the channel assignment message.

Typically the paging channel does not support soft handoff. Thus, the above-mentioned fading problem occurs. This is typically neutralized by having a higher paging channel radiation than the traffic channel. Since one paging channel can handle call origination and termination of many traffic channels, this higher power minimizes capacity loss. To support soft handoff in the paging channel, the system sends the same information to the paging channel of all cells, thereby dramatically reducing the overall capacity of the paging channel.

While in the idle state, the mobile station may be allowed to perform a handoff. Typically, whenever a signal level received from one cell is sufficiently above another, the mobile station performs a handoff. This idle handoff is done especially before the mobile station monitors the slot. However, there may be cases where the mobile station cannot select the correct cell before the slot begins and must constantly monitor the remaining cells. During the system access state, the mobile station is not allowed to perform idle handoff.

However, when the mobile station is in the system access state, there may be a case where the change in the signal-to-noise ratio (Ec / Io ₁ ) is so fast that the message error rate is so high that the mobile station cannot correctly receive the signaling message sent on the paging channel. Can be. As a result, the mobile station may not receive a channel assignment message. This prevents the call origination from being successful. The IS-95-A returns to the mobile station idle state by leaving the system access state if the mobile station does not receive any paging channel message for one second. Thus, the mobile station does not receive channel assignment messages and call origination is not successful.

Similar problems exist when a mobile station is first assigned to a traffic channel. IS-95-A allows only one base station to be assigned to the mobile station. If another cell is powerful or more powerful, the mobile station may not successfully receive the forward traffic channel. As a result, the call may be dropped. The problem is that the mobile station is assigned to a traffic channel with one active set member and is not in soft handoff.

In IS-95-A, the following steps must occur for the mobile station to initiate soft handoff. First, the mobile station must determine if the pilot of another base station is above a predetermined energy threshold. Secondly, the mobile station should send a pilot strength measurement message. Third, the infrastructure must set up a handoff to send a handoff direction message to the mobile station. Depending on the environment and implementation, this may take hundreds of milliseconds to 1 second or more.

Thus, although soft handoff is generally supported in IS-95-A systems, soft handoff is not supported when the mobile station is in system access. Thus, what is needed is a system that allows for soft handoff that can provide increased reliability and other benefits in the system access process when the mobile station is in system access.

Summary of the Invention

A method of performing a handoff in a mobile communication system having a first communication device in communication with a plurality of second communication devices, wherein the first communication device makes a handoff from one or more of the second communication devices to the first communication device. Sending a handoff list of a second communication device to perform. The channel assignment message is sent from each of the second communication device in the handoff list. The handoff list includes a base station controlled by a single controller that determines which of the second communication devices sends a channel assignment message.

The base station in the handoff list has a pilot signal and the energy level of the pilot signal is measured by the first communication device. The measured energy levels are sent to the controller and compared by the controller to provide a list of energy levels of the base station according to the comparison. The energy level list is sent to the first communication device and the handoff is performed according to the energy level list.

The present invention also describes a variant that may improve operation in a paging and access channel. The present invention thus enables handoff while the mobile station is in system access. This allows the mobile station to receive a base station with a high signal to noise ratio so that the message error rate is low. This prevents call setup from dropping due to not receiving the paging channel.

Another feature of the present invention is that the infrastructure enables the measurement of which base station sends a channel assignment message to the mobile station. This ensures that mobile stations can handoff to different base stations and that traffic channels are allocated at the new base station without delay.

Another feature of the present invention is that the infrastructure can measure which base station is in the mobile station's active set before the mobile station is assigned a traffic channel. An active set is a set of base stations that provide the most powerful signal to a mobile station. This allows the infrastructure to measure whether there are enough resources to put the mobile station in soft handoff before the traffic channel is assigned to the mobile station. This is useful because a mobile station near the cell boundary can immediately request to be placed into soft handoff before being assigned a traffic channel. In addition, this minimizes call drop due to the rapid change in the signal-to-noise ratio described above.

Finally, the above features have special use in Priority Access and Channel Assignment (PACA) operations where a user has access to limited communication resources in accordance with specified user priorities. Although the present invention has been described with respect to a CDMA system, the present invention is applicable to other cellular or satellite communication systems.

Brief description of the drawings

The features, objects, and advantages of the invention will become more apparent from the drawings and detailed description, respectively correspondingly identified by like reference numerals.

1 is a block diagram of a communication system of the present invention that includes a mobile station communicating with a group of base stations.

2 is a layout of a cell corresponding to the base station of FIG. 1.

3A-3D show a list of base stations sent to mobile stations in a communication system.

4 shows a pilot (Ec / Io) for a mobile station moving between two base stations.

Detailed description of the preferred embodiment

In FIG. 1, a communication system 14 is shown. The communication system 14 includes a mobile station 2 including a control processor 18 and a base station 26a-o. In the embodiment, base station 26a corresponds to cell 36a in FIG. 2. Thus, for example, base station 26a provides a coverage area for cell 36a. As it is powered up, the mobile station 2 is in a system measurement substate. The system measurement processor (not shown) selects a system to perform the acquisition attempt and provides the receiver 8 with the necessary frequency information. Although not shown separately, the system measurement processor can be realized with the control processor 18. The control processor 18 can be realized as a microcontroller or microprocessor operating under program control stored in a memory.

In the embodiment, the mobile station 2 moves to the pilot acquisition substate to demodulate the pilot signal based on the acquisition parameter received in the system measurement substate and attempts to acquire the CDMA pilot signal according to the acquisition parameter. The signal (if present) is received at the antenna 4 and passed through the duplexer 6 to the receiver 8. The receiver 8 downconverts and amplifies the received signal, converts the analog signal into digital, and delivers it to the searcher 10. Searcher 10 attempts to obtain a pilot signal by checking a pseudo random noise (PN) offset. PN offset is tested by demodulating the signal according to the PN offset hypothesis and measuring the energy of the demodulated signal. Search hardware design and implementation for CDMA acquisition is known in the art and is described in detail in the above-mentioned US Pat. No. 5,109,390.

When the searcher 10 detects a pilot signal with energy above a predetermined threshold, the mobile station 2 is in a sync channel acquisition substate and attempts to acquire a sync channel. Typically, a synchronization channel, such as broadcast by a base station, contains basic system information such as system identification (SID) and network identification (NID), but most importantly provides timing information to the mobile station 2. The mobile station 2 adjusts the timing according to the synchronization channel information and then enters the mobile station idle state.

Upon successful acquisition of the synchronization channel, the mobile station 2 starts to monitor the paging channel according to the predetermined paging format. The mobile station 2 demodulates the signal based on a predetermined Walsh sequence reserved for paging channel transmission. For example, if the obtained pilot signal is transmitted by the base station 26a, the mobile station 2 monitors the paging channel according to the timing information provided by the synchronization channel and using a predetermined Walsh sequence.

Referring to Figures 3A-3D, a base station list 50, 60, 70, 80 is shown that is sent to mobile station 2 to allow idle handoff in communication system 9 of the present invention. In IS-95-A, the neighbor list 50 is provided to the mobile station 2 by the base stations 26a-n of the neighbor list message. The neighbor list 50 is also referred to as NGHBR_LIST_BASE 50. NGHBR_LIST_BASE 50 is a list of base stations 26b-k that are geographically close to base station 26a that can provide a strong signal to mobile station 2, which is also a candidate for idle handoff of mobile station 2.

The base station controller 32 serves to provide information between the base stations 26a-o, that is, selectively provides information from the main telephone exchange (not shown) to the base stations 26a-o and the base stations 26a-o. ) To provide internally generated messages. It should be noted that the present invention is equally applicable when some base stations in the neighbor list 50 are not controlled by the same base station controller 32.

If the pilot signal obtained by the mobile station 2 is transmitted by the base station 26a after receiving the overhead information, the mobile station 2 transmits its mobile identification number to the base station 26a to the base station 26a. It may be registered. The mobile station 2 then goes into an idle state and, after successfully registering with the base station 26a, monitors its assigned paging channel in slot paging mode. If registration is not performed, the mobile station 2 is also in an idle state and monitors its assigned paging channel transmitted by the base station 26a in slot paging mode.

In the slot paging mode, the base station 26a transmits any paging or signaling information destined for the mobile station 2 at predetermined time intervals called time slots. In the embodiment, the time slot and paging channel are determined according to the hashing function of the mobile identification number known for the base station 26a and the mobile station 2 as it is registered.

In the present invention, the base station 26a transmits, to the mobile station 2, the handoff list 60 of the base station that the mobile station is allowed to perform idle handoff while in the system access state. Handoff list 60 is also referred to herein as LIST_BASE 60. The base station in LIST_BASE 60 is typically a subset of the base stations of NGHBR_LIST_BASE 50 and typically uses the same base station controller 32. For example, NGHBR_LIST_BASE 50 may include all base stations 26b-k, while LIST_BASE 60 may include only base stations 26b, 26c, 26g, 26h.

When the mobile station 2 originates a call, the message generator 20 generates an outgoing message and sends it to the access channel. Message generator 20 may be realized with a microprocessor programmed to perform the described functions. Although shown as a separate element, message generator 20 may be implemented within control processor 18. The outgoing message is received and demodulated by the base station 26a which the mobile station is currently monitoring. In response to receiving the originating message, each base station in LIST_BASE 60 sends a channel assignment message indicating a traffic channel through which communication can be performed. Typically, the Walsh channel used to communicate with the first base station in LIST_BASE 60 does not need to be the same as the Walsh channel used to communicate with the second base station in LIST_BASE 60. Because multiple base stations transmit a channel assignment message, mobile station 2 can perform idle handoff while in system access state and still receive a channel assignment message after sending an outgoing message to any base station in system access state. can do.

In an alternative embodiment, mobile station 2 sends the originating message to base station 26a and then waits for acknowledgment of the originating message. Until the mobile station 2 receives the acknowledgment, the mobile station 2 is not allowed to perform a handoff. However, after the mobile station 2 receives the acknowledgment, it is possible to perform idle handoff with any base station in the LIST_BASE 60.

In another alternative embodiment, mobile station 2 is co-pending under the name “Random Access Channel”, filed March 12, 1994, and assigned in US Patent Application No. 08 / 412,648, assigned to the assignee of the present invention. As described in detail, send an outgoing message using the procedure described in IS-95-A. If an acknowledgment is not received from base station 26a within a predetermined timeout, the mobile station attempts to increase its transmit power and resend the message. If the mobile station 2 does not receive an acknowledgment from the base station 26a after a predetermined number of attempts and another base station, for example, the base station 26b is more powerful, the mobile station 2 idles with respect to the base station 26b. Allow handoff to be performed and resume sending outgoing messages.

In one embodiment, each base station in LIST_BASE 60 sends a channel assignment message indicating only a traffic channel for communication with itself. In an alternative embodiment, each base station 26a-i in LIST_BASE 60 may be assigned the same channel assignment indicating the traffic channel used to communicate with a particular base station as well as the traffic channel used for communicating with all base stations within LIST_BASE 60. Send a message. This requires a base station in LIST_BASE 60 to communicate on the available traffic channel via base station controller 32. By providing a channel assignment message from a plurality of base stations, the success rate of the channel assignment process is greatly improved.

The above process allows the infrastructure to set up soft handoff and includes members of one or more active sets in the channel assignment message. Instead of first communicating with one base station and then moving to soft handoff, it is possible for the mobile station 2 to immediately receive traffic communication from two or more base stations and immediately to soft handoff. This accelerates the process by which the mobile station 2 goes into soft handoff, improves the performance of the communication system 9, and minimizes call drop due to low forward traffic channel signal to noise ratio.

In one embodiment of this process, the base station sets up soft handoff with all base stations in LIST_BASE 60. In an alternative embodiment of this process, the base station sets up a soft handoff with a subset of base stations in LIST_BASE 60 and sends the information in the channel assignment message necessary for the mobile station 2 to be in soft handoff. This information includes the identification of the base station subset. In IS-95-A, the pilot PN offset identifies the base station.

The paging message sent by the base stations 26a-i is received at the antenna 4 of the mobile station 2. The received message is then provided to the receiver 8 where the signal received via the duplexer 6 is downconverted and amplified. The downconverted message is provided to demodulators 12a-j that demodulate the received message. The control processor 18 according to the information from the searcher 10 selects an input paging channel or a channel to be demodulated by the mobile station 2. In one embodiment, demodulators 12a-j monitor only one base station.

The searcher 10, which cooperates with the control processor 18, may determine that another base station is better. The demodulator then demodulates the signal received from the other base station. Since mobile station 2 receives an assignment message from one or more base stations, mobile station 2 can perform idle handoff in a system access state. In another embodiment, mobile station 2 monitors all base stations in LIST_BASE 60 and demodulates the signals identified in LIST_BASE 60.

In one preferred embodiment, LIST_BASE 60 is not provided separately from neighbor list 50 in the neighbor list message. Instead, a compound list 70 is provided that includes all members of the neighbor list 50. In addition, the flag 72 is included in the compound list 70 to indicate which member of the neighbor list 50 is a LIST_BASE 60 member. In one embodiment, one of the reservation values 72 in the overhead message is used to indicate which of the base stations specified in the neighbor list message is in the LIST_BASE 60. In IS-95-A, base station reservation value 72 is provided in an overhead message to specify which member of neighbor list 50 is in LIST_BASE 60.

In an embodiment, the IS-95-A neighbor list message includes a pilot PN offset for the base station in NGHBR_LIST_BASE 50 and includes an indication of which base station in the neighbor list message is in LIST_BASE 60. A pilot PN sequence for the current base station is transmitted to provide a reference to the mobile station 2 to identify another base station PN offset.

As mentioned above, all base stations in the LIST_BASE 60 need to send a channel assignment message to the mobile station 2. This causes the mobile station 2 to be handed off and thus increases the success rate of the channel assignment process and also requires additional paging channel capacity for all call setups.

A modified procedure that reduces the impact on paging channel capacity sends a base station pilot list to mobile station 2 that is above a predetermined power threshold. This list is LIST_MOBILE 80, also referred to as energy list 80. In one embodiment, searcher 10 demodulates the pilot signal by first selecting the PN offset of the base station in LIST_BASE 60 followed by the PN offset of the base station in NGHBR_LIST_BASE 50 and then following the remaining PN offset. A method for providing an optimized search priority selection is described in US Pat. No. 5,267,261, supra.

In an embodiment, the searcher 10 demodulates the received signal according to the pilot PN offset and measures the energy of the demodulated pilot. The energy value is provided to the control processor 18. Control processor 18 compares the energy of the demodulated signal with a threshold and compiles energy list 80. The energy list 80 includes a PN offset list that is greater than or equal to the energy threshold used by the control processor 18. The energy list 80 can be designated as LIST_MOBILE 80. The LIST_MOBILE 80 is compiled and then sent to the access channel and received by the base station 26a which the mobile station 2 is monitoring. In an embodiment, LIST_MOBILE 80 is included in the outgoing message.

In yet another embodiment, LIST_MOBILE 80 is received by one or more base stations 26a-o. LIST_MOBILE 80 is provided to the base station controller 32. In the preferred embodiment, the threshold used by the mobile station 2 to determine whether it includes a base station in the LIST_MOBILE 80 is sent by the base stations 26a-o as part of the overhead message. In a preferred embodiment, the threshold may be a T_ADD value sent in an IS-95-A system parameter message. This T_ADD value is currently determined by the IS-95-A mobile station to determine whether to send an IS-95-A pilot strength measurement message in the traffic channel to a base station indicating that the mobile station has detected a pilot exceeding the T_ADD value. It is used.

4, when the mobile station 2 moves from the base station 26a to the base station 26b, an Ec / I ₀ for IS-95-A pilot channel broadcast by the base stations 26a and b is shown. . As indicated by the area 38, if the mobile station 2 is completely within the area of the base station 26a, the pilot channel of the base station 26b is below the T-ADD level. Similarly, if mobile station 2 is completely within the area of base station 26b, the pilot channel of base station 26a is below the T-ADD level. When mobile station 2 is in area 38, it does not report base station 26b in an outgoing message. Similarly, when mobile station 2 is in area 41, it does not report base station 26 in the outgoing message.

When mobile station 2 is in area 39, the pilot (Ec / Io) for base station 26b is greater than or equal to T-ADD and the mobile station reports base station 26b in an outgoing message. Similarly, when the mobile station is in area 40, the pilot (Ec / Io) for base station 26a is greater than or equal to T-ADD and mobile station 2 reports base station 26a in the outgoing message. Preferred embodiments use Ec / Io as described in IS-95-A for this measurement. However, other measurements of signal strength or signal to noise ratio known in the art are equally available.

In the preferred embodiment, mobile station 2 is allowed to perform idle handoff only for base stations in LIST_MOBILE and LIST_BASE 60. The base station set in both lists may be designated as LIST_BOTH. This has two advantages. First, the infrastructure is only needed to transmit channel assignment messages of base stations that are identified as candidates for idle handoff by the mobile station and are allowed to handoff the mobile station. This is the set of base stations given in LIST_BOTH. This significantly reduces the additional messages required. Second, LIST_MOBILE provides the base station controller 32 with a pilot list that is T_ADD or greater. This allows the infrastructure to identify which base station is a member of the mobile station's active set. Thus, base station controller 32 may set up soft handoff only for base stations in LIST_MOBILE when the mobile station is assigned to a traffic channel.

In an alternative embodiment, mobile station 2 sends the base station in LIST_BOTH to the base station in the originating message. This reduces the amount of information to be transmitted from the mobile station 2. In addition, the infrastructure can set up soft handoff and include one or more members of the active set in the channel assignment message. The channel assignment message includes the PN offset of the base station in the active set. Instead of first communicating with one base station and then moving to a soft handoff that may not be possible due to capacity or other limitations, the mobile station 2 may immediately be in soft handoff and receive traffic communications from two or more base stations immediately. Can be.

For example, if mobile station 2 is at location 37 in cell 36a that is near the boundary with cell 36b, LIST_MOBILE identifies the pilot PN offset of base station 26b. The channel assignment message sent by base stations 26a and b identifies the traffic channel for use by mobile station 2 for dedicated communication between base stations 26a and b and mobile station 2. One or more demodulators 12a-j are adjusted to receive traffic channel information from base station 26a and other demodulators 12a-j are adjusted to receive traffic channel information from base station 26b. The plurality of demodulators 12a-j begin to demodulate the traffic channel signals transmitted by the base stations 26a, b. The demodulated signal is applied to a diversity combiner 34 that combines the received signals to provide a measure of enhanced transmission data.

In practicing the method of the present invention, the mobile station 2 does not perform an idle handoff until it receives an acknowledgment for the timeout or transmitted message indicating that the acknowledgment period is complete. This allows the mobile station 2 to receive an acknowledgment for its access channel probes. In addition, this allows the base station 26a to transmit its access channel probe to generate an ack other than the one being generated by the base station controller 32. This reduces delays and makes the call setup process faster. In addition, if the mobile station 2 is in the system access state and performs idle handoff after the ack timeout is completed, the mobile station 2 must restart the access channel probe transmission procedure. This would be the same as if the mobile station 2 sent a new outgoing message.

In an alternative embodiment, mobile station 2 performs idle handoff for the base station in LIST_BASE 60 before receiving the acknowledgment. Thus, all base stations in LIST_BASE 60 must send an ac and base station controller 32 must assist in generating the ac. In a modification of this alternative embodiment, the mobile station 2 may perform an idle handoff for the base station in the LIST_MOBILE before receiving the acknowledgment. Similarly, all base stations in LIST_MOBILE must transmit an acknowledgment and thus base station controller 32 must be involved in generating an ac.

In a preferred embodiment, the present invention provides the possibility that the channel assignment message is sent by the base station 26a and not by the mobile station 2. Base station 26a may have received an outgoing message from mobile station 2 but mobile station 2 may not have received a channel assignment message that acknowledges receipt of an outgoing message from base station 26a. Even if there is no acknowledgment of the message, the mobile station 2 may, for example, perform an idle handoff with respect to the base station 26b.

The base station 26b sends the channel assignment message to the mobile station 2, while the mobile station 2 retransmits the originating message. In an embodiment, when an Ack message is sent, an indication of which message is being accompanied is accompanied. The mobile station 2 ignores the channel assignment message unless the indicator corresponds to the most recently sent outgoing message. The present invention includes several ways to correct this problem. One solution is for the base station 26b to use the same Ack indicator in the outgoing message received by the base station 26a. This is done by sending the acknowledgment value from base station 26a to base station 26b via base station controller 32. In an alternative embodiment, if the mobile station 2 receives the channel assignment message, it can stop sending the access probe and tune to the channel specified by the channel assignment message.

In an improved embodiment, the paging channel configuration of all base stations that mobile station 2 is allowed to handoff (relative to base stations in LIST_BASE 60) are the same. Base stations that do not support this functionality will not be included in LIST_BASE 60.

This method can also be used to support PACA. PACA features are known in the art and described in detail in "TIA / EIA / IS-53-A Cellular Features Description". When PACA is implemented, the mobile station 2 is given priority over other mobile stations in obtaining a traffic channel when no traffic channel is available. In particular, the mobile station 2 sends an outgoing message that includes the PACA feature code and the dialing number. If the traffic channel is immediately available, the mobile station 2 is assigned to the traffic channel. If the traffic channel is not immediately available and the mobile station 2 is authorized to use the PACA, the base station monitoring the mobile station 2 places the request of the mobile station 2 in the PACA queue. For example, base station 26a may place the request in a queue. Alternatively, the PACA queue can be managed by the base station controller 32. The position in the queue depends on the priority of the PACA request and the request duration. When a traffic channel becomes available, the request at the head of the PACA queue is allocated to the traffic channel.

When the request of the mobile station 2 is in the PACA queue, the mobile station 2 can receive a periodic message informing the user of the queue state of the mobile station 2. One problem with PACA is that the infrastructure needs to know which cell the mobile station 2 is currently using to determine whether the channel is free. In most systems this requires the mobile station 2 to register or resend an outgoing message each time it performs an idle handoff. Due to the rapid transition between CDMA base stations, mobile station 2 may register or retransmit an outgoing message while crossing the boundary between base stations. The second consideration with respect to CDMA is that it may be in soft handoff shortly after mobile station 2 is assigned to the traffic channel. The allocation may not be successful unless the resource is available at multiple base stations to support the call.

As the modified outgoing message described above, the mobile station 2 represents another base station that should be in the active set of the mobile station, and the set of base stations from which the mobile station 2 detected a strong pilot signal. In one embodiment, mobile station 2 sends LIST_MOBILE and the base station measures LIST_BOTH. In yet another embodiment, the mobile station 2 sends LIST_BOTH. This allows the infrastructure to determine whether resources are free at all base stations needed for the PACA call. To reduce the outgoing message rate, the base station in LIST_BOTH allows the mobile station to move without resending the outgoing message. When this feature is involved, the infrastructure needs to send queue status information of all base stations in LIST_BOTH. If the mobile station 2 is out of coverage of the base station in LIST_BOTH, the mobile station 2 retransmits the outgoing message.

Description of the preferred embodiments is provided so that any person skilled in the art can use or make the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, where the limited principles may be applied to other embodiments without the use of creativity. Accordingly, the present invention should not be limited to the embodiments shown herein but should be construed to the widest scope consistent with the novel features and principles.

Claims (44)

delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete A method of performing handoff in a mobile communication system having one or more first communication devices in communication with a plurality of second communication devices (SCDs), the method comprising: (a) transmitting from one or more of the SCDs to the first communication device a handoff list that is allowed to hand off when the first communication device is in a system access state, wherein: Sending a handoff list, the handoff list comprising an SCD controlled by a single controller not located in the one or more first communication devices; And (b) sending a channel assignment message from each SCD in the handoff list to the one or more first communication devices. The method of claim 18, And wherein said single controller determines the SCD used to send said channel assignment message. The method of claim 18, SCD on the handoff list transmits a pilot signal, (c) at the first communication device, measuring an energy level of the pilot signal; (d) sending the measured energy level to the single controller; (e) comparing the transmitted energy levels; And (f) according to the comparison, providing a list of energy levels of the SCD. The method of claim 20, Transmitting the energy level list to the first communication device. The method of claim 21, And performing a handoff according to the energy level list. The method of claim 22, And performing a handoff only to the SCD included in both the handoff list and the energy level list. The method of claim 20, Sending, to the first communication device, a neighbor list of SCDs located geographically close to the first communication device, The handoff is performed after the step of sending an origination message from the first communication device to one or more SCDs. The method of claim 24, And the handoff list and the neighbor list are transmitted together. The method of claim 24, And the neighbor list comprises an SCD controlled by a plurality of controllers. The method of claim 24, SCD on the handoff list includes a subset of SCD on the neighbor list. The method of claim 18, After the step (b), when the first communication device is in an idle mode (access state), performing a handoff of the first communication device from one of the SCDs to another of the SCDs; Is accompanied by a handoff method. The method of claim 18, The step (a) is performed after the step of synchronizing with at least one of the SCD by the first communication device. An apparatus for performing handoff in a mobile communication system, A first communication device (FCD); A plurality of second communication devices (SCDs) in communication with the FCD; And With one or more controllers, One or more of the SCDs sends a handoff list of other SCDs to the FCD that the FCD is allowed to handoff when the FCD is in system access; At least one SCD in the handoff list sends a channel assignment message to the FCD for communication with each SCD included in the handoff list, And the at least one controller is used to control all SCDs included in the handoff list. The method of claim 30, Further comprising one or more controllers used to control all SCDs included in the handoff list, Wherein each SCD in the handoff list individually transmits a channel assignment message to the FCD for communicating with the respective SCD. The method of claim 31, wherein And a controller determines one SCD or a plurality of SCDs used to transmit the channel assignment message. The method of claim 30, One or more controllers used to control one or more of the SCDs included in the handoff list; And A device communicatively coupled to the FCD, the device being used to measure an energy level of a pilot signal received by the FCD from any SCD, the measured energy level being transferred to the one or more controllers. More equipped And said at least one controller compares said energy levels and assists in delivering a ranked energy level list for said SCD to said FCD. The method of claim 33, wherein And handoff is performed according to the energy level list. The method of claim 34, wherein And handoff is performed only to the base SCD included in the handoff list. An apparatus having at least one first communication device (FCD), The FCD is, Processing apparatus; A message generating device coupled to the processing device; A modulation device coupled to the message generating device; A transmission device coupled to the modulation device; A receiving device; A plurality of demodulation devices communicatively coupled to the processing device and the receiving device to demodulate a signal received from the receiving device; A search device coupled to the processing device and the receiving device; A combiner coupled to the demodulation device and used to generate one signal from signals received from the demodulation device; A duplexing device coupled to the transmitting device and the receiving device; One or more antennas coupled to the duplexing device; And A plurality of second communication devices (SCD) in communication with the FCD, One or more of the SCDs sends a handoff list of other SCDs to the FCD that the FCD is allowed to handoff when the FCD is in system access; At least one SCD in the handoff list sends a channel assignment message to the FCD for communication with each SCD included in the handoff list. An apparatus having at least one first communication device (FCD), The FCD is, Processing means for processing data; Message generating means coupled to the processing means for generating a message; Modulation means coupled to said message generating means for modulating a signal; Transmission means coupled to the modulation means for transmitting a signal; Receiving means for receiving a signal; Demodulation means communicatively coupled to the processing means and the receiving means to demodulate a signal received from the receiving means; Search means coupled to said processing means, said demodulation means and said receiving means, for searching for a desired signal; Combining means coupled to the demodulation means, the combining means being used to generate a signal from signals received from the demodulation means; Duplexing means coupled to the transmitting means and the receiving device, the signals being received and transmitted simultaneously; One or more antenna means coupled to the duplexing means to receive a signal; And A plurality of second communication devices (SCD) in communication with the FCD, One or more of the SCDs sends a handoff list of other SCDs to the FCD that the FCD is allowed to handoff when the FCD is in system access; At least one SCD in the handoff list sends a channel assignment message to the FCD for communication with each SCD included in the handoff list. A method of performing handoff in a mobile communication system having one or more first communication devices in communication with a plurality of second communication devices (SCDs), the method comprising: (a) transmitting from one or more of the SCDs to the first communication device a handoff list of SCDs that the first communication device is allowed to handoff when the first communication device is in a system access state; And (b) sending a channel assignment message from each of the SCDs in the handoff list to the one or more first communication devices. The method of claim 38, And the channel assignment message transmitted from each SCD in the handoff list includes a channel assignment message to be used only for communication with each transmission SCD. A method of performing handoff in a mobile communication system having one or more first communication devices in communication with a plurality of second communication devices (SCDs), the method comprising: (a) transmitting from one or more of the SCDs to the first communication device a handoff list that is allowed to hand off when the first communication device is in a system access state, wherein: Sending a handoff list, the handoff list comprising an SCD controlled by a single controller not located in the one or more first communication devices; And (b) sending a channel assignment message from each of the SCDs in the handoff list to the one or more first communication devices. The method of claim 38, Transmitting to the first communication device a neighbor list of SCDs located geographically close to the first communication device. The method of claim 38, Step (b) is followed by performing a handoff of the first communication device from one of the SCDs to another of the SCDs. An apparatus for performing handoff in a mobile communication system, A first communication device (FCD); And A plurality of second communication devices (SCD) in communication with the FCD, One or more of the SCDs sends a handoff list of other SCDs to the FCD that the FCD is allowed to handoff when the FCD is in system access; At least one SCD in the handoff list sends a channel assignment message to the FCD for communication with each SCD included in the handoff list. The method of claim 43, Wherein each SCD in the handoff list individually sends a channel assignment message to the FCD for communicating with the respective SCD.

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