MXPA99004193A - Seamless handoff system and method - Google Patents

Abstract

A multicellular communications system where transmission between a roving subscriber and multiple base station transceivers is maintained throughout the territory. Each mobile subscriber unit has global code seeds for the entire communications system or an updated neighbor list and continuously searches to access available cell base stations while maintaining a communication link with one base station. Candidate cell base stations are interrogated and communicated with. The candidate base station that requires less transmitting energy from the mobile subscriber unit is closely monitored. When the mobile subscriber unit can successfully communicate with less transmit power than currently required by the present base station, the mobile subscriber unit is handed off to the candidate base station transceiver.

Description

SYSTEM AND METHOD OF TRANSFER WITHOUT INTERRUPTION BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to communication systems. More specifically, the invention relates to a system and method for controlling the transfer between individual cells for a mobile subscriber in a multicellular communication environment.

Description of the Previous Technique The transfer is a general term that describes the function of transferring a communication signal for a mobile subscriber from a base station transceiver to another base station transceiver as the subscriber moves in the cellular territory. There are two general methods of carrying out the transfer in a cellular communication system, soft and hard transfer. The quality may suffer using any of the transfer methods as the mobile subscriber changes base stations between individual cells.

Hard transfer typically occurs near the boundaries between cells. Through continuous measurements of the signal strength received from a mobile subscriber, the base station where the subscriber has established communication determines whether the power is reduced below a nominal value close to the cell limit. The hard transfer to a candidate cell base station occurs instantaneously without interrupting the call in progress. The power received at the candidate cell base station is much greater than that needed to ensure an ideal transfer. The difference between the current cell power and the candidate cell power significantly reduces the ability to interfere with other users. A smooth transfer is translated through a given distance interval from the current and candidate cell base stations. In the soft transfer method, the user connects to both the base stations as they move near the common cell boundary. The switching decision is made based on the reception of the mobile subscriber's pilot signal. A central switch center decides at which point of the base stations it should be interrupted. The shared communication is done during a finite period of time, during which the transmission of the current and candidate cells is required.

In a typical CDMA communication system, the plurality of signals is transmitted within the same frequency band. Frequency reuse not only applies to users in the same cell but also to those in all other cells. Since the same sequences are used, the power levels transmitted from the mobile subscriber and the base station must be monitored closely. If they do not strictly adhere to the power control, the total transmission interference and the total number of usable channels are adversely affected. Therefore, the number of signals that can be transmitted and received successfully is associated with the total power of all users. Both soft and hard transfer methods have drawbacks. Hard transfer can potentially suffer from high disconnection rates. The soft transfer method requires a duplication of the transmission resources from the current base station and a base station from at least one candidate cell. The mobile subscriber must establish two current communication links, so it requires twice the transmission power which would otherwise be required. The increase in transmission power wastes power and air capacity and contributes to the total interference of the system while the transfer is taking place. The power or energy transmitted in a CDMA system by each user must be kept at a minimum necessary to transport information and to minimize interference with other users. Careful control of the transmission power also contributes to an extended use of portable devices that rely on battery power. In addition, the diversity in the combination of duplicate signals in the central switching center is problematic unless the delay from both cell base stations is almost identical. Consequently, there is a need for an efficient, fast and reliable method of transfer.

BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a multicellular communication system wherein transmission is maintained between a shifting subscriber and multiple base station transceivers. Each mobile subscriber unit has global code seeds for the entire communications system and continuously seeks access to base stations of available cells and at the same time maintains a communication link with a base station. The candidate cell base stations interrogate and communicate with the mobile subscriber unit. The candidate base station that requires at least the transmission power from the mobile subscriber unit is monitored closely. When the mobile subscriber unit can communicate successfully with less transmit power than currently required by the current base station, the mobile subscriber unit provides the decision to be transferred to the candidate base station transceiver. Accordingly, an object of the present invention is to provide an efficient system and method for transfer between individual cells in a multicellular communications environment. A further objective of the invention is to provide a system and method that allows a mobile subscriber unit to be transferred between cells in a multicell transmission system with reduced complexity and interference. Other objects and advantages of the invention will become apparent to those familiar with the art upon reading the detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a transfer system embodying the present invention. Figure 2 is a transfer system embodying the present invention. Figure 3 is a flowchart of the transfer method embracing the present invention when it is synchronized.

Figure 4 is a flowchart of the transfer method that encompasses the present invention when it is not synchronized. Figures 5A and 5B are flow diagrams of the transfer method encompassed by the present invention. Figure 6 is the mobile subscriber unit using the system and method of the present invention.

DESCRIPTION OF THE PREFERRED MODALITY The present invention is described with reference to the figures of the drawings in which like numbers represent like elements throughout them. In figures 1 and 2 a system diagram illustrating the transfer system of method 15 of the present invention is shown. Four cells 17, 19, 21 and 23 of a multicell telecommunications system 15 are shown with their respective base station transceivers 17", 19 ', 21" and 23'. For clarity, none of the individual cells has been sectorized. A mobile subscriber unit 25 is shown near a common boundary between two of the individual cells 17 and 21. In operation, assuming that the mobile subscriber unit 25 communicates with the first base station 17 'as the mobile subscriber unit 25 moves from one cell position to another, the mobile subscriber unit 25 continues to communicate with the first mobile subscriber unit. base station 17 'while the mobile subscriber unit 25 performs an analysis of the transmissions of other base stations 19', 21 'and 23' within the system 15. At this point, only one traffic channel has been established between the unit -25 of mobile subscriber and base station 17 '. In operation, the mobile subscriber unit 25 of the present invention constantly receives transmissions from the base stations 17 ', 19', 21 'and 23' and others within the communication range of the mobile subscriber unit 25. The mobile subscriber unit 25 can search continuously to find a new communication link which requires less transmission power, or search only when the transmission power to the existing link approaches an almost maximum threshold. In order to establish a new communication link, the mobile subscriber unit 25 must first be synchronized with a candidate base station transceiver. Synchronization is the process by which the mobile subscriber unit 25 must align its locally generated pseudorandom code with the phase shifted code received from the candidate base station. The transmission from the candidate base station includes a short code, which is transmitted by the candidate base station transceiver and is used by the mobile subscriber unit 25 to determine the phase ambiguity for synchronization. The mobile subscriber unit 25 locally generates a replica of the global pilot signal that is transmitted from the base station. The mobile subscriber unit 25 then correlates the received signal with the locally generated replica of the global pilot code. If the received and locally generated pilot codes are aligned, the code phase has been found, otherwise, the mobile subscriber unit 25 switches to the local code stage and continues searching until a match is found. The mobile subscriber unit 25 performs the search from a set of global code seed stored in memory to determine whether the received pilot code matches one of the seeds of the global code. The memory of the mobile subscriber unit 25 may contain a small "neighbor list" or may include all of the global codes used in the system 15. As shown in Fig. 2, the neighbor list may include cells 19, 21, 23, 27, 29, and 31 in the first row, and twelve cells 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, and 55, in the second row. The neighbor list typically includes the global codes 18 closest to the existing link base station since the mobile subscriber unit 25 most likely makes a transition with one of these cells. Global seeds are assigned to the base stations and used in the service area of the base station to broadcast the global channels. Primary seeds are also provided to the base stations from which the base station generates the assigned seeds. These assigned seeds are assigned to individual links between the base station and the subscriber units and are used to disseminate assigned channels. The global code seeds are preprogrammed with each mobile subscriber unit 25. The mobile subscriber unit 25 may alternatively acquire the global code seeds by periodically receiving an updated updated list for each base station transceiver in the system 15. The mobile subscriber unit 25 receives the plurality of short codes transmitted from other subscriber stations. base and uses them to accelerate the acquisition of global pilot signals from base stations. The mobile subscriber unit 25 continuously searches all the global code seeds in memory until it finds a match by verifying the diffusion of short codes by each base station. The mobile subscriber unit 25 should only examine the short code to determine the global code for the cell. If the mobile subscriber unit 25 does not recognize a short code, it addresses an attempt to synchronize with the base station and examines another received code. When the synchronization between the mobile subscriber unit 25 and the candidate base station is completed, which will be described in greater detail later in the specification, the mobile subscriber unit 25 transmits another short symbol length code while gradually increasing the transmission power level. The mobile subscriber unit 25 monitors the candidate base station in search of a signal recognition, which acts as a "traffic light" to determine whether the base station receives and recognizes the short code. The increase in transmission power ceases under two conditions: 1) if the transmission power of the short code is within a predetermined switching differential for a finite period of time when compared to the current power level of the pilot signal with respect to the existing base station; or 2) the candidate base station recognizes the detection of the short code and informs the mobile subscriber unit 25 by means of the traffic light. The switching differential between the energy levels of the existing link and the candidate link is to avoid changes or switching back and forth between the two base stations when both power levels are close in magnitude. If the short transmission code requires a higher transmission power level compared to the power level of the current pilot, the transfer does not occur. The transfer process does not require the base stations to be synchronized. However, if the base stations are synchronized, it is easier for the mobile subscriber unit 25 to detect and synchronize with a candidate base station. If the base stations are not synchronized, the base stations transmit a short synchronization code in addition to the global pilot signal. The mobile subscriber unit 25 uses a multi-stage acquisition process when the mobile subscriber unit 25 first acquires the short synchronization code and then uses the base information obtained to accelerate the acquisition of the global pilot. In a system where the base stations are synchronized, there is no need for a short synchronization code, and the mobile subscriber unit 25 can quickly acquire the global pilot since the relative phase difference between the pilot of base stations of Neighbor cell is small. Referring now to Figures 3, 4 and 5A-5B, there is shown a flow chart illustrating the preferred method with synchronized or unsynchronized base stations. When the cell base station transceivers are synchronized (as shown in Figures 3, 5A-5B), process 15 (step 101) begins upon receiving a short code broadcast sequence from the base station transceivers of cell within the communication range. The set of global code seeds stored within each mobile subscriber unit 25 (step 103) are used to generate the short codes and compared against the broadcast of short codes received from a candidate cell base station (step 105a). The mobile subscriber unit 25 can acquire the overall pilot signal of a candidate base station transceiver in less than 10 ms and at the same time maintain the current communication link. To acquire a pilot signal from an unsynchronized candidate cell base station transceiver, the transceiver may approach 3 seconds. Since the non-synchronous case is unacceptably long, a multi-level search is carried out to synchronize the minimum amount of time. If the cell base stations are not synchronized (as shown in Figures 4, 5A-5B), the mobile subscriber unit 25 (step 101) compares the short codes generated from the stored global code seeds (step 103) against short code broadcasts received from candidate cell base stations. The mobile subscriber unit 25 searches through the stored set of global code seeds (step 105b) until a match is found with one belonging to the base station transceiver serving the particular cell. If the mobile subscriber unit 25 can not find a match with the broadcast short code (step 107) the mobile subscriber unit 25 does not continue with the short code of that particular base station and tries another one (step 109). If a match is found (step 107) the mobile subscriber unit 25 performs a search to acquire a fast broadcast channel (step 111). The mobile subscriber unit 25 uses the phase information obtained from the short synchronization code to resolve the phase ambiguity of the overall pilot signal and to determine the fast broadcast channel. If the search is unsuccessful (step 113) the mobile subscriber unit 25 attempts another global code seed (step 109). The mobile subscriber unit 25 again seeks to acquire the global pilot signal (step 115). If the global pilot signal can not be acquired (step 117) from the short synchronization code, another global seed is attempted (step 109). Once the mobile subscriber unit 25 acquires a global pilot signal corresponding to a synchronized or unsynchronized candidate cell base station transceiver (step 117), the mobile subscriber unit 25 transmits a short symbol length code to the candidate base station while performing a gradual increase in the transmission power level (step 119). As the power level is slowly increased, the mobile subscriber unit 25 monitors the transmit power (step 121) and the reverse semaphore from the candidate base station (step 123) to determine whether the candidate transceiver receives and recognizes the short code of symbol length transmitted. The transmission of the short code from the mobile subscriber unit 25 ceases when the transmission power of the short code is within the predetermined switching differential compared to the power levels of both communication links, or when the transceiver of the The candidate cell base station recognizes the detection of the transmitted short code by observing the traffic light (step 123). If the mobile unit maintains and searches through the complete list of global codes instead of a neighbor list, very often the tested code will not belong to the base station in close proximity. For example, there may be a total of 64 global codes, but a mobile unit may receive only some of them from the neighboring base stations. The transfer decision is controlled by the mobile subscriber unit 25 which requires less air capacity during the transfer and comparison with the methods described in the prior art. Since the decision for the base controlling the mobile subscriber unit 25, the system is not fixed as specified above. The system 15 is flexible with respect to the cell limits that change in reaction to air capacity within the individual cells. The mobile subscriber unit 25 establishes a link to the cell base station that requires the minimum transmit power from the mobile subscriber unit 25 to maintain a communication link. The mobile subscriber unit 25 sends a long-weight pilot signal (step 125) to the chosen candidate cell base station that includes an instruction that is further transported to a radio distribution unit. The radio distribution unit maintains a record of which base station for each mobile subscriber unit is currently associated. The radio distribution unit directs the communication link from a landline to an appropriate cell base station transceiver as the mobile user moves through various cells. The sent message indicates a transfer. During this brief moment, the mobile subscriber unit 25 is still attached to a current cell base station transceiver (step 127). The communication link with the original cell base station transceiver controls the transmission power of the mobile subscriber unit 25. However, the candidate cell base station transceiver also sends power control instructions. The power control instructions from the candidate cell base station transceiver decrease the power power of the mobile subscriber unit 25 based on the candidate communication link. The mobile subscriber unit 25 leaves (stage 129) the first communication link with the current cell base station transceiver and reassumes the power control and communication of the previous candidate, now the only link to the transceiver cell base station (step 131). Figure 6 shows a system that incorporates the method. As described above, the method acquires two separate communication links: 1) an alternative link to search for new candidate base stations 61a, and 63a and 2) a pre-existing communication link 61b and 63b. The mobile subscriber unit 25 includes a receiver 65 that can eliminate the broadcast 67a, 67b in at least two different channels. The maintenance symmetry is a transmitter 69 which can broadcast 71a, 71b at least two channels associated with the diffusion eliminators 67a, 67b. While maintaining a pre-existing communication link, the mobile subscriber unit 25 receives available broadcasts 73 from candidate base stations. The present invention stores the transfer process code without interruption within the ROM 75 and executes the code in a high-speed microprocessor 77 which controls the diffusion eliminators 67a, 67b and the diffusers 71a, 71b so as to they can store and reapply the global code seeds for the immediate neighboring cell sites of the RAM 79 to constantly acquire an alternative communication link. The pre-existing communication link 61b, 63b includes voice processing 81, the discussion of which is beyond the scope of this description. The microprocessor 77 interrogates 83, 85 the receiver 65 and the transmitter 69 to monitor the reception of response codes from the candidate base stations and also to compare the transmission power during the gradual increment stage. The process is preprogrammed in the ROM 75 and is loaded and executed in the microprocessor 77 continuously when the mobile subscriber unit 25 is in operation. When a communication link is found which requires less transmission power, the microprocessor 77 switches 91 to an alternative link 61a, 61b, maintaining a constant transmission of voice and data and at the same time retaining two simultaneous data communications. The mobile subscriber unit 25 searches for a communication link that requires less transmission power and always compares the prospective communication link with the pre-existing communication link. At no point during the process, two transmission data communication links are established. Switching is instantaneous between the current cell base station transceiver and the candidate cell base station transceiver. As described, the above belongs to a cellular network which has not been sectorized. If sectorization is used, which effectively multiplies the number of cell base stations, the transfer method of the present invention can be carried out in much less time. When it is transferred within sectors of the same cell, the base stations that serve the different sectors within the cells are synchronized and a limited search process is performed. A small portion of each global code is searched before performing a comprehensive search that covers the entire global code. This allows the mobile subscriber unit 25 to acquire a candidate base station in less time. Although the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as set forth in the following claims will be apparent to those familiar with the art.

Claims (10)

1. A method for a mobile subscriber to control the cellular transfer in a multicellular communication system CDMA while moving between individual cells, each cell is associated with a base station which transmits a global pilot signal, where the subscriber has a link of pre-existing communication with one of the base stations, the method is characterized in that it comprises the steps of: a) receiving a plurality of transmissions from a plurality of candidate base stations with which a pre-existing link has not been established; b) selecting a received transmission associated with a candidate base station; c) try to acquire a global pilot signal from the selected transmission; d) if the global pilot can not be acquired, select a received transmission associated with a different candidate base station; e) repeating steps b-d until a global pilot associated with the selected candidate base station is received; f) having a gradual change in the transmitting power of the mobile subscriber while transmitting a short access code from the mobile subscriber to the selected candidate base station; g) monitors the transmission to the selected candidate base station; h) detecting a signal transmitted back from the candidate base station; i) comparing the power transmitted to the selected candidate base station with the power transmitted to the pre-existing communication link; j) if the power transmitted to the selected candidate base station is equal to or greater than the power transmitted to the pre-existing communication link, repeat the steps b, otherwise, transmit a large access pilot to the selected candidate base station and establish a new communication link; and k) abandon the pre-existing communication link.

2. The method according to claim 1, characterized in that it comprises the step of making the new communication link the preexisting communication link.

3. The method according to claim 2, characterized in that the step of selecting a received transmission further comprises the steps of: a) selecting a global code seed associated with one of the plurality of candidate base stations from a plurality of seeds of global code stored in a memory; b) examine in search of a global pilot using the global code seed; and c) performing the search at a maximum expected phase shift between the pre-existing communication link in the selected candidate base station.

4. The method according to claim 3, characterized in that it further comprises the step of synchronizing the transmission between the mobile subscriber and the selected candidate base station.

5. The method according to claim 4, characterized in that the step of synchronizing further comprises the steps of: a) selecting a global code seed associated with one of the plurality of candidate base stations from a plurality of global code seeds stored in a memory; b) acquiring a short synchronization code from the candidate base station using a selected global seed; c) if the short synchronization code can not be acquired, select a received transmission associated with a different candidate base station; d) repeating steps b-c until a global pilot associated with a selected candidate base section is received; e) acquiring a fast broadcast channel based on the phase information obtained from the received short synchronization code; f) if the fast broadcast channel can not be acquired, select a received transmission associated with a different candidate base station; g) repeating steps b-f until a global pilot associated with a selected candidate base station is received; and h) acquiring a global pilot using the phase information obtained from the fast diffusion channel.

6. A mobile subscriber system that controls the cell transfer in a multicell CDMA communication system while traveling between individual cells, each cell is associated with a base station which transmits a global pilot signal, where the subscriber has a link pre-existing communication with one of the base stations, the system is characterized in that it comprises: means for receiving a plurality of transmissions from a plurality of candidate base stations with which a pre-existing link has not been established; means for selecting a received transmission associated with a candidate base station; a means to try to acquire a global pilot signal of the selected transmission; means for selecting the received transmission associated with a different candidate base station if the overall pilot can not be acquired; means for gradually changing the transmitting power of the mobile subscriber while transmitting a short access code from the mobile subscriber to the selected candidate base station; means for monitoring the transmission power to the selected candidate base station; means for detecting a signal transmitted back from the candidate base station; means for comparing the transmission power with the selected candidate base station with the transmission power to the pre-existing communication link; means for transmitting a large access pilot to the candidate base station selected to establish a link in new communication if the power transmitted to the selected candidate base station is equal to or greater than the transmitted power of the pre-existing communication link; Y a means to leave the pre-existing communication link.

7. The system according to claim 6, characterized in that the new communication link becomes the preexisting communication link.

8. The system according to claim 7, characterized in that the means for selecting a received transmission further comprises: means for selecting a global code seed associated with one of a plurality of candidate base stations from a plurality of code seeds globally stored in a memory; a means to examine a global pilot using the selected global code seed; and means for performing the search at a maximum expected phase shift between the pre-existing communication link and the selected candidate base station.

9. The system according to claim 8, characterized in that it further comprises means for synchronizing the transmission between the mobile subscriber and the selected candidate base station.

10. The system according to claim 9, characterized in that the means for synchronizing further comprises: means for selecting a global code seed associated with one of the plurality of candidate base stations from a plurality of global code seeds stored in a memory; means for acquiring a short synchronization code, from the candidate base station using the selected global seed; means for selecting a received transmission associated with a different candidate base station if a short synchronization code can not be acquired; means for acquiring a fast broadcast channel based on the phase information obtained from the received short synchronization code; means for selecting a received transmission associated with a different candidate base station if the fast broadcast channel can not be acquired; and a means to acquire a global pilot using the phase information obtained from the fast broadcast channel.