Patent Publication Number: US-2005141452-A1

Title: Base station for facilitating handover in TDMA communications

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This is a divisional application of co-pending application Ser. No. 09/413,658 filed on Oct. 6, 1999 entitled Method and Apparatus Handover in TDMA Mobile Communications invented by Jan Vancraeynest, Attorney Docket Number U001 100087 (former docket no. 16296-706) which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention generally relates to cellular communications systems having mobile subscribers, and more particularly, to a method of controlling handover in the communications of cell site stations with a mobile unit in a TDMA cellular telephone system.  
      2. Description of the Related Art  
      Cellular communications systems use a series of sites, known as base stations, to communicate with mobile subscribers. A mobile telephone unit typically communicates with the closest base station, or the base station having the best signal quality. A cellular network often includes a set of many base stations, each base station communicating with mobile telephones in an area known as a cell. As a mobile subscriber moves from one cell to another, eventually the mobile telephone begins to communicate with a new base station covering the new area and ends communication with the old base station covering the old area in which the mobile subscriber was located. This process of beginning communication with a new base station and ending communication with an old base station is known as handoff or handover.  
      Various systems and techniques have been developed to address problems in handoff. Two examples of patents addressing hand off in cellular communication are U.S. Pat. No. 5,822,313 by Malek and U.S. Pat. No. 5,323,466 by Kojima, both of which are incorporated herein by reference in their entirety. In some systems, simultaneous communication links are established between the mobile unit and the base stations. In some prior systems, the handover process involved the use of dual transceivers in a mobile unit. Such a dual transceiver adds cost, size and power consumption, and reduced reliability to the mobile unit.  
     SUMMARY OF THE INVENTION  
      One embodiment of the invention is a mobile communication system including a plurality of base stations. The system includes a plurality of mobile telephones. Each mobile telephone communicates with at least a base station from the plurality of base stations using a timeslot of a frame. The frame has a plurality of timeslots for the base station to use to communicate with the various of the plurality of mobile telephones in a cell covered by the base station. Each mobile telephone has a single radio capable of communication on various frequencies.  
      A mobile telephone may establish communication with a first base station on a particular timeslot in a frame. If another timeslot is available in the frame in a second base station, communication may be established with the second base station on another timeslot in the frame. The another timeslot is not next to a particular timeslot if the frequency used on the particular timeslot and the another timeslot are different. If the frequencies used on the particular timeslot and the another timeslot are the same, communication may be established with the second base station on the another timeslot in the frame where the another timeslot is next to the particular timeslot or not next to the particular timeslot.  
      In various embodiments of the invention, the circuitry that causes communication  20  to be established with the second base station may be comprised by mobile telephone or the equipment coupled to the base stations. Communication established on a particular timeslot may include use of the timeslot for downlink communication on one half frame and for uplink communication on the next half frame.  
      The circuitry may attempt to establish communication with the second base station if quality of communication with the first base station is below a particular level. Circuitry may drop communication with a base station of quality if communication with the base station is below a particular level. The system capacity may be controlled by adjusting a combination of (a) the particular level of quality of communication with the first base station below which the circuitry attempts to establish communication with the second base station and (b) a particular level of quality of communication with a base station below which the circuitry drops communication with the base station. Such combination may be adjusted automatically based on utilization of system resources and desired communication link reliability.  
      The second base station may be selected from among a set of base stations based on signal quality between the base stations and the mobile telephone. The second base station may be selected as a base station having the best signal quality between the base station and the mobile telephone coming from among base stations other than the first base station. The mobile telephone may have a list of base stations ordered based on signal quality between base stations and the mobile telephone, and the mobile telephone may include circuitry that selects the second base station as the first available base station on the list. The list may be ordered based on received signal strength indicator (RSSI), frame error rate (FER), or a combination thereof.  
      The system in alternative embodiments includes circuitry that reallocates a timeslot in the second base station to allow the reallocated timeslot to be used by the mobile telephone. The second base station may include circuitry that provides the mobile telephone an estimate of the time to prepare or free-up the another timeslot for communication with the second base station.  
      An embodiment of the invention is a mobile telephone. The mobile telephone includes a single radio not capable of communicating on different frequencies on adjacent timeslots of frame and circuitry that monitors signal quality with various base stations. The mobile telephone also includes circuitry that, when a mobile telephone is in communication with a base station using a particular timeslot or frame, causes the mobile telephone to request communication with a second base station. The mobile telephone is caused to request communication with the second base station based on measured single quality of the respective base stations. A timeslot used to communicate with the second base station is not next to the particular timeslot of the frame if the frequencies used on the particular timeslot and the another timeslot are different. A timeslot used to communicate with the second base station is next to the particular timeslot or not next to the particular timeslot if the frequencies used on the particular timeslot and the another timeslot are the same. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIGS. 1A-1C  illustrate a mobile communications system illustrating different connectivity as a subscriber in a mobile communications system moves from the coverage of one base station to another, according to an embodiment of the invention.  
       FIGS. 2A-2C  are a timing diagram illustrating communications activity between a subscriber, base stations, and base station controller, according to an embodiment of the invention.  
       FIG. 3  is a simplified flow chart of a handover process, according to an embodiment of the invention.  
       FIG. 4  shows a sample half-frame structure following the Personal Handyphone System standard (also known as ARIB RCR STD-28), showing time available in the ramp bits to switch channels between timeslots, according to an embodiment of the invention.  
       FIG. 5  is a block diagram of a personal station, according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      The handover process in mobile communications presents some challenges. For example, as a mobile telephone switches from communicating with a first base station to communicating with a second base station, interruption in the communication may occur. Such interruption may be audible, in the form of a click, other noise or simply silence. Avoiding this problem by providing dual radios for allowing for simultaneous communication with multiple base stations is expensive.  
      There are also performance impacts from the practice of adding enough timeslots in a frame so that there will always be two non-adjacent timeslots which can be used for communications (i.e., at least seven timeslots per half frame). For example, there may be increased round-trip delay for a given timeslot length. Further, such an approach is not always practical or desirable especially if it is required to have compatibility with the existing system standards that use fewer than seven timeslots per frame. One embodiment of the invention is a system in which there are no more than six timeslots per half frame.  
      An embodiment of the invention helps to address some of these problems by communicating with more than one base station on different timeslots. The timeslots are selected in order to accommodate limitations of the mobile telephone equipment. For example, the mobile telephone may not be able to transmit on different frequencies on adjacent timeslots. Such mobile telephones may be limited because their radios have slow frequency synthesizers that are capable of using the same frequency only on adjacent timeslots. The base station, however, is able to communicate using different frequencies on adjacent timeslots. Therefore, according to one embodiment of the invention, a timeslot is selected in the second base station that is not adjacent to the timeslot used by the first base station, if such a timeslot is available only on a different frequency from the one used by the first base station. Alternatively, a timeslot may be selected as one adjacent to the one used by the current base station if such timeslot uses the same frequency as is used by the timeslot used by the first base station. If such timeslot is not available in the second base station, but another timeslot is, the second base station will reallocate its timeslots to make available a suitable timeslot for the mobile telephone from the first base station.  
      An embodiment of the invention is directed to a time division multiple access (TDMA) system. In a TDMA system, multiple conversations are handled by dividing time up into a series of frames. Each frame is divided into a set of timeslots. A burst of data from each conversation may be transmitted on a timeslot of each frame. Thus, by dividing each frame into timeslots, multiple conversations can be handled on a shared communications channel.  
      According to an embodiment of the invention, whether a mobile telephone attempts to connect to another base station or attempts to disconnect with the current base station is determined by a predetermined level of communications quality with respect to base stations. For example, according to an embodiment of the invention, a mobile telephone attempts to communicate with a second base station if signal quality of communication with the first base station is below a predetermined threshold level. If communication quality with the first base station is below a predetermined threshold level, then communication with the first base station is dropped. The threshold values of signal quality for dropping or adding a station may be different for different base stations. Also, the base stations themselves may broadcast the threshold values, and the mobile telephones will initiate or drop communication with base stations according to such broadcasted threshold values.  
      The threshold levels for initiating or dropping connections with base stations determine the number of mobile telephones that are communicating with more than one base station. As more mobile telephones communicate with more base stations, fewer base station resources are available. Thus, the availability of system resources may be controlled by selecting the threshold levels.  
      According to one embodiment of the invention, the mobile telephone maintains an ordered list of base stations according to link quality, potential of the wireless link between the base station and mobile telephone signal quality. Such signal quality may be measured as RSSI or FER, other metrics, or a combination thereof. When the mobile telephone determines that it should initiate communication with another base station, it selects the second base station from the list of base stations, choosing the base station having the best link quality first. If that base station is not available, another base station is selected also based on link quality. Such a process may be continued until an available base station is found. Alternatively, the mobile telephone may attempt to communicate with a base station with which it has already attempted to communicate after trying another base station or stations.  
       FIG. 1A  shows subscriber PS A   216  beginning to move from coverage area  212  to coverage area  213 . A handover process will be initiated to allow subscriber PS A   216  (personal station) to begin communicating with another base station and terminate communications with the current base station.  
       FIG. 1A  includes radio port controller (RPC)  210 , which is coupled to base station  214  and base station  215 . RPC  210  is coupled to wired network  211 . Thus, mobile subscribers in the coverage areas of the respective base stations are able to communicate through the base stations, and then through RPC  210 , to wired network  211 . Note that base stations may also be referred to as radio ports (RPs). Wired network  211  may represent the public switched telephone network (PSTN), or other communications network. Thus, mobile subscribers can communicate with a telecommunications network through the base stations and RPC  210 . As shown, subscriber PS A   216  is currently communicating with base station  214  over a wireless link  240 . Subscriber PS A   216  is currently in coverage area  212  of base station  214 . This coverage area  212  overlaps with coverage area  213  of base station  215 . Subscriber PS B   217  is currently communicating with base station  215  via wireless link  241 .  
       FIG. 1A  shows that subscriber PS A   216  is communicating over a wireless link  240  with base station  214  via timeslots within respective frames. As shown, subscriber PS A   216  is communicating using for communications traffic timeslot  221  of frame  228  and the same timeslot  227  of frame  229 . Frame  228  as shown includes timeslots  218 - 221 , and frame  229  includes timeslots  224 - 227 . Since each frame shows four timeslots, corresponding slots  221  and  227 , which are both the fourth timeslots of their respective frames, are the same timeslot. In an actual system, a frame may contain a larger number of timeslots. Also included in a frame is typically a control signal, here shown as C 1    218  and C 1    224 . Such control signal may occur only on certain intervals of frames, rather than every frame, according to the parameters of the system. Arrow  242  points in the direction of coverage area  213  from coverage area  212  and indicates that subscriber PS A   216  is moving toward coverage area  213  from coverage area  212 .  
      Subscriber PS B   217  is also communicating with a local base station. As shown, subscriber PS B   217  is communicating via wireless link  241  with base station  215 . This communication traffic is also being carried by a timeslot on a frame. As shown here, communication is taking place on timeslot  233  of frame  238  and the same timeslot, timeslot  237  of frame  239 . Such communication, in this example, is taking place at a different frequency from the one at which communication is taking place between subscriber PS A   216  and base station  214 . Subscriber PS B   217  is also receiving and/or transmitting control information on a designated timeslot of a frame, shown here as C 2 , timeslot  231  and C 2 , timeslot  235 .  
      The handoff between base station  214  and  215  occurs, according to an embodiment of the invention, in a manner that helps to allow for low cost equipment, lower weight, and high reliability. Such advantages are achieved through the choice of a timeslot with which the mobile subscriber communicates with additional base stations. Through judicious choice of timeslots, cost, weight, and power requirements of additional, more complex equipment can be avoided.  
      The initiation of the handoff of subscriber PS A   216  between base station  214  and  215  may occur based on signal quality in the communication between the subscriber and the respective base stations. For example, the initiation of communication may occur when signal quality with the second base station is above a certain level. The termination of communication may occur when signal quality with either the first or second base station is below a certain level.  
       FIG. 1B  shows subscriber PS A   216  communicating with both base stations, base station  214  and base station  215 . Such configuration may have resulted from subscriber PS A   216 &#39;s movement from the coverage area of base station  214  (area  212 ) to the coverage area of base station  215  (area  213 ). As signal quality of potential link  250  with base station  215  improves, according to one embodiment of the invention, subscriber PS A   216  requests a communications link with base station  215 .  
      As shown in  FIG. 1B , subscriber PS A   216  is communicating with timeslot based wireless links  240  and  250  with base stations  214  and  215  respectively. A different timeslot is used to communicate with each of the respective base stations. As shown, timeslot  221  in frame  228 , which subscriber PS A   216  is using in communication with base station  214 , is next to timeslot  232  of frame  238 , which is used for communication in wireless link  250  with base station  215 . That is timeslot  221  is the fourth timeslot of the freame, and timeslot  232  is the third timeslot of the frame. The frequency used to communicate in these timeslots is the same for timeslot  221  and timeslot  232 . This allows a radio of subscriber PS A   216  to use the adjacent timeslots, even when such radio Is not capable of quickly switching between frequencies. A base station has multiple separate radios and thus, for example, timeslot  232  and adjacent timeslot  233  have different frequencies for communicating with the different respective subscribers, subscriber PS A   216  and subscriber PS B   217 . According to another example, subscriber PS A   216  may communicate with different base stations using different frequencies, provided that the timeslots on which the communication takes place are not adjacent. Base station equipment or other equipment in the system may make a determination whether to initiate communication in a timeslot adjacent to one currently in use by the subscriber or one not adjacent, depending on availability of timeslots and frequencies and/or interference conditions from other users of such timeslots in the system.  
       FIG. 1C  shows a system according to the embodiment of the invention in which subscriber PS A   216  has moved out of the coverage area of base station  214  then has dropped base station  214 . Thus, now subscriber PS A   216  is communicating via wireless link  250  with base station  215  and is no longer communicating with base station  215 . Subscriber PS A   216  communicates with base station  215  via timeslot  232  of frame  238 , and the same timeslot  236  of frame  239 . Subscriber PS B   217  communicates via wireless link  241  with base station  215  using timeslot  233  of frame  238  and the same timeslot  237  of frame  239 . Different frequencies are used on timeslots  232  and  233 . Arrow  242  shows that subscriber PS A   216  has moved from coverage area  212  of base station  214 .  
       FIGS. 2A-2C  are a timing diagram illustrating communications activity between a subscriber, base stations, and base station controller, according to an embodiment of the invention.  FIGS. 2A-2C  illustrate a handover in which subscriber (communication with subscriber represented by PS trace  162 ) is initially communicating with a first base station (RP 1  trace  160 ) and is handed off to a second base station (RP 2  trace  161 ). Timeslots are numbered as positions  1 - 144 . Communication with the radio port controller (RPC) is represented by RPC (trace  163 ). A radio port controller is coupled to both the first base station and second base station and communicates with a telecommunications network such as PSTN, thus allowing a mobile subscriber to communicate with the public switched telephone network.  
      In parallel to the communication with the mobile telephone equipment represented by trace PS  162 , the first base station and the second base station (represented by RP 1  and RP 2 ) are communicating with other mobile telephone equipment. For example, traffic timeslot T 2  on timeslots  4  and  8  of trace RP 2   161  (e.g., at positions  4 ,  8 ,  12 ,  16 , etc.) represent communication with another mobile telephone equipment.  
      According to an embodiment in the invention, the handover is initiated based on signal quality in the link or potential link with the respected base stations. As shown here, thresholds exist for initiating a connection (connect_destination_RP_threshold  152 ) and for dropping communication (drop_origin_RP_threshold  153 ). When signal quality of communication with the first base station (trace  150 ) drops below the connect_destination_RP_threshold  152 , the mobile subscriber equipment attempts to make a connection with another base station. When the signal quality with the initial base station drops below the drop_origin_RP_threshold  152 , the base station drops communication with the original base station, or initiates the termination of communication with the initial base station.  
      The second base station is selected based upon potential signal quality with the second base station. According to one embodiment of the invention, such signal quality is measured by the mobile telephone measuring control signals from various base stations. The mobile telephone may maintain an ordered list of various base stations, ordered based on respective signal quality. Such lists may be updated periodically by the mobile telephone.  FIG. 2A  shows such measurements being taken (e.g., at positions  17 - 20 ).  
      The traces shown include a series of TDMA frames and each frame includes a number of timeslots. As shown here, each time frame includes eight timeslots constituting four down-link slots followed by four up-link slots. Each timeslot may use a different frequency, subject to constraints of the mobile telephone unit. The mobile telephone can use a different frequency only on non-adjacent timeslots. The base station can use an arbitrary frequency on each timeslot. As shown here, each timeslot in the down link corresponds to a timeslot in the up-link four timeslots later (e.g. positions  4  and  8  of trace PS  162 ). One of the timeslots on the down-link and one of the timeslots on the up-link are reserved for the control channel at a single frequency F control . The control timeslot is transmitted every N time frames and different base stations transmit the control channel in different slots so that only one base station transmits the control carrier at anyone time in an area in which it may cause interference to control channel transmissions from other base stations. In  FIG. 2A , a super frame of N=3 frames is shown for illustrative purposes. Those skilled in the art will appreciate that a practical system using TDMA frame structure would more likely use a super frame of 20 frames. As shown, base station  1  shown in trace  160  transmits its control channel on timeslot number  1  and receives it on timeslot number  5 . In the example of  FIGS. 2A-2C ,  12  base stations with control channels C 1 -C 12  are accommodated in consecutive timeslots every 3 frames. It will be appreciated that the control channel may also be implemented through means other than the separate timeslots shown in the illustrative example.  
      The radio port controller (RPC), to which the base stations are connected, assigns timeslots and carrier frequencies as well as providing the interface between the frames transmitted over the air and land line network. Depending on the type of interface to the land line wired network, the RPC may perform multiplexing, framing, adding error correction coding, de-framing, de-multiplexing, etc. The RPC time line trace  163  does not include such operations and merely illustrates which frames are received (RX, for the wire network to the base stations) and transmitted (TX, from the base stations to the wire network) by the RPC. The base stations connected to the RPC are synchronized (e.g. by the RPC) so that the boundaries of timeslots fall within the limits of the guard time intrinsically built into TDMA timeslot structure.  
      As shown in  FIG. 2A , the subscriber telephone is communicating first with a first base station, represented by RPI (trace  160 ). As shown, communication is on a given timeslot, shown here, for example, as T 1  on position  4  (the base station transmitting to the subscriber equipment) and T 1  on position  8  (the subscriber equipment transmitting to the base station). A control communication takes place, for example, on timeslot  1  and timeslot  5  (positions  1  and  5  respectively).  
      On the idle timeslot nonadjacent to the timeslot used for traffic, the mobile telephone equipment monitors the link quality from the other base stations. Depending on the layout of the control timeslot used by the individual base stations, the mobile telephone equipment may skip one frame of communications every N frames to monitor the 2 base stations whose downlink control slot may be adjacent to the timeslot which the mobile telephone equipment is currently using for communications. By shifting the frame in which the mobile telephone equipment skips the communications with the base station relative to the repeat period of the control channel transmissions and successive super frames, the mobile telephone equipment can monitor all the base stations in the system. This introduces loss of only one frame which is unnoticeable to the subscriber if the frame length is chosen short enough and N is chosen large enough. This is indicated by instances marked {circle over ( 1 )} and {circle over ( 9 )}, e.g. positions  16 ,  20 ,  32 ,  84  and  100 . For example, at position  16  the mobile telephone equipment is monitoring signal quality from base stations in subsequent timeslots and thus does not communicate with traffic with the base station during that timeslot. In  FIGS. 2A-2C  for brevity&#39;s sake the processor shifting and the skipping frame is shown only in the first two instances of the N frames. Moreover, if the mobile telephone equipment adheres to a fixed rule of choosing which frame it skips communications, the RPC can substitute a replica of the last valid timeslot as a substitute for the missing timeslot and further reduce the impediment to voice quality as perceived by the wired network connection.  
      As shown in  FIG. 2A , the second base station is communicating with another mobile telephone equipment using another frequency, the same timeslot at which the mobile telephone equipment is communicating with the first base station. For example, see positions  4  and  8  of trace  161  in which the second base station is communicating with another mobile telephone equipment while the first base station, represented by trace  160 , is communicating on another frequency. As shown, for example, in position  4 , the telephone equipment represented by trace  162  is communicating on one frequency represented by T 1  with the first base station represented by trace  160  and T 1 , while the second base station, represented by trace  161 , is communicating with a second telephone equipment on a second frequency represented by T 2 . Throughout this diagram, T 1  represents traffic transmission at a first frequency and T 2  represents traffic transmission on a second frequency to an unrelated mobile telephone (i.e., first and second frequency may be the same).  
      Eventually the signal quality of communication between the telephone equipment and the first base station may degrade such that it is below the connect threshold. As shown here, trace  150  drops below connect_destination_RP_threshold  152 . As shown here, this takes place around position  24  and  25 . As a result, the telephone equipment sends a link establishment request to the base station with the best link quality, the destination base station in the example of  FIG. 2A , this is RP 2  at instant {circle over ( 2 )} (position  30 ). The mobile telephone equipment then advises the destination base station of the telephone equipment&#39;s current timeslot and frequency on the first base station. Thus C 2  at position  30  represents a control message from the telephone equipment requesting a link establishment. Depending on the location of the telephone equipment&#39;s control channel relative to the communication timeslot used by the telephone equipment, this message may require an interruption of one frame in the communications with the first base station. This interruption is unnoticeable in a system with judiciously chosen N (large) and frame length (short). While unnecessary for handover to another base station connected to the same RPC (because the RPC knows which timeslot and frequencies are used by the mobile telephone equipment), according to one embodiment of the invention, the current timeslot and frequency are included in the message requesting the link establishment to allow for connection with a base station not belonging to the same radio port controller.  
      The second base station checks whether it has a timeslot available which is different from the one already in use by the mobile telephone equipment and with interference conditions such that the same frequency can be used if the free timeslot is adjacent to the one already in use by the mobile telephone equipment with the first base station. The table below illustrates the allowed timeslot and frequency combinations if the first timeslot is used for the control channel (A represents a particular frequency, frequency A):  
                                                   Timeslot/Frequency Already   Allowed Timeslot/Allowed           in Use to Communicate   Frequency to Communicate           with First Base Station   with Second Base Station                          Timeslot 2/Frequency A   Timeslot 3/Frequency A or               Timeslot 4/Any Frequency           Timeslot 3/Frequency A   Timeslot 2/Frequency A or               Timeslot 4/Frequency A           Timeslot 4/Frequency A   Timeslot 2/Any Frequency or               Timeslot 3/Frequency A                      
 
      If a suitable timeslot is available, the second base station sends a link assignment directive to the mobile telephone equipment. From the next frame onward, the mobile telephone equipment communicates over two timeslots, the original timeslot to the first base station and the newly assigned timeslot to the second base station. For example in  FIG. 2B  at position  50 , C 2  represents a control message from the second base station sending a link assignment to the mobile telephone equipment. Subsequently the mobile telephone equipment is communicating with both base stations. For example, in position  59  (traces  161  and  162 ) the mobile telephone equipment is communicating on frequency one with the second base station, and in position  60  (traces  160  and  162 ) the mobile telephone equipment is communication with the first base station on the same frequency. These traffic communications are taking place over the same frequency and thus are allowed to occur on adjacent timeslots.  
      For example, the second base station may reallocate timeslots to accommodate one subscriber if the second base station does not have a timeslot available different from the one already in use by the mobile telephone equipment, but does have a timeslot available which is the same as the timeslot already in use by the mobile telephone equipment, it will send a command to a second PS which is using the different timeslot directing the second mobile telephone equipment to switch to the same timeslot as the first mobile telephone equipment thus freeing up the different timeslot for use by handover from the first mobile telephone equipment. The RPC knows which timeslots and frequencies are in use by a base station and the topology of the base station. Therefore it can make a good estimate of which of the mobile subscribers could be switched in order to make room for the handover without violating interference constraints and how frequencies may be reassigned. The RPC can then calculate how long the process of clearing the timeslot for the handover will take. The RPC can advantageously use this knowledge to clear available timeslots in advance of likely handovers. The base station responds to the link establishment request from the telephone equipment in such a situation with a link assignment reject message containing a time estimate of how many frames will be needed to clear a suitable timeslot at which the first telephone equipment is invited to retry its link assignment request. Upon receipt of the link assignment reject message, the first mobile telephone equipment will then revert to communicating with the first base station in order to minimize lost frames.  
      The mobile telephone equipment then tries to communicate with another (third) base station if another base station with suitable link quality is available. Such attempt is the initiation of a link assignment process. If link assignment is successful on the third base station, the mobile telephone equipment sends a message to cancel link establishment to the second base station so that the second base station knows that it does not need to reserve a timeslot for the first mobile telephone equipment. If the link assignment attempt on the third base station is unsuccessful, the mobile telephone equipment revisits the second base station or the third base station after expiration of the time estimate received in the link assignment reject message. If the time estimates from the different base stations end up in the same TDMA time frame, the base station preferentially retries the link assignment process with the base station which has the best link quality. Using this process the mobile telephone equipment queues its link assignment with multiple base stations (3 being the example in the illustrative sequence above), each of which is a suitable handover candidate.  
      After receiving the link assignment message, the mobile telephone equipment starts to communicate with the second base station in the same frame as the first base station, using a different timeslot for each link. Based on the respective qualities of the links between the different base stations, the mobile telephone equipment selects the best timeslot for further processing, between the timeslots used for the different base stations. The radio port controller (RPC) receives data from both base stations and forwards the frame with the lowest frame error rate onto the network. In the down-link, the RPC sends incoming frames from the network to both base stations to which the mobile telephone equipment is connected. As shown here for example at position  59 , RPC is using traffic from a different timeslot than it had in previous frames, based on the better quality of the signal from second base station as compared to the first base station. At position  68  the traffic from the first base station is selected for further transmission onto the network, presumably because the signal quality from the first base station is better than that of the second base station at that point in time.  
      As shown in positions  59 - 108  ( FIGS. 2B and 2C ) the mobile telephone equipment is communicating traffic with both base stations, the first base station and the second base station on timeslots  3 ,  4 ,  7  and  8 . For reception the mobile telephone equipment uses 3 and 4, and for transmission it uses timeslots  7  and  8 .  
      The mobile telephone equipment drops communication with a base station when the link quality of communication with that base station is too low. As shown here, the mobile telephone equipment drops communication with a base station when the quality of the communication with the base station is below a particular threshold (drop_origin_RP_threshold  153 ). As shown, trace  150 , which represents signal quality in the link between the mobile telephone equipment and the first base station, drops below the drop_origin_RP_threshold  153  around position  97 . The mobile telephone equipment sends a link cancel message to the base station for which the signal quality has dropped below the threshold. As shown here in  FIG. 2C , the link cancel message is sent at position  101  as control C 1  message from PS (trace  162 ) to the first base station (trace  160 ). The RPC sends a link cancel acknowledgement to the mobile telephone equipment, and then the mobile telephone equipment drops connection to the base station. Alternatively, the link may have deteriorated to the point that the link cancel message or its acknowledgement are lost. In such a case, the mobile telephone equipment automatically drops the link to the first base station and the RPC; automatically drops the link to the mobile telephone equipment from the first base station on its own initiative upon detecting that the link quality as seen by the first base station has dropped below the drop_origin_RP_threshold_at_RP. At this point, the mobile telephone equipment is fully connected to the other base station on every frame and the RPC no longer needs to select between frames between the two base stations, regardless of whether the link to the first base station is dropped in a control fashion or through default by exceeding a predetermined threshold. The threshold drop_origin_RP_threshold is chosen to be at a predetermined link quality level which corresponds to the threshold between good and poor subjective voice quality, according to an embodiment in the invention.  
      As shown here, starting with position  111 , the mobile telephone equipment is communicating only with the second base station, represented by trace  161 . The RPC acknowledges that the first base station has been dropped at position  121  marked with a {circle over ( 6 )}. At this point, the mobile telephone equipment is completely handed over to the second base station.  
      The effect on system quality can be throttled by how much larger a connect_destination_RP_threshold is chosen than the drop_origin_RP_threshold. If connection_destination_RP_threshold is set very high, a mobile telephone equipment will be connected to multiple base stations almost all the time. Under such condition, the system capacity is effectively cut in half compared to a system in which there is no seamless handover. In such a situation in which a mobile telephone equipment is connected to multiple base stations almost all the time, fairly reliable handover is provided even in cases where the signal from one base station dropped suddenly such as may occur when a subscriber walks or turns around a corner in an urban setting. If the connect_destination_RP_threshold is set only slightly larger than the drop_origin_RP threshold, then the mobile telephone equipment looks for a connection to a second base station only when it can reasonably assume that handover will be needed. Such a situation would occur for example when a subscriber moves from one cell to another cell. If connect_destination_RP_threshold is set to small, the process for establishing a link to the second base station may not have enough time to complete before the signal quality from the originating base station becomes unusable. Such a situation may occur when a subscriber moves fast relative to the cell size. The best setting for connect_destination_RP_threshold is a compromise between the ability to respond to sudden changes in signal strength and reduction in system capacity.  
      A further reduction in mobile telephone equipment power consumption may be advantageously achieved by restricting the search for the second base station to those times when link quality from the first base station is below a threshold, say search_for_other_RP_threshold (search_for_other_RP_threshold&gt;connect_destination_RP_threshold&gt;drop_origin_RP_threshold). As long as the link quality on its active timeslot is above search_for_other_RP_threshold, the mobile telephone equipment turns off its receiver on idle timeslots, according to one embodiment of the invention.  
       FIG. 3  is a simplified flow chart of a handover process, according to an embodiment of the invention.  FIG. 3  shows different approaches of various embodiments of the invention which may be used individually or in various combinations. For example, it is shown that a timeslot for communication between a mobile telephone equipment and a second base station is selected as a timeslot next to the one being used to communicate with the first base station only if such timeslot is on the same frequency as the one being used to communicate with the first base station. Another aspect of the flow chart is the use of a threshold to determine whether the mobile telephone equipment should attempt to connect with a second base station. Yet another aspect of the flow chart is the use of a threshold to determine whether communication with the first base station should be dropped. Also shown is the reallocation of a timeslot being used by other mobile telephone equipment in a base station in order to allow mobile telephone equipment to communicate with both base stations. An example where such reallocation may occur is where the timeslot available on the second base station is the same timeslot being used by the mobile telephone equipment to communicate with the first base station. In such a situation, for example, the second base station may reallocate the other mobile telephone equipment that it is communicating with to the timeslot thus freeing another timeslot for use by the first mobile telephone equipment. When such reallocation is attempted, a message may be provided to the first mobile telephone equipment that indicates an estimate of the time to switch timeslots and make a timeslot for handover available. Another aspect of the flow chart is the measurement of signal quality of other base stations (RPs ordered by signal quality). Such measurement may be used to select the next base station to which to attempt to connect.  
      As shown in  FIG. 3 , the mobile telephone equipment (PS) updates the list of base stations (RPs) ordered by signal quality (block  310 ). If signal quality is not below the particular threshold, then the mobile telephone equipment continues to update its list of base stations ordered by signal quality (block  310 ). If signal quality of communication with the current base station is below a particular threshold (block  311 ), then proceed toward connecting with another base station. In particular, it is determined whether a base station (RP) is available with suitable signal quality for which the mobile telephone equipment (PS) has not already queued up a link assignment request (block  312 ).  
      If a base station is available with suitable signal quality for which the mobile telephone equipment has not already queued up a link assignment request, then the mobile telephone equipment (PS) sends a link establishment request to the base station (RP) with the best signal quality (block  313 ). The message includes the current timeslot and frequency used by the mobile telephone equipment to communicate with the current base station (block  313 ). Next determine whether a timeslot is available on the destination base station, and (1) whether it is different from the current timeslot with a suitable interference condition and, (2) if it is adjacent to the current timeslot, whether it is at the same frequency (block  319 ). If these conditions are met, then the destination base station (RP) sends a link assignment to the mobile telephone equipment, letting the mobile telephone equipment know it may connect to the base station (block  322 ). The mobile telephone equipment sends a cancel link establishment to base stations (RPs) that have outstanding queued-up link assignment requests (block  324 ). Then, the mobile telephone equipment (PS) communicates with both base stations (RPs) (block  325 ).  
      If a timeslot was not available in the destination RP, different from the current timeslot and with suitable interference condition or, if adjacent to the current timeslot at the frequency (block  319 ), then it is determined whether the current timeslot is available in the destination base station (RP) (block  320 ). If such current timeslot is not available in the destination base station (RP), then the destination base station (RP) sends a link assignment reject message without queuing the mobile telephone equipment (block  321 ). Next, the mobile telephone equipment returns to determining whether there is a base station available with suitable signal quality for which the mobile telephone equipment has not already queued up a link assignment request (block  312 ).  
      If a current timeslot is available in the destination base station (RP) (block  320 ), then the destination base station sends a link assignment reject to the mobile telephone equipment queuing a link assignment request including a time estimate for how many time frames the mobile telephone equipment (PS) has to wait before a suitable timeslot will be cleared (block  323 ). Next, the destination RP sends a TCH (traffic channel) switch command(s) to move the other mobile telephone equipment to the current timeslot. By moving the other telephone equipment to the current timeslot, a timeslot different from the current timeslot is freed for the mobile telephone equipment for a handover (block  326 ). In the meantime, the mobile telephone equipment continues to determine whether a base station is available with suitable signal quality for which the mobile telephone equipment has not already queued up a link assignment request (block  312 ).  
      If such a base station is not available with suitable signal quality for which the mobile telephone equipment has not already queued up link assignment request (block  312 ), the mobile telephone equipment determines whether there are any queued up link assignment requests outstanding (block  315 ). If such link assignments outstanding are not present, then no handover is available (block  314 ). If such a link assignment request is outstanding, the mobile telephone equipment waits until the first queued up link assignment request served is expired (block  316 ). If the signal quality of the first available base station with a link assignment is still good (block  317 ), then the mobile telephone equipment sends a link establishment request to such base station, and such request includes the current timeslot and frequency used to communicate with the current base station (block  318 ). Next, proceed to block  319  to check whether a timeslot is available in the destination base station different from the current timeslot and with suitable interference condition and if adjacent to the current timeslot at the same frequency (block  319 ). Then, proceed as described above with relation to what happens after completing block  319 .  
       FIG. 4  shows a sample half-frame structure following the personal handy phone system standard (also known as ARIB RCR STD-28), showing the relatively short time available in the ramp bits to switch channels between timeslots, according to an embodiment of the invention. A set of timeslots as would be used in a communication with a mobile telephone are shown (slot  1   401 , slot  2402 , slot  3403 , and slot  4   404 ). Each slot, according to the embodiment shown, is 0.625 ms in length. The channel bit rate is 384 Kbps. A typical half-frame includes ramp bits 405 (4 bits), start symbol 406 (2 bits), preamble 407 (6 bits), unique word 408 (16 bits), channel identifier 409 (4 bits), slow associated control channel 410 (16 bits), information 411 (160 bits), and cyclic redundancy check 412 (16 bits). The particular format of timeslots may be different, depending on the embodiment of the invention. For example, embodiments of the invention may apply to systems in which timeslots contain information in a digital format as well as to systems where timeslots contain information in an analog format.  
      The above description of a timeslot applies to certain embodiments of the invention. Other embodiments of the invention are directed to systems with other timeslot formats, including formats in which timeslots contain information in a digital format, as well as formats where timeslots contain information in an analog format.  
       FIG. 5  is a block diagram of a personal station, according to an embodiment of the invention. A personal station  500  may have one of various types of architectures. The architecture shown is architecture according to one embodiment of the invention. A user communicates with the station by a microphone  501 , which is coupled to an audio processor  502 . Audio processor  502  is coupled to a speaker  520 . A timeslot selector  503  receives information from audio processor  502  and passes such information to modulator  504 , which is coupled to RF processor  505 . Timeslot selector  503  also receives control from command parser/controller  509 . RF processor  505  passes signals to demodulator  508 , which forwards information to command parser/controller  509  and timeslot quality judge/timeslot selector  517 . Timeslot quality judge/timeslot selector  517  has an input to audio processor  502 . Receive signal strength threshold detector  507  is coupled into command parser/controller  509 . Control command parser/controller  509  programs the RX signal strength threshold detector  507  to report to parser/controller  509  when the signal exceeds the following thresholds: search_for_other_RP_threshold  510 , connect_to_destination_RP_threshold  511 , and drop_origin_RP_threshold  512 . Command parser/controller  509  is coupled into timeslot selector  503 , timeslot quality judge/timeslot selector  517 , and power save mode  514 . Frequency synthesizer  506 , which receives a control from command parser/controller  509 , is coupled to RF processor  505 . RF processor  505  is coupled to RX signal strength threshold detector  507  and to antenna  515 . Personal station  500  also includes a list of outstanding link assignment requests  520  and a list of radio ports (base stations) ranked by signal quality  513 .  
      Alternative Embodiments  
      The techniques and systems described above may also apply to a system in which mobile telephone equipment connects to more than two base stations. In such a configuration, according to one embodiment, every N-th frame is used for communications to the N-th base station and data streams for N base stations are interleaved.  
      The methods and systems described above may apply in a system in which the time frames are interleaved in a hierarchical fashion for devices connecting to different RPCs. Such a configuration, synchronization of the base stations is achieved other than by the RPCs, according to an embodiment of the invention.  
      Having now described the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.