Abstract:
An apparatus and method for controlling the timing of control bursts in a wireless communications network maps the timing of control bursts on a multiframe by multiframe basis. In one implementation, a method of controlling base station transmissions in a wireless network in accordance with a multi-frame timing structure is provided. The method is defined as a succession of y frames, each frame including a succession of x time-slots, and includes allocating a frame/time-slot number pair to set the timing of a first-type control burst for a first multi-frame, and changing at least one of the (i) frame number and (ii) frame number and time-slot number of the frame/time-slot number pair for a second multi-frame so that the timing of the first-type control burst is different during the second multi-frame relative to the first multi-frame.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of wireless communications. 
     2. Description of Related Art 
     The segment of the wireless communications community which supports TDMA (Time Division Multiple Access)-based networks embodied in the IS-136 standard, through the UWCC (Universal Wireless Communication Consortium), has undertaken an evolution of the IS-136 TDMA standard toward a 3G (third-generation) wireless network which supports high-speed packet data services such as Internet/intranet access and other multimedia applications. As a major step in this evolution, the UWCC has decided to adopt GSM (Global System for Mobile Communication) EDGE (Enhanced Data for Generic Packet Radio Service Evolution)-based TDMA technology to support high-speed packet data service, and ultimately facilitate global roaming of network subscribers. The EDGE-based technology for packet data service being adopted by the IS-136 TDMA community, called EDGE-COMPACT, nominally is characterized by three 200 kHz air-interface channels, ⅓ frequency reuse (i.e., each base transceiver station (BTS) for a three sector cell being allocated the same three frequency channels as other BTSs), time-division of each 200 kHz frequency channel into eight time slots, and a  52  multiframe (frames being numbered  0  . . .  51  ) control signaling structure similar to that used in standard GSM networks. A  52  multiframe structure is a sequence of 52 frames, where each frame consists of x consecutive time-slots of the air-interface channel (e.g., x=8). 
     By using three 200 kHz carriers and. ⅓ frequency reuse, it is anticipated that EDGE-COMPACT can be deployed in approximately 600 kHz of spectrum. A key characteristic of EDGE-COMPACT which makes ⅓ frequency reuse feasible, and which is a significant departure from standard GSM implementations, is highly accurate time synchronization among all network BTSs down to the symbol level (i.e., within +/−3.69 μs), and “time-grouping” of sectors to reduce interference between packetized control data transmitted from proximate BTSs. More specifically, EDGE-COMPACT requires that each sector be designated to one of a plurality (e.g., three or four) of time-groups to protect control data “bursts,” such that when a sector assigned to a first time-group transmits certain control data, sectors assigned to other time-groups are idle. A burst duration is one time-slot of one frame. 
     Like current implementations of GSM, a mobile station (MS) must first synchronize with a BTS to enable packet data communication in EDGE-COMPACT. To enable such synchronization, a BTS transmits PSCH (Packet version of Synchronization Channel) control bursts which the MS uses for timing acquisition. PSCH control bursts also contain the base station identifier code (BSIC) and other information needed by the MS to select/reselect a serving BTS. An MS must continually detect PSCH control bursts from a plurality of neighboring BTSs. PSCH control bursts may also be referred to as CSCH (COMPACT Synchronization CHannel) control bursts. 
     Each BTS also transmits PFCCH (Packet version of a Frequency Correction Channel) control bursts which an MS monitors for the purpose of accurately tuning to the central frequency of the corresponding air-interface to channel. PFCCH control bursts may also be referred to as CFCCH (COMPACT Frequency Correction CHannel) control bursts. An MS typically needs to tune the central frequency of an air-interface channel only when the MS is initially powered On. In accordance with EDGE-COMPACT, each BTS further transmits other types of control information, including CPBCCH (COMPACT Packet Broadcast Control Channel) bursts and CPCCCH (Compact Packet Common Control channel) bursts. 
     Recent proposals for implementing the EDGE-COMPACT concepts discussed above have assumed that PFCCH and PSCH bursts should occur on the same frame from multiframe to multiframe, and specifically that each BTS should transmit PFCCH bursts on frame  25  and PSCH bursts on frame  51  of the  52  multiframe ( 0  . . .  51 ) control signaling structure. While such static timing of control channel bursts is suitable for standard GSM, the time synchronization between BTSs which facilitates ⅓ frequency reuse in EDGE-COMPACT will result in infrequent PSCH burst decoding opportunities by an MS, specifically only during one frame of each multiframe. Therefore, because of the time needed to tune to a different frequency channel, each MS will typically only have an opportunity to decode the BSICs (contained in PSCH bursts) of BTS transmissions which occur on a single frequency channel during each multiframe, and, thus, will typically require at least three multiframes to “see” the PSCH bursts occurring on all three frequency channels of the six strongest non-serving BTSs which are required-for reselection. More frequent PSCH decoding opportunities for a MS could result in decreased BTS reselection times. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus and method which controls the timing of control bursts in a wireless communications network by time-mapping control bursts on a multiframe by multiframe basis so that mobile stations will have increased opportunities to decode control information which is transmitted by network BTSs on different frequencies. 
     In one embodiment, the present invention is implemented in a wireless network which supports packet data service using three air-interface channels (e.g., each channel corresponding to 200 kHz of spectrum), ⅓ frequency reuse, time division of each air-interface channel into x time slots (e.g., x=8), a y multiframe control signaling structure (e.g., y=52), time synchronization among BTSs, and time-grouping of cell sectors. Each cell sector is grouped into one of z time groups (e.g., y=3 or 4) to protect control data transmissions, such that when a sector assigned to a first time-group transmits certain control data, sectors assigned to other time groups are idle. In contrast to standard GSM control signaling structures in which control data bursts occur on the same frame each multiframe, control channel bursts are time-mapped on a multiframe by multiframe basis to form a timing pattern which provides an MS with more opportunities to receive and decode the control bursts transmitted on different frequency channels. 
     In one specific implementation, each BTS transmits a PSCH burst during an assigned time-slot of either frame  25  or frame  51  of a  52  multiframe ( 0  . . .  51 ) control signaling structure, and transmits a PFCCH burst during an assigned time-slot in frame  51  when the PSCH burst is transmitted in frame  25  and in frame  25  when the PSCH burst is transmitted in frame  51 . The BTS will re-map the frame locations of the PSCH and PFCCH bursts during a subsequent multiframe as a function of the frequency channel being utilized so that PSCH bursts occur during frames  25  and  51  respectively on at least one frequency channel. In this way, an MS will be able to detect PSCH bursts being transmitted on all three frequency channels within two multiframes. Thus, the MS can perform all control measurements needed for reselection in a reduced amount of time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
     FIG. 1 illustrates a wireless communications network configuration suitable for implementing embodiments of the present invention; 
     FIG. 2 is a general block diagram of an exemplary base station transmitter suitable for implementing time-based mapping of control bursts according to embodiments of the present invention; 
     FIG. 3A illustrates an exemplary multiframe structure in which control channel bursts for sectors in a first time group are mapped in accordance with an embodiment of the present invention; 
     FIG. 3B illustrates an exemplary multiframe structure in which control channel bursts for sectors in a second time group are mapped in accordance with an embodiment of the present invention; and 
     FIG. 3C illustrates an exemplary multiframe structure in which control channel bursts for sectors in a third time group are mapped in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is an apparatus and method which controls the timing of control bursts in a wireless communications network by time-based mapping control bursts on a multiframe by multiframe basis so that each MS has an increased number of opportunities to receive control data from a plurality of network BTSs transmitting on different frequency channels. An illustrative embodiment of the apparatus and method according to the present invention is described below. 
     Referring now to FIG. 1, there is shown an exemplary wireless communications network suitable for implementing embodiments of the present invention. As shown in FIG. 1, a network area  90  is divided into a plurality of three-sector cells, each provided with a BTS  110 - 1  , . . . ,  110 - 7  which serve MSs therein. A first BTS  110 - 1  serves sectors  112   a ,  112   b , and  112   c ; a second BTS  110 - 2  serves sectors  114   a ,  114   b , and  114   c ; a third BTS  110 - 3  serves sectors  116   a ,  116   b , and  116   c ; a fourth BTS  110 - 4  serves sectors  118   a ,  118   b , and  118   c ; a fifth BTS  110 - 5  serves sectors  120   a ,  120   b , and  120   c ; a sixth BTS  110 - 6  serves sectors  122   a ,  122   b , and  122   c ; and a seventh BTS  110 - 7  serves sectors  124   a ,  124   b , and  124   c . Each BTS  110 - 1 , . . . ,  110 - 7  is connected to a BTS controller (not shown) which manages the wireless network, and serves as an interface between network BTSs and between the wireless network and a separate network, such as a Public Switched Telephone Network. Each BTS  110 - 1 , . . . ,  110 - 7  is nominally assigned three frequency channels, e.g., three 200 kHz channels, so that each sector served by the BTS is characterized by a different frequency channel. In the exemplary configuration of FIG. 1, sectors  112   a ,  114   a ,  116   a ,  118   a ,  120   a ,  122   a , and  124   a  are served by their respective BTS on frequency channel F 1 ; sectors  112   b ,  114   b ,  116   b ,  118   b ,  120   b ,  122   b , and  124   b  are served by their respective BTS on frequency channel F 2 ; and sectors  112   c ,  114   c ,  116   c ,  118   c ,  120   c ,  122   c , and  124   c  are served by their respective BTS on frequency channel F 3 . Therefore, the network configuration illustrated in FIG. 1 incorporates a ⅓ frequency reuse scheme. 
     A To enable ⅓ frequency reuse while ensuring the integrity of certain packet control data transmitted by each of the BTSs  110 - 1 , . . . ,  110 - 7 , each BTS is time-synchronized and utilizes a y multiframe (e.g., y=52) control signaling structure. Furthermore, each sector is assigned to one of z time groups to protect packet control data transmissions, such that when a sector assigned to a first time-group transmits certain control data, sectors assigned to other time-groups can be forced idle. For the network configuration illustrated in FIG. 1, sectors  112   a ,  114   c ,  116   c ,  118   b ,  120   a ,  122   a , and  124   c  are assigned to time group T 1 ; sectors  112   b ,  114   a ,  116   a ,  118   c ,  120   b ,  122   b , and  124   a  are assigned to time group T 2 ; and sectors  112   c ,  114   b ,  116   b ,  118   a ,  120   c ,  122   c , and  124   b  are assigned to time group T 3 . Although three time groups are used in a configuration of FIG. 1, it is possible to use more or less time groups. As will be described in more detail below, such time-grouping of sectors is used to control when the BTSs  110 - 1 , . . . ,  110 - 7  can transmit control bursts on each of the three frequency channels F 1 , F 2 , and F 3 . Accurate time synchronization among BTSs, e.g., down to the symbol level, allows such time-grouping to be realized for packet control data protection. 
     Referring to FIG. 2, there is shown a general block diagram of a BTS transmitter  100  suitable for implementing the multiframe control scheme with time-mapped control bursts according to an embodiment of the present invention. As shown in FIG. 2, the BTS transmitter  100  includes a packet scheduler unit  110 , a baseband processing unit  130 , and a radio frequency (RF) processing unit  140 . The packet scheduler  110  includes a logical multiframe generating unit  112  which receives a plurality of signals input 1 , . . . , input N , including for example voice/data traffic to be transmitted to MSs being served by the BTS transmitter  100  as well as control information from a switching center (not shown). The logical multifram generating unit  112  is connected to a traffic/control scheduler  114  and outputs traffic and control packets, such as PFCCH, PSCH, CPBCCH, and CPCCCH control data discussed above, in a logical multiframe format in accordance with traffic/control scheduling information received from the traffic/control scheduler  114 . As described in more detail below, the traffic/control scheduler  114  maps PSCH and PFCCH bursts to frame locations of a y multiframe control signaling structure on a multiframe-by-multiframe basis so that the frame location for PSCH and PFCCH changes during a sequence of multiframes. 
     The baseband processing unit  130  receives the output of the packet scheduler  110  and maps the logical multiframe packets onto physical timeslots/frames, for example in a conventional manner. The RF processing unit  140  receives the output of the baseband processing unit  130  which generates an RF transmission signal, Tx, using an assigned RF channel, which is output to a transmitter antenna  200 . 
     As described above, EDGE-COMPACT is nominally characterized not only by three frequency channels, ⅓ frequency reuse, and time synchronization, but is also based on time-division of each air-interface frequency channel into eight time slots and a  52  multiframe ( 0  . . .  51 ) control signaling structure similar to that used in standard GSM networks. FIG. 3A is a conceptual diagram of a  52  multiframe structure, in which a matrix is formed of eight columns, respectively representing eight time slots TS 0 -TS 7 , and a number of rows which each represent either an individual frame or a block of frames in the  52  multiframe ( 0  . . .  51 ). As shown in FIG. 3A, each frame is made up of eight consecutive time-slots. 
     To protect control bursts between proximate BTSs transmitting on the same frequency channel, each time group T 1 , T 2 , and T 3  is assigned to one of the eight time slots TS 0  . . . TS 7 . In the exemplary implementation illustrated in FIG. 3A, time group T 1  is designated for TS 1 , time group T 2  is designated for TS 3 , and time group T 3  is designated for TS 5 . Certain frame sequences are grouped into blocks, such that frames  0 - 3  form block B 0 , frames  4 - 7  form block B 1 , frames  8 - 11  form block B 2 , frames  13 - 16  form block B 3 , frames  17 - 20  form block B 4 , frames  21 - 24  form block B 5 , frames  26 - 29  form block B 6 , frames  30 - 33  form block B 7 , frames  34 - 37  form block B 8 , frames  39 - 42  form block B 9 , frames  43 - 46  form block B 10 , and frames  47 - 50  form block B 11.    
     Control burst timing for sectors in time group T 1  will next be described with reference to the  52  multiframe structure illustrated in FIG.  3 A. Initially, during TS 0 , TS 2 , TS 4 , and TS 6 , (i.e., the even-numbered time-slots) of any of frames  0 - 24  and  26 - 50 , there are no restrictions on transmissions for sectors in time group T 1 , or any other time groups. All base stations are idle during TS 0 , TS 2 , TS 4 , and TS 6  of frames  25  and  51 . Therefore, the synchronized  52  multiframe structure only affects transmissions during the even numbered time slots TS 0 , TS 2 , TS 4 , and TS 6  in frames  25  and  51 . During TS 1  of frames in B 0  sectors in time group T 1  transmit CPBCCH bursts, and sectors in the other time groups T 2  and T 3  are forced idle to protect the control data of sectors in time group T 1  for the exemplary ⅓ frequency reuse. In return, sectors in time group T 1  are forced idle during TS 3  and TS 5  of frames in block B 0  to protect control bursts in time-groups T 2  and T 3 , respectively. Although not shown in FIG. 3A, for four time-group networks, sectors in time group T 1  are also forced idle during TS 7  of frames in block B 0  to protect control bursts in an additional time-group T 4 . No control restrictions are placed on sectors during blocks B 1 , B 2 , B 4 , B 5 , B 7 , B 8 , B 10 , and B 11 , so that these blocks may be used for transmitting normal data traffic. In other words, the time-grouping of sectors is not used to protect data traffic, only control bursts. For example, sectors in all time groups T 1 , T 2 , and T 3  may transmit data during any of the empty frame blocks shown in FIG.  3 A. 
     During frame  12 , sectors in all three time groups T 1 , T 2 , and T 3  may transmit PTCCH bursts, which are monitored by each MS for continuous time synchronization. Sectors in time group T 1  transmit CPCCCH bursts during TS 1  of frame in blocks B 3 , B 6 , and B 9 , and sectors in other time groups are forced idle during TS 1  of frames in blocks B 3 , B 6 , and B 9  to protect the CPCCCH bursts transmitted for sectors of time group T 1 . In return, sectors in time group T 1  are forced idle during TS 3  and TS 5  of frames in blocks B 3 , B 6 , and B 9  to protect the CPCCCH control bursts for sectors in time groups T 2  and T 3 , respectively. An “X” in a matrix location of FIG. 3A is intended to illustrate the concept that sectors in time group T 1  are forced idle during TS 3  and TS 5  of frame in blocks B 0 , B 3 , B 6 , and B 9  to protect control bursts of sectors in time groups T 2  and T 3 . 
     During TS 1  of frames  25  and  51 , sectors in time group T 1  transmit either a PSCH burst or a PFCCH burst. The traffic/control scheduler  114  determines whether a PSCH or a PFCCH burst is transmitted during TS 1  of each of frame  25  and frame  51  as a function of the frequency channel assigned to the particular sector of time group T 1  and the multiframe number in a cycle of consecutive multiframes. More specifically, the PSCH and PFCCH bursts for time group T 1  sectors on a first frequency channel will be re-mapped relative to the PSCH and PFCCH bursts from time group T 1  sectors on at least one other frequency channel. In this way, no matter which of the three frequency channels a mobile subscriber terminal monitors during frame  25  of a first multiframe, it will be able to detect PSCH bursts being transmitted on all three frequency channels within successive multiframes. An exemplary time-mapping of PSCH bursts as a function of assigned frequency channel and multiframe number is illustrated in Table 1 below. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary Mapping of PSCH Bursts 
               
             
          
           
               
                   
                 F1 
                 F2 
                 F3 
               
               
                   
                   
               
             
          
           
               
                 Multiframe 0 
                 PSCH on frame 25 
                 PSCH on frame 25 
                 PSCH on 
               
               
                   
                   
                   
                 frame 51 
               
               
                 Multiframe 1 
                 PSCH on frame 25 
                 PSCH on frame 51 
                 PSCH on 
               
               
                   
                   
                   
                 frame 25 
               
               
                 Multiframe 2 
                 PSCH on frame 51 
                 PSCH on frame 25 
                 PSCH on 
               
               
                   
                   
                   
                 frame 25 
               
               
                 Multiframe 3 
                 PSCH on frame 51 
                 PSCH on frame 51 
                 PSCH on 
               
               
                   
                   
                   
                 frame 25 
               
               
                 Multiframe 4 
                 PSCH on frame 51 
                 PSCH on frame 25 
                 PSCH on 
               
               
                   
                   
                   
                 frame 51 
               
               
                 Multiframe 5 
                 PSCH on frame 25 
                 PSCH on frame 51 
                 PSCH on 
               
               
                   
                   
                   
                 frame 51 
               
               
                 Multiframe 6 
                 PSCH on frame 25 
                 PSCH on frame 25 
                 PSCH on 
               
               
                   
                   
                   
                 frame 51 
               
               
                 Multiframe 7 
                 PSCH on frame 25 
                 PSCH on frame 51 
                 PSCH on 
               
               
                   
                   
                   
                 frame 25 
               
               
                 . 
               
               
                 . 
               
               
                 . 
               
               
                 Multiframe 1023 
                 PSCH on frame 51 
                 PSCH on frame 51 
                 PSCH on 
               
               
                   
                   
                   
                 frame 25 
               
               
                 Multiframe 0 
                 PSCH on frame 25 
                 PSCH on frame 25 
                 PSCH on 
               
               
                   
                   
                   
                 frame 51 
               
               
                 . 
               
               
                 . 
               
               
                 . 
               
               
                   
               
             
          
         
       
     
     In the exemplary time-mapping shown in Table 1, during a first multiframe  0 , sectors on channel F 1  and channel F 2  will transmit PSCH bursts on frame  25  (and PFCCH bursts on frame  51 ), and sectors on channel F 3  will transmit PFCH bursts on frame  51  (and PFCCH bursts on frame  25 ). During a second multiframe  1 , PSCH and PFCCH bursts for sectors on channels F 2  and F 3  are re-mapped such that sectors on channel F 2  will transmit PFCH bursts on frame  51  and sectors on channel F 3  will transmit PSCH bursts on frame  25 . In a third multiframe  2 , the PFCH and PFCCH bursts of sectors on channels F 1  and F 2  flip so that sectors on channel F 1  transmit PSCH bursts on frame  51 , and sectors on channel F 2  again transmit PSCH bursts on frame  25 . As can be seen from the pattern of PSCH bursts shown in Table 1, an MS (which knows the mapping scheme described above) has up to twice the number of opportunities to decode neighbor PSCH bursts as compared to when PSCH bursts appear only in frame  51 . In this way, a MS will be able to monitor the PSCH bursts on all three frequency channels F 1 , F 2 , and F 3  by the end of two multiframes. Using the sequence illustrated in Table 1, an MS will have a ⅓ probability of taking a full two multiframes to see PSCH bursts of all frequencies and all time groups, and a ⅔ probability of taking only 1.5 mutliframes to see all such PSCH bursts. 
     In the PSCH/PFCCH burst time-mapping cycle shown in Table 1, the sectors on channel F 2  flip PSCH/PFCCH bursts each multiframe. Sectors on channels F 1  and F 3 , however, flip PSCH/PFCCH bursts only every three multiframes, and not on the same multiframe. It should be apparent that numerous other time-mapping sequences of PSCH/PFCCH bursts are possible to achieve the same goal of increasing mobile subscriber terminal opportunities for decoding PSCH bursts of all frequency channels. For example, sectors on channel F 1  may be controlled to flip PSCH/PFCCH bursts every multiframe, while sectors on channels F 2  and F 3  flip PSCH/PFCCH bursts every three multiframes. 
     FIG. 3B illustrates the multiframe control signaling configuration from the perspective of sectors in time group T 2 . As shown in FIG. 3B, sectors in time group T 2  transmit CPBCCH bursts during TS 3  of frames in block B 0 , transmit CPCCCH burst during TS 3  of frames in blocks B 3 , B 6 , and B 9 , and transmit either a PSCH or PFCCH burst during TS 3  of frames  25  and  51 . During such time-slots, sectors in other time groups are forced idle to protect the control bursts of sectors in time group T 2 . In return, sectors in time group T 2  are forced idle during TS 1  and TS 5  of frames in blocks B 0 , B 3 , B 6 , B 9 , frame  25 , and frame  51 . In all other respects, the control signaling structure, including the PSCH/PFCCH burst mapping scheme illustrated in exemplary Table 1, are the same for T 2  sectors as for T 1  sectors described above. 
     FIG. 3C illustrates the multiframe control signaling configuration from the perspective of sectors in time group T 3 . As shown in FIG. 3C, sectors in time group T 3  transmit CPBCCH bursts during TS 5  of frames in block B 0 , transmit CPCCCH bursts during TS 5  of frames in blocks B 3 , B 6 , and B 9 , and transmit a PSCH or PFCCH burst during TS 5  of frames  25  and  51 . As discussed above, sectors in other time groups are forced idle during TS 5  of frames in blocks B 0 , B 3 , B 6 , B 9 , frame  25 , and frame  51  to protect the control bursts of sectors in time group T 3 . In return, sectors in time group T 3  are forced idle during TS 1  and TS 3  of frames in blocks B 0 , B 3 , B 6 , B 9 , frame  25 , and frame  51  to protect control bursts of other time groups. In all other respects, the control signaling configuration for sectors in time group T 3 , including the scheme for mapping PFCH/PFCCH bursts illustrated in the exemplary Table 1, is the same as that discussed above. 
     As compared to prior proposals which require sectors in all time groups and on all frequency channels to transmit PFCCH bursts in frame  25  and PSCH bursts in frame  51  of a  52  multiframe ( 0  . . .  51 ) control signaling structure, time-base mapping of PSCH/PFCCH bursts on a multiframe by multiframe basis allows each MS to decode PSCH information for all significant neighbors on three frequencies, or average over 40% more frequently. 
     It should be apparent to those skilled in the art that various modifications and applications of the present invention are contemplated which may be realized without departing from the spirit and scope of the present invention.