Patent Publication Number: US-2005128978-A1

Title: Method and apparatus for compressed mode communication

Description:
FIELD OF THE INVENTION  
      The present invention pertains to framed signaling, and more particularly to a method and apparatus utilizing compressed mode operation for framed signals.  
     BACKGROUND OF THE INVENTION  
      Third generation wireless mobile user equipment will support dual radio access technology, such as by supporting communication over 3G (third generation) systems, such as wideband code division multiple access (WCDMA) systems, and 2G systems, such as Global Systems for Mobile communications (GSM) systems. Such user equipment will be required to acquire and maintain knowledge of multiple radio frequency domains with regard to signal strength of serving and adjacent cells, interference, and synchronization. When such user equipment is operating in idle mode, which is the mode where the user equipment is not engaged in dedicated communication with a serving cell, the implementation of such procedures is straightforward.  
      However, where the user equipment is engaged in dedicated communication on a serving cell of one system, requiring that it both receive and transmit signals, there may be a lack of time available during which measurements or synchronization of the other systems supported by the equipment can take place. For example, if user equipment is engaged in dedicated communication with a serving cell on the Universal Terrestrial Radio Access (UTRA) domain using frequency division duplex (FDD), the user equipment must transmit during each available frame period. This limits the time available for performing measuring and synchronization with a cell of a GSM system.  
      To overcome this problem, third generation partnership project (3GPP) specification section 25.212 specifies “compressed mode” operation, during which the mobile user equipment, or the network, may transmit during only a portion of a frame in order to allow measurement and/or synchronization during the other portion of the frame. However, this specification requires transmissions to be performed using a smaller spreading factor, thereby necessitating a 3 dB greater transmission power to achieve a suitable bit-error rate (BER). The specified method thus severely impacts the capacity of the cell, as the number of devices operating in compressed mode will be limited by the increased power requirements.  
      The 3GPP specification describes three methods for reducing the signal length to create a transmission gap for compressed mode operation. Puncturing, by which data redundancy is removed for a compressed frame to allow transmission within a shorter time period. This technique allows more data to be transmitted at the expense of error correction capability. A second technique is spreading factor reduction, by which the spreading factor is reduced by a factor of 2, thereby requiring half the time to transmit a given amount of data. However, such a reduction is at the expense of processing gain, which is applicable to both the uplink and the downlink. A third method of reducing the signal length is higher layer scheduling.  
      What is needed is an improved compressed mode communication. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The various aspects, features and advantages of the present invention will become more fully apparent from the following Detailed Description with the accompanying drawings described below.  
       FIG. 1  is schematic representation of a cellular communication system having different overlapping systems.  
       FIG. 2  is a circuit schematic in block diagram form illustrating user equipment and four base stations.  
       FIG. 3  illustrates a compressed mode frame allocation.  
       FIG. 4  illustrates an improved compressed mode frame allocation.  
       FIG. 5  illustrates user equipment operable in compressed mode.  
       FIG. 6  illustrates a base station operable in compressed mode.  
       FIG. 7  illustrates signal flow between the user equipment and the base station.  
       FIG. 8  illustrates another improved compressed mode frame allocation.  
       FIG. 9  is a flow diagram illustrating operation of a base station to assign portions of a frame for compressed mode communication.  
       FIG. 10  is a flow diagram illustrating an alternate operation of a base station to assign portions of a frame for compressed mode communication.  
       FIG. 11  is a flow diagram illustrating operation of user equipment and base station to allocate the portion of the frame for compressed mode communication.  
       FIG. 12  is a flow diagram illustrating an alternate operation of a user device and a base to allocate the portion of the frame for compressed mode communication.  
       FIG. 13  illustrates another embodiment of a compressed mode frame allocation.  
       FIG. 14  illustrates a compressed mode pattern. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      A complementary compressed mode method and apparatus facilitate evaluation of one communication system while communicating in another communication system. User equipment devices ( 108 ,  110 ) are assigned to different portions of a frame during compressed mode operation.  
      A cellular communication system  100  ( FIG. 1 ) is illustrated including a first communication system  102  covering a plurality of cells  101  (only some of which are numbered). By way of example, each cell can be considered to represent the coverage area of a base station. The first system may for example be a UTRA system, and in particular either a UTRA FDD or UTRA time division duplex (TDD) system. The communication system  100  also includes a second communication system  104  covering a plurality of cells  103  (only some of which are numbered), each cell representing the coverage are of a base station. The second system may for example be Global System for Mobile communications (GSM) or a second generation (2G) code division multiple access (CDMA) system. The cellular communication system may include additional or other communication systems, as the communication systems may operate according to any known wireless communication system specification such as the GSM, CDMA, wideband code division multiple access (WCDMA), time division multiple access (TDMA), Group Packet Radio System (GPRS), EDGE, or the like. The user equipment  108 , or user equipment device, may be a cellular radiotelephone, a personal digital assistant, a modem, an accessory, or the like, and may support either single mode or multimode operation, and thus may be capable of operating in one or more than one communication protocol and/or one or more than one frequency band.  
      The user equipment  108  includes transceiver  204  ( FIG. 2 ) and a controller  205 . In general, the transceiver  204  enables the user equipment to effect a wireless communication link with base stations, such as base station  200  of cell  101 ′, base station  230  of cell  101 ″, base station  210  of cell  103 ″, and base station  220  of cell  103 ′. Each base station includes a transceiver  206 ,  212 ,  222 ,  232 , for wireless communication and a controller  208 ,  214 ,  224 ,  234  for controlling the operation of the base station and establishing a communication link with the mobile switching center  240 . The mobile base stations (e.g.,  200 ,  220 ,  220 ,  230 ) and the mobile switching center  240  are a network supporting wireless communications.  
      In operation, as the user equipment  108 ,  110  moves through the system, hand-off will occur according to ordinary operating techniques, which are well known in the art. For multi-mode user equipment, such as those operating over a plurality of different communication air interfaces, the user equipment  108 ,  110  will be required to acquire and maintain knowledge of multiple radio frequency domains, and may for example maintain knowledge of signal strength of serving and adjacent cells, interference information, and synchronization information, as is known to those of ordinary skill in the art.  
      While the user equipment  108 ,  110  establishes a dedicated communication link with a base in the first communication system  101 , which is illustrated as a UTRA system, the user equipment will at least occasionally be required to evaluate system  104 , a GSM system. This may for example occur when the user equipment  108  moves to the edge of cell  101 ′,  101 ″ adjacent cell  103 ′. In the illustrated embodiment, cell  103 ′ covers an area not served by communication system  102 , and thus user equipment  108  will need to be handed off from base station  200  to base station  220 . In order to support the measurement and synchronization processes that user equipment  108  must perform while engaged in communications with base station  200  of cell  101 ′, at least the uplink communications between the user equipment  108  and base station  200  are made in compressed mode.  
      More particularly, the user equipment  108  ( FIG. 1 ) will be required to obtain knowledge of multiple radio frequency domains, and attend to inter-domain measurement and/or synchronization tasks for the second communication system  104  while has an established link with communication system  102 . In compressed mode, these tasks are performed during a time period when the user equipment  108  is not in dedicated communications with communications system  102 . One timing diagram for accomplishing this is illustrated in  FIG. 3 , showing timing for user equipment  108 ,  110 , referred to in this figure as Mobile A ( 108 ) and Mobile B ( 110 ). In compressed mode Mobile A and Mobile B both transmit with a reduced spreading factor, and higher power, such that communications with one system (e.g., base station  200 ) occur in the first half of the frame. During the second half of the frame, Mobile A and Mobile B may evaluate the other system. Evaluation may for example comprise making inter domain measurements, obtaining synchronization with the other system, or another system evaluation (e.g., base station  220 ).  
      Thus, in compressed mode, a transmission gap is created during which the user equipment may perform measurements without encountering a scheduling conflict. A scheduling conflict would otherwise occur where the user equipment attempts to perform two tasks simultaneously with a single transceiver path. Additionally, compressed mode occurs without subjecting the system to prohibitively high levels of self-interference, as in the case of inter-mode measurements that may occur at the same time in the same or a close frequency band.  
      In normal mode, the CDMA signals from user equipment  108 ,  110  are separated from one another by a channel identification code on the uplink. The signals in the downlink are also separated by a channel identification code. During compressed mode, the channel identification code (e.g., an orthogonal code in CDMA) still isolates the signals, but the information rate is effectively “sped up” by a factor of 2 in response to the spreading factor being reduced by ½. It is necessary to increase uplink power for the user equipment  108 ,  110  in compressed mode to compensate for the loss of processing gain due to the lower spreading factor. A significant problem encountered with a system operating according to  FIG. 3  is the number of user equipment devices that can operate in compressed mode is severely limited.  
      As used herein, in a “compressed mode pattern,” a certain number of frames having transmission gaps are followed by a certain number of frames that do not have transmission gaps, and this pattern repeats with a periodicity of a certain number of frames. Compressed mode pattern thus refers to: the number of time slots during which transmission occurs within the period of a given frame; the number of time slots during which compression does not occur within the period of a given frame; the number of compressed frames in which compressed transmissions occur during time slots of a given frame; and the number of non-compressed frames in which compressed transmission does not occur during the time slots of a given frame.  
      An example of a transmission pattern is illustrated in  FIG. 14 . The illustrated pattern comprises 12 frames, 2 compressed frames followed by 10 non-compressed frames. Within the compressed frames, there are 15 slots, the first 4 and last 4 of which are available for transmission and the middle  7  of which the transmitter is turned off. Those skilled in the art will recognize that many other transmission patterns are possible.  
      A significantly improved system for compressed mode operation is illustrated in  FIG. 4 . In  FIG. 4 , Mobile A ( 108  in  FIG. 1 ) communicates in the first portion of the frame and Mobile B ( 110  in  FIG. 1 ) communicates in the second portion of the frame. While Mobile A communicates with one base station  200  of cell  101 ′ of the first communication system  102 , Mobile B performs inter-domain measurement and/or synchronization with base station  220  of cell  103 ′. In the second portion of the frame, Mobile B communicates with a base station  200  and Mobile A performs inter-domain measurement and/or synchronization. The compressed mode continues for N frames, where N is an integer. N can be any number greater than 0, and may for example be 2, such that measurements may be made in 2 consecutive frames followed by 10 frames that are not compressed, thereby providing a 12-frame pattern. Although 2 user equipment devices are illustrated, more than 2 user equipment devices can be allocated to each of the first portion and the second portion of the frame, each of the user equipment devices having a respective orthogonal code, and because all of the user equipment is not communicating within the same portion of the frame, the number of devices that can operate in compressed mode is significantly increased.  
      Although the first portion and the second portion may be allocated from many different groups of slots, one pattern envisioned is to divide the frame into 15 slots. The first portion comprises 7 slots that are allocated to a plurality of devices separated by orthogonal codes. The second portion comprises the last 7 slots that are allocated to another group of devices, also separated by orthogonal codes. The third portion is a separation slot in the middle of a frame. In one embodiment, it is envisioned that the devices allocated to the first portion will have different orthogonal codes than the devices in the second portion. The slots are preferably of equal length.  
      To examine the effects of amplitude variations based on the type of pattern (on/off sequence) selected, a simulation was used to generate various compressed mode patterns in terms of the radio frequency (RF) envelope shape, and then a Fourier transform was used to determine the spectral properties of the envelope. The 7-1-7 slot compressed mode pattern was found to have favorable spectral characteristics when compared to other patterns when user equipment were paired.  
      In particular, Fourier analysis was used to compute the spectrum of the RF envelope having the maximum allowable rise time and a decay time of 25 μs. The following simulation used compressed mode patterns, each of which has a repetition period of 12 frames, i.e. 2 compressed frames followed by 10 uncompressed frames, which pattern was repeated. The inventors found a significant degree of cancellation of spectral components under 100 Hz (frequency number approximately 125) for the 7-1-7 pattern. While there are many other combinations of patterns that may be compared and utilized with the invention, the 7-1-7 pattern using a repetition period of 12 frames resulted in lower uplink interference.  
      The user equipment  108  will now be described in greater detail with reference to  FIG. 5 . The user equipment includes transceiver  204 , which may be implemented using any suitable wireless transceiver known in the art. The controller  205  includes a physical layer  504 , a medium access control (MAC) layer  506 , a radio link controller (RLC) layer  508 , and a radio resource control layer (RRC). The physical layer  504  maps the transport channels to physical channels and exchanges coded and modulated baseband signals with the RF transceiver. The channels may be identified by frequency, code (such as in a CDMA system) or time (such as in a TDMA system), or by any two or more of frequency, time and code.  
      The MAC layer  506  maps logical channels from the RLC  508  to transport channels in the physical layer. The RLC  508  controls the transmission link over the radio medium.  
      The RRC layer  510  controls radio operation of the user equipment  208  (or  210 ). RRC layer  510  includes a control message recognizer  514 , which outputs downlink messages to the control message parser  516 . Compressed mode control messages are input to the uplink compressed mode controller  518 . The compressed mode controller generates message acknowledgements, which are input to the uplink user data path for communication to the base station. The uplink compressed mode controller also generates compressed mode control information, pattern assignment information, and resource assignment and measurement scheduling, which is determined as described in greater detail hereinbelow. The physical layer includes an uplink compressed mode pattern and assignment manager  520  responsive to the compressed mode pattern received from the uplink compressed mode controller  518 . An uplink transmission controller  522  communicates via the radio frequency transceiver  204  under the control of resource assignments received from the uplink compressed mode controller  518 . The physical layer further includes a measurement acquisition unit  524 , responsive to the measurement schedule from the uplink compressed mode controller  518  for acquiring measurements and communicating the measurements to the measurement processor  526  in the radio resource controller  510 .  
       FIG. 6  illustrates the base stations in cellular communication system  100 , and is represented herein be base station  200  ( 210 ,  220 ,  230 ). The base station  200  includes RF transceiver  206 . The controller  208  includes a physical layer  604 , MAC  606 , RLC layer  608 , and RCC layer  610 . The physical layer maps transport channels to basic physical channels, and generates coded and modulated baseband signals. The MAC layer maps logical channels to transport channels. The RLC layer controls radio bearers or transmission links over the radio medium.  
      The RCC layer  610  controls radio resources. The RCC includes an uplink compressed mode controller  614 , which receives downlink signal data and generates uplink signal data. The compressed mode controller communicates with the uplink traffic scheduler  616 . Additionally, the uplink compressed mode controller communicates the measurement schedule to the measurement acquisition unit  612 . A measurement processor  618  receives the measurements from the measurement acquisition unit  612 .  
      The operation of the system will now be described with reference to  FIG. 7 . Initially, the user equipment devices  208 ,  210 , (represented by Mobile A and Mobile B) send a message containing measurement capability information element to the network (base station  200 ). The information element may be part of multiple messages, and contain the need, purpose and direction for the type of compressed mode requested. When the RRC layer  610  is making an assignment of radio resources to user equipment being scheduled for uplink transmission, the uplink compressed mode controller  614  in the network RRC  610  chooses pairs of user equipment devices and assigns them to complementary patterns and the same starting frame number. The characteristics of the uplink compressed mode are sent in an assignment message containing the compressed mode information IE which includes: compressed mode pattern; starting frame number; starting timeslot within frame; pattern period; and the maximum number of repetitions. In the example of  FIG. 4 , Mobile A will be assigned to transmit in timeslots TS 0 -TS 6  and Mobile B will be assigned to timeslots TS 8 -TS 14 . Thereafter, Mobile A and Mobile B transmit uplink blocks using the assigned communication slots in the current communication domain (e.g., communication system  102 ) and measure and/or synchronize on the other communication domain (e.g., communication system  104 ). The procedure repeats for the duration of the frames indicated.  
       FIG. 9  illustrates operation of the base station of system  102  to assign time slots according to one embodiment. In particular, as user equipment is added to compressed mode, the network first determines how many user equipment devices are operating in compressed mode and transmitting in portion  1  of the frame (time slots TS 0 -TS 6 ) for frames  1  and  2 , as indicated in step  902 . The network then determines how many user equipment devices are operating in compressed mode and communicating the second portion of the frame (time slots TS 8 -TS 14 ) for frames  1  and  2 , in step  904 . The newest addition to compressed mode operation in frames  1  and  2  is then assigned by the network to the time slots having the fewest number of user equipment devices operating in compressed mode, in step  906 . The assignment is stored in the network and communicated to the user equipment in step  908 .  
      An alternate embodiment is illustrated in  FIG. 10 . In this embodiment, when the subscriber equipment initiates compressed mode operation, the network determines in step  1002  to which portion (e.g., time slots TS 0 -TS 6  or time slots TS 8 -TS 14 ) the user equipment that last initiated compressed mode operation in the same frames was assigned in step  1004 . The network assigns the new user equipment device to the other portion of the frames, in step  1006 .  
      Yet another alternate embodiment for assigning compressed mode operation is illustrated in  FIG. 11 . In  FIG. 11 , both the network and the user equipment operation is described, as in this embodiment the assignment is not made at the network, but rather the user equipment and the base station both determine the compressed mode slots for the user equipment using a deterministic value known to both the user equipment device and the base station. Thus, in this embodiment, a predetermined deterministic value known to both the user equipment and the network selects the portion of the frame to which the user equipment is assigned. In particular, compressed mode operation is initiated in step  1102 . The deterministic value is checked in step  1104 . If the deterministic value is a  1 , the user device will conduct compressed mode operation in portion  1  of the time slot, as indicated in step  1108 . If  1 , as determined in step  1106 , the compressed mode communication will take place in the second portion of the frame, and measurements will be made in the first portion of the frame.  
      It is envisioned that the deterministic value may be any value known to the user equipment and the base station, and may for example be a particular bit of the subscriber equipment IMEI, such as the last bit of the subscriber equipment IMEI. An alternative deterministic value could be a predetermined bit of a signal communicated from the user equipment to the network, such as a bit stored in memory in the user equipment. Another alternative can be a random or pseudo-random number generated by circuitry in the user equipment and known to base station.  
       FIG. 8  illustrates an alternate frame allocation embodiment. In the embodiment of  FIG. 8 , the mobile user equipment slot assignments vary from frame to frame. Thus, in frame  2  (F 2 ), Mobile A is transmits in the first portion and Mobile B transmits in the second portion. In frame  3  (F 3 ), Mobile B transmits in the first portion and Mobile A transmits in the second portion. In the illustrated example, Mobile A and Mobile B transmit in different portions of the frame. Those skilled in the art will recognize that the assignments for each of Mobile A and Mobile B may advantageously be random, or pseudo random, such that in some frames Mobile A and Mobile B will communicate on one system and measure in the other system during the same time slots of a frame. Those skilled in the art will also recognize that there will be more than 2 user equipment devices operating on the system, such that many other mobiles are transmitting in each of the first and second portions of the frame. It is envision that the compressed modes illustrated in this application can apply to systems operating at capacity, with all the user equipment devices operating in compressed mode.  
       FIG. 12  illustrates operation of the user equipment and base station, wherein each use a deterministic value to determine the frame portion for communication such that the base station need not determine and assign the value to the subscriber device, and providing the compressed mode frame allocation shown in  FIG. 8 . In  FIG. 12 , the portion assignment for communication with base station  200  will change in each frame in a pseudo-random manner. The compressed mode is initialized in step  1202 . The deterministic value for the first frame is determined in step  1204 . For example, the deterministic value can be based on the encryption sequence that is generated for encrypting data in digital communication systems, which is a pseudo random number known to the user equipment device and base station, can be used as the basis for making the slot assignment for a user equipment device in a frame. Another alternative is for the user equipment device and the network to each use a predetermined bit of a synchronized linear shift register that steps through every number (or every number except all zeroes) as the deterministic value. For example, the last bit of the sequence selected to be the basis for the slot assignment can be used as the deterministic value.  
      In step  1206 , the controller in the user equipment and the base station determines whether the deterministic bit is a 1 or a 0. If the bit is a 0, then the communication in compressed mode will be in portion  1  for the initial frame. If the bit is a 1, the communications in the compressed mode will use portion  2  for the initial frame, as indicated in step  1208 . The controller waits for the next frame in step  1214 . If the next frame is after the last frame of the compressed mode sequence, as determined in step  1216 , the compressed mode communication ends. If the frame is not after the last frame of the compressed mode sequence, the controller identifies the deterministic value for the next frame in step  1218 , and returns to decision step  1206 . This process will be repeated for the compressed frames in the compression pattern.  
      With complementary compressed mode, the uplinks are still processed by reducing their spreading factor by ½, except that instead of being isolated from one another by orthogonal codes, they are now temporarily isolated in a manner of a slotted physical access mechanism. Additionally, compressed mode operation can be assigned to pairs of user equipment devices, each with a complementary pattern.  
      The present invention has significant technically and commercially desirable attributes. It reduces the peak-to-peak amplitude variations arriving at the Node-B receiver from a given pair of user equipments assigned to symmetrical compressed mode. This results in lower self-interference on the uplink, and therefore greater cell capacity. Additionally, a timeslot pattern and period can be selected that demonstrates surprising spectral characteristics for the RF envelope to be optimized.  
       FIG. 13  illustrates a system wherein radio resource assignments are made for a universe of 4 user equipment devices that would previously have been allocated to 2. This is accomplished by assigning the same channel identification code, such as the spreading or orthogonal code of a CDMA system, to Mobile B and Mobile C, and another channel identification code to Mobile A and Mobile D. The user equipment operates in compressed mode to increase the capacity of the system, by assigning the same code to a pair of user equipment devices that operate within different portions of the frame. This assignment can be utilized in those frames where compressed mode is used as described herein above, and in systems where compressed mode is not utilized for measurement and synchronization activity on other domains. In both cases, the compressed mode is used to increase capacity for best-effort packet transmission by an amount approaching 66%. The 66% improvement is dictated by the requirement that 1 of 3 frames must be transmitted in a non-compressed manner. Thus, an intelligent algorithm permits user equipment devices to achieve a substantially higher capacity for best-effort packet transfer mode as more user equipment devices may be assigned to the same code and multiplexed in time as well as by code. It is expected to be especially useful in the case of a best effort packet data transmission where an intelligent complementary compressed mode scheduler would produce pairs of radio resource assignments and compressed mode patterns based on the radio resource availability as well as uplink signal quality. In this manner, multiple user equipment may share the same codes on the uplink.  
      While the present inventions have been described in a manner that enables those of ordinary skill in the art to make and use the inventions, it will be understood and appreciated that there are many equivalents to the exemplary embodiments disclosed herein and that modifications and variations may be made without departing from the scope and spirit of the inventions, which are to be limited not by the exemplary embodiments but by the appended claims.