Abstract:
Techniques and systems for estimating the spreading factor of data in a channel in a spread spectrum radio communication system is described. An illustrative method includes communication between a base station and a mobile station that takes place over a multirate data channel having a corresponding control channel. The control channel is transmitted in parallel with the data channel and is decoded to extract control information in order to decode the data channel. An illustrative method and system for estimating the spreading factor of data in a channel in a spread spectrum radio communication system includes a transmitter and a receiver, wherein the transmitter transmits a data unit at one of a plurality of spreading factors over a data channel and transmits in parallel over a control channel a control unit including information for decoding the data unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a spread spectrum communication system, and more specifically, to spreading factor estimation in a spread spectrum communication system. 
     2. Description of the Prior Art 
     In a spread spectrum system, a modulation technique is used which spreads the information signal over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to carry the information signal. For example, if the information signal is a voice signal, it may have a bandwidth of only a few kilohertz but, during transmission, it&#39;s energy could be spread so as to be transmitted over a channel 5 MHz wide. This is accomplished by modulating the information signal with a wideband encoding signal. The information signal is then recovered by remapping the received spread spectrum into its original bandwidth. 
     Spread spectrum systems can be multiple access communication systems. One type is a code division multiple access (CDMA) system. In a CDMA system, users of the system can simultaneously use the same wideband physical communication channel (for example, the same 5 MHz part of the spectrum) with the signals between one group/pair of users being differentiated from that of another by a unique spreading code. 
     The present invention is a particularly applicable to a cellular system. A highly schematic cellular architecture is shown in  FIG. 1 . The system comprises a plurality of macrocell base stations  10  (only an exemplary  10   a ,  10   b ,  10   c  being shown) providing service within a corresponding macrocell  12  (only an exemplary  12   a ,  12   b ,  12   c  being shown). The system also comprises a plurality of mobile stations  14  (only an exemplary  14   a  and  14   b  are shown in the macrocell  10   a ). Each base station  10  communicates with the mobile stations  14  on a CDMA channels at a frequency F 1  and a bandwidth of 5 MHz, the communication channel (s) carried out between a base station  10  and a mobile station  14  in the service area thereof being defined by at least one unique spreading code. 
     SUMMARY OF THE INVENTION 
     The present invention is concerned with communications between a base station and a mobile station taking place over a multirate data channel having a corresponding control channel which is (i) transmitted in parallel with the data channel and which (ii) needs to be adequately decoded to extract control information in order to properly decode the data channel. This situation is illustrated in  FIG. 1  in the downlink direction between the base station  10   a  and the mobile station  14   a . The data channel is labelled DPDCH and the control channel is labelled DPCCH is labelled. 
     The present invention provides a method of estimating the spreading factor of data in a channel in a spread spectrum radio communication system comprising a transmitter and a receiver, wherein the transmitter transmits a data unit at one of a plurality of spreading factors over a data channel and transmits in parallel over a control channel a control unit comprising information for decoding said data unit. 
     The method includes steps of 
     decoding an initial portion of the control unit, 
     decoding an initial portion of the data unit at an assumed one of the plurality of spreading factors, and 
     calculating the received power of the initial portions of the control unit and the data unit to make an estimate of the spreading factor used to transmit the data unit. This situation is illustrated in  FIG. 1  in the downlink direction between the base station  10   a  and the mobile station  14   a . The data channel is labeled DPDCH and the control channel is labeled DPCCH. 
     By estimating the correct spreading factor used to transmit the data unit based on decoding only on an initial portion of the control unit and the data unit, the data unit can thereafter be properly decoded. Provision for the buffering of a whole data unit need not be made. It is also an advantage that, for the control channel, the transmission power need not be so high nor coding so powerful, because the information for decoding the data unit is not the only indicator of the spreading factor used to transmit the data unit. 
     The data in the data unit and the control unit is preferably interleaved. The length of the data unit and the control unit corresponds to the interleaving interval. For example, when the data is interleaved over one system frame, the control unit and the data unit each occupy one system frame. Moreover, when data is interleaved over a number of frames, the control unit and the data unit occupy that number of frames. In this case, because the spreading factor is constant over an interleaving interval, when the second and subsequent frames of a data unit are transmitted their spreading factor is already known. In one embodiment, the initial portion of the data unit can comprise one system frame. 
     Preferably, the lowest of the possible spreading factors is used to decode the initial portion of the data unit. By using this spreading factor, even if the data was actually transmitted with a higher spreading factor, the integrity of the data remains in tact even if it is poorly noise filtered. 
     The present invention is also a spread spectrum radio communication system, comprising a transmitter which transmits a data unit at one of a plurality of spreading factors over a data channel and transmits in parallel over a control channel a control unit comprising information for decoding the data unit, and a receiver comprising a decoder for decoding an initial portion of the control unit, a decoder for decoding an initial portion of the data unit at an assumed one of the plurality of spreading factors; and means for calculating the received power of the initial portions of the control unit and the data unit to make an estimate of the spreading factor used to transmit the data unit. 
     The present invention can be applied to especially, but not exclusively, to W_CDMA uplink. 
     In the context of the present invention, the term ‘estimating the spreading factor’ is used. It will be appreciated by those skilled in the art that by determining the spread factor (essentially a layer  1 ) quantity, the bit rate of data coming from the layer is also, in effect, being determined, the bit rate being a straightforward and known function of the amount of repetition applied by the channel coding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are hereinafter described with reference to the accompanying drawings, in which: 
         FIG. 1  shows a diagram of a cellular system useful for explaining the present invention; 
         FIG. 2  shows a diagram of a mobile station transmitter architecture; 
         FIG. 3  shows a diagram of a base station receiver architecture; 
         FIG. 4  shows the frame structure of the DPCCH and DPDCH from an air interface perspective; 
         FIGS. 5(   a - c ) show the signal constellations for the receiver of  FIG. 3  with the DPDCH channel transmitting at three different power levels/spreading factors; and 
         FIG. 6  shows the frames shown in  FIG. 4  from a user service perspective. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring back to  FIG. 1 , in order for the base stations  10  to communicate with the mobile stations or radiotelephones  14 , that is to set up, release and maintain connections therebetween, a number of functions need to be achieved over the air in both the uplink and the downlink. These functions are carried out by means of logical channels. 
     Generically, the basic functions to be carried out are (i) synchronization, where the mobile station locks onto the timing of a base station, enabling it to decode other channels; (ii) broadcast, where, for the purposes of initialization, the mobile station decodes system and cell specific information e.g. cell identities, spreading codes, access channel and neighboring cells lists; (iii) random access, where the mobile station can initiate a service request; (iv) paging, whereby an incoming service can be directed to the mobile station; (v) dedicated channel control, necessary for carrying signalling information such as handover measurements, service adaptation information, and power control information; (vi) traffic, necessary for carrying a wide variety of user-service data. Thus, generally logical channels exist corresponding to each of the functions (i) to (vi). 
     These basic functions can be mapped into physical channels, wherein the precise choice of how the logical channel are mapped into the physical channels will be highly system dependant. 
     In the system of the illustrated embodiment, the downlink comprises three common channels: a primary and a secondary common control physical channel (CCPCH), and a synchronisation channel (SCH) (function (i) above). The downlink also includes dedicated physical data channels (DPDCH) (function (vi) above) and physical control channel (DPCCH) (function (v) above). The primary CCPCH incorporates the point to multipoint broadcast control channel (BCCH) (function (ii) above). The secondary CCPCH comprises a forward access channel (FACH) and a paging channel (PCH) (function (iv) above). The FACH is used for carrying control information to a mobile station when the network knows the location cell of the mobile station. 
     The uplink comprises one common channel, the random access channel (function (iii) above). The uplink also includes dedicated physical data channels (DPDCH) (function (vi) above) and physical control channel (DPCCH) (function (v) above). 
     When a mobile station, say  14   a , is first powered up it initializes and registers with the network using the SCH to acquire synchronization to the strongest base station, which in this case is  14   a . Once synchronization has occurred the mobile station  14   a  detects the CCPCH, reading the system and cell specific BCCH information. From the BCCH, the mobile station  14   a  acquires codes permitting it to make a call request with the network. After initialization, the mobile station enters idle mode and waits to be paged by an incoming service, for example, by an incoming call, or for the user to request a service, for example place an outgoing call. 
     Services for the user are provided using the previously mentioned DPDCH and the DPCCH. Each of these physical channel consists of 10 ms frames, each frame comprising 15 slots. In one mode, services are provided with the frames operating in a mode hereinafter referred to as the multirate mode. In this mode, the PDCH carries the user-service data at a data rate which is constant within a single frame, but may vary from frame to frame. The DPCCH carries control information necessary to decode the DPDCH. Specifically, each frame of the DPCCH includes a transport format indication TFI which carries information indicating the data rate of the corresponding frame of the DPDCH. The DPCCH also carries power control symbols, pilot symbols and service parameter information for the corresponding frame of the DPDCH. The DPCCH is transmitted at constant data rate. 
       FIG. 2  shows the transmitter  30  architecture of a mobile station for transmitting data on these two physical channels. The base station comprises a DPDCH baseband processor  32  for baseband processing data for transmission on the DPDCH, and a DPCCH baseband processor  34  for baseband processing data for transmission on the DPCCH. Each baseband processor  32 ,  34  is operable to provide the conventional baseband processing operations, including, for example, convolution coding, turbo coding, puncturing/repetition and interleaving. 
     The data from each baseband processor  32 ,  34  is fed to a spreading modulation element  36 . Within the spreading modulation element  36 , the data for the DPCCH is spread by PN code Cd in a spreading element  38  and scaled by a factor Ad in scaling element  40  to give a signal I, and the data for DPDCH is spread by PN code Cc in spreading element  42  and scaled in scaling unit  44  by a factor Ac to give a signal Q. The codes Cd and Cc are orthogonal variable spreading factor codes. The signals I, Q are then fed to a quadrature modulator (QPSK)  46  to produce a signal I+jQ. This signal is then spread again by a PN scrambling code Cscramb in spreading element  48  which is a complex user-specific scrambling code to give signal R. The codes Cd and Cc are for channelization. 
     The multiplexed and spread signal R is then upconverted to the frequency F, power amplified and transmitted by RF section  50 . 
       FIG. 3  shows the receiver  60  architecture for the receiver of the base station. The receiver  60  comprises an RF section  80  for demodulating the received signals into the I,Q parts. A power estimator unit  75  provides an estimate of the power of I and Q and feeds this information to a baseband processing unit  65 . As described below in more detail below, the baseband processing unit of the receiver  60  is able to use the power estimates I and Q to calculate an estimate of the spreading factor/data rate. 
     The frame structure of the multirate mode is illustrated in  FIG. 4 . 5 exemplary 10 ms frames are shown, the DPCCH frames are labelled  101  to  105 , and the corresponding DPDCH frames are labelled  201  to  205 . 
     If  FIG. 4  is considered as a simple example of a uplink transmission, from the air interface (layer  1 ) perspective, the user data stream is transmitted on DPDCH as three data units. Data unit  1  is transmitted over frames  1  and  2  at the highest power, P 1  (and hence lowest spreading factor); data unit  2  is transmitted over frames  3  and  4  at a lesser power, P 2 ; and data unit  3  is transmitted over frame  5  only at the lowest power, P 3  (and hence highest spreading factor). The data rate of the blocks is changed by changing the length of the spreading codes or using parallel spreading codes in the spreading modulation unit  36  or by puncturing/repetition in the DPDCH baseband processor  32 . Because of the interleaving operation in the baseband processor  32 , the user service data is interleaved over both frames  1  and  2  in block  1 , over both frames  3  and  4  in block  2 , and over only frame  5  in block  3 . Similarly, the data stream in the DPCCH, notably the FCH, is interleaved over frames  1  and  2 , frames  3  and  4 , and frame  5 , corresponding to the data units in the DPCCH. As explained above though, the transmission power on this channel is constant, P 0 . 
     The receiver  60  knows a priori the set of possible ratios of data channel receive power to control channel receive power. Expressed in other words, it may be thought that the receiver  60  knows the set of absolute transmission powers/spreading factors, because the channel attenuation of the data channel and the control channel is approximately the same, the corresponding received powers are related to the corresponding transmission power by the same factor of proportionality. Hence, the set of possible ratios of data channel power to control channel power as transmitted are the same as the set possible of possible powers on reception. 
     Thus, in this way, the transmission power and hence the spreading factor can be estimated in principle.  FIGS. 5(   a - c ) show the signal constellations for received power is P 3 ′, P 2 ′ and P 1 ′, respectively. It will be appreciated that because the control and data channels are subjected to varying degrees of attenuation, the magnitude of the signal constellations vectors varies, but because the attenuation is approximately the same, their angles remain the same. 
     Referring again to  FIG. 4 , in order to estimate the relationship between the received power of the control channel and the data channel before the spreading factor used to transmit the data channel can be decoded from the control channel DPCCH, the data channel DPDCH signal is decoded assuming the lowest of the set of allowed spreading factors. With this assumption, the samples from the first 20% or so of frame  1  of the data channel DPDCH are decoded and averaged to give a power estimate Pda. Over the same interval, the control channel DPCCH is also decoded at its known, fixed spreading factor. The samples decoded from each channel are squared and averaged to give an estimate Pca. The ratio Pda/Pca will correspond more closely to one of P 3 ′/P 0 ′, P 2 ′/P 0 ′ or P 1 ′/P 0 ′ and hence yield an estimate of the corresponding spreading factor. Once an estimate of the spreading factor is so obtained, decoding of the data channel begins at the estimated spreading factor and hence little buffering is needed. In this way, both frames  1  and  2  are decoded. The process is then repeated for data unit  2 , and subsequently for data unit  3 . 
     It will be appreciated that as the number of frames in a data unit increases the advantage of not having to buffer the whole data unit to properly to decode the TFI becomes more and more significant. 
     Because communication between the base station  10   a  and the mobile station  14   a  takes place over a multirate data channel DPDCH having a corresponding control channel DPCCH which is transmitted in parallel and carries information about the data on the data channel, this channel architecture can be exploited advantageously in accordance with the described preferred embodiment of the invention to flexibly bundle a variety of user services into the data channel according to the priority of the services and the current data rate supportable by the data channel. For example, if there are four sets of user service data which need to be transmitted, say services  1  to  4  and, for convenience of explanation, the priority of the services is also in numerical order (whereby service  1  is the highest priority and service  4  is the lowest priority), then these services could be transmitted in accordance with the preferred embodiment of the invention as shown in  FIG. 6 . In  FIG. 6 , the same data units  1  to  3  of  FIG. 4  are considered from a user services perspective. In data unit  1 , transmitted with the power P 1 , where the spreading factor is the lowest and hence the data rate the highest, all services  1  to  4  are being transmitted. In data unit  2 , which is at a lower power P 2  and lower data rate, only higher priority services  1  and  2  are transmitted. In data unit  3 , which is at the lowest power P 2  and lowest data rate (highest spreading factor), only the highest priority service  1  is transmitted. Although for diagrammatic simplicity the services are shown in consecutive, separate time segments, in practice, each service is evenly interleaved over the respective data unit. 
     In other embodiments, the whole data unit can be decoded before estimation of the data rate/spreading factor because this may lead to better estimation.