Abstract:
A method and apparatus for generating a frame quality indication of a frame of a received turbo encoded wireless signal is provided. A turbo decoder is used to decode a demodulated frame and provide a decoded frame. A turbo encoder is used to encode the decoded frame to provide a re-encoded frame. A modulator modulates the re-encoded frame. The frame quality is assessed based on a degree of correlation between the demodulated frame and the re-encoded modulated frame. The advantages include a less complex receiver due to reduced computational load, and potentially greater payload capacity in frames due to elimination of Yamamoto bits.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the first application filed for the present invention. 
     MICROFICHE APPENDIX 
     Not applicable. 
     TECHNICAL FIELD 
     The invention relates to spread spectrum communications and in particular to methods and apparatus for providing frame quality indication in decoding received coded spread spectrum signals. 
     BACKGROUND OF THE INVENTION 
     Multiple access modulation techniques are some of the most efficient techniques for utilizing the limited wireless frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). 
     CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum communications system uses a modulation technique that spreads a transmitted signal over a wide frequency band, this frequency band is substantially wider than the minimum bandwidth required to transmit the signal. The transmission signal is spread over the wide frequency band according to a spreading code associated with an intended receiver. 
     In a spread spectrum communications system, multiple spread spectrum signals are transmitted simultaneously in the same frequency band. A particular spread spectrum receiver determines which signal is intended for that particular receiver using a unique spreading code associated with the particular receiver. The spread spectrum signals in the frequency band not associated with the particular spreading code of the particular receiver appear as background noise to that receiver. The intended signal is to be discerned from the background noise. 
     Although spread spectrum transmission techniques provide excellent multiplexing transmission efficiencies compared to other signal transmission techniques, an induced signal degradation is experienced by a particular spread spectrum signal due to noise created by the other simultaneously transmitted spread spectrum signals. This is a limiting factor affecting the multiplexing efficiency of spread spectrum communications systems. This noise can be characterized as white noise and can be modeled using a Gaussian noise model referred to as additive white Gaussian noise (AWGN). 
     One of the major technical challenges in communicating effectively and reliably over wireless links is overcoming signal interference. There are may types of signal interference, each of which affects a spread spectrum signal differently. One type of interference affects the amplitude of the spread spectrum signal as it is received at a receiver. Another type of interference affects the phase of the spread spectrum signal as it is received at the receiver. Yet another type of interference affects the ability of the receiver to discern the signal from background noise such as thermal noise and other radio frequency pick-up. This last type of noise interference can be modeled as additive white Gaussian noise applied to the spread spectrum signal. 
     In general, noise, be it due to other spread spectrum signal transmissions in the frequency band or noise interference, reduces the signal-to-noise ratio (SNR) of a received spread spectrum signal. A reduced SNR makes it difficult to demodulate a particular spread spectrum signal intended for the receiver. This has a negative impact on multiplexing efficiency. 
     Considering that increasing multiplexing efficiency is a desired goal in spread spectrum communications, it is therefore of a competitive advantage to determine the reliability of wireless links employing spread spectrum communications techniques. Typically a transmitted signal employing spread spectrum techniques has a transmission structure: the payload is digital, and the stream of data conveyed between a spread spectrum transmitter and a spread spectrum receiver is divided into frames. The frames have an associated frame transmission rate and an associated frame transmission time. A frame quality indication is used to specify the reliability of reception of frames at the receiver. 
     Another spread spectrum communications area in which the reliability of a spread spectrum communications link is important, is the control of a soft handover of an established spread spectrum communications session. Typical mobile telephony implementations use mobile telephones with limited reception capabilities to exchange frames over a spread spectrum frequency bandwidth with a base station within a limited distance from the base station. Typically the distance is limited to one over which the amplitude of the spread spectrum signal diminishes to a value which makes the spread spectrum signal indistinguishable from background noise for the purposes of demodulation/decoding. Given this arrangement, such a spread spectrum communications session is said to take place within a cell. Therefore, as a mobile telephone nears a border of the cell, the mobile telephone experiences a degradation in the reception of the spread spectrum signal equivalent to a contamination by white noise. Consequently, determination of frame quality can also be used to initiate soft handover of spread spectrum communications sessions. Smooth soft handovers are essential to a mobile telephone end user&#39;s experience. 
     Techniques for assessing the reliability of wireless links employing spread spectrum transmission techniques have been proposed. These techniques address a variety of sources of interference and have yielded varied levels of success. 
     U.S. Pat. No. 5,802,105 which issued on Sep. 1, 1998 to Tiedemann, Jr. et al. describes a system which transmits a test sequence of digital data over a wireless communications channel established between a transmitter and a receiver. The accuracy of transmission over the wireless communications channel is determined by comparing the received test sequence to a replica test sequence generated at the receiver. While the teachings of Tiedemann, Jr. et al. have merit, in implementing these teachings a considerable fraction of a transmitted payload stream is dedicated to determining the accuracy of transmission. A considerable amount of computation is also required at the receiver to enable determination of the accuracy of transmission. Besides, the accuracy of the indicator is dependent on an overall accuracy of transmission. 
     U.S. Pat. No. 5,790,596 which issued Aug. 4, 1998 to Sexton describes a system which estimates a slowly changing channel parameter such as channel gain and phase shift to improve receiver performance for subsequent transmissions. Sexton teaches the use of frame quality indicating information transmitted as part of the transmission signal. The frame quality indicating information is processed subsequent to demultiplexing the transmitted signal and used to selectively accept data frames. Rejected data frames are discarded. Sexton also teaches the use of an encoder block in the receiver to convolute and interleave a de-interleaved, de-convolved and demultiplexed accepted data frame to determine a degree of degradation of the transmitted signal with respect to channel gain or phase distortions. A portion of the re-encoded frame is compared with the received frame to compute the slowly changing channel parameter. The computed channel parameter is fed back to the receiver to improve reception of future transmissions on the theory that the channel parameter will remain substantially constant for a period of at least two received, accepted frames. While the teachings of Sexton have merit, frame acceptance is dependent solely on the transmitted frame quality indicator bits. This contributes to overhead and may result in the rejection of good frames. A considerable amount of computation is also required at the receiver in processing the frame quality indicating information in addition to computing the channel parameter. 
     Therefore there is a need for methods and apparatus for providing an efficient frame quality determination for a received spread spectrum signal subjected to noise induced distortions which does not rely on an overhead of sacrificial frame quality information in the transmitted data frames. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method of assessing the frame quality of a received wireless signal and improving multiplexing efficiency by eliminating a need for transmitting frame quality information along with the payload. 
     According to an aspect of the invention, a method of indicating the frame quality of a received frame of a received wireless signal is provided. The method comprises five steps. The received frame is demodulated using a demodulator and the demodulator provides a demodulated frame. The demodulated frame is decoded using a decoder and the decoder provides a decoded frame. After decoding, the decoded frame is encoded using an encoder matched to the decoder and the encoder provides a re-encoded frame. The re-encoded frame is modulated by a modulator matched to the demodulator and the modulator provides a re-modulated frame. The frame quality indication is computed based on a comparison between the received frame and the re-modulated frame. 
     According to another aspect of the invention, a receiver adapted to indicate the frame quality of a received frame of a received wireless signal is provided. The receiver comprises: a demodulator adapted to demodulate a received frame and provide a demodulated frame, a decoder adapted to decode the demodulated frame and provide a decoded frame, an encoder matched to the decoder, the encoder being adapted to encode the decoded frame and provide a re-encoded frame, a modulator matched to the demodulator, the modulator being adapted to modulate the re-encoded framed and provide a modulated signal, a correlator adapted to correlate the received frame with the re-modulated frame providing a correlation level and a decision circuit adapted to compute the frame quality indication of the received frame based on the correlation level. 
     According to another aspect of the invention, a frame quality indication generator is provided. The frame quality indication generator is adapted to process a received frame of an encoded wireless signal and a decoded frame resulting from the encoded wireless signal being demodulated by a demodulator and decoded by a decoder. The frame quality indication generator comprises: an encoder matched to the decoder, the encoder being adapted to encode the decoded frame and provide a re-encoded frame, a modulator matched to the demodulator, the modulator being adapted to modulate the re-encoded frame and provide a re-modulated frame, a correlator adapted to correlate the received frame with the re-modulated frame and provide a correlation level, and a decision circuit adapted to compute the frame quality indication of the received frame based on the correlation level. 
     According to another aspect of the invention, a method of initiating soft handover of a wireless communication session between base stations is provided. The method uses a frame quality indication to selectively initiate a soft handover. 
     According to another aspect of the invention, a method of decoding a received wireless signal is provided. The method uses a frame quality indication to selectively accept decoded frames. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
     FIG. 1 is a schematic diagram showing components of a wireless communication system and a process in accordance with the invention by which a frame quality indication is provided; 
     FIG. 2 is a schematic diagram showing components of another wireless communication system and process in accordance with the invention by which a frame quantity indication is provided; and 
     FIG. 3 is an exemplary graph showing the characteristics of a typical received turbo encoded spread spectrum signal conveying data and transmission error rejection characteristics afforded by the invention. 
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with a preferred embodiment of the invention, components and a process providing an efficient frame quality indication are shown in FIG.  1 . The frame quality is assessed based on demodulated frames. 
     A transmitter  10  having a turbo encoder  12  and a spread spectrum signal modulator  14 , provides a turbo encoded spread spectrum transmitted signal  16 . As the transmitted signal  16  propagates towards a receiver  17 , the transmitted signal  16  is subject to degradation due to interference which may be represented as white noise indicated by reference numeral  18 . Typically the white noise  18  combines with the transmitted signal  16  in an additive manner. A received spread spectrum signal  19  is picked-up by the receiver  17  using an antenna (not shown). 
     The received spread spectrum signal  19  is divided up into constituent received frames  20  by front-end processing  21 . A received frame  20  is demodulated according to methods known in the art by employing a demodulator  22 . Demodulator  22  is adapted to provide a demodulated frame  24 . The demodulated frame  24  is decoded by a turbo decoder  26  according to methods known in the art. The turbo decoder  26  is adapted to output a decoded frame  28 . 
     According to the invention, a copy of the decoded frame  28  is encoded by a turbo encoder  30  according to methods known in the art. The turbo encoder  30  is matched to the turbo decoder  26  of the receiver  17 . The turbo encoder  30  is adapted to provide a re-encoded frame  32 . A correlator  34  is used to correlate the demodulated frame  24  with the re-encoded frame  32 . An example of a method of correlating frames will be described below in detail. The correlator  34  is adapted to provide a correlation level  36 . 
     As described above, the correlator  34  correlates the demodulated frame  24  with the re-encoded frame  32 . Processing time is required to decode the demodulated frame  24  into the decoded frame  28  and also to encode the decoded frame into the re-encoded frame  32 . In accordance with one implementation of the invention, the frame reception rate and frame transmission time can be such that the demodulator  22  still provides a copy of the demodulated frame  24  at its output, stored, for example, in a register or the like, for correlation purposes at the time the re-encoded frame  32  is provided. In accordance with another implementation of the invention, a copy of the demodulated frame  24  is stored in an intermediary frame store  44 . The preferred frame store  44  would permit an increased throughput at the cost of an increased component count. 
     In accordance with another embodiment of the invention, components and another process providing an efficient frame quality indication are show in FIG.  2 . In this implementation the same spread spectrum transmitter  10  is used. The frame quality is assessed based on a correlation between modulated frames. Consequently, the re-encoded frame  32  is processed by a modulator  48  which is matched to demodulator  22 . The modulator  48  is adapted to provide a re-modulated frame  50 . 
     A decision circuit  38  common to both of the above-described embodiments, and having at least one register (not shown) to store at least one threshold  40  is adapted to compare the correlation level  36  against the at least one threshold  40 . The decision circuit  38  is further adapted to provide a frame quality indication  42  of the demodulated frame  24 . 
     Although the embodiments of the invention shown in FIGS. 1 and 2 use turbo coding techniques, the invention can be implemented using other coding techniques, such as Viterbi-convolutional coding techniques. For such an embodiment, multiplexing efficiency can be increased by eliminating the transmission of Yamamoto bits traditionally used to provide frame quality indication. 
     The modulated frame  50  and a copy of the received frame  20  are correlated by a correlator  52 . The correlator  52  is adapted to provide the correlation level  36 . The frame store  44  may be used to retrievably store a copy of each received frame  20  for correlation purposes. A decision circuit  38  common to both of the above embodiments has at least one register (not shown) for storing at least one threshold  40 . The decision circuit is adapted to compare the correlation level  36  against the at least one threshold  40 , and to provide a frame quality indication  42  related to the demodulated frame  24 . 
     Although the embodiment of the invention illustrated in FIG. 3 uses turbo coding techniques, that embodiment can also be implemented using other coding techniques, such as Viterbi-convolutional coding. As noted above, multiplexing efficiency may be increased by eliminating the transmission of Yamamoto bits traditionally used to provide a frame quality indication. 
     The frame quality indication generator  46  shown in FIG. 1 may be provided as an after-market add-on for a spread spectrum receiver that uses turbo coding techniques. The add-on equipment includes the turbo encoder  30 , the correlator  34 , the decision circuit  38 , and optionally the frame store  44 . The frame quality indication generator  54  shown in FIG. 2 may likewise be provided as after-market add-on equipment for a spread spectrum receiver that uses turbo coding techniques. The frame quality indication generator  54  includes the turbo encoder  30 , modulator  48 , correlator  52 , the decision circuit  38 , and optionally the frame store  44 . 
     As will be explained below, the decision circuit  38  may use two thresholds to provide a hard frame quality indication  42 . If two thresholds are used, a first threshold level is greater than a second threshold level and: if the correlation level is greater or equal to the first threshold level, the frame quality is considered “acceptable”; if the correlation level is less than the first threshold level but greater or equal to the second threshold level, the frame quality is considered “indeterminent”; and, if the correlation level is less than the second threshold level, the frame quality is considered “unacceptable”. In practice, frames are rejected unless the correlation level is greater than the first threshold level. Otherwise, the frames are rejected. The two lower levels are used to control soft handover of a spread spectrum communications session, as will be explained below in more detail. 
     The decision circuit  38  may also use only one threshold  40  to provide a hard frame quality indication  42 . The threshold  40  is a level against which the correlation level  36  is compared. If the correlation level  36  is greater or equal to the threshold level  40 , the frame quality is considered “acceptable”. If the correlation level  36  is less than the threshold level  40 , the frame quality is considered “unacceptable”. Frames are accepted if the frame quality indication is greater than the threshold and rejected if it is less than the threshold. Data transport over a wireless link employing spread spectrum techniques can use this implementation of the invention to selectively accept decoded data frames  28  processed by a receiver  17  of a mobile receiver. 
     The apparatus and methods described above can also be used to initiate a soft handover of a spread spectrum communications session between base stations as a mobile receiver  17  moves towards and across a boundary between two cells. The frame quality indication  42  provided with a first and a second threshold may be used to initiate and perform a soft handover of a spread spectrum communications session for the mobile receiver  17  crossing a boundary between two cells. If the correlation level  36  is greater or equal to the first threshold level, then soft handover is suppressed. If the correlation level  36  is less than the first threshold level but greater or equal to the second threshold level, then soft handover is initiated. If the correlation level  36  is less than the second threshold level, then soft handover is performed. 
     The frame quality indication  42  provided by one threshold level  38  may also be used to provide an initiation of a soft handover of a spread spectrum communications session for a mobile receiver  17  crossing a boundary between two cells. In this case, if the correlation level  36  is greater than or equal to the threshold level  40 , a soft handover is suppressed. If the correlation level  36  is less than the threshold level  40 , then soft handover is initiated and performed. 
     Although the above-described embodiments use one or two threshold levels, there is no limitation on the degree of specificity of the frame quality indication. More thresholds may be used to provide a continuous range of specificity for a soft frame quality indication. 
     FIG. 3 is a graph showing characteristics of a typical received turbo encoded spread spectrum signal conveying data, and a transmission error rejection window afforded by the invention. In this example 1000 frames of 256 bits were transmitted between a spread spectrum transmitter and a spread spectrum receiver. Turbo coding was used. Frame quality indication was provided according to the invention for the purposes of conveying data over a spread spectrum communications link. 
     Each of the received constituent demodulated bits transmitted in each frame can be represented by received samples {r k =t k +n k ,1≦k≦N, where {t k ,1≦k≦N represents the transmitted bits and {n k ,1≦k≦N represents the additive noise. The noise variance σ n   2 , for the purposes of this example, can be set to 1 without loss of generality. Each of the constituent estimated re-encoded bits can be represented by samples {s k ,1≦k≦N. One, of the most commonly used modulations for turbo coding applications is binary phase shift keying (BPSK). For BPSK, the correlation output is set to          ∑     k   =   1     N            r   k            s   k     .                              
     This value is normalized as follows:          γ   =                ∑     k   =   1     N            r   k          s   k                       ∑     k   =   1     N          t   k   2              =       1   λ                 ∑     k   =   1     N            r   k          s   k                    ,                          
     where        λ   =       ∑     k   =   1     N          t   k   2                              
     is constant for BPSK modulation. Therefore {γ,0≦γ≦1 represents a normalized correlation level. 
     The results shown in FIG. 3 relate to a simulation test using a turbo coding technique. Two symmetrical parallel concatenated convolutional coders were employed. The encoders each used a generator polynomial expressed as G=[1(1+D+D 3 )/1+D 2 +D 3 ]. A coding rate of        R   =     1   3                            
     was used with a random interleaver. As is well known, interleavers are used to guard against bursty interference. At a signal-to-noise ratio (SNR)              E   b       N   0       =     1                 dB       ,                          
     the achievable bit error rate was BER=3.4×10 −4  and the frame error rate was about 4.8×10 −2 . 
     FIG. 3 shows two correlation curves A and B. Curve A represents received frames that did not correspond to the transmitted frame content, and curve B represents received frames that correctly corresponded to the transmitted frame content. Curves A and B were generated by comparing the transmitted frame content to the decoded frame content. The simulation mimicked a real environment typical of the intended operating environment for the spread spectrum communications system described above. Curve A represents a variation of the probability of transmitting an erroneous frame corresponding to a correlation level computed as described above. Curve B represents a variation of the probability of transmitting a correct frame corresponding to a correlation level as described above. The graph shows a fuzzy type result. It is apparent that if a threshold of about 0.83 is used, 100% of the frames received with a normalized correlation level less than this threshold are erroneous. It is also apparent that 100% of the correct frames have a normalized correlation level above this threshold. A small fraction of erroneous frames also have a normalized correlation level above this threshold. 
     The result is that all correct frames can be accepted by employing the teachings of this invention using one hard threshold. A large fraction of erroneous frames are rejected and only a small fraction of erroneous frames are accepted. Typically the ratio of rejected to accepted erroneous frames was found to be 9:1. These results can be improved if a complex white Gaussian noise model is used to characterize noise present during demodulation and decoding of a particular spread spectrum signal induced by other simultaneously transmitted spread spectrum signals, background noise and signal loss due to signal dissipation over a distance between the transmitter and the receiver. 
     Although the invention has been described with reference to spread spectrum signals and turbo encoders, it will be understood by those skilled in the art that the invention may be applied to any wireless telecommunication that uses frames to transmit data. 
     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.