Abstract:
A method for synchronizing receivers that receive turbo encoded signals to a received signal. Turbo encoding may enable signals to be decoded at a much lower signal to noise ratio than previously practical. A traditional method of synchronizing a receiver to an incoming signal is to use a slicer to determine a received symbol and then to compare the determined symbol to the incoming waveform, in order to adjust the phase of the slicer with respect to the incoming signal. At signal low levels, at which turbo encoded signals may be decoded, this slicing method may be prone to errors that may disrupt the synchronization of the receiver to the incoming signal. By replacing the slicer by a Viterbi decoder with zero traceback (i.e., one which does not consider future values of the signal only past values) a prediction as to what the incoming signal is can be made. Because the Viterbi decoder can consider past signal values it can predict the present symbol being received with higher reliability than by using a slicer, which considers only the present value of the incoming signal.

Description:
CROSS REFERENCE TO RELATED PATENTS/PATENT APPLICATIONS 
       [0001]    Continuation priority claim, 35 U.S.C. § 120 
         [0002]    The present U.S. Utility Patent Application claims priority pursuant to 35 U.S.C. § 120, as a continuation, to the following U.S. Utility Patent Application which is hereby incorporated herein by reference in its entirety and made part of the present U.S. Utility Patent Application for all purposes: 
         [0003]    1. U.S. Utility Application Ser. No. 09/729,443, entitled “Viterbi slicer for turbo codes,” (Attorney Docket No. BP1235), filed Dec. 4, 2000, pending, which claims priority pursuant to 35 U.S.C. § 119(e) to the following U.S. Provisional Patent Application which is hereby incorporated herein by reference in its entirety and made part of the present U.S. Utility Patent Application for all purposes:
       a. U.S. Provisional Application Ser. No. 60/168,809, entitled “Viterbi slicer for turbo codes,” (Attorney Docket No. BP1235), filed Dec. 3, 1999, now expired.       
 
     
    
     BACKGROUND OF THE INVENTION 
       [0005]    1. Technical Field of the Invention 
         [0006]    The invention relates generally to communication systems; and, more particularly, it relates to signal acquisition and tracking within such communication systems. 
         [0007]    2. Description of Related Art 
         [0008]    As coding technology improves, signals can be decoded with lower signal to noise ratios. Decreasing signal levels that can be decoded require receivers that can acquire and track at lower signal levels. There is therefore a need in the art for receiver technology to enable the acquisition and tracking of signals at lower signal levels. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to apparatus and methods of operation that are further described in the following Brief Description of the Several Views of the Drawings, the Detailed Description of the Invention, and the claims. Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings. 
         [0010]    It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only embodiments of the invention by way of illustration of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0012]      FIG. 1  is a graphical illustration of a prior art communications system. 
           [0013]      FIG. 2  is a graphical illustration of a communication system in which the coding section comprises a turbo encoder. 
           [0014]      FIG. 3  is a graphic illustration of a communication system according to an embodiment of the invention. 
           [0015]      FIG. 4  is a graphic illustration of a communication system according to an embodiment of the invention. 
           [0016]      FIG. 5  is a graphic illustration of a conventional carrier loop. 
           [0017]      FIG. 6  is a graphic illustration of a carrier loop according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1  is a graphical illustration of a communications system. In  FIG. 1 , data  101  is provided to an encoder  103 . The encoder codes the data and then provides it to a transmitter  105 . The transmitter modulates the coded data on a carrier frequency, amplifies the resultant signal and broadcasts it to a relay satellite  107 . The relay satellite  107  then rebroadcasts the data transmission to a receiver  109 . The received signal is then provided by the receiver  109  to a mixer  113 . A voltage controlled oscillator  123  provides a mixer signal to the mixer with the result that the coded signal is translated to a baseband signal. The coded baseband signal comprises the data and the coding added by encoder  103 . The transport interface of the signal from (and including) the transmitter  105  to (and including) the receiver  109  is referred to as a channel  111 . 
         [0019]    The coded data from the multiplier  113  is filtered (filter not shown) and provided to a slicer  115 . The slicer  115  extracts symbols from the coded data stream and provides it to a decoder  119 . The decoder  119  decodes the symbols and creates a data stream  121 . A phase detector  117  compares the symbol found by the slicer  115  with the value input to the slicer  115 . By comparing the signal input to the slicer  115  to the actual symbol found by the slicer in the phase detector  117 , the phase detector  117  detects whether the slicing process is leading or lagging the actual symbol value detected within the data stream. The phase detector  117  can then adjust the voltage controlled oscillator  123  to adjust the mixer signal provided to the multiplier  113  to match the carrier signal. 
         [0020]      FIG. 2  is a graphical illustration of a communication system in which the encoder  103  replaced by a turbo encoder  200 . The turbo encoder  200  accepts data  201 . The data is then encoded in a first trellis encoder  203 . The data is also interleaved by an interleaver  205  and provided to a second trellis encoder  207 . The second trellis encoder  207  may be identical to the first trellis encoder  203 , but it may also be different. The outputs of trellis encoders  203  and  207  are then punctured by switch  209 . In other words, switch  209  selects between the output of trellis encoder  203  and trellis encoder  207 . The punctured output of turbo encoder  200  is then provided to a channel  211 . 
         [0021]    The signal received from the channel is then coupled into a multiplier  213 , and the received signal is mixed with a mixer signal (as provided by the voltage controlled oscillator (VCO)  223 ), which replicates the carrier signal. The slicer  215  slices the symbols from the data stream, and the phase detector  217  detects the difference between the sliced symbol found at the output of the slicer  215  and the value input to the slicer  215 . The output of the phase detector  217  then adjusts the VCO  223  in order to correct the carrier signal being mixed in multiplier  213 . The output of the slicer  215  is then coupled into turbo decoder  219  to decode the turbo encoded data. 
         [0022]    Turbo encoder  200  is a parallel concatenated encoder. Parallel concatenated codes (“turbo codes”) allow communications systems to operate near the Shannon capacity. However, when operating in this region, the signal to noise ratio may be very low. This low signal to noise ratio (E S /N O ) can make synchronization with a received signal difficult. If the channel symbol error rate is greater than 1:10 (i.e., one out of ten transmitted signals is decoded incorrectly), a decision directed loop, such as illustrated in  FIG. 2  (comprising the slicer  215  and phase detector  217 ) can fail. Such parallel concatenated codes (“turbo codes”) can operate in this region. 
         [0023]    In order to improve the accuracy, the slicer  215  may be replaced by a Viterbi decoder as illustrated in  FIG. 3 . Viterbi decoders typically produce the most likely channel symbol based on past data, present data and (depending on trace-back depth) future data. A Viterbi decoder uses the past and future data as well as correlations within the data to produce a current symbol that is more likely to be correct than if only the present data is used (such as with a typical data slicer). In the embodiment illustrated in  FIG. 3 , future data is not available, so the Viterbi decoder  301  will examine past and present data in order to produce a symbol, which is more likely to be accurate than one determined by a slicer mechanism such as illustrated in  FIG. 2 . A Viterbi decoder  301  is more likely to make an accurate decision as to what the symbol being decoded is based on a history of inputs than can a slicer, which makes a decision based on only the present input. 
         [0024]    The turbo encoder  200 , however, is a parallel concatenated encoder. Turbo encoder  200  comprises two trellis encoders separated by an interleaver  205 . Any number of trellis encoders separated by interleavers may be used, but two are shown for sake of simplicity. 
         [0025]    The interleaver  205  accepts the data  201  and interleaves or shuffles the data before providing it to the trellis encoder  207 . As a result, the data provided by the lower leg of the turbo encoder comprising the trellis encoder  207  is out of sequence and must be resequenced. For this reason, switch  303  is added to the Viterbi decoder  301  so that only the symbols from trellis encoder  203  or trellis encoder  207  are used by the phase detector  217  to adjust the controlled oscillator  223 . The delay introduced by interleaver  205  makes it impractical for the Viterbi decoder  301  to use symbols from both sides of the turbo encoder  200  without a buffering and delay mechanism at the input of the Viterbi decoder. Switch  303  will select every other symbol. Either a symbol from trellis encoder  203  will be selected or a symbol from trellis encoder  207  will be selected by switch  303 . 
         [0026]      FIG. 4  is a graphical illustration of a communication system according to an embodiment of the invention. In  FIG. 4 , the turbo encoder  403  has been modified by placing an inverse interleaver in series with trellis encoder  207 . The inverse interleaver  401  unscrambles the order of the data which had been scrambled by the interleaver  205 , after it has been trellis encoded. By utilizing inverse interleaver  401 , every symbol can be used by the Viterbi decoder  301  in order to synchronize the frequency of the VCO  223 . 
         [0027]      FIG. 5  is a graphic illustration of a conventional carrier loop. 
         [0028]      FIG. 6  is a graphic illustration of a carrier loop according to an embodiment of the invention 
         [0029]    As mentioned above, when using “turbo codes,” the constituent codes are often trellis codes. Each of the constituent codes can be decoded with a conventional Viterbi decoder. For example, in one embodiment, when using the iterative decoding procedure, the soft input/soft output decoding algorithm is used. 
         [0030]    If instead of slicing the soft decisions at the decision point in the receiver, a sequence detector is used, decisions can be made with improved accuracy. A conventional decision directed carrier loop is shown in  FIG. 5 . One embodiment of the invention replaces the slicer with a Viterbi decoder. The improved receiver is shown in  FIG. 6 . In the case of “turbo codes”, where two of more trellis codes are concatenated together, multiple Viterbi decoders are needed, all operating in parallel. In order to avoid the large delay inherent in Viterbi decoding, a limited traceback depth can be used. In fact, the traceback depth can be set to zero. The Viterbi decoder works on the incoming soft decisions and produces the most likely channel symbol based on past data and (depending on traceback depth) future data. 
         [0031]    Although shown for a decision directed carrier loop, embodiments of the invention are also applicable to decision directed timing loops, and decision directed automatic gain control (AGC) loops. An extension of this invention can be used for decision feedback equalization (DFE). To extend to DFE, instead of one channel symbol being decoded, a vector of the most likely channel symbols is produced based on the internal Viterbi metrics. This vector of channel symbols is loaded in parallel into the DFE. This process is repeated for each new symbol. 
         [0032]    Moreover, in one aspect of the present invention, a method of processing signals includes receiving first and second signals each being modulated on a carrier signal, the first signal preceding the second signal in time, multiplying each of the first and second signals with a reference signal having a reference frequency, adjusting the multiplied first signal based on the multiplied first and second signals, comparing the adjusted first signal to the multiplied first signal, and adjusting the reference frequency as a function of the comparison. 
         [0033]    In another aspect of the present invention, a receiver includes an oscillator having a reference signal output with a tunable reference frequency, a multiplier to multiply a first signal with the reference signal, and to multiply a second signal, succeeding the first signal in time, with the reference signal, the first and second signals each being modulated on a carrier frequency, a decoder to adjust the multiplied first signal based on the multiplied first and second signals, and a detector to compare the adjusted first signal with the multiplied first signal, the detector being adapted to tune the reference frequency as a function of the comparison. 
         [0034]    In yet another aspect of the present invention, a receiver includes an oscillator having a tuning input, a multiplier having a first input to receive a signal, and a second input coupled to the oscillator, the signal comprising a first signal and a second signal succeeding the first signal in time, the first and second signals each being modulated on a carrier frequency, a decoder having an input coupled to the multiplier, and an output, and a detector having a first input coupled to the decoder input, a second input coupled to the decoder output, and an output coupled to the tuning input of the oscillator. 
         [0035]    In a further aspect of the present invention, a receiver includes oscillator means for generating a reference signal having a tunable reference frequency, multiplier means for multiplying a first signal with the reference signal, and multiplying a second signal, succeeding the first signal in time, with the reference signal, the first and second signals each being modulated on a carrier frequency, decoder means for adjusting the multiplied first signal based on the multiplied first and second signals, and detector means for comparing the adjusted first signal with the multiplied first signal, the detector means comprises tuning means for tuning the reference frequency as a function of the comparison. 
         [0036]    In yet a further aspect of the present invention, a method of processing signals having a first and second symbol each representing a constellation point, the first symbol preceding the second symbol in time, includes quantizing the first symbol to its nearest constellation point as a function of the first and second signals, comparing the first symbol to the quantized first symbol, and adjusting a reference frequency as a function of the comparison. 
         [0037]    In still a further aspect of the present invention, a receiver to receive a signal including first and second symbols each representing a constellation point, the first symbol preceding the second symbol in time, includes a decoder to quantize the first symbol as a function of the first and second symbols, a detector to compare the first symbol to the quantized first symbol, and an oscillator having a tunable output as a function of the comparison. 
         [0038]    In another aspect of the present invention, a communications system includes a transmitter to transmit a signal including first and second symbols each representing a constellation point, the first symbol preceding the second symbol in time, and a receiver including a decoder to quantize the first symbol as a function of the first and second symbols, a detector to compare the first symbol to the quantized first symbol, and an oscillator having a tunable output as a function of the comparison. 
         [0039]    Although a preferred embodiment of the present invention has been described, it should not be construed to limit the scope of the appended claims. Those skilled in the art will understand that various modifications may be made to the described embodiment. Moreover, to those skilled in the various arts, the invention itself herein will suggest solutions to other tasks and adaptations for other applications. It is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.