Abstract:
A data communications receiver incorporating soft decision error correction decoding and a means to set the amplitude or threshold for quantized soft decisions in a near-optimum manner to a soft decision decoder. In one embodiment the means for setting the amplitude or threshold measures the soft decisions from a quantizer and marks data bits as weak or strong. The gain or threshold is automatically adjusted to achieve a desired fraction if each marking. The desired fraction is chosen as the value that optimizes performance of the soft decision decoder.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to data communications and more specifically to control of gain, amplitude or threshold in a data communications receiver with soft decision decoding to obtain an optimum signal level into a soft decision decoder. 
     Data transmission and reception systems that operate on noisy channels use any of a variety of coding and modulation schemes known in the art. Many of these schemes allow a data communications receiver to make symbol decisions directly on the output of a demodulator and feed the decisions to an error detection or correction decoder. Improved performance in such a system is obtained by obtaining soft decision information about the reliability of those received symbols and using that information in the decoder. 
     If the decoder is implemented in hardware, it is important to conserve logic gates in the decoder by using the minimum acceptable number of bits of precision to represent the soft decision information. It is well known in the art that three-bit quantization produces nearly the same error rates as infinite precision soft decision decoding if the signal and noise amplitudes are appropriate for the quantization thresholds. 
     When a data communication receiver uses quantized soft decision information with an error correction decoder, some means must be used to set the amplitude of the signal at the quantization point. Too large or too small amplitude does not capture the full benefit from the soft decision information, resulting in more bit errors than the optimum amplitude setting. Amplitude control is typically achieved by automatic gain control circuits that require an amplitude detector or noise level measurement. In many cases, a simpler method of control is desirable. 
     The prior art generally uses an automatic gain control (AGC) circuit that operates to maintain a constant measure of either signal plus noise or noise alone. An Advanced Hardware Architectures, Inc Application Note ANRS07-0795“Soft Decision Thresholds and Effects on Viterbi Performance” indicates that a noise variance AGC is desirable, but that a signal plus noise AGC is easier to implement. A Qualcomm Application Note AN1650-2 “Setting Soft-Decision Thresholds for Viterbi Decoder Code Words from PSK Modems” states that the quantizer thresholds are based on noise alone and that the AGC should operate on noise. Practical AGC algorithms do not operate on noise alone but use signal plus noise or typically on signal alone. The application note illustrates a signal plus noise controlled feedback AGC. 
     U.S. Pat. No. 5,566,191 discloses a soft decision decoding method using a Viterbi decoder. The need to control the level of an input signal is described and a feedforward AGC rather than the more typical feedback AGC is disclosed. The AGC feeds forward a signal dependant on the amplitude of the received signal to correct soft decision likelihood metric values in the Viterbi decoder. 
     U.S. Pat. Nos. 5,214,675 and 5,379,324 disclose noise variance estimation for use in receiver that corrects for Rayleigh fading in a multipath channel. 
     What is needed is a simple gain control function to set the input to a soft decision decoder at a near-optimum level to obtain the best performance in a data communications system. 
     SUMMARY OF THE INVENTION 
     A data communications receiver incorporating soft decision decoding is disclosed. The data communications receiver includes a demodulator for demodulating an input signal, a means connected to the demodulator for producing strong and weak data symbols from the demodulated input signal and for controlling a desired fraction of strong and weak data symbols, and a soft decision decoder for receiving and decoding the strong and weak data symbols to produce a data output. 
     In one embodiment of the present invention, the means for producing the strong and weak data symbols from the demodulated signal may be a soft decision quantizer. The means for controlling the desired fraction of strong and weak data symbols to the soft decision decoder comprises a strong/weak indication function connected to the soft decision quantizer for providing strong and weak indications. An averaging function connected to the strong/weak indication function averages the strong/weak indications. A comparison function connected to the averaging function is used to compare the average of strong/weak indications to a desired fraction of weak signals to generate a gain error signal. A gain control function generates a gain control signal from the gain error signal received from the comparison function. A variable gain element connected to the input of the receiver and to the gain control function varies the gain with the gain control signal to maintain the desired fraction of strong and weak data symbols to the soft decision decoder. 
     In another embodiment of the present invention feed forward gain control is used rather than feedback. The demodulator provides a multiple bit digital word output indicative of the demodulated input signal. The means for producing strong and weak data symbols of the demodulated input signal and for controlling the desired fraction of strong and weak data symbols provided to the decoder comprises strong/weak indication functions, averaging functions, a threshold value, comparison functions, a logic function, and a bit shifter. Each strong/weak indication function has an input connected to a hard decision bit output from the demodulator and another input connected to another output bit of the demodulator. The averaging functions connected to outputs of the strong/weak indication functions average the strong/weak indications. A fixed threshold value is used to set a threshold to control the desired fraction of strong/weak indications. The comparison functions connected to the averaging functions and to the threshold value are used to indicate which strong/weak indications exceed the threshold. A logic function connected to the comparison functions is used to select an index number indicative of which strong/weak indications drop below a threshold and to generate a shift control signal. A bit shifter connected to the multiple bit digital word output of the demodulator and to the logic function receives the shift control signal to select bits from the demodulator that provide the desired fraction of weak and strong symbols to the soft decision decoder. The bit shifter functions as a gain control means. 
     In another embodiment of the present invention, the threshold is varied rather than the gain. The means for producing strong and weak data symbols of the demodulated input signal and for controlling the desired fraction of strong and weak data symbols further comprises a first greater/than test function, an absolute value circuit, a plurality of greater/than test functions, a threshold control function, and scaling functions. The first greater/than test function has a first input connected to the demodulator to receive the demodulated input signal and a second input connected to a reference to provide a hard decision indication. The absolute value circuit connected to the demodulator receives the demodulated input signal to provide an absolute value output of the demodulated input signal. In the plurality of greater/than test functions, each has a first input connected to the absolute value circuit output. A second greater/than test function in the plurality of greater/than test functions provides a strong/weak indication. The threshold control function is connected to the strong/weak indication to provide a variable threshold output level in accordance with the strong/weak indication. The scaling functions are connected to the variable threshold output and to a second input of the greater/than test functions. The scaling functions scale the thresholds to provide a desired fraction of strong and weak data symbols from the greater/than test functions to the soft decision decoder. 
     It is an object of the present invention to provide a means to set the amplitude or threshold for quantized soft decisions in a near-optimum manner. 
     It is an object of the present invention to set the amplitude or threshold for quantized soft decisions with a simple method of control. 
     It is an advantage of the present invention to measure the fraction of the soft decisions which mark bits as weak or strong and to adjust the gain to achieve a desired fraction of each marking. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention in conjunction with the appended drawings wherein: 
     FIG. 1 is a block diagram of a data communications receiver employing soft decision decoding and an automatic gain control systems as is known in the art; 
     FIG. 2 is diagram showing the operation of a three-bit quantizer used in soft decision decoding systems; 
     FIG. 3 is a block diagram showing an embodiment of a gain control circuit of the present invention using feedback to control the input amplitude; 
     FIG. 4 is a block diagram showing another embodiment of a gain control of the present invention using feedforward to select soft decision bits to control the input to a soft decision decoder; and 
     FIG. 5 is a block diagram showing another embodiment of a gain control of the present invention using feedback to adjust the thresholds of a quantizer. 
    
    
     DETAILED DESCRIPTION 
     A data communications receiver  100  known in the art is shown in FIG.  1 . An input signal to the receiver  100  may be an analog signal transmitted from a remote transmitter (not shown) that contains modulated digital data and error correction coding. The input to the data communications receiver  100  may be received from other stages (not shown) of the receiver such as intermediate frequency stages. The data communication receiver  100  may include a demodulator  120 , a quantizer  130 , and a decoder  140  for error detection and correction to produce a data output signal. In the example of FIG. 1, the demodulator  120  may be a PSK demodulator and the decoder  140  may be a Viterbi decoder. Other forms of modulation and error correction coding may be used. An automatic gain control (AGC) function may be incorporated in the receiver  100  to maintain a constant input level to the quantizer  130 . The AGC may comprise an AGC block  105  that provides a gain control signal proportional to the signal level output from the demodulator  120  and a variable gain or scaling function  115  to vary the level of the input signal to the demodulator  120  as is known in the art. 
     In FIG. 1 the input data signal may be corrupted with Gaussian or other noise when received at the input of the receiver  100 . The noisy input signal is demodulated in the demodulator  120  and sent to the quantizer  130 . The quantizer  130  may be a two-level quantizer that determines whether the received signal with noise is a zero or a one thus making what is known in the art as a hard decision. The quantizer  130  may be a 1-bit analog to digital (ADC) converter. The output of the quantizer  130  is the passed to the Viterbi decoder  140  for processing. The decision made by the quantizer  130  may be in error due to the added noise. In FIG. 2 curve  210  shows the distribution of an input logic 1 signal with added Gaussian noise and curve  215  shows the distribution of an input logic 0 with added Gaussian noise. To reduce potential errors that exist in two-level hard decision quantization, the input signal may be quantized with greater than two levels. Adding additional levels of quantization to quantizer  130  provides the decoder  140  more information about the signal. With a three-bit quantizer  130 , eight levels (three bits) are available that provide a level of confidence that the received signal is a logic one or a logic zero. Decoding in the decoder  140  with a quantizer with more than two levels is known in the art as soft decision decoding. With soft decoding decoder  140  becomes a soft decision decoder and quantizer  130  is referred to as a soft decision quantizer. 
     The output of the quantizer  130  in FIG. 1 is typically of limited precision, with three bits being the most common, in order to minimize the hardware needed in the soft decision decoder  140 . A three-bit soft decision quantizer may be implemented as a 3-bit ADC. FIG. 2 shows operation of a three-bit quantizer. Scale  220  shows thresholds T of the three-bit quantizer for logic 1 (0, T, 2T, 3T) and for logic 0 (0, −T, −2T, −3T, and −4T). Table  225  on the right of FIG. 2 represents a soft decision output code from the quantizer  130  corresponding to an input signal represented by curves  210  and  215 . For example if an input logic 1 plus noise point  212  on curve  210  is received, the signal is at threshold level 3T on the scale  220 . This example condition corresponds to a soft decision code word 011 in table  225  of FIG. 2. A strong data symbol or bit may be represented by a soft decision code word 011 or 010 corresponding to 3T or 2T respectively in table  225  for logic 1 and 101 or 100 corresponding to −3T or −4T respectively for logic zero. The soft decision code words in-between the strong data symbols correspond to weak data symbols. The most significant bit b 2  in the table  225  may be a hard decision bit. A strong/weak symbol indication may be determined by the exclusive OR of b 2  and b 1  of table  225  in FIG.  2 . 
     A high gain in the receiver before the quantizer  130  relative to current received signal and noise amplitudes, will produce mostly the largest positive (011) and negative values (100) at the quantizer  130  output and most of the data symbols will be labeled as strong. A low gain will produce mostly the smallest positive and negative values and most of the symbols will be labeled as weak. An optimum gain will produce a full range of quantized values. This discussion refers to values that are at least half of the maximum quantized value as strong and less than this value as weak. Other values may also be used. 
     An automatic gain control (AGC) function of some type is used with the data communications receiver  100  to produce the optimum gain. In FIG. 1 the AGC function includes AGC control block  105  and the variable gain function  115  as previously discussed. With additive white Gaussian noise, rate {fraction (1/2 )}coding, and three-bit quantization it has been determined that the threshold level T is best set to about 0.50σ x  where σ x  is the standard deviation of the noise. The AGC function should keep the noise level at the soft decision quantizer  130  set at this level. For binary phase shift keyed (BPSK) modulation the threshold is 
     
       
           T =0.5{square root over (N o /2)}  Equation 1 
       
     
     where N 0  is the noise. 
     As previously discussed, prior art attempts at implementing an AGC have included noise AGC and signal plus noise AGC. Noise AGC offers the best performance but is difficult to implement so signal plus noise AGC or just signal AGC is commonly used. 
     Instead of using signal, noise, or signal plus noise to control an AGC to maintain a proper level into the quantizer, an embodiment of the present invention uses a measured fraction of strong and weak quantizer symbols in a feed back fashion to adjust the gain in such a way as to keep the optimum level into the quantizer. If more weak symbols are produced than desired, the gain is increased. If fewer weak symbols than desired are produced, the gain is decreased. Another embodiment of the present invention uses feed forward indications of strong and weak quantization symbols to select bits from a high-precision digital demodulator output to use as quantization outputs for the soft decision decoder  140 . This can be viewed as feed forward gain control. Another embodiment of the present invention uses the strong and weak quantizer symbols to adjust quantization thresholds in the quantizer instead of gain in the receiver. These embodiments are described in the following paragraphs. 
     A wide variety of implementations is possible for the mechanization of a gain control circuit in one embodiment of the present invention. A simple implementation of the first embodiment employing feedback is shown in FIG.  3 . Portions of the block diagram of FIG. 1 are used in FIG.  3 . Quantizer  130  Is a soft decision quantizer and decoder  140  is a soft decision decoder in FIG. 3 as described in conjunction with FIG.  1 . In this embodiment, a strong/weak indication function  150  provides a strong/weak indication by using an exclusive OR logic gate operating on the bits b 2  and b 1  of the quantized soft decision value from soft decision quantizer  130  as shown in the table  225  of FIG.  2 . An averaging function  160  averages the strong/weak indications by the accumulation of up and down indications in a counter. If a weak indication is given, the counter  160  counts up to increase gain In the receiver. The gain that is applied to the received input signal is proportional to the current counter value. The output of the counter  160  may have an offset subtracted at comparison function  165  to set a desired fraction of strong and weak symbols input to the soft decision decoder  140 . The comparison function implementation may be used to adjust the fraction of weak symbols to approximately 50% or other desired value as discussed above. 
     The output of the comparison function  165  is a gain error signal that is passed to a gain control function  110 . The gain control function may include a digital to analog converter (DAC) to convert the digital gain error signal into an analog error signal. The analog error signal may then be applied to a variable gain or scaling function  115  to vary the level of the input signal to the demodulator  120 . The variable gain or scaling function  115  may be at the input as shown in FIG. 3 or at any location in the signal path of the receiver  100  such as between the input to the demodulator  120  and the antenna (not shown). The variable gain function may also be located after the demodulator  120  and before the quantizer  130 . The scaling function  115  may be any variable gain control element used in AGC systems known in the art such as a field effect transistor (FET) amplifier or a digital signal processing (DSP) digital multiplier factor. By controlling the signal level input to the quantizer  130 , the desired measured fraction of strong and weak data symbols to the soft decision decoder  140  is obtained. 
     The exact logic used to accomplish the detection of strong and weak symbols will depend on the number system used for representing the demodulator  120  outputs. Two&#39;s complement representation is primarily used in illustrating this invention but other forms such as sign and magnitude could accomplish the same results. 
     It is obvious that the principles of this invention apply to both binary and non-binary symbols, such as quadrature phase shift keyed (QPSK), so long as a quantized soft decision value is produced that is amplitude dependent. 
     An alternate embodiment of the present invention that employs feedforward gain control is shown in FIG.  4 . This embodiment feeds forward a determination of which bits in a high-precision digital output word to use as the quantized soft-decision bits in an n-bit quantization function  131 . The embodiment of FIG. 4 utilizes three bits but other numbers of bits may be used. A demodulator  121  provides a multi-bit digital output word that has enough range to cover all signal and noise level variations of the input signal. Circuit functions within the block  131  replace the quantizer  130  of FIG.  3 . Exclusive OR functions  150 ,  151 , and  152  are connected to a sign bit hard decision output  122  and to each of the next lower order output bits of the demodulator  121  to provide strong/weak indications. An output on a strong/weak indication exclusive OR  150  connected to the hard decision bit  122  and exclusive OR  151  on the next lower order bit  123  provides a strong indication with a high signal input level. An output on the hard decision bit  122  exclusive OR  150  and a low order bit  124  exclusive OR  152  provides a weak indication due to a low signal input level. The outputs of the exclusive OR functions  150 ,  151 , and  152  are connected to averaging functions  160  such as digital counters to average the exclusive OR outputs. The averaging functions  160  average the number of strong/weak indications to obtain a gain update at a rate lower than the data symbol rate. The outputs of the averaging functions  160  are connected to comparison functions  180  where the average counts are compared to a predetermined threshold value  185 . The gain may be set for a block of data symbols independently from other blocks in this embodiment. 
     The outputs of each of the comparison functions  180  in FIG. 4 are connected to a bit select logic function  190 . The logic function may be a conventional combinational logic circuit that selects an index number of the highest order bit from the demodulator  121  that drops below the threshold value  185  or exceeds the threshold value depending on the chosen number system. The output of the logic function  190  is connected to a shift control input of a bit shifter  195 . The bit shifter  195  may be a commonly available multiplexer. The shift control is used to select the three output bits from the demodulator  121  that are to be used by the soft decision decoder  140 . If the receiver gain is too high a large number of strong indications from the comparison functions  180  are obtained as indicated by the number of average count output levels that above the threshold  185 . The bit select logic function  190  causes the bit shifter  195  to select higher order demodulator outputs to the soft decision decoder  140 . The bit shifter  195  functions as a gain control by shifting the multiple bit digital word by a variable amount before providing it to the soft decision decoder  140 . In this fashion, the desired measured fraction of strong and weak symbols to the soft decision decoder  140  is made. The threshold  185  is used to set the desired fraction. 
     Another embodiment of the present invention is shown in FIG.  5 . This embodiment automatically adjusts the soft decision thresholds in a quantizer  132  using a feedback means rather than adjusting the gain before the quantizer  130  of FIG. 3 to achieve a desired fraction of strong and weak soft decision symbols. Circuit functions within the quantizer block  132  replace the quantizer  130  of FIG.  3 . In FIG. 5, the output of the demodulator  120  is connected to a first “greater than” (GT) test function  210  that may be an analog or digital comparator known in the art. The output of the demodulator  120  is compared to a zero threshold level in the GT test function  210  to make a hard decision. If the input to the GT test function  210  is greater than zero, the hard decision bit value is a logic one as indicated by the GT function  210  output. The demodulator  120  output is also connected to an absolute value function  220  to convert all input levels to positive outputs to provide a measure of how strong or weak the data symbol is. The absolute value function  220  may be an operational amplifier circuit known in the art for analog circuit implementations or digital logic known in the art if the demodulator  120  produces multi-bit digital values. The output of the absolute value function  220  is connected to an input of a plurality of GT test functions  230  and a second GT test function  232  for comparison to a threshold. The plurality of GT test functions  230  may include three or more depending on the desired level of quantization. The output of the GT test functions  230  form the remainder of the soft decision outputs that together with the hard decision output from GT test function  210  are passed as the quantized soft decision symbol outputs to the soft decision decoder  140 . An output from the second GT test function  232  is used as a strong/weak indication. This strong/weak indication is connected to a threshold control  250  that may an up/down counter. The output of the threshold control  250  is connected to the input of each of a plurality scaling functions  240 . The other input of each of the GT test functions  230  and the second GT test function  232  is connected to the outputs of the scaling functions  240 . Each scaling function  240  may be set to values that provide a range of thresholds that divide the range of demodulator output values into multiple subranges. Typically, values distributed over a range above and below unity will be used. The second GT test function  232  is near the middle of the input range from the demodulator  120  and has its threshold scaling function set at unity. 
     In FIG. 5 when the input signal level is high into the demodulator  120 , the strong/weak indication from the second GT test function  232  provides a strong indication and the threshold control  250  counts up. The counter output may be used in digital form or converted to an analog signal with a digital to analog converter in the threshold control  250 . The threshold increases as the counter counts up increasing the threshold level to the scaling functions  240 . The threshold to the GT test functions  230  and the second GT test function  232  is increased thus reducing the percentage of strong indications. In this fashion, the desired measured fraction of strong and weak symbols to the soft decision decoder  140  is made as the input signal and noise levels vary. 
     It is believed that the automatic gain control for soft decision decoding of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages, the form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.