Soft Decision Viterbi Decoding Research Articles

Bit-by-bit soft-decision decoding of trellis-coded M-DPSK modulation

We present a simple soft-decision decoding method for use with trellis-coded M-ary differential phase-shift keying (M-DPSK). The demodulator assigns a separate confidence level to each bit associated with an M-ary symbol, and these confidence levels are based only on the phase of the received symbol. Decoding is performed by the standard soft-decision Viterbi decoder that is normally used for binary modulation. Our soft-decision decoder is much simpler to implement than the typical trellis decoder, but we show that the performance is approximately the same.

Rational rate punctured convolutional codes for soft-decision Viterbi decoding

We present rational rate k/n punctured convolutional codes (n up to 8, k=1, /spl middot//spl middot//spl middot/, n-1, and constraint length /spl nu/ up to 8) with good performance. Many of these codes improve the free distance and (or) weight spectra over previously reported codes with the same parameters. The tabulated codes are found by an exhaustive (or a random) search.

A 1-Gb/s, four-state, sliding block Viterbi decoder

To achieve unlimited concurrency and hence throughput in an area-efficient manner, a sliding block Viterbi decoder (SBVD) is implemented that combines the filtering characteristics of a sliding block decoder with the computational efficiency of the Viterbi algorithm. The SBVD approach reduces decode of a continuous input stream to decode of independent overlapping blocks, without constraining the encoding process. A systolic SBVD architecture is presented that combines forward and backward processing of the block interval. The architecture is demonstrated in a four-state, R=1/2, eight-level soft decision Viterbi decoder that has been designed and fabricated in double-metal CMOS. The 9.21 mm/spl times/8.77 mm chip containing 150 k transistors is fully functional at a clock rate of 83 MHz and dissipates 3.0 W under typical operating conditions (V/sub DD/=5.0 V, T/sub A/=27/spl deg/C). This corresponds to a block decode rate of 83 MHz, equivalent to a decode rate of 1 Gb/s. For low-power operation, typical parts are fully functional at a clock rate of greater than 12 MHz, equivalent to a decode rate of 144 Mb/s, and dissipate 24 mW at V/sub DD/=1.5 V, demonstrating extremely low power consumption at such high rates.

Performance evaluation of COFDM for digital audio broadcasting. I. Parametric study

This paper describes the performance of COFDM (coded orthogonal frequency division multiplexing) for digital audio broadcasting (DAB) over various multipath mobile radio channels. COFDM is a channel coding and modulation scheme which mitigates the adverse effects of fading by using wideband multicarrier modulation combined with time interleaving and a convolutional error correcting code. A guard interval is inserted at the transition between successive symbols to absorb the intersymbol interference created by the time domain spread of the mobile radio channel. The decoding process is performed using differential demodulation in conjunction with a soft decision Viterbi decoder. The COFDM evaluation is done by means of computer simulations. A parametric study of the proposed COFDM system is done in this first of a series of papers to determine the optimal system parameter values for operation at a radio frequency of 1.5 GHz. The results show that the parameter values proposed in the standard are indeed well suited for operation at 1.5 GHz. Using the set of optimal parameter values found, the COFDM performance is then evaluated in mobile radio channels typical of different geographical environments. Simulation results confirm the benefit that the system gains from frequency diversity found in the urban and hilly terrain areas.

A supervised learning neural network coprocessor for soft-decision maximum-likelihood decoding

A supervised learning neural network (SLNN) coprocessor which enhances the performance of a digital soft-decision Viterbi decoder used for forward error correction in a digital communication channel with either fading plus additive white Gaussian noise (AWGN) or pure AWGN has been investigated and designed. The SLNN is designed to cooperate with a phase shift keying (PSK) demodulator, an automatic gain control (AGC) circuit, and a 3-bit quantizer which is an analog to digital convertor. It is trained to learn the best uniform quantization step-size Delta (BEST) as a function of the mean and the standard deviation of various sets of Gaussian distributed random variables. The channel cutoff rate (R(0)) of the channel is employed to determine the best quantization threshold step-size (Delta(BEST)) that results in the minimization of the Viterbi decoder output bit error rate (BER). For a digital communication system with a SLNN coprocessor, consistent and substantial BER performance improvements are observed. The performance improvement ranges from a minimum of 9% to a maximum of 25% for a pure AWGN channel and from a minimum of 25% to a maximum of 70% for a fading channel. This neural network coprocessor approach can be generalized and applied to any digital signal processing system to decrease the performance losses associated with quantization and/or signal instability.

Burst error statistics of simulated Viterbi decoded BFSK and high-rate punctured codes on fading and scintillating channels

The burst error statistics of a soft-decision Viterbi decoder were simulated and obtained for the case when the transmitted signal is: (1) encoded with constraint length=3, rate 1/2 convolutional codes and rate 2/3, 3/4, 4/5, and 7/8 punctured convolutional codes; (2) modulated via noncoherent BFSK for the AWGN channel; and (3) subjected to slow and non-selective scintillation/fading modeled by the Nakagami-m and Rician pdfs. These statistics are evaluated to determine how Viterbi decoder burst error statistics vary with the channel and code parameters, and to compare the statistics of the Nakagami-m and Rician fading models (Morris and Chang 1990). The results have important implications for the design of interleaved or noninterleaved concatenated coding schemes, and to the development of burst error models and simulators for scintillating and fading channel environments. This extends the earlier burst-error-statistics results of Deutsch and Miller (1981), and Miller, Deutsch, and Butman who looked at the nonfading case for BPSK modulation and coherent demodulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Soft-decision Viterbi decoding with diversity combining for multi-beam mobile satellite communication systems

Diversity combining methods for mobile satellite communication systems employing convolutional encoding and soft-decision Viterbi decoding are evaluated. Computer simulation clarifies that the pre-Viterbi-decoding maximal ratio combining method has better performance than other methods in Rician fading channels. The simulation results agree with the Pe (bit error probability) performance derived from the numerical analysis for slow fading using the approximate Pe performance of Viterbi decoding in AWGN (additive white Gaussian noise) channels and the probability density function of Rician fading. Applying this diversity method to the multi-beam mobile satellite communication systems, a satellite beam diversity reception scheme is proposed. A computer simulation result shows that the proposed scheme decreases the packet error rate of the control signals to less than 1/100 around the satellite beam boundary. >

Dynamic adaptation of quantization thresholds for soft-decision viterbi decoding with a reinforcement learning neural network

Two reinforcement learning neural network architectures which enhance the performance of a soft-decision Viterbi decoder used for forward error-correction in a digital communication system have been investigated and compared. Each reinforcement learning neural network is designed to work as a co-processor to a demodulator dynamically adapting the soft quantization thresholds toward optimal settings in varying noise environments. The soft quantization thresholds of the demodulator are dynamically adjusted according to the previous performance of the Viterbi decoder, with updates occurring in fixed intervals (every 200 decoded bits out of the Viterbi decoder.) To facilitate implementaiton in digital hardware, each weight of the neural network and related parameters are specified as binary numbers. Computer simulation results demonstrate that, on average, the performance of a Viterbi decoder on an AWGN channel with nonuniformly-spaced soft decision thresholds dynamically adjusted by these neural networks is better than the performance of a Viterbi decoder with uniformly-spaced thresholds. This approach may be used for a variety of other digital communication applications such as channel estimation, adaptive equalization, and signal acquisition.

Advanced modulation and channel coding tecniques for digital HDTV via satellite in the 20 GHz range

Three advanced modulation and channel coding techniques, making use of soft-decision Viterbi decoding, are evaluated over a simulated satellite channel affected by noise and interference to assess the feasibility of Wide RF-band digital HDTV emission systems (W-HDTV) in the 20 GHz range, capable of providing near studio quality. These techniques are evaluated, for different bit-rates (i.e. 140, 105 and 70 MBit/s), in the context of a possible ‘common frequency allocation’ by the WARC 1992 of the frequency range 21.4-22 GHz. This would allow a very efficient use of the spectrum by assigning the overall bandwidth (on one or both polarisations) to all service areas. The systems are compared in terms of required C/ N for high quality pictures and for service continuity, protection ratios and number of channels available in 600 MHz, using both polarisations (i.e. frequency reuse).

Improved HF modem using convolutional coding

The paper describes the design of a digitally implemented modem for use over HF (2-30 MHz) radio channels. As a completely digital approach offers the prospect of a number of significant improvements, a new digital multifrequency-shift-keying (MFSK) modem has been developed that employs a novel digital processing procedure termed code-assisted bit synchronisation (CABS). Signal detection is achieved via a set of noncoherent correlators, whilst symbol synchronisation and error correction are performed using a soft-decision Viterbi decoder. Practical tests of modem show that it operates effectively under both additive white Gaussian noise and real HF channel conditions.

Burst error statistics of simulated Viterbi decoded BPSK on fading and scintillating channels

The author presents and analyzes burst error statistics of a soft-decision Viterbi decoder when the transmitted signal is encoded with the 313 (3, 1/2) or 31123 (5, 1/2) convolutional codes, modulated via coherent binary phase-shift keying (BPSK) for the additive white Gaussian noise (AWGN) channel, and subjected to slow and nonselective scintillation/fading modeled by the Nakagami-m distribution. These statistics were generated by Monte-Carlo simulations, and presented in terms of burst error length average and quantile (90 and 99%) statistics versus SNR (E/sub b//N/sub 0/) parameterized by the fading intensity parameter m. The results indicate how Viterbi decoder burst error statistics vary with the fading/scintillation intensity m for Nakagami-m channels, and, consequently, provide information important to the design of interleaved or noninterleaved concatenated coding schemes for such channel environments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A 140-Mb/s, 32-state, radix-4 Viterbi decoder

A 140-Mb/s, 32-state, radix-4, R=1/2, eight-level soft-decision Viterbi decoder has been designed and fabricated using 1.2- mu m double-metal CMOS. The architecture of the add-compare-select (ACS) array is based on a restructuring of the conventional radix-2 trellis into a radix-4 trellis. Radix-4 units, consisting of four 4-way ACS units, process two stages of the constituent radix-2 trellis per iteration. A four-way ACS circuit achieves an iteration delay 17% longer than comparable two-way ACS circuits, resulting in a factor of 1.7 increase in throughput. A ring-based ACS placement and state metric routing topology achieves an area efficiency comparable to radix-2 designs. In a process referred to as pretrace-back, one stage of lookahead is applied to the trace-back recursion, combining two radix-4 trace-back iterations into a single radix-16 iteration based on 4-b decisions. This allows implementation of trace-back using one compact, single-ported decision memory, organized as a cyclic buffer. A 7.30-mm*8.49-mm chip containing 146000 transistors achieves a radix-4 iteration rate of 70 MHz. >

Coding and optimum baseband combining for wide-band TDMA indoor wireless channels

Optimum baseband combining with coherent BPSK/QPSK is an attractive technique for wide-band TDMA portable communication radio links in view of its ability not only to combat dispersive fading but also to cancel cochannel interference. The performance of such a scheme for data rates ranging from 1 to 20 Mbits/s is investigated. It is shown that convolutional coding with soft-decision Viterbi decoding can be a viable alternative to increasing the space diversity order even in the absence of interleaving if the effective diversity order is sufficiently large. A number of performance curves for various combinations of transceiver and channel parameters are presented.

A convolutional single-parity-check concatenated coding scheme for high-data-rate applications

The coding scheme uses a set of n convolutional codes multiplexed into an inner code and a (n,n-1) single-parity-check code serving as the outer code. Each of the inner convolutional codes is decoded independently, with maximum-likelihood decoding being achieved using n parallel implementations of the Viterbi algorithm. The Viterbi decoding is followed by additional outer soft-decision single-parity-check decoding. Considering n=12 and the set of short constraint length K=3, rate 1/2 convolutional codes, it is shown that the performance of the concatenated scheme is comparable to the performance of the constraint length K=7, rate 1/2 convolutional code with standard soft-decision Viterbi decoding. Simulation results are presented for the K=3, rate 1/2 as well as for the punctured K=3, rate 2/3 and rate 3/4 inner convolutional codes. The performance of the proposed concatenated scheme using a set of K=7, rate 1/2 inner convolutional codes is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of Convolutionally Coded OOK and PPM Signalling in a Direct Detection Optical Communication

The performance of convolutionally coded On-Off Keying (OOK) and M-ary Pulse Position Modulation (M-PPM) systems is evaluated for avalanche photodetection receiver. Results are presented in terms of bit error probability and achievable coding gains for both hard decision and soft decision Viterbi decoding. The computed results predict that for the soft decision Viterbi decoding with code constraint length K=6, binary PPM, 4-ary PPM, 8-ary PPM and OOK achieve coding gain of 5.34 dB, 8.11 dB, 9.83 dB and 7.83 dB respectively relative to the uncoded binary PPM. The results of hard decision decoding are approximately 2.5–3.0 dB less than those for the soft decision case.

High rate concatenated coding systems using bandwidth efficient trellis inner codes

High-rate concatenated coding systems with bandwidth-efficient trellis inner codes and Reed-Solomon (RS) outer codes are investigated for application in high-speed satellite communication systems. Two concatenated coding schemes are proposed. In one the inner code is decoded with soft-decision Viterbi decoding, and the outer RS code performs error-correction-only decoding (decoding without side information). In the other the inner code is decoded with a modified Viterbi algorithm, which produces reliability information along with the decoded output. In this algorithm, path metrics are used to estimate the entire information sequence, whereas branch metrics are used to provide reliability information on the decoded sequence. This information is used to erase unreliable bits in the decoded output. An errors-and-erasures RS decoder is then used for the outer code. The two schemes have been proposed for high-speed data communication on NASA satellite channels. The rates considered are at least double those used in current NASA systems, and the results indicate that high system reliability can still be achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of interleaved trellis-coded differential 8-PSK modulation over fading channels

The performance of trellis-coded differential octal phase-shift keying (coded 8-DPSK) with differentially coherent detection and soft-decision Viterbi decoding is investigated. A suitable receiver is presented whose signal processing is based on Nyquist signaling, requiring only one complex sample per modulation interval. Symbol synchronization and automatic frequency control are performed in a decision-directed way. Bit-error-rate (BER) performance over Gaussian, Rayleigh, and Rician channels is determined by means of computer simulations. The performance of coded 8-DPSK on the Gaussian channel is shown for a four-state convolutional trellis code. The unquantized outputs of up to three symbol detectors with delays of 1, 2, and 3 symbol periods are used for metric computation. The coding gain which includes losses due to timing and frequency synchronization errors is found to be 2.5 dB at BER=10/sup -5/ with respect to uncoded 4-DPSK. Much larger gains are achieved for fading channels if interleaving is applied. Using an eight-state trellis code the performance is determined on Rayleigh and Rician channels for various Doppler spreads and interleaver sizes. >

Development of FEC and ESC performance characteristics for IDR carriers greater than 10 Mb/s

AbstractThe requirement for restoration of high capacity fibre optic cables and the new tariffs approved for INTELSAT digital services has created a requirement to develop performance characteristics for IDR carriers greater than 10 Mb/s. This paper presents the trade‐offs used in this development, and estimates the coding gains needed to meet these requirements. A number of FEC schemes are evaluated and a rate 3/4 convolutional code with soft decision Viterbi decoding is selected. Engineering service circuits (ESC) performance characteristics for IDR carriers greater than 10 Mb/s are also presented.

New short constraint length rate (n-1)/n punctured convolutional codes for soft-decision Viterbi decoding

Punctured convolutional codes with good performance are reported for rates (n-1)/n with n=5, 6, 7, 8 and constraint lengths k=2, 3, . . ., 6. The tabulated codes are nonsystematic binary convolutional codes with maximum free distance and minimum information weight, for use with soft-decision Viterbi decoding.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interleaved convolutional coding for the turbulent atmospheric optical communication channel

The coding gain of a constraint-length-three, rate one-half convolutional code over a long clear-air atmospheric direct-detection optical communication channel using binary pulse-position modulation signalling is directly measured as a function of interleaving delay for both hard- and soft-decision Viterbi decoding. Maximum coding gains theoretically possible for this code with perfect interleaving and physically unrealizable perfect-measurement decoding were about 7 dB under conditions of weak clear-air turbulence, and 11 dB at moderate turbulence levels. The time scale of the fading (memory) of the channel was directly measured to be tens to hundreds of milliseconds, depending on turbulence levels. Interleaving delays of 5 ms between transmission of the first and second channel bits output by the encoder yield coding gains within 1.5 dB of theoretical limits with soft-decision Viterbi decoding. Coding gains of 4-5 dB were observed with only 100 mu s of interleaving delay. Soft-decision Viterbi decoding always yielded 1-2 dB more coding gain than hard-decision Viterbi decoding.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>