衰落信道下卷积码的软判决译码

本文将交织、信道输出软信息、信道特征软信息相结合,研究Rayleigh衰落信道下卷积码的Viterbi软判决译码,在利用和不利用信道状态信息(CSI)两种情况下给出软判决的支路度量计算公式和性能仿真结果,计算了卷积码的性能上界;研究了解调器输出量化门限(量化级数目、量化间距)对译码性能的影响.结果与硬判决译码及AWGN信道的相关结论进行了比较.     

Comparison of convolutional and turbo codes for OFDM with antennadiversity in high-bit-rate wireless applications

This letter presents simulation results for the application of turbo coding to an OFDM system with diversity. First, results are presented for convolutional and Reed-Solomon codes. It is shown that even with short constraint lengths, convolutional codes have the potential to outperform Reed-Solomon codes, provided that sufficient precision is used in the soft decoder. We then evaluate the performance of turbo codes under slow fading conditions and study the effects of varying codeword size. Increasing codeword size theoretically provides better interleaving between the two component codes. However, this advantage is less clear when the fading rate is significantly lower than the symbol rate, which is typical of the high-data-rate systems considered here. Under such conditions, the advantage of using two component convolutional codes in turbo codes is limited. A single convolutional code with a long constraint length may be a better choice     

Effect of Time Diversity on the Forward Link of the DS-CDMA Cellular System

A direct-sequences code division multiple access system has been accepted as a digital cellular standard (IS–95) in North America [1]. This digital cellular standard employs a powerful rate 1/2, constraint length 9, convolutional code in its forward link. It is well-known that in a Rayleigh fading channel the performance of a channel code depends very heavily on the interleaving depth and the relative variations of the channel characteristics. In slow fading channels, since the input symbols to the channel decoder are highly correlated, the bit-error-rate at the output of the channel decoder may be unacceptably high. Interleavers of large dimensions can reduce the correlation of the input signal to the channel decoder at the expense of an intolerable delay. In this paper we examine the performance of the IS–95 system, at the mobile receiver, for different channel fade rates. Also, we present a simple time diversity technique which employs multiple receive antennas. The multiple receive antennas in this case generate a fast fading effect and thus improves the performance of the channel decoder significantly.     

Comparison of two convolutional orthogonal coding techniques forCDMA radio communications systems

For CDMA (code-division multiple-access) wireless communications systems, orthogonal signaling with noncoherent detection has been proposed when a reference phase is not available. In this paper, orthogonal signaling using Walsh signals is considered. Performance of the channel using error-correcting coding together with 1) bit-by-bit interleaving or 2) Walsh word-by-word interleaving is investigated. Simulation results show that for several types of fading and non-fading channels with practical amounts of interleaving and with practical power control, the second technique with complete soft decision decoding that has added complexity in the decoder gives significant reduction in the required signal to interference ratio (SIR) over the first technique     

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     

Channel coding for satellite mobile channels

The deployment of channel coding and interleaving to enhance the bit-error performance of a satellite mobile radio channel is addressed for speech and data transmissions. Different convolutional codes (CC) using Viterbi decoding with soft decision are examined with inter-block interleaving. Reed-Solomon (RS) codes with Berlekamp-Massey hard decision decoding or soft decision trellis decoding combined with block interleaving are also investigated. A concatenated arrangement employing RS and CC coding as the outer and inner coders, respectively, is used for transmissions via minimum shift keying (MSK) over Gaussian and Rayleigh fading channels. For an interblock interleaving period of 2880 bits, a concatenated arrangement of an RS(48,36). over the Galois field GF(256) and punctured PCC(3,1,7) yielding an overall coding rate of 1/2, provides a coding gain of 42dB for a BER of 10^?6, and an uncorrectable error detection probability of 1–10^?9.     

Frame error rates for convolutional codes on fading channels andthe concept of effective Eb/N0

Estimation of Viterbi decoder performance over channels with time-varying received signal levels is the subject of this paper. This work is motivated by a desire to obtain good estimates of the frame error rate (FER) for convolutional codes with bit-level interleaving over fading channels subject to practical power control algorithms. The convolutional code performance is quantified through the FER and effective E_b/N₀. where the latter is defined as the E_b/N₀ on an additive white Gaussian noise (AWGN) channel that results in the same FER. Given a received vector of (time-varying) E_b/N₀ values, we compute analytic estimates for the probability of frame error and the effective E_b /N₀ for a Viterbi decoder and interleaver combination. In particular, we validate our analysis using the R=1/3 convolutional code and interleaver used on the IS-95 CDMA reverse channel. Comparisons with simulations show that even for E_b/N₀ vectors with very large variations, our proposed estimates are good to within 0.2 dB for the effective E_b/N₀, giving FER estimates within a factor of two-five of the simulations     

卷积编码及译码在数字通信系统中的仿真

本文介绍了利用点对点（哪）数字通信系统模型，推导卷积编码和Viterbi译码的非线性传输函数的方法以及对Viterbi译码软判决和硬判决的性能分析。通过Matlab中的Simulink仿真模块，对系统模型进行了建模，其仿真结果表明。增大卷积编码和Viterbi译码的约束长度可以提高误码性能。最后，得到了Viterbi译码在软判决和硬判决条件下的误码曲线。     

A new block interleaver and noniterative decision feedback decodingenhance performance of IS-95 CDMA uplink or similar M-ary system

This paper investigates the combination of a new block interleaver design and a noniterative decision feedback receiver/decoder for IS-95 code-division multiple-access (CDMA) uplink in additive white Gaussian noise and a Rayleigh fading multipath environment. The new interleaver design yields better performance than the currently used IS-95 interleaver, with E_b/N₀ improvements of 0.27 to 0.76 dB. At the same time, the new interleaver design improves the effectiveness of a noniterative decision feedback receiver/decoder herein presented. The performance of the proposed design is measured via computer simulation for both coherent and noncoherent detection. A mathematical justification is also given to show the reason why the performance is improved using the proposed design. Based on these results, it is concluded that the use of the new interleaver design with noniterative decision feedback decoding can improve the quality of service and/or capacity of a modified cellular IS-95 system or a system using a similar concatenation of convolutional coding, interleaving, and orthogonal Walsh modulation. Although the approach presented is applied to the uplink of IS-95, it can also be applied to similar M-ary systems     

Generating series and performance bounds for convolutional codes over burst-error channels

We present an analytical method for evaluating performance bounds for a communication system that employs a convolutional code, a block interleaving with finite interleaving depth, a binary channel that exhibits statistical dependence in the occurrence of errors, and a decoder that implements the Viterbi (1979) algorithm with Hamming metric. The main idea is to apply combinatorial methods to derive a formula for bounds to first-event and bit error probabilities in terms of coefficients of a generating series. The method is used to investigate the tradeoff between coding parameters and interleaving depth to achieve a required performance.     

On the performance of concatenated convolutional code and Alamouti space-time code with noisy channel estimates and finite-depth interleaving

In this paper, performance of the Alamouti space- time code (STC) [1] and performance of the concatenation between the convolutional code and the Alamouti STC are derived. In order to describe realistic performance issues, we assume that the channel estimates are calculated from linear filters using noisy pilot symbols. We also assume non-quasi- static channels, spatially correlated transmit antennas and finite- depth interleaving. Two types of receivers are investigated for the Alamouti STC, namely, the linear-combining space-time decoder (LC-STD) and the maximum-likelihood space-time decoder (ML- STD). Two types of receivers are investigated for the concatenated system, namely, the LC-STD with the ML convolutional decoder and the joint Alamouti and convolutional ML decoder. The results have shown that the LC-STD is more sensitive to the Doppler spread than the ML-STD. However, since the ML- STD is very sensitive to the channel estimation error, the gains provided by the decoder in fast fading channels will be offset unless an optimized channel estimator is employed. Performance comparisons between the Alamouti systems and the SISO systems indicate that, when the system environment is not ideal, the SISO systems may outperform the Alamouti systems. Lastly, analytical results are compared with simulation results to illustrate the accuracy of the analysis.     

Systolic array processing of the Viterbi algorithm

Results on efficient forms of decoding convolutional codes based on the Viterbi algorithm by using systolic arrays are presented. Various properties of convolutional codes are discussed. A technique called strongly connected trellis decoding is introduced to increase the efficient utilization of all the systolic array processors. Issues dealing with the composite branch metric generation, survivor updating, overall system architecture, throughput rate, and computational overhead ratio are also investigated. The scheme is applicable to both hard and soft decoding of any rate b/n convolutional code. It is shown that as the length of the code becomes large, the systolic Viterbi decoder maintains a regular and general interconnection structure as well as moderate throughput rate gain over the sequential Viterbi decoder     

级联纠错编码DS/BPSK系统性能分析与仿真

为了研究级联编码对DS/BPSK系统性能的影响,设计了一种基于RS与卷积码级联编码的DS/BPSK系统,分析了该级联编码直扩系统的性能,仿真了在纠错编码参数发生改变时的系统性能。仿真结果表明,级联编码的DS/BPSK系统性能比单码编码的DS/BPSK系统性能要好。在RS码参数保持不变时,降低卷积码的码率,采用软判决方式译码以及多比特软判决,能有效提升系统性能;同时,交织在一定程度上能改善系统的性能。     

Adaptive trellis-coded multiple-phase-shift keying for Rayleighfading channels

An adaptive scheme for trellis-coded modulation of MPSK signals, called adaptive trellis-coded multiple-phase-shift keying (ATCMPSK), is proposed for slowly Rayleigh fading channels. The adaptive scheme employs a slightly modified rate 1/2 convolutional encoder and the corresponding Viterbi decoder to realize a family of codes of different rates which are employed according to channel conditions. During poor channel conditions, trellis-coded QPSK (TCQPSK) together with repetition schemes are employed. As channel conditions improve, higher rate schemes such as trellis-coded 16 PSK are used. An interleaving/deinterleaving method suitable for the adaptive scheme is proposed. Theoretical bounds for the error performance and an exact expression for the throughput of the proposed adaptive scheme are derived, and are compared against simulation results. Simulations have been performed to measure the performance of the scheme for different parameters and some nonideal conditions. It is shown that ATCMPSK results in considerable improvement in bit-error-rate (BER) performance of MPSK signals. Under ideal conditions, gains in the range of 3-20 dB are achieved over conventional fixed rate pragmatic trellis-coded schemes     

Correction of extrinsic information for iterative decoding in a serially concatenated multiuser DS-CDMA system

The system under study is a coded asynchronous DS-CDMA system with orthogonal modulation in time-varying Rayleigh fading multipath channels. Information bits are convolutionally encoded, block interleaved, and mapped to M-ary orthogonal Walsh codes, where the last step is essentially a process of block coding. This paper aims at tackling the problem of joint iterative decoding of this serially concatenated inner block code and outer convolutional code and estimating frequency-selective fading channels in multiuser environments. The (logarithm) maximum a posteriori probability, (Log)-MAP criterion is used to derive the iterative decoding schemes. In our system, the soft output from inner block decoder is used as a priori information for the outer decoder. The soft output from outer convolutional decoder is used for two purposes. First, it may be fed back to the inner decoder as extrinsic information for the systematic bits of the Walsh codeword. Secondly, it is utilized for channel estimation and multiuser detection (MUD). We also show that the inner decoding can be accomplished without extrinsic information, and in some cases, e.g., when the system is heavily loaded, yields better performance than the decoding with unprocessed extrinsic information. This implies the need for correcting the extrinsic information obtained from outer decoder. Different schemes are examined and compared numerically, and it is shown that iterative decoding with properly corrected extrinsic information or with non-extrinsic/extrinsic adaptation enables the system to operate reliably in the presence of severe multiuser interference, especially when the inner decoding is assisted by decision directed channel estimation and interference cancellation techniques.     

A burst-error-correcting Viterbi algorithm

A dual-mode burst-error-correcting algorithm that combines maximum-likelihood decoding with a burst detection scheme is presented. The decoder nominally operates as a Viterbi decoder and switches to time diversity error recovery whenever an uncorrectable error pattern is identified. It is demonstrated that the new scheme outperforms interleaving strategies under the constraint of a fixed overall decoding delay. It also proves to be more powerful than known adaptive burst decoding schemes, such as the Gallager burst finding scheme. As the new method can be used with soft decision decoding, it is mainly intended for use on random-error channels affected by occasional severe bursts     

Performance of fast FH/MFSK signals in jammed binary channels

Soft-decision decoded performance of fast frequency-hopped (FH) M-ary FSK signals over partial band noise jammed binary channels is studied. The effects of metrics conversion, quantization, and the presence of regenerative nodes on the system's cutoff rate performance are investigated. It is found that the conversion from an M-ary metric to a binary one suffers only negligible degradations. For communication links without regenerative nodes, as expected, the infinite-bit soft-decision is the optimal metric, followed by finite-bit soft decision and then hard-decision. For those with regenerative nodes, however, the infinite-bit soft-decision is outperformed by the hard-decision, but the finite-bit soft-decision decoder still keeps its edge over the latter. The issue concerning the order of metric conversion and diversity combining is also analyzed. Numerical results indicate that the conclusion obtained by an earlier simulation report addressing a similar design alternative in fading channels is valid for jammed binary channels, as well. That is, the precombining metric conversion technique gives up only minor performance degradations when compared to the more sophisticated postcombining metric conversion technique     

Joint coding and decision feedback equalization for broadbandwireless channels

This paper introduces a new approach for joint convolutional coding and decision feedback equalization (DPE). To minimize error propagation, the DFE uses a combination of soft decisions and delayed tentative decisions to cancel intersymbol interference (ISI). Soft decisions are obtained by passing the DFE output through a (soft) nonlinear device. This simple method is shown to perform almost as well as an optimum soft feedback approach on wireless channels with diversity. Tentative decisions from the Viterbi decoder are used to cancel ISI due to multipath with large delays, thus remedying the increasing effect of error propagation in channels with large delay spreads. We consider the use of this soft/delayed feedback DFE (S/D-DFE) technique in broadband wireless channels (with delay spreads up to several tens of the symbol period) typical in high-bitrate mobile data applications. Simulation results indicate that the proposed joint coding and S/D-DFE technique performs to within 1-2 dB [in required signal-to-noise ratio (SNR)] of an ideal coded DFE without error propagation. When combined with antenna diversity and a reduced-complexity DFE concept with adaptive feedforward tap assignment, it provides high packet throughput against Rayleigh fading, severe delay spreads, and high Doppler rates     

Bit-Interleaving in Fast FH/SSMA Systems with M-FSK Modulation

The effects of bit-interleaving on the performance of convolutional codes and turbo codes in fast frequency- hop/spread-spectrum multiple-access systems with M-FSK modulation are investigated. It is observed that bit-interleaving induces two counter-acting forces on decoder performance. On the one hand, bit-interleaving disperses consecutive error bits caused by a noisy M-ary signal and makes the errors more correctable. On the other hand, the same measure makes it difficult for the decoder to make use of the bit dependency information. Both theoretical upper bounds and simulation results show that bit- interleaving degrades the performance of soft-decision decoded convolutional codes and turbo codes.     

块交错方案在移动图像通信系统中的应用

提出了一种适合于卷积码的新的交错方案块交错方案,并对采用块交错方案的(2,1,3)卷积码在移动图像通信系统中的抗干扰性能进行了计算机模拟,且与周期交错方案的结果做了比较.模拟结果表明周期交错方案不适合于卷积码,而采用块交错方案的(2,1,3)卷积码可明显改善图像质量,大大提高了快衰落移动信道环境中图像通信系统的可靠性.