Reduced-Complexity Approach to Iterative Detection of Coded SOQPSK

We develop a reduced-complexity approach for the detection of coded shaped-offset quadrature phase-shift keying (SOQPSK), a highly bandwidth-efficient and popular constant-envelope modulation. The complexity savings result from viewing the signal as a continuous-phase modulation (CPM). We give a simple and convenient closed-form expression for a recursive binary-to-ternary precoder for SOQPSK. The recursive nature of this formulation is necessary in serially concatenated systems where SOQPSK serves as the inner code. We show that the proposed detectors are optimal in the full-response case, and are near-optimal in the partial-response case due to some additional complexity reducing approximations. In all cases, the proposed detectors achieve large coding gains for serially concatenated coded SOQPSK. These gains are similar to those reported recently by Li and Simon, which were obtained using a more complicated cross-correlated trellis-coded quadrature modulation (XTCQM) interpretation.     

Shaped offset quadrature phase shift keying (SOQPSK) modulation scheme and its application in optical wavelength-division multiplexed (DWDM) transmission

This paper presents a new quaternary modulation scheme called SOQPSK. The principle on the optical SOQPSK generation is derived and analyzed, which is implemented by traditional Mach–Zehnder modulators. The performance of the optical SOQPSK modulated system is evaluated and compared with those of quadrature phase shift keying (QPSK) and offset QPSK (OQPSK) modulation systems via simulation, in terms of spectral efficiency, receiver sensitivity and density DWDM transmission performance. Simulations show that the novel modulation scheme improves spectral efficiency for DWDM transmission and provides better transmission performance than QPSK.     

成形正交相移键控及其最佳接收机的设计

采用一种分组差分编码的规则将QPSK表示成连续相位调制的形式,从而提出了一种适用于深空通信中使信号满足恒定包络要求的成形正交相移键控(SQPSK)调制方法.基于分析总结SQPSK的信源序列与同相、正交支路上基带波形对之间的映射关系,设计出了SQPSK的交叉相关网格编码正交调制等价形式的发射机和相对应的最佳接收机.仿真结果表明,SQPSK的带宽效率和误比特率性能均要优于成形偏移正交相移键控(SOQPSK).     

High rate concatenated coding systems using multidimensionalbandwidth-efficient trellis inner codes

A concatenated coding system using two-dimensional trellis-coded MPSK inner codes and Reed-Solomon outer codes for application in high-speed satellite communication systems was proposed previously by the authors (ibid., vol.37, no.5, p.420-7, May 1989). The authors extend their results to systems using symbol-oriented, multidimensional, trellis-coded MPSK inner codes. The concatenated coding systems are divided into two classes according to their achievable effective information rates. The first class uses multidimensional trellis-coded 8-PSK inner codes and achieves effective information rates around 1 b/dimension (spectral efficiency 2 b/s/Hz). The second class employs multidimensional trellis-coded 16-PSK inner codes and provides effective information rates around 1.5 b/dimension (spectral efficiency 3 b/s/Hz). Both classes provide significant coding gains over an uncoded reference system with the same effective information rate as the coded system. The results show that the symbol-oriented nature of multidimensional inner codes can provide an improvement of up to 1 dB in the overall performance of a concatenated coding system when these codes replace bit-oriented two-dimensional codes     

迭代译码的级联Reed-Solomon乘积码与卷积码

该文提出用Reed Solomon(RS)乘积码作为外码,卷积码作为内码的级联码方案并且内外码间用Congruential向量生成的交织图案对RS码符号进行重排列。对此级联码采用的迭代译码基于成员码的软译码算法。当迭代次数达到最大后,通过计算RS码的校正子,提出一种纠正残余错误的方法,进一步提高了系统的误比特性能。仿真结果表明,在AWGN信道中与迭代译码的级联RS/卷积码相比,当误比特率为1e-5时,新系统的编码增益大约有0.4 dB。     

Adaptive coded modulation for fading channels

We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation designs are separable. Therefore, trellis and lattice codes designed for additive white Gaussian noise (AWGN) channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains. We first describe the methodology for combining coset codes with a general class of adaptive modulation techniques. We then apply this methodology to a spectrally efficient adaptive M-ary quadrature amplitude modulation (MQAM) to obtain trellis-coded adaptive MQAM. We present analytical and simulation results for this design which show an effective coding gain of 3 dB relative to uncoded adaptive MQAM for a simple four-state trellis code, and an effective 3.6-dB coding gain for an eight-state trellis code. More complex trellis codes are shown to achieve higher gains. We also compare the performance of trellis-coded adaptive MQAM to that of coded modulation with built-in time diversity and fixed-rate modulation. The adaptive method exhibits a power savings of up to 20 dB     

New rate-1/3 trellis codes with 8-PSK signals for bandlimited AWGN channels

New rate-1/3 linear trellis (convolutional) codes with 8-PSK modulation for bandlimited additive white Gaussian noise (AWGN) channels are reported. The simulated bit error probability performance of the trellis-coded modulation schemes with unquantised Viterbi decoding shows useful coding gains over the uncoded coherent BPSK having the same bandwidth efficiency of 1 bit/s/Hz and the schemes are suitable for use in many practical bandlimited channels.<>     

High-rate concatenated space-time block code M-TCM designs

In this paper, a new technique to design improved high-rate space-time (ST) codes is proposed based on the concept of concatenated ST block code (STBC) and outer trellis-coded modulation (M-TCM) encoder constructions. Unlike the conventional rate-lossy STBC-MTCM schemes, the proposed designs produce higher rate ST codes by expanding the codebook of the inner orthogonal STBC. The classic set partitioning concept is adopted to realize the STBC-MTCM designs with large coding gains. The proposed expanded STBC-MTCM designs for the two-, three-, and four-transmitter cases are illustrated. Simulation results show the proposed STBC-MTCM designs significantly outperform the traditional ST-TCM schemes. Furthermore, decoding complexity of the proposed scheme is low because signal orthogonality is exploited to ease data decoding.     

瑞利衰落信道下的网格酉空时调制的研究

酉空时是一种多天线系统信号调制方案,可以在不知道信道状态下进行解调。文中将网格编码与酉空时星座结合运用,性能仿真证明了网格酉空时调制系统非常适合无信道状态信息的无线通信系统,网格编码在一定程度上改进了系统的比特误码率性能。将网格编码下的酉空时调制与未编码的酉空时调制进行对比,结果显示在10^-4的比特误码率下可以得到10～15dB的编码增益。     

基于MSK和LT码的联合迭代译码算法

传统的简单级联编码调制系统在译码时会造成软信息损失.提出了一种基于MSK和LT码的联合软迭代译码算法,设计了算法的系统模型.利用LT码的软译码和MSK调制的SISO算法,进行联合软迭代译码,提高了编码调制系统的性能.仿真结果表明:在误码率为10-4时,提出的算法比传统的简单级联编码调制算法约有1.5 dB的编码增益.     

Delayed decision-feedback sequence estimation

The authors delayed-decision-feedback sequence estimation (DDFSE) for uncoded PAM signals is considered. Estimates on the performance of the algorithm are given, and simulation results are provided for several examples. A more general form of DDFSE applicable to coded modulation systems is also presented. As an example, detection of trellis-coded QPSK (quadrature phase-shift keyed) signals over intersymbol interference channels is discussed     

Performance of interleaved trellis-coded differential 8-PSKmodulation 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^-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     

Noncoherent iterative decoding of spectrally efficient coded modulations

In this paper, we consider possible solutions for noncoherent decoding of concatenated codes with spectrally efficient modulations. Two main classes of schemes are considered. A first class is obtained by concatenating parallel coding schemes with differential encoding. A second class considers serially concatenated coding structures and possible schemes derived from turbo trellis coded modulation (t-tcm), which do not employ differential encoding. In the first case, at the receiver side we consider separate detection and decoding, while in the second case we consider joint detection and decoding. The major problem connected with such an iterative decoding procedure is that taking into account an augmented channel memory leads to an intolerable trellis size, and hence to an impractical decoding complexity. Reduced-complexity techniques suited to iterative decoding become fundamental, and we consider a recently proposed state-reduction technique. This way, the performance of a coherent receiver is approached, by keeping the number of receiver states fixed.     

A Turbo Detection and Sphere-Packing-Modulation-Aided Space-Time Coding Scheme

A recently proposed space-time block-coding (STBC) signal-construction method that combines orthogonal design with sphere packing (SP), referred to here as STBC-SP, has shown useful performance improvements over Alamouti's conventional orthogonal design. In this contribution, we demonstrate that the performance of STBC-SP systems can be further improved by concatenating SP-aided modulation with channel coding and performing demapping as well as channel decoding iteratively. We also investigate the convergence behavior of this concatenated scheme with the aid of extrinsic-information-transfer charts. The proposed turbo-detected STBC-SP scheme exhibits a "turbo-cliff" at E_b/N₀=2.5 dB and provides E_b/N₀ gains of approximately 20.2 and 2.0 dB at a bit error rate of 10 ^-5 over an equivalent-throughput uncoded STBC-SP scheme and a turbo-detected quadrature phase shift keying (QPSK) modulated STBC scheme, respectively, when communicating over a correlated Rayleigh fading channel.A condensed version of this paper was presented at VTC'04 Fall, LA, USA     

Optimum Modulation and Channel Filters for Nonlinear Satellite Channels

This is a comparative study of three modulation schemes: QPSK, Offset QPSK(OQPSK), and MSK, in nonlinear satellite channels with adjacent channel interference. Two kinds of typical satellite channel models are used for performance evaluation: one is an INTELSAT type narrow band model and the other is a domestic type wide band model. The transmitter filter and receiver filter are optimized for each combination of modulation scheme, channel model, and an input back-off condition for nonlinear amplifiers. All the combinations considered here have the optimum division of the Nyquist shaping between transmitter filter and receiver filter in common. The filters are a wide band sharp cut-off transmitter filter with flat inband response and a gently rolled-off Nyquist receiver filter with an aperture equalizer (x/sin xor1-(2x)^{2}/cos xequalizer). With these optimized filters, comparisons are made with respect to the bit error rate (BER) performance and phase jitter of the recovered carrier. OQPSK and MSK show superior BER performance to QPSK in the wide band model. However, in the narrow band model, QPSK shows the best BER performance among the three modulation schemes. The phase jitter in the recovered carrier is small for all modulation schemes in the wide band model. But, in the narrow band model, the phase jitter in OQPSK and MSK is four times larger than in QPSK. Generally QPSK is the most preferable modulation scheme in the narrow band model: OQPSK and MSK are most preferable for the wide band model.     

Serial concatenation of LDPC codes and differential modulations

In this paper, we consider serially concatenated schemes with outer novel and efficient low-density parity-check (LDPC) codes and inner modulations effective against channel impairments. With a pragmatic approach, we show how to design LDPC codes tailored for simple and robust modulation formats, such as differentially encoded (DE) modulations. The LDPC codes are optimized through the use of a recently proposed analysis technique based on extrinsic information transfer (EXIT) charts. In particular, we optimize, through a "clever" random walk in the parametric space, the degree distributions of the outer LDPC codes, obtaining significant insights on the impact of such distributions on the performance of the proposed concatenated schemes. The optimization is carried out for transmission over both the additive white Gaussian noise channel and a noncoherent channel. The performance predicted by the EXIT chart-based optimization is confirmed by results obtained via computer simulations, considering phase-shift keying and quadrature amplitude modulation at the transmitter side, and iterative demodulation/decoding at the receiver side. The significance of the proposed optimized design of LDPC-coded schemes with DE modulations is validated by the fact that standard nonoptimized LDPC codes perform poorly when used together with inner DE modulations.     

Concatenated coding with inner trellis codes for fading channels

It is known that concatenated coding can significantly enhance the performance of digital communication systems operating over fading channels when compared to uncoded or single-coded systems. The price to be paid for such an improvement, however, is a substantial increase in the required bandwidth. In this paper, we consider the use of a concatenation scheme in which the inner code is a trellis-coded modulation (TCM) system. The analysis is carried out for two different outer codes: a binary Golay code and a non-binary Reed-Solomon code. Results obtained for the Rayleigh and Rician fading channels through analytical bounds indicate that the use of this system does provide a significant reduction in the bit error probability, a fact that is also verified through computer simulation. Unlike a traditional concatenated system, the proposed method achieves the coding gain while maintaining acceptable bandwidth efficiency.     

Performance of trellis-coded CPM with iterative demodulation anddecoding

Interleaved trellis-coded systems with full response continuous-phase modulation (CPM) are considered. Upper bounds on the bit-error rate performance are derived for coherent detection on the additive white Gaussian noise and flat Rayleigh fading channels by considering the trellis code, interleaver, and CPM modulator as a serially concatenated convolutional code. A coherent receiver that performs iterative demodulation and decoding is shown to provide good bit error performance. Finally, a noncoherent iterative receiver is proposed and is shown to perform close to the coherent iterative receiver     

Bandwidth-efficient turbo trellis-coded modulation using puncturedcomponent codes

We present a bandwidth-efficient channel coding scheme that has an overall structure similar to binary turbo codes, but employs trellis-coded modulation (TCM) codes (including multidimensional codes) as component codes. The combination of turbo codes with powerful bandwidth-efficient component codes leads to a straightforward encoder structure, and allows iterative decoding in analogy to the binary turbo decoder. However, certain special conditions may need to be met at the encoder, and the iterative decoder needs to be adapted to the decoding of the component TCM codes. The scheme has been investigated for 8-PSK, 16-QAM, and 64-QAM modulation schemes with varying overall bandwidth efficiencies. A simple code choice based on the minimal distance of the punctured component code has also been performed. The interset distances of the partitioning tree can be used to fix the number of coded and uncoded bits. We derive the symbol-by-symbol MAP component decoder operating in the log domain, and apply methods of reducing decoder complexity. Simulation results are presented and compare the scheme with traditional TCM as well as turbo codes with Gray mapping. The results show that the novel scheme is very powerful, yet of modest complexity since simple component codes are used     

Design of serially concatenated systems depending on the block length

Based on extrinsic information transfer (EXIT) charts, the convergence behavior of iterative decoding is studied for a number of serially concatenated systems, such as a serially concatenated code, coded data transmission over an intersymbol interference channel, bit-interleaved coded modulation, or trellis-coded modulation. Efficient optimization algorithms based on simplified EXIT chart construction are devised to find irregular codes improving the convergence of iterative decoding. One optimization criterion is to find concatenated systems exhibiting thresholds of successful decoding convergence, which are close to information-theoretic limits. However, these thresholds are approached only for very long block lengths. To overcome this problem, the decoding convergence after a fixed, finite number of iterations is optimized, which yields systems performing very well for short block lengths, too. As an example, optimal system configurations for communication over an additive white Gaussian noise channel are presented.