Convolutionally coded binary PSAM for Rayleigh fading channels

Recently proposed, pilot symbol assisted modulation (PSAM) uses a known pilot sequence to derive amplitude and phase references at the receiver. The authors present convolutional coding for such systems and derive the exact pairwise error probability and the Chernoff upper bound of it. A comparison among PSAM, coherent and differential detected coded systems indicates that, even at 5% Doppler fading rate, coded PSAM requires 3.5 dB more than the ideal coherent case but less than the differential case.<>     

Optimum Pilot Symbol Assisted Modulation

Optimum detectors for pilot symbol assisted modulation (PSAM) signals in Rayleigh and Rician fading channels are derived. Conventional PSAM as used on Rayleigh fading channels is also employed on Rician fading channels. It is shown that the conventional PSAM receiver is optimal for binary phase shift keying in Rayleigh fading but suboptimal for Rician fading and suboptimal for 16-ary quadrature amplitude modulation in Rayleigh fading. The optimum PSAM signal detector uses knowledge of the specular component and also jointly processes the pilot symbols and the data symbol. The performance of the optimum detector is analyzed and compared with that of the conventional detector. It is concluded that substantial gains can be achieved by exploiting knowledge of the specular component while joint processing of the data symbol with the pilot symbols may offer small benefits.     

A Comparative Synchronizers Study of a Class of PSAM Frame

Pilot symbol assisted modulation (PSAM) is amethod to reduce the effects of fading in mobilecommunications by periodically inserting knowing symbolsin the data stream. The receiver uses the pilot symbols to estimate channel state information. In orderto do this, the pilot symbols must first be located inthe noisy received sequence. This paper investigates theperformance of a class of PSAMs frame synchronizers which have the form of a quadratic correlationfilter and threshold test. Computer simulations are usedto show that these synchronizers can be designed to havea robust performance over a large range of Doppler spreads and SNR, to achieve anarbitrarily high probability of correct acquisition, andto maintain low computational complexity.     

Design and analysis of transmitter diversity using intentionalfrequency offset for wireless communications

Coded modulation (usually with interleaving) is used in fading channel communications to achieve a good error performance. The major benefit from using coded modulation in fading channels is achieved if each code symbol of a codeword (or coded sequence) suffers statistically different fading (preferably independent fading). However, in many applications of mobile communications (e.g., in a metropolitan environment), a low vehicle speed (and hence, a small Doppler spread, f _D) is very common. With a small Doppler spread, ideal or close-to-ideal interleaving is no longer feasible and all code symbols of a codeword would suffer highly correlated fading especially in stationary fading (f_D≈0). Coded modulations will thus suffer seriously degraded performance. Previous performance analyses based on ideal interleaving are not accurate when a small Doppler spread is encountered and the much used union bound error probability analysis is loose for small Doppler spreads. To rectify this situation, this paper presents an improved performance analysis of coded modulations with correlated fading and pilot-symbol-assisted modulation (PSAM). Transmitter diversity can generate the necessary time-varying fading to maintain the effectiveness of a coded signaling scheme which this paper examines in detail using an intentional frequency offset between antennas. This work found that proper selections of the intentional frequency offset and interleaving depth can lead to good performance with traditional coded modulations (if enough antennas are used) using essentially the same simple demodulation structure as used in the traditional single-antenna PSAM     

A Novel Wavelet Packet Division Multiplexing Based on Maximum Likelihood Algorithm and Optimum Pilot Symbol Assisted Modulation for Rayleigh Fading Channels

Wavelet packet division multiplexing (WPDM) is a high-capacity, flexible, and robust multiple-signal transmission technique. In this paper, a novel WPDM system based on optimum pilot symbol assisted modulation (OPSAM) and a maximum likelihood (ML) algorithm is studied for Rayleigh fading channels. The ML detecting algorithm and the new discrete wavelet packet transform structure, which is based on pilot symbol assisted modulation (PSAM) using a least mean squares algorithm, are two novel aspects of the presented system. An expression for the bit error rate of the WPDM scheme on quadrature phase-shift keying (QPSK) is derived in the presence of flat fading and Gaussian noise. It is demonstrated by simulation results that the OPSAM WPDM scheme can provide greater immunity to flat fading channels and Gaussian noise than the OPSAM orthogonal frequency division multiplexing scheme, the differential QPSK WPDM scheme, and the normal PSAM WPDM scheme.     

Performance of MMSE Receiver Based CDMA System with Higher Order Modulation Formats in a Fading Channel

This paper studies the effect of using higher order modulation formats on the performance of minimum mean-squared error (MMSE) receiver based direct-sequence (DS) code-division multiple access (CDMA) systems at different loading levels in additive white Gaussian noise (AWGN) and slow fading channels. The performance of BPSK, QPSK, and 16QAM modulation formats are compared and analytical and simulation results are presented in terms of the bit error rates (BER) for these different modulation formats. A comparison of the rejection of the near-far effects for each modulation scheme is also presented. The main contribution of this paper is in showing that user capacity may be increased by using higher order modulation schemes to cause the MMSE receiver to operate away from the interference limiting region. In particular it is shown that under high loading levels, 16QAM outperforms QPSK and BPSK for identical bandwidth and information rate, while at moderate loading levels, QPSK represents the best option. A combination of pilot symbol assisted modulation (PSAM) and linear prediction are used to estimate the fading process. A general structure of the MMSE receiver capable of demodulating a wide range of digital modulation formats in this type of environment is presented.     

A space-time coding modem for high-data-rate wirelesscommunications

This paper presents the theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach, data is encoded by a space-time (ST) channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem, as applied to the IS-136 cellular standard, are presented. We present the frame error rate (FER) performance results as a function of the signal-to-noise ratio (SNR) and the maximum Doppler frequency, in the presence of timing and frequency offset errors. Simulation results show that for a 10% FER, a 32-state eight-phase-shift keyed (8-PSK) ST code with two transmit and two receive antennas can support data rates up to 55.8 kb/s on a 30-kHz channel, at an SNR of 11.7 dB and a maximum Doppler frequency of 180 Hz. Simulation results for other codes and other channel conditions are also provided. We also compare the performance of the proposed STCM scheme with delay diversity schemes and conclude that STCM can provide significant SNR improvement over simple delay diversity     

Binary adaptive coded pilot symbol assisted modulation over Rayleigh fading channels without feedback

Pilot symbol assisted modulation (PSAM) is a standard approach for transceiver design for time-varying channels, with channel estimates obtained from pilot symbols being employed for coherent demodulation of the data symbols. In this paper, we show that PSAM schemes can be improved by adapting the coded modulation strategy at the sender to the quality of the channel measurement at the receiver, without requiring any channel feedback from the receiver. We consider performance in terms of achievable rate for binary signaling schemes. The transmitter employs interleaved codes, with data symbols coded according to their distance from the nearest pilot symbols. Symbols far away from pilot symbols encounter poorer channel measurements at the receiver and are therefore coded with lower rate codes, while symbols close to pilot symbols benefit from recent channel measurements and are coded with higher rate codes. The performance benefits from this approach are quantified in the context of binary signaling over time-varying Rayleigh fading channels described by a Gauss-Markov model. The spacing of the pilot symbols is optimized to maximize the mutual information between input and output in this setting. Causal and noncausal channel estimators of varying complexity and delay are considered. It is shown that, by appropriate optimization for the spacing between consecutive pilot symbols, the adaptive coding techniques proposed can improve achievable rate, without any feedback from the receiver to the sender. Moreover, channel estimation based on the two closest pilot symbols is generally close to optimal.     

Effect of channel estimation error on M-QAM BER performance inRayleigh fading

We determine the bit-error rate (BER) of multilevel quadrature amplitude modulation (M-QAM) in flat Rayleigh fading with imperfect channel estimates, Despite its high spectral efficiency, M-QAM is not commonly used over fading channels because of the channel amplitude and phase variation. Since the decision regions of the demodulator depend on the channel fading, estimation error of the channel variation can severely degrade the demodulator performance. Among the various fading estimation techniques, pilot symbol assisted modulation (PSAM) proves to be an effective choice. We first characterize the distribution of the amplitude and phase estimates using PSAM. We then use this distribution to obtain the BER of M-QAM as a function of the PSAM and channel parameters. By using a change of variables, our exact BER expression has a particularly simple form that involves just a few finite-range integrals. This approach can be used to compute the BER for any value of M. We compute the BER for 16-QAM and 64-QAM numerically and verify our analytical results by computer simulation. We show that for these modulations, amplitude estimation error leads to a 1-dB degradation in average signal-to-noise ratio and combined amplitude-phase estimation error leads to 2.5-dB degradation for the parameters we consider     

Optimal training for block transmissions over doubly selective wireless fading channels

High data rates give rise to frequency-selective propagation, whereas carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. To mitigate the resulting time- and frequency-selective (or doubly selective) channels, optimal training sequences have been designed only for special cases: pilot symbol assisted modulation (PSAM) for time-selective channels and pilot tone-assisted orthogonal frequency division multiplexing (OFDM) for frequency-selective channels. Relying on a basis expansion channel model, we design low-complexity optimal PSAM for block transmissions over doubly selective channels. The optimality in designing our PSAM parameters consists of maximizing a tight lower bound on the average channel capacity that is shown to be equivalent to the minimization of the minimum mean-square channel estimation error. Numerical results corroborate our theoretical designs.     

两路接收分集Turbo乘积编码PSAM系统的研究

提出具有两路接收分集Turbo乘积编码引导符号辅助调制(TPC-PSAM）的16QAM系统的实现方法。该系统将Turbo乘积编码和分集接收用于改善16QAM系统在平坦衰落信道中的性能。通过对衰落信道中的数值仿真，结果表明具有接收分集的TPC-PSAM系统比TPC-PSAM系统明显的改善，同时可以减小接收机在不同衰落信道中的性能差异。     

A two-symbols/branch MBCM scheme based on 8-PSK constellation under fading channels

A two-symbols/branch scheme of multiple block coded modulation (MBCM) is investigated under fading channels. Compared with a conventional scheme of block coded modulation (BCM), this two-symbols/branch MBCM scheme greatly increases the minimum squared Euclidean distance (MSED), minimum symbol distance (MSD), and minimum product distance (MPD). These three distances determine the bit-error-rate (BER) performance under either Gaussian or fading channels. A pilot symbol assisted fading compensation, as well as the techniques of symbol interleaving and branch weighting, are employed to combat the effect of channel fading. Through computer simulations, it is shown that large coding gains are obtained under both Rayleigh and Rician fading channels.     

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the system's interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on -steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.     

Frequency offset compensation of pilot symbol assisted modulationin frequency flat fading

Pilot symbol assisted modulation (PSAM) is a practical technique in mobile digital communications since it can provide high performance in fading with large constellations and it has a simple implementation. Maintaining high performance with PSAM requires an accurate estimate of the transmitted carrier frequency. This paper examines model-based frequency estimation for mobile digital communications with PSAM. Maximum-likelihood estimators (MLE), which include the model for the fading, are derived and compared with those in an additive white Gaussian noise (AWGN) channel, and performance hounds are computed. Reduced complexity frequency estimators based on the MLE are derived and the performance is quantified by Monte-Carlo simulation     

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.     

Pilot symbol assisted modulation and differential detection infading and delay spread

Pilot symbol assisted modulation (PSAM) has previously been shown to give good performance in flat fading, noise and cochannel interference. The present paper analyzes its performance in ISI due to frequency selective fading, and provides a similar analysis of differential detection for comparison. The paper also introduces a method for performing the formidable average over transmitted data patterns simply, and with an analytical result, PSAM is shown to be sensitive to RMS delay spread, though it always gives better performance than differential detection     

On the Power Allocation and System Capacity of OFDM Systems Using Superimposed Training Schemes

Channel estimation in multipath environments is typically performed using the pilot-symbol-assisted modulation (PSAM) scheme. However, the traditional PSAM scheme requires the use of dedicated pilot subcarriers and therefore leads to a reduction in the bandwidth utilization. Accordingly, this paper investigates a channel-estimation approach for orthogonal frequency-division multiplexing (OFDM) systems using a superimposed training (ST) scheme, in which the pilot symbols are superimposed onto the data streams prior to transmission. By using equally spaced pilot symbols of equal power and assuming that the number of pilots is larger than the channel order, it is shown that the channel-estimation performance is independent of the number of pilots used. The optimal ratio of the pilot symbol power to the total transmission power is analyzed to maximize the lower bound of the channel capacity. Overall, the current results show that the ST-based channel estimation schemes have a slightly poorer performance than the PSAM scheme but yield higher system capacity.     

Optimal Pilot Spacing and Power in Rate-Adaptive MIMO Diversity Systems with Imperfect CSI

In this paper, performance of a rate-adaptive multiple-input multiple-output diversity system with imperfect channel state information is analyzed and optimized. Space-time block coding is utilized to exploit spatial diversity and to thereby combat some of the fading in wireless channel communications. The remaining fading is tacked by means of adaptive coded modulation. The pilot symbol period and the power allocated to pilot and data symbols are optimized in such a way that the average spectral efficiency is maximized. At the same time, the instantaneous (with respect to predicted channel-signal-to-noise ratio) bit error rate is maintained below a desired level. The average time duration that the channel stays within a certain interval, the average fade region duration, is also derived. A numerical example is given for uncorrelated flat Rayleigh fading subchannels with Jakes spectrum     

Effect of channel-estimation error on QAM systems with antenna diversity

This paper studies the effect of channel estimation error and antenna diversity on multilevel quadrature amplitude modulation (M-QAM) systems over Rayleigh fading channels. Based on the characteristic function method, a general closed-form bit-error rate (BER) for M-QAM systems is presented. The effect of the inaccurate channel estimation on the performance for pilot-symbol-assisted modulation M-QAM systems with antenna diversity is investigated. Simulation results for M-QAM (M = 4, 16, 64, 256, etc.) show that the analytical method can accurately estimate the system performance. Moreover, numerical results show that with the antenna-diversity technique, the BER performance improves significantly, especially in perfect channel-estimation cases. It is also found that the channel-estimation error limits the benefit of antenna diversity. By increasing the length of the channel estimator and the amplitude of the pilot symbol, more accurate channel estimation can be achieved, so that the BER performance is improved.     

Combining PSA fading estimation techniques for TCM and diversity reception in Rician fading channels

In this paper, a novel pilot‐symbol‐aided (PSA) technique is proposed for fading estimation in the land mobile satellite fading channels. The proposed technique combines the fading estimates obtained from a bandwidth‐efficient technique and a conventional technique according to the signal‐to‐noise ratios (SNRs) of the fading estimates. To enhance the transmission quality, trellis‐coded modulation (TCM) and diversity reception are employed in the system, and the combined estimates are subsequently used to correct the channel fading effects, to weight the signals from different diversity branches, and to provide channel state information to the Viterbi decoder. Monte Carlo computer simulation has been used to study the bit‐error‐rate (BER) performance of the proposed technique on trellis‐coded 16‐ary quadrature amplitude modulation in the frequency non‐selective Rician fading channels. Results have shown that the proposed PSA technique requires a very low bandwidth redundancy to provide satisfactory BER performance at low SNRs, and thus is suitable for use with TCM and diversity reception to achieve both bandwidth and power‐efficient transmission. Copyright © 2002 John Wiley & Sons, Ltd.