Robust image communication over wireless channels

Typical image compression algorithms produce data streams which require a very reliable communication-they are not designed for transmission in an environment in which data may be lost or delayed, as provided by current and next-generation wireless communication networks. Compression and transmission provisions that avoid catastrophic failure caused by lost, delayed, or errant packets are therefore imperative in order to provide reliable visual communication over such systems. This robustness is obtained by modifying the source coding and/or adding channel coding. This article presents an overview of both lossy and lossless source coding techniques and combined source/channel techniques providing robustness, presenting examples of successful techniques     

Adaptive-rate coding for frequency-hop communications over Rayleighfading channels

Because of the variability of the channels in frequency-hop wireless systems and networks, the performance of error-control coding can be improved by adapting the rate of the code to the channel conditions. In this paper, adaptive-rate error-control coding is investigated for slow frequency-hop communications with Reed-Solomon coding. Two methods are investigated that use decoder side information as a means for selecting the code rate. These methods are based on counts of errors and erasures, which are provided by the demodulator and the decoder. The performance of the adaptive-rate coding system is evaluated for channels with Rayleigh fading, partial-band interference, and thermal noise     

Power efficient multimedia communication over wireless channels

In this work, we introduce an approach for minimizing the total power consumption of a mobile transmitter due to source compression, channel coding and transmission subject to a fixed end-to-end source distortion. We illustrate our approach both on an abstract class of sources and channels and on a realistic H.263 video transmission system through a wireless channel. Performance under different channel environments and implementation schemes are investigated. Our numerical analysis shows that optimized settings can reduce the total power consumption by a significant factor and prolong battery life considerably compared with fixed parameter settings.     

Matched filter bound for multipath Rician-fading channels

This paper derives an infinite series and an integral for computing matched filter bounds for binary phase-shift keying (BPSK) communications over a multipath Rician-fading channel of discrete or continuous dispersion. Based on the derived bound we compare intrinsic frequency diversity (IFD) gains for channels of different Rician factors     

Pulse shaping for wireless communication over time- or frequency-selective channels

This letter investigates the influence of transmit and receive filtering on the design and associated performance of wireless communication systems. Based on derivations using power-series models of time- or frequency-selective channels, we present pulse-shaping filters which can be matched to the characteristics of the channel. The choice of filter parameters allows some degree of control over the received signal. The peak-to-average-power-ratio (PAPR) requirements in the transmitter are determined. Orthogonal frequency-division multiplexing (OFDM) represents an alternative method of achieving some of the stated objectives of the pulse-shaping methods developed. Consideration is given to PAPR, bandwidth efficiency, receiver complexity, and performance in terms of error probability for the derived pulses and related forms of OFDM.     

Error performance of OFDM signaling over doubly-selective Rayleighfading channels

A simple method for the evaluation of the error rate performance of OFDM signals transmitted over doubly-selective fading channels is described. The accuracy of the proposed technique is exemplified by simulation results     

Matched filter bound for OFDM on Rayleigh fading channels

A matched filter bound (MFB) analysis is presented, in which the normalised Doppler rate is unrestricted, and thus applicable to OFDM signalling. In contrast to the static channel case, the optimal matched filter receiver is shown to be time varying and the probability of error is shown to depend on the transmitted pulse shape     

Matched filter performance bounds for diversity combining receiversin digital mobile radio

By employing the technique known as the matched filter bound, the authors derive analytical expressions for the distribution and average of the bit-error-rate in an ideal space diversity mobile radio receiver. Each diversity branch receives from a frequency-selective Rayleigh fading channel of arbitrary delay profile, and is subjected to additive Gaussian noise of arbitrary spectral shape. Numerical results calculated from the analytical expressions give insight into the relative benefits of antenna diversity and wideband transmission over the mobile radio channel     

Synchronization over fading dispersive channels

An investigation is reported of an acquisition system for digital communication over fading dispersive channels that is optimum in the maximum-likelihood sense. A performance measure for the synchronizer in the acquisition mode is defined. Upper and lower bounds on the performance measure are derived. The effective bounds on parameters, such as signal-to-noise ratio, pulse width and modulation, number of pulses, spread of the channel, and the size of the resolution cell, on the performance of the synchronizer is investigated     

Performance analysis of cell search in W-CDMA systems over Rayleighfading channels

The three-step cell search has been considered for fast acquisition of the scrambling code unique to a cell in wideband code-division multiple-access (W-CDMA) systems. In this paper, the performance of the cell-search scheme is analyzed in Rayleigh fading channels. The system parameters for the cell-search scheme and the design parameters for the receivers are examined. Probabilities of detection, false alarm, and miss for each of the three steps are derived in closed form based on the statistics of CDMA noncoherent demodulator output. Through the analysis, the effect of threshold setting and post-detection integration for each step is investigated and the power allocation for the channels is considered. The optimal number of post-detection integrations for each step may depend on not only the power allocation for the channels related to the cell search, but also the false-alarm penalty time. Our analysis can be utilized for determining the values of the parameters. Also, the cumulative probability distribution of the average cell-search time is obtained. Finally, it is shown that the analysis could be used to obtain the distribution of the cell-search time graphically according to the distance between a mobile station and a base station by considering the propagation models for pass loss and the traffic distribution models     

Capacity bounds and power allocation for wireless relay channels

We consider three-node wireless relay channels in a Rayleigh-fading environment. Assuming transmitter channel state information (CSI), we study upper bounds and lower bounds on the outage capacity and the ergodic capacity. Our studies take into account practical constraints on the transmission/reception duplexing at the relay node and on the synchronization between the source node and the relay node. We also explore power allocation. Compared to the direct transmission and traditional multihop protocols, our results reveal that optimum relay channel signaling can significantly outperform multihop protocols, and that power allocation has a significant impact on the performance.     

Performance bounds of multihop wireless communications with blind relays over generalized fading channels

In this letter, efficient performance bounds for multihop wireless communications systems with non-regenerative blind relays over non-identical Nakagami-n (Rice), Nakagami-m and Nakagami-q (Hoyt) generalized fading channels, are presented. More specifically, the end-to-end signal-to-noise ratio (SNR) is formulated and upper bounded by using the well-known inequality between harmonic and geometric mean of positive random variables. This bound is used to study important system's performance metrics: i) the moments of the end-to-end SNR which are obtained in closed-forms, and ii) the outage probability and the average error probability for coherent and non-coherent modulations, which are accurately approximated using the moments-based approach. Furthermore, new analytical formulae are derived for the gain of previously proposed semi-blind relays in generalized fading environments. These kind of relays are used in numerical examples and computer simulations to verify the accuracy and to show the tightness of the proposed bounds.     

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     

Two-user dispersive communication channels (Corresp.)

The optimum transmitters for maximum information throughput when two independent users share a dispersive channel are investigated. The channel is assumed to be linear and time invariant and is described by a2 times 2matrix of transfer functions. The noise at each receiver is additive and Gaussian and may have a nonwhite spectrum. Assuming naive superposition (the interfering signal is treated as noise) it is shown that for many practical channels, frequency division multiplexing is optimum, and the structure of the optimum transmitters is obtained.     

Block Markov superposition transmission over turbulence channels in outdoor optical wireless communication

Based on the equivalent genie-aided system,the bit-error rate of the block Markov superposition transmission system was analyzed and the union bound of the bit-error rate of the genie-aided system was derived.When the bit-error rate was extremely low at which the Monte Carlo simulation was time-consuming,the union bound could be adopted to evaluate the performance of the block Markov superposition transmission system.Simulations under turbulence conditions from weak to strong are performed which show that,in the low bit-error rate region,performance of the block Markov superposition transmission system matches well with the lower bound.It is also illustrated that the derived union bound matches well with Monte Carlo simulation result.At the BER of 10^-5,block Markov superposition transmission system with transmission memory 2 performs within 1 dB away from the Shannon limit.     

Error-resilient transcoding for video over wireless channels

We describe a method to maintain quality for video transported over wireless channels. The method is built on three fundamental blocks. First, we use a transcoder that injects spatial and temporal resilience into an encoded bitstream. The amount of resilience is tailored to the content of the video and the prevailing error conditions, as characterized by bit error rate. Second, we derive analytical models that characterize how corruption propagates in a video that is compressed using motion-compensated encoding and subjected to bit errors. Third, we use rate distortion theory to compute the optimal allocation of bit rate among spatial resilience, temporal resilience, and source rate. Furthermore, we use the analytical models to generate the resilience rate distortion functions that are used to compute the optimal resilience. The transcoder then injects this optimal resilience into the bitstream. Simulation results show that using a transcoder to optimally adjust the resilience improves video quality in the presence of errors while maintaining the same input bit rate     

Bandwidth expansion for robust, low-complexity communication overfading dispersive channels

In this paper, we derive pulses that yield Nyquist or near Nyquist properties when convolved with the initial terms of the series model of a slow fading dispersive channel. For the f-power series these are the so called powers of Nyquist pulses. For a Karhunen-Loeve series, model pulses are found that maximize the ratio of the mean peak-sample energy to mean residual intersymbol interference energy for a given bandwidth expansion factor. Under certain conditions, the approach significantly lessens the need for, or complexity of, equalization. Other benefits attributable to the use of the derived pulses, including those related to timing recovery, are discussed     

Identification of dispersive communication channels using Barker codes

The identification of dispersive sommunication channels using short duration binary signals in considered in this paper. It is shown that Barker codes are the best known finite length binary test signals for the identification of the impulse response of base-bond channels using the cross-correlation technique. The identification of RF passband channels is performed by using RF pulse with Barker code phase modulation as test signals. The described identification method is suitable for adaptive digital communication systems operating in time varying channels. It is also suitable for channels that may show non-linear effects when long duration test signals are used for identification.     

Communication over fading dispersive channels with feedback

An intermittent on-off noiseless feedback scheme for binary communication over the slow- and fast-fading Rayleigh channels is proposed and analyzed. At high energy-to-noise ratios, doubling the number of feedback iterations yields a 3-dB power saving for the slowly fading channel. Power savings ranging from 1 dB for one feedback iteration to 9 dB for 16 iterations are typical for the fast-fading model. Also for the fast-fading model, by picking the optimum number of forward transmissions for each value of energy-to-noise ratio, the best achievable performance requires approximately 7.5 dB more energy than the minimum predicted by the rate-distortion bound. Also presented is a feedback communication system for wide-sense stationary, uncorrelated-scatterer, fading, and dispersive forward and feedback channels. The model used for both forward and feedback channels is Kennedy's. Upper and lower bounds on the error probability for block orthogonalM-ary communication are presented for this system.     

Orthogonal time-frequency signaling over doubly dispersive channels

This paper develops a general framework for communication over doubly dispersive fading channels via an orthogonal short-time Fourier (STF) basis. The STF basis is generated from a prototype pulse via time-frequency shifts. In general, the orthogonality between basis functions is destroyed at the receiver due to channel dispersion. The starting point of this work is a pulse scale adaptation rule first proposed by Kozek to minimize the interference between the basis functions. We show that the average signal-to-interference-and-noise (SINR) ratio associated with different basis functions is identical and is maximized by the scale adaptation rule. The results in this paper highlight the critical impact of the channel spread factor, the product of multipath and Doppler spreads, on system performance. Smaller spread factors result in lesser interference such that a scale-adapted STF basis serves as an approximate eigenbasis for the channel. A highly effective iterative interference cancellation technique is proposed for mitigating the residual interference for larger spread factors. The approximate eigendecomposition leads to an intuitively appealing block-fading interpretation of the channel in terms of time-frequency coherence subspaces: the channel is highly correlated within each coherence subspace whereas it is approximately independent across different subspaces. The block-fading model also yields an approximate expression for the coherent channel capacity in terms of parallel flat-fading channels. The deviation of the capacity of doubly dispersive channels from that of flat-fading channels is quantified by studying the moments of the channel eigenvalue distribution. In particular, the difference between the moments of doubly dispersive and flat-fading channels is proportional to channel spread factor. The results in this paper indicate that the proposed STF signaling framework is applicable for spread factors as large as 0.01.