Predictive downlink beamforming for wideband CDMA over Rayleigh-fading channels

A new approach to adaptive downlink beamforming to combat fast Rayleigh fading is presented. In this approach, the antennas at the base transceiver station form transmit beam patterns according to the prediction of downlink channels. The channel prediction is a linear prediction based on the autoregressive model, which is downsampled to extend the memory span given fixed model order. For a wideband code-division multiple-access downlink, pre-RAKE transmission is employed to achieve the multipath diversity gain. In particular, we combine pseudoinverse directions of arrival beamforming with pre-RAKE transmission to alleviate self-interference. The beamforming weights are adjusted within a downlink frame to compensate the predicted fading. We give measures of the prediction and beamforming performance and evaluate the impact of prediction errors on the downlink. Ray tracing simulations in a three-dimensional urban physical model show that the predictive downlink beamforming outperforms the conventional beamforming over Rayleigh-fading channels.     

End-to-end performance of transmission systems with relays over Rayleigh-fading channels

End-to-end performance of two-hop wireless communication systems with nonregenerative relays over flat Rayleigh-fading channels is presented. This is accomplished by deriving and applying some new closed-form expressions for the statistics of the harmonic mean of two independent exponential variates. It is shown that the presented results can either be exact or tight lower bounds on the performance of these systems depending on the choice of the relay gain. More specifically, average bit-error rate expressions for binary differential phase-shift keying, as well as outage probability formulas for noise limited systems are derived. Finally, comparisons between regenerative and nonregenerative systems are presented. Numerical results show that the former systems clearly outperform the latter ones for low average signal-to-noise-ratio (SNR). They also show that the two systems have similar performance at high average SNR.     

Optimal power allocation for relayed transmissions over Rayleigh-fading channels

Relayed transmission is a way to attain broader coverage by splitting the communication link from the source to the destination into several shorter links/hops. One of the main advantages of this communication technique is that it distributes the use of power throughout the hops. This implies longer battery life and lower interference introduced to the rest of the network. In this context, this paper investigates the optimal allocation of power over these links/hops for a given power budget. All hops are assumed to be subject to independent Rayleigh fading. Outage probability which is the probability that the link quality from source to destination falls below a certain threshold is used as the optimization criterion. Numerical results show that optimizing the allocation of power enhances the system performance, especially if the links are highly unbalanced in terms of their average fading power or if the number of hops is large. Interestingly, they also show that nonregenerative systems with optimum power allocation can outperform regenerative systems with no power optimization.     

Efficient video transmission over correlated Nakagami fadingchannels for IS-95 CDMA systems

This paper deals with the problem of efficient transmission of video signals over generalized fading channels in direct sequence-spread spectrum (DS-SS) code division multiple access (CDMA) systems. We first propose a modified version of the H.263 video codec incorporating a selective forward error correction (FEC) coding scheme combined with a forced intra-frame update mechanism. The modified codec results in the improvement of the average video and frame-to-frame performance. We further consider a coherent DS-CDMA system for the forward link (base-to-mobile) in both single-cell and multiple-cell environment. We provide performance evaluation results by both analysis, employing the Gaussian approximation, and computer simulations, using Monte Carlo error counting techniques. By integrating the proposed video codec with a coherent DS-CDMA system based upon the IS-95 standard, we investigate the performance of the video transmission over frequency-selective, correlated Nakagami fading forward-link channels employing a RAKE receiver. To simulate the fading channel, we have implemented in software a correlated Nakagami fading simulator based upon the principle of superposition of complex partial waves, an approach which replicates the wave propagation process in actual physical situations. A variety of performance evaluation results, both in single-cell and multiple-cell environment, are presented for a different number of resolving paths, cell user capacity, signal propagation characteristics, as well as for the presence of channel estimation errors. Heuristic explanations and interpretations of the trend of the obtained results are also given     

Cross-layer optimization for 3-D video transmission over cooperative relay systems

The rate budget constraint and the available instantaneous signal-to-noise ratio of the best relay selection in cooperative systems can dramatically impact the system performance and complexity of video applications, since they determine the video distortion. By taking into account these constrained factors, we first outline the signal model and formulate the system optimization problem. Next, we propose a new approach to cross-layer optimization for 3-D video transmission over cooperative relay systems. We propose procedures for estimation of the end-to-end instantaneous signal-to-noise ratio using an estimate of the available instantaneous signal-to-noise ratios between the source–destination, and source–relay–destination before starting to send the video signal to the best relay and destination. A novel approach using Lagrange multipliers is developed to solve the optimum bit allocation problem. Based on the rate budget constraint and the estimated the end-to-end instantaneous signal-to-noise ratio, the proposed joint source–channel coding (JSCC) algorithm simultaneously assigns source code rates for the application layer, the number of high and low priority packets for the network layer, and channel code rates for the physical layer based on criteria that maximize the quality of video, whilst minimizing the complexity of the system. Finally, we investigate the impact of the estimated the end-to-end instantaneous signal-to-noise ratio on the video system performance and complexity. Experimental results show that the proposed JSCC algorithm outperforms existing algorithms in terms of peak signal-to-noise ratio. Moreover, the proposed JSCC algorithm is found to be computationally more efficient since it can minimize the overall video distortion in a few iterations.     

Analysis of video transmission over lossy channels

A theoretical analysis of the overall mean squared error (MSE) in hybrid video coding is presented for the case of error prone transmission. Our model covers the complete transmission system including the rate-distortion performance of the video encoder, forward error correction, interleaving, and the effect of error concealment and interframe error propagation at the video decoder. The channel model used is a 2-state Markov model describing burst errors on the symbol level. Reed-Solomon codes are used for forward error correction. Extensive simulation results using an H.263 video codec are provided for verification. Using the model, the optimal tradeoff between INTRA and INTER coding as well as the optimal channel code rate can be determined for given channel parameters by minimizing the expected MSE at the decoder. The main focus of this paper is to show the accuracy of the derived analytical model and its applicability to the analysis and optimization of an entire video transmission system     

Rate control for robust video transmission over burst-errorwireless channels

We study the problem of rate control for transmission of video over burst-error wireless channels, i.e., channels such that errors tend to occur in clusters during fading periods. In particular we consider a scenario consisting of packet based transmission with automatic repeat request (ARQ) error control and a back channel. We start by showing how the delay constraints in real time video transmission can be translated into rate constraints at the encoder, where the applicable rate constraints at a given time depend on future channel rates. With the acknowledgments received through the back channel we have an estimate of the current channel state. This information, combined with an a priori model of the channel, allows us to statistically model the future channel rates. Thus the rate constraints at the encoder can be expressed in terms of the expected channel behavior. We can then formalize a rate distortion optimization problem, namely, that of assigning quantizers to each of the video blocks stored in the encoder buffer such that the quality of the received video is maximized. This requires that the rate constraints be included in the optimization, since violating a rate constraint is equivalent to violating a delay constraint and thus results in losing a video block. We formalize two possible approaches. The first one seeks to minimize the distortion for the expected rate constraints given the channel model and current observation. The second approach seeks to allocate bits so as to minimize the expected distortion for the given model. We use both dynamic programming and Lagrangian optimization approaches to solve these problems. Our simulation results demonstrate that both the video distortion at the decoder and packet loss rate can be significantly reduced when incorporating the channel information provided by the feedback channel and the a priori model into the rate control algorithm     

A joint source coding-power control approach for video transmission over CDMA networks

We consider future generation wireless code-division multiple-access (CDMA) cellular networks supporting heterogeneous compressed video traffic and investigate transport schemes for maximizing the number of users that can be supported in a single cell while simultaneously maximizing the reconstructed video quality of individual users. More specifically, we demonstrate that the network resources consumed by an individual user in a spread-spectrum CDMA network can be taken as the product of the allocated source-coding rate R/sub s/ and the energy per bit normalized to the multiple-access interference noise density /spl gamma//sub b/. We propose a joint source coding and power control (JSCPC) approach for allocating these two quantities to an individual user, subject to a constraint on the total available bandwidth, to simultaneously maximize the per-cell capacity while maximizing the quality of the delivered video to individual users. We demonstrate the efficacy of this approach using the ITU-T H.263+ video source coder, although the approach is generally applicable to other source-coding schemes as well. The results indicate a significant improvement in delivered quality-of-service (QoS), measured in terms of the end-user average peak signal-to-noise ratio, that can be achieved at a given level of network loading. Furthermore, we demonstrate that without an appropriate JSCPC strategy the traditional soft-capacity limit associated with CDMA networks is no longer present. Indeed, a precipitous decrease in performance can be expected with increasing load. We show that this behavior can be avoided with the proposed JSCPC approach, thereby significantly extending the useful capacity of the CDMA network while exhibiting a more graceful degradation pattern under increasing load.     

Transmit diversity in 3G CDMA systems

Transmit diversity (TD) is one of the key contributing technologies to defining the ITU endorsed 3G systems W-CDMA and cdma2000. Spatial diversity is introduced into the signal by transmitting through multiple antennas. The antennas are spaced far enough apart that the signals emanating from them can be assumed to undergo independent fading. In addition to diversity gain, antenna gain can also be incorporated through channel state feedback. This leads to the categorization of TD methods into open loop and closed loop methods. Several methods of transmit diversity in the forward link have been either under consideration or adopted for the various 3G standards. This article describes the concept of transmit diversity and explains the features of selected TD techniques     

An optimum power management scheme for wireless video service in CDMA systems

An optimum power management scheme is proposed for intra-frame refreshed image sequences of the wireless video service in code-division multiple-access (CDMA) systems. The end-to-end distortion of H.263 video data is first modeled when the error concealment schemes are employed. This distortion model takes into account the error propagation effects caused by the motion compensation. Then, based on the model, the target bit-error rates (BERs) of the image frames are optimized in such a way that the consumed power could be minimized under the constraint of maximum distortion. To satisfy the specified target BER requirement, an optimum power management scheme is proposed. Simulation results show that the proposed scheme considerably outperforms the conventional scheme in the sense of the decoded image quality. Additionally, the effects of the imperfect power control on the performance are also investigated.     

Guard sequence optimization for block transmission over linearfrequency-selective channels

We show that the optimum length-ν guard sequence for block transmission over a linear Gaussian-noise dispersive channel with memory ν is a linear combination of the N information symbols of the block. A closed-form expression for the optimum guard sequence is derived subject to a total average energy constraint on the information and guard symbols. The achievable channel block throughput with the optimum guard sequence is compared with that achievable with two common guard sequence types, namely zero stuffing and cyclic prefix     

Unequal error protection for MPEG-2 video transmission over wireless channels

This paper proposes an unequal error protection (UEP) method for MPEG-2 video transmission. Since the source and channel coders are normally concatenated, if the channel is noisy, more bits are allocated to channel coding and fewer to source coding. The situation is reversed when the channel conditions are more benign. Most of the joint source channel coding (JSCC) methods assume that the video source is subband coded, the bit error sensitivity of the source code can be modeled, and the bit allocations for different subband channels will be calculated. The UEP applied to different subbands is the rate compatible punctured convolution channel coder. However, the MPEG-2 coding is not a subband coding, the bit error sensitivity function for the coded video can no longer be applied. Here, we develop a different method to find the rate-distortion functions for JSCC of the MPEG-2 video. In the experiments, we show that the end-to-end distortion of our UEP method is smaller than the equal error protection method for the same total bit-rate.     

Distributed power control algorithms for asynchronous CDMA systems in frequency-selective fading channels

In Code Division Multiple Access (CDMA) radio environments, the maximum number of supportable users per cell is limited by multipath fading, shadowing, multiple access interference and near-far effects which cause fluctuations of the received power at the base station. In this context, power control and signal detection are essential to provide satisfactory Quality of Service (QoS) and to combat the near-far problem in CDMA systems. In this paper, we raised the uplink power control problem for a generalize asynchronous direct-sequence (DS) CDMA system that explicitly incorporate into the analysis: (1) the propagation delays in the network (generally neglected in the literature), (2) the adverse effect of multipath fading for wireless channels, and (3) the asynchronous transmissions in the uplink channels. This framework is used to propose a distributed power control strategy enhanced with linear multiuser receivers. It is shown that through a proper selection of an error function, the nonlinear coupling among active users is transformed into individual linear loops. A Linear-Quadratic-Gaussian (LQG) power control strategy is derived and compared with other approaches from the literature. Simulation results show that the uplink channel variations do not destroy the stability of these power control structures. However, delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. It is also shown that the use of multiuser detection at the base station can bring significant improvements to the performance of power control.     

Maximum-likelihood receiver of FFH/BFSK systems with multitonejamming and AWGN over Rayleigh-fading channels

A maximum-likelihood receiver for fast frequency-hopped binary frequency-shift-keying (FFH/BFSK) systems which operates in an additive white Gaussian noise (AWGN) environment and is subjected to multitone jamming over Rayleigh-fading channels is derived. Simulation results show that the proposed maximum-likelihood receiver outperforms existing receivers with the same channel and jamming conditions     

Robust VB 2D-CELP image transmission over CDMA Rayleigh fading channels

This paper describes a new error resilient analysis-by-synthesis coding scheme and its application for robust still image transmission over CDMA Rayleigh fading channels. For the purpose of providing reliable transmission, a type-I hybrid ARQ protocol is used in a concatenated Reed–Solomon/convolutional coding scheme. However, for delay-constrained applications, the protocol is used with a limited number of retransmissions in addition to limited RS interleaving. Results of coded images are provided for the uplink of a communication system, where a slowly Rayleigh fading channel is considered.     

Non-predictive multi-scaled based MLVQ video coding for robust transmission over mobile channels

A new non-predictive video codec for mobile applications is presented. The scheme omits the prediction step in the temporal axis and increases robustness of its transmission in the mobile channel. The same subbands from each frame in the group of frames (GOP) are joined together to exploit their spatial-temporal redundancies. The significant vectors of the joined subbands within the GOP are quantised using a novel multi-stage lattice vector quantisation. This process reduces quantisation errors and enhances the reconstructed frame quality. Experimental results are shown to be significantly better than H.263 and comparable to the current H.264 standards in an erroneous hilly terrain mobile environment using the TETRA channel simulator for some test video sequences.     

Rate-distortion optimized unequal loss protection for video transmission over packet erasure channels

Video transmission over networks often suffers from packet loss due to network congestions and stringent end-to-end delay constraints. In this paper, we develop a Rate-Distortion optimized Unequal Loss Protection (RD-ULP) scheme to combat packet loss. Based on packet-level transmission distortion modeling, we estimate the amount of contribution of each video packet to the reconstructed video quality, which defines the priority level of each packet. Unequal amounts of protection are then allocated to different video packets according to their priority levels and the dynamic channel conditions. The proposed RD-ULP resource allocation problem is formulated as a constrained nonlinear optimization problem. An optimization algorithm based on Particle Swarm Optimization (PSO) is then developed to solve the optimal resource allocation problem. Our extensive experimental results demonstrate the effectiveness of the proposed RD-ULP scheme, which outperforms existing methods by up to 2 dB in the reconstructed video quality.     

Optimal resource allocation for Medium Grain Scalable video transmission over MIMO channels

In this paper we investigate an optimal solution for adaptive H.264/SVC video transmission over Multiple-Input Multiple-Output (MIMO) channels.We first write the end-to-end distortion of the H.264/SVC video transmission over a diagonal MIMO channel. The total distortion is expressed following three physical layer parameters: power allocation, modulation spectral efficiency and Error Code Correction (ECC) code rate. Minimizing the total distortion is considered as an optimization problem containing both discrete and continuous variables.We use the Lagrangian method associated with Karush–Kuhn and Tucker conditions to find out the optimal continuous physical layer parameters. Concerting the discrete modulation spectral efficiency and ECC code rate, we exploit information of the MIMO system to remove all suboptimal configurations. Therefore, the optimal power allocation is computed only for a reduced number of discrete configurations.The performance of the proposed solution is evaluated over both statistical and realistic MIMO channels. Results show that the proposed solution performs an optimal resource allocation to achieve the best QoS regardless the channel conditions.     

A rate adaptation transcoding scheme for real-time video transmission over wireless channels

In this work, we investigate the problem of bit-rate adaptation transcoding for transmitting video over burst-error wireless channels, i.e., channels such that errors tend to occur in clusters during fading periods. In particular, we consider a scenario consisting of packet-based transmission with an Automatic Repeat reQuest (ARQ) error control and a feedback channel. With the acknowledgements received through the feedback channel and a statistical channel model, we have an estimate of the current channel state, and effective channel bandwidth. In this paper, we analyze the buffering implications of inserting a video transcoder at the wireless access point with the variable bit-rate channel as the target. We derive the conditions that both the source encoder and transcoder buffers have to meet for preventing the end decoder buffer from underflowing. Furthermore, we propose a bit-rate adaptation algorithm for VBR transcoders used in the wireless access point. Our experimental results demonstrate that the proposed algorithm can accurately control the bit-rate of the transcoded video stream and reduce the number of frames been skipped without violating the end-to-end delay requirement.