Transmit power optimization for video transmission over slowly-varying Rayleigh-fading channels in CDMA systems

An optimum transmit power management scheme is proposed for wireless video service in code-division multiple-access systems. The scheme minimizes the transmit power subject to the constraint on the distortion resulted from the loss of each packet and the error propagation effect caused by the motion compensation. It implicitly controls the target bit-error rate (BER) of the video packet according to the importance of the packet. The more important the packet is, the more securely it is transmitted over noisy channel. Furthermore, the scheme adjusts the target BER as if the water filling concept is applied to the problem of minimizing the transmit power. The simulation results show that the proposed scheme considerably outperforms the conventional schemes.     

Harmonic mean and end-to-end performance of transmission systems with relays

Closed-form expressions for the statistics of the harmonic mean of two independent and identically distributed gamma variates are presented. The probability density function of the harmonic mean of two F variates is also derived. These statistical results are then applied to study the performance of wireless communication systems with nonregenerative relays over flat Nakagami fading channels. More specifically, outage probability formulas for noise-limited systems as well as systems affected by interference are obtained. Furthermore, general expressions for average bit-error rates are also 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 ratios (SNRs). They also show that the two systems have similar performance at high average SNRs.     

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     

Another look at the BER performance of FFH/BFSK with productcombining over partial-band jammed Rayleigh-fading channels

Product combining (PC) is a well-known diversity technique that effectively combat the effect of partial-band jamming (PBJ) affecting fast-frequency hopping/frequency-shift-keying (FFH/FSK) systems. We present two approaches for the average bit error rate (BER) evaluation of FFH/FSK systems with PC over Rayleigh channels subject to PBJ. We first rely on the fact that the decision statistic at the output of PC receivers can be viewed as a product of F-variates to obtain the average BER in the form of a rapidly converging infinite series, which can be readily evaluated numerically for cases of practical interest. We then rely on the theory of H function random variables to present a second approach for the average BER evaluation of PC over partial-band jammed Rayleigh-fading channels. The final closed-form formula is expressed in terms of the Meijer's G function, which can be easily evaluated using common mathematical software for small values of the diversity order. Based on this result, we also develop another infinite series representation for the average BER. Numerical experiments show that the latter series representation offers a speed-up factor in evaluating the average BER for high values of the diversity order. The mathematical formalism is illustrated by numerical examples showing the effect of various parameters on the performance of the system     

Performance of Alamouti transmit diversity over time-varying Rayleigh-fading channels

We analyze the impact of a time-varying Rayleigh-fading channel on the performance of an Alamouti transmit-diversity scheme. We propose several optimal and suboptimal detection strategies for mitigating the effects of a time-varying channel, and derive expressions for their bit-error probability as a function of the channel correlation coefficient /spl rho/. We find that the maximum-likelihood detector that optimally compensates for the time-varying channel is very tolerant to time-varying fading, attaining full diversity order even for the extreme case of /spl rho/=0. In contrast, although lower in complexity, the suboptimal schemes suffer a diversity penalty and are thus suitable only for slowly fading channels.     

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.     

A simple DPSK receiver with improved asymptotic performance overfast Rayleigh-fading channels

In this letter we present a simple receiver for differential phase shift keying (DPSK) signals transmitted over fast, frequency flat, fading channels, which is characterized by a very low asymptotic error probability. The system performance analysis is carried out for a 2-DPSK signal     

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.     

Error probability minimizing pilots for OFDM with M-PSK modulation over Rayleigh-fading channels

Orthogonal frequency division multiplexing (OFDM) with pilot symbol assisted channel estimation is a promising technique for high rate transmissions over wireless frequency-selective fading channels. In this paper, we analyze the symbol error rate (SER) performance of OFDM with M-ary phase-shift keying (M-PSK) modulation over Rayleigh-fading channels, in the presence of channel estimation errors. Both least-squares error (LSE) and minimum mean-square error (MMSE) channel estimators are considered. For prescribed power, our analysis not only yields exact SER formulas, but also quantifies the performance loss due to channel estimation errors. We also optimize the number of pilot symbols, the placement of pilot symbols, and the power allocation between pilot and information symbols, to minimize this loss, and thereby minimize SER. Simulations corroborate our SER performance analysis, and numerical results are presented to illustrate our optimal claims.     

Power allocation in cognitive radio networks over Rayleigh-fading channels with hybrid intelligent algorithms

In this paper, we introduce a hybrid strategy which combines pattern search (PS) optimization and genetic algorithm (GA) to address the problem of power allocation in cognitive radio networks. Considering the fluctuating interference thresholds in cognitive networks, an approach for promoting the coexistence of licensed users and cognitive users is designed. Secondly, based on the analysis of transmission outage probability, a corresponding objective function with regard to the power allocation over Rayleigh fading channels is obtained. It is a difficult task to obtain this objective function directly by using traditional methods, such as common mathematical deduction or linear programming, due to the nonlinearity and complexity of the underlying optimization problem. Inspired by the concept of intelligent algorithms, we employ the scheme of combining PS optimization and GA method, which are both efficient intelligent algorithms to address this challenge. The advantage of this hybrid strategy is that it can overcome the instability problem of GA as well as the local convergency problem of PS method. Thus, the hybrid intelligent method can attain a global and steady outcome. We improve the performance of power allocation strategy with an acceptable increase in computation overhead. The numerical results are encouraging and show that the proposed approach is worthy of consideration in achieving complicated power optimization. Hence, we achieve steady and rational outcomes by applying the proposed hybrid strategy when traditional method is to be ineffective in addressing the nonlinear objective.     

Optimum detection over Rayleigh-fading, dispersive channels, withnon-Gaussian noise

The paper discusses the problem of detecting one out of M Gaussian correlated signals in impulsive noise, modeled as a compound-Gaussian, possibly correlated process. We first show that, for uncorrelated noise, the detection problem admits one and the same optimum-in the sense of attaining minimum error probability-solution, independent of the noise statistics: the optimum detector, in fact, amounts to an estimation block, aimed at measuring the short-time noise power spectral density (PSD), whose output is fed to a bank of estimators-correlators, each keyed to one of the M admissible waveforms and to the estimated noise PSD. We also give suggestions for realizing a suboptimum receiver, with reduced complexity, which again is canonical in its structure. As for the performance analysis, we focus on binary frequency-shift keying (BFSK) signaling: we provide numerical results for two particular channel correlations and for Cauchy-distributed noise. These results indicate that, like for the general Gauss-Gauss case, the performance depends on the energy contrast, as well as on the “time-bandwidth” product of the useful signal. Moreover, noise spikiness seems to negatively affect the performance, in the sense that heavier and heavier high-amplitude tails of the noise marginal distribution give rise to higher and higher error probabilities for fixed energy contrast and time-bandwidth signal product     

Performance analysis of three-branch selection combining over arbitrarily correlated Rayleigh-fading channels

The recent literature has thoroughly treated two-branch selection combining (SC) over correlated Rayleigh fading and three-branch SC over exponentially correlated Rayleigh fading. However, a long-standing open problem involves the three-branch SC performance over arbitrarily correlated Rayleigh fading. We solve this problem completely by deriving new infinite series expressions for the cumulative distribution function, the probability density function, and the moment generating function (mgf) of the three-branch SC output signal-to-noise ratio (SNR). The output mgf can be used to derive the average symbol-error rate for any two-dimensional digital modulations. The outage probability and the higher moments of the SC output SNR are also derived. These analytical results are canonical, in that the three-branch SC performance is now completely solved for arbitrary correlation. Some previous results are shown to be special cases of our new results.     

Cooperative diversity performance for OFDM transmission over partially jammed channels

Cooperative diversity is a transmission technique, where multiple terminals share their resources to form a virtual antenna array that realizes spatial diversity gain in a distributed fashion. In this paper, we focus on a performance evaluation for orthogonal frequency division multiplexing (OFDM) transmission in cooperative networks under partial-band jamming (PBJ) environments. We present a bit error rate (BER) analysis for a cooperative diversity system with amplifying-and-forward (AF) relays over partially jammed Rayleigh fading channels. In addition, a simple jamming mitigation technique, called relay-based sub-band shifting method, is proposed. Through this approach, each sub-band of the amplified OFDM symbol at the relay can be changed by the predefined shifting rule of each relay, and the jamming effects at the destination are partially removed. Simulation results show that the proposed method improves significantly the BER performance at a low signal-to-jamming ratio.     

Improved approximate maximum-likelihood receiver for differential space-time block codes over Rayleigh-fading channels

In this paper, an approximate maximum-likelihood (ML) receiver for differential space-time block codes is investigated. The receiver is derived from the ML criterion and is shown to mitigate error floor occurring in a conventional differential receiver very well. Because the receiver employs knowledges of signal-to-noise ratio (SNR) and fading rate, we study mismatched cases when these parameters are not accurate. It is shown that the receiver is more sensitive to the mismatched parameters when the fading rate is high. Then, a union bound on the bit error probability is derived. The bounds show good agreement with the simulation results at high fading rate and at high SNR. Finally, a modified receiver, denoted as multistage receiver, is proposed to compensate the so-called intrablock interference caused by the time-varying characteristic of the channel within a transmission block. The multistage receiver offers further reduction of error floor of about half order of magnitude as compared with an approximate ML receiver.     

Identification plus transmission over channels with perfectfeedback

We determine the region of all identification and transmission rate-pairs achievable over a discrete memoryless channel (DMC) with perfect and instantaneous feedback, for both randomized and deterministic encoding. As a by-product, we also have a new proof of Kemperman's (1973) strong converse to Shannon's coding theorem for DMC's with feedback     

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.     

Outage performance of cellular systems over arbitrary lognormalshadowed Rician channels

Techniques for analysing the outage performance for lognormal shadowed Rician fading channels are becoming of increasing importance as the concept of micro-cells is introduced into wireless personal communications. A general solution, however, has not yet been developed except for some special cases. The authors present an exact solution which takes the form of two-fold integrals, and is applicable to general situations without any restrictions     

Capacity maximizing MMSE-optimal pilots for wireless OFDM over frequency-selective block Rayleigh-fading channels

The location, number, and power of pilot symbols embedded in multicarrier block transmissions over rapidly fading channels, are important design parameters affecting not only channel estimation performance, but also channel capacity. Considering orthogonal frequency-division multiplexing (OFDM) systems with decoupled information-bearing symbols from pilot symbols transmitted over wireless frequency-selective Rayleigh-fading channels, we show that equispaced and equipowered pilot symbols are optimal in terms of minimizing the mean-square channel estimation error. We also design the number of pilots, and the power distributed between information bearing and pilot symbols, using as criterion a lower bound on the average capacity. Numerical results corroborate our theoretical findings.