Yogananda Isukapalli

An Analytically Tractable Approximation for the Gaussian Q-Function

In this letter we propose an approximation for the Gaussian Q-function that enables simpler evaluation of important communication system performance metrics. The approximation enables derivation of closed-form expressions for metrics such as average symbol, bit and block error probabilities which are known to be analytically involved as they require computation of the expectation of Q-function and its integer powers, for any m of Nakagami-m fading. The tightness of the approximation is verified by simulations. The usefulness of the approximation is demonstrated by obtaining a simple closed form expression for the average symbol error probability of differentially encoded QPSK in Nakagami-m fading.

Finite Rate Feedback for Spatially and Temporally Correlated MISO Channels in the Presence of Estimation Errors and Feedback Delay

In this paper, the problem of finite-rate feedback for spatially and temporally correlated Rayleigh fading multiple input single output (MISO) channels with estimation errors at the receiver and feedback delay is addressed. A model that captures estimation errors, feedback delay, and finite-rate quantization of the channel is developed. A novel codebook design algorithm that minimizes the loss in ergodic capacity is proposed. Simulation results show that the new codebook designed under the consideration of estimation errors and feedback delay outperforms the codebook designed assuming ideal conditions. Analysis for the loss in ergodic capacity for spatially i.i.d channels with channel estimation errors and delay (EED) is presented and validated through simulations.

Performance analysis of transmit beamforming for MISO systems with imperfect feedback

In this paper, we analyze the performance of transmit beamforming on multiple-antenna Rayleigh fading channels with imperfect channel feedback. We characterize the feedback imperfections in terms of noisy channel estimation, feedback delay, and finite-rate channel quantization. We develop a general framework, that is valid for an arbitrary two-dimensional linear modulation, to capture the aforementioned imperfections and derive the symbol and bit error probability expressions for both M-PSK and M-ary rectangular QAM constellations with Gray code mapping. We show that the proposed analytical formulation is valid for a frequency-domain duplexing system with/without finite-rate channel quantization and a time-domain duplexing system. We validate the accuracy of the analysis through simulations, and assess the relative effects of channel estimation inaccuracy, feedback delay, and finite-rate quantization on the symbol and bit error performances for various constellations.

Packet Error Probability of a Transmit Beamforming System With Imperfect Feedback

Average packet error probability (PEP) is an important error statistic for wireless communication system designers. In this paper, we address the problem of analytically quantifying the effect of channel estimation errors, feedback delay and channel vector quantization on the PEP of transmit beamforming multiple-input-single-output (MISO) systems in a spatially independent slow-fading wireless channel environment. We develop an accurate characterization of estimation errors as well as errors due to feedback delay, and tools relevant for deriving analytical expressions for the PEP. The modeling highlights the distinction between errors that arise due to channel estimation from those that arise due to feedback delay and represents an important departure from past work. Analytical expressions are derived for the PEP with BPSK signaling. The derived approximated closed-form analytical expression is complemented by simulations.

Use of the Newton Method for Blind Adaptive Equalization Based on the Constant Modulus Algorithm

We study the applicability of the second-order Newton gradient descent method for blind equalization of complex signals based on the constant modulus algorithm (CMA). The constant modulus (CM) loss function is real with complex valued arguments, and, hence, nonanalytic. We, therefore, use the framework of the Wirtinger calculus to derive a useful and insightful form of the Hessian for noiseless FIR channels and rederive the known fact that the full Hessian of the CM loss function is always singular in a simpler manner. For the implementation of a suboptimum version of Newton algorithm, we give the conditions under which the leading partial Hessian is nonsingular for a noiseless FIR channel model. For this channel model, we show that the perfectly equalizing solutions are stationary points of the CM loss function and also evaluate the leading partial Hessian and the full Hessian at a perfectly equalizing solution. We also discuss regularization of the full Newton method. Finally, some simulation results are given.

Performance Analysis of Finite Rate Feedback MISO Systems in the Presence of Estimation Errors and Delay

In this paper, we analyze the performance of transmit beamforming on multiple input single output (MISO) Rayleigh fading channels with imperfect channel feedback. We characterize the feedback imperfections in terms of noisy channel estimation, feedback delay, and finite rate channel quantization. We develop a novel model that captures the above mentioned three forms of channel imperfections with particular emphasis on modeling the impact of delay more precisely. As opposed to some previous attempts at combining the three forms of imperfection, this modeling approach shows that feedback delay is of a different nature than channel estimation errors. The performance criteria considered is the average symbol error probability of M1 times M2 rectangular QAM constellation. We also present simulation results in support of the derived analytical expressions.

Average SEP and BEP Analysis of Transmit Beamforming for MISO Systems with Imperfect Feedback and M-PSK Constellation

We analyze the performance of transmit beamforming on multiple-antenna Rayleigh fading channels with imperfect channel feedback. The feedback imperfections are characterized by noisy channel estimation, feedback delay, and finite rate channel quantization. We develop a general framework, valid for an arbitrary two-dimensional linear modulation, that captures the aforementioned imperfections, and derive the average symbol and bit error probability expressions for M- PSK constellation. We show that the proposed analytical formulation is valid for frequency-domain duplexing system with/without finite rate channel quantization and time-domain duplexing system. The simulation results show accurate agreement with the analytical expressions.

Analyzing the Effect of Channel Estimation Errors on the Average Block Error Probability of a MISO Transmit Beamforming System

We address the problem of analyzing the effect of channel estimation errors on the average block error probability (BLEP) of transmit beamforming multiple input single output (MISO) system in a slowly varying Rayleigh fading wireless channel. For this purpose, we develop an accurate model of estimation errors in a block fading context and derive an analytical expression quantifying the impact of channel estimation errors on feedback MISO systems. In particular, the block fading model implies that the channel estimate and its error component are fixed for the entire block and an appropriate performance criteria is the average BLEP, a more complex metric to analyze. The derived closed form analytical expression for the average BLEP is validated by simulations.

Ergodicity of Wireless Channels and Temporal Prediction

In this paper, we study the role of ergodicity in wireless channel prediction. Following the sinusoidal channel model, conditions under which the ergodic assumption is valid are presented. This sheds insight into when statistical channel models that employ ensemble averaging are appropriate. Due to the lack of ergodicity in a typical real world wireless channel, least squares prediction, an approach based on time averages is motivated as opposed to linear minimum mean squared error channel prediction, an approach based on ensemble averaging. We then study methods such as forward-backward and rank reduction for high quality channel prediction.

Exploiting the nature of extrinsic information in iterative decoding

Histograms of extrinsic information are used to explain the performance gain of standard turbo codes with number of iterations. The simulation results presented clearly show the Gaussian nature of the extrinsic information and they also establish a heuristic link with performance gain. Based on the histogram plots, a modified form of turbo encoding and decoding structure is proposed. The encoder is formed with three parallelly concatenated recursive systematic convolutional (RSC) encoding blocks separated by two random interleavers, subsequently a heuristic method is proposed about the way the extrinsic information is fed to the three blocks in the decoder. A new scaling of extrinsic information is proposed based on the observations made on new structure as well as performance gain in standard turbo structure.