Chandranath R. N. Athaudage

Performance Analysis of Dual-Hop OFDM Relay Systems with Subcarrier Mapping

In multihop OFDM relay systems the end-to-end average capacity can be increased by incorporating subcarrier mapping (SCM) at the relay nodes. In this paper, we propose an exact analytical technique of evaluating the average capacity of a dual-hop OFDM relay system with SCM in a Rayleigh fading channel. Closed-form expressions are derived for the probability density function of the end-to-end SNR of mapped subcarrier pairs. Comparison with simulation results confirms the accuracy of the proposed analytical technique. Also, the results show that an average capacity increase of the order of 10%-30% can be achieved using SCM in the low SNR regime. Moreover, the achievable percentage capacity increase with SCM is more when the average transmit power of the relay is less than that of the source.

Adaptive OFDM synchronization algorithms based on discrete stochastic approximation

This paper presents discrete stochastic approximation algorithms (DSA) for time synchronization in orthogonal frequency division multiplexing (OFDM) systems. It is shown that the discrete stochastic approximation algorithms can be effectively used to achieve a significant reduction in computational complexity compared to brute force maximum-likelihood (ML) methods for OFDM synchronization. The most important property of the proposed algorithms is their recursive self-learning capability-most of the computational effort is spent at the global or a local optimizer of the objective function. The convergence of the algorithms is analyzed. An adaptive version of the discrete stochastic approximation algorithm is also presented for tracking time-varying time delays and frequency offsets in time-selective fading channels. Detailed numerical examples illustrate the performance gains of these DSA-based synchronization algorithms.

Low-complexity channel estimation for wireless OFDM systems

Wireless communication systems incorporating coherent OFDM requires the estimation and tracking of the time-varying channel response for accurate demodulation of data at the receiver. In pilot-symbol-assisted (PSA) OFDM systems, the minimum mean-square-error (MMSE) estimator is a good channel estimation technique given that channel statistics are known. However, the major drawback of the MMSE estimator is its high computational complexity, which grows with number of pilots. Piecewise-linear interpolation is a low complexity solution to the channel estimation task. In this paper we derive exact expressions for the channel estimation error as a function of pilot spacing, channel statistics and channel noise, when piecewise-linear interpolation is incorporated. We compare the performance of piecewise-linear interpolation against that of a low complexity MMSE technique (piecewise-MMSE interpolation) under both matched and mismatched conditions of channel statistics. Results show that a piecewise-linear interpolator performs better than a mismatched piecewise-MSSE interpolator of same complexity.

Fairness Based Resource Allocation for Multi-User MIMO-OFDM Systems

This paper investigates the performance of a weighted proportional fair (WPF) based resource allocation algorithm for future multi-user (MU) multiple-input multiple-output (MIMO) orthogonal frequency-division multiple-access (OFDMA) systems with frequency-selective broadband channels. In future networks algorithms should be designed to allocate the systems resources (time, frequency, space and power) in a manner that exploits multi-user diversity and ensures each user is treated fairly. The proposed algorithm extends the original proportional fair (PF) scheme, which was designed for single-antenna single-carrier systems, to the MIMO-OFDMA downlink such that individual users quality of service (QoS) requirements are satisfied in terms of requested bit-rate and bit error rate (BER). Monte Carlo simulation results indicates that extending the algorithm to a frequency- and space-division network produces the desirable characteristics of a fair and efficient algorithm

A low complexity timing and frequency synchronization algorithm for OFDM systems

The paper presents a low complexity discrete stochastic approximation algorithm for time and frequency synchronization in OFDM systems. The proposed technique can track the conditions of a slowly time varying channel where synchronization parameters, namely the symbol timing and frequency offset, vary slowly with time. The most important property of the proposed algorithm is its self-learning capability - it spends most of the computational effort at the global minimizer of the objective function. In particular, we show that the algorithm achieves an /spl epsiv/=(1-/spl rho/)/(1+/spl rho/) reduction in computational cost (in terms of complex multiplications to be performed), where /spl rho/=N/sub cp//N is the ratio between cyclic prefix length (N/sub cp/) and number of subcarriers (N) of the OFDM system (e.g. N=512 and N/sub cp/=64 gives /spl epsiv/=78%). Numerical examples illustrate the synchronization accuracy of the proposed technique in terms of symbol timing and frequency offset estimation errors.

Probability of error of space-time coded OFDM systems with frequency offset in frequency-selective Rayleigh fading channels

In this paper, we analyze the performance of space-time coded (ST-coded) orthogonal frequency division multiplexing (OFDM) systems with carrier frequency offset (CFO) in a frequency-selective Rayleigh fading channel. Closed-form analytical expressions are derived for the symbol error probability (SEP) for M-PSK and M-QAM modulation schemes. The SEP expressions derived are valid and exact for OFDM systems with highly frequency-selective wireless channels where the subcarrier channel responses are i.i.d. with Rayleigh fading. Also, for the case where imperfect channel knowledge is used for space-time decoding, we derive expressions for constellation phase-rotation and post-equalized SINR (signal-to-interference-and-noise ratio) degradation due to CFO. The numerical results demonstrate the sensitivity of the receiver error performance to CFO in ST-coded OFDM systems.

Enhanced frequency synchronization for OFDM systems using timing error feedback compensation

A strategy of improving the frequency synchronization performance of the conventional maximum-likelihood (ML) algorithm for OFDM systems is proposed. In the conventional ML algorithm, the accuracy of the frequency offset estimation directly depends on the block timing estimation. In the proposed algorithm, the block timing error caused by the ML algorithm is estimated at the post-FFT stage of the OFDM receiver using pilot symbols. An improved frequency offset estimation is obtained from the ML algorithm with timing error feedback compensation. Simulation results show that the proposed algorithm can: (i) eliminate the error floor caused by the ML algorithm and (ii) achieve normalized frequency synchronization errors as low as 1 ppm (< 10/sup -6/) for both AWGN and multipath channels with high SNR.

On Power Allocation for Dual-Hop Amplify-and-Forward OFDM Relay Systems

The solutions to an optimal power allocation problem in dual-hop amplify-and-forward (non-regenerative) OFDM relay systems over given channel gains are provided with a joint transmit power constraint. We analytically derive the allocated power to each hop/subcarrier that satisfies the Karush-Kuhn-Tucker (KKT) conditions by solving simultaneous equations. The solutions imply that at most 3N - 1 possible power allocation candidates could exist on both source and relay nodes satisfying KKT conditions when N is the number of subcarriers. Based on the solution, a sub-optimal power allocation method that selects only significant allocated power in strong subcarriers is proposed and achieves higher capacity (achievable rate) than conventional sub-optimal methods. As for the feasibility, the number of candidates to be evaluated is N in the proposed method.

Model-based speech signal coding using optimized temporal decomposition for storage and broadcasting applications

A dynamic programming-based optimization strategy for a temporal decomposition (TD) model of speech and its application to low-rate speech coding in storage and broadcasting is presented. In previous work with the spectral stability-based event localizing (SBEL) TD algorithm, the event localization was performed based on a spectral stability criterion. Although this approach gave reasonably good results, there was no assurance on the optimality of the event locations. In the present work, we have optimized the event localizing task using a dynamic programming-based optimization strategy. Simulation results show that an improved TD model accuracy can be achieved. A methodology of incorporating the optimized TD algorithm within the standard MELP speech coder for the efficient compression of speech spectral information is also presented. The performance evaluation results revealed that the proposed speech coding scheme achieves 50%–60% compression of speech spectral information with negligible degradation in the decoded speech quality.

On the PAR reduction in OFDM signals using multiple signal representation

Multiple signal representation (MSR) techniques have been used to reduce the high peak-to-average power ratios (PAR) of orthogonal frequency division multiplexing (OFDM) signals. These includes partial transmit sequences (PTS), selected mapping (SLM), selective scrambling, and interleaving. All MSR techniques often improve the PAR statistics and are iterative in nature. The PAR reduction obtainable depends on the number of iterations performed, which also increases the complexity of the OFDM transmitter. However, a means to estimate the achievable PAR reduction for a given number of iterations has not been reported in the literature so far. This paper derives a bound on the achievable PAR when a MSR technique with a given complexity is used. Our analytical results show a clear asymptotic behavior of the PAR as the number of iterations is increased. Simulation results justify the significance and accuracy of the PAR bound derived.