C.R.N. Athaudage

University of Melbourne, Melbourne, Victoria, Australia

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Publications (41)14.91 Total impact

  • Shuhei Haraguchi · Masato Saito · C.R.N. Athaudage · Minoru Okada
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    ABSTRACT: We evaluate a dual-hop OFDM relay system with non-regenerative relay without FFT/IFFT processes at the relay node to reduce the processing delay and the complexity of the relay process in terms of the achievable rate and throughput. To maximize the achievable rate, we propose a power allocation scheme to each subcarrier of the source node and the whole subcarrier of the relay node. Due to the flexible power allocation at the source node, the proposed system can attain a higher achievable rate by the method in which more power is allocated to a weaker channel and more power is allocated to the source node when the second hop has stronger channel gains. When OFDM symbol duration is relatively large, the throughput performances that consider the delay due to OFDM signal buffering at the FFT-process and propagation delay reveal that the proposed power allocation scheme outperforms a dual-hop OFDM relay system with subcarrier mapping and sub-optimal power allocation scheme.
    No preview · Conference Paper · Oct 2009
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    Ming Zhang · Thushara D. Abhayapala · Dhammika Jayalath · David Smith · Chandranath R. N. Athaudage
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    ABSTRACT: This paper presents a novel matched rotation precoding (MRP) scheme to design a rate one space-frequency block code (SFBC) and a multirate SFBC for MIMO-OFDM systems with limited feedback. The proposed rate one MRP and multirate MRP can always achieve full transmit diversity and optimal system performance for arbitrary number of antennas, subcarrier intervals, and subcarrier groupings, with limited channel knowledge required by the transmit antennas. The optimization process of the rate one MRP is simple and easily visualized so that the optimal rotation angle can be derived explicitly, or even intuitively for some cases. The multirate MRP has a complex optimization process, but it has a better spectral efficiency and provides a relatively smooth balance between system performance and transmission rate. Simulations show that the proposed SFBC with MRP can overcome the diversity loss for specific propagation scenarios, always improve the system performance, and demonstrate flexible performance with large performance gain. Therefore the proposed SFBCs with MRP demonstrate flexibility and feasibility so that it is more suitable for a practical MIMO-OFDM system with dynamic parameters.
    Preview · Article · Feb 2009 · Journal on Advances in Signal Processing
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    Masato Saito · C.R.N. Athaudage · Jamie Evans
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    ABSTRACT: 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 3<sup>N</sup> - 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.
    Full-text · Conference Paper · Jan 2009
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    C.R.N. Athaudage · M. Saito · J. Evans
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    ABSTRACT: In this paper, we analyze the capacity of orthogonal frequency division multiplexing (OFDM) systems with carrier frequency offset (CFO) in frequency-selective Nakagami-m fading channels. Previous work on this topic has not taken into account the frequency selectivity of the channel. In this work, we have explicitly attributed the effect of channel frequency selectivity, i.e. frequency domain correlations, in evaluating the OFDM system performance in the presence of CFO. A closed-form expression is derived of the probability density function (PDF) of the signal-to-interference-and-noise ratio (SINR) in terms of CFO and channel correlation vector. Capacity is evaluated using numerical integration. The frequency-flat fading scenario and the perfectly frequency-selective fading (uncorrelated subcarriers) scenario form the two extremes, i.e. bounds, of the achievable OFDM capacity in the presence of CFO in Nakagami-m fading channels.
    Full-text · Conference Paper · Oct 2008
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    C.R.N. Athaudage · M. Saito · J. Evans
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    ABSTRACT: 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.
    Full-text · Conference Paper · Jun 2008
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    C.R.N. Athaudage · M. Zhang · A.D.S. Jayalath · T.D. Abhayapala
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    ABSTRACT: In this paper, we propose a low complexity detection scheme for MIMO systems incorporating spatial multiplexing. Optimal detection schemes such as maximum-likelihood (ML) detection of MIMO signals demands computational resources that are beyond the capabilities of most practical systems. Alternative reduced complexity MIMO detection techniques have been proposed, but the complexity of algorithmic schemes are in general much higher than that of the equalizer-based techniques, e.g. zero-forcing (ZF) or MMSE. On the other hand, equalizer- based techniques perform relatively poor in terms of error rate. In this paper, we propose a hybrid of an equalizer-based technique and an algorithmic search stage. Based on an error matrix and its probability density functions for different classes of error, a particular search region is selected for the algorithmic stage. As the probability of occurrence of error classes with larger search regions is small, overall complexity of the proposed technique remains low while providing a significant improvement in the error performance.
    Preview · Conference Paper · Jan 2008
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    Chandranath R. N. Athaudage · Min Zhang · Dhammika Jayalath · Thushara D. Abhayapala
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    ABSTRACT: Abstract—In this paper, we propose a low complexity detection scheme for MIMO systems incorporating spatial multiplexing. Optimal detection schemes such as maximum-likelihood (ML) detection of MIMO signals demands computational resources that are beyond the capabilities of most practical systems. Alternative reduced complexity MIMO detection techniques have been proposed, but the complexity of algorithmic schemes are in general much higher than that of the equalizer-based techniques, e.g. zero-forcing (ZF) or MMSE. On the other hand, equalizerbased techniques perform relatively poor in terms of error rate. In this paper, we propose a hybrid of an equalizer-based technique and an algorithmic search stage. Based on an error matric and its probability density functions for different classes of error, a particular search region is selected for the algorithmic stage. As the probability of occurrence of error classes with larger search regions is small, overall complexity of the proposed technique remains low while providing a significant improvement in the error performance.
    Preview · Article · Jan 2008
  • C.R.N. Athaudage · K. Sathananthan
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    ABSTRACT: In this paper, we analyze the lower bound on the error performance of space-time coded (Alamouti-coded) orthogonal frequency division multiplexing (OFDM) systems with carrier frequency offset (CFO) in a Rayleigh fading channel. It is shown that the frequency-flat fading causes the maximum correlation between the signal and the inter-carrier-interference (ICI) noise power. This channel scenario provides the lower bound on error performance. A semi-analytical procedure of calculating the symbol error probability (SEP) lower bound based on post-equalized signal-to-interference-and-noise ratio (SINR) distribution is provided. This procedure covers both M-PSK and M-QAM modulation schemes. The numerical results demonstrate the accuracy of the expressions derived for SEP lower bound
    No preview · Conference Paper · Oct 2006
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    Chandranath R. N. Athaudage · Jian Wang · A. Dhammika S. Jayalath
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    ABSTRACT: We propose adaptive subcarrier selection (ACS) as an efficient technique of improving the error performance of grouped linear constellation precoding (GLCP) OFDM in the low-to-medium SNR range (0-20 dB). Recently, GLCP-OFDM has been proposed to achieve frequency diversity (multipath diversity) in OFDM overcoming the problem of channel nulls (deep fades in frequency-domain). In GLCP-OFDM, symbol precoding is performed on subgroups of OFDM subcarriers rather than on the total number of OFDM subcarriers which is large in practice. This reduces the receiver decoding complexity while maintaining the high SNR frequency diversity benefit - though the error performance in the low SNR regime is worse than a non-precoded system. In contrast, the proposed GLCP-ACS-OFDM improves the error performance in the low-to-medium SNR range without increasing the receiver complexity. Simulation results demonstrate the superior performance of GLCP-ACS-OFDM over GLCP-OFDM for a practical range of SNR as applicable to mobile wireless communications
    Preview · Conference Paper · Sep 2006
  • P.D. Morris · C.R.N. Athaudage
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    ABSTRACT: 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 system's 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 user's 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
    No preview · Conference Paper · Jun 2006
  • C.R.N. Athaudage · R.R.V. Angiras
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    ABSTRACT: A detailed analysis of the performance of FFT-equalised (FFT-EQ) zero-padded OFDM (ZP-OFDM) systems in the presence of time and frequency synchronisation errors is given. In order to achieve low-complexity in equalisation, FFT-based demodulation and per subcarrier channel equalisation is considered for ZP-OFDM. It is shown that, unlike in cyclic-prefix OFDM (CP-OFDM), FFT-equalised ZP-OFDM is sensitive to time synchronisation error causing ICI. An exact expression for ICI is derived in terms of time and frequency synchronisation errors for both AWGN and frequency-selective Rayleigh fading channels. Moreover, based on the Gaussian approximation for the ICI noise, expressions are derived for the symbol-error-probability (SEP) of a FFT-EQ-ZP-OFDM system with time and frequency synchronisation errors in both AWGN and frequency-selective Rayleigh fading channels. Numerical results demonstrate the sensitivity of FFT-EQ-ZP-OFDM systems to synchronisation errors.
    No preview · Article · Jan 2006 · IEE Proceedings - Communications
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    Julynn Ong · Dhammika Jayalath · Chandranath R. N. Athaudage
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    ABSTRACT: This paper describes a novel low complexity time-frequency spreading of orthogonal block codes for MIMO-OFDM systems in highly time and frequency selective channels. The space-time codes (STC) and space-frequency codes (SFC) proposed in literature for MIMO-OFDM systems require a quasi-static channel, where the channel is assumed to be constant over a number of OFDM time symbols or OFDM subcarriers. In orthogonal block codes (OBC) this is equal to the number of transmit antennas Nt employed. For higher order space-diversity systems with Ntges4, it is likely that the assumption of constant channel within the OBC to be violated in fast fading channels resulting performance degradation. We propose an adaptive time-frequency spreading strategy that minimizes the decoding error rate in different types of fading channels. The adaptation technique incorporated is based on a fading interference matric, which is evaluated at the transmitter using channel fading statistics. Simulation results demonstrate the effectiveness of the proposed adaptive time-frequency spreading of OBCs.
    Preview · Article · Jan 2006
  • K. Sathananthan · C.R.N. Athaudage
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    ABSTRACT: We propose a low complexity frequency diversity technique for OFDM to improve its error performance in a frequency-selective wireless channel. Recently, linear precoding has been proposed to achieve frequency diversity (multipath diversity) in OFDM, which overcomes the problem of channel nulls (deep fades in frequency-domain). However, currently reported precoding techniques for OFDM have high computational complexity due to associated maximum-likelihood (ML) decoding or pseudo-inverse of large matrices (linear decoding). In the proposed diversity enhanced-OFDM (DE-OFDM), the decoding complexity is minimal as maximum ratio combining (MRC) is incorporated at the receiver to generate a decision variable for each repeatedly transmitted symbol over multiple subcarriers. Moreover, we show that DE-OFDM does not cause any effective data rate (throughput) loss as DE-OFDM using a higher-order modulation scheme (thus the same data rate as conventional OFDM) can provide a better error performance. Both analytical and simulation results are provided to demonstrate the efficiency of DE-OFDM in achieving frequency diversity benefit with low receiver complexity in multipath (frequency-selective) channels. DE-OFDM also outperforms the previously proposed ICI self-cancellation schemes in a frequency-selective fading channel with small frequency offset error.
    No preview · Conference Paper · Dec 2005
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    C.R.N. Athaudage · A.D.S. Jayalath
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    ABSTRACT: This paper proposes a novel non-data-aided maximum likelihood (ML) approach for the estimation of the residual timing error in OFDM receivers. The novel approach effectively utilizes the finite alphabet property of the received symbol constellation to perform a near perfect residual timing error estimation. Unlike some of the current techniques, the proposed approach requires no pilots and therefore is bandwidth efficient. Moreover, the reduced complexity version of the post-FFT ML algorithm minimizes the receiver computational burden. Simulation results show that the BER degradation due to residual timing error can be almost completely recovered for both AWGN and Rayleigh fading channel scenarios
    Preview · Conference Paper · Oct 2005
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    C.R.N. Athaudage · K. Sathananthan
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    ABSTRACT: 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.
    Preview · Conference Paper · Jun 2005
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    Vikram Krishnamurthy · Chandranath R N Athaudage · Dawei Huang
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    ABSTRACT: 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.
    Preview · Article · May 2005 · IEEE Transactions on Signal Processing
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    A.R. Varma · L.L.H. Andrew · C.R.N. Athaudage · J.H. Manton
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    ABSTRACT: Channel identification of a time-varying channel is considered using superimposed training. A sequence of known symbols with lower power is arithmetically added to the information symbols before modulation and transmission. The channel estimation is done exploiting the known superimposed data in the transmitted signal. Two iterative algorithms are considered in this paper: recursive least squares (RLS) and the expectation maximization (EM). Performance of the proposed algorithms is compared with a simple averaging scheme and the LMS algorithm. For short data blocks RLS outperforms EM, but with large blocks EM is superior
    Preview · Conference Paper · Mar 2005
  • K. Sathananthan · C.R.N. Athaudage
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    ABSTRACT: In this paper, we analyze the error performance of single-input-single-output (SISO) and space-time coded (ST-coded) orthogonal frequency division multiplexing (OFDM) systems with carrier frequency offset (CFO) in frequency-flat Rayleigh fading channels. Exact analytical expressions are derived for the symbol error probability (SEP) for BPSK and QPSK modulation schemes. The analysis presented has the following key features - it explicitly accounts for (i) the non-Gaussian nature of the intercarrier-interference (ICI) noise when the total number of subcarriers is small, and (ii) the dependency of the post-FFT signal and the ICI noise terms due to subcarrier correlations. Thus, the proposed method provides accurate SEP formulae, overcoming the inaccuracies involved in Gaussian approximation based methods reported in literature. The numerical results demonstrate the sensitivity of the receiver error performance to CFO in SISO-OFDM and ST-coded OFDM systems in flat Rayleigh fading channels. Also, the SEP results presented can be interpreted as the lower-bound of error performance in a general frequency-selective Rayleigh fading channel
    No preview · Conference Paper · Mar 2005
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    C.R.N. Athaudage · A.D.S. Jayalath
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    ABSTRACT: A novel cyclic-prefix based delay-spread estimation technique for wireless OFDM systems is proposed. In particular, the authors propose a technique for estimating the delays and powers of multipath components when the channel is sparse, i.e. a few strong multipaths distantly spaced in time, and a technique for estimating the RMS delay-spread when the channel has a large number of sample-spaced multipath components. The proposed techniques are based on a multiple-argument correlation function which exhibits change of gradient according to the delay path arrival pattern, i.e delay times and powers. Numerical results demonstrate the accuracy of the proposed techniques in estimating the relative timing and the power of delay paths for a sparse multipath channel, and the RMS delay-spread for a sample-spaced multipath channel. Moreover, the RMS delay-spread estimation can be used adaptively to operate the MMSE channel estimation process in the OFDM receiver at near optimum.
    Preview · Article · Jan 2005 · IEE Proceedings - Communications
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    Chandranath R N Athaudage · A. Dhammika S Jayalath
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    ABSTRACT: Estimation and tracking of the frequency-selective time-varying channel response is a challenging task for wireless communication systems incorporating coherent OFDM. In pilot-symbol-assisted (PSA) OFDM systems, the minimum mean-square-error (MMSE) estimator provides the optimum performance based on the channel statistics (channel correlation function and SNR). In OFDM systems, FFT-block timing error introduces a linear phase rotation to data modulated on individual subcarriers. An MMSE channel estimator designed only using the wireless channel statistics performs only sub-optimally when subcarrier phase rotations due to block timing errors are present. In this paper, we show that by using the block timing error statistics of the OFDM time-synchronizer the performance of the MMSE channel estimation can be significantly improved. Numerical results show that the bit-error-probability (BEP) performance degradation due to timing errors can be almost completely recovered by the proposed technique.
    Preview · Article · Jan 2005 · IEEE Transactions on Broadcasting