M. Münster

Co-channel interference suppression assisted adaptive OFDM in interference limited environments

This paper investigates the feasibility of adaptive OFDM (AOFDM) transmissions in co-channel interference limited environments upon invoking the sample matrix inversion (SMI) algorithm. The subcarrier based signal-to-noise ratio (SNR), which can be derived from the SMI algorithms weights is shown to be in most cases an effective measure for controlling the modulation mode adaptation. A simple pilot based scheme is introduced, which allows channel parameter estimation on an OFDM symbol-by-symbol basis. The system BER performance is shown to be improved by an order of a magnitude due to combining AOFDM with interference suppression.

Parallel-interference-cancellation-assisted decision-directed channel estimation for OFDM systems using multiple transmit antennas

The number of transmit antennas that can be employed in the context of least-squares (LS) channel estimation contrived for orthogonal frequency division multiplexing (OFDM) systems employing multiple transmit antennas is limited by the ratio of the number of subcarriers and the number of significant channel impulse response (CIR)-related taps. In order to allow for more complex scenarios in terms of the number of transmit antennas and users supported, CIR-related tap prediction-filtering-based parallel interference cancellation (PIC)-assisted decision-directed channel estimation (DDCE) is investigated. New explicit expressions are derived for the estimators mean-square error (MSE), and a new iterative procedure is devised for the offline optimization of the CIR-related tap predictor coefficients. These new expressions are capable of accounting for the estimators novel recursive structure. In the context of our performance results, it is demonstrated, for example, that the estimator is capable of supporting L=16 transmit antennas, when assuming K=512 subcarriers and K/sub 0/=64 significant CIR taps, while LS-optimized DDCE would be limited to employing L=8 transmit antennas.

A turbo-coded burst-by-burst adaptive wide-band speech transceiver

Turbo-coded burst-by-burst adaptive orthogonal frequency division multiplex (AOFDM) wide-band speech transceivers are proposed. A constant throughput adaptive OFDM transceiver was designed and benchmarked against a time-variant rate scheme. The proposed joint adaptation of source-codec, channel-codec, and modulation regime results in attractive, robust, high-quality audio systems, capable of conveying near-unimpaired wide-band audio signals over fading dispersive channels for signal-to-noise ratios (SNR) in excess of about 5 dB.

Performance of SDMA multiuser detection techniques for Walsh-Hadamard-spread OFDM schemes

Multiple reception antenna assisted space division multiple access (SDMA) techniques designed for wireless OFDM systems have recently drawn wide interest. These techniques facilitate the implementation of effective multiuser detection algorithms at the receiver, such as minimum mean-square error (MMSE) detection, successive interference cancellation (SIC), parallel interference cancellation (PIC) or maximum likelihood (ML) detection. We investigate the performance of the MMSE and PIC detection techniques in the context of a Walsh-Hadamard spreading assisted SDMA OFDM system.

MMSE channel prediction assisted symbol-by-symbol adaptive OFDM

Subband adaptive orthogonal frequency division multiplexing (OFDM) constitutes an effective method of compensating for the frequency selective fading incurred by mobile receivers. The channel transfer function estimation error increases both as a function of the Doppler frequency and that of the subband modem mode signalling delay. This leads to a degradation of the systems performance. In this article a 2D-MMSE channel-predictor is investigated as a means of compensating for these effects, which incorporates the scheme proposed Tufvesson, Faulkner and Maseng, (see IEEE Proc. of 6th International Workshop on Intelligent Signal Processing and Communications Systems, Melbourne, Australia, p.57-33, November 1998) into an adaptive OFDM scheme.

Second-order channel parameter estimation assisted cancellation of channel variation-induced inter-subcarrier interference in OFDM systems

OFDM transmission is known to be sensitive against carrier frequency offsets, as well as against the channel impulse response (CIR) variations incurred during an OFDM symbol period. Both of these phenomena lead to inter-subcarrier interference (ICI). Here, we concentrate on the latter phenomenon. ICI can be considered as an additional noise contribution, which linearly depends on the signal power, and thus it potentially limits the achievable BER at relatively high SNRs. In this contribution, we propose a second-order time-domain CIR tap variation estimator, which relies on both the received signal as well as on tentative symbol decisions. Our simulation results, obtained in the context of a system employing decision-directed channel-prediction, demonstrate that, with the aid of the proposed algorithm, ICI cancellation can be performed even in high Doppler frequency channel scenarios. This is achieved without inserting additional time-domain pilot symbols.

HIPERLAN-like turbo coded multi-user detection assisted OFDM based interactive video telephony

Multiuser detection assisted, multiple transmit antenna based OFDM arrangements are studied in the context of HIPERLAN 2-like systems. It is demonstrated that the system user capacity can be improved with the aid of unique spatial user signatures, hence supporting a multiplicity of users. The maximum likelihood sequence estimation (MLSE) detection algorithm outperformed the minimum mean square error (MMSE) scheme by about 5 dB in terms of the required signal-to-noise ratio (SNR) and this performance gain manifested itself also in terms of the system improved video performance.

Conclusion and Further Research ProblemsOFDM and MCCDMA for Broadband Multiuser Communications WLANS and Broadcasting. L.Hanzo, Mnster, T. Keller and B.J. Choi,

This chapter contains sections titled: Summary of Part I Summary and Conclusions of Part II Summary and Conclusions of Part III Closing Remarks

OFDM Transmission over Gaussian ChannelsOFDM and MCCDMA for Broadband Multiuser Communications WLANS and Broadcasting. L.Hanzo, Mnster, T. Keller and B.J. Choi,

This chapter contains sections titled: Orthogonal Frequency Division Multiplexing Choice of the OFDM Modulation OFDM System Performance over AWGN Channels Clipping Amplification Analogue-to-Digital Conversion Phase Noise Chapter Summary and Conclusion

Channel Estimation for MultiUser OFDMOFDM and MCCDMA for Broadband Multiuser Communications WLANS and Broadcasting. L.Hanzo, Mnster, T. Keller and B.J. Choi,

This chapter contains sections titled: Motivation The SDMA Signal Model on a Subcarrier Basis Multi-User Multiple Reception Antenna Aided OFDM Least-Squares Error Decision-Directed Channel Estimation Frequency Domain Parallel Interference Cancellation Aided DDCE RLS-Adaptive Parallel Interference Cancellation Aided DDCE Chapter Summary and Conclusion List of Symbols Used in Chapter 16