Alexandr Kuzminskiy

Finite amount of data effects in spatio-temporal filtering for equalization and interference rejection in short burst wireless communications

Abstract In a burst-based wireless TDMA communication system, the accessible amount of data used by signal processing algorithms for adjusting the coefficients of spatio-temporal filters may not be large enough to allow global convergence of either training-based or blind techniques. Semi-blind algorithms offer an attractive solution to overcome this problem by merging these techniques together. A review of some existing training-based, blind and semi-blind techniques is presented. Their applicability in short burst wireless systems is discussed.

Second-order asynchronous interference cancellation: regularized semi-blind technique and non-asymptotic maximum likelihood benchmark

An asynchronous interference cancellation problem is addressed where training and working intervals are available that contain the desired signal and arbitrary overlapping interference. A regularized non-iterative estimation of the antenna array coefficients is proposed which employs the autocorrelation matrix estimation as a weighted sum of the autocorrelation matrices estimated over the training and working intervals. A non-asymptotic second-order statistic maximum likelihood (ML) benchmark is developed, which is based on estimation of the structured covariance matrices over the training and working intervals for the Gaussian data model. It is shown by means of simulation in the TDMA and OFDM environments that the regularized semi-blind solution significantly outperforms the conventional estimators and demonstrates performance close to the proposed benchmark.

Iterative channel estimation for EGPRS

The Enhanced Data GSM Evolution (EDGE) standard will increase the data rates in a GSM system significantly. An 8 PSK modulation scheme is proposed which is more sensitive to impairments in the transmission medium than GMSK modulation in the GSM system. Channel estimation plays a crucial part on the receiving end to overcome those impairments. We investigate different iterative channel estimation (ICE) techniques. In order to reduce complexity sub-optimum algorithms are addressed. Comparisons with the GSM system show that not all algorithms are suitable for the EDGE system. Simulation results show that with ICE the BLER at 10% in EGPRS can be improved by almost 2 dB in a hilly terrain environment.

Downlink beamforming subject to the equivalent isotropic radiated power constraint in WLAN OFDM systems

Range extension by means of downlink beamforming in a wireless local area network (WLAN) orthogonal frequency-division multiplexing (OFDM) system is addressed. A maximum ratio (MR) combining beamformer scaled according to the equivalent isotropic radiated power (EIRP) constraint adopted in the European regulations, is compared to the conventional total power (TP) restricted solution in typical propagation conditions. It is shown that the EIRP-scaled MR beamformer demonstrates significant performance degradation compared to the TP case. A joint optimization problem for the beamforming weights over all the sub-carriers subject to the EIRP constraint is formulated based on maximization of the minimum signal-to-noise ratio (SNR) over all the working sub-carriers at the receiver. A two-stage sub-optimal solution is proposed that exploits optimization over groups of non-adjacent sub-carriers and normalization to maintain the overall EIRP constraint. Its efficiency is illustrated by means of comparison with the EIRP-scaled MR beamformer and another sub-optimal algorithm based on the conventional grouping of highly correlated sub-carriers in the IEEE 802.11 environment.

Comparison of linear and MLSE spatio-temporal interference rejection combining with an antenna array in a GSM system

By using multiple receiving antennas and spatial or spatio-temporal processing techniques cochannel interference (CCI) may be suppressed. The maximum likelihood (ML) estimator requires a spatio-temporal model of the CCI. The model may be inaccurate because of estimation errors or difficulties in the parametrization of the generally asynchronous CCI. In this case linear spatio-temporal filtering based on training or semi-blind algorithms can be competitive compared to the ML-based algorithms. This paper compares the performance of ML and linear spatio-temporal techniques in synchronous and asynchronous interference-limited GSM urban scenarios. The best results for the asynchronous CCI and/or propagation channels with fading are obtained with a linear spatio-temporal semi-blind filtering technique based on minimization of the dual least squares constant modulus criterion.

Semi-blind spatio-temporal processing with temporal scanning for short burst SDMA systems

Abstract A semi-blind approach for the adjustment of spatio-temporal filter coefficients in short burst spatial division multiple access (SDMA) systems is proposed. The case of imperfect time localization of the desired burst of data is examined. The known temporal structure of a burst of data is used to search and locate the appropriate data processing time frame, which corresponds to the optimal, under some criterion, delay at the output of the spatio-temporal filter. This approach is different than symbol synchronization which is not well defined in the spatio-temporal case due to multipath propagation. Training-based and semi-blind off-line algorithms with temporal scanning are formulated and compared by means of computer simulations. The performance improvement for the proposed semi-blind algorithms and their potential for practical applications are clearly demonstrated.

Blind source separation with randomized Gram-Schmidt orthogonalization for short burst systems

A blind source separation problem for short burst systems is addressed by means of a constant modulus technique under orthogonal constraints. It is shown that a conventional Gram-Schmidt orthogonalization procedure normally exploited in similar applications may cause a non-uniform misadjustment distribution among the receiver outputs leading to an overall performance degradation. We propose a modified algorithm, based on random reordering of the weight vectors before the orthogonalization stage, and demonstrate its efficiency by means of simulations in a short burst MIMO environment.

Downlink throughput enhancement of IEEE 802.11a/g using SDMA with a multi-antenna access point

We address the problem of downlink throughput improvement for IEEE 802.11a/g systems by using a modified access point equipped with multiple antennas. The main restriction is that the standard terminals should not be modified in any way. An alternating time-offset space division multiple access (SDMA) solution is proposed for a conference room scenario. A simulation is based on channel models approved by the IEEE 802.11 Standards Group and takes into account the main features of the IEEE 802.11a/g standard. It is demonstrated that a near doubling of downlink capacity can be achieved in a conference room environment.

Downlink SDMA for IEEE 802.11A/G: A Means for Improving Legacy Mobile Throughput Using a Multi-Antenna Access Point

We address the problem of downlink throughput improvement for IEEE 802.11a/g systems by using a modified access point (AP) equipped with multiple antennas. The main restriction is that the standard terminals should not be modified in any way. An alternating time-offset space division multiple access (SDMA) solution is considered for a conference room scenario. A simulation is based on channel models approved by the IEEE 802.11 Standard Group and takes into account the main features of the IEEE 802.11a/g standard. It is demonstrated that a near doubling of downlink capacity can be achieved in a conference room environment

Cross-Layer Design of Uplink Multiple-Antenna Interference Cancellation for WLAN with CSMA/CA in Open Access Networks

The potential of multi-antenna interference cancellation receiver algorithms for increasing the uplink throughput in WLAN systems such as 802.11 is investigated. The medium access control (MAC) in such systems is based on carrier sensing multiple-access with collision avoidance (CSMA/CA), which itself is a powerful tool for the mitigation of intra-system interference. However, due to the spatial dependence of received signal strengths, it is possible for the collision avoidance mechanism to fail, resulting in packet collisions at the receiver and a reduction in system throughput. The CSMA/CA MAC protocol can be complemented in such scenarios by interference cancellation algorithms at the physical (PHY) layer. The corresponding gains in throughput are a result of the complex interplay between the PHY and MAC layers. It is shown that semi-blind interference cancellation techniques are essential for mitigating the impact of interference bursts, in particular since these are typically asynchronous with respect to the desired signal burst. This is illustrated by cross-layer simulations in open access network (OAN) scenarios involving intra- and (impolite) inter-system interference.