Marius Sirbu

Adaptive equalization of time-varying MIMO channels

This paper addresses the problem of adaptive multiple-input multiple-output (MIMO) equalization of time-varying channels (TVC).The proposed technique is based on Kalman filtering and decision feedback equalization. The time-varying channels can be estimated and tracked using a Kalman filter type of algorithm. An equalization technique for MIMO channels is derived based on a novel decision feedback equalizer structure. Moreover, we present a method to estimate the measurement and state noise variances used by the Kalman filter because they are crucial parameters in obtaining a reliable performance.The performance of the algorithm is investigated in simulations using realistic channels generated based on the COST 207 model. The results show that the algorithm performs well even in low SNR conditions or in difficult channel conditions.

Delay estimation in long-code asynchronous DS-CDMA systems using multiple antennas

The problem of propagation delay estimation in asynchronous long-code DS-CDMA multiuser systems is addressed. Almost all the methods proposed so far in the literature for propagation delay estimation are derived for short codes and the knowledge of the codes is exploited by the estimators. In long-code CDMA, the spreading code is aperiodic and the methods developed for short codes may not be used or may increase the complexity significantly. For example, in the subspace-based estimators, the aperiodic nature of the code may require subspace tracking. In this paper we propose a novel method for simultaneous estimation of the propagation delays of several active users. A specific multiple-input multiple-output (MIMO) system model is constructed in a multiuser scenario. In such model the channel matrix contains information about both the users propagation delays and channel impulse responses. Consequently, estimates of the delays are obtained as a by-product of the channel estimation task. The channel matrix has a special structure that is exploited in estimating the delays. The proposed delay estimation method lends itself to an adaptive implementation. Thus, it may be applied to joint channel and delay estimation in uplink DS-CDMA analogously to the method presented by the authors in 2003. The performance of the proposed method is studied in simulation using realistic time-varying channel model and different SNR levels in the face of near-far effects, and using low spreading factor (high data rates).

Multichannel Estimation and Equalization Algorithm for Asynchronous Uplink DS/CDMA

This paper addresses the problem of channel estimation andequalization in asynchronous uplink DS/CDMA. We propose a completereceiver structure which contains a multichannel estimator suitablefor time-varying channels, a MMSE equalizer and a propagation delayestimator. The multichannel estimator is a stochastic gradientalgorithm which does not require any knowledge about the propagationdelays. It estimates a matrix which contains a linear combination ofthe impulse response of each channel and the propagation delays. Thisestimate is used to derive a MMSE equalizer. In the despreading stageknowledge on the integer part (chip period multiple) of thepropagation delays is needed. Two reduced complexity schemes forpropagation delay estimation are presented. The channel estimatorconvergence is studied in stationary conditions while the overallalgorithm performance is demonstrated by using simulations intime-varying channel conditions.

A novel method for time delay acquisition in satellite navigation systems

We propose a novel code acquisition method for satellite navigation systems. We consider that the system uses long-codes where the navigation message is spread with a code with period much longer than the symbol period. The fine delay acquisition is performed by minimizing the mean square error (MSE) between the received signal and a locally generated replica of the spreading code. The most important advantage of the proposed method is that it is able to estimate fractional delays (i.e., a fraction of the sampling period) whereas correlation based techniques can only estimate delays up to the sampling period. In our case, the sampling period is equal to the chip period.

On the feasibility of blind equalization for EDGE systems

The main goal of this paper is to find whether or not blind equalization can be used in EDGE (enhanced data rate for GSM evolution) systems in the perspective of increasing the data rate. An EDGE system uses 8-PSK modulation as well as the binary GMSK modulation defined for GSM. The main problem is that GMSK modulation is nonlinear, but most blind equalization techniques are derived for linear modulation. Linear approximation of the real GMSK signal has to be performed in order to apply blind equalization. It is of interest to determine if the input sequence resulting from linearisation and the derotation in the receiver is white, which is a key assumption in many blind equalization algorithms. We show that the whiteness property is preserved. We make a comparison among three blind algorithms: two fractionally spaced and one based on baud-rate HOS. The simulation results indicate that blind equalization performs well and the algorithms have potential as a feasible solution to channel tracking in EDGE systems.

Channel estimation and tracking in asynchronous uplink DS/CDMA using multiple antennas

The channel estimation and tracking as well as the equalization for the uplink in asynchronous long code DS/CDMA is our concern in this paper. We derive a novel algorithm for multiple channel estimation. It is an adaptive algorithm which allows for tracking the time variations of the multiple channels simultaneously. A MMSE equalizer is derived also and it is used together with the proposed channel estimator in a decision directed (DD) mode. The performance of the proposed method is evaluated in nonstationary channel conditions.

Recursive semi-blind equalizer for time-varying MIMO channels

This paper addresses the problem of semi-blind multi-input multi-output (MIMO) equalization of time-varying channels by employing recursive filtering methods. Based on a realistic channel model described in COST 207 project, we derive a state-space model that characterizes the behavior of the channel in time. The channel estimation and tracking are performed using a Kalman filter method, and a decision feedback equalizer derived using MMSE criterion is used to perform the equalization.

A block-Toeplitz VCMA equalizer for MIMO-OFDM systems

In this paper we consider the problem of blind recovery of multiple OFDM signals in a MIMO system. The equalization algorithm is based on composite criterion which is able to cancel both inter-symbol interference (ISI) and inter-user interference (IUI). We propose a modified version of vector constant modulus algorithm (VCMA) which exploits the Toeplitz structure of the equalization matrix, in order to cancel the ISI. The IUI is mitigated by a second decorrelation criterion.

Blind multi-user receiver for MIMO-OFDM systems

This paper addresses the problem of blind multiuser equalization for OFDM. A vector constant modulus algorithm (VCMA) is developed for a specific multiple input-multiple output (MIMO) system. The proposed algorithm relies on a combined criterion, to cancel both intersymbol interference (ISI) and interuser interference (IUI). ISI is minimized by using a constant mean block energy criterion, while IUI is reduced by using a decorrelation criterion. Reliable performance is achieved with relatively fast convergence and small steady-state error, as well as low computational complexity.

Multichannel stochastic gradient method for adaptive equalization in uplink asynchronous DS/CDMA

The paper addresses the problem of channel equalization in asynchronous uplink DS/CDMA systems. An MMSE equalizer is derived based on our proposed channel estimation algorithm (Sirbu, M. and Koivunen, V., Proc. IEEE Int. Conf. on DSP, 2002). For channel estimation, there is no need for knowledge of the propagation delays, whereas for despreading purposes only the chip period multiple of the propagation delays has to be estimated. The proposed scheme processes all user signals simultaneously. The performance is studied based on simulations in non-stationary channel conditions.