Erik Lindskog

Space-time block coding for frequency-selective channels

We describe the so-called time-reversal space-time block coding (TR-STBC) transmission scheme for communication systems with multiple transmit antennas operating over frequency-selective channels. TR-STBC can be seen as an extension of the orthogonal space-time block codes for flat fading channels. We show that using TR-STBC, a linear filtering at the receiver can achieve an approximate decoupling of the space-time channel into scalar and independent frequency-selective channels and hence any standard maximum-likelihood sequence detector (MLSD) can be used for equalization. Numerical examples are provided to illustrate the performance of the TR-STBC transmission scheme.

Space-time block coding for channels with intersymbol interference

The downlink of many wireless communication systems can be a MISO channel. An important problem for a MISO channel is how to code across space and time to obtain the same ML receiver as for the corresponding SIMO channel. For fiat fading channels, space-time block coding (STBC) is a recent breakthrough solution to this problem. STBC has been generalized to channels with intersymbol interference (ISI) for the case of two transmit antennas and one receive antenna. In this paper we first revisit the generalized STBC scheme of et al. (2000), and show that it has the same appealing properties as the standard STBC for flat fading channels. Then we go on to present an extension of the scheme in Lindskog to ISI channels with any number of transmit and receive antennas.

Multi-channel maximum likelihood sequence estimation

In mobile radio communications, antenna arrays can be used to improve the quality and/or the capacity of the communication system. The combination of an antenna array and maximum likelihood sequence detection (MLSE) is studied. Different realizations of the multi-channel MLSE are presented. Although equivalent in performance, it is pointed out that one of them, the multi-dimensional matched filter approach, is superior in terms of computational complexity when more than one antenna is used. For completeness, temporally colored noise is included in the formalism.

Reduced rank channel estimation

A space-time wireless communication channel can be decomposed into a set of filters, each consisting of a scalar temporal filter followed by a single spatial signature vector. If only a small number of such filters is necessary to accurately describe the space-time channel, we call it a reduced rank channel. We consider different methods of exploiting this property to improve the channel estimation and subsequent space-time equalization. Three methods have been studied, a maximum likelihood reduced rank channel estimation method and two different signal subspace projection methods. The first method projects the channel estimate onto an estimate of the signal subspace. The second, which is the new method proposed here, projects the received data onto the same estimate of the signal subspace. Simulations indicate that even though the maximum likelihood reduced rank method has the smallest channel estimation errors, the BER of the detector based on this model exceeds the BER of the detectors based on the channel models obtained using either of the signal subspace projection methods. The best performance is obtained using the proposed method, which also has the lowest complexity.

Spatio-temporal equalization for multipath environments in mobile radio applications

Combined spatial and temporal equalization using an antenna array combined with a decision feedback equalization scheme is investigated. In particular a TDMA type system with a relatively short training sequence is considered. Three algorithms are introduced. The first two algorithms are based on indirect schemes, where the channels to each receiver antenna element are identified. The identified channels and the correlations of the residuals are then used for the tuning of the beamformer/equalizer coefficients. The spatio-temporal correlations of the residuals are used in the first algorithm while in the second algorithm only the spatial correlations of the residuals are considered. The third algorithm forms a number of beams by using mixtures of different delayed versions of the training sequence as reference signals. It then performs temporal equalization by combining the outputs from the different beamformers, with appropriate delays. This latter algorithm requires less computations for the tuning of the equalizer, at the expense of a performance degradation in general. The algorithms are evaluated with simulations of multipath scenarios involving co-channel interference.

Robust decision feedback equalizers

Design equations are presented for robust and realizable decision feedback equalizers, for IIR (infinite impulse response) channels with colored noise. Given a probabilistic measure of model uncertainty, the mean MSE (mean square error), averaged over a whole class of possible models, is minimized. A second type of robustification, which reduces the error propagation due to the feedback, is also introduced. The resulting design equations define a large class of equalizers, with DFEs (decision feedback equalizers) and linear equalizers based on nominal models being special cases.<<ETX>>

Combatting co-channel interferers in a TDMA system using interference estimates from adjacent frames

Suppression of co-channel interferers in a TDMA cellular phone system by the use of an antenna array is studied. Especially the problem of suppressing co-channel interferers that appear outside the training sequence is addressed. It is observed that if a cochannel interferer is present during the data sequence but not during the training sequence of a frame, then it will be present during the training sequence of an adjacent frame. The interferer plus noise spectrum of adjacent frames is utilized in order to suppress such co-channel interferers. The effect on the performance is illustrated with an example scenario for the GSM system.

Combined spatial and temporal equalization using an adaptive antenna array and a decision feedback equalization scheme

The purpose of this paper is to investigate the use of combined spatial and temporal equalization, in particular for short training sequences. The motivation for combining spatial and temporal equalization is the existence of multipath propagation and co-channel interference. Our main concern is to obtain good performance yet low complexity. We will suggest a low complexity algorithm utilizing a circular antenna array. Although it has inferior performance in an asymptotic sense, it turns out to be superior to the general solution for short training sequences. This conclusion is supported by simulations where a number of algorithms are evaluated for different scenarios involving co-channel interference.

Reduced rank space-time equalization

In wireless communication systems with antenna arrays the spatio-temporal channel can often be described by a low-rank model. By exploiting this information, corresponding low rank equalizers with reduced complexity can be designed. By applying such low rank equalizers to a set of uplink measurements, we demonstrate that the performance loss associated with the lower complexity is small.

Auto-calibrating adaptive array for mobile telecommunications

A major concern for adaptive antennas is the calibration of hardware. Here we consider the calibration of an analog beamformer (ABF), which calculates the weights in a digital signal processor (DSP), but weights the RF signals using a hardware beamformer. Two novel methods are presented of performing a calibration of this adaptive antenna during normal operation. This will mitigate temperature drift and aging of active components. Both methods uses a feedback of the beamformer output signal, to calculate the error in the beamformer output or to identify the parameters in the temperature drift. The algorithms are transparent to normal antenna operation and are computationally simple. Simulations show that when using these auto-calibration methods, the performance is independent of variation in hardware parameters when a realistic temperature drift is introduced. The proposed algorithms are intended for use in a time-division multiple access (TDMA) mobile telephone system, but the methods are applicable in radar systems also.