Fabio Sterle

Widely linear decision-feedback equalizer for time-dispersive linear MIMO channels

The paper deals with the transmission over multiple-input/multiple-output channels exhibiting time-dispersion. A minimum mean-square error equalizer based on widely linear processing combined with the decision-feedback (DF) strategy is implemented via finite-impulse-response filters. The proposed equalizer provides considerable performance gain at the expense of a limited increase in computational complexity. The performance analysis has been carried out accounting for mismatch conditions always present in practice. The results confirm the stronger sensitivity of the DF-based equalizers with respect to the feedforward-based ones when system parameters are not accurately known.

Widely Linear MMSE Transceivers for MIMO Channels

This paper considers the joint design of the transmitter and the receiver for linear multiple-input multiple-output (MIMO) channels when the channel state information (CSI) is available at both sides of the link. Assuming that the data symbols are mapped into points of fixed signal constellations, a transceiver structure employing widely linear (WL) filters, rather than linear ones, is proposed in order to exploit the statistical redundancy of some constellations, i.e., the correlation between the signals and their conjugate version. WL filters linearly and independently process both the in-phase and the quadrature components of the input signals and, with a limited increase in the computational complexity, allows one to improve the system performances. It is shown that, by representing the basic system model in terms of in-phase and quadrature components, the proposed structure can be derived by utilizing the already existing procedure for the design of the linear transceiver according to the minimum-mean-square-error (MMSE) criterion. The performances of the WL and linear transceivers are compared in terms of mean-square error, symbol error rate, and achieved multiplexing gain.

MMSE WL Equalizer in Presence of Receiver IQ Imbalance

In this correspondence, with reference to the transmission over a linear time-dispersive channel, we address the problem of the in-phase and quadrature-phase (IQ) imbalance compensation when a single-carrier modulation scheme is used. Low-cost fabrication technologies and high data-rate transmissions render the conventional receivers very sensitive to the imperfections of their analog stage and, hence, proper countermeasures need to be adopted. Since the IQ imbalance renders the received signal rotationally variant, we propose to resort to the widely linear (WL) filtering in the synthesis of the receiver. More specifically, the synthesis of the WL receiver is performed by assuming perfect knowledge of the IQ imbalance parameters, but a new blind algorithm for IQ imbalance compensation, which outperforms the existing blind technique, is also proposed.

MMSE equalization in presence of transmitter and receiver IQ imbalance

The wide spread of low-cost fabrication technologies gives rise to unpredictable imperfections associated with the analog stages which perform the frequency conversion. More specifically, it is well known that such stages suffer from the in-phase (I) and quadrature (Q) imbalance. In this paper, with reference to a single-carrier time-dispersive noisy channel, we address the receiver design when both the transmitter and the receiver are affected by the IQ imbalance, and we propose to resort to the widely linear filtering in order to improve the performances of the conventional minimum mean square error (MMSE) linear equalizer. The results show that the adoption of WL filters allows one to achieve considerable gains both in terms of MMSE and symbol error rate, with a limited increase in the computational complexity of the equalization stage.

Widely linear MMSE equalizer for MIMO linear time-dispersive channel

The paper deals with transmission over multiple-input/multiple-output (MIMO) communication systems where, due to the large demand of high bit-rate telecommunication services, is required to take into account the time-dispersion of the channel. Recently, widely linear processing, which can be considered as a generalization of linear processing, has been proposed for single-input/single-output (SISO) and MIMO zero-memory equalizer. In this paper, widely linear processing has been extended to the general case of minimum mean square error (MMSE) equalizer for both finite impulse response (FIR) and recursive structures. Simulation results (with reference to recursive structure) show that the proposed equalizer significantly outperforms the linear MMSE equalizer and decision feedback one in operative conditions of practical interest where the conditions of joint circularity of input and output signals are not satisfied (e.g., DS/WCDMA employing BPSK scheme).

ML estimation of receiver IQ imbalance parameters

In this paper, we address the in-phase (I) and quadrature (Q) imbalance estimation and compensation at the receiver stage of modern communication systems. Low-cost fabrication technologies and high data-rate transmissions render the conventional receivers very sensitive to the imperfections of their analog stage and, hence, proper countermeasures need to be adopted. Since the IQ imbalance can be completely compensated provided that the IQ parameters are known at the receiver stage, in this paper we address the blind estimation of such parameters. According to the maximum likelihood (ML) criterion, we derive the optimum blind estimator for a QAM transmission as well as for a circularly symmetric Gaussian one and, therefore, we evaluate, by means of computer simulations, their performances in terms of mean square estimation error.

Constellation design for widely linear transceivers

Constellation design has been previously addressed by assuming that there is a linear equalizer at the receiver side. However, the widely linear equalizer is well known to outperform the linear one with no significant complexity increase; we derive optimum and suboptimum techniques for constellation design in presence of such an equalizer. The proposed techniques adapt the circularity properties of the transmitted signals to the specific channel to be equalized; their performance analysis shows that also the simplest suboptimum procedure provides significant improvements over a fixed-constellation scheme.

Multistage Widely-Linear DF Equalizers for MIMO Channels

The paper addresses the low-complexity design of minimum mean square error (MMSE) multistage decision-feedback (DF) equalizers for multiple input/multiple output (MIMO) channels. More specifically, the multistage DF equalization strategy is combined with the widely-linear (WL) processing in order to exploit the properties of the rotationally variant transmitted signals. The optimization (in the MMSE sense) of WL-DF equalizer is re-examined in terms of the classical least-square approach and it is generalized to thecase of the MIMO multistage DF receivers, which require at least one equalization stage to initialize the input of the feedback filter. The performance analysis is aimed at comparing the performances of the linear multistage DF equalizers and their WL counterparts in terms of the symbol error rate (SER), the number of equalization stages required to minimize the SER and the number of floating point operations required to optimize the MMSE feedforward and feedback filters.