Alexander Lampe

Receivers with widely linear processing for frequency-selective channels

We propose several equalization schemes based on widely linear processing (WLP). The received signal and its complex conjugate are separately filtered and the results are linearly combined. It is shown that WLP yields a gain in performance if the (noiseless) received signal can be interpreted as the convolution of a real-valued data sequence and an equivalent complex-valued intersymbol interference channel impulse response. Such a model applies to, e.g., amplitude-shift keying, offset quadrature amplitude modulation, and binary minimum-shift keying-type modulation. We consider receivers without and with decision feedback. Finite impulse response filters are derived for these structures, which are optimum with respect to the zero-forcing and minimum mean-squared error (MMSE) criteria, respectively. In the MMSE case, adaptive algorithms for filter adjustment are given. Infinite filter orders are investigated in order to obtain analytical performance results. Furthermore, suboptimum trellis-based detection with widely linear preprocessing is briefly discussed. It is demonstrated analytically and by numerical examples that widely linear schemes may outperform conventional schemes significantly, depending on the considered application.

MIMO precoding for decentralized receivers

This paper presents a modified version of Tomlinson-Harashima type precoding applicable to a setting with decentralized receivers, e.g. in a DS-CDMA downlink scenario. Precoding, i.e., nonlinear pre-equalization, is an interesting strategy for significantly simplifying signal processing at the receiver side (mobile terminals) at only moderate additional complexity at the base station.

Equalization concepts for Alamouti's space-time block code

In this paper, we develop receiver concepts for transmission with space-time block codes (STBCs) over frequency-selective fading channels. The focus lies on Alamoutis space-time block-coding scheme, but the results may be generalized to other STBCs as well. We show that a straightforward combination of conventional equalizers and a space-time block decoder is only possible if at least as many receive antennas as transmit antennas are employed, but not for the practically interesting case of pure transmit diversity, for which space-time coding had been originally developed. This restriction is circumvented by our approach. Here, the structural properties of the transmit signal of space-time block coding, which is shown to be improper (rotationally variant), are fully used. For this, equalizers with widely linear (WL) processing are designed, such as a WL equalizer, a decision-feedback equalizer with WL feedforward and feedback filtering, and a delayed decision-feedback sequence estimator with WL prefiltering. Simulation results demonstrate that the proposed concepts may be successfully employed in an enhanced data rates for GSM evolution (EDGE) receiver, especially for pure transmit diversity. Here, significant gains can be observed, compared with a conventional single-input single-output transmission.

Tomlinson-Harashima precoding in space-time transmission for low-rate backward channel

In this paper, Tomlinson-Harashima precoding, a nonlinear pre-equalization technique, is proposed for transmission over multiple-input/multiple-output channels. Instead of equalizing intersymbol interference (temporal equalization) here spatial equalization, i.e., equalization of multi-user interference is performed. If only a low-rate backward channel is available for communicating channel state information back from the receiver to the transmitter, a compromise precoder setting, calculated from (medium-term) average channel knowledge in combination with linear residual equalization at the receiver side is proposed. Compared to an optimal adjustment of the precoder, i.e., perfect channel state information at the transmitter, only small losses have to be accepted.

A novel iterative multiuser detector for complex modulation schemes

A novel multiuser detector for direct sequence code division multiple access is proposed. The receiver performs iterated soft decision interference cancellation (ISDIC) based on multiuser interference suppression filters designed for minimization of the mean-square error. Assuming a complex modulation format, we show that the multiuser interference becomes rotationally variant in the course of the iterations. Regarding this rotational variance in the design of the multiuser interference suppression filter, the presented iterative multiuser detector achieves significant performance gains compared with conventional ISDIC employing a standard minimum mean-squared error filter which is optimum only for rotationally invariant multiuser interference.

Iterative multiuser detection with integrated channel estimation for coded DS-CDMA

A practically interesting approach for iterative channel estimation, multiuser detection, and single-user decoding based on maximum a posteriori symbol-by-symbol estimation for direct sequence/code-division multiple-access (DS-CDMA) is proposed. The receiver relies on the output of a bank of matched filters for each user and each path, and combines interference cancellation with iterated soft-decision feedback to improve channel estimation accuracy and data symbol reliability in course of a few iterations. We show that in this way, near single-user channel phase and amplitude estimation accuracy is achieved for frequency-selective fading channels, even in highly loaded systems, and illustrate that reliable data symbol estimation can be performed.

Asymptotic analysis of widely linear MMSE multiuser detection-complex vs real modulation

We give the asymptotic signal-to-interference ratio for the so-called widely linear minimum mean-squared error multiuser receiver for transmission of multiple users to a common receiver with DS-CDMA. Observing that widely linear multiuser detection virtually doubles the spreading factor, we show that transmission with real-valued channel symbols can lead to a higher spectral efficiency than with complex-valued channel symbols when each user employs its own complex-valued random spreading sequence.

Equalization with widely linear filtering

The concept of widely linear estimation is applied to the equalization problem. Widely linear equalizers and decision-feedback equalizers with widely linear feedforward filtering are presented. For mobile communication channels significant performance gains can be achieved compared to conventional equalizers.

Widely linear equalization for space-time block-coded transmission over fading ISI channels

We develop receivers for transmission with space-time block codes (STBCs) over fading intersymbol interference (ISI) channels. The focus lies on Alamoutis STBC, but the results may be generalized to related codes. It turns out that a straightforward combination of conventional equalizers and a space-time block decoder is only possible if at least as many receive antennas as transmit antennas are employed, but not for the practically interesting case of pure transmit diversity. This restriction is circumvented by our approach. Equalizers with widely linear (WL) processing are designed, exploiting the structure of Alamoutis STBC, which is shown to produce an improper (rotationally variant) transmit signal. The presented receivers are especially well suited for high-level modulated signals, which are used in third-generation time-division multiple access (TDMA) mobile communications standards such as EDGE (enhanced data rates for GSM evolution). In contrast to block-based STBCs which have recently been proposed for ISI channels, our approach of combining a symbol-based STBC and equalization can be easily accommodated to channels with time-variant behaviour inside a transmission burst. Furthermore, for our scheme, the present burst structure of the EDGE system can be retained.

Iterative interference cancellation for DS-CDMA systems with high system loads using reliability-dependent feedback

In this paper, coded transmission over time-variant multipath Rayleigh-fading channels employing direct-sequence code division multiple access (DS-CDMA) is considered. Assuming ideal knowledge of the actual channel state and randomly chosen spreading sequences, we show that iterative multi-user interference suppression based on adapted minimum mean-squared error (MMSE) filters combined with serial successive cancellation and single user decoding can reach near optimum performance within a few iteration cycles. This holds even for doubly loaded systems, i.e., when the number of users K in the system is two times as large as the spreading factor N. Further, we evidence that for sufficiently reliable symbol estimates soft decision feedback can be replaced by hard decision feedback without any performance degradation but with significant savings in complexity.