COMM-6.1

ROBUST DETECTION IN ASYNCHRONOUS CDMA MULTIPATH FADING CHANNELS
YONG-SUK MOON, MOSTAFA KAVEH, LAURIE B NELSON (UNIV. of MINNESOTA)

This paper presents a robust detection approach in the presence of transmission delay estimation errors in asynchronous CDMA frequency selective Rayleigh fading channels. The transmission delays in asynchronous channels need to be estimated in practice and are subject to estimation errors, which may cause severe performance degradation for most detectors assuming perfect synchronization. As the fundamental framework of the robust detector, an asynchronous subspace detection scheme based on oblique projection is introduced and examined in comparison with the conventional RAKE receiver and the multipath decorrelator. The performance degradation of the subspace detector due to delay estimation errors is also investigated. The robust detector shows reliable performance even when the errors are much larger than those of delay estimation algorithms like MUSIC, and it is not affected by the near-far problem.

COMM-6.2

Performance and Complexity Trade Off in CDMA Multiuser Detection
Mohammed H Nafie, Ahmed H Tewfik (University of Minnesota)

In this paper, we describe a new tree-based CDMA receiver that can optimally trade complexity for detection performance. We describe a technique for designing receivers with linear complexity (including the optimal linear detector and decorrelator). We then explain how we can increase performance at the expense of a minimal increase in complexity. We show that as complexity increases to the level of that of the optimal receiver, our design approach automatically produces the optimal receiver. We also explain how our approach can be used with a minimum-mean-square-error design criterion and coded CDMA transmission. Finally, we illustrate with several examples the superiority of the receivers designed with our approach and discuss their advantages.

COMM-6.3

Realizable MIMO decision feedback eqaulizers
Claes Tidestav, Anders Ahlen, Mikael Sternad (Signals and systems, Uppsala university)

A general MMSE decision feedback equalizer (DFE) for multiple input-multiple output channels is presented. It is derived under the constraint of realizability, requiring finite smoothing lag and causal filters both in the forward and feedback link. The proposed DFE, which provides an optimal structure as well as optimal filter degrees, is obtained as an explicit solution in terms of the channel and noise description. A generalization of the scalar zero-forcing DFE is also presented. Conditions for the existence of such an equalizer are discussed. We argue that when no zero-forcing equalizer exists, poor near-far resistance of the corresponding optimal minimum mean square error DFE can be expected. A numerical example indicates the potential advantages of using a decision feedback equalizer with appropriate structure. The main improvements are obtained at moderate to high signal-to-noise ratios.

COMM-6.4

Iterative Nonlinear MMSE Multiuser Detection
Sridhar Gollamudi, Yih-Fang Huang (University of Notre Dame)

This paper introduces the notion of nonlinear MMSE multiuser detection and shows that MMSE signal estimates followed by single user detectors yield MAP (or minimum probability of error) decisions for CDMA signals. Iterative solutions are proposed for nonlinear MMSE estimation. If interferers' codes are assumed to be known by the signal estimators for each user, the MMSE solution is shown to be a fixed point which is reached in one iteration, but is computationally intractable. If interferers' codes are not assumed to be known, the resulting iterative multiuser detector (called random-code NMIC) is shown to be of the same order of complexity as a conventional multistage interference canceler. Furthermore, the nonlinear MMSE solution is shown to be a fixed point of the random-code NMIC. Particular solutions are presented for the cases of BPSK and M-ary orthogonal spread spectrum systems. Audio demonstrations of random-code NMIC performance can be found at http://www.nd.edu/~aspect/.

COMM-6.5

An Integrated Signal Processing Framework for Multiuser CDMA Communications
Akbar M Sayeed (University of Wisconsin-Madison)

The major signal processing challenges in wireless CDMA systems stem from time-varying multipath propagation effects, multiaccess interference (MAI), and the complexity of the DSP algorithms. We propose a framework based on canonical multipath-Doppler coordinates for addressing these issues in an integrated fashion. The canonical coordinates are derived from a fundamental characterization of channel propagation dynamics in terms of discrete multipath-delayed and Doppler-shifted copies of the transmitted waveform. These delayed and Doppler-shifted spreading waveforms constitute a natural fixed basis and dictate a canonical low-dimensional processing. In addition to providing a robust vehicle for channel modeling and estimation, the framework facilitates exploitation of dispersion effects for MAI suppression via subspace-based processing in the canonical coordinates. Finally, the low-dimensional subspace formulation affords a direct handle on the complexity of the DSP algorithms.

COMM-6.6

Constrained minimum-BER multiuser detection
Xiaofeng Wang, Wu-Sheng Lu, Andreas Antoniou (Dept. of Electrical & Computer Engineering, University of Victoria)

A new linear multiuser detector for binary signaling in code-division multiple-access communication systems is described. The new detector directly minimizes the bit-error rate (BER) subject to a set of reasonable constraints. It is shown that any local minimum of the constrained BER cost function is a global minimum; hence a robust constrained minimization algorithm always leads to a detector with good performance. Although the proposed detector cannot be shown to be optimal among linear multiuser detectors because of the constraints imposed, our analysis and simulations indicate that it always outperforms the decorrelating detector and is optimal for most realistic systems.

COMM-6.7

On the Multichannel Characteristics of a 1.8 GHz Smart Antenna System Using a Circular Array in realistic Non-stationary Wireless Scenarios
Weidong Yang, Adnan Kavak, Guanghan Xu (Dept. of Electrical & Computer Engineering, Univ. of Texas at Austin)

In mobile communications, smart antenna systems that utilize an antenna array and perform advanced signal processing techniques can achieve greater channel capacity and improve link quality by selective receiving/transmission at the base station. However, development of adaptive signal processing alogrithms for smart antenna system applications require the accurate knowledge about the multichannel propagation characteristics. A few experimental results on the spatial signature variation of a uniform linear array(ULA) at 900 MHz have been reported. This paper presents the experimental results on the channel propagation characteristic variations of a 1.8 GHz smart antenna system using a uniform circular array(UCA) in moving mobile scenarios. The results indicate that the stability of the direction-of-arrivals (DoAs) of multipath components in all scenarios, and the unstability of the spatial signatures in scenarios with strong multipaths.

COMM-6.8

Canonical Space-Time Processing in CDMA Systems
Eko N Onggosanusi, Akbar M Sayeed, Barry D Van Veen (University of Wisconsin-Madison)

We propose a canonical space-time receiver structure for wireless communications based on front-end processing with respect to a fixed basis that is independent of the true channel parameters. The basis is dictated by a canonical characterization of channel propagation dynamics in terms of discrete multipath delays, Doppler shifts, and directions of arrival that capture the essential degrees of freedom in the received signal. In addition to dramatically reducing the number of channel parameters to be estimated, performance analysis demonstrates that canonical space-time receivers can deliver optimal performance at substantially reduced complexity compared to existing designs.

COMM-6.9

Space-Time Multiuser Detection in Multipath CDMA Channels
Xiaodong Wang (Texas A&M Univ.), Vincent Poor (Princeton Univ.)

The problem of multiuser detection in multipath CDMA channels with receiver antenna array is considered. The optimal space-time multiuser receiver structure is first outlined, followed by linear space-time multiuser detection methods based on iterative interference cancellation. Single-user-based space-time processing methods are also considered and are compared with the multiuser approach. It is seen that the proposed multiuser space-time processing techniques offer substantial performance gains over the single-user-based methods, especially in a near-far situation.

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Last Update: February 4, 1999 Ingo Höntsch