3:30, SAM-P3.1
DIVERSITY SIGNAL RECEPTION VIA SOFT DECISION COMBINING
Q. ZHANG, P. VARSHNEY
The problem of diversity signal reception using quantized data is considered. This problem arises in situations where multiple receivers are employed over a geographically broad area. We propose a method for the design of the quantizers that are used by the receivers to make soft decisions. These quantizers minimize the mean-squared error between the quantizer output and the log-likelihood ratios associated with the receiver observations. The soft decisions are then combined at a central location to yield the final decision. Our quantizers are applied to the coherent detection of a BPSK signal over Rayleigh fading channels. Simulation results show that using coarse quantizers yields close to optimum performance.

3:30, SAM-P3.2
JOINT SPATIAL AND TEMPORAL DELTA-SIGMA MODULATION FOR WIDEBAND ANTENNA ARRAYS AND VIDEO HALFTONING
D. SCHOLNIK, J. COLEMAN
Extending an existing architecture for delta-sigma conversion of vector inputs, we suggest spectrally shaping quantization noise jointly in temporal and spatial frequency domains with delta-sigma modulation, and we examine the application of the idea to wideband antenna or acoustic arrays and to halftoning of video imagery.

3:30, SAM-P3.3
COD: BLIND PATH SEPARATION WITH LIMITED ANTENNA SIZE
X. ZHANG, S. KUNG
This paper proposes a generalized multipath separability condition for subspace processing and derives a novel COD (Combined Oversampling and Displacement) algorithm to utilize both spatial and temporal diversities for path separation and DOA estimation. A unique advantage lies in its ability to cope with the situation where the number of multipaths is much larger than that of antenna elements, which arises in many practical situations. The traditional data matrix or any of its horizontally expanded versions cannot yield a sufficient matrix rank to satisfy the condition, when there is antenna deficiency. Neither can a vertical expansion via oversampling, except when there is no overlapping among intra-user paths (a much stronger condition than the asynchrony condition). The COD strategy solves the antenna deficiency problem by combining vertical expansion with temporal oversampling and horizontal expansion with spatial displacement. Another unique advantage of COD is its multiplicity of eigenvalues which greatly facilitates the later signal recovery processing. The paper first analyzes the theoretical footings for COD and follows with some illustrative simulation results in noisy channels.

3:30, SAM-P3.4
LIMITS ON LINEAR DETECTOR PERFORMANCE FOR CLOSE USERS IN A WIRELESS MULTIPATH ENVIRONMENT
H. JONES, R. KENNEDY, B. HART, P. RAPAJIC
In multiuser wireless systems using multiple antennas in a multipath environment it has been asserted that whenever users are at least a distance of half the carrier wavelength apart (lambda/2), it is possible to recover their respective signals with appropriate signal processing. This paper significantly strengthens this remarkable assertion by: 1) showing explicitly that separability of the users' signals is possible using the simplest class of linear detectors; 2) showing that the lambda/2 rule of thumb can be quite conservative with meaningful signal separability achievable at separations around lambda/10; and 3) identifying critical geometrical properties, such as position of users and multipath geometry that permit the best separability.

3:30, SAM-P3.5
A NEW APPROACH TO POWER ADJUSTMENT FOR SPATIAL COVARIANCE BASED DOWNLINK BEAMFORMING
H. BOCHE, M. SCHUBERT
When using downlink beamforming at the base station of a cellular system, care must be taken to assure that all users satisfy the given SINR requirements. Generally, this leads to a computationally prohibitive joint optimization problem, therefore decoupled techniques based on eigenvalue decomposition of the spatial covariance have been proposed. However, these techniques require additional power adjustment of the steering vectors in order to satisfy the given SINR requirements. Conventional power control schemes are not suited for this purpose because the SINRs primarily depend on the shape of the radiation patterns. In this paper we propose a new power adjustment algorithm, which adjusts the beamforming weights, such that all users have the same SINR. Additionally, it is possible to limit the maximum radiated user power.

3:30, SAM-P3.6
JOINTLY OPTIMAL DOWNLINK BEAMFORMING AND BASE STATION ASSIGNMENT
M. BENGTSSON
We present an algorithm that jointly determines the optimal downlink beamformers and the optimal assignment of each mobile to a base station. The optimality criterion is based on a systems perspective; provide sufficient quality of service for all users, transmitting as little excess power as possible. Since the algorithm is centralized and requires knowledge about all the channels in the system, it may be infeasible in a practical implementation. However, it provides the ultimate benchmark in system evaluations. Numerical examples show substantial gain compared to ordinary base station assignment.

3:30, SAM-P3.7
TRACKING PERFORMANCE OF A STOCHASTIC GRADIENT ALGORITHM FOR TRANSMIT ANTENNA WEIGHT ADAPTATION WITH FEEDBACK
B. BANISTER, J. ZEIDLER
In this paper the tracking performance of a transmit antenna weight adaptation algorithm applied in a fading channel is considered. The system uses a stochastic gradient algorithm incorporating gradient sign feedback from a receiving unit to adjust the transmit weights. The feedback is simply a bit indicating which of two normalized perturbed transmission weights delivers greater power to the receiver. A reasonable performance measure is defined and an analytic estimate of the performance in an AR1 vector fading channel is derived and compared to simulations.

3:30, SAM-P3.8
PREDICTION ERROR BASED FEEDBACK FOR DOWNLINK TRANSMIT BEAMFORMING
S. NAGARAJ, Y. HUANG
This paper examines a downlink transmit beamforming scheme recently proposed by the authors. The idea is based on the use of an adaptive channel estimator at the base-station and requires the mobile to selectively feed back the value of the prediction error. This paper proposes a joint maximum-likelihood and set-membership filtering algorithm for adaptive channel estimation that provides robustness against incorrect feedback due to mobile decision errors. The amount of power and bandwidth-saving possible with this scheme is quantified via an uplink capacity analysis.

3:30, SAM-P3.9
DIRECTION-TIME TRANSMISSION FOR FAST FADING VECTOR CHANNEL
H. LI, E. POWERS, G. XU
Transmit diversity and space-time coding have received lots of research activities in recent years. In this paper, we propose a direction-time transmit scheme for fast block fading channel exploiting parametric structure of the time-varing vector channel. Specular propagation and local scattering are assumed at the base-station and mobile station respectively. The partial channel knowledge needed for the transmit design includes: number of specular paths, path direction vectors, length of coherent interval. Transmission matrix codewords are formed according to those partial channel information such that multiple channels are induced and symbols are spreaded over different transmission directions Diversity gain and blind channel estimation are achieved simultaneously at the mobile station through equalization.

3:30, SAM-P3.10
QUASI-STATIC ANTENNA ARRAY PROCESSING FOR RAPIDLY TIME-VARYING CHANNELS
T. THOMAS, F. VOOK
This paper explores the use of quasi-static frequency-domain antenna combining weights for multi-user (e.g., SDMA) or multi-stream (e.g., MIMO) communication systems operating in rapidly time-varying frequency-selective channels. By implementing combining weights that are constant across a time slot but are updated from slot-to-slot (i.e., "quasi-static"), great computational complexity savings can be realized compared to calculating new weights at each data block within a time slot. The quasi-static weights are computed by first modeling the time-varying channel for each user as the superposition of multiple time-invariant channels, called Doppler channels. The weights are then calculated based on all users' Doppler channels. These new weights work by using some of the degrees of freedom of the antenna array to suppress time variations as well as multiple access interference. Despite being fixed across a time slot, these weights can equalize and suppress SDMA interference even when the channel varies significantly over the slot.

3:30, SAM-P3.11
PERFORMANCE ANALYSIS AND COMPARISON OF MRC AND OPTIMAL COMBINING IN ANTENNA ARRAY SYSTEMS
B. RAO, M. WENGLER, B. JUDSON
In this paper we examine the statistical properties of the output signal to interference plus noise ratio (SINR) of a spatial combiner where the spatial weights used are either the Maximal Ratio Combiner (MRC) weights or the Optimal Combining (Weiner-Hopf) weights. The channels are modeled as slow flat Rayleigh fading channels and multiple interferers are assumed present. In particular, the modified F-distribution is introduced to provide an exact characterization of a MRC receiver and to bound the performance of the OC receiver. Simulation results are provided to support the analytical results and to provide insight.