This letter presents two results for multiuser wireless systems employing dirty-paper coding strategies along with greedy scheduling over the broadcast multiple-input multiple-output channel. Specifically, an efficient and suboptimal downlink scheduler is proposed to approximate the maximum sum rate using equal power allocation, and it is shown to approach the maximum sum rate of optimal power allocation for a large number of users under optimal scheduling. The second result demonstrates that the average maximum sum rate can be tightly upper bounded when spatial multiplexing is maximized.

This paper investigates peak-to-average power ratio (PAR) reduction with constrained transmit power for spatially precoded downlink transmission, based on orthogonal frequency-division multiplexing (OFDM). We generalize the concept of multiuser Tomlinson-Harashima (T-H) precoding and propose a nonlinear spatial precoder, termed a complex sphere precoder (CSP), which has the same throughput and error performance as the T-H precoder, but achieves both a low PAR and low average transmit power. Our study shows that a sequential CSP over individual tones achieves a significant PAR reduction while requiring a moderate transmit power increase. The suboptimal per-tone lattice search constitutes a good compromise between performance and complexity.

This paper considers packet transmission over a multiple-input-and-multiple-output (MIMO) Gaussian broadcast channel (GBC). Specifically, we compare and contrast two transmission strategies built on dirty paper coding (DPC) and the Bell labs layered space-time architecture (BLAST), respectively, under channel-aware scheduling, and demonstrate the advantage of a distributed MIMO architecture at a system performance level. For the DPC-based strategy, we also present an efficient suboptimal scheduling algorithm using equal transmit power allocation across users with a maximum order of spatial multiplexing which achieves near optimal scheduling performance for a sufficient number of users.

We consider the use of multiple antennas at the transmitter and/or the receiver to provide open-loop spatial diversity in a multiuser wireless data network. With channel quality information (CQI) available to the transmitter, and by always scheduling the transmission to the active user having the best channel conditions at the time of scheduling, another form of diversity, termed multiuser diversity, is obtained in a data system. This paper provides an analysis of the interaction between these two forms of diversity. From a network point of view, we prove that the asymptotic sum rate, in the limit of a large number of active homogeneous users and subject to the same average total transmit power, is inversely related to the number of transmit antennas for independent and identically distributed (i.i.d.) flat Rayleigh fading channels. In the case of i.i.d. flat Rician fading, the asymptotic sum rate also depends inversely on the number of transmit antennas, but directly on the number of receive antennas. Numerically, we show that the total diversity gain is also constrained by finite CQI quantization and channel fading statistics.

A global optimization technique is applied to solve the optimal transmitter placement problem for indoor wireless systems. An efficient pattern search algorithm - DIviding RECTangles (DIRECT) of Jones et al.- has been connected to a parallel three-dimensional radio propagation ray tracing modeler running on a 200-node Beowulf cluster of Linux workstations. Surrogate functions for a parallel wideband code-division multiple-access (WCDMA) simulator were used to estimate the system performance for the global optimization algorithm. Power coverage and bit-error rate are considered as two different criteria for optimizing locations of a specified number of transmitters across the feasible region of the design space. This paper briefly describes the underlying radio propagation and WCDMA simulations and focuses on the design issues of the optimization loop.

This paper presents comparisons between STTD (transmit diversity in WCDMA), downlink beamforming, and a system which combines both transmit diversity and beamforming.