Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing this scheme in a complete wireless system.

Several aspects of the design and optimization of coded multiple-antenna transmission diversity methods for slowly time-varying channels are explored from an information-theoretic perspective. Both optimized vector-coded systems, which can achieve the maximum possible performance, and suboptimal scalar-coded systems, which reduce complexity by exploiting suitably designed linear precoding, are investigated. The achievable rates and associated outage characteristics of these spatial diversity schemes are evaluated and compared, both for the case when temporal diversity is being jointly exploited and for the case when it is not. Complexity and implementation issues more generally are also discussed.

High data rate wireless communications, nearing 1 Gigabit/second (Gbps) transmission rates, is of interest in emerging Wireless Local Area Networks (WLANs) and home Audio/Visual (A/V) networks. Designing very high speed wireless links that offer good Quality-of-Service (QoS) and range capability in Non-Line-of-Sight (NLOS) environments constitutes a significant research and engineering challenge. Ignoring fading in NLOS environments, we can, in principle, meet the 1Gbps data rate requirement with a single-transmit single-receive antenna wireless system if the product of bandwidth (measured in Hz) and spectral efficiency (measured in bps/Hz) is equal to 10 9. As we shall outline in this paper, a variety of cost, technology and regulatory constraints make such a brute force solution unattractive if not impossible. The use of multiple antennas at transmitter and receiver, popularly known as multiple-input multiple-output (MIMO) wireless is an emerging cost-effective technology that offers substantial leverages in making 1Gbps wireless links a reality. This paper provides an overview of MIMO wireless technology covering channel models, performance limits, coding, and transceiver design.

: This paper extends the traditional Clarke/Jakes model for a frequency flat fading process in a land mobile radio system to facilitate the examination of coherent space-time demodulation systems. The work develops a space-time correlation function using a ring of scatterers model around the mobile unit. The resulting correlation function permits the investigation of a variety of issues concerning base station configurations in space-time systems. The interrelationship of the fading process between the space and the time domain is explored. A detailed example regarding the effects of antenna separation in a receiver diversity system is considered. A set of design rules for interleaving depth and antenna separation in a space-time modem are presented and quantified. 1 Introduction Diversity techniques provide significant performance improvements for fading channels. Typical diversity techniques include time diversity, frequency diversity and spatial diversity. Designs with combinations...

Abstract—Coping with time-selective fading channels is challenging but also rewarding, especially with multiantenna systems, where joint space-Doppler diversity and coding gains can be collected to enhance performance of wireless mobile links. These gains have not been quantified, and space-time coded systems maximizing joint space-Doppler benefits have not been designed. Based on a parsimonious basis expansion model for the underlying time-selective (and possibly correlated) channels, we quantify these gains in closed form. Furthermore, we develop space-time-Doppler coded systems that guarantee the maximum possible space-Doppler diversity, along with the largest coding gains within all linearly coded systems. Our three novel designs exploit knowledge of the maximum Doppler spread, and each offers a uniquely desirable tradeoff, including high spectral efficiency, low decoding complexity, and high performance. Our analytical results are confirmed by simulations and reveal the relative of merits of our three designs in comparison with an existing approach. Index Terms—Basis expansion channel model, diversity, fading, phase sweeping, space-time coding, time-varying channel. I.

This paper studies the error propagation effect that is caused by certain ambiguities in joint data detection--channel tracking algorithms for transmission diversity schemes. Here, we use a Space-Time (ST) receiver based on the Maximum A Posteriori (MAP) method that takes into account the channel estimation error assuming the unknown channel to have a given complex multivariate Gaussian probability density function (pdf) (i.e., a Ricean channel). The decision criterion that is expressed in quadratic form represents either a linear detector or a non-coherent-non-linear detector in extreme cases. Then, the channel pdf for the next iteration is updated by estimates of the second order statistics of the channel coefficients, and a very simple decision-directed adaptive algorithm is derived for adaptive channel estimation. The adaptive algorithm can efficiently track a fast Rayleigh fading channel and as a result achieves robust performance. However, the occurrence of two types of ambiguities initiated in deep fades result in error propagation.

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Coping with time-selective fading channels is challenging, but also rewarding especially with multiantenna systems, where joint space-Doppler diversity and coding gains can be collected to enhance performance of wireless mobile links. These gains have not been quanti ed, and space-time coded systems maximizing joint space-Doppler bene ts have not been designed. Based on a parsimonious basis expansion model for the underlying time-selective (and possibly correlated) channels, we quantify these gains in closed form. Furthermore, we develop space-time-Doppler coded systems that guarantee the maximum possible space-Doppler diversity, along with the largest coding gains within all linearly coded systems. Our three novel designs exploit knowledge of the maximum Doppler spread, and each oers a uniquely desirable tradeo among: high spectral eciency, low decoding complexity, and high performance. Our analytical results are con rmed by simulations that not only validate the channel model, but also reveal the relative of merits of our three designs in comparison with an existing approach.

In this paper, we will demonstrate that a system employing a space time convolutional code (STCC) can be imnplemented with only a single transmit antenna when there are multiple receive antennas. The idea is to transmit more than one symbol from a single transmit antenna during a symnbol period by superimposing the encoded symnbols on top of each other. This objective is achieved by inducing randomness into the system, that creates additional channel paths, called virtual paths. The design of such an approach is studied for slow Rayleigh fading channels. One immediate application of this approach is to model an n x m multiple-input mutiple-output (MIMO) system as an equivalent group of n distinct 1 x m systems. Consequently, we demonstrate that STCCs with high spectral efficiencies can be designed utilizing QPSK STCCs as component codes for each 1 x m system. Simulation results evaluate the performance of this technique for the case of two transmit antennas and several different number of receive antennas, a spectral efficiency of4 bits/s/Hz, and slow Rayleigh fading channels.

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