SPATIAL MULTIPLEXING IN CELLULAR MIMO-CDMA SYSTEMS WITH LINEAR RECEIVERS OUTAGE PROBABILITY AND CAPACITY

INTRODUCTION

THE idea of using multiple receive and transmit antennas has emerged as one of the most significant technical breakthroughs in modern wireless communications. A large suite of techniques, known collectively as multiple input multiple output (MIMO) communications, have been developed in the past several years to exploit the resulting multidimensional channel. Significant spectral efficiency advantages can be achieved by exploiting the multidimensional MIMO channels in point-to-point communication.

For future cellular systems to compete in mobile data market with emerging technologies like 802.16e/WiMax in the medium to long-term, multi-antenna transmission and reception will be required to achieve the requisite high data rates. Since any well-designed cellular system is by nature interference-limited and it is in the strong interest of service providers to provide universal frequency reuse and high per-cell loading, MIMO systems, especially spatial multiplexing, will need to function reliably in an interference-limited environment.

Initial investigations on MlMO systems with co-channel interference can be seen in, which quantified the throughput of multicell MIMO systems with spatial multiplex¬ing by computer simulations. They showed that co-channel interference could seriously degrade the overall capacity of a spatial multiplexing system to the point of negligible im¬provement over single input multiple output (SIMO) systems.The authors showed that it is in fact preferable to have all users utilize only a fraction of the available substreams in an interference-limited MIMO system with linear receivers and single user detection.

The common conclusion of these papers is that the independent data streams effectively become independent interferers, and without any extra diversity, sufficient degrees of freedom to combat this co-channel interference are not available, unless the number of receive antennas is very large. Consequently, it is possible that although spatial multiplexing has a fundamental capacity advantage relative to transmit diversity, that this advantage is lost in cellular MIMO systems with linen receivers if any extra diversity is not provided. The most straightforward way to provide extra diversity is to exploit spatial diversity. However, there is a fundamental tradeoff between spatial multiplexing and spatial diversity in the limited degrees of freedom.

Spread spectrum is a likely candidate for the extra diversity because it can simultaneously provide frequency diversity and robustness to interference. It should also be noted that in MIMO spread spectrum (MIMO-CDMA) systems a larger number of transmit antennas can give a higher processing gain for a given data rate, which provides additional degrees of freedom for suppressing co-channel interference.

Furthermore, CDMA systems with low or unity spreading factors can be viewed as general interference-limited systems. For these reasons of current relevance and generality, the effectiveness of spatial multiplexing in cellular MIMO-CDMA systems with linear receivers needs to be carefully investigated. This paper provides a general framework for analyzing the outage capacityl of cellular MIMO-CDMA systems and investigates effectivenes of spatial multiplexing in terms of outage rapacity.

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