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3527 results in Wireless Communications

Page 169 of 177

  • 10 - SPACE-TIME CODING FOR INFORMED TRANSMITTERS

  • Erik G. Larsson, Royal Institute of Technology, Stockholm, Petre Stoica, Uppsala Universitet, Sweden
  • Book:
    Space-Time Block Coding for Wireless Communications
    Published online:
    31 October 2009
    Print publication:
    08 May 2003, pp 212-222

  • Summary

    All the discussion so far in this book (with the exception of Sections 3.3 and 6.1) has focused on the case when the channel is unknown to the transmitter. In this chapter, we will briefly study how partial channel knowledge at the transmitter can be used to improve the system performance. In particular we will study linearly precoded STBC.

    Introduction

    If the transmitter knows the channel, then it is optimal from an error probability point of view to use what we referred to in Section 6.1 as beamforming. In the case of one receive antenna (nr = 1), beamforming amounts to transmitting a symbol weighted by the conjugate transpose of the channel, h* (for nr = 1, H becomes a row vector hT). Although doing so might lack the interpretation of beamforming in a strict physical sense (depending on the antenna configuration), it shares the main signal processing attributes thereof.

    For a given transmit power, the performance obtained via transmit diversity using STBC is in general inferior to that of beamforming, or receive diversity, by a factor that is sometimes called the “array gain.” Loosely speaking, this is so since space-time coding methods spread power uniformly in all directions in space, while beamforming uses information about the channel to steer energy in the particular direction of the receiver.

  • 11 - SPACE-TIME CODING IN A MULTIUSER ENVIRONMENT

  • Erik G. Larsson, Royal Institute of Technology, Stockholm, Petre Stoica, Uppsala Universitet, Sweden
  • Book:
    Space-Time Block Coding for Wireless Communications
    Published online:
    31 October 2009
    Print publication:
    08 May 2003, pp 223-234

  • Summary

    This short chapter is aimed at highlighting some issues related to the use of space-time coding in a multiuser environment. We will see that in general, the use of a space-time code changes the statistical properties of the transmitted signal compared with conventional transmission. We will also describe some simple techniques for multiuser interference suppression in a system that uses orthogonal STBC.

    Introduction

    Since the radio spectrum is a finite resource, the radio frequencies will always be shared. For this reason, all users in a system will suffer from co-channel interference, that is, disturbing radio signals from other users who use the same carrier frequency. The capacity of a system is related to how often, or how densely, the carrier frequencies are reused in the system. The more densely the frequencies are used, the higher the system capacity, but also the higher the level of co-channel interference. Therefore a signal processing algorithm that can suppress co-channel interference at the receiver, or maintain a functional communication link at a higher interference level, can also increase the system capacity. In a cellular system, the sharing of frequencies is usually coordinated by the network, whereas for some indoor local area networks, and so-called ad-hoc networks, it may be harder to assign radio resources in a coordinated manner; consequently the problem of mitigating co-channel interference may be even more important for such networks.

Page 169 of 177