Diversity and Decoding in Non-Ideal Conditions

Nowadays, there are many products that provide personal wireless services to users who are on the move. Multiple antenna diversity is usually required to make a wireless link more reliable. User terminals have to be small enough to consume and emit low power. As a result, antennas cannot be spaced far apart enough to have independent and diverse branches for the received signals. Another issue affecting diversity gain is the unbalanced branches due to different locations or different polarizations of the antennas. The average signal power received from those unbalanced branches is different. Both the branch correlation and power imbalance degrade the benefits of diversity reception. Therefore, it is very important to investigate such effects before applying diversity reception to practical mobile or wireless radio systems. There have been a significant numbers of theoretical researches reported in the area of diversity systems and combining techniques. Some papers considered diversity systems with the correlated branches as in the references. The problems of correlated and unbalanced branches are addressed in (Dietze et al., 2002) and (Mallik et al., 2002) for the two-branch diversity system and for the Rayleigh fading channel. This chapter will address both the effects of branch correlation and power imbalance for generic L branches diversity system. The diagonalization transformation is used in the performance analysis for diversity reception with the correlated Rayleigh-fading signals in (Fang et al., 2000)-(Chang & McLane, 1997). Here, the diagonalization transformer is introduced as a linear transformer implemented before the diversity branches are being combined, which can transform the correlated and balanced branches to the uncorrelated and unbalanced ones, and vice versa. A real world simulation system is included in the chapter, which has the extended result of the paper (Vasana & McLane, 2004). Most analyses assume that the fading signal components are correlated in diversity branches but the noise components are independent in the branches. However, the external noise and interference that come with the fading signals are correlated. Plus, the coupling of diversity branches has the same effect on both signal and noise components. Some paper assumes that the dominant noise and interference have the same correlation distribution as the fading signals (Chang & McLane, 1997). This chapter assumes a generic case, in which the noise components are correlated with a correlation equal or smaller than the correlation between signal components. If the transmitted signal is u(t), the received signal from the kth branch can be expressed as: