Diversity Techniques and Fade Mitigation

The purpose of any communication system is to reliably transfer information between the source and destination. As a signal propagates through a wireless channel, it experiences random fluctuations in time, due to changes in reflections and attenuations. Thus, the channel characteristic of the channels change randomly with time. The average signal strength received by an antenna element over a local area in the propagation environment can be quite large, but during some time intervals it is not uncommon for the instantaneous signal levelin a multipath environment to fall 30 dB or more below its mean level. A substantial decrease in Signal to Noise Ratio (SNR) occurs in a flat fading channelwhen all arrivingmultipath components add destructively at the receiver antenna. In this case, the receiver is essentially experiencing in a deep fade or signal null. To cope with these results, during these time periods the receiver requires an alternate signal path to the transmitted signalwith a sufficiently large SNR in order to reliably decipher the desired signal. Accordingly, diversity is achieved by using the information on the different branches available to the receiver in order to increase the SNR at the decoding stage. The additional branches increase the probability that at least one branch, or the combined branch outputs, produce a sufficiently high SNR to permit reliable decoding of the useful message at the receiver. There are several domains to produce additional diversity branches; themain ones are antenna, time, and frequency domains. Space or site diversity refers to the method of transmission or reception, or both, in which the effects of fading are minimized by the simultaneous use of two or more physically separated antennas (or sites). Antenna diversity requires multiple antennas at the receiver and is therefore usually bulkier. However, operating at high frequency bands allows for the size reduction of antenna elements, and it becomes feasible to have multiple antennas not only at the base stations, but also on the mobile handset. Time diversity takes advantage of the dynamics of the channel; at some point in time the received signal might be in a deep fade, while at a later time the channel has changed significantly such that the received SNR is at an acceptable value. Frequency diversity is implemented by transmitting information on more than one carrier frequency. The rationale behind this technique is that frequencies separated bymore than the coherence bandwidth of the channel are uncorrelated and thus do not experience the same fading. Generally, this chapter focuses on all diversity domains and mitigation techniques in general against the blockage/multipath and atmospheric effects for satellite communication systems. More specifically, the satellite diversity concept is introduced and some results are presented for Code Division Multiple Access (CDMA) based systems. Furthermore, several other issues of the diversity concept applied in satellite communication systems, as Multiple-Input Multiple-Output (MIMO) and space time coding, Power Control (PC), Adaptive Coding and Modulation (ACM), are analyzed. Concerning the blockage/ multipath mitigation, recent results for joint fading mitigation, Intermediate Module Repeater (IMR) multipath diversity and combining techniques are presented.