Joint Adaptive Transmission and Switched Diversity Reception - Adaptive Signal Processing in Wireless Communications - page 155

1

2

Diversity
combiner

Receiver

Transmitter

L

FIgure 6.1 Block diagram of a receiver space diversity system.

6.2.2 System Model

In this work, we consider a receiver space diversity system as shown in Figure 6.1. In
particular, L antennas are used at the receiver to create differently faded replicas of the
transmitted signal. Because of the hardware complexity constraint that is imposed by
MM-wave systems, we assume that the receiver can only process a single selected diver-
sity branch. We adopt a discrete-time implementation for the proposed transmission
system. More specifically, short guard periods are periodically inserted into the trans-
mitted signal. During these guard periods, the receiver performs a series of operations,
including diversity path estimations and their comparisons, in order to select the appro-
priate diversity branch and the suitable adaptive modulation mode to be used during
the subsequent data burst reception. Once these decisions are made at the receiver, the
adaptive modulation mode is fed back to the transmitter via an error-free reverse chan-
nel before the guard period ends. After that, the transmitter and receiver are configured
accordingly throughout the subsequent data burst transmission.

6.2.3 Channel Model
We assume that the length of the guard period plus data burst is of the order of the chan-
nel coherence time, and therefore, the faded signal amplitude remains constant during
each guard period-data burst pair and de-correlates after that. We also assume that
the received signal on each antenna branch experiences an independent and identically
distributed (i.i.d.) fading process. As such, the faded SNR, denoted by γ i , i = 1, … , L , on
each diversity branch shares a common probability density function (PDF) and cumu-
lative distribution function (CDF). In Table 6.2 , we summarize the PDF, p γ ( x ), and the
CDF, F γ ( x ), of the received SNRs under three popular fading models: Rayleigh, Rice, and
Nakagami- m . In Table 6.2, γ - is the average SNR, Γ(·) is the Gamma function [31, section
8.31], I 0 (·) is the modified Bessel function of the first kind with zero order [31, section
8.43], Γ(·,·) is the incomplete Gamma function [31, section 8.35], and Q 1 (·,·) is the first-
order Marcum Q -function [32].

6.3 Joint Adaptive Modulation

and Selection Combining

In this section, we consider the analysis of a joint adaptive modulation and selection
combining system. We first present the mode of operation of the transmission system