MIMO Systems: Principles, Iterative Techniques, and Advanced Polarization - Adaptive Signal Processing in Wireless Communications - page 111

10 0

M R = 1

10 −1

M R = 2

10 −2

M R = 4

10 −3

10 −4

Tu rbo−MAP

M R = 3

Tu rbo−PIC

0 2 4 6 8 10 12 14

E b /N 0 (dB)

FIgure 4.10 Comparison of turbo-PIC and turbo-MAP detectors, M T = 4, (5, 7) 8 channel code,
QPSK modulation, 64 channel uses per frame.

by nonorthogonal schemes, hence relaxing the conditions on signal constellation and
channel coding. Here, a simple (suboptimal) iterative detector can be used for ST decod-
ing, as explained in the previous subsection, and we may approach the optimal detection
performance after few iterations. Nevertheless, the detector remains more complex, as
compared to the OSTBC case. However, this increased Rx complexity is quite justified;
using such an appropriate nonorthogonal ST scheme and iterative detection, we obtain
a considerable gain in performance with respect to OSTBC choice [64-66]. Results in
[65, 66] have also confirmed that the gain obtained by using nonorthogonal with respect
to orthogonal schemes is still considerable, and even more important when channel esti-
mation errors are taken into account.

4.4.4.1 Case Study
We consider the case of a (2 × 2) MIMO system, Gray bit/symbol mapping and random
interleaving, as well as the Rayleigh flat block-fading channel model with N c = 32 inde-
pendent fades per frame. The number of channel
uses corresponding to a frame is 768. The NRNSC
channel code (133, 171) 8 is considered with rate
R c = 1/2. The ST schemes we consider are shown
in Table 4.1, where η is the spectral efficiency in
units of bps/Hz. As the OSTBC scheme, we con-
sider the Alamouti code [51]. Using the formu-
lation of LD codes that we presented in section
4.4.2, we have Q = T = 2, R STC = 1, and

Table 4.1 Different ST Schemes for a
(2 × 2) MIMO System with η = 2 bps/Hz
ST
Scheme

R STC Modulation R c

Alamouti 1 16-QAM 1/2

V-BLAST 2 QPSK 1/2

GLD 2 QPSK 1/2