mohatta2015.pdf

THE MATH 133
results, DFE and MLSD complexity have increased due to the need for most feedback
taps and a larger state space, respectively. Compared to the previous figure,
performance of each equalizer improves due to the lower delay spread of the channel.
HALF-SYMBOL-SPACED CHANNEL
IQ' 1
LU
m
IQ 2
10" 3 -2 0 2 4 6 8 10 12
Eb/N0 (dB)
Figure 6.11 BER vs. E b /N 0 for QPSK, root-raised-cosine pulse shaping (0.22 rolloff),
static, two-tap, half-symbol-spaeed channel, with relative path strengths 0 and —1 dB, and
path angles 0 and 90 degrees, 3 feedback taps.
6.3.6.2 16-QAM results Let's return to the static, two-tap, symbol-spaced channel
with relative path strengths 0 and —1 dB and path angles 0 and 90 degrees.
Only now consider 16-QAM instead of QPSK. Modem BER is shown as a function
oîE b /N t) (SNR) in Fig. 6.12. Both analytical (REF) and simulated (MFB) results
are provided, the latter created through perfect subtraction of ISI from the previous
and subsequent symbols.
Relative to the QPSK results in Fig. 6.10, we see that all curves have shifted
right. This is because 16-QAM pays a penalty in sending more than one bit on
each orthogonal dimension. Otherwise, trends are similar.
6.3.6.3 Fading results Now we consider fading channels. Consider the two-path,
symbol-spaced fading channel (TwoTSfade). Each path experiences independent
fading with average relative powers 0 and —1 dB. The phase angle settings are nó
longer important, as the fading introduces a random, uniformly distributed angle. A
block fading channel model is used, in which 2000 fading realizations are generated.
For each realization, 200 symbols (plus edge symbols) are transmitted through the
channel, noise is added, and the resulting signal is demodulated.
Matched filter bound results were generated one of two ways. One is a semianalytical
approach (labeled REF). For each fading realization, a channel gain is