mohatta2015.pdf

THE MATH 107
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Eb/NO (dB)
Figure 5.3 BER vs. Eb/N 0 for QPSK, root-raised-cosine pulse shaping (0.22 rolloff),
static, two-tap, symbol-spaced channel, with relative path strengths 0 and —1 dB, and path
angles 0 and 90 degrees, DFE results.
The observations for MMSE and MISI DFE parallel those for MMSE and MISI
LE we saw in the previous chapter.
1. MMSE DFE performs better than MISI DFE. At high SNR, the performance
becomes similar, as ISI dominates.
2. At low SNR, MMSE DFE, MF and the MFB become similar, as noise dominates.
3. At low SNR, MISI DFE performs worse than the MF, because MISI DFE
focuses on ISI when noise is the real problem.
MMSE LE and MMSE DFE are compared in Fig. 5.4. At high SNR, MMSE
DFE performs better because most of the time it perfectly subtracts ISI from past
symbols. The combining weights focus on signal energy collection and suppression
of ISI from future symbols only. The combining weights for MMSE LE must also
try to suppress ISI from past symbols.
At low SNR, the MMSE DFE makes decision errors, which affect future decisions.
This problem is referred to as error propagation. As a result, performance is worse
than MMSE LE, which suppresses past symbol ISI through filtering.
Results for fractionally spaced equalization and for fading channels are given in
Chapter 6.