mohatta2015.pdf

132 MAXIMUM LIKELIHOOD SEQUENCE DETECTION
Modem BER as a function of E b /N t) (SNR.) is shown in Fig. 6.10. At low
SNR, all receivers perform similarly, as performance is noise-limited. At high SNR,
MLSD performs better than DFE and LE. DFE and LE try to remove ISI through
filtering and/or subtraction. As a result, signal energy collection is sacrificed for ISI
suppression. MLSD does not have to trade signal energy, as it accounts for what
the channel does to the signal rather than trying to partially undo it in some way.
-2 0 2 4 6 8 10 12 14
Eb/NO (dB)
Figure 6.10 BER vs. E b /N 0 for QPSK. root-raised-cosine pulse shaping (Ü.22 rolloff).
static, two-tap. symbol-spaced channel, with relative path strengths 0 and —1 dB, and path
angles 0 and 90 degrees, single feedback tap.
6.3.6.1 Fractionally spaced equalization results We now consider a two-path, halfchip
spaced channel with relative path strengths 0 and —1 dB and path angles 0
and 90 degrees. The LE results correspond to 61 7'/2-spaced taps centered on the
first signal path. The DFE results use a 31-tap, T/2-spaced forward filter with the
first tap aligned with the first signal path. Also, ISI from 3 past symbols is removed,
which removes most of the ISI. The MLSD results use a fractionally spaced, ideal
matched filter front end and L s = 4 (ISI from 3 past symbols is accounted for). DFE
and MLSD performance is slightly worse when only 2 past symbols are considered.
Note that in both cases, due to the ringing of the pulse shape, ISI from even earlier
symbols is still present, though small. While this was ignored, it could be included
in the forward filter and matched filter designs, respectively. In the latter case, this
residual ISI could be modeled as nonstationary noise, giving rise to a matched filter
in colored noise.
Results are provided in Fig. 6.11. Relative performance of the different equalizers
is similar to the symbol-spaced case. However, unlike the previous symbol-spaced