mohatta2015.pdf
signal processing from power amplifier operation control point of view
signal processing from power amplifier operation control point of view
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132 MAXIMUM LIKELIHOOD SEQUENCE DETECTION<br />
Modem BER as a function of E b /N t) (SNR.) is shown in Fig. 6.10. At low<br />
SNR, all receivers perform similarly, as performance is noise-limited. At high SNR,<br />
MLSD performs better than DFE and LE. DFE and LE try to remove ISI through<br />
filtering and/or subtraction. As a result, signal energy collection is sacrificed for ISI<br />
suppression. MLSD does not have to trade signal energy, as it accounts for what<br />
the channel does to the signal rather than trying to partially undo it in some way.<br />
-2 0 2 4 6 8 10 12 14<br />
Eb/NO (dB)<br />
Figure 6.10 BER vs. E b /N 0 for QPSK. root-raised-cosine pulse shaping (Ü.22 rolloff).<br />
static, two-tap. symbol-spaced channel, with relative path strengths 0 and —1 dB, and path<br />
angles 0 and 90 degrees, single feedback tap.<br />
6.3.6.1 Fractionally spaced equalization results We now consider a two-path, halfchip<br />
spaced channel with relative path strengths 0 and —1 dB and path angles 0<br />
and 90 degrees. The LE results correspond to 61 7'/2-spaced taps centered on the<br />
first signal path. The DFE results use a 31-tap, T/2-spaced forward filter with the<br />
first tap aligned with the first signal path. Also, ISI from 3 past symbols is removed,<br />
which removes most of the ISI. The MLSD results use a fractionally spaced, ideal<br />
matched filter front end and L s = 4 (ISI from 3 past symbols is accounted for). DFE<br />
and MLSD performance is slightly worse when only 2 past symbols are considered.<br />
Note that in both cases, due to the ringing of the pulse shape, ISI from even earlier<br />
symbols is still present, though small. While this was ignored, it could be included<br />
in the forward filter and matched filter designs, respectively. In the latter case, this<br />
residual ISI could be modeled as nonstationary noise, giving rise to a matched filter<br />
in colored noise.<br />
Results are provided in Fig. 6.11. Relative performance of the different equalizers<br />
is similar to the symbol-spaced case. However, unlike the previous symbol-spaced