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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64 ZERO-FORCING DECISION FEEDBACK EQUALIZATION<br />
the number of receive antennas exceeds the number of active codes, then we can<br />
tolerate ISI from the next symbol period, allowing us to use more received samples.<br />
This is usually not the case. With MIMO (same codes used on both transmit<br />
antennas), a ZF solution is possible if the number of receive antennas meets or<br />
exceeds the number of transmit antennas.<br />
For OFDM, ISI from future symbol periods is avoidable as long as the delay<br />
spread is less than the length of the CP (assuming the CP is discarded). ISI<br />
within a symbol period is avoided due the properties of the symbol waveforms.<br />
However, with MIMO, there is ISI between symbols transmitted from different<br />
transmit antennas but sharing the same subcarrier frequency. Like the CDM case,<br />
a ZF solution is possible if the number of receive antennas equals or exceeds the<br />
number of transmit antennas.<br />
Note: It is possible to use a CP with CDM. A rectangular windowing function<br />
would normally not be used, and the chip pulse shape would be root-Nyquist. As<br />
in the OFDM case, inter-block interference would be avoided by discarding the CP,<br />
as long as the delay spread is not too large.<br />
As for TDM, the possibility of multiple receive antennas provides additional flexibility<br />
in achieving a zero-forcing solution. In fact, multiple zero-forcing approaches<br />
become possible.<br />
In the remainder of this section, we will focus on two scenarios. The first is<br />
TDM with multiple transmit (cochannel) and receive antennas and dispersion. The<br />
second is the MIMO scenario (no dispersion), which is also of interest for both CDM<br />
and OFDM.<br />
3.4.1 Dispersive scenario and TDM<br />
We will continue with the assumptions of the previous section, but introduce multiple<br />
transmit and receive antennas. The model in (3.31) becomes<br />
N, , L-l<br />
r m μ £ V s « 0 Σ ëï )sii) (m -e) + u m , (3.36)<br />
¿,=i (m-¿) ) (3.37)<br />
t=i e=i<br />
which, assuming the previous decisions are correct, can be modeled as<br />
¿=1<br />
Notice that we have assumed that we are detecting all symbols from all transmitters.<br />
Often we are only interested in symbols from one transmitter. The other<br />
transmitters generate interference, which is referred to as cochannel interference.