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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MORE MATH 167<br />
If the BCJR algorithm is used, the bit likelihoods can be computed directly from<br />
the joint state transition/data probabilities by summing transition probabilities<br />
corresponding to a particular bit value.<br />
7.3.3 Joint demodulation and decoding<br />
With full joint demodulation and decoding, we would need to consider each possible<br />
codeword and form a sequence metric. This is usually highly complex, motivating<br />
approximate approaches.<br />
One popular approximate approach is turbo equalization. With turbo equalization,<br />
the equalizer and decoder interact with one another, as illustrated in Fig. 7.2.<br />
The first time the equalizer runs, it assumes all symbol values are equi-likely. Soft<br />
bit values are passed to the decoder as usual, without adjustment. The decoder<br />
then performs decoding, but produces some additional information. It determines<br />
likelihoods associated with the modem bit values. These likelihoods are adjusted<br />
to capture only the information learned from the decoding process (the information<br />
provided by the equalizer is removed). The adjusted or extrinsic information<br />
is then used by the equalizer to process the received samples a second time. The<br />
results are adjusted and then fed to the decoder for a second decoding process. The<br />
process continues for either a fixed number of iterations or until an error detection<br />
code determines that there are no errors. We have shown the adjustments as subtractions,<br />
which assumes the soft bit information is in the form of a log-likelihood<br />
ratio (LLR).<br />
equalizer<br />
decoder<br />
+ H-<br />
Figure 7.2<br />
Turbo equalization.<br />
Turbo equalization was originally formulated for use with MAPSD. However,<br />
there have also been formulations with LE, referred to as linear turbo equalization.<br />
7.4 MORE MATH<br />
The basic mathematics have been covered previously. For systems in which symbols<br />
are sent in parallel (CDM, MIMO), a vector form of the MAP algorithm can be<br />
used.<br />
7.5 AN EXAMPLE<br />
The LTE downlink, which employs OFDM, is used as an example in Chapter 2.<br />
Here we consider the case in which two transmit and two receive antennas are used