Some concatenated and iterative decoding approaches for continuous phase modulation
Several concatenated and iterative decoding approaches are used to
improve the power efficiency of continuous phase modulation (CPM).
First a symbol interleaver is used for serially concatenated CPM
(SCCPM). Convergence of iterative decoding for this scheme can be
earlier (lower SNR: Signal-to-Noise-Ratio) compared to bit
interleaved SCCPM. Then a ring convolutional code (CC) is used as
the outer code in this symbol interleaved SCCPM. By setting the
denominator of the CPM modulation index equal to the alphabet size
of the system (CPM, interleaver and ring CC), a natural
combination of them is achieved. Further improvements on earlier
convergence or error floors are achieved with this scheme.
Finally, irregular repeat accumulate (IRA) codes with CPM (IRCPM)
are considered. CPM now takes the role of the accumulator. This
scheme shows even earlier convergence than SCCPM.
To analyze the error floor performance, we derive the union bound
for symbol interleaved SCCPM. In this bound analysis, we have to
consider the order of nonzero permuted difference symbols that are
not identical. We also derive the union bound for IRCPM from its
equivalent structure. This structure introduces a virtual
interleaver before CPM. Thus, the union bound analysis for SCCPM
is now applicable.
serially concatenated codes
Continuous phase modulation