| Summary: | 碩士 === 國立交通大學 === 電信研究所 === 83 === Frequency hopping systems are often divided into slow FH and
fast FH systems. The latter class has a hopping rate of $k$
hops/symbol where $k > 1$ while for the former class $k \le 1$.
The anti-jam (AJ) capability of a fast FH system is superior to
that of a slow FH system for it has an implicit coding gain
through the use of diversity. To enhance a slow FH system's AJ
capability forward error-control (FEC) coding is employed. It
is well known that soft-decision decoding hard-decision
decoding in an AWGN channel by an average margin of 2 dB. But
the soft-decision decoding gain is often much more impressive
when operates in a non-AWGN channel. This thesis studies a
special slow FH system, one that uses MFSK modulation and RS
code. We do not consider a full soft-decision decoder due to
the complexity consideration. Instead, we use a single-pass
errors-and-erasures decoder. Two jamming scenarios are studied:
partial-band noise jammer (PBNJ, in Chapter 3) and band
multitone jammer (BMTJ, in Chapter We examine two schemes for
generating a decision to erase an unreliable symbol (erasure
insertion) so that the error correcting capability can be
increased. One of them is called the direct test (DT) and the
other is borrowed from Viterbi's ratio threshold test (RTT).
Another issue under investigation is the effect of finite
interleaving length, which was usually neglected in performance
analysis. Although we consider block interleaver only our
results can easily be used for systems using convolutional
interleaver by simple modifications on parameter values.
Numerical results are presented to compare the effectiveness of
DT and RTT and show the relationships amongst the hopping rate,
the interleaver size and the coding rate. Most of our analysis
concentrates on the case when the signal size ($M$) and the
codeword symbol field size ($|GF(q)|=2^{\ell}$) are equal. An
alternative design option $M \neq q$, $\ell/ (log_2 M)=$
~integer is briefly addressed (Section 3.3.5).
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