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Spread spectrum communication systems allow many users access to the same
frequency band simultaneously. In an SFH system, each user broadcasts
m channel symbols (m > 1) on a specific sub-band before
hopping to a new sub-band. The time spent in a specific sub-band is
termed a dwell. As the number of users increases, so does the
probability that two users will broadcast in the same sub-band at the same
time (this is termed a hit). In addition, noise will corrupt
messages in any real channel. Forward error correcting codes, such as Reed-Solomon (R-S) codes, are often used to combat errors introduced by hits and other channel noise. The nature of these codes allows the decoder to reconstruct damaged messages with redundant information. This research project will study the effectiveness of error and erasure decoding with the parity-bit method for MPSK (M=2,4,8) and DPSK in AWGN and interfering channels.
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The performance of the system will be measured with regard to various
factors. An extended (n, k) R-S code will be used to
introduce redundancy into the channel symbols allowing for correction of
e errors and s erasures with 2e+s < (n-k+1) [1]. To take advantage of
error and erasure decoding, information about which symbols to erase must
be provided to the decoder. The use of a parity bit with each channel
symbol is one way to provide this information. If no information about
which symbol to erase is provided, the decoder will not be able to erase channel symbols and will use error only decoding. Analysis will be performed for BPSK, QPSK, and 8-PSK systems. When one user (or very few) occupies a channel, the AWGN model provides a realistic analysis. When enough users are present to cause frequent hits, a user will experience catastrophic interference in the dwell(s) where a hit has occurred. Both situations will be analyzed.
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[1] Wicker, Stephen B., Error Control Systems for Digital
Communication and Storage, Upper Saddle River, New Jersey: Prentice
Hall, 1995.
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