| Summary: | 碩士 === 國立交通大學 === 電信研究所 === 85 === Multi-carrier (MC) or multi-tone (MT) transmission, which is
often referred to asOrthogonal Frequency Division Multiplexing
(OFDM), is an effective signalingmethod against frequency-
selective fading. On the other hand, the code divisionmultiple
access (CDMA) technique has attracted a great deal of interest
recently,because its spatial diversity combining capability and
its potential to offer a systemcapacity larger than those
achieved by other more conventional multiple access (MA)
techniques. The combination of MC and CDMA techniques can
therefore provide enhancedanti-multipath and anti-fading
capability while accommodating a large number of activesystem
users.In this thesis, we analyze the frequency and time response
of OFDM systems. We discusstwo classes of OFDM/CDMA systems:
those spread across time and those spread acrossfrequency
(subcarriers). Time- and frequency-selective Rayleigh fading
channels are considered.etc.--and channel conditions--delay
spread, maximum Doppler shift, etc.--on the systemperformance
are examined. Guard time insertion can eliminate intersymbol
interference (ISI)if its length is longer than the maximum delay
spread. In a guard interval, we can eitherinsert a periodic
extension of the desired message or leave it empty. The former
alternative,however, may incurs cyclic interchannel
interference. Hence, an accurate estimation of thechannel
condition and a judicious selection of the system parameters are
needed to achievethe optimal performance.We also propose a new
equalization method of OFDM/CDMA using different
updatealgorithms in training mode and tracking mode. Simulation
results show thatthis new equalizer is effective.
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