| Abstract | Multicarrier code division multiple access (MC-CDMA) is a technique that combines
orthogonal frequency division multiplexing (OFDM) and code division multiple access
(CDMA). Since MC-CDMA system provides an efficient method of frequency diversity, it is
suitable to a frequency selective rayleigh fading environment. This technique becomes a
potential technique for future wireless networks. In addition, MC-CDMA is especially useful
in the downlink transmission. However, similar to OFDM system, the transmitted signal
from MC-CDMA system also exhibits a very high Peak to Average Power Ratio (PAPR)
when using a large number of subcarriers. Therefore, the amplifiers should be operated in
large linear ranges to avoid nonlinear distortion. This leads to very inefficient amplification
and very expensive transmitters. Thus, P APR reduction techniques are very significant for
multicarrier transmission systems.
Consequently, several P APR reduction methods have been proposed, but most of
techniques are considered with single user OFDM such as Clipping, Selected Mapping,
Partial Transmit Sequence, and so on. In MC-CDMA systems, the P APR reduction technique
is mostly based on the type of spreading sequences.
Partial Transmit Sequence (PTS) technique is an efficient approach and a distortionless
scheme for reducing P APR. Due to its flexibility, the PTS technique works with an arbitrary
number of subcarriers. In addition, it works without restriction on the type of modulation
scheme. Thus, the investigation of P APR in MC-CDMA system combined with PTS
technique is very attractive.
In this thesis, the P APR reduction in synchronous downlink MC-CDMA using PTS
technique is investigated with a different number of active users. Moreover, WalshHadamard
(WH) sequence and Orthogonal Gold (OG) sequence are used as the spreading
sequences in MC-CDMA system. From the simulation results, when the number of active
users is varied, P APR reduction of this system also varies depending on the P APR
characteristics of spreading sequence. Although PTS technique is very efficient in reducing
PAPR, high PAPR reduction is achieved at a cost of increased system complexity.
Furthermore, to compromise between P APR reduction and system complexity, the
suboptimal algorithms are used for combining partial transmit sequences. Based on
simulation results, we can reduce system complexity by more than 90% at the expense of
little P APR performance degradation. |