The effects of frequency offsets and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading are investigated in this research. Random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to determine the covariance matrix of the interference-plus-noise vector. An analytic expression for the average signal-to-interference-plus-noise ratio (SINR) is obtained. Furthermore, average bit error probability (BEP) based on the Gaussian approximation is also derived. We show that the system degrades significantly if the frequency offset of the desired user is present, while the system is insensitive to interferers’ frequency offsets. In consequence, it is crucial to estimate and compensate the desired user’s synchronization imperfection. Finally,
design tradeoff among the number of sub-carriers, fading correlations, and inter-carrier interference (ICI) are presented in simulation results.
The effects of inter-carrier interference (ICI) and aperiodic
random spreading sequences on the performance of asynchronous
multicarrier code division multiple access (CDMA) systems with
correlated fading among sub-carriers are investigated in this
research. To obtain the maximal ratio combining (MRC) filter,
random parameters including asynchronous delays, correlated
Rayleigh fading and spreading sequences are averaged to find the
unconditional covariance matrix of the interference-plus-noise
vector. We demonstrate that the ICI in the system proposed by
Kondo and Milstein can be mitigated by
assigning a common random spreading sequence over all sub-carriers
for each user, rather than using a set of distinct spreading
sequences. Moreover, the analytic expression for the bit error
probability (BEP) can be obtained with the Gaussian approximation.
Simulation results are used to demonstrate the accuracy of our
analysis. Finally, various design tradeoffs including the number
of sub-carriers, fading correlations, ICI and multipath effect are
also presented in simulation.
The performances of direct sequence-code division multiple access (DS-CDMA), multicarrier-CDMA (MC-CDMA) and multicarrier-direct sequence-CDMA (MC-DS-CDMA) systems under different channel conditions are compared in this work. In a frequency-selective slowly fading channel, MC-CDMA and MC-DS-CDMA outperform DS-CDMA, since the former two systems partition the frequency band into sub-channels, each of which has a nearly constant frequency response. Thus, MC-CDMA and MC-DS-CDMA do not suffer much from the multipath effect. The performance of MC-CDMA and MC-DS-CDMA can be further differentiated in severe fading conditions. In a frequency-selective fast fading channel, the larger spreading ratio of MC-DS-CDMA in the time domain prevents the chip duration of a sub-carrier from being longer than the channel coherence time. Hence, the sub-carrier orthogonality is maintained in MC-DS-CDMA, leading to its better performance in this case.