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Abstract Multicarrier and multiple input–multiple output (MIMO) techniques have become popular in wireless communications over multipath fading channels in recent years. This thesis firstly considers the characterization and simulation of fading mobile radio channels for MIMO multicarrier systems. Secondly, the performance of spread-spectrum multicarrier (MC) code-division multiple-access (CDMA) with multirate transmission is analyzed. Thirdly, the analysis is applied to ultra-wideband (UWB) orthogonal frequency-division multiplexing (OFDM) systems enhanced with frequency-domain code-division multiplexing (CDM). The response of a small-scale fading channel is derived as a function of time, transmit and receive antenna positions, and subcarrier frequency, which leads to a tapped delay-line model with time-, space-, and frequency-selective taps. The taps are modeled as a sum of a deterministic line-of-sight or dominant scattered path and a zero-mean Gaussian part composed of a number of unresolvable scattered paths and, therefore, are Rice fading. The Gaussian parts have the desired temporal and spatiospectral correlations generated by time-correlation shaping filtering and a space-frequency correlation transformation, respectively. The simulator achieves a good accuracy while retaining a reasonable computational complexity. The generic performance analysis of MC-CDMA includes both the multicode and variable spreading factor (VSF) multirate schemes that are inherent for CDMA and capable of providing efficient support for services of different required data rates. The analysis also takes into account the intersymbol interference caused by the multipath delay components exceeding a guard interval, which is commonly omitted in the literature by assuming the guard interval to be longer than the maximum delay spread. Results comparing and pointing out notable differences in the error rate performance of the two multirate schemes in conjunction with six different combining techniques are presented for a synchronous downlink and both a synchronous and asynchronous uplink. The analysis of CDM-enhanced OFDM-UWB involves first a single piconet with different combinations of the VSF and multicode schemes. Frequency-domain spreading is found to improve the performance remarkably when a sufficient spreading factor and a suitable subcarrier combining method are used. Subsequently, CDMA of simultaneously operating piconets (SOPs) with either the VSF or multicode scheme is considered. While both multirate schemes result in a similar performance when the number of SOPs is large, notable differences arise when there are only a few SOPs.