In this paper, a performance analysis of orthogonal frequency-division multiplexing (OFDM) based ultra-wideband (UWB) systems with frequency-domain spreading is presented. A continuum of schemes from a plain nonspread OFDM scheme to a maximally spread multicode scheme via medium-spread combinations of the variable spreading factor (VSF) and multi- code transmissions is considered. The channel is assumed to be Rayleigh fading with exponentially decaying delay power profile. Furthermore, the analysis takes into account the intersymbol interference (ISI) caused by the multipath delay components exceeding the guard interval. Numerical results demonstrate that frequency-domain spreading can improve the performance remarkably when sufficient spreading factors and a suitable combining method is used.

In this paper, a performance analysis for the frequency-domain code-division multiple-access (CDMA) of multiple simultaneously operating piconets (SOPs) in orthogonal frequency-division multiplexing (OFDM) based ultra-wideband (UWB) systems is presented. Two different strategies for the accommodation of multiples SOPs, namely, the variable spreading factor (VSF) scheme with minimal spreading and the multicode scheme with maximal spreading, were considered. The channel is assumed to be Rayleigh fading with exponentially decaying delay power profile. Furthermore, the analysis takes into account the intersymbol interference (ISI) caused by the multipath delay components exceeding the guard interval. Numerical results demonstrate that the frequency-domain CDMA can be employed to accommodate multiple SOPs while maintaining acceptable performance when a suitable subcarrier combining method is used. The VSF and multicode schemes result in a similar performance when the number of SOPs is large, while notable differences are pointed out when there are only a few SOPs.

A statistical characterisation of a small-scale fading mobile radio channel is presented and a fading simulator for the simulation of multiple input–multiple output multicarrier systems is introduced. The derivation of the equivalent lowpass channel as a function of time, transmit and receive antenna positions and subcarrier frequency, leads to a tapped delay-line model with time, space and frequency-selective taps. The taps are modelled 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 are therefore Rice fading. The Gaussian parts can have the desired temporal and spatiospectral correlations generated by time-correlation shaping filtering and a space–frequency correlation transformation, respectively. The structure and details of the simulator are presented, and the simulator is shown to achieve good accuracy while retaining a reasonable computational complexity.

Multicarrier code-division multiple access (MC-CDMA) is a promising multiplexing technique for future mobile radio systems, which require multirate transmission capability to efficiently support various services with wide range of data rates. In this paper, a generic performance analysis for the multicode and variable spreading factor multirate schemes in MC-CDMA is presented. Results comparing and pointing out notable differences in the error rate performance of these multirate schemes in conjunction with six different combining techniques are presented for the synchronous downlink and both the synchronous and asynchronous uplink. Furthermore, the analysis takes into account also the intersymbol interference caused by the multipath delay components exceeding the guard interval, which is commonly omitted in the literature.

In this paper, a generic performance analysis for the multicode and variable spreading factor (VSF) multirate schemes in a multicarrier code division multiple access (MC-CDMA) system is presented. In addition to the downlink channel, the analysis includes also both the synchronous and asynchronous uplink channels. Furthermore, the analysis takes into account the intersymbol interference (ISI) caused by the multipath delay components exceeding the guard interval, which is commonly omitted in the literature. Numerical results comparing and pointing out notable differences in the error rate performance of the two multirate schemes, different combining techniques, and spreading sequences are presented

A statistical characterization of a small-scale fading mobile radio channel is presented, and a fading simulator for the simulation of multiple input - multiple output (MIMO) multi-carrier systems is introduced. The derivation of the equivalent lowpass channel as a function of time, transmit and receive antenna locations, and subcarrier frequency, 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 (LOS) or dominant scattered path and a zero-mean Gaussian part composed of a number of unresolvable scattered paths, and are therefore Rice fading. The Gaussian parts can have the desired temporal and spatiospectral correlations generated by time correlation shaping filtering and a space-frequency correlation transformation, respectively. The simulator is shown to achieve a good accuracy while retaining a reasonable computational load.

Combining of space-time coded modulation (STCM) and phase sweeping transmit diversity (PSTD) is considered. The potential performance gain of space-time codes for fast fading channels can be utilized also in quasistatic fading channels by introducing quickened fading that can be obtained by employing PSTD. In addition to using separate PSTD antennas for each STCM signal, another approach where the additional PSTD antennas are shared between the STCM signals is proposed. Both approaches are shown to improve significantly the error rate performance of two considered space-time codes, which were proposed for fast fading channels, in slow Rayleigh fading.

The performance of space-time block and trellis codes proposed for narrowband systems is evaluated in wideband code division multiple access (CDMA) system operating over a frequency-selective Rayleigh fading channel. The evaluation is done by both analysis and computer simulations. The considered space-time codes are shown to provide considerable performance improvement over uncoded single-antenna transmission at high signal-to-interference-plus-noise ratio (SINR), but at high system loads the performance is pronouncedly limited in frequency-selective channels due to increased interference.