The ability to achieve high bit rates at low error rates over wireless links is severely restricted by the propagation characteristics of the wireless environment where signals typically arrive at the receiver via a scattering mechanism resulting multiple propagation paths with different time delays, amplitudes and phases. This causes a spread in delay times which imposes a limit on the maximum transmission rate. These restrictions manifest themselves as intersymbol interference which leads to the introduction of an irreducible error floor. Therefore, without countermeasures to mitigate the delay spread impairments, the information rate is usually limited to be under 1 Mbps when user mobility prevents steady line-of-sight conditions....[ Read more ]

The ability to achieve high bit rates at low error rates over wireless links is severely restricted by the propagation characteristics of the wireless environment where signals typically arrive at the receiver via a scattering mechanism resulting multiple propagation paths with different time delays, amplitudes and phases. This causes a spread in delay times which imposes a limit on the maximum transmission rate. These restrictions manifest themselves as intersymbol interference which leads to the introduction of an irreducible error floor. Therefore, without countermeasures to mitigate the delay spread impairments, the information rate is usually limited to be under 1 Mbps when user mobility prevents steady line-of-sight conditions.

In this thesis, we propose a high-speed transmission technique called multicode modulation with interference cancellation technique to improve signal quality over non-line-of-sight frequency-selective fading channels. In this technique, the high-rate data are serial-to-parallel converted into low-rate data streams which are then modulated with Walsh and Pseudo-noise concatenated codes. After using Rake receivers to process the received waveform, interference cancellers are applied to lower the bit error rate floor. Numerical results show that significant performance improvement can be achieved. Using this technique, high transmission rates can be supported over a multipath fading channel having only non-line-of-sight components. Potential applications of this modulation method include those that require variable and high peak transmission rates such as the wireless Ethernet connection for multimedia communications.

We study the performance of multicode modulation for single-user access in greater detail. Comparisons with performance using multicarrier modulation are also made. In addition, we also propose several strategies for variable-rate multiuser wireless access. We found that similar performance can be achieved by both multicode and multicarrier modulations. However, multicode technique appears to be more flexible in handling multiple access because both CDMA and PDMA/TDMA options are available.