||The third generation wireless mobile and personal communication systems are expected to support not only high quality voice, but also data, facsimile, and video services on networks such as wireless LANs. To achieve such an objective, the next-generation personal communication systems will need to be able to support a very high speed data transmission along with a wide range of services. However, obtaining high bit rates at low error rates over wireless channels is a difficult task. Transmission over wireless and mobile channels is severely restricted by the propagation characteristics of the wireless environment. Signals typically arrive at the receiver via a scattering mechanism resulting in multiple propagation paths with different time delays, amplitude, and phases. Therefore, a time delay spread also occurs and this imposes a limit on the maximum transmission rate. This multipath propagation also produces inter-symbol interference (ISI) which leads to the introduction of an irreducible error floor. Recently, a new modulation technique, known as multicode modulation, has been developed for high-speed data transmission over wireless environments. In this method, the incoming high-rate data stream is divided into a number of parallel low-rate bit streams as in multitone modulation. However, each low-rate data stream is then modulated by a code, which is a combination of orthogonal codes and spreading sequences, on a single carrier. The processing gain is properly selected so that the required bandwidth will be the same as that of the original high-rate data stream; thereby, gaining the inherent benefit of multipath rejection without expanding the bandwidth of the original high-rate stream. Potential applications of this method include those that require variable and high transmission rates such as wireless Ethernet connection for multimedia communications. In this thesis, we investigate the use of channel coding, such as convolutional codes and turbo codes, in multicode modulation over multipath Rayleigh fading channels. In addition, we propose various multicode modulation schemes which employ different combination of orthogonal codes and signature sequences. In order to improve the system performance, we also consider the use of antenna diversity. Since the conventional multicode system uses a RAKE receiver in each sub-channel, the complexity of the receiver can be quite high. Hence, we propose a new low-complexity receiver structure for the multicode modulation system which is based on the concept of Fast Walsh Transform or Fast Fourier Transform. There may be one possible disadvantage of multicode modulation; namely, the peak-to-average power ratio can become large when the number of sub-channels is large. Hence, power amplifiers which are linear in a wide range are required for the amplification of these signals. Thereby, reducing the efficiency of the power amplifier. We thus propose a new orthogonal code structure that can reduce the peak-to-average power ratio.