1. Multiple-access communications for future wireless systems

Due to the substantial increase of internet users and with the introduction and development of new services, high-speed access in the future generations of wireless systems is an important issue. Consequently, broadband systems with bandwidths much wider than that of the 3rd-generation systems are required in order to meet future requirements. Hence, compatibility with both the emerging Broadband Access Networks (BRAN), which has been opted for a multi-carrier, Orthogonal Frequency Division Multiplexing (OFDM) based solution and the existing 2nd- and 3rd-generation CDMA systems is an important consideration.

The objective of this research is to investigate the relevant techniques in the context of FH/MC DS-CDMA, including modulation/demodulation, error-control, synchronization, equalization, multiuser interference suppression, adaptive detection, etc.

2. Multi-carrier CDMA techniques

Multi-carrier CDMA is an enabling technology for future mobile phones. Future mobile phone services are likely to be endowed with crystal-clear integrated digital TV, digital Hi-Fi radio, high-speed internet browsing as well as the video phone capabilities. All these services require efficient transmission of multimedia traffic. Multi-carrier CDMA is likely to be the transmission method of future mobiles due to its bandwidth efficiency and inherent diversity over fading channel. However, the multi-carrier signals show highly varying power envelope waveforms, which hinder the popular employment of multi-carrier CDMA.

Our research focuses on the study this phenomenon and to provide some practical solutions for it. Another important topic is to use adaptive modulation, exploiting the channel quality fluctuations, so that more multimedia traffic can be exchanged using the same bandwidth.

3. Ubiquitous antenna based systems

According to this concept a high number of antennas are distributed over the service area. Each antenna and the base station are connected via optical fiber, conveying the radio signals directly over the optical-fiber cable.The ubiquitous antenna not only reduce the required number of radio base stations, which reduces the cost of the overall system, but also allow us to employ sophisticated signal processing techniques such as diversity reception, interference cancellation, and multi-user detection.

This research is to study and design simple interference cancellation algorithms for the ubiquitous antenna.

4. Wireless multi-user OFDM systems

In a typical multi-cellular environment a signal transmitted from a handset to the base station is subjected to a variety of impairments. The most obvious impairment is the contamination of the transmitted signal by thermal noise at the receiver. Secondly, the signal quality suffers from multipath propagation, which implies that several delayed replicas of the same signal arriving at the receiver antenna. This inflicts inter-symbol interference, which heavily degrades the signal quality and hence must be compensated by equalization. In addition the channel might impose time variant fading due to the mobility of the users or the deflectors and caterers. In a multi-cellular environment we additionally encounter interference from other users and base stations.

This research is concerned with the aspects of channel parameter estimation as well as synchronization mechanisms.

5. Source matched adaptive transmission

The required bit rate, the acceptable error rate and delay of a wireless communication link varies depending on the type of information transmitted. Upon varying the transmission power, the forward error correction code and the modulation format, we can improve the bandwidth efficiency of wireless systems.

This work is oriented to investigate the control for the transmission schemes according to the type of information.