Low Earth Orbit Satellite (LEOS) communications face problems such as multipath fading, Inter-Symbol Interference (ISI) and rapidly changing environments. These problems are less tolerable in LEOS than in terrestrial communications. It is because the distance between a mobile unit and a LEOS is far more than the distance between a mobile unit and a terrestrial basestation. Thus, LEOS systems provide a considerably smaller link margin for fading than do terrestrial systems. The goal of this research is to enhance the performance of Time Division Multiple Access (TDMA) LEOS communication systems by combating these problems. This is one of the most challenging tasks in modem wireless communications.
Given the future usage of high frequency bands, multi-element antenna technologies for handheld terminals will become feasible and that is a new area of research. This study investigates employing multi-element antenna systems to improve the system performance of LEOS handheld/portable terminals. Multi-element antenna systems include adaptive antennas and antenna diversity. To further improve the system performance, this research also investigates ways of optimally combining adaptive equalisation or coding and interleaving technology with multi-element antenna systems. Traditional multi-element antenna systems research has concentrated on basestation applications and human body influences on antennas have not been covered. In this research, human body influences on antennas are also studied.
Computer modelling is the primary research method used and it involves simulation of signal generation and transmission, mobile channel modelling and simulation of reception systems. In this research, existing system parameters were used for signal generation and transmission simulation. For channel modelling, both statistical and geometrical approaches were used. Since multi-element antenna systems are the centre of the present research and controlling correlation coefficients between received signals in a multi-element system through channel modelling is also important, this research introduces new ways to control the correlation coefficient values for both statistical and geometrical models. On the other hand, the analysis of reception system performance shows that the combined system of antenna diversity, coding and interleaving outperforms the traditional omni-directional antenna system. The combined system requires on average 20dB lower Signal to Noise Ratio (SNR) than the omni-directional antenna system to provide 10"^ BER when the time and antenna correlation coefficients of the combined system have uniform distribution. This result was obtained when the combined system was under a flat Rayleigh fading environment.
In frequency selective fading environments, ISI exists. Equalisers can be employed together with Maximum Ratio Combiner (MRC) or Adaptive Antenna (AA) systems to improve the SNR. Decision feedback equalisers were used in this research. The equaliser consists of feedforward and feedback filters. A model with a novel structure was proposed in this research. The model has two feedback filters in parallel with a MRC sub-system and is followed by a feedback filter. At 15dB SNR, the BER of the model with this parallel structure is 27.6 and 8.2 times lower than the BER of the omni-directional antenna system and the maximum ratio combiner system. When the MRC sub-system in the parallel structured model is replaced by the adaptive antenna sub-system, the new model also outperforms other combination systems.
In addition to computer simulated channel models, different multipaths and interference environments were generated by an experimental test-bed. Using real data collected from the experimental measurements, multi-element antenna systems (including the parallel structured models) were examined. The experimental studies were conducted under the influences of the human body and with the received signals coming from high and low elevation angles. The experimental results obtained are consistent with the simulation results. Moreover, human body influences on magnetic loop and electric field antennas on handheld/portable terminals were also studied and the possibility of using field component diversity was investigated. By performing computer simulations and experimental studies, the present research proposed innovative solutions at a system level. It was found that multi-element antenna systems with adaptive signal processing can improve the performance of TDMA LEOS handheld and portable terminals. This research is one of the first systematic studies dedicated to overcoming the problems of multipath fading, ISI, small link margins and rapidly changing environments for LEOS systems. With the future trend of using high frequency bands for high-speed data communications, the research results have great potential to be fransferred to other terrestrial applications.