Multi-carrier systems such as orthogonal frequency division multiplexing (OFDM) are significantly affected by peak-to-average-power ratio (PAPR). Unfortunately, the high PAPR inherent to OFDM signals envelopes will occasionally drive high power amplifiers (HPAs) to operate in the nonlinear region of their characteristic curve. The nonlinearity of the HPA exhibits amplitude and phase distortions, which cause loss of orthogonality among the subcarriers (SCs), and hence, inter-carrier interference (ICI) is introduced in the transmitted signal. The ICI power is proportional to the amplitude of the signal at the amplifier input and it may cause a considerable bit error rate (BER) degradation. A plethora of research has been devoted to reduce the performance degradation due to the PAPR problem inherent to OFDM systems. Some of the reported techniques such as amplitude clipping have low-complexity; on the other hand, they suffer from various problems such as in-band distortion and out-of-band expansion. Signal companding methods have low-complexity, good distortion and spectral properties; however, they have limited PAPR reduction capabilities. Advanced techniques such as coding, partial transmit sequences (PTS) and selected mapping (SLM) have also been considered for PAPR reduction. Such techniques are efficient and distortionless, nevertheless, their computational complexity is high and requires the transmission of several side information (SI) bits. In this thesis, a new low-complexity scheme is proposed based on the PTS that employs two inverse fast Fourier transforms (IFFTs) and two circulant transform matrices, in order to reduce complexity and improve the system performance. Furthermore, the low-complexity scheme is simplified by omitting one of the circulant transform matrices in order to reduce both the computational complexity and the number of SI bits at the cost of a small reduction in PAPR and BER performance. It is well known that, accurate PAPR estimation requires oversampling of the transmitted signal, which in turn results in increased complexity. More importantly, minimising the PAPR does not necessarily minimise the distortion produced by the nonlinearity of the HPA. Therefore, minimising PAPR does not necessarily imply that the BER will be minimised too. Efficient and less complex schemes for BER reduction of OFDM systems in the presence of nonlinear HPA and/or carrier frequency offset (CFO) are proposed. These proposed techniques are based on predicting the distortion introduced by the nonlinearity of HPA and/or CFO. Subsequently, techniques such as the PTS and SLM are invoked to minimise the distortion and BER. Three distortion metrics are adopted in this thesis: inter-modulation distortion (IMD), peak interference-to-carrier ratio (PICR) and distortion-to-signal power ratio (DSR). Monte Carlo simulations will confirm that the DSR and PICR are more reliable than the PAPR and IMD for selecting the coefficients of the PTS and SLM to minimise the BER. Furthermore, complexity analyses demonstrate that the proposed schemes offer significant complexity reduction when compared to standard PAPR-based methods. A closed form solution for accurate BER for the OFDM signals perturbed by both the HPA nonlinearity and CFO was derived. Good agreement between the simulation results and the theoretical analysis can be obtained for different HPA parameters and CFOs. Finally, efficient approaches to reduce the impact of nonlinear power amplifiers with respect to the BER of OFDM systems are proposed. These are approaches based on: the well-established PAPR schemes, a power amplifier model and a simple single point cross correlator. The optimum phase sequence within the proposed approaches is selected by maximising the correlation between the input and output of the power amplifier model. Simulation results have confirmed that the BER using the proposed approaches is almost identical to the DSR, while the complexity is reduced significantly for particular system configurations.

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This thesis targets higher frequency satellite systems (Ku-band and above) that suffer from severe atmospheric impairments and require advanced techniques to compensate for the effects of rain attenuation. Keeping a classical approach, based on a worst case sizing, results in fixed over-sizing of systems leading to reduced efficiency and unreasonable costs. It is therefore necessary that these systems include adaptive Fade Mitigation Techniques (FMTs) to counteract propagation impairments in real-time and use system resources efficiently. The combination of the management of radio resources and the deployment of Fade Mitigation Techniques appears to be an outstanding challenge for Broadband Satellite Networks. DVB-RCS and DVB-S standards have not primarily been defmed considering the details of rain FMTs; they have to be adapted to support FMTs, so the study of efficient FMTs and their integration within the standards is necessary. Adaptive transmission techniques, such as Adaptive Coding and Modulation (ACM) , are mostly introduced as a way of achieving efficient Bandwidth on Demand (BoD). But they can also be a good framework for deploying FMTs and can provide high availabilities for powerful connections. Employing Burst Length Control (BLC) can be a useful extension of ACM to permit its dynamic range to be increased in the case of power-limited return channels, for which high -order modulation schemes cannot be used. A design methodology of a combined FMT is proposed and presents the complex problem of system availability depending not only on the combined FMT performance, but also on the resource allocation scheme performance. A resource allocation protocol that incorporates FMT implementation within its dynamic capacity assignment strategy is developed and tested in the presence of bursty traffic when multiple links experience simultaneous fading.

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This thesis considers the physical layer performance of the Satellite High Speed Downlink Packet Access (S-HSDPA) system via ground based relays or intermediate Module Repeaters (IMRs). The work evaluates the modifications needed for the terrestrial HSDPA system to a satellite architecture by change of certain significant parameters within a high dispersive IMR environment to deliver high data throughput to satellite mobile terminals. Initial results for S-HSDPA with different terminal capabilities and modulation formats have been obtained. Two diversity techniques, namely receive antenna diversity and space time transmit diversity (STTD) were also investigated. The impact of increasing the number of multicodes transmission for the S-HSDPA system has also been considered. In all of these results, we have shown the capacity to be insufficient to support an economic operational system. In order to investigate improved capacity we have analyzed two advance receiver techniques using channel equalizers and multipath interference cancellers. The performances of two low complexity chip-level adaptive equalizers (CPICH NLMS equalizer and Griffiths' equalizer) equalizer) and the multipath interference canceller (MPIC) have been compared with the conventional RAKE receiver for the S-HSDPA system in an IMR environment. It has been shown that the equalizers can increase the throughput in comparison to the conventional Rake receiver while incurring minimum additional complexity. Thus it is concluded that advanced terminal receivers will be necessary in any practical satellite system. Finally, a study of the orthogonality factor (which parameterizes the intracell interference), as a crucial parameter in the calculation of downlink satellite power has been performed in the IMR environment. Results of the orthogonality factor are presented for multicode S-HSDPA transmission for simple receivers and the use of receive antenna diversity. Using the equalizer based receivers it is shown that the orthogonality factor statistics can be significantly lowered as compared to the conventional matched filter receiver.

Satellites will play an indispensable role in the deployment of commercial networks to meet an increasing demand for supporting multimedia services at high data rates. Next generation satellite systems, operating at high frequency bands offer large bandwidth and are able to provide broadband services. To interface satellite links with existing terrestrial networks for providing communication access to a variety of users directly, several performance issues need to be addressed. Current thesis presents a technically viable satellite-integrated network model that is efficient in carrying broadband services to users over a wide scattered area. Accurate prediction of attenuation level is necessary for a reliable network model to operate with required service availability. Long term rainfall data has been analysed to characterise attenuation level at a selected region such as Dhaka. It is shown that rainfall is highly seasonal and attenuation level is quite high during monsoon. However, the seasonal behaviour of rainfall can be exploited to improve the link availability. Radar and rain gauge measurements at Sparsholt are also used to find rain cell size distribution, which is an important factor in site diversity implementation to combat severe rain fade. It is found that convective rain cell has extension in the region of 10 km. The network model is designed with dimensioning the effective bandwidth to support a number of users over the satellite link by taking into account the multimedia traffic characteristics. Concatenated coding, a robust coding scheme is implemented to improve the link quality at a level required to deliver broadband services. The ITU-T performance objectives of 7.5×10 for CLR and 1.4×10 for CER over satellite links are met at a required Eb/No of 2.95 dB and 2.88 dB respectively. Different enhancement mechanisms for optimum TCP performance are implemented to combat the large propagation delay associated with a satellite link. It is revealed through the simulation that TCP performance over a satellite link is as efficient as terrestrial links with these enhancement mechanisms Finally, the overall performance of the designed network is evaluated through link budget analysis and simulation. An innovative downlink power control strategy has been implemented to maintain the link during the rainiest months. The interference level due to high power satellite transmission in the designed system is also calculated to protect other existing communication links sharing the same frequency bands. A feasible broadband network designed with characterising propagation as well as networking issues will efficiently deliver broadband communication services to a large population promptly and in a cost-effective manner. Such a network solution will be in the realm of current R & D towards broadband satellite networks.