Wireless transmission protocols using relays for broadcast and information exchange channels

Relays have been used to overcome existing network performance bottlenecks in meeting the growing demand for large bandwidth and high quality of service (QoS) in wireless networks. This thesis proposes several wireless transmission protocols using relays in practical multi-user broadcast and information exchange channels. The main theme is to demonstrate that efficient use of relays provides an additional dimension to improve reliability, throughput, power efficiency and secrecy. First, a spectrally efficient cooperative transmission protocol is proposed for the multiple-input and singleoutput (MISO) broadcast channel to improve the reliability of wireless transmission. The proposed protocol mitigates co-channel interference and provides another dimension to improve the diversity gain. Analytical and simulation results show that outage probability and the diversity and multiplexing tradeoff of the proposed cooperative protocol outperforms the non-cooperative scheme. Second, a two-way relaying protocol is proposed for the multi-pair, two-way relaying channel to improve the throughput and reliability. The proposed protocol enables both the users and the relay to participate in interference cancellation. Several beamforming schemes are proposed for the multi-antenna relay. Analytical and simulation results reveal that the proposed protocol delivers significant improvements in ergodic capacity, outage probability and the diversity and multiplexing tradeoff if compared to existing schemes. Third, a joint beamforming and power management scheme is proposed for multiple-input and multiple-output (MIMO) two-way relaying channel to improve the sum-rate. Network power allocation and power control optimisation problems are formulated and solved using convex optimisation techniques. Simulation results verify that the proposed scheme delivers better sum-rate or consumes lower power when compared to existing schemes. Fourth, two-way secrecy schemes which combine one-time pad and wiretap coding are proposed for the scalar broadcast channel to improve secrecy rate. The proposed schemes utilise the channel reciprocity and employ relays to forward secret messages. Analytical and simulation results reveal that the proposed schemes are able to achieve positive secrecy rates even when the number of users is large. All of these new wireless transmission protocols help to realise better throughput, reliability, power efficiency and secrecy for wireless broadcast and information exchange channels through the efficient use of relays

Performance analysis of diversity techniques in wireless communication systems: Cooperative systems with CCI and MIMO-OFDM systems

This Dissertation analyzes the performance of ecient digital commu- nication systems, the performance analysis includes the bit error rate (BER) of dier- ent binary and M-ary modulation schemes, and the average channel capacity (ACC) under dierent adaptive transmission protocols, namely, the simultaneous power and rate adaptation protocol (OPRA), the optimal rate with xed power protocol (ORA), the channel inversion with xed rate protocol (CIFR), and the truncated channel in- version with xed transmit power protocol (CTIFR). In this dissertation, BER and ACC performance of interference-limited dual-hop decode-and-forward (DF) relay- ing cooperative systems with co-channel interference (CCI) at both the relay and destination nodes is analyzed in small-scale multipath Nakagami-m fading channels with arbitrary (integer as well as non-integer) values of m. This channel condition is assumed for both the desired signal as well as co-channel interfering signals. In addition, the practical case of unequal average fading powers between the two hops is assumed in the analysis. The analysis assumes an arbitrary number of indepen- dent and non-identically distributed (i.n.i.d.) interfering signals at both relay (R) and destination (D) nodes. Also, the work extended to the case when the receiver employs the maximum ratio combining (MRC) and the equal gain combining (EGC) schemes to exploit the diversity gain

Forwarding strategies and optimal power allocation for coherent and noncoherent relay networks

In fading wireless channels, relays are used with the aim of achieving diversity and thus overall performance gain. In cooperative relay networks, various forwarding techniques like amplify and forward (AF) and decode and forward (DF) are used at the relay for better throughput and improved BER performance than traditional multihop systems. In a power constrained environment, the performance can be further improved by using an optimal power allocation strategy. The relative position of the relay with respect to the source and destination also has an immense effect on the efficacy of the relay.;We position the relay at various positions in a planar grid, with the position of source and destination being fixed, and we investigate the effect that the positioning of the relay has on a relaying system. We use our three terminal model to optimize the power allocation under total transmit power constraint, to maximize the instantaneous signal-to-noise ratio (SNR) at destination, and thus achieve improved throughput and BER performance, while using AF and DF protocols. We evaluate the performance of our system for both coherent and noncoherent modulation in a Rayleigh block fading channel. Quadrature phase shift keying (QPSK) is used in the coherent case and 4-Frequency shift keying (4-FSK) is used in the noncoherent case.;Previous works involving power allocation schemes have mainly concentrated on optimizing information theoretic quantities like capacity and outage probability. We derive expressions for instantaneous SNR using our model and optimize the power allocation based on that, with the final aim of achieving improved uncoded BER. Analytical expressions of the instantaneous SNR at the destination are derived for both AF and DF. These expressions are numerically optimized to obtain an optimum power allocation strategy for each position of the relay in both the AF and DF schemes using coherent or noncoherent detection.;We compare the performance of the AF and DF protocols based on their positional BER and throughput at different received SNR and notice that our power optimized schemes outperform existing power control schemes at certain areas. Finally we also identify the shape and area of the regions where relaying would provide performance gains for both the protocols at different received SNRs

Diversity Techniques in Free-Space Optical Communications

Free-space optical (FSO) communication offers significant technical and operational advantages such as higher bandwidth capacity, robustness to electromagnetic interference, a high degree of spatial confinement (bringing virtually unlimited reuse and inherent security), low power requirements, and unregulated spectrum. FSO communication can be deployed as an efficient solution for a wide range of applications such as last-mile access, fiber backup, back-haul for wireless cellular networks, and disaster recovery among others. Although FSO system have many appealing features, they have rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In this dissertation, we investigate different diversity techniques to boost the performance of FSO systems in the presence of the atmospheric turbulence-induced fading. In Chapter 3, we investigate receive diversity in coherent FSO systems considering both turbulence-induced amplitude and phase fluctuations under weak turbulence regime. To mitigate the wavefront phase distortion effect, modal compensation is deployed at the receiver. Under the assumption of Rician channel that models the combined effects of the atmospheric fading and modal compensation, we derive outage probability and diversity- multiplexing tradeoff of such systems. Our results show that, at high signal to noise ratio (SNR) regime, the diversity gain as great as the number of receiving apertures is achieved. Moreover, it is found that the modal compensation provides finite-SNR diversity advantages in coherent receivers. In Chapter 4, we investigate multi-hop transmission (serial relaying) as a form of diversity technique to combat atmospheric fading in coherent FSO systems. Utilizing the same channel model as in Chapter 3 and considering decode-and-forward relaying strategy, we quantify the outage probability and the finite-SNR diversity-multiplexing tradeoff of this relaying scheme. Exploiting the fact that fading variance is distance-dependent in the atmospheric channel, our results demonstrate that the multi-hop transmission takes advantage of the resulting shorter hops and yields significant performance improvements in the presence of fading. In Chapter 5, we study hybrid-ARQ protocols in coherent FSO communications over Gamma-Gamma atmospheric fading channels. We investigate and compare the performance of three hybrid-ARQ protocols in terms of the outage probability and throughput. Furthermore, we characterize the outage performance at high-SNR regime by diversity and coding gains. Our results provide insight into the performance mechanisms of different hybrid-ARQ protocols in coherent FSO systems and demonstrate that hybrid-ARQ significantly improves the outage performance of a coherent FSO system particularly in strong turbulence regime. In Chapter 6, we investigate parallel relaying in an intensity modulation/direct detection (IM/DD) FSO system. Assuming Gamma-Gamma fading model, we analyze both decode-and-forward and amplify-and-forward modes of cooperation. Focusing on high SNR regime, we investigate the outage probability and characterize it by the diversity and coding gains. The diversity-multiplexing tradeoff expression of each cooperation mode is also derived. Our performance analysis reveals that large energy savings can be achieved through the use of parallel relaying in FSO systems

Communications over fading channels with partial channel information : performance and design criteria

The effects of system parameters upon the performance are quantified under the assumption that some statistical information of the wireless fading channels is available. These results are useful in determining the optimal design of system parameters. Suboptimal receivers are designed for systems that are constrained in terms of implementation complexity. The achievable rates are investigated for a wireless communication system when neither the transmitter nor the receiver has prior knowledge of the channel state information (CSI). Quantitative results are provided for independent and identically distributed (i.i.d.) Gaussian signals. A simple, low-duty-cycle signaling scheme is proposed to improve the information rates for low signal-to-noise ratio (SNR), and the optimal duty cycle is expressed as a function of the fading rate and SNR. It is demonstrated that the resource allocations and duty cycles developed for Gaussian signals can also be applied to systems using other signaling formats. The average SNR and outage probabilities are examined for amplify-and-forward cooperative relaying schemes in Rayleigh fading channels. Simple power allocation strategies are determined by using knowledge of the mean strengths of the channels. Suboptimal algorithms are proposed for cases that optimal receivers are difficult to implement. For systems with multiple transmit antennas, an iterative method is used to avoid the inversion of a data-dependent matrix in decision-directed channel estimation. When CSI is not available, two noncoherent detection algorithms are formulated based on the generalized likelihood ratio test (GLRT). Numerical results are presented to demonstrate the use of GLRT-based detectors in systems with cooperative diversity