Relay Operation Modes

As mention to the operation modes of RSs, two general types of RSs have been classified:

transparent RSs and non-transparent RSs, which are with respect to the knowledge in MSs. In the transparent mode, an MS is not aware of whether or not it communicates with the network via an RS; while in the non-transparent mode, an MS is aware of whether or not it is communicating via an RS. In IEEE 802.16j and 802.16m, they are simply referred as non-transparent RSs and transparent RSs [9], [13], while in 3GPP LTE-Advanced they are named as Type-I RSs and Type-II RSs, respectively [10].

Non-Transparent Relay (Type I Relay)

A non-transparent RS operates like a BS, except that there is no backhaul connection to the core network. All control signaling and data transmissions between a BS and an MS are forwarded by RSs in the non-transparent mode [9], [10], [13]. The non-transparent RS controls its own cell, which appears to an MS as a separate cell, so that an MS can camp on the RS, just like camping on the BS. The non-transparent RS can operate in both centralized and distributed scheduling in the in-band or out-band of the BS operation. Besides the throughput enhancement, the non-transparent RS may be located at the cell edge to cover the area that is originally not covered by the BS, which is known as coverage extension.

Transparent Relay (Type II Relay)

On the other hand, in the transparent relay mode, the control signaling from a BS can directly reach MSs and only data traffic is forwarded by an RS [9], [10], [13]. The transparent RS is usually located within the coverage of the BS and is under the BS’s supervision so that only centralized scheduling is supported in the transparent mode. The MS should camp on the BS and it may not be aware of the existence of the RS. The main objective of the transparent relay is to enhance the user throughput and to increase the system capacity by achieving multipath diversity.

1.5 Problem and Motivation

The concept of relay-assisted cellular networks, the potential benefits, and the detailed relaying technologies are discussed in Section 1.4. With the basic knowledge, the crucial step in developing relay-assisted cellular networks is to fully evaluate their performance from both

operations of RSs, the reuse pattern among RSs, and the scheduling method of data transmissions among BSs, RSs, and MSs are important design issues reflecting the system performance. How RSs effectively and efficiently assist cellular systems has been a topic of extensive research in both academia and industry.

From the previous literature, however, the performance evaluation of relay-assisted cellular networks has been mostly done for very specific system scenarios, such as fixed number of RSs and locations, fixed reuse pattern, or seeking optimal RSs’ positions with a simplified one-dimensional model. Furthermore, although several works have been done to evaluate the system performance in some environments which are especially suitable for deploying RSs such as the Manhattan-like environment, previous research has mostly focused on the aspects of coverage extension and end-to-end user throughput enhancement.

In this dissertation, we aim to fully investigate a general relay-assisted network in a multi-cell environment both in the downlink and in the uplink from the information-theoretic point of view. By jointly considering RSs’ locations, reuse patterns, path selections and resource allocation, in the downlink, two types of quality of end-user experience (QoE): fixed bandwidth allocation (FBA) and fixed throughput allocation (FTA), are investigated along

with two path selection methods: spectral efficiency (SE) based and signal-to-noise-plus-interference ratio (SINR) based; in the uplink part, two performance

measures: average MS’s power consumption and uplink system spectral efficiency, are optimized.

In addition, from the practical point of view, we aim to evaluate the overall system capacity enhancement for the relay-assisted network in the Manhattan-like environment with new scheduling methods proposed for the system where directional antennas are equipped at the BSs and RSs.

Detailed literature review, problem formulation, and optimized solutions corresponding to each topic will be provided in the following sections.

1.6 Organization of the Dissertation

This dissertation consists of three themes. The first part is to investigate the downlink performance and optimization of relay-assisted cellular networks in multi-cell environments.

The second part aims to look into the uplink performance and optimization of a relay-assisted cellular network. The third part studies resource scheduling with directional antennas for multi-hop relay networks in a Manhattan-like environment.

The rest of this dissertation is organized as follows. In Chapter 2, the adopted system setups, including multi-cell architecture, relaying technology, propagation models, antenna configurations, power setting criteria, path selection methods, and frequency reuse patterns for downlink performance evaluation, are addressed. The joint optimizations based on GA for maximizing system capacities of different system configurations are proposed. The numerical results and the summary are then provided and discussed. In Chapter 3, those of system settings for uplink evaluation are presented. Both the minimization of average MS’s transmit power and the maximization of uplink system capacity are formulated and solved by the proposed GA-based algorithm and MAI estimation algorithm. Also, the numerical results of uplink optimization and the summary are indicated. In Chapter 4, we first show the system setups and the propagation models adopted for multi-hop relay networks in a Manhattan-like environment. We propose two efficient resource scheduling methods with directional antennas equipped in BSs and RSs to enhance the system capacity under these settings. The

Chapter 2

Downlink Performance and Optimization of Relay-Assisted Cellular Networks in Multi-cell Environments

In this chapter, we investigate the downlink performance limits of a general relay-assisted network with optimized system parameters in a multi-cell environment. A genetic algorithm (GA) based method is proposed for joint optimization of system parameters, including the number of RSs and their locations, frequency reuse pattern, path selection and resource allocation so as to maximize the system spectral efficiency. Two types of quality of end-user experience (QoE), i.e., fixed bandwidth allocation (FBA) and fixed throughput allocation (FTA), are studied along with two path selection methods, i.e., spectral-efficiency (SE) based and signal-to-noise-plus-interference ratio (SINR) based. The background, system setups, the objective function, the proposed optimization algorithm and the numerical results are completely described in the following subsections.