1.1.1 Throughput-Oriented Relay Selection Rules for Single Relay Case
The objective of this study is to design relay selection rules achieving higher through-put while maintaining link reliability. In the literature, many studies have shown that outage probability can be improved by deploying relay stations [7–13]. However, how to choose a relay to achieve higher throughput is rarely discussed. Transmission through a relay station may lower the throughput since it needs two transmission phases. One is from the source to a relay station and the other is from the relay station to the destination. Therefore, how to choose the appropriate relay station to get higher system throughput is an important issue.
We can use an example to illustrate the relay selection problem. As shown in Fig. 1.1, there are N possible relay nodes between the source and the destination.
When choosing the relay node close to the source, the throughput in the relay link from the source is higher than that in the link to the destination. As a result, the overall link throughput of the two-hop links will be limited by the lower-throughput.
By contrast, when choosing the relay node close to the destination, the throughput in the relay link from the source is lower than that in the link to the destination.
Therefore, the overall link throughput of the two-hop links will be limited by the link from the source to the relay. Therefore, how to choose appropriate relays to achieve higher throughput is a crucial question.
We propose two partner selection methods in this thesis. The first one is to calculate throughput corresponding to each relay, and then choose the relay achieving the maximal throughput. Though this method can achieve the highest throughput, computation cost is quite high. In the second method, we first compare the SNRs
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Figure 1.1: A source-destination pair with multiple relay candidates
of the link from the source to relay with that from the relay to the destination, and designate the smaller one as the bottleneck SNR associated with that relay. The bottleneck SNR of each relay is recorded and compared. The relay with the largest bottleneck SNR is selected. Both methods have the similar throughput performance.
However, the outage probability of the second method is better than that of the first one. Meanwhile, compared to the conventional signal-based partner selection, the proposed bottleneck SNR approach can achieve higher throughput at a cost of small SNR degradation.
1.1.2 Throughput-Oriented Relay Selection Rules for Multi-ple Relays Case
Furthermore, we examine the performance of the considered relay selection rules in the multi-relay case. We find that at the same consumed power level the outage probability and throughput performance in the multi-relay case is indeed worse than
those in the one relay case. This is because the multi-relay case yields more power consumption and a higher probability of selecting inappropriate relays.
1.1.3 Power Distribution for Throughput-Oriented Relay Se-lection Rules
In the literature, many studies have shown that at the same consumed power level power distribution in a two-hop relaying network can obtain better outage perfor-mance [7, 14, 15]. We would like to examine if power distribution also works well for throughput-oriented relay selection rules. In the traditional method, the transmit power allocated in the link from the source to the relay is the same as that in the link from the relay to the destination. Now, we suggest a simple power distribution algorithm to adjust transmit power for relay nodes when the number of the relays increases. In the suggested power distribution, the transmit power of each relay is inversely proportional to the number of relays, and the sum of the total transmit power from the relay is equal to the transmit power from the source. Our results show that at the same consumed power level the proposed power distribution can improve the outage probability, while maintaining throughput even in the multi-relay case. This is because power allocation can eliminate unnecessary power in the second transmission phase with multiple relays.
1.1.4 Thesis Outline
The rest of the thesis are organized as follows. Chapter 2 introduces the backgrounds on the multi-hop relay in the wireless systems and defines the considered performance metrics. In Chapter 3, we discuss the proposed relay selection rules and the algorithm of power distribution ,and the simulation platform. In Chapter 4, we describe the two proposed relay selection rules. In Chapter 5, we further discuss the two relay
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selection rules in the multiple relays case. In Chapter 6, we investigate the impact of the power allocation on the two proposed relay selection rules. At last, Chapter 7 gives the concluding remarks and suggestions for the future works.
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CHAPTER 2
Background and System Model
In this chapter, we introduce some background knowledge on the cooperative multi-hop relay networks and define a few performance metrics used in this thesis.
2.1 Background on Relay Stations
Multi-hop relay networks were originated from the ad-hoc and peer-to-peer networks.
Recent years, the applications of multi-hop relay networks progress significantly. As the result, the relaying techniques have become more advanced and much complicated.
The notable features of multi-hop relay network are discussed as follows:
2.1.1 Radio Resources Selection
In a relay network, the radio resources are allocated to the link form the source to the relay and the link from the relay to the destination. There are three kinds of resource allocation schemes [16] as shown below:
• Relaying in the time-domain scheme: The same carrier frequency is operated on both sides of the relay station. The frame structure is used to connect nodes via a time-multiplexing channel. Each user uses a different time-slot to transmit data in the same time division multiple access (TDMA) frame. A TDMA frame
is subdivided into three segments in the relay network, one for the direct source-destination link, one for the source-relay link and one for the relay-source-destination link.
• Relay in the frequency-domain scheme: This scheme operates source and relays at two different carrier frequencies.That is, source transmits data to relay with carrier frequency f1, and then relay transmits the received data to destination with carrier frequency f2. This will increase the complexity of the hardware and frequency management.
• Relaying in the hybrid time/frequency domain scheme: The concept was inves-tigated in [17]. The basic idea is that the relays periodically switches between carrier frequency f1 and f2. No additional devices are needed for this scheme.
However, the fast frequency switching has to be supported, which will increase hardware complexity.
2.1.2 Relay Function Selection
In general, relay station can be classified into two schemes according to its function:
(1) Amplify-and-Forward (AF): the relay station received signal and then simply amplify the signal before retransmission; (2) Decode-and-Forward (DF): the relay stations decode received signal and then re-encode the signal before retransmission [18, 19]. In the DF scheme, if the relay node can not correctly detect the massage send from the source then the relay stop forwarding to the destination. However, in the AF scheme the relay do not detect the signal from the source and forward the received signal to the destination as long as the signal arrived. DF scheme ensure the quality of the forwarding signal from the relay to the destination.
2.1.3 Macro-Diversity Function
In this work we assume that the signals from the relay to the destination are syn-chronous. Therefore we need the property called “Macro-Diversity”. Macro-diversity means that the transmitter (source or relay) can serve more than one user at the same time. The receiver (relay or destination) may also communicate with more than one transmitter [20–22]. Therefore the destination can receive multiple copies of the message from the relays and obtain the diversity gain.
2.1.4 Relay Deployment Strategies
In the infrastructure-based networks, the access links are generally sheltered with the buildings or at indoors. This phenomenon is called the non-line-of-sight (NLOS) transmission. However, deploying relays can improve the transmission in the NLOS environment. There are two strategies that can be used to achieve line-of-sight trans-mission. One is to deploy relays in a vast space according to a careful plan. The other is to increase the antenna height of relays. These two strategies are not mu-tually exclusive. Therefore, a relaying network can employ both methods to deploy relays.