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Title Reliability Improvement of Cooperative Relay Networks under Time-Varying Channels
Degree Ph.D.
Author Jeehoon Lee
Advisor Kiseon Kim
Graduation Date 2015.08.25 File
    Date 2017-03-06 12:05

In recent years, cooperation technologies among geographically distributed nodes or users have been emerged as a new communication paradigm. This mainly comes from two flows of communication fields. The first thing is the advent of ad-hoc and sensor networks with many new applications, where a source requires the assistance from other nodes to forward or relay information to a desired destination. The second thing is the demands of very high data rate with communication reliability. In current fourth-generation (4G) wireless networks, multiple-input multiple-output (MIMO) technologies are considered as a powerful approach to meet the demands. In practice, most users either have difficulty with multiple antennas installed on small-size devices, or the propagation environment of wireless link may not support the requirements for applying MIMO technologies. To overcome the limitations of achieving MIMO gains, cooperative schemes in a distributed fashion are required beyond traditional point-to-point communications.

Diversity is a promising technique for reliability improvement in cooperative relay networks. The relaying nodes essentially operate in either amplify-and-forward (AF) and decode-and-forward (DF) relaying modes, which are basic for various evolved relaying schemes. Basically, such cooperative relaying schemes provide a (cooperative) diversity gain, but are affected by channel conditions of source-to-relay (SR) and relay-to-destination (RD) links. In real wireless environments for half-duplex based cooperative relay networks, all the channels vary with time, and thus repeated transmission through source-to-destination (SD) link instead of forwarding by a relaying node can also provide a (time) diversity gain. That is, a cooperative relaying scheme is not always better option than the non-cooperative case but it depends on channel conditions from a practical viewpoint. Thus, researches on cooperative relaying schemes with the consideration of time-varying property of SD link are required. In addition, for the case that multiple SD pairs share one relaying node with analog network coding (ANC), the channel conditions of overhearing links (OLs) have a decisive effect on the system performance. Such issues make us to pay attention to reliability improvement of various cooperative relay networks.

The first research issue is to investigate the effectiveness of cooperative relaying schemes under practical time-varying channels. It is well known that the diversity order of two can be obtained by a help of a single relaying node. In practical wireless environments, since a wireless channel between a transmitter and a receiver varies with time, transmitting the same signal twice through SD link in cooperative relay networks can provide the same diversity order as a cooperative relaying scheme. From such a point, the effective ranges of AF and DF-based relaying schemes will be described from a statistical viewpoint. In addition, we propose three effective protocols for selecting better node between source and relay depending on the node to select in half-duplex based cooperative relay networks, and analyze the outage performances of all the proposed protocols. Simulation results show that all the proposed protocols has better outage performance than conventional AF and DF-based relaying schemes. In particular, one of the three protocols can provide additional diversity gain for a special case. More detailed explanations will be described later.

The second research issue is about error propagation problem in multiple access relay networks (MARNs). When a DF-based relaying scheme is utilized in cooperative relay networks, the diversity loss may be occurred by propagating the error caused by poor channel condition of SR link. In particular, such a problem is more serious in MARNs due to increased probability of decoding failure at the relay with multiple SR links. Due to time-varying property of wireless channel, any source in MARNs may be considered as a helper of other sources for forwarding the network-coded message like a relaying node, and also perform a role of relaying for itself because repeated transmission through the direct link may provide a time diversity gain. Thus, all nodes except for the common destination can be considered as candidates for relaying in MARNs, and thereby the error propagation problem caused by relaying node may be resolved, as well as we can expect performance improvement. From such points, we propose a novel selection DF relaying protocol for MARNs, which selects the node for forwarding the network-coded message among multiple sources and one relay under time-varying channels with some feedback information. Simulation results show that the proposed protocol can not only improve outage performance, but also provide additional diversity gain. Furthermore, although the proposed protocol requires two feedback information with one short packet, but has very similar outage performance with those of comparable relaying protocols employing global CSI information.

The third research issue is about analog network coding (ANC) in X-shaped two-way relay networks. The ANC technology is an effective way of increasing network throughput by simultaneously transmitting packets from sources to relay during the first transmission phase, and then simply amplifying-and-forwarding the superposition of the received packets from the relay to the corresponding receivers during the second transmission phase, thereby reducing the number of required transmission phases. Herein, in order each destination to decode the desired signals from the superimposed signals, the destinations should first estimate the interfering signals through the corresponding overhearing links (OLs) during the first transmission phase, and then remove them from the superimposed signals after the second transmission phase. The estimation results at the destinations strongly depend on the quality of the OLs, and have a decisive effect on system performance. In addition, since the variation of relay location results in distance variation of source-to-relay and relay-to-destination links, the relay location is deeply related to the received signal-to-noise power ratios at the destinations. From such points, we analyze impact of channel quality of the OLs on power allocation strategy and relay location improving the sum rate. Also, we compare the sum rate of the ANC technology with that of a conventional relaying scheme, and by doing so we derive a criterion for determining the effective scheme between them. All the analytical results are validated by simulations.

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