Design and Analysis of Duplexing Modes and Forwarding Protocols for OFDM(A) Relay Links

Relaying, i.e., multihop communication via so-called relay nodes, has emerged as an advanced technology for economically realizing long transmission ranges and high data rates in wireless systems. The focus of this thesis is on multihop multiuser systems where signals are modulated with orthogonal frequency-division multiplexing or multiple access, i.e., OFDM(A), and relays are infrastructure-based network nodes. In general, the thesis contributes by investigating how to operate relay links optimally under spectrum, transmit power and processing capability limitations, as well as how to improve signal processing in relays by exploiting other advanced concepts such as multiantenna techniques, spectrum reuse, transmit power adaptation, and new options for multicarrier protocol design. The first theme is the design and analysis of duplexing modes which define how a relay link reuses allocated frequency bands in each hop. Especially, the full-duplex relaying mode is promoted as a feasible, advantageous alternative to conventional half-duplex operation while its inherent loopback self-interference is recognized and dealt with: New techniques are developed and evaluated for the mitigation of self-interference while transmit power control is proposed as a countermeasure against residual distortion. Finally, the fundamental criteria to prefer one mode over the other are determined which not only serves as a comparison between the modes but allows for opportunistic mode switching during operation. The second theme is the design and analysis of forwarding protocols which specify signal processing done between reception and retransmission in a relay. The study identifies a comprehensive set of design options for OFDM(A) relaying protocols and evaluates their combinations. Similar analysis is conducted for evaluating the duplexing modes and multiuser scheduling. Furthermore, a new analytical framework is developed for studying the channel delay spread characteristics of non-regenerative multihop OFDM links in conjunction with inter-carrier and inter-symbol interference as well as time synchronization error. Throughout the thesis, the design of relaying modes and protocols is conducted together with statistical performance analysis which facilitates discerning comparisons between existing and proposed concepts. Especially, all the contributions are established by means of closed-form expressions derived for the considered systems. The results cover all essential system variations, including amplify-and-forward and decode-and-forward protocols as well as downlink and uplink relaying reflecting their differences in infrastructure-based relaying.