In this chapter, Distributed Spatial Modulation (DSM) and Distributed Spatially Modulated Space-Time Block Code (DSM-STBC) are elaborately described. DSM and DSM-STBC are two new cooperative wireless protocols for multi-relay networks, which are based on the principle of Spatial Modulation (SM). The distinguishable feature of DSM lies in improving the reliability of the source via distributed diversity and by increasing the aggregate throughput of the cooperative network, since new data is transmitted during each transmission phase. This is achieved by encoding the data transmitted from the source into the spatial positions of the available relays and by exploiting the signal domain for transmitting the data of the relays. In DSM-STBC, SM and Space-Time-Block-Codes (STBCs) are synergistically combined for a distributed scenario. The distinguishable feature of DSM-STBC lies in offering throughput enhancement, by achieving the same order of diversity and having to activate the same number of relay nodes as conventional distributed STBC schemes. At the destination, demodulators robust to demodulation errors at the relays are developed, and their end-to-end error probability and achievable diversity are studied. With the aid of Monte Carlo simulations, DSM and DSM-STBC are compared against state-of-the-art cooperative protocols, and they are shown to provide better performances.

In this chapter, we have proposed a novel SUDAS with the objective of achieving the 10 Gbit/s data rate goal set by 5G for indoor UEs. The proposed SUDAS exploits the benefits of the licensed and unlicensed frequency bands simultaneously. In particular, it translates the spatial multiplexing in the licensed bands into frequency multiplexing in the unlicensed bands for boosting the end-to-end data rate via VMIMO. It is expected that the proposed SUDAS can further enhance the system performance when advanced resource allocation technique is employed. Besides, we have also discussed some potential application scenarios where deployment of SUDAS appears to be beneficial. Also, we have investigated different potential realizations of SUDAS and the corresponding implementation challenges. It is expected that the proposed SUDAS is able to bridge the gap between the current technology and the high data rate requirement of the next generation communication systems.

As explained in many chapters of this book, relaying in principle means indirect data transmission from a source node to a destination node via intermediate relay nodes (RNs), e.g., not so unlike repeaters, gap-fillers, or signal boosters do in old-fashioned wireless systems. On the other hand, the field of information theory has addressed many classical questions related to the performance of idealistic relay links. Relay transceivers in these original research areas conventionally operate in a full-duplex (FD) mode, while later studies on cooperative communication deemed it impossible to begin with and focused almost invariably on half-duplex (HD) relaying. However, modern relay transceivers such as those assumed in the context of this chapter are much more advanced than simple analog amplifiers in terms of their processing capabilities so that, e.g., they can be made robust to interference. Time has thus become right for us to return to the origins of relaying by promoting FD operation as a practical and highly efficient design choice whose technical difficulties can be now overcome. With this background, this chapter provides an overview of essential aspects to be considered when introducing FD operation into relaying systems. Two particular contributions are central for this development, namely introducing transmit power control for FD relaying and comparing the performance of FD relay links to that of their HD counterparts. The main focus herein is on infrastructure relaying although the considered systems have their relatives with cable-connected distributed antennas and mobile relays instead. Thus, the considered relay transceivers are fixed nodes and belong to infrastructure deployed by a network operator. This scope choice differs from earlier mainstream literature which is inclined toward ad hoc mobile relaying by user terminals. When discussing cellular systems, the link elements may be referred to as a base station (BS), a RN, and a user equipment (UE). The BS and RNs form a “backhaul link” while UEs are always connected to a “service link”. There is typically only one relay per each group of destinations (except for special hand-over situations) while backhaul links may involve multihop relaying. Furthermore, the study is formulated in an implicit context to concern a single subcarrier within orthogonal frequency-division multiplexing (OFDM) transmission.

This chapter has discussed energy efficiency issues and comparisons among relay protocols. A widely applicable system model has been introduced along with relevant energy efficiency metrics. In addition, the importance of using models that can compute the overall energy consumption (not just the transmitted radio-frequency power) has been emphasized. We have discussed in detail the energy efficiency optimization of different relay protocols includingAF,DF and CF. Mathematical methods including a general fractional programming framework to minimize the energy consumption ratio have been presented and the best energy efficiency regions for deploying relays have also been presented. This chapter has also studied the benefits of using relaying to form VMIMO network configurations. The performance trade-offs for different relay protocols in this scenario have been discussed and compared. The spectral and energy efficiency comparisons of different relay setups with varying numbers of antennas have also been discussed.

The rapidly growing wireless networks come with great challenges such as fading degradation, spectrum scarcity and energy constraints. This chapter introduces advanced relaying techniques in cognitive radio networks in which the primary and the secondary systems actively seek for cooperation opportunities, that can deal with these challenges in an effective way. We focus on cooperation at both information level and energy level, such that the primary users' (PUs) performance can be boosted, the secondary users (SUs) have much higher chance for spectrum access, and energy flow is optimized to benefit both systems.

In this chapter, we investigate the problem of physical-layer security in wireless networks with cooperative relays. In Sections 11.2 and 11.3, we study the notion of deaf cooperation to reinforce security of transmission in wireless relay networks. We distinguish between two main approaches to deaf cooperation, namely, the cooperative jamming (CJ) approach and the noise forwarding (NF) approach. In a CJ scheme, a helping interferer transmits Gaussian noise when it can hurt the eavesdropper more than it can hurt the legitimate receiver, and hence improves the achievable secure information rate. The idea of introducing artificial noise in a Gaussian wiretap channel by a helper node was introduced in References 5-7. In relay networks with secrecy constraints, the role of CJ was further investigated, e.g., in References 8-10. References 11-13 proposed CJ strategies for multiple-antenna relay networks. On the other hand, in the NF scheme which was introduced in Reference 14, the relay node sends a dummy (context-free) codeword drawn at random from a codebook that is known to both the legitimate receiver and the eavesdropper to introduce helpful interference that would hurt the eavesdropper more than the legitimate receiver.

In this chapter, we extend our previous works [24, 25] to a more general scenario of relay interference channels with energy harvesting relays equipped with multiple antennas. It is worth pointing out that implementing multiple antennas at the relays is particularly appealing in practice since it can not only boost the amount of harvesting energy at the relay in the first hop but also enhance the strength of the received signal at the destination by exploiting the transmit diversity technique in the second hop. The power splitting technique is assumed to implement at relays. Specifically, each relay node splits the signal received from all sources into two parts according to a power splitting ratio: one part is sent to the information processing unit, and the rest is used to harvest energy for forwarding the received information in the second time slot. We consider that each link's performance is characterized by its achievable rate and thus regard the sum-rate of all links as a network-wide performance metric. However, the network-wide sum-rate maximization problem of the considered system is shown to be analytically non-tractable due to its complexity and non-convexity. Motivated by this, we propose a heuristic two-stage network optimization approach. Specifically, considering the energy harvesting feature of the relays, we first figure out that the maximum ratio combining/maximal ratio transmission (MRC/MRT) scheme would be a particularly appropriate signal processing technique for the relays in practice owing to its low complexity and light requirement of channel state information (CSI). In the second stage, we optimize the power splitting ratios of the relays with the selected MRC/MRT technique.

The main goal of this chapter has been to introduce the reader to the most recent research developments on joint processing multibeam satellite communications. Since a joint processing enabled multibeam satellite constitutes a cooperative amplify-and-forward relay, these developments are closely related to the cooperative relaying literature. Nevertheless, as in detail described herein, the inherent nature of the satellite channel emanates fundamental differentiations in the proposed signal processing methods. Three main practical limitations have been overviewed. First, the need to jointly process signals over a fixed framing structure, coined as framed-based precoding. Second, the proper consideration of the non-idealities of real amplifiers and the effect they can have on precoded signals. Finally, but equally important, the detailed design of receivers that can operate in an interference-limited environment and adequately estimate the channel state of each user, so that precoding at the transmitter can be ultimately performed. If a single take home lesson needs to be gained from this overview would be that cooperative satellite relaying from high above is a mature technology from a research perspective and has reached the doorstep of practical demonstration and implementation.