Spectral efficiency of a communication system refers to the information rate that the system can transmit reliably over the available bandwidth (spectrum) of the communication channel. Enhancing the spectral efficiency is without doubt a major objective for the designers of next generation wireless systems. It is evident that the telecommunications industry is rapidly growing due to the high demands for ubiquitous connectivity and the popularity of high data rate multimedia services. As well-known, wireless channels are characterized by temporal and spectral fluctuations due to physical phenomena such as fading and shadowing. A well-established approach to exploit the variations of the fading channel is opportunistic communication, which means transmitting at high rates when the channel is good and at low rates or not at all when the channel is poor. Furthermore, in the last few years, the research focus has turned into exploiting the broadcast nature of the wireless medium and the potential gains of exploiting the interaction (cooperation) between neighboring nodes in order to enhance the overall capacity of the network. Cooperative communication will be one of the major milestones in the next decade for the emerging fourth and fifth generation wireless systems. Cooperative communication can take several forms such as relaying the information transmitted by other nodes, coordinated multi-point transmission and reception techniques, combining several information flows together using network coding in order to exploit side information available at the receiving nodes, and interference management in dense small cell networks and cognitive radio systems to magnify the useful information transmission rates. We propose to exploit all sources of capacity gains jointly. We want to benefit from old, yet powerful, and new transmission techniques. Specifically, we want to examine optimal resource allocation and multiuser scheduling in the context of the emerging network architectures that involve relaying, network coding and interference handling techniques. We like to call this theme opportunistic cooperative communication. With the aid of opportunistic cooperative communication we can jointly exploit many sources of capacity gains such as multiuser diversity, multihop diversity, the broadcast nature of the wireless medium and the side-information at the nodes. We suggest exploring opportunistic cooperative communication as the choice for future digital communications and networking. In this direction, we introduce the topic of buffer-aided relaying as an important enabling technology for opportunistic cooperative communication. The use of buffering at the relay nodes enables storing the received messages temporarily before forwarding them to the destined receivers. Therefore, buffer-aided relaying is a prerequisite in order to apply dynamic opportunistic scheduling in order to exploit the channel diversity and obtain considerable throughput gains. Furthermore, these capacity gains can be integrated with other valuable sources of capacity gains that can be obtained using, e.g., multiuser scheduling, network coding over bidirectional relays, and interference management and primary-secondary cooperation in overlay cognitive radios systems. The gains in the achievable spectral efficiency are valuable and hence they should be considered for practical implementation in next generation broadband wireless systems. Furthermore, this topic can be further exploited in other scenarios and applications.


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