3.1.1
Cooperative Communication
The large demand for mobile access and advances in wireless hardware technology have caused tremendous progress in wireless communication networks over the past decades. In previous years, many attempts to attain reliable and high data-rate communication over the wireless channels have been failed because of different wireless channel character- istics such as shadowing, multi-path fading, and path loss effects. Due to these effects, the wireless channel quality varies in space, time, and frequency. Therefore, techniques used in traditional wireline communications are not always suitable for wireless channels. One of the techniques to overcome such an issue is a transmit/receive diversity technique, where a diversity gain can be achieved in space, time or frequency domains. With a good knowledge of CSI, more resources such as power, time, and frequency can be allocated to users with stronger CSI and users with unreliable channels are allocated with fewer resources. Yet while CSI is not available, one can implement space-frequency or space- time resource allocation policies to improve the reliability of the network. Particularly, recent advances in MIMO antennas benefit in achieving spatial diversity gains in modern wireless networks [17,89–92]. On the other hand, the cost and size of many wireless network applications, such as mobile phones, sensors, etc., are limited and installation of
(a)
Amplify-and-forward.
(b)Detect-and-forward.
Figure 3.1. The basic cooperative relaying schemes: amplify-and-forward and decode-and-forward.
multiple antennas on such devices can be impractical. In such a circumstance, a desirable and promising alternative technique can be when single antenna mobile users can coop- erate with each other to build up a distributed antenna system to obtain spatial diversity gains. This method is called cooperative or relaying communications [93].
In cooperative communication systems, users help one another to relay their mes- sages to a destination user [94–100]. On that basis, cooperative users can achieve spatial diversity gains by imitating a distributed antenna array of MIMO systems. Thus, due to wireless channel characteristics, different cooperative users experience various fading effects while relaying each others’ data to the destination user, which consequently im- proves transmission reliability.
Many relaying techniques have been studied in the literature, namely, AF [101,102], DF [101,103], coded cooperation [104], selective relaying (SR) [105], compress-and- forward (CF) [105] and so on. All relaying techniques work in a half-duplex mode, which means that, at each time slot, only one user serves as a source node, whereas other users act as relay nodes to forward messages from the source to the destination. Depending on the system requirement, any cooperative user can act as either a source or a relay. Fig. 3.1 illustrates AF and DF relaying schemes, which are the most basic and widely adopted relaying schemes. In both schemes, in the 1st time slot, the source transmits its data to both the destination and the relay. In the 2nd time slot, while the AF relaying scheme is implemented, the received signal at the relay is amplified and forwarded to the destination. Then, the destination combines the both received signal from the source and the relay to increase the performance of the signal detection. If the DF relaying scheme is adopted, within the 2nd time slot, the relay detects the received signal from the source
3.1.2
Cognitive Relaying Networks
CR is a promising technique that can utilize the wireless spectrum usage, the main idea of which is to allow SUs to broadcast in licensed frequency bands when no communication activity is required for PUs. The benefits of cooperative communications have also been applied in CR [106–110]. A relay network adopted to CR enhances the throughput and coverage area of the entire network [106]. In cognitive relaying networks (CRNs), the SN can obtain better throughput by implementing the main two approaches. The first one is while the PUs and the SUs cooperate with each other, whereas, in the second approach, the cooperation happens only among the SUs [107]. The authors in [108] compared the performance of licensed and unlicensed bands with that of the conventional relay network. The results showed that adopting cooperation in CR shows outstanding performance in wireless relay networks by reducing inter-cell interference. Another CRN studies [109] has developed the best relay selection technique in an underlay CRN, where obtained results showed that the CRN achieves the same full selection diversity order as traditional networks. Moreover, an increase in the number of relays resulted in a better OP compared to conventional relay networks. Furthermore, an underlay dual-hop DF CRN in Nakagami-m fading channels was studied in [110], where each relay demodulates the received signal and forwards it to the destination. The authors in [111] derived the OP of underlay CRN which operates in generalized-K and generalized-gamma distributions.
In the wireless medium, a large number of distributions exist that can well repre- sent the statistics of the wireless channel. For example, the long-term signal variation can be described by the Log-normal distribution, while the short-term signal variation can be presented by various other distributions, i.e., Rice, Rayleigh, Hoyt, Weibull, and Nakagami-m. All studies in the literature of CRNs were mainly adopted Rayleigh, Ri- cian and Nakagami-m fading channels, while neglecting other distributions such as Log- normal, Negative exponential, Weibull, etc. By motivated from the research above, the rest of this Chapter considers a study of CRNs over the general α − µ distribution that fully embraces all the aforementioned channel distributions [112].