Employing multiple antennas at the transmitting and/or receiving nodes can assist in mitigating the effects of fading. In communication networks, such as wireless sensor networks, IoT entities, wireless body area network, etc., installation of multiple antennas at nodes may not be feasible due to hardware size constraints. In such scenarios, extra nodes can be deployed between the transmitting and receiving nodes to improve the end-to-end performance. These extra nodes are generally called relaysand can form virtual antenna array in order to improve the diversity gain [22]. In communication systems like cellular networks, fixing multiple antennas at nodes (including relays) can further enhance the systems performance [23,24]. In systems where relay nodes are introduced between the source and destination nodes to improve the diversity gain are known asparallel relay systems. Scenarios can also exist when the destination node is not in the coverage range of the source node or the channel between source and destination is blocked such that the source data cannot be conveyed to the destination.
Under such circumstances, multiple relay nodes can be arranged in between source and destination nodes such that the source data is communicated to the destination in multiple hops and hence the coverage range is extended. The systems employing arrangement of relays in multiple hop fashion are commonly known as multihop relay systems [25, 26]. The hybrid arrangement of relay nodes is also appealing when there is a requirement of diversity enhancement in individual hop [26]. In networks such as device-to-device communications, usually user devices act as the relay and cooperate with each other to convey their data to the destination(s). Such user devices are generally called cooperative relays.
1.2 An Overview of Relay Systems
The advantage of relaying in terms of improved performance comes at the cost of increased over- head, which is caused by the requirement for additional channel state information (CSI), relay se- lection, and coordination. The increment in overhead in relay systems results in higher end-to-end delay as compared to point-to-point communications. In the literature, considerable efforts are made to investigate methods for minimization of the end-to-end delay [27–29], etc. Particularly, in parallel relay systems, the end-to-end delay can be minimized by appropriately modeling the transmission duration, applying suitable relay selection methods, etc. [27, 28]. The end-to-end delay in multihop relay systems is minimum when relays are placed at an equal distance [29]. Usually, half-duplex mode of operation is considered for the relays which imposes low system throughput. Schemes such as two- way relaying [30], full-duplex relaying [31], mimicking full-duplex relaying [32], etc., can be useful in improving the throughput, though this comes at the cost of increased system complexity. Network coding in relaying systems is also a potential candidate in improving the system throughput [33–35].
The relay nodes basically employ amplify-and-forward (AF) and decode-and-forward (DF) proto- cols to process the received signal. In AF, relays simply amplify the received signal with some gain and forward it further, whereas in DF, they first decode the received data, re-encode it and then forward.
Depending on whether the gain by which relays forward the amplified signal is fixed or varying, AF protocol is called fixed-gain AF or variable-gain AF, respectively. Fixed AF requires no knowledge of instantaneous channel state information (CSI), whereas CSI of source-to-relay links are required at the relays in variable-gain AF. DF protocol is sub-categorized as fixed DF and selective DF. In fixed DF, relays forward all the decoded data, while in selective DF only the correctly decoded data is for- warded. Correct decoding at the relay nodes can be accomplished using cyclic redundancy check codes or threshold-based checking. AF protocol has lower complexity, whereas DF protocol outperforms AF protocol at high SNRs [26, 29].
1.2.1 Three-node Relay System
Three-node relay system has a relay node assisting the source and the destination nodes. In case the direct link connecting the source to destination node is deeply faded, then the relay node is useful for coverage extension. This arrangement of nodes is widely known as two-hop relay system. Further, if the direct link exists, the relay node assist in improving the diversity gain by means of diversity combining schemes such as selection combining (SC), equal-gain combining (EGC), and maximum
1. Introduction
D S
D S
D S
R
T
P StoD
T
P/2 StoD(at 1stantenna) P/2 StoD(at 2ndantenna)
T
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(a.1) 1x1 System (a.2) 2x1 System (a.3) Relaying
(b.1) 1x1 System (b.2) 2x1 System (b.3) Relaying
(a) System Model
(b) Transmission Technique 1st 2nd
T
Figure 1.2: System model and transmission technique of 1×1 system, 2×1 system and relaying system.
ratio combining (MRC) [36]. In two-hop relay system, fixed-gain AF may outperform variable-gain AF at low to medium SNRs but exhibits relatively poor performance at high SNRs [37]. Fixed DF can outperform AF (both fixed-gain and variable-gain) at low to medium SNRs, although at high SNRs, these protocols perform identically [37]. In system with direct link, selective DF protocol possesses better performance than the other protocols [22].
Performance Comparison of Selective DF: Here we compare the performance of a three- node relay system with that of SISO (single-input and single-output) and MISO systems. Figure 1.2 presents the system model and transmission techniques for three systems: 1) SISO, where source node S and destination nodeDare mounted with single antenna, 2) MISO, where nodeS is equipped with two antenna and node Dwith single antenna, and 3) three-node relay system, where each node has a single antenna. SISO and MISO systems are represented by 1×1 system and 2×1 system, respectively.
In the relay system, the relay nodeRemploys selective DF protocol and operates in half-duplex mode.
1.2 An Overview of Relay Systems
0 5 10 15 20 25 30
10−5 10−4 10−3 10−2 10−1 100
SNR (dB)
Average SER
1×1 system
Relaying (Selective DF) 2×1 system
Figure 1.3: Average SER comparison of a three-node selective DF relay system with direct transmission and 2×1 MISO system for BPSK modulated data under Rayleigh fading.
The direct link between the source and destination nodes exists and the two signals arriving at the destination via the direct link and relayed link are combined using MRC principle. Each transmission requires T time. Hence, the relaying system requires a total 2T time of the end-to-end transmission.
This causes a reduction in throughput by half as compared to the other two systems. The total transmission power in each system is taken as P. In 1×1 system, node S transmits with power P whereas in 2×1 system, each transmitting antenna at node S transmit with power P/2. In the relay system, source and relay nodes transmit with power P/2. The background noise is assumed to be additive white Gaussian noise (AWGN) with mean zero and variance N0. The mean power of the fading gain is assumed to be unity. Hence, the received SNR isP/N0. Data is considered to be binary PSK (BPSK) modulated and the fading environment is modeled by Rayleigh distribution. These three systems are emulated inMATLABand the corresponding simulated average SER results are presented in Figure 1.3. We assume unit transmission time duration, that is,T = 1 for simulation. In Figure 1.3, we observe that the diversity gain of 2×1 system is better than that of the other two systems. The relay system exhibits diversity gain close to 2×1 system. 1×1 system possesses diversity order (DO) of one, whereas the other two systems have a DO of two. DO is diversity gain at high SNRs and can be
1. Introduction
interpreted as the negative slope of average SER versus SNR curves on the log-log scale. We conclude that the performance of a communication system having a source and a destination, each equipped with a single antenna can be improved considerably by introducing a relay node in the system. The relay system and 2×1 MISO system perform close and both have DO of two.
Conventionally, it is assumed that nodes are powered by infinite energy capacity or rechargeable batteries. Although, in practice, energy storage is finite and in some applications communicating nodes are physically unreachable for recharging and replacing batteries when exhausted. In such circumstances, the end-to-end communication terminates if there is no sufficient amount of energy at any of the transmitting nodes. Renewable energy resources, such as wind, solar, mechanical vibration, etc., are looked upon as prominent solutions for scavenging energy at the energy-constrained nodes and extending the communication lifetime [38]. Though the quantity of energy arrival is dependent on geography, weather and many other factors, which make these resources unpredictable and unreliable [39]. This limits the applicability of these energy harvesting (EH) solutions when there is a requirement to transmit vital information but harnessed energy is insufficient to support the communication.
Wireless EH in such scenarios is being considered as an effective solution for providing energy to energy-constrained nodes when required. It has attracted the interest of a vast group of researchers.
Wireless EH in relay systems is also a prominent area of research [40, 41]. In the next section, we briefly discuss about wireless EH and its applicability in communication systems.