In the present scenario, an energy efficiency has become a matter of prime importance for wireless networks. To meet the demands of an increased capacity, an improved data rate, and a better quality of the service of the next-generation networks, there is a need to adopt energy-efficient architectures. Along with these requirements, it is also our social responsibility to reduce the carbon footprint by reducing the power consumption in a wireless network. Hence, a green communication is an urgent need. In this paper, we have surveyed various techniques for the power optimization of the upcoming 5G networks. The primary focus is on the use of relays and small cells to improve the energy efficiency of the network. We have discussed the various scenarios of relaying for the next-generation networks. Along with this, the importance of simultaneous wireless power and information transfer, massive multiple input multiple output, and millimeter waves has been analyzed for 5G networks.

Power Optimization in 5G Networks: A Step Towards GrEEn Communication

Employing energy harvesting (EH) at the relay with simultaneous wireless information and power transfer is promising to prolong lifetime of energy-constrained relay networks. In this letter, considering multicarrier decode-and-forward (DF) relay network with time-switching (TS) based relaying, we investigate the optimization problem which jointly designs TS ratios of EH and information-decoding at the relay, TS ratio of signal forwarding from relay to destination as well as power allocation (PA) over all subcarriers at source and relay. Our objective is to maximize end-to-end achievable rate of DF relay networks subject to transmit power constraint at source and EH constraint at relay. We propose to decouple the optimization problem into a convex problem and a quasi-convex problem. For the quasi-convex problem, we propose to solve it by bisection search. Simulation results demonstrate that our proposed joint TS and PA optimization scheme outperforms the scheme with fixed TS ratio of EH.

Joint Time Switching and Power Allocation for Multicarrier Decode-and-Forward Relay Networks with SWIPT

In this paper, a coordinated multiple-relays-based cooperative communication scheme is proposed to improve the physical-layer security. In order to benefit the relays in forwarding the signals for defending against the eavesdropping attacks, the interactions between the source and the multiple relays are modeled as a single-leader multiple-followers Stackelberg game. The source plays as the leader to coordinate the relays, including the phase of signals forwarded by the relays and the transmit power of the relays, for maximizing the secrecy capacity of the system. An algorithm is developed for the relays to find an optimal price allocation to achieve the fairness among the multiple relays based on the egalitarian welfare solution, and an approximate optimal strategy of source (i.e., phase coordinated vector and power allocation) is studied. The closed-form intercept probability of the proposed scheme is derived. Numerical studies demonstrate that the proposed scheme can greatly improve the utilities of both the source and multiple relays over that resulted from the Nash equilibrium scheme and rand scheme, which means that the relays are more willing to participate in the cooperative communication, and the source can achieve better secure transmission based on the proposed scheme. It is also shown that the proposed scheme performs much better in defending against the eavesdropping attacks than those existing schemes, for example, the single relay selection scheme, the opportunistic relay selection scheme, the optimal relay scheme, and cooperative jamming scheme.

Coordinated Multiple-Relays Based Physical-Layer Security Improvement: A Single-Leader Multiple-Followers Stackelberg Game Scheme

Cooperative communication systems (CCS), which make use of the intermediate relays between the transmitter and the receiver, have been employed as an effective technique to combat the channel fading and to enhance the system performance. Cooperative systems have some drawbacks such as high latency and low bandwidth efficiency. To alleviate the negative effects of these factors, relay selection criteria is employed in cooperative communication systems to increase overall bandwidth efficiency and decrease latency. Relay selection in the cooperative systems enables the source to cooperate with the single relay node rather than multiple relay nodes which reduce the overall bandwidth efficiency. To prestige the benefit of cooperation, an efficient relay selection method is needed. In this paper, we propose a novel relay selection method, maximum link source-relay-destination (MLSRD) method in cooperative communication systems. The design goal of MLSRD is to improve the bandwidth efficiency. The result shows the MLSRD outperform cooperative communication systems without MLSRD in the term of the Bandwidth efficiency.

Bandwidth efficiency analysis of cooperative communication with Reactive Relay Selection

Orthogonal Frequency Division Multiplexing for Wireless Communications

In multihop relay networks with a large number of hops, it is challenging to allocate the available resources optimally among the relay nodes due to the large number of required control signals between the controller and the relay nodes. In addition, the latency caused by this signaling can result in using outdated resource-allocation information as the channels are time varying. Hence, a distributed resource-allocation scheme is proposed for multihop decode-and-forward (DF) relay systems, with low complexity in analytical computation and practical implementation. Based on the type of division multiplexing in time or frequency, this scheme can be used to obtain the optimal per-hop time slot length or bandwidth, respectively. This distributed iterative scheme needs no additional resource for optimization. The required communication to fulfill this optimization is only with the adjacent nodes of each relay and can be performed through the forwarding and acknowledgment packets (ACKs). It is shown that this scheme converges to the global optimal solution with an exponential convergence rate. The performance of this scheme is also investigated for the time-variant transmission channels by computer simulations.

Distributed Resource Allocation for Multihop Decode-and-Forward Relay Systems

Although cooperative relaying is generally considered as an attractive technique to improve system performances, the additional power consumption of a relaying link becomes a consistent concern. In order to characterize the power consumption of the dual-hop opportunistic relaying with power control, an exact expression for the cumulative distribution function of the total required transmission power is derived in this paper. Our analysis is based on the scenario that the best relay is selected among a set of decode-and-forward relays with homogeneous spatial Poisson distribution, such that the source-plus-relay power consumption is minimized. The required transmission power is evaluated by including the randomness of both path loss related to the locations of relays and the channel fading caused by multipath propagation. The analytical solution is verified by computer simulations and is used to compare the required power of relaying and direct links under different channel conditions. The results show that the dual-hop relaying is advantageous in long-term power saving by requiring a bounded average transmission power, while the power for the direct link communication is unbounded in some fading environments. Moreover, the required transmission power for the opportunistic relaying is less than that of the direct link with a reasonable relay density, therefore the cooperative communications can provide the power efficient transmission when the relay network is not sparse.

Performance analysis of decode-and-forward dual-hop opportunistic relaying with power control

In amplify-and-forward (AF) relay systems, multi-hop transmissions over multiple frequency selective fading channels result in an accumulation of delay spread, which could lead to strong intersymbol interference (ISI) at the destination since there is no proper channel compensation at the relay nodes. If orthogonal frequency division multiplexing (OFDM) with fixed-length guard interval (GI) is used in these systems, then the length of the GI has to be extended to the longest possible channel delay spread to avoid ISI. However, using such a long GI lowers the effective data transmission throughput. To this end, we propose a novel OFDM system with adaptive GI for AF relay networks to reduce the overall transmission overhead by dynamically choosing the suitable GI length. More specifically, the GI is adaptively selected at the transmitter from a variable-length orthogonal code set, which provides a GI sequence with proper length, depending on the accumulated channel duration. By exploiting the orthogonal property between different GI sequences, the proposed adaptive GI scheme can be implemented without any extra control signal transmitted by the source to notify the destination about the GI used. This differs from existing adaptive GI studies which require such an additional control signal and thus introduce extra system overhead. Numerical results show that the proposed adaptive scheme (without additional control signal) can achieve the same symbol error rate (SER) performance as the conventional adaptive GI approaches (with control signal). This implies that the proposed scheme can further save the control signaling overhead without any SER performance loss.

An Efficient OFDM with Adaptive Guard Interval for Amplify and Forward Relay Systems

The acquisition of accurate channel estimation is important for Orthogonal Frequency-Division Multiplexing (OFDM) system to reliably recover the transmitted data symbols. Decision-directed channel estimation (DDCE) has been utilized to improve the accuracy of conventional training-based channel estimation. However, this method can only provide limited improvement when the data decision feedback comprises a large portion of decision errors at low signal-to-noise (SNR) range. A new Iterative DDCE (IDDCE) scheme with reliable data feedback selection is proposed in this paper. The unreliable data decisions are eliminated on the subcarriers where noise components are enhanced due to the impact of frequency selective channel. An explicit expression of the variance of channel estimation error is derived, based on which the optimal selection criteria are also derived to minimize the variance. Simulation results show that the accuracy of channel estimation is significantly improved with the proposed algorithm.

Optimal reliable data feedback selection based iterative decision-directed channel estimation for OFDM system

In cooperative amplify-and-forward (AF) relay networks, the system performance over multipath channels is impacted by both frequency-selective fading and delay spread. However, most relay selection (RS) schemes choose the best relay node only based on the channel gain while ignoring the delay spread effect on the performance. If orthogonal frequency division multiplexing (OFDM) is used in AF relay networks, the cyclic prefix (CP) length has to be extended to tolerate the accumulated delay spread from source via relay to destination due to the lack of the channel compensation at relay nodes. A long CP, which is the transmission overhead, redeces both the effective data transmission throughput and the overall system transmission efficiency. Therefore, an appropriate RS scheme in cooperation multiple-relay networks should not only enhance the overall transmission reliability but also minimize the relay overhead. To this end, we propose a variable-CP based RS scheme for AF relay networks to maximize the transmission efficiency by dynamically choosing the most suitable relay node. In the proposed scheme, a normalized effective throughput is defined as the selection criterion which depends on both the end-to-end channel gain and the accumulated delay spread. Based on this criterion, the best relay link is selected by achieving the tradeoff between the transmission reliability and overhead. Both the theoretical analysis and simulation results show that when the channel delay spread varies, the proposed scheme can dramatically improve the the effective data transmission throughput compared to the maximum signal-to-noise-ratio schemes with variable/fixed CP.

Xin Gao

Papers

Distributed Resource Allocation for Multihop Decode-and-Forward Relay Systems

Performance analysis of decode-and-forward dual-hop opportunistic relaying with power control

An Efficient OFDM with Adaptive Guard Interval for Amplify and Forward Relay Systems

Optimal reliable data feedback selection based iterative decision-directed channel estimation for OFDM system

Relay selection scheme with adaptive cyclic prefix for cooperative amplify-and-forward relay