# Resource Allocation for Relay-Based OFDMA Power Line Communication System
## 1 Introduction  
The orthogonal frequency division multiplexing (OFDM) combined with dynamic resource allocation technology can dynamically allocate the subcarriers to every user and allocate the transmit power on each subcarrier according to the channel state. Applying OFDM into the power line communication (PLC) system can effectively eliminate the multi-path, attenuation, time-varying and frequency selectivity of the power line channel, therefore, effectively improving the resource utilization, quality of service and data transmission rate [1; 2; 3; 4; 5]. Motivated by the above technique, OFDM-based PLC systems have become increasingly attractive. Due to the long transmission distance of the multi-conductor cable, it is necessary to adopt a relay for signal amplification and forwarding in the PLC system. The diagram of the relay-aided PLC system is shown in Figure 1, we can see that the base station (BS) of the communication network is connected to the Internet through the access point, and the BS is connected to the power grid by the coupling circuit on the low voltage side of the power transformer. The signal of the BS is transmitted to each PLC gateway through the multi-conductor cable, and the gateway is connected to the user terminal through the power line. Therefore, the network is composed of outdoor broadband access and indoor broadband interconnection. The resource allocation in the relay-aided OFDM PLC system is a joint optimization problem, including the power control, subcarrier assignment, relay selection, etc., therefore, when referring to the relay-aided OFDM PLC system, it is usually considered challenging due to the complexity [6]. The essence of the problem is how to optimally allocate subcarriers tousers and allocate power to subcarriers [7; 8; 9; 10; 11]. Recently, a variety of multi-user dynamic resource allocation algorithms have been proposed with different optimization objectives or constraints for different communication systems. Ref. [12] proposed a cooperative wireless power communication network protocol, in which the time allocation of hybrid access point downlink transmission and user uplink transmission were jointly optimized for two different aspects. Ref. [13] proposed a multiuser OFDM system named simultaneous wireless information and power transfer, which can transfer information and power separately on different subcarriers. The optimal design for resource allocation is investigated to maximize the sum rate with a minimum transferred power constraint. In [14], for OFDM downlink distributed multiple input single output (MISO) systems in frequency selective fading channels, two power allocation strategies are proposed to solve the power constraint of a single antenna unit. In [15], the energy-saving user scheduling and power control in downlink multicell multiuser orthogonal frequency division multiple access (OFDMA) networks is investigated only with the channel distribution information, which can solve the resource allocation problem subject to the minimum throughput constraint of each user, even if the channel gain is the same for all the subcarriers. In [16], a stochastic optimization programming in dynamic wireless self-backhaul small cell networks is formulated to investigate the spectral efficiency maximization by both considering the resource reuse and allocation, additionally, the random and finite traffic loads are also considered to keep the system stable. The resource allocation problem in two-tier heterogenous networks with massive MIMO is investigated in [17], which used cellular frequency bands and millimeter wave frequency bands for wireless backhaul links. A block diagonalization based precoding scheme is proposed to eliminate the multi-user and inter-tier interference and a power allocation optimization problem is formulated to maximize the downlink sum rate of the system subject to per-small cells clusters power and users' quality of service requirements.