01.12.2019 | Research | Ausgabe 1/2019 Open Access

# On signal processing scheme based on network coding in relay-assisted D2D systems

## 1 Introduction

## 2 System model

_{1},C

_{2},...C

_{j},...C

_{N}] and [R

_{1},R

_{2},...R

_{i},...R

_{M}], respectively. The transmitter and receiver of the DUEs are denoted by S and D respectively. It is assumed that the cell is overloaded; that means resources are not idle and cannot be separately allocated to DUEs, and only CUEs’ uplink resources can be reused [23]. DUEs can communicate directly or with RA-D2D mode according to channel quality and communication requirements. For each of the two communication modes, at most, one CUE’s uplink resource can be reused. In RA-D2D communication mode, transmitter-to-relay and relay-to-receiver transmissions are finished in two time slots, and transmitter-to-relay and relay-to-receiver links reuse the same uplink resource. As a result, the CUE whose uplink channel is reused will generate interference to the relay UE and the receive DUEs. In addition, assume that all UEs in the cell are under the control of the BS, which can acquire all channel state information, control DUEs’ communication mode, select the reused resource, and optimize the relay selection.

_{0}is a constant whose value corresponds to the implemented path loss model, d

_{a,b}is the distance from the transmitter to the receiver, α is is the path loss exponent, \( \beta _{a,b}^{s} \) is slow fading obeying the logarithmic distribution, and \( \beta _{a,b}^{f} \) is fast fading obeying the exponential distribution [26].

## 3 The proposed joint scheme

### 3.1 RA-D2D signal processing scheme based on network coding

_{C}and P

_{C}are the transmit power of the DUEs and the CUE respectively. h

_{sr}, h

_{cr}, and h

_{cd}are the channel gains between the users; n

_{SR}and n

_{DD}are additive white Gaussian noises.

### 3.2 Optimal relay selection strategy

_{NC}of the system is calculated in Section 3.1. Subsequently, we will introduce how to calculate the relay survival time and end-to-end delay and discuss the optimization problem of relay selection scheme with different criteria considered [27].

_{1},E

_{2},...E

_{i},...,E

_{M}denote the remaining battery capacities of the M RUEs. The survival time of RUE R

_{i}can be obtained according to

_{i}stands for the discharge current. According to the relationship among voltage, current, and power, if the discharge voltage and the corresponding power consumed are known, the discharge current can be acquired. In this paper, the discharge voltage is a predefined parameter and the total power used for data transmission at each transmit UE can be calculated according to the remaining battery capacity. α is constant to indicate the non-linear effect, and its value is around 1.3.

_{s}ΔT is the number of packets arriving at the device in one time slot, λ

_{s}is the arrival rate, and \( p_{sd}^{\left (i,j \right)} \) and \( p_{se}^{\left (i,j \right)} \) are the packet dropping rate and packet error rate, respectively.

_{d}is the SINR at the receiver D. The end-to-end rate- and RUE survival time-based relay selection models are

## 4 Simulation results and discussions

Parameters | Value |
---|---|

Cell radius (R) | 500 m |

Path loss exponent (α) | 4 |

Path loss constant (K _{0}) | 0.01 |

Noise power \(\left ({\sigma ^{2}_{N}}\right)\) | − 174 dBm/Hz |

Transmit power of UEs | 24 dBm |

Distance between source and destination DUEs | [0,200](m) |

Slow fading coefficient \(\left (\beta ^{s}_{a,b}\right)\) | Log-normal distribution with standard deviation of 8 dB |

Fast fading coefficient \(\left (\beta ^{f}_{a,b}\right)\) | Exponential distribution with unit mean |