Adaptive resources assignment in OFDM-based cognitive radio systems

International Journal of Electrical and Computer Engineering (IJECE) Vol. 9, No. 3, June 2019, pp. 1935~1943 ISSN: 2088-8708, DOI: 10.11591/ijece.v9i3.pp1935-1943  1935 Adaptive resources assignment in OFDM-based cognitive radio systems Shirin Razmi, Naser Parhizgar Department of Electrical and Computer Engineering, Islamic Azad University, Shiraz Branch, Shiraz, Iran Article Info ABSTRACT Article history: Spectrum efficiency of orthogonal frequency division multiplexing (OFDM)based cognitive radio (CR) systems can be improved by adaptive resources allocation. In resources allocation, transmission resources such as modulation level and transmission power are adaptively assigned based on channel variations. The goal of this paper is maximize the total transmission rate of secondary user (SU). Hence, we investigate adaptive power and modulation allocation to achieve this purpose. For power allocation, we investigate optimal and conventional methods and then introduce a novel suboptimal algorithm to calculate the transmission power of each subcarrier. In addition, for adaptive modulation, we consider two kinds of modulations including multi-quadrature amplitude modulation (MQAM) and multi-phase-shift keying (MPSK). Also, simulation results are indicated the performance of our algorithm. Received May 19, 2018 Revised Nov 12, 2018 Accepted Dec 10, 2018 Keywords: Adaptive modulation Cognitive radio Interference constraints OFDM Power allocation Copyright © 2019 Institute of Advanced Engineering and Science. All rights reserved. Corresponding Author: Naser Parhizgar, Department of Electrical Engineering, Islamic Azad University, Shiraz Branch, Shiraz, Fars Province, Iran. Email: 1. INTRODUCTION The frequency band is an important resource in wireless communication. Due to increase wireless systems, frequency band has been scare more and more. Therefore, novel methods have been introduced to improve the spectrum performance for overcoming this challenge [1]. Cognitive Radio (CR) is a novel and powerful technique to increase spectrum band performance. In the CR the unlicensed user or the secondary users SU can utilize the frequency spectrum that originally allocated to a licensed user or the primary users (PU). The main challenge for SUs is keep the amount of interference that introduces on the PUs than specified threshold [2]. Several scenarios have introduced by researchers for CR systems. In the one of the most important scenario, that is considered in this research, SUs can use unoccupied parts of spectrum bands between PUs bands. In this scenario, the PUs and SUs are assigned in adjacent bands [3]. Therefore, due to this vicinity, adjacent channel interference (ACI) is produced on both PU receiver (PUR) and SU receiver (SUR) [4]. Hence, it is so important to consider this interference to guarantee the performance of both PU and SU systems. Because of main advantages of OFDM technique, it is used by SUs to utilize the unoccupied portion of spectrum bands [5]. This kind of system is named OFDM-based CR system. As we know, adaptive resources assignment is a technique to increase the performance of the communication systems. Therefore, we consider this technique to improve the performance of the OFDM-based CR system. Therefore, we consider adaptive power allocation that the transmission power of each subcarrier is adaptively assigned based on channel variation [6]. In power allocation, more power is allocated to channels with better channel fading gain and less power allocated to channels with worse channel fading gain [7]. Also, we consider adaptive modulation in this paper. In the adaptive modulation, the modulation level is changed adaptively [8]. Journal homepage: http://iaescore.com/journals/index.php/IJECE 1936  ISSN: 2088-8708 The idea behind adaptive modulation is that the transmitter can use the channel in optimum mode regardless channel situations [9]. Due to importance of this research topic, resources allocation in OFDM-based CR systems has investigated in some papers. In the [3] and [4], researchers investigated power allocation in the OFDM-based CR systems and introduced the algorithm for power allocation. Authors in [5], consider effect of the mutual interference on the CR system. In addition, in [10] and [11] authors introduced suboptimal algorithms for power allocation. Although suboptimal algorithms have worse performance than the optimal algorithm but due to their low-complexity procedures, they are the better candidate for practical usages. In [12] authors considered both adaptive modulation and power allocation and introduced a suboptimal power allocation algorithm for OFDM-based CR systems. In the [8] and [12], researchers considered MQAM modulation technique for transmitting data. In the both papers, modulation level is changed adaptively, based on the channel state information. In this research, we first introduce the optimal method for allocating the transmit power and then introduce a novel suboptimal algorithm in the OFDMbased CR systems. In addition, conventional power allocation methods such as water filling and uniform loading algorithms are described in this paper. In the above papers, only MQAM modulation level was investigated while in this paper, we consider two kinds of modulation including MQAM and MPSK. Both modulation schemes are used in modern communication systems, therefore, we compare the performance of them in OFDM-based CR systems. This compression helps to researchers to select the best modulation technique for future researches and applications. The rest of this paper is organized as follows; in Section 2, we introduce the system model and optimum power allocation. Our suboptimal algorithm is introduced in Section 3. Water filling and uniform loading algorithms are described in Section 4. In Section 5, numerical results are presented. 2. SYSTEM MODEL The model of the system is shown in Figure 1, where a SU is located between L PUS. As discussed is the previous section, the SU uses OFDM to use frequency holes. Therefore, the secondary user divides frequency holes into N flat subcarriers with a bandwidth Δf. Spectrum allocation is behind of this paper scope, hence, we assume spectrum allocation has done and the values of Δf and N are known. SU transmitter (SUT) utilizes ideal Nyquist pulse. Each spectrum band of PU is equal to B. the maximum value of interference that SU can introduce on each PUR is equal to i-th. Figure 2, indicates system model in a spatial domain. hiss is the channel fading gain of SUT- SUR channel over i-th subcarrier. hℓsp is the channel fading gain of SUT - ℓ-th PUR channel. Figure 1. System model in frequency domain Figure 2. System model in spatial domain We consider two kinds of modulation. It is worth to note that the SU can use only one kind of these modulation schemes at the same time; i.e. the SU can use MQAM or MPSK, but it cannot use both of them simultaneously. On the other wo (...truncated)