Recent Advances on Radio-Frequency Design in Cognitive Radio

Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2016, Article ID 9878475, 16 pages http://dx.doi.org/10.1155/2016/9878475 Review Article Recent Advances on Radio-Frequency Design in Cognitive Radio H. M. El Misilmani, M. Y. Abou-Shahine, Y. Nasser, and K. Y. Kabalan ECE Department, American University of Beirut, P.O. Box 11-0236, Beirut 1107 2020, Lebanon Correspondence should be addressed to Y. Nasser; Received 22 October 2015; Accepted 24 January 2016 Academic Editor: Symeon Nikolaou Copyright © 2016 H. M. El Misilmani et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. With the growth of mobile data applications, the spectrum allocation is becoming very scarce. To ease congestion and boost speeds, cognitive radio (CR) is currently seen as a major solution and expected to be the key player in the new wireless technologies. In this paper, we will start by introducing the cognitive radio systems, followed by exploring the challenges in designing RF engine, along with an investigation of its antennas, amplifiers, oscillators, and the components that are expected to operate over a wide range of frequencies. 1. Introduction With the increasing demand for high data rates requiring high resources, such as energy and frequency bandwidth, cognitive radio (CR) is thought of as a very promising solution. The basic operating principle of CR device relies on a cycle of observation, analysis, and decision and an opportunistic access to the available bandwidth. Hence, a CR device, usually referred to as secondary terminal, has to firstly sense the existence of a primary transmission and opportunistically transmits whenever a frequency/time slot is vacant. If the authorized (primary) terminal restarted transmission, the secondary terminal jumps off into a different band or alters its transmission parameters so that it does not affect the primary transmission [1]. In practice, the cognitive radio devices are expected to sense the occupancy of any channel at any band in the entire spectrum and autonomously adapt to the primary transmission [2]. This continuous (or discontinuous) sensing process on a large bandwidth imposes different constraints on the radio-frequency front-ends of the secondary terminal. More precisely, these requirements constrain strict issues on antenna design, low noise amplification, frequency synthesizers providing a carrier frequency from tens of megahertz to about 10 GHz, mixing spurs, and spectrum sensing. Broadband and tunable antennas, multiband amplifiers, RF filters, broadband direct-conversion mixers, baseband filters, and ADCs/DACs are needed to realize software-defined cognitive radio equipment. These RF components are expected to operate over a wide range of frequencies [3]. In cognitive radio, a reconfigurable radio front-end can be programmed to transmit, steer to any band, tune to a channel of any bandwidth, and receive any acceptable modulation scheme. The ability to design linear and spectrally agile components and architectures in the radio-frequency frontend of the transceiver is considered a primary technological concern in cognitive radio architectures. The objective of this paper is to revise the major concerns of the front-end design of a CR system including the antennas, amplifiers, and oscillators. In the literature, very few works have investigated the design issues of the CR frontend from end-to-end. Moreover, a complied review of the different constraints has been very little tackled. We cite, for instance, the work in [4] as a first attempt to harmonize all these constraints. Hence, this paper belongs to the research works which could be used as a reference for the RF engineers working on CR. To complete this work, we detail and investigate the challenges to overcome in the next few years in order to design a complete and smoothly tunable RF frontend for CR applications. 2 International Journal of Antennas and Propagation Wsub = Wground y d x a b L ground Table 1: Operating bands achieved when switches are ON/OFF [6]. L feed1 Wupper Switch L sub Wlower L feed2 } } Wfeed1 Wfeed2 Switch 1 Switch 2 Switch 3 Switch 4 Frequency band (GHz) ON ON ON ON 2.50–3.40 and 3.81–5.32 ON ON ON OFF 2.49–4.26 and 5.90–6.25 ON ON OFF ON 2.49–4.36 ON ON OFF OFF 2.50–4.38 ON OFF ON ON 3.79–6.71 OFF ON ON ON 3.82–6.38 OFF OFF ON ON 4.06–6.36 71 Figure 1: A proposed sensing and communicating antenna configuration in [5]. 15.5 13.5 11 Switch 3 2. Antennas for Cognitive Radio Cognitive radio communication is envisaged to be a new paradigm of methodologies for enhancing the performance of radio communication systems through the efficient utilization of radio spectrum. A key enabler for realization of a cognitive communication system and one of its main challenges is the capability of reconfigurability in the underlying hardware and the associated protocol suite. From the antenna design perspective, the demand for multiwideband antennas which can be easily integrated with the communication system is continuously increasing. Reconfigurable and frequency agile architectures are mostly designed nowadays in order to solve the broad frequency allocation and to reduce the number of functional blocks. Intensive work has been done in designing antennas for cognitive radio applications. The use of wideband antennas for spectrum sensing and narrowband antennas for transmission has been proposed by the research community [16, 17]. The sensing and transmitting antennas could be also found in the structure, as shown in Figure 1. In general, there are three different categories of reconfigurable antenna: (1) Frequency reconfigurable antennas: the frequency of the antenna is tuned to have single multifunctional antenna as a small terminal for many services, with radiation pattern remaining unchanged while the frequency is changing [18–20]. (2) Radiation patterns reconfigurable antennas: the antenna can steer its radiation patterns beams to different direction. The frequency remains unchanged while the radiation pattern changes upon the system requirements [21–23]. (3) Polarization reconfigurable antennas: providing an additional degree of freedom to improve link quality as a form of switched antenna diversity with improved signal reception performance in a multipath fading environment [24]. So far, reconfigurable antennas for cognitive radio communications can be classified into three types: electronically reconfigurable antennas, mechanically reconfigurable antennas, and optically reconfigurable antennas. 8 24.5 Top Switch 1 1.0 FR4 epoxy Switch 4 Switch 2 19 1.6 Bottom 2.7 11 2.7 8 19 Figure 2: A proposed sensing and communicating antenna configuration in [6] (dimension is mm). 2.1. Electronically Reconfigurable Antennas. Electro (...truncated)