830 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 Performance Improvement of a Wideband MIMO System by Using Two-Port RLWA Daniele Piazza, Student Member, IEEE, Michele D’Amico, and Kapil R. Dandekar, Senior Member, IEEE Abstract—In this letter, we demonstrate the performance achiev- able with a highly reconfigurable leaky-wave antenna (RLWA) when used as a building block of a wideband multiple-input–mul- tiple-output (MIMO) communication system. The antenna design consists of a periodic structure, composed of cascaded composite right/left-hand (CRLH) microstrip unit cells with tunable elec- trical characteristics, that is capable of steering its radiation beam over a wide angular range. Field measurements collected in an indoor environment employing an array of dipoles at the transmitter and the RLWA and anarray of dipoles at the receiver are used to measure the RLWA achievable gains. We show that the proposed antenna is capable of providing significant gains, with respect to a traditional array of dipoles, while reducing the space occupied by the antenna on the communication device. Index Terms—Metamaterials, multiple-input–multiple-output (MIMO), reconfigurable antennas, smart antennas. I. INTRODUCTION R ECONFIGURABLE antennas have been recently sug- gested as an ideal solution for improving the level of di- versity of multiple-input–multiple-output (MIMO) communica- tion systems without increasing the number of antennas em- ployed at the receiver and at the transmitter [1], [2]. In partic- ular, recent studies have shown that the higher the number of uncorrelated antenna configurations, the higher the communi- cation system diversity level and the higher the achievable per- formance [1]. Different reconfigurable antenna solutions capable of dy- namically changing their radiation pattern and polarization have been proposed and studied for MIMO systems [2]–[6]. In [7], the authors demonstrated a two-port reconfigurable leaky-wave antenna (RLWA) that allows the beam of each port to be steered across a wide angular range. Ideally, an infinite number of radiation patterns can be excited at each port while keeping good matching and high isolation between the two ports. The goal of this letter is to determine the performance achiev- able with this highly reconfigurable array when employed in a Manuscript received April 28, 2009, revised June 10, 2009. First published July 07, 2009; current version published July 28, 2009. D. Piazza is with the Department of Electrical and Computer Engineering, Drexel University, Philadelphia, PA 19104 USA, and also with the Department of Electrical and Computer Engineering, Politecnico di Milano, Milano 20133, Italy (e-mail: dp84@drexel.edu). M. D’Amico is with the Department of Electrical and Computer Engineering, Politecnico di Milano, Milano 20133, Italy. K. R. Dandekar is with the Department of Electrical and Computer Engi- neering, Drexel University, Philadelphia, PA 19104 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2009.2026594 Fig. 1. Two-port RLWA and the tunable CRLH unit cell design. 2 2 802.11n-like MIMO communication system. Field mea- surements have been collected in line-of-sight (LOS) and non- line-of-sight (NLOS) scenarios to determine the system achiev- able channel capacity and to quantify the level of power saving achievable for a fixed transmission rate with the RLWA com- pared to a reference nonreconfigurable antenna system. II. ANTENNA SYSTEM The reconfigurable antenna system is a microstrip composite right/left-hand (CRLH) leaky-wave antenna composed of 25 cascaded metamaterial unit cells. The antenna design and the structure of each unit cell composing the leaky-wave antenna are shown in Fig. 1. To achieve the CRLH behavior, we imple- mented the unit cell by inserting an artificial series capacitance and a shunt inductance into a conventional microstrip line by means of interdigital capacitor and a shorted stub, respectively [8]. To dynamically tune the handedness of the unit cell, two varactor diodes ( ) are placed in parallel with the microstrip series interdigital capacitor, and one varactor diode ( ) is placed in series with the shunt inductor similar to the design in [8]. Unlike from the design in [8], in this unit cell structure, two independent bias networks are used to separately tune the var- actors (“S” bias) and (“SH” bias) in order to keep the unit cell Bloch impedance close to 50 for good impedance matching. A capacitor ( pF) is used to decouple the two dc bias networks, and quarter-wave transformers are em- ployed to prevent the RF signal from flowing to dc ground. Dif- ferent combinations of applied voltages “S” and “SH” are used 1536-1225/$26.00 © 2009 IEEE Authorized licensed use limited to: Drexel University. Downloaded on August 4, 2009 at 11:21 from IEEE Xplore. Restrictions apply.