Design of a Practical and Compact mm-Wave MIMO System with Optimized Capacity and Phased Arrays

Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2014, Article ID 608345, 9 pages http://dx.doi.org/10.1155/2014/608345 Research Article Design of a Practical and Compact mm-Wave MIMO System with Optimized Capacity and Phased Arrays Tommaso Cella,1 Pål Orten,2 and Torbjörn Ekman3 1 NTNU and UniK, 7491 Trondheim, Norway ABB and UniK, 1396 Billingstad, Norway 3 NTNU, 7491 Trondheim, Norway 2 Correspondence should be addressed to Tommaso Cella; Received 21 February 2014; Revised 7 May 2014; Accepted 2 June 2014; Published 24 June 2014 Academic Editor: Joonhyuk Kang Copyright © 2014 Tommaso Cella 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. In this paper we evaluate the feasibility of short range outdoor mm-wave MIMO links in the 70 GHz portion of the E-band (71– 76 GHz). We use phased arrays in order to strongly reduce the impact of the multipath components, thus making the channel mainly line-of-sight (LOS). We design the array using a simple patch as a single element and simulate the performances for a 200 m link and a MIMO system with equal element spacing at the transmitter and the receiver. Each node of the MIMO system consists of a uniform rectangular array (URA) where the single element is a patch antenna, in order to achieve higher gains and narrow beams. Such configuration is much more compact compared to the antennas currently employed for the same bandwidth. We optimize the interelement distances at the transmitter and the receiver and evaluate the capacity achievable with different array sizes. The results show that, for the proposed link budget, capacity up to 29 bit/s/Hz is achievable at a range of 200 m, with practical dimensions. We also show that the beamforming capabilities make the design much more flexible than the single reflector antenna systems. In the last part of the paper, we verify that our antenna can also operate in rainy conditions and longer ranges. 1. Introduction Research on mm-wave communications has been significantly increased during the last years. Even though mmwave extends from 30 to 300 GHz, there are three bands to which we usually refer, which are the V-band (57–66 GHz), the E-band (71–76 GHz and 81–86 GHz), and the W-band (92–95 GHz). The main difference between mm-wave communications compared to lower frequency bands is that they experience a much higher attenuation. This is due to the increased free space propagation loss and the atmospheric absorption. However, while the E-band and the W-band exhibit a reasonable atmospheric absorption, the V-band suffers significant attenuation due to the oxygen absorption [1]. Due to that, the V-band is primarily investigated for indoor communications, while most of the commercially available transceivers intended for outdoor applications are designed in the E-band. Several studies have demonstrated that communications at those bands are mainly LOS [2]. Some possible applications include 4 G/LTE backhaul, disaster recovery, remote storage access, and fiber extension. The commonly used antenna elements are parabolic reflectors, with very high antenna gains, up to 50 dBi. The antenna diameters are in the order of 30–60 cm with very narrow beams, down to less than 1∘ half power beamwidth (HPBW) [3]. The dimensions vary depending on the range and the link budget. Such antennas are very sensitive to positioning, and installing them is not trivial. In addition, the link is fixed and limited to one transmitter and one receiver. One of the main reasons for the increased research in the mm-wave bands is the demand for higher capacity. An attractive method to improve the system capacity is the use of MIMO [4] taking advantage of multiplexing gain as multiple information streams are sent from different transmit antennas towards different receive antennas at the same frequency. In order to get spatial multiplexing at lower frequencies, rich multipath is needed. The 2 main advantage of using MIMO at mm-wave bands is that by having a proper inter-element spacing in both transmitting and receiving antennas, multiple independent streams, and thus high capacity, can be obtained, even in LOS [5]. Different LOS MIMO systems have been proposed and tested indoor [6] and outdoor [7, 8]. Another unique characteristic of mmwave band communications is that highly directional beams can be obtained using relatively small antennas. In this paper, we propose a feasibility study of a LOS MIMO radio link aided by high gain antennas implemented using URA. In our design, the transmit and receive antennas consist of multiple uniform rectangular array (URA) [9] with proper spacing to achieve LOS MIMO. The URA provide additional gain through narrow beams. The single antenna element in the URA is a patch. Such antennas are small at those frequencies and cheaper compared to the reflector antennas. In addition, due to the planar shape of the antennas, their placement could be easier. Such combinations of advanced MIMO schemes and practical designs have not been presented by other authors for outdoor scenarios, to our knowledge. Millimeter wave antenna systems have been recently tested indoor [10] and outdoor [11]. We show the possibility of using patch antenna arrays for short range outdoor MIMO links, which provide gain comparable to the commonly used parabolic reflectors, guaranteeing enough received power. In addition, we limit the contribution of the multipath components by using a sufficient number of antenna elements. This reduces the array HPBW, thus obtaining characteristics comparable to a LOS channel. We will show later in this paper that such a configuration is much smaller than using reflector antennas. The antenna would be possible to produce with much lower cost and could be easily located due to the planar shape of such antennas. In addition to the benefits of using MIMO, the system we propose can take advantages of the use of beamforming. The beamforming capabilities of each URA could provide more flexible links, because each array could focus the beam towards receivers placed in different locations, avoiding the time consuming and costly positioning of very narrow beam antennas [12]. Such a system could be used for communications in enterprises, for disaster recovery or fast deployed fiber replacement. We focus our study on the 70 GHz portion of the E-band (71–76 GHz). In the results section, we will first show the performances achievable for a range up to 200 m. The geometry of the proposed system is shown in Figure 1. We design the array and simulate the mm-wave channel with ray tracing. We then show that high capacity could be obtained for a range longer than 200 m, if particular attention is taken during the array design, even during rainy conditions. The novelty of the proposed system li (...truncated)