530 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 57, NO. 2, FEBRUARY 2009 On the Applicability of MIMO Principle to 10-66GHz BFWA Networks: Capacity Enhancement through Spatial Multiplexing and Interference Reduction through Selection Diversity Konstantinos P. Liolis, Student Member, IEEE, Athanasios D. Panagopoulos, Member, IEEE, Panayotis G. Cottis, and Bhaskar D. Rao, Fellow, IEEE Abstract—This paper investigates the applicability of multiple- input-multiple-output (MIMO) technology to broadband fixed wireless access (BFWA) systems operating in the 10-66GHz frequency range. In order to employ the MIMO principle at these frequencies, the spatial channel benefits that may arise from the rainfall spatial inhomogeneity are more relevant since multipath is insignificant. Therefore, a special MIMO/BFWA channel may be implemented if every subscriber is equipped with multiple antennas and communicates with multiple base stations. The exact relationship between conventional MIMO and the proposed 10-66GHz MIMO/BFWA channels is established. Then, emphasis is put on two different topics from the field of MIMO applications: (i) capacity enhancement for spatial multi- plexed MIMO/BFWA systems; and (ii) interference reduction for MIMO/BFWA diversity systems employing receive antenna selec- tion. More specifically, in the first case, a communication-oriented single-user capacity analysis of a 2 × 2 MIMO/BFWA spatial multiplexing system is presented, the relevant optimal power allocation policy is explored and useful analytical expressions are derived for the outage capacity achieved in the asymptotically low and high SNR regions. The effect of feedback on the capacity is investigated and quantified through Monte Carlo simulations. In the second case, a 2 × 2 MIMO/BFWA diversity system with receive selection combining is considered and its efficiency to mitigate intrasystem/intersystem cochannel interference over the downstream channel is studied from a propagation point of view. A general analytical prediction model for the interference reduction obtained by such a 2×2 MIMO/BFWA diversity system is presented along with a numerical validation. Index Terms—Broadband fixed wireless access (BFWA), cochannel interference, multiple-input-multiple-output (MIMO), outage capacity, rain fading, selection diversity, spatial multiplex- ing, WiMAX. Paper approved by A. Lozano, the Editor for Wireless Network Access and Performance of the IEEE Communications Society. Manuscript received August 22, 2006; revised February 16, 2007. 50% of this work was supported by UC Discovery grant nos. Cor02-10109 and Com04-10176. K. P. Liolis was with the Digital Signal Processing Laboratory, Department of Electrical and Computer Engineering, University of California, San Diego (UCSD), La Jolla CA 92093-0407 USA. He is now with the Wireless & Satel- lite Communications Group, School of Electrical and Computer Engineering, National Technical University of Athens (NTUA), 9 Iroon Polytechniou Street, Zografou, Athens 15780, Greece (e-mail: kliolis@mail.ntua.gr). A. D. Panagopoulos and P. G. Cottis are with the Wireless & Satellite Communications Group, School of Electrical and Computer Engineering, National Technical University of Athens (NTUA), 9 Iroon Polytechniou Street, Zografou, Athens 15780, Greece (e-mail: thpanag@cc.ece.ntua.gr). B. D. Rao is with the Digital Signal Processing Laboratory, Department of Electrical and Computer Engineering, University of California, San Diego (UCSD), La Jolla CA 92093-0407 USA. Digital Object Identifier 10.1109/TCOMM.2009.02.060474 I. I NTRODUCTION B ROADBAND fixed wireless access (BFWA) is employed for the transmission of a plethora of high data rate multimedia and IP services to stationary users over distances of several km. It is a competitive alternative to relevant wireline technologies, such as digital subscriber line (xDSL) and cable, and a promising, cost-effective solution to the so called “last-mile” problem [1]. Recently, BFWA systems have evolved and matured to the degree of being standardized by the IEEE 802.16 Working Group in the U.S. as well as by the ETSI HiperMAN Committee in Europe. On top of that, the WiMAX forum was established to promote and certify interoperable products based on the standards addressing the 2-11GHz [2] and 10-66GHz [3] frequency ranges. For the past decade or so, multiple-input-multiple-output (MIMO) wireless communication systems have received much attention due to their promise of significantly higher data rates compared to their single antenna counterparts at no cost of extra transmit power and frequency spectrum [4], [5]. So far, research on MIMO technology and, specifically, on its appli- cability to BFWA systems has been mostly concerned with the sub-11GHz band [1], [2] where the propagation conditions are such that channel spatial multiplexing/selectivity is feasible. Propagation phenomena in the 10-66GHz frequency range are quite different than those encountered in the 2-11GHz range. At frequencies above 10GHz, line-of-sight (LOS) be- tween the base station (BS) and subscriber station (SS) is deemed a practical necessity and higher antenna directivities are exploited at the SS [6]. Multipath is insignificant, while attenuation from atmospheric precipitation is more important. Rainfall is the dominant fading mechanism and exhibits sig- nificant spatial inhomogeneity within the distances of interest. Although multipath is negligible, this paper investigates the applicability of MIMO technology to 10-66GHz BFWA systems and proposes two different system architectures, which promise significant performance gains over the rele- vant single-input-single-output (SISO) cases. A key feature of conventional MIMO systems operating below 10GHz is their ability to turn multipath, normally a pitfall of wireless transmission, into a benefit for the user. Multipath makes the channel spatially selective [5]. At frequencies above 10GHz, the required channel separability might arise from the spatial 0090-6778/09$25.00 c  2009 IEEE