Quadrature Cosine-Basis Beamforming in Wireless Networks using Dual-Polarized Circular Multimode Antenna Arrays ¨ Ozg¨ ur Ertu˘ g Gazi University Electrical and Electronics Engineering Department Telecommunications and Signal Processing Laboratory 06570, Ankara, TURKEY Abstract—Co-channel interference in wireless networks has a spatial nature due to geographically distributed users in the coverage area and has to be suppressed using beamfoming techniques to achieve higher spectral and power efficiencies. Beamforming with conventional center-fed dipole arrays has several drawbacks such as low array gain, low spatial selectivity due to wide main beamwidths and possibility of occurence for grating lobes due to spatial aliasing. Motivated with these drawbacks of DPA-BMF, a conventional beamforming method using dual-polarized circular multimode microstrip antennas, denoted as quadrature cosine-basis beam- forming (QCB-BMF), is proposed in this work that achieves much higher beamforming performance than DPA-BMF at comparable compactness and improved ergonomy with respect to DPA-BMF as well as higher spectral and power efficiency when used in interference-limited wireless communication networks. I. I NTRODUCTION Spatial digital beamforming; i.e. spatial filtering, with an- tenna arrays is a technology mainly used to suppress the spatial interference seen by a desired user at the front-end of a digital receiver to improve received signal-to-interference ratio (SINR) of the desired user and consequently the achievable data rate and link reliability. Classically, spatial beamforming is employed using uniform linear arrays of center-fed dipole antennas (DPA-BMF) which has several drawbacks. First of all, dipole antennas are electrically-large antennas that are quite hard to fit into space-limited modems and terminal/access points used in wireless networks. Secondly, the beamforming performance using dipole antennas is quite low; since the array gain that determines the output SINR is low, spatial selectivity is low due to large 3-dB beamwidths and the first sidelobes are high with respect to the main lobe. Motivated by the aforementioned drawbacks of conventional phased-arrray beamforming and direction-finding with center- fed dipole arrays which we denote as DPA-BMF throughout the sequel, we propose in this paper a beamforming technique using dual-polarized circular multimode microstrip antenna arrays, that is termed as quadrature cosine-basis beamforming (QCB-BMF), that achieves much higher beamforming perfor- mance than conventional dipole-array beamformers in terms of array gain and spatial selectivity and consequently that offers higher spectral and power efficiency when used in co-channel interference-limited wireless communication systems. The paper is organized as follows. In Section II, we present the physical structure of dual-polarized circular multi- mode microstrip antenna arrays used for quadrature cosine- basis beamforming. In Section III, quadrature cosine-basis beamforming methodology is presented using dual-polarized circular multimode antenna arrays. In Section IV, numerical results and discussions in terms of comparative array responses and beamforming suppression factors between DPA-BMF and QCB-BMF are provided. Section V. finally concludes the paper. II. DUAL-POLARIZED CIRCULAR MULTIMODE MICROSTRIP ANTENNA ARRAYS The antenna array proposed for use in beamforming in this work is a dual-polarized circular multimode microstrip antenna array, an element of which is presented in Fig. 1. The circular microstrip antenna as usual is a multi-layered structure with a dielectric layer between the radiating conductor and the ground plane. Generation of higher-order modes in circular microstrip patch antenna structures is well presented in the literature first by Vaughan in [6]. The radius of a microstrip antenna excited at TM 0m mode is given by [6]: r m = χ m λ c 2π  ǫ (m) r (1) where ǫ r is the dielectric constant used in the patch antenna and χ m is the first zero of the derivative of the mth order Bessel function of first kind; i.e. χ m = J ′ m (x)| x=0 , as tabulated in Table I. Based on the radius formula and the fundamental formula λ c f m = c, where c =3 × 10 8 m/s is the speed of light in free space, the resonance frequency of each mode that might be generated in a circular microstrip antenna can be derived as: f (m) r = χ m c 2πr m  ǫ (m) r (2) 2011 17th Asia-Pacific Conference on Communications (APCC) 2nd – 5th October 2011 | Sutera Harbour Resort, Kota Kinabalu, Sabah, Malaysia 978-1-4577-0390-4/11/$26.00 ©2011 IEEE 588