RADIOENGINEERING, VOL. XX, NO. X, XX 2013 1 Spectral Efficiency Improvement in IEEE 802.11n MIMO-OFDM WLAN Communication Systems using Compact Space-Multimode Diversity Stacked Circular Microstrip Patch Antenna Arrays Asuman Yavanoğlu 1 , Özgür Ertuğ 1 ,Erdem Yazgan 2 1 Gazi University, Electrical and Electronics Engineering Department, Telecommunications and Signal Processing Laboratory (TESLAB), Ankara, TURKEY 2 Hacettepe University, Electrical and Electronics Engineering Department, Ankara, TURKEY asavascihabes@gazi.edu.tr , ertug@gazi.edu.tr , yazgan@hacettepe.edu.tr Abstract. The support of high spectral efficiency MIMO spatial-multiplexing communication in OFDM-WLAN systems conforming to IEEE 802.11n standard requires the design and use of compact antennas and arrays with low correlation ports. For this purpose, compact space- multimode diversity provisioning stacked circular multimode microstrip patch antenna arrays (SCP-ULA) are proposed in this paper and their performance in terms of spatial and modal correlations, ergodic spectral efficiencies as well as compactness with respect to antenna arrays formed of vertically-oriented center-fed dipole elements (DP-ULA) and dominant-mode operating circular microstrip patch antennas (CP-ULA) are presented. The lower spatial and modal correlations and the consequent higher spectral efficiency of SCP- ULA with ML detection over statistically-clustered Kroenecker-based correlated Rayleigh fading channels with respect to DP-ULA and CP-ULA at significantly lower antenna and array sizes represents SCP-ULA as a promising solution for deployment in terminals, modems and access points of next-generation high-speed 802.11n MIMO-OFDM WLAN systems. Keywords IEEE 802.11n MIMO-OFDM WLAN, spectral efficiency, correlation, multimode antenna, spatial multiplexing, Kroenecker channel model. 1. Introduction In the concurrent and next-generation communication systems, the spectral efficiency and transmission quality can be vastly enhanced by multiple-input multiple-output (MIMO) communication techniques [1]. In communication systems employing MIMO spatial-multiplexing, higher data rates can be achieved when there are a large number of scatterers between the transmit and receive antennas i.e. rich-scattering environment. However, the spatial correlation between transmit and receive antenna ports that is dependent on antenna-specific parameters such as the radiation patterns, the distance between the antenna elements as well as the channel characteristics such as unfavourable spatial distribution of scatterers and angular spread severely degrades the capacity and quality achievable by MIMO spatial-multiplexing systems. The space consumption of MIMO antennas is especially vital in applications such as access points, modems and end-user terminal equipments (laptops, PDAs etc.) of WLAN and WIMAX systems. When regularly spaced antenna elements are used in MIMO systems, the correlation between the antenna elements in a space diversity system and hence the channel capacity and transmission quality are dependent on the distance between antenna array elements, the number of antenna elements and the array geometry. However, due to the physical constraints and the concerns on ergonomics and aesthetics, the distance between antenna elements in practice can not be extended beyond a certain level which limits the use of space-only diversity MIMO spatial-multiplexing systems to achieve the desired spectral efficiencies and transmission qualities. As an alternative solution to achieve compactness in MIMO systems, the use of pattern diversity [2,3], multimode diversity [4,5], and polarization diversity [6,7] techniques in conjunction with space diversity are proposed in the literature. Besides polarization diversity that is well-known, multimode and pattern diversity techniques that are less addressed in antenna engineering community are achieved by using higher-order mode generation in antenna structures and in general microstrip, biconical, helical, spiral, sinous and log-periodic antenna structures are amenable to higher-order mode generation. In this manner, the higher-order modes generated in a single antenna structure with directional radiation patterns resulting in low