TopIntroduction
In this work we propose space-multimode (SM-SCP-ULA) and space-multimode-polarization stacked circular patch uniform linear antenna arrays (SMP-SCP-ULA) for deployment in size-constrained access points, modems and end-user terminal equipments of IEEE 802.11n MIMO-OFDM WLAN systems at 5.8 GHz. The performance gain of higher-order modal SMP-SCP-ULA in terms of spatio-modal power correlations, ergodic spectral efficiencies and compactness gains are compared with dominant-mode operating circular patch antenna arrays (CP-ULA) and center-fed dipole arrays (DP-ULA).
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 (Telatar,1999). 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 (Goldsmith, 2007; Biglieri, 2010). 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 (Harshal, 2014). However, due to the physical constraints and the concerns on ergonomics and aesthetics, the distance between antenna elements in practice cannot 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 (Foschini, 1998; Forenza, 2006), multimode diversity (Sanchez, 2008; Svantesson, 2002), and polarization diversity (Mukherjee, 2007;Waldschmidt, 2003) 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, sinuous and log-periodic antenna structures are amenable to higher-order mode generation (Yavanoglu, 2011). In this manner, the higher-order modes generated in a single antenna structure with directional radiation patterns resulting in low spatial correlation in angle space are used as diversity ports in a MIMO system within a compact space. In pattern diversity on the other hand that is slightly different than multimode diversity, orthogonal radiation patterns generated on distinct antennas that are co-located at the phase-centers are generated and used as diversity ports.