Abstract
Small cell networks (SCN) have emerged as a viable solution for improving the spectral efficiency in order to satisfy the growing demand for high data rate mobile network. SCNs consist of multiple short range base station (BS) to cover small areas. The BS is typically known as femtocell BS. Polarisation mismatch loss between the BS and mobile station (MS), and inter-cell interference between BSs can be the performance limiting factors for SCN deployment in non-cluttered open space. This work covers the antenna design for the femtocell BS and channel characterisations within a SCN environment. Two designs for quadrifilar helix antenna (QHA) gain improvement using parasitic loop have been proposed. The designs are based on parasitic meandered loop (PML) and parasitic quadrifilar helix loop (PQHL). These parasitic loops are able to improve the boresight gain by up to 1.8 dB. Another design that is evaluated in this work is the switched parasitic QHA (SPQHA). By using parasitic elements at the side of the QHA, it gives a low complexity beam steering capability with up to 35° beam tilt. This feature is useful in cooperative SCNs to improve coverage and minimise interference. The performance of BS antennas with different polarisations against mobile station (MS) under random human handling in a real environment has been evaluated. Results show that polarisation mismatch between the BS and MS can be severe due to lack of signal depolarisation in short range communication. Results also show that a circular polarised BS antenna can be a good compromise to minimise polarisation mismatch loss in a SCN environment. A second field measurement has been conducted to evaluate the performance of the SPQHA in a real environment. Results have shown that SPQHAs are able to provide a high diversity gain. With local parasitic switching on one BS, 8 dB diversity gain can be achieved. With global parasitic switching on two BSs, 13 dB diversity gain is obtained. Furthermore, MIMO antenna selection using SPQHAs has also been shown to be able to match the performance of a 8-elements QHA-based MIMO setup. As a result, MIMO SPQHA can reduce the number of RF-chains required as compared to a full MIMO setup.