Beamforming in 3G and 4G Mobile Communications: The Switched-Beam Approach

The technology and deployment of modern mobile communications systems, should adapt to the continuous and rapid growth of wireless data traffic. Besides the increase in bandwidth, the addition of cell sites and sectors, and the enhancement of air interface capabilities, smart antennas play also a substantial role in the improvement of wireless systems’ performance. When we refer to smart antennas, we mean structures of multiple antenna elements at the transmitting and/or the receiving side of the radio link, whose signals are properly processed, in order to better exploit the mobile radio channel and enhance the communications performance. During the last decade there has been intensive research on Multiple Input-Multiple Output (MIMO) systems, which are antenna formations that involve processing at both sides of the link (Jensen & Wallace, 2004). Depending on the signal processing methods and the adaptive schemes used, smart antenna techniques can be separated into three broad categories: a) Diversity, b) Spatial Multiplexing (SM), and c) Beamforming. Roughly speaking, beamforming aims at improving Signal to Interference plus Noise Ratio (SINR), diversity aims at reducing the variations in the SINR experienced by the receiver, while SM aims at sharing SINR in high SINR scenarios (3G-Americas, 2009). Transmit and receive diversity are used in order to mitigate the problem of multipath fading, enhancing the reliability of a wireless link. In SM, which is the most popular transmission scheme of MIMO systems, multiple data streams are transmitted in parallel, increasing the data transmission rate. Beamforming uses an antenna array1 to transmit/receive energy in a specific direction, increasing the cellular capacity and coverage. Although it is an early smart antenna technology, beamforming is still supported by the latest 3GPP releases (3rd Generation Partnership Project www.3gpp.org), namely the LTE (Long Term Evolution) and LTE advanced. Operators though seem reluctant to incorporate smart beamforming techniques into base stations, mainly due to the cost and complexity of such implementations. The current research concerning 4G mobile systems is mostly concentrated on MIMO processing and space-time coding algorithms. However, antenna arrays and beamforming have still the potential to contribute in the enhancement of modern cellular systems.