Enhancing cellular network capacity with adaptive antennas

Unlike conventional cellular antennas, which broadcast energy over the entire cell, adaptive antennas are antenna arrays that confine the broadcast energy to a narrow beam (Figure 1). The advantages of directing the broadcast energy into a narrow beam are increased signal gain, greater range of the signal path, reduced multipath reflection, improved spectral efficiency, and increased network capacity. There are also some disadvantages, the main one being the need to continuously track the angular position of mobile terminals in the cell. In a conventional cellular network, a single base-station antenna defines the cell parameters and is the focus of all radiated communication. This includes the transmission and reception of revenuegenerating data and voice traffic, as well as the broadcasting of system-related information that is necessary for operating the network—information that must be received continuously and simultaneously by every mobile terminal operating within the cell. System-related information includes cell identity, the frequencies in use within the cell, frequency-hopping sequences, maximum power levels, and so on. An adaptive antenna design that increases system capacity requires that the conventional base-station antenna be replaced by one or more adaptive antenna arrays. Instead of flooding the cell with radiated information from a single source, adaptive antennas fill the cell with several narrow signal beams (typically four or eight). An immediate consequence of this new approach is that a different downlink strategy must be applied; that is, more complex information must be used for transmission from the base station to the mobile terminals in the cell. This is because the system needs to know • which beam direction reaches which mobile terminals; and • how it can get system information to every mobile terminal simultaneously. Two main downlink strategies meet these requirements. The first calls for a beam to be directed toward the mobile terminal; the second requires a beam to be selected from a set of fixed beams (Figure 2). In either case, the downlink beam relies on estimating the direction of arrival (DOA) of the uplink from the mobile terminal. The algorithms that determine the most suitable beam or beam path for downlink are thus vital elements of the adaptive antenna solution. Several different antenna architectures (with different levels of complexity) can be used for directing radiated energy from an antenna into a narrow beam. For example, the phase front on the antenna elements that correspond to a beam can be generated at baseband using digital beam forming or it can be generated at radio frequency using a passive network or phase shifters. A main advantage of using a beam from a passive network is that it does not require phase coherency between the radio transmitter and the beam former. While numerous system architectures exist for adaptive antennas—including separate antenna systems for uplink and downlink—Ericsson favors three approaches, each of which appears to offer the appropriate trade-off between system-level performance and the complexity and cost of implementation: • multibeam or switched-beam architecture with a passive beam-forming network;