Cobham Antenna Systems develops antenna for ultra fast MIMO
The MBA6-3.5DS45/2045 Multi-beam Hub Base Station Antenna has been developed by Cobham Antenna Systems, Microwave Antennas to enable multiple input multiple output (MIMO) radio system operators to meet their goal of 1 Gbps/km2 anywhere within a cell.
Designed for high speed MIMO 4G (and beyond) urban access for backhaul, this was originally a European Union funded project that has become a finished and available multi-beam antenna with additional military and security beneficial applications.
Projected deployment for this Hub Base Station antenna system is for it to communicate with a network of ‘below-rooftop’ Access Base Stations using existing structures. The system strategy with these access base stations will allow self-backhauling. This multi-beam antenna has 90˚ coverage (a 90˚arc) utilising six individual narrow beam patterns each with a half power beamwidth of 15˚. MIMO capability is provided with +/-45˚ polarisation in each of the six 15˚ sectors.
The advantage of this format is that a narrow beam can create high density coverage within a specific narrow area allowing for a high level of frequency re-use whilst being far more efficient that installing six separate narrow beam directional antennas. By positioning four multi-beam antennas in a square formation, complete 360˚ coverage is achieved as this provides 24 dual-polarised beams. The current system benefits from meeting ETSI specification for multi-beam antennas (EN 302 326-3 V.1.1.2) provides low side-lobe levels for greater system efficiency.
The antenna measures 586 mm high by 456 mm wide by 76 mm deep and it provides 2 x 6 beams each with 17 dBi peak gain. The antenna element comprises a single cross-dipole assembly operating at a centre frequency of 3.5 GHz interlocked in a configuration which provides a slant dual polar beam. Eight sets of these assemblies are fed in phase through a stripline feed to create the single 110˚ sector antenna which forms the basis for the array. There are 8 sector antennas in the complete assembly.
In order for the 6 (x2 polarisations) narrow beams to be formed to cover the 90˚ arc, two 8x8 Butler matrix beam forming devices are used to feed the separate ports of each antenna element. By a mechanism of fixed phase shifters and couplers the Butler matrices provide defined sets of phases into each of the 8 internal antenna elements which results in two sets of 6 skewed beams (The two outer ones covering +/-60º are not currently used).
Within the new unit the Butler matrix devices are fully integrated into the antenna eliminating the need for 16 phased-matched cables making for an efficient and cost-effective antenna. In this antenna the down-tilt is a nominal 2˚across the band of interest, i.e. 3.4 – 3.6 GHz with an elevation beamwidth of 10˚ to the half-power point.
For security and military applications the technology can be scaled to cover other frequency bands, with applications such as communications to multiple mobile platforms (ground or airborne).