Multi-beam Antenna Arrays for Base Stations in Cellular Communication Systems

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Multi-beam antennas will play an increasingly important role in future communications systems such as the fifth generation (5G) and sixth generation cellular systems. There will also be a shift to higher frequencies where more spectrum is available to accommodate wide-band high data rate systems. These factors will cause shifts in the technologies used in implementing advanced communications systems. Multi-beam antenna implemented with conventional network techniques are approaching physical limitations due to increasing losses and complexity. Quasi-optical techniques for implementing multi-beam antenna such as Luneburg lenses should be an attractive alternative. In this thesis, some of the advantages and limitations of these opposing technologies using as examples firstly, a multi-beam antenna implemented using a Butler matrix feed network which is approaching high frequency limitations of loss and complexity and secondly, a multi-beam antenna based on a spherical Luneburg lens implemented with an artificial dielectric material which is approaching low frequency limitations of excessive size, weight and anisotropy is explored. In the case of Butler matrix networks, control of beamwidth and sidelobes is studied as are implementation details such as network crossovers. In the case of Luneburg lenses, the effects of an economical layered construction and the characteristics of the artificial dielectric are examined in detail. Two prototype Butler matrix fed arrays and two prototype spherical Luneburg lenses were designed, manufactured and tested. The results of these tests are described in the text. An important qualitative difference between these two technologies is that to implement a two-dimensional (2D) beam space with beamforming networks requires a 2D array of networks. To provide dual polarization as is almost universally required in communications systems, all the hardware must be duplicated. The spherical lens solution naturally provides these capabilities with no duplication. In addition, the ohmic losses can be very low. These capabilities are explored in detail and simple construction methods for the artificial dielectric are presented that avoid the anisotropy that can mar such designs. In current advanced 5G antennas with 2D arrays using multiple-input and multiple-output (MIMO) where multipath propagation is exploited to enhance capacity are being used in preference to multi-beam antennas. Such systems require a separate radio for each array element. The high power consumption is seen as a disadvantage of such systems as is the high cost. The move to higher frequencies where propagation characteristics make MIMO less advantageous will likely lead to a trend to wider use of multi-beam antennas.
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