There are several applications for phased arrays in Army communication systems. This spans frequency bands ranging from UHF to Ka bands and performance requirements that include single and multiple beams, various scanning speeds, and different polarizations. The radiating apertures can be planar, conformal, or segmented and distributed over multi-faceted structures. Examples include distributed apertures on military vehicles, conformal aperture on an unmanned aerial vehicle (UAV) aircraft body, an antenna on an elevated post or tower on a stationary or mobile platform, and others.
This arrangement has the advantage of providing higher gain and thus lower power relative to the omni-directional antenna that is often used. Mechanical scanning has been used for Army vehicle Satellite-on-the-Move (SOTM) and Satellite-on-the-Pause (SOTP) systems with limited capabilities in scanning speeds and single-beam operations.
The beam former is an essential part of the phased array design. The complexity of beam forming is a function of the number of elements in the array, scanning steps, scanning range, and the number of beams in a multibeam system. Typically, two different beam forming types are used in these systems: RF and digital. Both have different limitations that depend on the mechanism of forming the beams. Multibeam RF beam formers can be implemented using phase-shifter-populated beam forming matrices. The smallest phase shift determines the scanning steps for RF beam formers, and the number of required phase shifters depends on the number of array elements and the number of simultaneous beams. This results in a high-loss, high-cost beam former design.
The loss and cost increase as the number of scanning steps increase for RF beam formers. Digital beam formers employ A/D converters followed by a processor, where the number of the scanning steps is not a major factor in the beam former complexity. However, digital beam formers typically require as many A/D converters as the number of array elements. The challenge is to design a low-loss beam former, which uses a small number of components, yet does not sacrifice performance.
The Distributed Beam Forming Concept
A low-cost, low-loss, and lightweight beam former for a distributed or conformal aperture antenna is presented here. The array is divided into modular subarrays, and its beam former is distributed in two stages. The first stage is at the subarray level with a small number of pre-set scanning positions that are realizable using a Butler matrix, printed Rotman lens or other switched time delay system. This stage ends with the multiplexed multibeam signals fed into a single A/D converter per sub-array. The second stage is a central digital beam former that operates on the combined outputs of the sub-arrays.
Whether the array is of planar or conformal aperture, it will be replaced by a distributed aperture configuration with a base-band digital network that is used to combine signals from the distributed apertures. The concept is shown in Figure 1 for receive mode of operation. The signal received by each sub-array is downconverted, A/D converted, and combined with other sub-arrays’ signals via a base-band cable. A digital signal processor will perform phase compensation when combining the signals from the different subarrays. This will allow for the correction for the path loss and LNA differences for each subarray.
Figure 1 shows the two beam forming stages. In the first stage, the outputs of the radiating elements of the subarray are fed to a LNA and are demultiplexed to the individual channels of the different beams. Each subarray module uses a multibeam beam forming matrix or a single-beam beam forming network that can be designed with pre-set scanning directions. The beam former for this stage can be realized using a printed Butler matrix, a printed Rotman lens, or other switched time delay system. The absence of variable phase shifters or variable delay lines with associated high-loss switches reduces the losses incurred by the sub-array beam former. The outputs of the different beams are multiplexed after the first stage to allow for using a single downconverter and a single A/D converter per sub-array.
In the second stage, the A/D converter outputs are combined and then digitally demultiplexed. A single A/D per sub-array versus one per array element presents a significant saving, which results in reduced power consumption, number of components, and cost. In the transmit mode of operation, the reverse process is employed, where a common processor will transmit the phased signals to the sub-arrays. The signals received by the sub-arrays will go through the pre-set beam forming and are retransmitted such that the signals are combined in space to form the desired beam.
The pre-set scanning directions of the sub-arrays depend on the orientation of the sub-array in the conformal structure. At the digital beam former stage, the phases imposed per sub-array are determined based on an optimization process that takes into account the scanning directions of the sub-arrays, which are in turn selected based on the required scanning steps and overall scanning range of the whole array.
Stages of a Distributed Beam Former
The first stage in the distributed beam former uses an RF beam former that has pre-set scanning directions. In a conformal array or a large planar array that is divided into sub-arrays, the pre-set scanning directions may be different for different sub-arrays. In addition, each beam former can support multiple beams. A switch matrix controls the pre-set scanning directions for the different beams according to the sub-array location in the whole array and according to the desired beam scanning direction of the whole array. Two of the possible realizations of the first stage beam former are the Butler matrix and the Rotman lens (see Figure 2).
Butler matrix is a passive device for the realization of pre-set phase front with equal phase differences among the output ports. This is achieved through a multistage circuit configuration that uses a symmetric arrangement of hybrids and phase shifters. The phase shifters are usually realized as passive delay lines. For multiple beams or for a switchable phase fronts for different beam direction, a switch or a switch matrix is inserted at the beam ports of the matrix. While the numbers of input and output ports of the matrix are equal, the number of usable beam ports may be far less. The binary and equal numbers of the matrix input and output ports may pose a limitation on using it, and the number of hybrids may be excessive if the number of ports is large.
The realization of the pre-set beam former using Rotman lens alleviates some of the inherent problems associated with Butler matrix. The lens also has the advantage of operating over wide bandwidths. However, its design and optimization can be more involved. The numbers of input and output ports are subject to geometric rather than configuration restrictions. The lens elements on the beam side correspond to scanning directions and each element illuminates the array side with a certain taper to produce the desired phase front. This puts restrictions on the element sizes that may restrict the number of preset scanning directions. As is in the Butler matrix option, a switch matrix controls the location of the preset beams for the sub-array to satisfy the scanning requirements of the whole array.
The second stage of the distributed beam former is a digital beam former that operates on all the beams simultaneously. To reduce the number of A/D converters, the beam outputs that use different channels are multiplexed in the frequency domain and the aggregate sum is downconverted and converted to a digital signal using a single A/D converter per sub-array. Digital cables connect the A/D outputs at the sub-arrays to the central digital processor for the whole array. The multi-channel input is demultiplexed and digital phase shifting is applied to individual channels to produce the right phase front for each channel. The digital phase shifters can operate with high resolutions and can compensate for errors that accumulate in the signal paths. The outputs of the individual digital beam formers are multiplexed in the time domain and then sent to the common receiver, or sent as individual channels to separate receivers.
The concept for a distributed beam former lends itself to conformal or multi-faceted arrays. The concept has the following features:
- Two-stage beam forming that will produce scanning directions at small increments over a wide scanning range.
- Low-loss beam forming, which results from using pre-set fixed scanning directions in the first stage and digital cabling over the longest parts of the signal distribution.
- Low-cost beam forming, which results from using fewer A/D converters and modular sub-array designs.
- Conformal or multi-faceted array structure that can be stationary or mounted on vehicles.
Butler matrix and Rotman lens were discussed, but other low-loss passive implementations may also be used. The optimization of the beam forming at the two stages is essential for the overall array performance. This includes the pre-set scanning directions in the first stage, which may have significant effects on the RF losses and on the cost of that modular beam former. The concept was presented for a one-dimensional linear or piecewise linear array and one-dimensional scanning. It can be expanded to planar or piece-wise planar arrays and two-dimensional scanning.
This article was written by Amir I. Zaghloul of Virginia Tech’s Bradley Department of Electrical and Computer Engineering (ECE), Blacksburg, VA, and Ozlem Kilic of The Catholic University of America, Washington, DC. For more information, visit Virginia Tech’s ECE at www.ece.vt.edu/.