Feature: Communications design
(2) The maximum beam angle resolution θRES_MAX of this array
in one dimension when 6-bit 5.6° LSB PSs are used behind every antenna element, would be approximately ≅ 0.056° by Equation 3.
(3) Approximately 1.3ps LSB TTDs would be required to replace
5.6° LSB PSs to have a 0.056° beam angular resolution at 12GHz according to Equation 4, which is used for conversion between time and phase shift.
(4) The beamwidth value is considerably greater than the beam
angular resolution, even at very small scan angles, and placing PSs on the same line with TTDs to compensate for beam angular resolution would introduce additional beam squint and resolution degradation into the system. In practice, the reason for finer TTD resolution is to have
lower quantisation sidelobe levels (QSLL) rather than finer beam angular resolution. As the frequency goes higher, designing a TTD with a required time resolution to meet the target QSLL criteria gets rather more difficult than designing a PS with a required phase resolution; hence, PSs can be a companion of TTDs to achieve the target QSLL while still having an acceptable level of beam squint.
Cross-polarisation Another reason to implement PSs and TTDs in the same ESA could be to mitigate beam squint whilst designing a system with cross-polarisation capability. Cross-polarisation is generated by setting a 90° phase shift between the V and H feeds of antenna elements. Ensuring as close to a 90° difference between feeds over the desired cross-polarisation bandwidth is essential to have good cross-polarisation isolation for healthy operation. Due to having a constant phase over frequency, PS-based ESAs have wideband cross-polarisation capability (Figure 2), unlike TTD-based ESAs that can have 90° between feeds only at a single frequency; see Figure 3. The architecture in Figure 4 can be used to apply cross-
polarisation whilst mitigating beam squint. TTD coverage is set by the maximum delay ΔtMAX
between
the most distant elements of the whole array at the lowest operation frequency. By Equation 5, this is around 2.45ns for the example array in Figure 1.
(5)
When cross-polarisation is not required, there are a couple of things to consider when using TTDs instead of PSs behind
Figure 4: A true time delay on common leg and phase shifters behind the V and H feeds of antenna elements, to optimise beam squint whilst having wideband cross polarisation capability
Figure 1: 1024 (32 × 32) element array partitioned into 16 subarrays consisting of 8 × 8 elements
Figure 2: Non- squint-free
wideband cross- polarisation with phase shifters behind the V and H feeds of the antenna elements
Figure 3: Squint- free narrowband cross polarisation with true time delays behind the V and H feeds of the antenna elements
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