Page 17 - EngineerIT February 2022 UPDATED

ICT – MILLIMETRE WAVE TECHNOLOGY

output matching and bias settings of
the beam former at specific frequencies
can result in up to 13 dBm rms output
power, while maintaining EVM and ACPR
specifications. However, it is not possible
to maintain this level of performance
across a wide range of frequencies
and multiple units. Alternatively, if
the appropriate conditions are met
(the saturated power level of the PA is
maintained above 21 dBm), the use of
DPD consistently achieves an output
power of more than 14 dBm across the
band of interest.
When specifying a mmWave
array, the EIRP per beam is a core
requirement. If the power per element
is relatively small, many elements are
needed to achieve the target EIRP,
which in turn increases the cost,
power and size of the array. The more
elements deployed in the array, the
Figure 5: Far field horn antenna as DPD observation receiver. narrower the resulting beam. Narrower
beams are not always desirable; they
64 PAs in each beam (Figure 3). That up to 200 MHz bandwidth. Future ADI make pointing the beam and tracking
means each PA gets the same signal as transceivers will support at least 400 the mobile users more challenging.
the other 63 PAs, except for a relative MHz of BW with DPD. The plot in Figure 6 illustrates how the
phase shift that is applied for beam Our analysis found that mmWave number of required elements and the
steering. If a single DPD loop is wrapped array DPD can boost the beam EIRP by array DC power consumption change as
around the cluster of 64 PAs, then we more than 3 dB (in a range of 1,5 dB a function of DPD improvement from 0
only need a total of four DPD loops for to 3,2 dB) over the frequency range of dB to 3 dB, while maintaining a target
the entire AiB256 array. Essentially, the 26,5 GHz to 29,5 GHz. Optimising the EIRP of 60 dBm.
DPD loop is wrapped around each beam
as opposed to each PA. We refer to this
as an array DPD to distinguish it from
sub-6 GHz DPD, which has a dedicated
DPD loop per PA.
The observation receiver must
“look” into the boresight of the
beam, where the signals from all PAs
add in-phase, so it can correct the
distortion caused by the cumulative
far field aggregation of 64 PAs. Our
early stage evaluation used a far field
horn antenna as the DPD observation
receiver, as shown in Figure 5, and
demonstrated that a single DPD loop
could be wrapped around a beam to
improve the EVM and ACPR. Future
ADI products may include an integrated
observation path to simplify the DPD
implementation.
The DPD set-up uses ADRV9029
integrated transceivers with built-in
CFR and DPD capabilities for signals Figure 6: Required number of elements and DC power as a function of DPD improvement.

EngineerIT | February 2022 | 15