Page 16 - EngineerIT February 2022 UPDATED

ICT – MILLIMETRE WAVE TECHNOLOGY

and width are 6 × (λ/2) = 2 meters and 2 the PA at average power levels 6 dB to Implementation and value of the
× (λ/2) = 0.33 meters, respectively. The 9 dB below its peak power capability. DPD in mmWave arrays
gain of the antenna for each polarisation, Operating the PA in such deep back-off In the AiB256, there are 256 transmit
assuming 5 dBi gain per dipole element, is regime results in very low efficiency, and receive chains capable of generating
approximately 10 × log(12) + 5 dBi = 15.8 often less than 10%. two or four beams with 128 or 64 PAs
dBi. If each PA puts out 40 W (46 dBm) of Efficient PA architectures such as deployed in each beam. Like sub-6 GHz
RF power, the EIRP per polarisation is 46 Doherty maintain high efficiency across systems, the EVM requirements for the
dBm + 3 dB (2 columns) + 15.8 dBi = 64.8 6 to 9 dB below their peak power, mmWave bands are 8% and 3.5% for
dBm. This level of EIRP would be expected but they are considerably less linear 64-QAM and 256-QAM modulations,
to provide good coverage over distances compared to class AB PAs. If deployed respectively. However, the ACPR
of several kilometres at 900 MHz. without any linearisation technique, requirements in mmWave are much less
Now let’s consider the 28 GHz they fail to meet the error vector stringent than sub-6 GHz; they are 28
AiB256 with 128 antenna elements per magnitude (EVM) and adjacent channel dBc for 28 GHz band and 26 dBc for 39
polarisation, arranged in eight rows power ratio (ACPR) required by the GHz band in 3GPP standard 38.104.
and 16 columns, as shown in Figure application. One of the most popular Each class AB PA in an ADMV4828
1. Assuming λ/2 distance between linearisation techniques is DPD, which is beam former can deliver 21 dBm of peak
the elements and 5 dBi gain per widely used in sub-6 GHz systems. power. Operating the PAs on ADMV4828
element, the overall antenna gain is Sub-6 GHz systems require the at approximately 12 dBm rms output
approximately calculated as 10 × log EVM to be less than 8% and 3.5% for power leaves 9 dB headroom to the
(128) + 5 dBi = 26 dBi. Compared to the 64-QAM and 256-QAM modulations, peak power and results in both the EVM
900 MHz example, the antenna gain is respectively, for 3GPP standard 38.104. 1 and ACPR requirements being achieved.
10.2 dB higher. However, this comes at To meet these EVM requirements, the At 12 dBm (16 mW) output power, each
the expense of narrower beam width. PAPR of the signal should be maintained transmit chain consumes around 300
The 3 dB beam width is only 12° in the between 6 dB to 9 dB. The ACPR should mW of power, resulting in 5% efficiency.
vertical plane and 6° in the horizontal normally be less than –45 dBc for Some of the power in the transmit
plane. Such a narrow beam is not able 3GPP standard 38.104. In the previous chain is consumed by the variable phase
to cover a typical 120° sector all at example of a 900 MHz 4Tx/4Rx radio shifters that are necessary for beam
once. The solution is to first find the with 40 W of rms output power per forming. Each receive path, including
active UEs in the cell coverage area, transmitter, if the power amplifiers the variable phase shifters, consumes
point the beam to them and track their are to be operated in linear region to around 125 mW of DC power.
movement in the cell. The 5G standards meet the EVM and ACPR requirements, Based on the above power numbers,
specify the beam acquisition and their efficiency is normally less than it is clear that the share of the PA power
tracking procedures, which is outside 10%. This means each of the four consumption in a mmWave radio relative
the scope of this article. To calculate PAs consume more than 400 W of DC to the total DC power consumption
the EIRP of this radio, let’s assume each power to put out 40 W of RF power. is much smaller compared to a sub-6
transmit path puts out 13 dBm of RF Therefore, just the four PAs alone GHz radio. This raises the question of
power. The total power per polarisation consume more than 1600 W! This has whether a mmWave radio could still
is 13 dBm + 10 × log(128) = 34 dBm. huge implications on the size, cooling, benefit from the DPD or not?
Combined with 26 dBi antenna gain, the reliability and operating expenses To answer this question, one needs
total EIRP per polarisation is 34 dBm + (OPEX) of the radio. In contrast, using to propose a suitable DPD architecture
26 dBi = 60 dBm. In a typical outdoor a Doherty PA in combination with the in mmWave. Simple extension of the
deployment scenario, this level of EIRP crest factor reduction (CFR) and DPD DPD implementation form sub-6 GHz
covers up to a few hundred meters at techniques results in PA efficiencies systems to mmWave, requires a DPD
28 GHz. greater than 40%. This means that each loop around every one of the PAs. In our
PA consumes less than 100 W of DC AiB256 example, that would mean 256
Value of the DPD in sub-6 GHz power to put out 40 W of RF power. DPD loops! Obviously, implementing 256
systems The four PAs in the radio consume DPD loops is very expensive and power
5G and 4G wireless standards are based less than 400 W of DC power. The rest hungry. Since each PA puts out a small
on OFDM signals with an inherently of the radio normally consumes less amount of power (12 dBm typical), the
high peak-to-average power ratio than 50 W of DC power. Therefore, overall system efficiency with DPD is
(PAPR). To amplify and transmit these the PA power consumption makes up most likely less than a system without
signals with high fidelity and to avoid more than 85% of the total DC power DPD.
polluting the adjacent channels, care consumed by the radio, even when Fortunately, there is an elegant
must be taken not to compress or clip Doherty amplifiers with DPD and CFR solution to this problem. AiB256 could
the signal peaks. This requires operating are deployed. put out a maximum of four beams using

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