Page 53 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

power and efficiency of another device technology As an example, the minimum energy of the TX array
supports the requirements. is plotted in terms of the number of elements in
Fig. 2. A total overhead power consisting of the RFU
A notional link between a BS and UE can be
mathematically analyzed to understand the digital and LO is assumed to be 100 mW. Additionally, we
beam-former power consumption. As more assume the PA gain is 20 dB. Using current state-of-
channels are added, the digital beam-former the-art numbers for the DC to RF efficiency of the
increases the RFU and DAC power linearly. modulator and the PA, 1% and 10%, respectively,
However, the PA output power decreases given a the base station (EIRP = 75 dBm) is optimized for
fixed EIRP constraint. more than 1000 elements. Moreover, the minimum
power is around 220 W. As expected from (2), the
By adding the power consumption of the RFU output power per PA is around 10 dBm based on the
(which includes DAC and IF signal generation) and overhead of 100 mW and the efficiency of 10%. On
the LO power (which includes LO distribution, the other hand, if the PA efficiency is improved to
multiplier, buffer and mixer power consumption), 40%, the number of elements reduces (by roughly a
an optimization is found based on basic circuit factor of 2) but the optimum PA output power
parameters for the PA (drain) efficiency and the increases to around 16 dBm.
overhead of the per-element RF upconversion.
On the other hand, the UE (EIRP = 45 dBm) is

η MOD EIRP minimized for 32 elements at a total power of 7 W
N OPT = √ (1)
η PA G ANT (P RFU +P LO ) and output power per element again of 10 dBm.
This is the same as the BS since the assumptions
Minimum energy is found when the PA power in (2) do not change between the BS and UE in this
consumption offsets the overhead power exercise. With improvements in the PA efficiency,
consumption for the RFU and DAC. Not only is the the number of elements decreases to 16 elements
power consumption significantly reduced, the size and the overall power is around 3 W and the
of the array can be reduced by a factor of two average power per element is again 16 dBm. The
through the smaller number of elements. When we critical insight is that improvements in efficiency
consider this optimization in terms of the number also increase the average output power demands
of elements, the optimum output power per per element.
element is
While these power consumptions for the array
P OUT,OPT = η PA (P RFU + P ) (2) might seem high, considering that the BS MIMO
LO
beam-former could support more than 100 users
η MOD
Consequently, the optimum output power is a (load factor of ¼), the power is amortized by the
fraction of the DC power consumed in the RFU and throughput to each user. If each user receives a
LO based on the PA efficiency. 10Gb/s QPSK stream, the overall energy efficiency
of each BS beam amounts to around 200 pJ/b which
is comparable to the energy efficiency of
communications at lower frequency bands. Notably,
the UE achieves similar energy efficiency.
Consequently, the PA requirements in the BS or UE
are not excessively different nor are they
particularly onerous from the standpoint of design
compared to LmmW. Most interestingly, the design
demands higher output power per element as the
efficiency improves and suggests examination of the
device technologies for the TX PA. For the purposes
of further analysis, we will consider a peak power of
20 dBm as a target per element output power to

account for the PAPR.
Fig. 2 - Optimization of the number of elements under 3. POWER AMPLIFIER CONSTRAINTS
different EIRP conditions demonstrating the output power
per element in dBm and the total array power consumption The Power Added Efficiency (PAE) and output
in dBW. (Assumes GANT = 5 dB). power are directly related to properties of the

© International Telecommunication Union, 2021 41