Page 13 - EngineerIT February 2022 UPDATED

ICT – MILLIMETRE WAVE TECHNOLOGY

Why millimetre wave requires a

different approach to DPD and how

to quantify its value

By Hossein Yektaii, Wireless System Architect; Patrick Pratt, Algorithm Design Engineer and Frank Kearney, Engineering Manager

Introduction
In this article
In the 5G New Radio standard, In addition to reduced latency and higher reliability, the exponential increase in demand
millimetre wave (mmWave) for higher data throughput has been one of the most powerful driving forces behind 3GPP
frequencies, in addition to sub-6 5G NR standard.
GHz frequencies, are utilised to While the 4G LTE systems were deployed in sub-3 GHz bands, the allocation of the
enhance throughput. The use of new spectrum in the 3 GHz to 5 GHz range in recent years has enabled wider channel
mmWave frequencies provides bandwidths (BW) in 5G NR. Compared to the 4G LTE, the maximum channel bandwidth
unique opportunities for a drastic has increased from 20 MHz to 100 MHz in sub-6 GHz frequencies. Besides wider channel
increase in data throughput while bandwidth, multiple transmit and receive antennas and, ultimately, massive MIMO
presenting new implementation technology, has further increased the spectral efficiency. While all these improvements
challenges. This article explores help to deliver higher data throughputs, the fundamental limitation—the relatively small
architectural differences between amount of allocated sub-6 GHz spectrum—continues to limit the peak throughput for
sub-6 GHz and mmWave base station individual users to less than 1 Gbps.
radios, with particular emphasis In 5G NR, for the first time in the history of the 3GPP standards, millimetre wave
on the challenges and benefits of frequencies between 24,25 GHz to 52,6 GHz are allocated for cellular mobile applications.
implementing digital predistortion This new frequency range is referred to as FR2, in contrast to the sub-6 GHz frequencies
(DPD) on these systems. While termed FR1. There are much larger swathes of spectrum available in FR2 relative to FR1. A
digital is a well established single channel in FR2 could be as large as 400 MHz, enabling unprecedented throughput.
technique commonly used in sub- However, the use of mmWave frequencies brings new implementation challenges to both
6 GHz wireless communication the base station (BS) and user equipment (UE). The most significant of these challenges
systems to improve the power are higher path losses and lower PA output powers, making the link budget between the
efficacy, most mmWave radios do base station and UE quite challenging.
not use DPD. Using a prototype 256 Path loss between BS and UE is defined as P l [dB] = 10log 10 (P t/P r), where P t and
element mmWave array, built with P r are the transmitted and received power, respectively. In free space, the received
ADI beam formers and transceivers, power is a function of distance and wavelength, also known as Friis’ formula, where P r
we are able to demonstrate that (d,λ) = P t G t G r (λ/4πd)², and G t and G r are the transmitter and receiver antenna gains,
DPD improves the effective isotropic respectively. λ is the wavelength and d is the distance between transmitter and receiver.
radiated power (EIRP) by up to 3 In a typical wireless communication environment, due to reflection off nearby objects
dB. This allows for a 30% reduction and loss through construction material, the path loss is much more complex to model and
in the number of array elements, estimate. However, to form an understanding of higher path loss at mmWave frequencies
relative to an array without DPD, for compared to sub-6 GHz, let us assume free space propagation, similar antenna gains, and
the same target EIRP. equal distances between base station and user equipment. Using this approach, the path
The purpose of this article is loss at 28 GHz compared to 900 MHz is 10xlog (28000/900)² = 29.8 dB higher!
to draw a comparison between a It is not uncommon for base station power amplifiers at sub-6 GHz frequencies to
traditional sub-6 GHz macro-cellular output tens of watts of RF power with efficiencies above 40%. This is enabled by the
and a mmWave base station radio adoption of high efficiency PA architectures such as Doherty and the use of advanced
and antenna design. It further covers digital predistortion techniques. In contrast, the highly linear class AB mmWave PAs
how these design differences impact typically output less than 1 W of RF power and have single digit efficiencies. At mmWave
DPD implementation in mmWave frequencies, these operating conditions exacerbate the link budget challenges between
arrays relative to sub-6 GHz radios. BS and UE. The solution to both challenges—the larger path loss and lower power per
PA—is the more accurate delivery of power to specific spatial locations. This is achieved

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