Page 52 - 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

ago [4]. As millimeter-wave systems are reaching fundamental limits on efficiency for device
commercial adoption based on RF beam-formers, technologies above 100 GHz and highlights the
interest has pivoted into the further capabilities of transistor improvements. Section 4 discusses
digital beam-formers in conventional 5G bands for advanced III-V transistor nodes and the possibilities
mMIMO and preliminary studies have begun to in these nodes. Section 5 discusses how attempts to
demonstrate multi-beam test beds [4]. However, seek optimal transistor embedding networks to
the feasibility of mMIMO systems above 100 GHz improve gain struggle to improve efficiency, and
requires evaluation of the benefit from both signal Section 6 presents recent work that has
processing and the short wavelength of the signals, demonstrated how one particular process, InP HBT,
particularly for systems where area is a limitation. has the potential to significantly increase efficiency
in the 100-300 GHz bands.

2. DIGITAL BEAM-FORMING

In the UmmW bands, the BS and UE for a
communication link would have different
requirements for the PA. To produce multiple
beams for different users, the digital beam-former
in the BS is illustrated in Fig. 1. As opposed to an RF
beam-former, each PA is supported by an individual
pair of DACs and RF Upconverter (RFU) unit. The
architecture translates into higher Peak to-Average
Fig. 1 - Architectural partition for large-scale integration of Power Ratio (PAPR) requirements in the digital
transmitters in UMMW bands where 2D arrays prohibit
direct integration of the full transceiver behind a single beam-former for the PA. By simulating the sum of
antenna element. random uncorrelated QPSK waveforms associated
with several users, the aggregate PAPR
The architecture of a UmmW digital beam-former is asymptotically approaches 12 dB. High PAPR
illustrated in Fig. 1 to move the DSP and suggests relatively high peak power and linearity
Intermediate Frequency (IF) signal generation requirements for the PA.
blocks in scaled CMOS away from the 2D transmit
(or receive) front end in an array. An RF The UmmW band poses a unique problem for the
upconverter allows the IF (or baseband) to be packaging of a digital transmit array. To occupy a 2D
shifted to the RF band where a Power Amplifier (PA) array site of only 1mm by 1mm, very compact
generates the appropriate output power level. This electronic ICs must fit into a 2-Dimensional (2D)
architecture might be substantially different than a array without a 3D packaging solution [7]. More
sub-6 GHz or 28 GHz array where the element importantly, the power consumption of a UmmW
spacing is relaxed relative to the amount of front end is constrained to prevent a substantial
integration on a single silicon chip and multiple thermal load that must be dissipated with the small
polarizations and even transmit/receive might be area with the large numbers of elements.
fit within a single array site.
The PA poses the most significant problem for the
This paper reviews the Transmitter (TX) and PA energy efficiency of a UmmW digital beam-forming
constraints for digital beam-formers that would array. While an all-silicon solution might be
support future mMIMO deployments and details the attractive to integrate the large number of elements,
output power requirements and how these can be the PA performance in silicon processes is limited in
translated into an efficient PA design using terms of output power and efficiency. Prior work
commercially available III-V and Si transistor demonstrated single digit efficiency in CMOS and
technologies today [6]. Recent work supports the SiGe processes for 15-20 dBm output power in the
possible rapid improvement in circuit design above power amplifier [8][9][10]. With single digit
100 GHz that may usher in an era of energy efficient efficiency, the power requirements across 100s of
communication and sensing electronics for the elements are tremendous, particularly given the
UmmW bands. Section 2 will review the digital dense array of power amplifiers at 140 GHz, and
beam-former architecture and highlight the lends to an insurmountable heat removal challenge.
different demands in the Base Station (BS) and the Consequently, the transmit digital beam-former
User Equipment (UE). Section 3 reviews the illustrated in Fig. 1 might include a III-V PA if the

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