IEEE Solid-States Circuits Magazine - Fall 2021 - 45

techniques for 5G New Radio (NR), focusing
on mm-wave frequency bands
for handset applications. It begins with
frequency generation requirements
and an LO topology comparison, followed
by a phase-locked loop (PLL) architecture
discussion and circuit design
techniques for key building blocks, including
phase detectors (PDs), oscillators,
and frequency multipliers.
LO Generation Topology for 5G NR
5G NR is a radio access technology
(RAT) developed by the 3GPP for the
fifth-generation mobile network. The
3GPP specification 38 series provides
the technical details behind NR, which
is the RAT beyond LTE [2]. The first
3GPP specification was available in
2017, and the latest release, 16, was
completed in June 2020. Figure 2
shows the spectrum allocation of 5G
NR worldwide [3]. The allotment consists
of two parts, i.e., frequency range
1 (FR1), which is below 7 GHz, and
frequency range 2 (FR2), at mm-wave
frequency. In FR1, both frequency
duplexing division and time duplexing
division (TDD) are used, which
requires separate PLLs for the receiver
(Rx) and transmitter (Tx). On the contrary,
FR2 uses only TDD, in which the
uplink and downlink harness the same
spectrum frequencies but at different
times. Therefore, the LO generation
circuit for the Tx and Rx can be shared
to minimize the number of PLLs. The
PLL locking time is also relaxed (e.g.,
~100 μs), as it doesn't need to relock
during Tx and Rx switching.
Currently, FR2 consists of multiple
bands across 24.25-29.5 GHz and 37-
43.5 GHz worldwide, as depicted in
Figure 2. The first commercial launch
of 5G phones was in 2019, supporting
the sub-6-GHz band, followed by
28- and 39-GHz dual-band support in
2020. Since there are more and more
new bands available in different countries
for FR2, future 5G phones need
to support more bands across a wider
spectrum at mm-wave frequency,
which imposes new design challenges
on LO generation. The choices for the
LO topology and frequency plan are
crucial to mitigate these difficulties.
For simplicity, let us assume a homodyne
transceiver architecture [see
Figure 3(a)]. It is commonly used
for 2/3/4G and 5G sub-6-GHz cellular
transceivers. In it, quadrature LO
signals tunable from 24.25-29.5 GHz
and 37-43.5 GHz are required to support
entire FR2 bands. According to
CMOS inductor-capacitor (LC) oscillators
published in the literature, a typical
achievable tuning range is ~30% at
these frequencies, with decent PN performance
[4]-[6]. Although switchedinductor
or multimode oscillators
using a transformer-coupled topology
have demonstrated more than a 50%
tuning range, their PNs could vary
more than 10 dB across the tuning
range [7], making them impractical
due to the extremely stringent PN requirement
for 5G NR.
Alternatively, multiple oscillators
can be used to cover the wide tuning
range, at the cost of a larger chip area.
Besides the tuning range challenge,
it is also not a trivial task to generate
quadrature LOs at such high carrier
frequencies. In a sub-6-GHz design,
10 ms
1 Gb/s
3 GHz
Connections:
100,000/km2
Latency
Latency
Peak Data Rates
Available Spectrum
4G
Connection Density
(a)
5G
Connection Density
Connections:
1 million/km2
1 ms
Peak Data Rates
Available Spectrum
10 Gb/s
30 GHz
Complex
Modulations
OFDM
Higher
256 QAM
Frequencies
Wider
Bandwidth
100/400 MHz
1.6 GHz (CA)
(b)
FIGURE 1: (a) A comparison of 5G and 4G technologies. (b) Characteristics of 5G New Radio OFDM: orthogonal frequency-division multiplexing;
QAM: quadrature amplitude modulation; FR: frequency range; MIMO: multiple-input, multiple output; CA: carrier aggregation.
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2021
45
Multiple
Antenna
Techniques
FR1: <7.125 GHz
FR2: 24.25~50 GHz
Phased-Array Antenna
MIMO FR1: 8 × 8
MIMO FR2: 2 × 2

IEEE Solid-States Circuits Magazine - Fall 2021

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