Figure 1 illustrates just some of the po-
tential test points when planning a test
regime for 5G equipment. Baseband
transceivers work with the quadrature
modulation IQ waveforms for which lin-
earity, IQ signal calibration and signal
conditioning into intermediate frequen-
cies are all key test points. Engineers
need to have access to highly linear test
equipment, 5G waveform generation and
test sets capable of working with the
high-bandwidth IQ waveforms. Likewise,
the RF transceivers, which feature up
and down conversion to mmWave from
IF, need stringent testing. Signal integri-
ty, amplifier efficiency, output power and
the removal of unwanted harmonics and
phase noise artefacts are all test metric
candidates.
Apart from working on different frequen-
cies, the above highlighted aspects of
the signal chain have much in common
with traditional 4G equipment. However,
delivering 5G capabilities relies on the use
of beamformers and front-end modules
(FEMs).
Beamforming is more critical at mmWave
frequencies due to higher propagation
loss characteristics. The benefit of using
these frequencies, however, is that the
antenna element sizes are much smaller,
enabling a high number of elements to be
constructed in a relatively small physical
space, making beamforming a viable op-
tion for increased antenna gain. Through
the use of a hybrid approach, using ana-
logue phase shifters and digital circuitry,
infrastructure equipment can direct the
signal towards the intended receivers, re-
sulting in a higher received signal strength
that increases the operational range, and
a reduced bit error rate (BER).
The measurement challenge is in how to
characterise and obtain optimal levels of
performance out of these new IC-based
beamforming components and FEMs.
Comprising mixers, filters, power ampli-
fiers and low-noise amplifiers, the need
to achieve high-bandwidth linearity while
maintaining energy efficiency is crucial.
Digital pre-distortion (DPD) is typically
employed to aid transmit linearisation,
but it does require the test equipment to
generate and measure signals of up to
five times the desired bandwidth, signif-
icantly more than that encountered in 4G
testing systems.Reciprocity of the transmit and receive
paths needs examination too. For exam-
ple, the power amplifiers introduce ampli-
tude and phase shifts when they are driv-
en into compression. Also, the tolerances
of RF components such as variable atten-
uators, variable gain amplifiers and phase
shifters can create unequal phase shifts
between channels, potentially impacting
the phase coherency of the FEM.Beamforming test systems for 5G need
to sweep across a wide spectrum and be
able to test the maximum linear output
and compression behaviour of each path.
A fast, bi-directional multi-port switching
test solution is a prerequisite for any 5G
development and production test envi-
ronment.Over-the-air (OTA) testing is the ideal
method of measuring real-world perfor-
mance and characterisation of the beam-
former and FEMs. Such tests determine
how well the device under test (DUT) can
focus its power output in specific direc-
tions and examine how well a beam is
formed. This is achieved by a spatial scan
sweeping one of the orthogonal coordi-
nates from 0 to 360º and the other one
from 0 to 180º.An example OTA measurement set-up
is the NI mmWave OTA reference solu-
tion – see Figure 2. It comprises an NI
high-bandwidth mmWave vector signal
transceiver (VST) for waveform generation
and analysis, high-gain antennas, and an
RF anechoic chamber with a highly accu-rate real-time motion-controlled positioner.A test sequencer allows engineers to set
up a configuration to characterise and
validate the beamforming capabilities of
the DUT. A comprehensive set of soft-
ware provides a range of visualisation op-
tions for the measured data, an example
of which is shown in Figure 3.5G looks set to radically change how we
use cellular wireless networks. It will un-
leash a host of new network services and
also promises to deliver fast, low-laten-
cy fixed wireless networks for industrial
applications and campus deployments.
Implementing a comprehensive test and
measurement approach, from develop-
ment through to production, will be es-
sential to a successful deployment.http://www.ni.comAbout the author:
David Hall has served in multiple roles
throughout his career including applications
engineering, product management, and
product marketing for automated test and
RF instruments. Hall is a known expert on
subjects such as 5G, the Internet of Things,
autonomous vehicles, and software-defined
instrumentation.Feature: 5G reality
dpaonthenet.net 11
Figure 3: Beamforming measurement visualisation (source NI)Figure 2: NI mmWave OTA reference solution diagram (source NI)