Beam steering will allow transmitters to use less power than omnidirectional radios by concentering signal power where it’s needed

BY MARTIN ROWE, Senior Technical
Editor, Test & Measurement
EDN and EE Times
www.edn.com, www.eetimes.com

5G New Radio (5GNR) will rely heavily on beam steering to maximize
data throughput, especially when used to send and receive millimeter-wave
(mmWave) frequencies. Starting at about 24 GHz, mmWave signals offer tremendous
bandwidth as compared with microwave signals. But that comes at a price: signal
losses. Beam steering, also called beamforming, minimizes losses by
concentrating signals at their targets rather than transmitting signals in all
directions. The technique uses multiple antennas that can be simulated through
software.

The array antenna shown in Fig. 1 consists of
256 antennas arranged in a 16 × 16 grid transmitting 64-QAM data signal at 28
GHz. Keysight Technologies joined with Anokiwave and Ball Aerospace to
demonstrate beam steering at the 2018 International Microwave Symposium.

Fig. 1: An array of 256 antennas uses a 16 × 16 grid to steer the beam
toward a target. Image: Martin Rowe.

The array is driven by 64 Anokiwave quad-core ICs, wherein
each IC drives four antenna elements. The antenna can form a single steerable
beam using all 256 elements or four independently steerable beams using 64
elements each.

Beam steering can use analog (RF), digital, or hybrid
approaches. In analog beam steering (Fig. 2), a single analog-to-digital converter (ADC)
provides the signal for eight antennas.

1118_Special_5G_Fig-2

Fig. 2: Analog antenna systems use a single ADC for all antennas. Image:
Anokiwave.

While analog beam steering minimizes the number of ADCs, it
has problems such as phase shift, RMS phase error, and RMS amplitude error as a
function of frequency. Plus, all signal processing — phase shifting and signal
attenuation/amplification — must take place at the antenna. Thus, it’s
practical only with highly integrated ICs. You also lose flexibility in the
number of beams that can be formed.

A full digital signal chain uses a dedicated ADC/DAC pair per
antenna for transmit/receive. Such an architecture provides the most beamforming
flexibility, but it’s impractical. The high cost of the ADCs and excessive heat
that they generate make digital beam steering unattractive. Thus, a hybrid
approach is taking hold. Fig. 3 shows the same eight antennas from Fig. 2 driven by two ADCs handling four antennas each. This
approach provides good beamforming flexibility while eliminating the challenges
of digital beam steering.

1118_Special_5G_Fig-3

Fig. 3: In a hybrid system, each ADC generates signals for multiple
antennas. Image: Anokiwave.

Logan Minard, customer engineer at Anokiwave, explained the
signal chains used in the company’s ICs (Fig. 4): “Common”
refers not to a common signal return but to the ADC and DAC circuits included
in the transmit and receive signal chains, respectively. Analog signals from
the ADCs go to Wilkinson power dividers, followed by amplitude and phase control and a power
amplifier (PA). Another switch connects the signal to the antenna.

On the receive side, the received signal first comes through
a low-noise amplifier (LNA), then through a Wilkinson combiner and temperature
compensator, which adjusts signal gain based on temperature, he continued.

1118_Special_5G_Fig-4

Fig. 4: A block diagram of the Anokiwave AWMF-0108 shows the transmit and
receive signal chains. Image: Anokiwave.

In the Microwave Symposium demonstration, the beam steers ±30°
from boresight, though the array is capable of steering up to ±60° from
boresight. “Once you get beyond 60° from boresight, excessive scan loss
occurs, making the antenna gain drop below acceptable levels,”
said Minard.
Designing a
beam-steering system requires simulating the antenna array, the radio components
(amplifiers, filters, mixers, phase shifters, etc.), and the transmission
channel. How do you get started?

Modeling phased-array antennas and other 5GNR components doesn’t
mean that you have to develop your own models. Modeling and simulation software
from several companies can crunch the numbers for you. Just feed them your
design parameters, including:

  • Frequency
  • Antenna gain
  • Sidelobe level
  • Material property such as permittivity
  • Array geometry
  • Weighting of each antenna in an array
  • Phase-shift equalization levels

To design an array, start by modeling a single antenna element,
then scale those characteristics up to form an array. Fig. 5
shows models of elements, an 8 × 8 array, and the sidelobe levels from the
array.

1118_Special_5G_Fig-5

Fig.
5: Using COMSOL Multiphysics software, you can model a slot-coupled microstrip
patch antenna array. Image: COMSOL Multiphysics.

Simulation software lets you enter a range of parameters and step
through them while viewing the signal pattern. In MATLAB’s Antenna Toolbox, you
can enter a range of values in a simulation app or you can write scripts to
automate the process and see the results as parameters change.

Software represents the simulated antenna patterns using colors
and distances from the antenna. The colors in Fig. 6
highlight the directivity from a single element in units of decibels relative
to an isotropic antenna (dBi).

1118_Special_5G_Fig-6

Fig.
6: Software such as MATLAB simulates the radiated pattern from a single antenna
in units of dBi. Red indicates the strongest difference of signal relative to
the isotropic antenna. Image: MathWorks.

While it’s important to know the expected response of a
phased-array antenna, what happens if a cell should fail? “Simulation software can
produce volumetric and surface plots of each dimensional component of an
electric field, which can help to diagnose the abnormal behavior of networks
and individual array elements,” said Jiyoun Munn, technical product manager of
the RF Module at COMSOL.

Phased arrays may consist of sub-arrays, as in the case of hybrid
systems. “Failed subarrays can also be simulated,” added Rick Gentile, product
manager for Phased Array System Toolbox and Signal Processing Toolbox at MathWorks.
“With a failed subarray, many elements may be missing because a
transmit/receive module is shared.”

Phased arrays will
likely become commonplace as 5G rollouts begin. Larger arrays, such as 64 × 64
and larger, will be used in base stations and for mmWave frequencies in small
cells. Smaller arrays such as 2 × 2 or 4 × 4 will likely be incorporated into
handsets.