IEEE Spectrum November, 2014 - 20

antennas broadcasting on the same
frequency and decodes the inevitable cross talk on the receiving
end using digital signal processing.
OAM multiplexing sends multiple
channels of information along a
single beam without any interference between them. That means
once the phase plate at the receiver
unwinds the helical beam into its
component channels, they don't
have to undergo further cleanup.
One of OAM radio's early critics,
Lund University radio-systems professor Ove Edfors, remains unconvinced, however. While he doesn't
question Willner's results, Edfors
remains incredulous that radiobased OAM could be made practical for long-range communications.
Without the assistance of impractically large antennas to transmit
and receive them over long distances, Edfors says now, "signals
carried on...OAM components rapidly become useless from a communication point of view."
Fabrizio Tamburini, one of the
scientists behind the 2012 Venice
OAM experiment, sees a lot of promise in the latest work, saying that
"very good ideas can come from it."
Tamburini is now working on ways
to refine OAM for use in telecommunications and other industries.
If OAM pans out, the technology
could be adopted in places where
high-speed, line-of-sight wireless
connections are in demand, such
as for wireless backhaul in cellular
networks, suggests Willner. OAM
could be a good fit for transmitting
data among "a dense network of
small base stations without...the
stringing of fiber to connect them
to the core network," he says. He
and USC colleague Andy Molisch
also see the potential for OAM in
data centers. "With better equipment, [transmission rates] could
go much higher," Willner says.
"A radio back haul like that
could be a huge pipe for data
centers or building-to-building
connections."
OAM techniques might also
be used in other fields, such
as microscopy, Willner says.
"There are potential applications outside of communications.
We're going to continue learning
how to tailor and manipulate the
structure of waves in ways we've
never thought of before."
-i a n ch a n t

Twisted radio
waves sent
32 gigabits per
second across
2.5 meters of air,
about 30 times
as fast as an
LTE wireless
connection

nOV 2014

nORTh aMERICan

A version of this article appeared
online in September.

SPECTRUM.IEEE.ORG

miCrowaVe
stetHosCope
Lets pHysiCiaNs
peer iNto
tHe LuNgs
A stick-on sensor can measure vital
signs and lung fluid

When a person's heart is failing, water
begins to build up in the lungs, making it
increasingly difficult to breathe. The sensation,
patients say, is like drowning.
Deciding whether it's safe for that patient to go
home can depend on whether the water level goes
back down. But today the options for measuring lung
water are cumbersome, such as taking chest X-rays,
inserting a tube into the lung, doing blood tests, or

HaWaii Center for advanCed CommuniCationS/tHe univerSitY of HaWaii (2)

Proving the technology works at
high data rates with radio waves
is important, because those frequenc ies a re less a f fec ted by
obstacles and atmospheric conditions than optics and could have
broader commercial applications.
Willner and his team used four
antennas to send eight channels of
data. Those beams of data were sent
through specially shaped "spiral
phase plates," plastic plates that
don't absorb the beams but do cause
them to change their shape, twisting them slightly. The twisted waves
are then gathered by a multiplexer
and sent through a single transmitter aperture. Since each wave has
a slightly different OAM, they can
travel along a shared axis without
interfering with one another.
The combined beam, which takes
on a helical shape, travels through
another aperture at the receiver,
after which it is split back into four
beams by a demultiplexer. The four
beams then pass through another
set of spiral phase plates. These
plates are inverted versions of the
first set, which undo the initial twisting and prepare the waves to deliver
their data payload.
This isn't the first time radio waves
have been used to demonstrate the
potential of OAM. Italian and Swedish researchers in 2012 used the same
principles to send a pair of radio
waves sharing a single frequency
between two islands in Venice. At
the time, some communications
engineers criticized that work, suggesting it was not significantly different than existing multiple-input,
multiple-output (MIMO) techniques.
Willner says this latest study demonstrates that there are clear implementation differences between
convent ional MIMO and OA M
multiplexing. MIMO sends different streams of data from different

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Table of Contents for the Digital Edition of IEEE Spectrum November, 2014