IEEE Spectrum July, 2016 - 42

nology. Our full-duplex receiver can
operate at any frequency between
0.8 and 1.4 gigahertz, and the RF selfinterference canceler suppresses the
transmitter interference, for a variety of
antenna types, over a bandwidth that is
about 10 times as great as what you can
get with existing, conventional cancellation techniques. We achieved this 10x
performance advantage with just two
N-path filters in the bank. That's good
enough to make it compatible with many
advanced wireless standards, including
LTE and Wi-Fi. More filters would enable
even wider cancellation bandwidths.
Another advantage of our frequencybased cancellation scheme is its compatibility with existing wireless systems,

one small paCkage: this is the world's first full-duplex transceiver on a chip. it
cancels interference with the familiar technique of frequency-domain equalization.

assigned to that band. Again, to use the
audio analogy, it's like dialing up the bass
and dialing down the treble, and doing
whatever else it takes to get the output
to match the input signal.
The next step is to automate this
weighting process so that the output
changes accordingly as the environment
changes. Of course, these environmental
changes are dynamic, fluctuating from
second to second, so the process has to
be automated. Though we have shown
some initial and promising demonstrations of such automation, there is more
work to be done here.
We desig ned a protot y pe of t he
receiver-cum-canceler and fabricated
it using 65-nanometer CMOS tech42

jul 2016

North AmericAN

which use what's called multiband
frequency-division duplexing. As mentioned earlier, FDD is a half-duplex strategy in which the transmitter and the
receiver operate at the same time but
at different frequencies. It requires
duplexer filters to separate the transmitter and the receiver signal at the
common antenna. Because these offchip duplexer filters cannot be tuned,
today's smartphones use a separate
duplexer filter to support each of the
FDD bands-and 4G LTE suppor ts
25 bands, therefore requiring 25 separate duplexer filters!
You can reduce the bulk and cost of
the radio component of a cellphone by
replacing those filters with just a few

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tunable duplexers, but such duplexers
typically are less effective in isolating
the transmitter from the receiver than
their fixed-frequency counterparts. Consequently, the receiver is particularly
prone to transmitter self-interference.
And that's where self-interference cancellation comes in.
The entry point for self-interference is
right at the antenna, and it would be
wonderful to suppress the interference
there, before it has a chance to leak into
the receiver. The main challenges are
to keep the antenna compact-say, for
use in a cellphone-and make sure that
the self-interference doesn't come back
every time the electromagnetic environment changes. In other words, we need
the antenna to be smart.
Such a smart antenna can manipulate the radio wave's obvious electronic
characteristics-amplitude, phase, and
frequency-but also the extra dimension:
wave polarization. A radio wave is really
two fields joined at the hip, one electrical, the other magnetic-hence the word
"electromagnetic." Each field oscillates
at a given frequency, and the oscillation
of the electric field induces the magnetic
field, and vice versa. The two fields are
perpendicular, and the way the pair of
them are oriented in space is called their
polarization. Electromagnetic waves of
different polarization can pass through
each other without interference.
Krishnaswamy and his Ph.D. student
Tolga Dinc were able to use polarization
for duplexing within a pair of compact
antennas (configured for 4.6 GHz), one for
the transmitter and one for the receiver.
We were able to place them right next to
each other because the waves that were
coming and going to the two antennas
were orthogonally polarized with respect
to each other, which effectively isolated
them. But though this isolation minimizes
self-interference, it does not eliminate it
entirely. That's why we also installed a port
in the receiving antenna that's copolarized
with the transmitting antenna. The port
samples a small portion of the transmitted signal, conditions the signal through a
filter, and then passes it on to the receiver
port. Result: near-perfect cancellation.
Because the filter | cO nTI n u e d O n pag e 53

JiN Zhou (2)

echo cancellation across a very wide
band of frequencies for a full-duplex
wireless radio.
In our system, a bank of N-path filters
taps a small portion of the transmitter
signal. Next it divides that RF signal into
two frequency bands (though more than
two is also quite feasible). Then it conditions the signal in each of those bands to
mimic the self-interference that's arriving at the receiver.
This multiband approach divides the
bandwidth into bite-size chunks, a divideand-conquer strategy that makes it easier
to condition each chunk of bandwidth-
that is, to adjust it for power and for phase.
Circuitry performs the conditioning for
each band according to the weights

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Table of Contents for the Digital Edition of IEEE Spectrum July, 2016