in-band full-duplex radio transceivers with imperfect rf ... · components: analysis and enhanced...
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In-band Full-Duplex Radio
Transceivers with Imperfect RF
Components: Analysis and
Enhanced Cancellation Algorithms
Dani Korpi, Lauri Anttila, and Mikko Valkama
Tampere University of Technology, Department of Electronics and
Communications Engineering, Finland
4.6.2014
9th International Conference on Cognitive Radio Oriented
Wireless Networks, DUPLO Workshop
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Outline
• Introduction
• RF impairments in a typical full-duplex
transceiver
• Overall feasibility with linear digital cancellation
• Enhanced digital cancellation algorithm
• Waveform simulations
• Conclusion
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Introduction
• Simultaneous trans-
mission and reception
at the same center fre-
quency is an appealing
scheme
– Increased data rate,
MAC level benefits,
etc.
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Introduction (cont.)
4.6.2014 4
• However, it has
also its down-
sides
– Increased com-
plexity due to
self-interference
cancellation, RF
impairments,
etc.
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RF impairments
• The self-interference (SI) signal is distorted in
numerous ways within the transceiver chain
– This affects the accuracy with which the SI
signal can be regenerated for cancellation in
the digital domain
– Using only linear processing results in
insufficient digital SI cancellation
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Full-duplex transceiver model
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• Complex conjugate of the input signal is summed
on top of it with certain attenuation:
𝑥𝐼𝑄 𝑡 = 𝑔1 𝑡 ∗ 𝑥 𝑡 + 𝑔2 𝑡 ∗ 𝑥∗(𝑡)
• Image rejection ratio (IRR) is the power difference
between the direct and image component
IQ imbalance
• It has been observed that imbal-
ance between I- and Q-branches
is a serious concern in in-band full-
duplex transceivers
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Nonlinear distortion
• Amplifiers distort the signal
nonlinearly if the input power
is too high
– Due to the high power of the SI signal, even a
mild distortion can be significant interference for
the weak received signal of interest
• Power of nth order nonlinear distortion can be
approximated with the well known equation: 𝑃𝑁𝐿,𝑛𝑡ℎ = 𝑃𝑜𝑢𝑡 − (𝑛 − 1)(𝐼𝐼𝑃𝑛 − 𝑃𝑖𝑛)
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Quantization noise
• If the power of the SI signal at the input of the
ADC is very high, also quantization noise can
be a serious issue
– Only little dynamic range is available for the
received signal of interest
• The quantization noise floor can be expressed as 𝑃𝑞 = 𝑃𝐴𝐷 − 𝑆𝑁𝑅𝐴𝐷𝐶.
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Transmitter-induced thermal
noise
• Something that has not been studied before
• The active components in the TX chain and
RF cancellation path produce additional
thermal noise
– Part of it is cancelled by RF cancellation
• With high levels of analog SI attenuation, TX-
induced thermal noise is not an issue
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Typical parameters
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Component Gain (dB) IIP2 (dBm) IIP3 (dBm) NF (dB)
PA (TX) 27 - 13 5
VM -10 - - 20
LNA (RX) 25 43 -9 4.1
IQ Mixer (RX) 6 42 15 4
VGA (RX) 0-69 43 14 4
Parameter Value
SNR requirement 10 dB
Bandwidth 12.5 MHz
Sensitivity level -88.9 dBm
Received signal power -83.9 dBm
Antenna separation 40 dB
RF cancellation 30 dB
ADC bits 12
IRR (RX & TX) 30 dB
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Overall feasibility with linear
digital cancellation
• SI mirror image
is dominating
• Also PA-induced
nonlinearities
are problematic
• TX-induced
thermal noise is
not an issue in
this case
4.6.2014 12
-5 0 5 10 15 20 25-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Transmit power (dBm)
Pow
er
of diff
ere
nt sig
nal c
om
ponents
(dB
m)
Antenna separation: 40 dB, RF cancellation: 30 dB
pSI
pSI,im
pn,RX
pn,TX
pNL,TX
pNL,RX
pq
pSOI
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Joint cancellation algorithm
• The previous observations motivate the
development a digital cancellation algorithm
capable of modeling both IQ imaging and
nonlinear distortion
• The simplest way to do this is just to combine
widely-linear and nonlinear cancellation
algorithms
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Joint cancellation algorithm
(cont.)
• Assuming that there is only IQ imaging in the
transceiver chain, the SI signal in the digital
domain is of the form
𝐲𝐴𝐷𝐶,𝐼𝑄 = 𝐗𝐡1 + 𝐗∗𝐡2 = 𝐗 𝐗∗ 𝐡1
𝐡2= 𝐗𝑎𝑢𝑔𝐡𝑎𝑢𝑔
where 𝐗 is a convolution data matrix
constructed from the original transmit signal
𝑥 𝑛 , 𝐗∗ is its element wise complex conjugate
and 𝐡𝑎𝑢𝑔 is the total channel response
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Joint cancellation algorithm
(cont.)
• Assuming only a nonlinear PA, the observed
SI signal is of the following form: 𝐲𝐴𝐷𝐶,𝑁𝐿 = 𝚿𝐟𝑒𝑓𝑓
where 𝚿 is a convolution data matrix
constructed from basis functions 𝜓𝑝 𝑥 𝑛 =
𝑥 𝑛 𝑝−1𝑥 𝑛 and 𝐟𝑒𝑓𝑓 consists of the
corresponding responses of the different
basis functions.
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Joint cancellation algorithm
(cont.)
• A simple way to approximate the combined
effect of IQ imbalance and nonlinear PA is to
write the observed signal as 𝐲𝐴𝐷𝐶 = 𝐗𝑎𝑢𝑔𝐡𝑎𝑢𝑔 + 𝚿 𝐟 𝑒𝑓𝑓 + 𝐳 = 𝐗𝑎𝑢𝑔 𝚿 𝐡𝑡𝑜𝑡 + 𝐳 = 𝚿𝑎𝑢𝑔𝐡𝑡𝑜𝑡 + 𝐳
where the accent ~ denotes the removal of the
linear SI term and 𝐳 is the additional noise.
• Thus, by excluding the cross terms arising from
the cascade of the PA and IQ mixers, a simple
SI signal model can be derived 4.6.2014 16
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Joint cancellation algorithm
(cont.)
• The total effective channel response can be
estimated easily based on the signal model
• For instance, an estimate for the response
can be calculated with least squares as
𝐡 𝑡𝑜𝑡 = 𝚿𝑎𝑢𝑔𝐻 𝚿𝑎𝑢𝑔
−1𝚿𝑎𝑢𝑔
𝐻 𝐲𝐴𝐷𝐶
• The channel estimate can then be used to
cancel the SI signal:
𝐲𝑐𝑎𝑛𝑐 = 𝐲𝐴𝐷𝐶 − 𝚿𝑎𝑢𝑔𝐡 𝑡𝑜𝑡
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Waveform simulations
• Same transceiver model as before, with the
same parameters
• OFDM signal
• All the nonidealities are modelled in the
simulations, including RX nonlinearities and
quantization noise
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Waveform simulations (cont.)
• A significant perfor-
mance gain is at-
tained even with this
type of a simple joint
cancellation scheme
• RX nonlinearities
and the cross terms
decrease the SINR
with high transmit
powers
4.6.2014 19
-5 0 5 10 15 20 25-20
-15
-10
-5
0
5
10
15
Transmit Power (dBm)
SIN
R (
dB
)
Antenna separation: 40 dB, RF cancellation: 30 dB,M = 10, N = 10000
Widely-linear Cancellation
Nonlinear Cancellation
Joint Cancellation
Linear Cancellation
No Self-interference
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Conclusion
• There are several sources of nonidealities,
which make in-band full-duplex
communications a challenging concept
• Typically, IQ imbalance and PA-induced
nonlinear distortion are the most harmful
impairments
• It was shown that even a simple joint
cancellation scheme can help in preventing the
SINR decrease caused by these nonidealities
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Thank you!
• Questions or comments?
4.6.2014 21
Supporting references
[1] D. Korpi, T. Riihonen, V. Syrjälä, L. Anttila, M. Valkama, and R. Wichman, “Full-duplex
transceiver system calculations: Analysis of ADC and linearity challenges,” IEEE
Transactions on Wireless Communications, 2014.
[2] L. Anttila, D. Korpi, V. Syrjälä, and M. Valkama, “Cancellation of power amplifier induced
nonlinear self-interference in full-duplex transceivers,” in Proc. 47th Asilomar Conference
on Signals, Systems and Computers, Nov. 2013, pp. 1193–1198.
[3] D. Korpi, L. Anttila, V. Syrjälä, and M. Valkama, “Widely-linear digital self-interference
cancellation in direct-conversion full-duplex transceiver,” IEEE JSAC special issue on Full-
Duplex Wireless Communications and Networks, 2014.