Full duplex radio communication allows simultaneous two-way communication using the same frequency band. This doubles the spectral efficiency compared to half-duplex communication, which requires allocating separate frequency bands or time slots for the two directions of communication. However, full duplex faces significant challenges from self-interference caused by the high-power transmitted signal overwhelming the weak received signal. Recent research has developed interference cancellation techniques using passive suppression, analog cancellation, and digital cancellation to mitigate self-interference and realize the benefits of full duplex radio communication.

1. Full Duplex Radio Communication
Domain: Wireless Communication Research
Dr. Varun Kumar
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2. Outlines
1 Introduction to Communication
2 Difference Between Half-duplex and Full Duplex
3 Introduction to Full Duplex Radio Communication
4 References
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3. Introduction to Communication
⇒ Communication is a process for sending information/signal/data from
source to sink.
⇒ As per the nature of signal (analog and digital), it is categorized into
two parts.
Analog communication ⇒ When analog signal is transmitted from
source to sink.
Digital communication ⇒ When digital signal is transmitted from
source to sink.
⇒ By default all signals are analog in nature, whereas for convenience in
signal processing operation, this signal is converted into digital.
⇒ Digital signal is a man-made signal.
⇒ Digital signal has lots advantageous features compare to analog, like
noise immunity, storage, encryption and many more.
⇒ In modern wired and wireless communication; digital signal is the raw
ingredient for effective communication.
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4. Fundamental of half duplex (HD) and full duplex (FD)
Fundamental of half duplex and full duplex
Example:
⇒ Let A and B are two device ( may be base station (BS) and user
equipment (UE) ) communicate for 1 sec with 100 KHz bandwidth
availability, where other parameters are supposed to be constant.
⇒ Both devices are supposed to be identical in all respects.
⇒ Half duplex :
1 Time division duplexing (TDD)
2 Frequency division duplexing (FDD)
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5. Continued–
⇒ TDD scheme : As per above consideration, let τAB sec time is
reserved for communication from A → B and remaining time τBA sec
for communication from B → A. Mathematically,
τAB + τBA = 1
During communication, either from A → B or B → A, the total
bandwidth utilization remain 100 KHz as per above consideration.
⇒ During time 0 to τAB, no communication can happen from B → A.
⇒ Similarly, from τAB to 1 sec no communication can happen from
A → B.
⇒ FDD scheme : As per above consideration, let ωAB KHz bandwidth is
reserved for communication from A → B and remaining bandwidth
ωBA KHz for communication from B → A. Mathematically,
{fc − ωAB, fc + ωAB} = 100 KHz ⇒ ωAB + ωBA = 100 KHz
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6. Continued–
⇒ fc is the carrier frequency for bi-directional communication (Ex- single
carrier communication).
⇒ FDD scheme utilize the whole allocated time, i.e τ = 1 sec, as per
above consideration.
⇒ No communication occurs from A → B, when frequency range lies
between the range (fc, fc + ωBA)
⇒ Similarly, no communication occurs from B → A, when frequency
range lies between the range (fc − ωAB, fc)
Full duplex communication
⇒ 100 % utilization of time as well as frequency resource. On the other
side half duplex scheme either utilize 50 % of time resource or 50 %
frequency resource.
⇒ Spectrum efficiency is just double the half duplex scheme.
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7. Full duplex communication
⇒ If spectrum is kept constant for half and full duplex schemes then the
number of users support becomes double in full duplex scheme.
Shannon’s capacity
In case of FD communication
CFD = log2 1 +
S
I + N
(1)
On the other side, the Shannon’s capacity for HD is
CHD =
1
2
log2 1 +
S
i0 + N
(2)
where, S → signal power, {I, i0} → interference, N → Noise
→ Both FD and HD scheme affected by the of interference and noise.
→ FD radio has double achievable rate, if {I, i0} ≈ 0.
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8. Practical difficulty for full duplex communication
→ From (1) and (2), I >> i0 → Major obstacle of FD communication.
→ In the absence of self-interference full duplex radio outperform to the
half duplex even if SNR is very low.
→ (1) and (2) shows the channel capacity, when same amount of
wireless resources are utilized from A → B and B → A.
Practical difficulty for full duplex communication
⇒ Bi-directional communication occurs at same time-frequency slots
that causes huge amount of interference.
♦ Phase noise
It arises due to the rapid, short term, random phase fluctuations that
occur in a signal.
In time domain these random fluctuations or instabilities called as
phase jitter.
Power amplifier non-linearity
In-phase and quadrature-phase (I/Q) imbalance
⇒ Self interference occurs due to limited RF isolation.
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9. Continued–
⇒ In co-operative network, if SNR is high then HD relaying outperform
in terms of outage behavior. Because FD relaying suffers with high
distortion, noise amplification.
⇒ In FD communication, a single node let A or B utilize the time and
frequency resource at full scale. In other words, node A acts as a
transmitter and receiver for full time slot and full scale bandwidth.
⇒ Let at t = τ0, bi-directional communication begins then high power
transmitted signal acts as an interfere for the low power received
signal.
⇒ If no signal isolation mechanism is available then interference plus
noise is much higher than the signal power, which is one of most
difficult job of FD communication.
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10. Interference cancellation techniques
Passive suppression
Circulator: A three port micro-wave device that separate high power
transmitted signal and low power received signal
Analog cancellation
It does the phase noise correction or estimate the fixed delay.
Digital cancellation
It eliminates linear and non-linear distortion
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11. References
D. Bharadia, E. McMilin, and S. Katti, “Full duplex radios,” in Proceedings of the
ACM SIGCOMM 2013 conference on SIGCOMM, 2013, pp. 375–386.
B. Debaillie, D.-J. van den Broek, C. Lavin, B. van Liempd, E. A. Klumperink,
C. Palacios, J. Craninckx, B. Nauta, and A. P¨arssinen, “Analog/rf solutions
enabling compact full-duplex radios,” IEEE Journal on Selected Areas in
Communications, vol. 32, no. 9, pp. 1662–1673, 2014.
M. Chung, M. S. Sim, J. Kim, D. K. Kim, and C.-B. Chae, “Prototyping real-time
full duplex radios,” IEEE Communications Magazine, vol. 53, no. 9, pp. 56–63,
2015.
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