1. Co-time Co-frequency Full Duplex for 802.11 WLAN
July 2013
Co-time Co-frequency Full Duplex for WLAN
Date: xx
Authors:
Name
Affiliations
Address
Phone
Hongliang Bian
China Telecom
No.118 Xizhimennei Street, Xicheng District, Beijing ,China
Yonggang Fang
ZTE USA
Bo Sun
ZTE Corp.
Yunzhou Li
Tsinghua Univ.
Hongliang Bian (China Telecom)

2. July 2013
Abstract
HEW is aimed to improve the system efficiency of WLAN. This proposal makes a brief introduction about a new PHY technology namely Co-time Co-frequency Full Duplex (CCFD). The purpose is to share the up to date information and to facilitate further discussion on this technology.
Hongliang Bian (China Telecom)

3. July 2013
Background
Next generation WLAN network will accommodate more users and provide higher user throughput in unit area.
Heterogeneous APs will be deployed densely in unit area for high data rate coverage in hotspot.
Current WLAN spectrum is limited, therefore new technologies which can further improve spectrum efficiency is worth consideration.
Hongliang Bian (China Telecom)

4. Co-time Co-frequency Full Duplex (CCFD)
July 2013
Co-time Co-frequency Full Duplex (CCFD)
Traditional duplexing is accomplished by conducting transmission and reception in different time or frequency, which limits radio resource usage.
In CCFD, transmission and reception can be conducted simultaneously using the same frequency, which promotes the spectrum efficiency significantly.
The main challenge in CCFD is to suppress the interference between transmitting and receiving.
Time slots or carriers split for transmitting or receiving separately now can be combined for transmitting and receiving simultaneously. Improve by up to 100% spectrum efficiency!
Key point: interference cancellation.
While the CCFD can improve the spectrum efficiency, self interference and interference between each pair of transmitting and receiving points are the key issues to be solved.
Hongliang Bian (China Telecom)

5. Interference cancellation
July 2013
Interference cancellation
Current studies mainly focus on self interference cancellation which is specific to point to point communication.
There are three types of self interference cancellation schemes:
Antenna cancellation;
Analog cancellation;
Digital cancellation
Hongliang Bian (China Telecom)

6. Antenna cancellation (1/2)
July 2013
Antenna cancellation (1/2)
Asymmetric antenna placement
Distance difference from two transmit antennas to receive antenna is an odd number of λ/2, which provide a π phase difference between the two self interferences.
Factors that influence antenna cancellation performance:
Placement accuracy of transmit antennas
Power balance of self interferences
Signal bandwidth
Distance mismatch effect on antenna cancellation[1]
Amplitude mismatch effect on antenna cancellation[1]
Hongliang Bian (China Telecom)

7. Antenna cancellation (2/2)
July 2013
Antenna cancellation (2/2)
Symmetric antenna placement
Benefits of symmetric placement compared to asymmetric placement:
Receive cancellation and transmit cancellation can be combined with placement of both symmetric Rx antennas and symmetric Tx antennas.
Symmetric antenna placement can be extended to support MIMO easily.
With a π phase inversion between two symmetric Rx/Tx antennas, self interference can be cancelled at Tx/Rx antenna.
The main factor needs to be considered is distance mismatch of the pair of Rx/Tx antennas.
Double antenna cancellation with receiving cancellation plus transmitting cancellation[2]
Hongliang Bian (China Telecom)

8. Analogy cancellation July 2013
Take the known interference and received signals as input and outputs the received signal with the self interference subtracted out.
Noise canceller chip can be used to adjust the amplitude and phase of the interference reference signal to match the self interference.[3]
Factors that influence analogy cancellation performance:
Accuracy of adjustment of the amplitude and phase of the interference reference signal. Adaptive algorithm to adjust the interference reference signal according to the output signal is key issue.
Hongliang Bian (China Telecom)

9. Digital cancellation July 2013
Given that the input signal doesn't block the ADC, digital cancellation can be applied to remove the self interference. Digital interference reference signal is derived from transmitting signal, then it is subtracted from the output of the ADC to remove the self interference.
The amplitude difference between self interference and received signal should lie in the range of the ADC.
Factors that influence analogy cancellation performance:
ADC dynamic range
SIR of the output of ADC
Quantification principle
Hongliang Bian (China Telecom)

10. July 2013
Summary
Based on the theoretic studies above, there are some challenges need further considerations before the technology can be applied in real network.
Performance of antenna cancellation is very sensitive to the AP placement, for individual AP which can’t be managed professionally it is very difficult to guarantee the performance.
Multi-path signal will degrade the performance of self interference cancellation, especially for indoor scenario where the multi-path signal is very strong.
Digital cancellation is mainly limited by the dynamic range and quantification principle of ADC. For amplitude sensitive modulation/demodulation schemes such as OFDM it needs higher dynamic range and specific quantification principle of ADC.
Since the analog cancellation and digital cancellation can be accomplished jointly, how to coordinate the analog cancellation and digital cancellation effectively?
Hongliang Bian (China Telecom)

11. July 2013
Conclusion
This presentation proposes a Co-time Co-frequency Full Duplex technology, which can improve the spectrum efficiency.
Proposal 1: Self interference cancellation should be further studied.
Proposal 2: In multi APs scenario interference between APs should to be cancelled effectively, which need further study.
Proposal 3: Performance estimation and challenges need further discussion.
Proposal 4: Impacts to current system and implementation scheme need careful consideration.
Hongliang Bian (China Telecom)

12. July 2013
References
[1] Choi J I, Jain M, Srinivasan K, et al.. Achieving single channel, full duplex wireless communication [C]. Proceedings of the 16th Annual International Conference on Mobile Computing and Networking (MobiCom '10), New York, 2010: 1–12.
[2] Khojastepour M A, Sundaresan K, Rangarajan S, et al.. The case for antenna cancellation for scalable full-duplex wireless communications [C]. 10th ACM Workshop on Hot Topics in Networks (HOTNETS '11), Massachusetts, USA, 2011:
[3] Jain M, Choi J I, Kim T M, et al.. Practical, real-time, full duplex wireless [C]. Proceedings of the 17th Annual International Conference on Mobile Computing and Networking (MobiCom '11), New York, 2011:
Hongliang Bian (China Telecom)

13. July 2013
Thank You!
Hongliang Bian (China Telecom)