Division Free Duplex in Small Form Factors. Leo Laughlin,ChunqingZhang, Mark Beach, Kevin Morris, and John Haine

Transcription

1 Division Free Duplex in Small Form Factors Leo Laughlin,ChunqingZhang, Mark Beach, Kevin Morris, and John Haine

2 Outline Duplexing Electrical Balance duplexers Active self-interference cancellation Electrical balance and active cancellation prototype transceiver Results and conclusions

3 The Data Explosion

4 Bi-directional duplex communication Current systems simply avoid this problem >20dBm <100dBm Time Division Duplexing (TDD) Frequency Division Duplexing (FDD) Transmitter Antenna Self Interference Receiver

5 Frequency Time Division Duplexing Transmitting and receiving on the same frequency at different times F c Time

6 Frequency Frequency Division Duplexing Transmitting and receiving on the different frequencies at the same time F 1 F 2 Time

7 Frequency In Band Full Duplex Transmitting and receiving on the same frequency at the same time F c Time

8 Frequency Division Duplex Band Agility LTE specification defines >30 FDD bands World phone would be: Large Costly Lossy

9 Frequency Division Duplex Band Agility A single tunable duplexing circuit for all bands is required Must provide Tx-Rx isolation in uplink and downlink bands

10 Full-duplex technologies for mobile devices? Requirements: Low cost. Small form factor. Tuneable/multiband. Circulators? Bulky Expensive Limited bandwidth/tunability Isolation limited by antenna return loss (could be as bad as 6dB!)

11 Full-duplex technologies for mobile devices? Requirements: Low cost. Small form factor. Tuneable/multiband. Separate antennas? Limited space for antennas on mobile device Does not scale well to MIMO Isolation depends on physical separation Isolation can be compromised by nearby objects/user Can be multi-band 71dB isolation 57dB isolation 34dB isolation E. Everett, A. Sahai, and A. Sabharwal, Passive Self-Interference Suppression for Full-Duplex Infrastructure Nodes, Wirel. Commun. IEEE Trans., vol. 13, no. 2, A. Sahai, G. Patel, and A. Sabharwal, Pushing the limits of full duplex wireless: design and real-time implementation, Rice Univ. Houston, TX, Tech. Rep. TREE1104, 2009.

12 Electrical Balance Duplexing A hybrid junction can isolate the transmitter and receiver to prevent self interference 0ᵒ Z ANT 0ᵒ Self Interference Cancellation Tx PA LNA Rx 0ᵒ 180ᵒ Z BAL High isolation achieved when Z BAL = Z ANT

13 Electrical Balance Duplexing High isolation can be achieved (theoretically unlimited) Z ANT Low cost and small form factor Single antenna On-chip duplexer Tx PA 0ᵒ 0ᵒ LNA Rx Widely Tuneable 0ᵒ 180ᵒ Loss in Txand Rx paths Z BAL Highly accurate impedance control is required Balancing impedance must adapt to variations in antenna impedance due to environmental effects.

14 Electrical Balance Duplexing

15 EB Duplexer isolation performance For an ideal hybrid the Tx-Rx gain is given by 1 4 Γ Γ BUT -the antenna reflection and the balancing reflection are functions of frequency Γ ω Γ ω SO to balance over entire system bandwidth we require Γ ω Γ ω for 15

16 Transmit to Receive Isolation Real antenna exhibits significant impedance variation with frequency

17 Measured EB Duplexer performance Γ ANT Tx PA LNA Rx Γ BAL 17

18 EB Duplexer performance The antenna is the critical factor determining duplexer isolation and bandwidth Isolation over a given bandwidth is limited by frequency domain variation in antenna and balancing impedances L. Laughlin, M. Beach, K. Morris, J. Haine, Optimum Single Antenna Full Duplex Using Hybrid Junctions IEEE Journal on Selected Areas in Communications. vol. 32, no. 9, Sept

19 Electrical Balance and Active RF Cancellation EB can be combined with signal cancellation methods to improve isolation and bandwidth Cancellation signal is actively injected to cancel remaining interference Residual self-interference after EB duplexing stage Self-interference is cancelled by actively injecting another signal prior to receiver input

20 Electrical Balance and Analogue Cancellation Rice University Stanford University Bristol University Frequency selective selfinterference channel model is applied in baseband processing

21 Electrical Balance and Analogue Cancellation Cancellation signal calculated using Tx signal and measured channel responses Self-interference channel Cancellation channel Cancellation signal = Tx signal x Self-interference channel Cancellation channel

22 Experimental setup Electromechanical impedance tuner National Instruments Vector Signal Transceivers MEMS tunable impedance

23 Experimental setup Hybrid implementation Off the shelf Hybrid Coupler SAGE Wireline Hybrid CMOS Hybrid can be implemented on chip Various implementation options for hybrid junction For isolation performance analysis the choice is arbitrary Off the shelf hybrid is used in this experimental setup S. H. Abdelhalem, P. S. Gudem, and L. E. Larson, Hybrid Transformer-Based Tunable Differential Duplexer in a 90-nm CMOS Process, Microw. Theory Tech. IEEE Trans., vol. 61, no. 3, 2013.

24 Experimental setup LTE-like physical layer facilitates frequency selective cancellation 15kHz subcarrier spacing, 20MHz bandwidth. Self-interference channel and cancellation channel estimation is performed at each subcarrier frequency. Frequency domain equaliser is used to generate cancellation signal Tx subcarriers Baseband DSP architecture IFFT Cyclic prefix Transmit signal Channel estimation and Frequency Domain Equalisation IFFT Cyclic prefix Cancellation signal Rx subcarriers FFT Remove Cyclic Receive signal

25 Experimental setup

26 Results Txpower = 10dBm -35dBm -73dBm 45dB isolation provided by EB duplexer 83dB isolation from combined EB and cancellation (cancellation = 38dB)

27 Results Architecture is widely tunable. Similar performance at 890MHz and 1890MHz Freq 890MHz 1890MHz Isolation 85dB 83dB

28 Antenna separation and Active Cancellation (ASAC) Vs. Electrical Balance and Active Cancellation (EBAC) AS = 57dB ASAC = 81dB EB = 45dB EBAC = 85dB AS = 71dB ASAC = 95dB AS = 34dB ASAC = 58dB EBAC outperforms ASAC in mobile device form factors However cancellation performance is over-optimistic (instrument grade hardware was used).

29 Conclusions Full duplex radios transmit and receive simultaneously on the same frequency, providing up to double the link capacity. Electrical balance duplexers provide high transmit to receive isolation, but bandwidth is limited by antenna impedance variation EB is well suited to mobile device implementation due to its use of low cost small form factor technologies Electrical balance has been combined with active analogcancellation to increase isolation and widen bandwidth Our EBAC prototype outperforms antenna separation full duplex architectures when separation is limited by small form factor.

30 Hardware Prototype

31 Questions?