Orbital Angular Momentum RadioA new possibility for Full Duplex Radio?

Ben Allen Alan TennantDepartment of Engineering Science Department of Electronic & Electrical Eng

University of Oxford University of Sheffield

Justin CoonDepartment of Engineering Science

University of Oxford

Acknowledgments:

Eduardo Cano, University of Bedfordshire

Q (Simon) Bai, University of Sheffield

• What is Orbital Angular Momentum Radio?

• The state of the art

• How may OAM-radio help towards Full Duplex?

Full duplex

• Spectral efficiency gain of x 2 MAX

• Three stages:

– Signal processing (cancellation)

– RF (separation / isolation & cancellation, saturation)

– Antennas (separation / isolation / leakage)

What is OAM?

Polarisation

(Spin angular momentum)

Zero spin = linear polarisation

Phase profile

(Orbital angular momentum)

Zero = polarisation only

Default angular velocity = carrier frequency

Can control this...faster / slower

Angular momentum is preserved in the absence of torque

Prior art

Astro physics → Fibre optic comms → Free space optical comms → Radio

RADIO

• First experiments (Venice, 2012) [1]

• Community debate (IEEE papers, 2012) [2] [3]

• First simulations & Experiments…some examples…

• Circular time-switched array (A Tennant, B Allen, 2012) [4]

• Circular array @ 10GHz (A Tennant, B Allen et al, 2014) [5]

• OAM signal coding and detection (E Cano, B Allen, 2014) [6][7]

• Spiral phase plates @ 60 GHz (S Walker, 2013) [8]

• Spiral phase plates @ 28 GHz (Yan...Molisch et al, Nature, 2014) [9]

• Spiral phase plates @ 60GHz (Allen et al…on going 2014)

Context

• May be implemented as “MIMO”

• Line-of-sight links & point-to-point / multi-point applications e.g.

wireless backhaul & data centre connectivity

• It is NOT polarisation or phase

• It IS a 3D spatial phase profile (DNA helix)

• It IS independent to polarisation i.e. additional degree of freedom

Challenges

• Detectors for non-aligned links

• Mitigating multipath

• Code / MUX design

• Limitations and strengths

• Cross-talk6

From [5]

Antenna Array Implementation

Data Encoding and OAM Generation

• 8-element circular array

• Angular velocity = carrier frequency

• OAM modes l = ±n/2 = ±4

• We use l = ±2

• Selected progressive phase shift to generate selected OAM mode

d

r

Data Encoding

Data symbol OAM Mode Phase

profile

00 l=1

01 l=-1

11 l=2

10 l=-2

‘Vortex’

Diameter increases with distance

-Depends on mode

-Sets link limits and detector configuration

Detection – phase gradient method

φ1

φ2

β

21 l

Realistic phase profiles

Example Results

From [8]

Spiral Phase Plate Implementation

Y Yan et al, “High-capacity millimetre-wave communications with

Orbital Angular Momentum”, Nature Communications, Sept 2014

32 Gb/sec @ 28 GHz carrier

2m link distance

Data multiplexing

3D printed 60 GHz SPP

(University of Bedfordshire)

28 GHz SPP used in [9]

• Point-to-point / multi-point wireless back haul in “5G” and beyond

• Use SPP to provide isolation hence relax spec of RF & signal

processing stages. Acts as a spatial matched filter.

• There are some significant challenges!

– Quantify improvement in isolation & dealing with leakage

– Cross-talk (about 20 dB so far)

– Size (depends on carrier frequency)

– OAM mode bandwidth

– Link distance (avoiding the ‘vortex’)

– Detection & multipath tolerance

– Multi-point applications

• But there has already been some decent progress

• Backhaul IS a bottleneck so we need SOMETHING!

How may OAM-radio help towards Full Duplex?

References1. F Tamburini et al, “Encoding many channels on the same frequency through radio

vorticity: first experimental test”, New Journal on Physics, 2012

2. O Edfors, A F Johanson, “Is Orbital Angular Momentum (OAM) based radio

communication an unexplored area?”, IEEE Trans. A&P, 2012

3. S M Mohammadi et al, “Orbital Angular Momentum in radio – a system study”, IEEE

Trans A&P, 2012

4. A Tennant, B Allen, “Generation of OAM radio waves using circular time-switched

array antenna”, IET Electronics Letters, 2012

5. Q Bai, A Tennant, B Allen, “Experimental circular phased array for generating OAM

radio beams”, IET Electronics Letters, 2014

6. F.E Mahmouli, S Walker, “4-Gbps Uncompressed Video Transmission over a 60-

GHz Orbital Angular Momentum Wireless Channel”, IEEE Wireless Coms Letters,

2013

7. B Allen et al, “Wireless data encoding and decoding using OAM modes”, IET

Electronics Letters, 2014

8. E Cano, B Allen, “Multiple-antenna phase-gradient detection for OAM radio

communications”, IET Electronics Letters, 2015

9. Y Yan et al, “High-capacity millimetre-wave communications with Orbital Angular

Momentum”, Nature Communications, Sept 2014