Optical Technique for

5G Wireless Access

Xavier FernandoRyerson Communications Lab

Toronto, Canadahttp://www.ee.ryerson.ca/~fernando

A Major Challenge in 5G

• 5G envisions up to 10 Gb/s

• The air interface has to be very short at Gb bit rates, due to limits in basic physics (very low energy per bit)

• 𝐸𝑏 =𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡 𝑃𝑜𝑤𝑒𝑟

𝐵𝑎𝑢𝑑 𝑅𝑎𝑡𝑒

Distance drastically drops with the bit rate -Pasquale Romano, 2Wire

Optical Techniques

• Bring the radio access points closer to the user

– Fiber-Wireless (Radio over Fiber) systems

– Optical feeders for distributed antennas

– Integration with PON and, HFC networks

• Optical wireless techniques

– Visible light communications (VLC)

– Infrared wireless communications

– Point to point/Distributed

– Indoor/outdoor

Key Issues in OW

• Primarily for downlink – IOT needs more uplink

• Line of sight and short range

• Very short channel coherent time (outdoors)

• Detection issue. Optical reception is difficult than RF receivers

• High level of ambient light noise (SNR < 0)

• Intensity modulated optical signals are scalars.

– Only the Amplitude can be changed (Unipolar) while radio signals are vectors

Multi-Systems ROF

Two (Fiber and Wireless) channels in series.

Two levels of modulation (optical and radio)

Multi system support

Ideally no signal processing at the ‘Radio Access Point (RAP)

Cellular

• Short air-interface - Low power transmission

•(Massive) MIMO capability

• Multiple antennas with a good ‘inter-

distance’ can be interconnected by fiber

•Wideband access

• Low delay spread – less/no ISI (flat fading)

• Frequent LOS path – less outage

•Coverage to special areas

• tunnels, mines, super markets etc.

• Support mm-wave bands

Fi-Wi Systems also offer:

2/1 Loss

Radio over Fiber

• All of the processing that enables internet traffic to turn into radio signals happens at a central station, so ROF is much cheaper to build, run and maintain than typical wireless distribution networks. It also means that new wireless standards – such as 5G wireless, and the latest Wi-Fi protocol, 802.11ac – can replace older standards simply by changing equipment at a central point -www.newscientist.com

“You could carry an entire

town’s wireless traffic over

a single fibre-optic cable”

- Jeff Heynen, an analyst at

Infonetics Research

ROF Deployment

• China Telecom is building a 5G-oriented cloud radio access network (C-RAN) fronthaul network in the north-east Liaoning province using OTN and ROF –(http://www.telecomtv.com)

• As of 2016, telecoms giant AT&T had 4000 systems deployed around the US, boosting mobile broadband coverage in areas like stadiums and shopping malls using ROF.

A Closer Look….

Y

Single

Mode

Fiber

Baseband

DataBaseband-RF

Modulation

RF-Optical

Modulation

Optical - RF

DemodulationGain

BPF

Antenna

200 THz1.8 GHz

RF-Baseband

Demodulation

YBaseband

Data

Radio

Access

Point

Central

Base

Station

Portable

Unit

Two Channels

in series

Research Directions

• Issue of two concatenated channels

– The cumulative SNR is a weighted sum of optical and electrical SNR

• Optical link nonlinearity issues

– Nonlinear, static optical channel is in series with time varying, dispersive wireless channel

• Microwave photonics

– Optical generation, up/down conversion, (de)multiplexing of radio signals

AM-AM & AM-PM Distortion

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5 3 3.5

Input RF Power (mW)

Ou

tpu

t R

F P

ow

er

(mW

)

0

20

40

60

80

100

120

140

160

Ph

as

e S

hif

t (D

eg

ree

s)

Output Power (mW) Phase (Deg)

A Unified Compensation

Hammerstein System

Fiber dispersion will rotate the phase of sidebands

o=1310 nm

0.02 nm = (3.6 GHz)

Transfer function of the fiber

Optical Carrier

RF Subcarrier

RF Bandwidth

Modulation

Depth ~ 0.2

RF Spectrum within the Fiber

H(f) = exp[-j()l(f-fo)2];

l: length, :Dispersion factor

Spectrum with 5 GHz RF Sidebands

Tuesday, October 31, 2017

Fiber Chromatic DispersionDispersion can cause 180o between the USB and LSB and lead to sideband cancellation – This is not a concern up to few GHz

100

101

102

103

104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Fiber Length [km]

Nor

mal

ized

Rec

eive

d R

F P

ower

f = 2.4 GHz f = 900 MHz

More Practical ROF Bus Network

All-Optical Demultiplexing

Radio-over-Fiber (ROF)

Cellular

Microcell900

MHz

WLAN

2.4 GHz

Electrical

MultiplexerOptical

Demultiplexer

Cellular

Base

Stations

Laser

Diode

Y

LNA

Y

LNA

photodiode

photodiode

RA

P

RA

PRAP: Radio Access Point

Wireless

LAN

All-Optical Demultiplexing

• Any RF subcarrier can be accessed at any point in the ROF network.

• Unnecessary loss, noise and distortion due to O/E and E/O conversion are avoided.

• The photodetector can have low bandwidth (matched to only one subcarrier)

• Significant cost reduction

• Works well with PON access network architecture

Tuesday, October 31, 2017

Grating Structure in the Fiber Core

Fiber Bragg

Grating

Input Signal

Reflected

Signal

Transmitted

Signal

Grating

Period, ΛλBragg = 2Λneff

Tuesday, October 31, 2017

• A highly reflective filter with a bandwidth in the sub-Pico meter range was imprinted using two highly reflective FBGs, which formed a resonator

• The overall length of the filter is 28mm

FBG-Based Resonance Filter

FBG1 FBG2

12 mm 12 mm

4 mm H2-loaded SMF-

28λB λB

Tuesday, October 31, 2017

Transmission Spectra of the Resonance Filter

• The stop bandwidth of the FBG was ~ 0.3 nm at -3 dB and five resonant peaks were created.

• The bandwidth of the resonant peak is determined by the length of the resonator and the reflectivity of the FBG.

-30

-25

-20

-15

-10

-5

0

1536.2 1536.3 1536.4 1536.5 1536.6 1536.7 1536.8

Wavelength [nm]

Tra

nsm

issi

on

[dB

] (a)

~73

pm

Filter Transfer FunctionThe spectrum of resonant peak (black trace) was obtained by

scanning the sideband over a 2 GHz range at 4 MHz per step.

The red trace was the calculated resonant spectrum from a

planer Fabry-Perot resonator.

The filter has a bandwidth

of

120 MHz at -3 dB

360 MHz at -10 dB

1.5GHz at -20 dB

The insertion loss is 0.8 dB

at the resonant peak.

Filter is polarization

sensitive

-27

-24

-21

-18

-15

-12

-9

-6

-3

0

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Tra

ns

mis

sio

n [

dB

]

Wavelength [pm]

Calculated F-P transmission vs measured one

Demux Experiment

Filtered Spectrum

The FBG filter aligned

to the LSB of the

900 MHZ peak

Tuesday, October 31, 2017

Selectivity of the Demultiplexer

0

10

20

30

40

50

60

-10 -8 -6 -4 -2 0 2 4

Lo

ss

Du

e t

o D

em

ult

iple

xer

[dB

]

Input RF Power (dBm)

About 25 dB from -8 to +2 dBm

2.4 GHz

900 MHz

Tuesday, October 31, 2017

Frequency Separation of the Filter

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

400 500 600 700 800 900 1000 1100 1200

log

(BE

R)

of

90

0 M

Hz

Second Subcarier [MHz]

• The BER performance of 900 MHz signal at the filter output

as the 2nd subcarrier was swept from 450 MHz to 1.1 GHz

• The BER level at 50 MHz separation is 2.72x10-6

o=1310 nm

0.02 nm = (3.6 GHz)

Transfer function of the fiber

Optical Carrier

RF Subcarrier

RF Bandwidth

Modulation

Depth ~ 0.2

Modulation Depth

• Modulation depth normally kept low to avoid extensive nonlinear distortion

• Carrier does not carry useful information, just floods the photodetector

Carrier Suppression

-16

-14

-12

-10

-8

-6

-4

-2

0

-50 -45 -40 -35 -30 -25 -20 -15 -10 -5

log(

BER

) o

f 9

00

MH

z

Received Optical Power [dBm]

With Filter

Without Filter

Narrow optical filters can be used to suppress un-modulated carrier

In this case sensitivity improvement ~7 dB

Single FBG based SCM Demux

f1 f2 f3

Important Parameters:

1. Freq. separation (fi - fj )

2. Slope of the FBG filter

3. Flatness of the filter top

4. Modulation depth

EX: If f2 = 2.4 GHz,

filter BW < 38.6 pm

FBG filter

BW

Transmission Characteristics of an FBG Measured by Agilent 8164A

-10

-9.5

-9

-8.5

-8

-7.5

-7

-6.5

-6

1553.8 1553.9 1554 1554.1 1554.2 1554.3 1554.4

Lo

ss (

dB

)

Wavelength (nm)

Center λ = 1554.184 nm

Δλ = 37 pm 3 dB

Spectrum with 2.4 GHz RF Signal

Conclusions

• Radio over Fiber is an attractive approach for wideband wireless access

• Fiber has ample bandwidth

• Lots of existing dim/dark fiber

• Supporting multiple standards is possible

• Major concerns are– High loss and noise due to concatenated channels

– Nonlinear distortion and limited dynamic range of the ROF link

• Some emerging areas like coherent modulation will improve the situation