Low Cost RF over Fiber Systems for Wireless LANs - MITcfp.mit.edu/publications/CFP_Presentations/Jun04/Penty... · 2004. 6. 22. · 256-QAM Transmission Fig: 256-QAM constellation

Centre for Photonic Systems

UNIVERSITY OFCAMBRIDGE

Low Cost RF over Fiber Systems for Wireless LANs

Ian White, Richard Penty, Peter Hartmann and Xin QianPhotonic Communication Research, Department of Engineering, University of Cambridge,

Trumpington Street, Cambridge, CB2 1PZ, United Kingdom

Alwyn SeedsDepartment of Electronic and Electrical Engineering, University College London,

Torrington Place, London, WC1E 7JE, United Kingdom



OutlineOutline

1. Introduction1. Use of RF of fiber systems in antenna remoting

2. Review of RF over fiber advances in recent years1. Use of low cost uncooled lasers2. Successful transmission of RF signals over both

single and multimode optical fiber3. Multiservice distribution

3. Future challenges1. Hybrid fibre data/radio networks2. High bandwidth (~10 Gb/s) short haul wireless links

4. Conclusions and future work



Introduction: Distributed Antenna System (DAS)Introduction: Distributed Antenna System (DAS)

Customers: demand for 100% indoor mobile coverage, especially in office buildings but also in locations such as airports, shopping malls, …

Distributed Antenna System (DAS)

Coverage: Penetration from outdoor coverage is unreliable and insufficient, so installation of indoor system is inevitable.

Network operators: want to provide this coverage, but at low installation and maintenance cost.

!!!!Analogue fibre-optic DAS



Introduction: FibreIntroduction: Fibre--optic DASoptic DAS

Analogue fibre-optic DAS

Low installation cost

• For d>100m, installation of fibre is preferred to coaxial cable

• Preinstalled fibre base may be used

• Directly transmitting RF signals significantly reduces cost and complexity of remote antenna units (RAU)

Low maintenance cost

• RAU is inherently transparent to transmitted signals

• Technology updates only have to be carried out at central distribution units (CDU)

• No need to modify or even replace RAU



StateState--ofof--art Laser Performanceart Laser PerformanceFig: L-I characteristic for 25…85 °C Fig: Optical spectrum at 60mA and 85 °C

• Threshold current is 6mA at 25°C and 31mA at 85°C• Slope efficiency at 60mA is 0.143 W/A at 25°C and 0.054 W/A at 85°C• Single-mode operation over the 25°C - 85°C range, with typical SMSR of 45dB

1308 1310 1312 1314 1316 1318 1320 1322-80

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-10

0

Waveleng th [nm]

Opt

ical

pow

er in

0.1

-nm

ban

dwid

th [d

Bm

]

85 °C

45 dB

0 10 20 30 40 50 60 70 80 90 1000

2

4

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10

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14

Forward current [mA]

Opt

ical

pow

er fr

om fi

ber p

igta

il [m

W]

25 °C

45 °C

65 °C

85 °C



Linearity Assessment: Frequency Response and IntermodulationLinearity Assessment: Frequency Response and Intermodulation

Two-tone setup: Tones separated by 1MHz, Centre frequency from 1GHz to 20GHz Power swept from –15dBm to +10dBmLaser biased at 60mA, package temperature of 25°C and 85°C

Fig: Fundamental and IM3 at 25°C Fig: Fundamental and IM3 at 85°C

1 2 3 4 5 6 7 8 9 10-100

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Ce ntre fre que ncy [GHz]

Out

put e

lect

rical

pow

er [d

Bm

]

Fundame ntal

Third-orde r inte rmodulation

85 °C

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0

Ce ntre frequency [GHz]

Out

put e

lect

rical

pow

er [d

Bm

]

Fundame ntal

Third-orde r inte rmodulation

25 °C



1 2 3 4 5 6 7 8 9 1070

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Ce ntre fre que ncy [GHz]

Spu

rious

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dyn

amic

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e [d

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z2/3

] 85 °C

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Centre frequency [GHz]

Spu

rious

-free

dyn

amic

rang

e [d

B H

z2/3

]

25 °C

Linearity Assessment: SpuriousLinearity Assessment: Spurious--Free Dynamic Range (SFDR)Free Dynamic Range (SFDR)

Highest SFDR-values reported so far for uncooled DFB lasers

25°C SFDR is greater than 103dB·Hz2/3 over 1-20GHz range, max = 114dB·Hz2/3 (4GHz)SFDR decreases towards higher frequencies, because it incorporates link gain

85°C SFDR is greater than 90dB·Hz2/3 over 1-10GHz range, max = 104dB·Hz2/3 (2GHz)SFDR shows a minimum in the vicinity of the relaxation frequency

Fig: SFDR at 25°C Fig: SFDR at 85°C



256256--QAM TransmissionQAM Transmission

Fig: 256-QAM constellation diagram after transmission over 15km SMF

Fig: EVM of DFB-laser as a function of SMF link distance

• Transmitted signal is 10MS/s 256-QAM, carrier amplitude of –10dBm• Fibre-optic link assessed for laser-temperatures of 20°C and 70°C and carrier

frequencies of 2GHz and 5GHz• EVM < 1.9%• Increase in EVM < 0.1% / km SMF• Elevated temperature results in only a small excess EVM

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SMF distance / km

EV

M /

%

2GHz, 20°C5GHz, 20°C2GHz, 70°C5GHz, 70°C



Transmission of 3GPP WTransmission of 3GPP W--CDMACDMA

Fig: QPSK constellation diagram of single user-channel after transmission over 15km SMF

Fig: EVM of DFB-laser as a function of SMF link distance for 3GPP W-CDMA uplink and downlink signals

Standard test pattern no. 1• 16 QPSK modulated user-channels, each 30kb/s• Several low-rate synchronisation channels• carrier frequencies of 1.93GHz (uplink) and 2.14GHz (downlink)

0.0

0.5

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0 5 10 15 20

SMF distance / km

EV

M /

%

1.94GHz, 20°C2.13GHz, 20°C1.94GHz, 70°C2.13GHz, 70°C

• EVM requirement for 3GPP W-CDMA: 17%• EVM after transmission over 15km SMF: < 2.1%



MultiMulti--Antenna Remoting using Single Mode FibreAntenna Remoting using Single Mode Fibre

0

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0 20 40 60 80 100 120 Number of antennae

EVM

[%]

QPSK16QAM32QAM64QAM

Bias T

35 mA

M-QAM

Splitter

Splitter

Splitter

PolarisationController SOA

Single Mode Fibre

O/E

O/E

O/E

O/E

Transmitter

Bias T

35 mA

M-QAM

PolarisationController SOA



Long Haul Remoting using Amplifier SMF LinksLong Haul Remoting using Amplifier SMF Links

0

5

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25

0 20 40 60 80 100 120Fibre Length [km]

EVM

[%]

Without SOA

With SOA at the end ofthe linkWith SOA after 50km

SOA – Semiconductor Optical Amplifier



Summary of SMF ResultsSummary of SMF ResultsTwo-tone intermodulation experiments• 25°C SFDR>100dB·Hz2/3 IIP3>20dBm for 1-20GHz range• 85°C SFDR>90dB·Hz2/3 IIP3>18dBm for 1-10GHz range

!Highest values reported so far

Transmission of wireless signals• If operation range of laser is properly chosen, the device adds only little

excess EVMSignal EVM requirement EVM after 15km SMF256-QAM N/A < 1.6% at 20°C,



RF over MultiRF over Multi--mode Fibremode Fibre

frequency, GHz

rela

tive

resp

onse

, dB

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MULTIMODE FIBRE RESPONSE (1km; 1300nm)

3G

802.

11b

802.

11a



IEEE 802.11b WLAN MMF DemonstratorIEEE 802.11b WLAN MMF Demonstrator

Schematic of the WLAN demonstrator setup

-10dBLD DL PD DL

PD UL LD UL+30dB

+30dB

Uplink(MMF)

EthernetSwitch

AccessPoint

Downlink(MMF)

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Bias Current [mA]SN

R [d

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1m MMF100m MMF400m MMF700m MMF1000m MMF



Multiservice Transmission of IEEE 802.11a/gMultiservice Transmission of IEEE 802.11a/g

+

Wired LAN

.11a

.11g

+

RF combiningnetwork

Local transceiverunit

Remote antennaunit

.11a.11a

.11g

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of 5

4Mbp

s O

FDM

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nal [

%]

20°C

70°C

CoaxComposite EVM of IEEE 802.11a signal from wireless access point within IEEE specification up to 1375m MMF link length



Future OpportunitiesFuture OpportunitiesHybrid Optical/RF networks – an optimum scenario?

Fibre Core

Distributor

Transponder

Transponder

Fibre Radio Interface

Distributor500 Mb

it/s

1 Gbit

/s

30

250 m

100

300 m

104

m

100700

m

245 m

100700 m

245 m

100700 m

245 m

100

700 m 245 m

Distributor

...100 M

bit/s

100 Mbit/s

20 Mbit/s

1 Gbit/s

1 Gbit/s

30

300 m

500 Mbit/s100 Mbit/s

REACH architecture

10 Gbit/s

Fibre Radio Interface

1 Gbit/s

104m

Distributor

100 Mbit/s

...

- Use of core fibre network with 10 Gb/s link rates

- Use of directed UWB RF links to achieve low cost large bandwidth last drop to the user

-Use of RF over fibre techniques to allow raw transmission to the edge of the fibre network

-Particular interest in UWB or 20,40,60 GHz bands



Future OpportunitiesFuture Opportunities

Scope of Hybrid Optical/RFNetwork

1. End-drop bit rate to the user of 100 or 500 Mb/s. 2. Achieved by demuxing a 10 Gbit/s fibre channel rate to

10 x 1 Gbit/s rates. 3. The 1 Gbit/s downstream rate is further split into two

500 Mbit/s streams. 4. Stream one delivers 500 Mbit/s over 300 m (radio free

space) distance to a single user. 5. Stream two at 500 Mbit/s provides 100 Mbit/s per user to

5 users through 5 beams; with 300 m range.

Fibre Core

Distributor 1 Gb

it/s50

300 m

104

m

1 Gbit/s

30

300 m

500 Mbit/s100 Mbit/s

1 Gbit/s

104m

Distributor

100 Mbit/s

..



3G

+

+ RF combiner

Local unit

Remote unit

10GbE

10Gbit/s

Music

Movie

Data

Bias T

Bias

RF

Ultra-high speed wireless access



0.40.0 0.8 1.2 1.6 2.0 2.4 2.8

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frequency / GHz

resp

onse

/ dB

• Good quality 200 Mb/s eye diagrams achieved for carrier frequencies up to 3.8 GHz i.e. 20 x F3dB

1.4 GHz 2.6 GHz 3.8 GHz

1.5ns/div

• Baseband channel transmission is limited by the bandwidth of the first null

• Propose to transmit several channels which are located in the passband regions of the fibre

Passband Transmission at 20x the Fibre Bandwidth



5Gb/s Baseband Channel

5Gb/s SCM Channel

Back-Back (50mV/div, 150ps/div)

Back-Back (40mV/div, 150ps/div) After 300m (25mV, 150ps)

After 300m (50mV, 150ps)

• 5Gb/s signals are not adversely affected by transmission over 300m of 62.5µm MMF

10 10 Gb/sGb/s MMF Link DemonstrationMMF Link Demonstration



• Subcarrier Multiplexing can be employed to achieve the ultra-high data rate

• 5Gbit/s/channel x 2 channels = 10Gbit/s• 2.5Gbit/s/channel x 4 channels = 10Gbit/s• 1.25Gbit/s/channel x 8 channels = 10Gbit/s

• M-QAM modulation schemes can also be utilised to improve the spectral efficiency and potentially further increase the capacity

• E.g. for 16QAM, 625MHz/channel x 4bit/Hz x 4 channels = 10Gbit/s

802.11b/g2G 3G

2GHz 2.5GHz1.5GHz

802.11a

5GHz 6GHz5.5GHz3GHz 10GHz 10.5GHz

UWB

-41.3dBm

Implementation using Ultra-wide Band Space



0

24

68

10

1214

1618

20

Channel 1 Channel 2 Channel 3 Channel 4

EVM

[%]

Fibre 1 16QAMFibre 2 16QAM

Reference Value

EVM of 10Gbit/s transmission over a typical MMF of 300m length

Output constellation and eye diagram of 16QAM

• Channel distribution • Channel 1: 4.0GHz

• Channel 2: 6.5GHz



Input constellation and eye diagram of 16QAM

Modelling results of SCM transmission characteristics



ConclusionsConclusionsWe have shown that …

• … state-of-the-art uncooled DFB lasers show very high linearity performance at both high temperatures and high frequencies

• … few km transmission is possible over MMF and 100 km over SMF

We wish to study whether …… hybrid optical RF links will provide an optimum wireless distribution network

for broadband data… what are the limits to bandwidth… how dynamic bandwidth allocation is achieved... what impact may result from WDM… how can data and wireless networking be unified at the physical level



AcknowledgementsAcknowledgements

• UK EPSRC/DTI LINK FRIDAY and WOWS projects

• Agilent Technologies and Bookham for providing some of the laser diodes used in these projects

• Amardeep Uppal of Agilent Technologies for providing the loan of the Agilent E4438C VSG and the Agilent 89640 VSA.

• Professor J M Elmirghani, University of Swansea for collaborations on the hybrid optical/RF architecture

Low Cost RF over Fiber Systems for Wireless LANs - MITcfp.mit.edu/publications/CFP_Presentations/Jun04/Penty... · 2004. 6. 22. · 256-QAM Transmission Fig: 256-QAM constellation

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