1. Warsaw University of Technology
Radio-over-Fiber
Systems
Prof. Bogdan Galwas
Warsaw University of Technology
RoFS-Pforzheim 2007

2. R-o-F – Basic Structure of System
Data transmission between Central Station and Base Station via fiber link
Data transmission between Base Station and Terminal via radio link
Mobile
wave
Terminal
Base
Station
Data
Input
Fiber
Central
Output
Optical Transceiver
Data Output
Data Input
Radio System
Fiber link
RoFS-Pforzheim 2007

3. R-o-F – Basic Structure of System
Central Station transmits optical carriers (fO) modulated at RF (fC & data) over fiber links toward remote base stations
Photodiode PD converts the optical signal into an electrical RF signal (fC,2fC... nfC & data)
RF signal is amplified and transmitted by an antenna (fC,2fC... nfC & data)
RoFS-Pforzheim 2007

4. Outline of lecture: 1. Introduction
2. Optical Transmission of μwave Signals
3. Optical Generation of μwaves
4. Optical- μwave Mixing
5. Examples
6. Conclusions
RoFS-Pforzheim 2007

5. 1. Introduction LD MSM P-I-N EAM EOM
Modulation frequency of of laser diodes LD is limited to the 40 GHz by the internal resonance between the electrons and photons.
The push-pull principle solves partially these problems.
Two types of the external optical modulators are widely used:
The electro-optic (EOM) ridge-type travelling wave LiNbO3 Mach-Zender modulators,
Electro-absorption (EAM) optical modulators.
The new types of travelling-wave PIN photodectors have moved the bandwidth above 100 GHz .
Special constructions of Metal-Semiconductor-Metal photodetectors have bandwidth above 300 GHz. LD
P-I-N
MSM 3 10 30
100
300
f [GHz]
EAM
EOM
RoFS-Pforzheim 2007

6. 2. Optical... Analog optical link (1)
Problem: microwave signal (fRF,PIN,POUT ) is transmitted by an analog optical link.
Fiber
fRF,PIN WN
Photodetector WO
fRF,POUT
fOPT,PT
L,=+j
fOPT,PR
Laser
The simplest technique for the distribution of the RF signal modulated with date is an intensity modulation scheme via direct modulation of laser. We will discuss the overall gain G of the system.
Modulation Transmission Detection
RoFS-Pforzheim 2007

7. 2. Optical... Analog optical link (2)
Attenuation by fiber is simply expressed:
IL [mA]
POPT [mW]
SL [W/A]
POPT(t) t
ID [A]
POPT[W]
RD[A/W]
ID [mA]
Principle of operation of optical analog link with direct intensity modulation of laser optical power.
Gain of analog link:
RoFS-Pforzheim 2007

8. 2. Optical... Analog optical link (3)
An intensity modulation of laser optical power may be realised by external electrooptical modulator.
Laser PO
Fiber
fOPT,PT
L,=+j
fOPT,PR
Photodetector WO
fRF,POUT
fRF,PIN WN
V(t) 1 T t
POPT
SMZ[V-1] V
ID [A]
RD[A/W]
POPT t
I(t)
RoFS-Pforzheim 2007

9. 2. Optical... Analog optical link (4)
The transmission of M-Z modulator can be described as:
In the point of inflexion of the T(V) characteristic there is a long straight line section at V0 = V/2 and with a slope SMZ :
Gain of analog link is proportional to the level of optical power P0 :
RoFS-Pforzheim 2007

10. 2. Optical... Analog optical link (5)
Analog link with external electro-optical modulator offers high gain
Photodetector current [mA]
Gain G[dB]
-10
-20
-30
0,01
0,1 1 10
100 20 30
High SL laser
Typical link
External modulation
Laser modulation
RoFS-Pforzheim 2007

11. 2. Optical Transmission of μwaves (1)
a). A conventional FO link in which the data signal is up-converted by the MMW carrier reference before laser bias current modulation
b) The date signal and carrier are transmitted separately over different FO links. Separation of signals can significantly increase dynamic range
DATA
SIGNAL
PHOTO-
-DIODE
FIBER
LASER
DIODE
OUTPUT
CARRIER
REFERENCE
DATA
SIGNAL
FIBER
LASER
DIODE
OUTPUT
CARRIER
REFERENCE
GAIN
PHOTO-
-DIODE
RoFS-Pforzheim 2007

12. 2. Optical Transmission of μwaves (2)
c) The photodiode is used as W mixer. This solution reduces numbers of elements and local oscillator power
d) The photo-detector output signal is filtered and carrier reference signal is separated, next amplified and directed to the W mixer
FIBER
OUTPUT
GAIN
PHOTO-
-DIODE
DATA
SIGNAL
LASER
DIODE
CARRIER
REFERENCE
FIBER
LASER
DIODE
OUTPUT
CARRIER
REFERENCE
GAIN +
FILTER
DATA
SIGNAL
PHOTO-
-DIODE
RoFS-Pforzheim 2007

13. 2. Optical Transmission of μwaves (3)
e) It is also a conventional FO link in which the data signal is up-converted by the W carrier reference and external modulator is used
f) The structure of the circuit was discussed earlier, external modulator is also used.
DATA
SIGNAL
FIBER
OUTPUT
CARRIER
REFERENCE
LASER
DIODE
PHOTO-
-DIODE EO
MODUL.
DATA
SIGNAL
OUTPUT
CARRIER
REFERENCE
GAIN
FIBER
LASER
DIODE
PHOTO-
-DIODE EO
MODUL. EO
MODUL.
RoFS-Pforzheim 2007

14. 2. Optical Transmission of μwaves (4)
LD - 1=1,3 m
5 GHz DM
x 8
Mux
Demux
40-58 GHz
0-18 GHz
PD - 1=1,3 m
LD - 1=1,5 m
PD - 1=1,5 m
Amp
Very interesting and professional system for transmission signal from GHz millimeter-wave region by optical link.
RoFS-Pforzheim 2007

15. 2. Optical ... Chromatic-Dispersion Effect
Laser optical power is modulated to generate an optical field with the carrier and two sidebands
If the signal is transmitted over fiber, chromatic dispersion causes each spectral component to experience different phase shifts depending on the fiber-link distance L, modulation frequency fRF, and dispersion parameter D[ps/nm.km]
At the PIN output the amplitude of the mm-wave power is given by:
RoFS-Pforzheim 2007

16. 2. Optical ... Chromatic-Dispersion Effect
PRF = 0 at frequency fTO, where N = 1, 3, 5...
Problem: The standard amplitude modulation of optical carriers generates double-sideband signals.
Due to the chromatic dispersion effects the sidebands arrived at the BS are phase shifted.
In consequence periodical fading of PFR is observed.
The techniques of Optical Single-Sidebands OSSB generation have been developed.
RoFS-Pforzheim 2007

17. 2. Optical... Subcarrier Multiplexing
Selective
Terminal
Base
Station
Fiber
Central
Optical Transceiver
Data 1 – f1
M U X fN
Data N
Data 2
Data 1 f2 f1
Data 2 – f2
Subcarrier multiplexing may be used for multichannel transmission
RoFS-Pforzheim 2007

18. 3. Optical Generation...Optical mixing
Photodetector is responsive to the photon flux, is insensitive to the optical phase.
Two optical signals (EM fields): the first signal:
The second signal, local oscillator, ELO, |ALO|, fLO i LO. The signal directed to the photodetector:
Photocurrent I is proportional to the incident power P and detector’s sensitivity R :
- PS and PLO are the powers, is intermediate frequency.
The name of the process: optical mixing, optical heterodyning, photomixing, coherent optical detection.
Signal fS
Intermediate
Frequency fIF
Coupler
3dB, 1800
Local
Oscillator fLO
Photodetector
RoFS-Pforzheim 2007

19. 3. Optical Generation...Two Optical Carriers
Process of optical mixing may be used for generation of microwave frequency signal.
The simplest way is to use 2 lasers with frequency f1 and f2, to transmit the optical signals by fiber to a photodiode and to extract the intermediate frequency fIF.
Data
Laser f2 f1
Coupler
f1, f2,
fopt
Carier
& Data
Amp
f1- f2
fIF
One optical signal may be modulated by data.
The spectrum of optical signals must be “pure”, it is not easy to satisfy this condition .
RoFS-Pforzheim 2007

20. 3. Optical Generation...Two Optical Carriers
It is possible to construct a specially modified distributed feedback semiconductor laser (DFB) in which oscillation occurs simultaneously on two frequencies, for two modes.
f1, f2,
Double-mode
Laser
f1 & f2
fopt
Microwave
Signal
Amp
f1- f2
fIF
Dual-Mode DFB semiconductor laser for generation of microwave signal
The mode separation is adjusted to the desired value by proper choosing the grating strength coefficient.
RoFS-Pforzheim 2007

21. 3. Optical Generation...Two Optical Carriers
The spectral purity of the microwave signal may be really improved by synchronising the laser action.
The master laser is tuned by stable microwave source of frequency f.
Tunable
Master Laser
fOPT  nf
Fiber PD
Slave
Laser 1 f
Laser 2
fOPT + 10f
fOPT - 10f
20f
The slave laser 1 and laser 2 are synchronized for different sidebands: upper sideband fOPT + 10f, and lower sideband fOPT - 10f,
The frequency of output signal is equal to 20 f..
RoFS-Pforzheim 2007

22. 3. Optical Generation of μwaves
Laser on Nd:LiNbO3 electrooptical material placed inside microwave cavity changes its frequency of optical oscillation.
M-Z
Modulator
Date
(fSi  Bi)
fOPT
f0  fm fm
Laser Nd:LiNbO3
Laser inside microwave
cavity
Microwave Generator
f0  (fPi  Bi)
Optical transmitter with Nd:LiNbO3 laser with frequency modulated by Microwave Generator and with external Mach-Zehnder modulator
RoFS-Pforzheim 2007

23. 3. Optical Generation of μwaves
It is possible to transmit reference frequency fREF and to control a frequency of VCO by Phase Detector and PLL system.
With using frequency multiplication process we can obtain every frequency from millimetre-wave region. PD
Amp
fREF, PIN
fOUT= n m fREF
POUT >> PIN
VCO
x n
Frequency
Multiplier m
Divider
Phase
Detector
Complex and universal circuit for optical controlling of frequency from millimetre-wave region.
RoFS-Pforzheim 2007

24. 4. Optical- μwave Mixing (1)
Transmission of an optical power by Mach-Zehnder interferometer may be written as:
Above formula will be the the starting point for a theoretical analysis of nonlinear mixing processes. RF
PIN
Coplanar Line
Planar Optical Waveguide
POUT a) V C A B V b)
TMAX
T(V) V0
RoFS-Pforzheim 2007

25. 4. Optical- μwave Mixing (2)
System to perform optical-microwave mixing process with the use of M-Z modulator
A combiner and bias circuit allow inputting the bias voltage and two alternating sine-form voltages into the modulator.
The amplitude of the first of them, called also the signal, is small. The second signal at the amplitude V2 plays role of a heterodyne and usually V2 >> V1.
Fiber
POUT [W]
Photodiode
Laser P0
Combiner
Filter
V2,f2 V0
M-Z Modulator
f1, f2, 2f1, 2f2, 2f1-f2, 2f2+f1, 2f2-f1
V1,f1
RoFS-Pforzheim 2007

26. 5. Examples (1)
Data
M-Z Modulator
Laser
Amp
.....
f1, f2,... fN f
Fiber
Remote Antenna
Receiver at Base Station
Optical link for transmitting the received signal to the base station.
RoFS-Pforzheim 2007

27. 5. Examples (2)
Data
M-Z Modulator
Laser
Antenna
Amp
.....
f1, f2,... fN f
Fiber
Transmitter at Base Station
Receiver at Remote Antenna
Optical link for transmitting microwave signal to remote antenna.
RoFS-Pforzheim 2007

28. 5. Examples (3)
Fiber m
Picocell
Millimetre-wave
radio signals
Optical coupler
Central
Station
Radio-over-fiber system delivers the broad-band services to the customers by a radio
RoFS-Pforzheim 2007

29. Block diagram of the system which uses dense WDM
5. Examples (4)
By using wavelength division multiplexing WDM techniques into the fiber access network each BS can be addressed by a different wavelength. f0
MUX
M-Z
Modul
Optical
Coupler & Filter

fIF1
LD1 1
fIF2
LD2 2
fIFN
LDN N
Base Station
PD1 xN
PD2
  
Transponder 1
Transponder 2
Block diagram of the system which uses dense WDM
RoFS-Pforzheim 2007

30. 5. Examples (5)
Amp
Base-station fC
fD,D
x N
Laser DFB fD
WDM 2 1
Photodiode
Customer
Unit
Block diagram of base-station circuit with multiplication of carrier frequency for full-duplex, mm-wave fiber-radio network
RoFS-Pforzheim 2007

31. T/R module 156 Mb/s/60GHz Transceiver
5. Example (60 GHz P-MP)
T/R module
Base Station
Central Station
T/R module 156 Mb/s/60GHz Transceiver
E/O system
T/R module BS
T/R module
Point-to-multipoint radio-over-fiber full duplex system transmits data between computer systems
RoFS-Pforzheim 2007

32. 5. Examples (60 GHz P-MP) Central Station λ2 λ1
EAM
EDFA PD LD λ1 λ2
DWDM Mux
Central Station
Base Station
156 Mb/s DPSK Modem
60 GHz Trans-ceiver
LD – Laser diode, EAM – Electro-absorption modulator, EDFA – Fiber amplifier, DWDM Mux – Multiplexer, PD Photodiode
RoFS-Pforzheim 2007

33. EAM – Electro-absorption modulator, PD - Photodiode
5. Examples (60 GHz P-MP)
Base Station λ1
156 Mb/s DPSK Modem
60 GHz Trans-ceiver PD λ2
EAM
156 Mb/s DPSK Modem
60 GHz Trans-ceiver
EAM – Electro-absorption modulator, PD - Photodiode
RoFS-Pforzheim 2007

34. The last experimental system
5. Examples (125 GHz/10 Gb/s) PD
EDFA
EOM LD
fM=62,5 GHz
fOPT f0 fM
2fM
DATA
125 GHz Receiver
DATA
Terminal
The last experimental system
RoFS-Pforzheim 2007

35. 6. Conclusions
Photonic technology opens new possibilities to generate and to transmit the microwave signals, especially in millimeter-wave region
New wideband communication systems are developed on the basis of mm-wave and optical technologies
The gap between what is theoretically possible and what we experimentally demonstrated has narrowed considerably in the last decade
RoFS-Pforzheim 2007