radio over fiber

1. Radio over Fiber and
Fiber-Wireless (Fi-Wi)
Systems

2. Cell
BS
Y
BS
Y
Current Macro Cellular Networks
BS
Y
BS
Y
Service area is divided into several Macro cells
Each cell is served by one base station
Several kilometers of cell radii
To PSTN

3. • More frequency-reuse & high capacity
• Wideband access (less multipath fading)
• Low power transmission, especially for 5G
• Coverage to special areas
• tunnels, mines, super markets etc.
• Support mm-wave bands
Micro and Pico Cells offer:
Air interface should be short for
Really broadband wireless access

4. Radio over Fiber (ROF) Solution
E/O Converter Fiber
Several low power Radio Access Points fed by fiber

5. ROF Architecture
• Optical fibers transmit the RF signal between
central-base station (CBS) and low power
Radio Access Point (RAP).
• The RAP then transmits the RF signal to
customer units through air.
• The RAPs only implement optical to RF
conversion and RF to optical conversion.
• No DSP(Digital Signal Processing) at RAP to
keep it simple

6. ROF in Sydney Olympics 2000
Tekmar BritecellTM
> 500 Remote Antennae
Over 500,000 wireless calls
Multi operator system (3 GSM
operators)
Multi standard radio (900/1800
MHz)
Dynamic allocation of network
capacity
In building and external Pico
cells

7. Some Existing Fi-Wi Systems

8. The ROF Link
Basics

9. A Closer Look….
Y
Single
Mode
Fiber
Baseband
Data
Baseband-RF
Modulation
RF-Optical
Modulation
Optical - RF
Demodulation
Gain
BPF
Antenna
200 THz
1.8 GHz
RF-Baseband
Demodulation
Y
Baseband
Data
Radio
Access
Point
Central
Base
Station
Portable
Unit
Regular
Wireless
System
Two Channels in Series

10. RAP Bridges Two Channels
• The Radio Access Point amplifies and
retransmits the RF signal (downlink)
• Cumulative SNR is the sum of two SNR’s
– Optical Channel SNR (OSNR)
– Wireless Channel Electrical SNR (ESNR)
OSNR ESNR

11. • The noise is added twice (at the optical and
wireless receivers) where the signal is weak.
• The overall SNR is the weighted sum of the
two SNRs and smaller than the smallest SNR.
Cumulative SNR

12. Two SNRs
• Week signal plus noise is amplified and
transmitted at the RAP
• More noise added in the air and at the
portable receiver
• Both signal and noise go through wireless
channel loss
• Optical and Radio noises dictate the SNR
• Acceptable SNR at the cell boundary dictates
the cell size

14. Losses and Gains
Optical output power from the laser:
• Gm is the modulation gain of the laser
• Zin is the input impedance.
• PRF;laser is the RF input power to the laser.
• The factor 1/2 comes because:
• the output optical power is proportional to
the driving current, while the input electrical
power is proportional to the square of the
driving current.

15. Optical output power from the laser can also be given as:
Optical link loss:

16. Received optical signal at the photo detector is:
Detector current is,
M is the avalanche gain
Zout is the output impedance of the receiver

17. Hence the optical link loss (seen in electrical domain)
= loss in the modulation/demodulation
+ loss in impedance mismatch
+ loss in fiber link (α dB/km) appears twice in the
electrical domain
• Total loss in the optical link (in dB) is:

18. Therefore the relationship between RF input & output power is:
Hence, the Optical SNR is,

20. OSNR Vs Fiber Length
B = 1.25 MHz,
RIN = -155 dB/Hz,
R=0.75 A/W,
α=0.5 dB/km

22. The received signal power is,
The received noise power is,
Hence the Cumulative SNR is,

23. If we assume the optical noise and electrical noise
powers are almost equal,
Worst case SNR will happen at the cell boundary

24. Optical Receiver Amp. Gain Vs
Wireless Path Loss
L = Path loss to the cell
boundary

25. SNR Sphere
• We can imagine an OSNR sphere around the RAP.
• At the sphere boundary Lwl = Gop.
• Well within this sphere (Lwl << Gop), the optical noise will
be dominant (the cSNR will at best be close to the OSNR).
• When the user is far away from the RAP, (Lwl >> Gop), cSNR
will decrease with the distance as usual.
• Note, at the sphere boundary cSNR = OSNR/2.

26. Some Observations
• There is an inherent inverse relationship
between the radio cell size and the fiber
length
• Loss and noise in the ROF link plays significant
role in system performance.
– If OSNR is bad, even if you are really close to the
antenna, performance will be poor
• There is circle surrounding RAP.
– Within the circle OSNR is limiting
– Out of this circle ESNR is limiting