1. DIGITAL PULSE INTERVAL MODULATION (DPIM) AS AN ALTERNATIVE MODULATION SCHEME FOR FREE SPACE OPTICS (FSO)

2. Intro to FSO Intra-city Fiber Optic Links
Fiber Optic Cable
Most Metropolitan Area Networks are inter-connected through high speed fiber optic links. The speed currently being offered as we all now is around 10Gbps. He is an example of a map of Southern Ontario & Western New York where the fiber optic cables are connecting cities from Syracuse to Ottawa to Toronto.

3. The Reasoning High-speed Access The Last Mile Problem?
Picture taken from: I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam 785nm and 1550nm in fog and haze for optical wireless communications, Optical Access Incorporated, San Diego
The main issue is high-speed access. An interesting note is that only 5 percent of the buildings in the United States are connected to fiber optic backbone, yet 75 percent of the other buildings are within one mile of fiber, hence the name ‘The Last Mile Problem’. As it can be seen from the diagram, the other building are connected to the fiber optic backbone through T1, ADSL, Cable Technologies through existing underground copper wires but can only achieve speeds ranging from 1.5 MBps – 6 MBps. In order to provide high speed access to the other buildings, fiber optic cables must be buried over the existing copper lines connecting the fiber optic backbone to the buildings. This is a very expensive solution ($10,000/km).

4. The Solution Free Space Optics
Picture taken from: I. I. Kim, and E. Korevaar, Availability of Free Space Optics (FSO) and hybrid FSO/RF systems, Optical Access Incorporated, San Diego
Free Space Optics is exactly the same as regular fiber optic communication except that the medium in which the light travels is not the fiber but free space (air). The transmitter (laser) and the photodetector (photodiode) are placed on building roof tops instead of burying fiber between the buildings. The major difference between fiber optic and free space optic communication is the attenuation. The attenuation in fiber is constant while the attenuation in free space (air) is dependent on weather conditions. Free Space optics can offer similar bit rates as fiber optics. Attenuation in FSO can be as good as fiber optics but it can also be extremely poor such that the link may be broken. More on this later.

5. The Solution (cont’d) High-speed Access (cont’d)
Picture taken from: I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam 785nm and 1550nm in fog and haze for optical wireless communications, Optical Access Incorporated, San Diego
Since a fiber optic link can be replaced by a free space optic link, all other building can now connect to the fiber optic backbone through FSO providing bit rates from 100Mbps to 1.25Mbps. Here all buildings are connected in mesh configuration.

6. The Solution (cont’d) Typical FSO Laser/Photodiode Systems
Photos taken from:

7. FSO Limitations Power Link Budget Equation PTX – Power Transmitted
PRX – Power Received
dTX – Transmit Aperture Diameter (m)
dRX – Receive Aperture Diameter (m)
D – Beam Divergence (mrad)
R – Range (km)
 – atmospheric attenuation factor (dB/km)

8. FSO Limitations (cont’d)
Atmospheric Attenuation
Table taken from: I. I. Kim, and E. Korevaar, Availability of Free Space Optics (FSO) and hybrid FSO/RF systems, Optical Access Incorporated, San Diego
Atmospheric attenuation is the most important factor determining the reliability of FSO. Attenuation is dependent on the weather conditions ( which is dependent on the city). Some Cities are very brutal for FSO (San Diego, CA) and some cities are excellent for FSO (Tuscan, AZ). This table shows the different attenuation numbers for different weather conditions 2 wavelengths. Atmospheric Attenuation dependence on wavelength is very miniscule due to the logarithmic relation. There are other benefits for 1550nm instead of 785nm, which I will discuss in a bit. As

9. FSO Limitations (cont’d)
TX/RX Alignment
TX/RX Misalignment
Unlike fiber optics, the precise alignment between the laser and the photodiode is another important factor for FSO. Here is a picture of laser spot at a very small distance away from the laser. However, as the laser moves through space the size of the spot grows! As it can bee seen here. For example, a 1.5mm laser spot diameter with a divergence angle of 1mrad, expands to a distance 100m away from the laser itself. And around 1km away the spot size can be around 1 – 5 meters. It can also be seen that first spot has its intensity confined in a smaller area than the second one. Obviously, the intensity at the center of the spot is always the highest and decreases as u move further away from the center. In this diagram, the amount of intensity/power being received from the laser beam is from the center. If there is any misalighnment, the power being received from the laser will not be from the center which will severely reduced the focused intensity. In this picture, a camera was mounted with the receiver to actually see the laser spot movements within the receiver sub-system.
Picture taken from: TD. A. Rockwell, and G. S. Mecherle, Optical Wireless: Low-cost, Broadband, Optical Access,
Fsona Communication Corporation, Richmond, BC

10. Limitation Solutions RF Back-up (Hybrid FSO/RF) Active Beam Tracking
There are many alternative solutions out there to combat the unpredictable atmospheric attenuation and TX/RX misalignment issues. One is the use of a RF back up system in case the optical link goes down. These system are called Hybrid FSO/RF systems. Unfortunately, the RF backup does not provide bit rates anywhere near to what the optical link can offer. For misalignment issues, some manufacturers provide active beam tracking such that when the laser or the photodiode sub-systems begins to go out of alignment, one of the sub-systems adjusts its direction to compensate. Both of these solutions drive up the overall cost of the FSO system.
Active Beam Tracking

11. Limitation Solutions (cont’d)
Increase Laser Power
Higher power received
Higher power per unit area
1550nm instead of 800nm
Increase Average Power Efficiency (APE)
Pulse Modulation Schemes can provide
higher average power efficiency at the
expense of higher BW requirement
Hence, increase Peak-APE
It is always desired to increase the power of any kind of transmitter to combat attenuation. However, there are always limitation in terms of how much power a laser can output. In terms of safety, any wavelength above 1400nm is absorbed by the lens and the cornea of our eyes. Hence, more power is 1550nm.
Even though PM schemes have a higher BW requirement, for OC there is enough BW to allow for this increase

12. Limitation Solutions (cont’d)
On-Off Keying (OOK)
Simplest solution based on intensity modulation
‘0’ – zero intensity, ‘1’ positive intensity
Popular Pulse Time Modulation Schemes for OC
Pulse Position Modulation (PPM)
Pulse Interval Modulation (PIM)

13. Pulse Time Modulation PPM Higher average power efficiency than OOK
Increases system complexity due to symbol-level
synchronization.
DPIM
Higher APE than OOK but a bit lower than PPM
No symbol-level synchronization required
Higher Information capacity
Data encoded as a number of time intervals between
successive pulses
Simplified receiver structure
High Information Capacity means more bits can be added for error correction, etc.

14. Pulse Time Modulation (cont’d)
Table taken from: A.R. Hayes, Z. Ghassemlooy, and N.L. See, The Effect of Baseline Wander on the Performance of
Digital Pulse Interval Modulation, 1999 IEEE

15. Pulse Time Modulation (cont’d)
M = log2L
Picture Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland
As you can see, less power is used for PPM and DPIM since the incoming data is stored in the time interval between 2 pulses. DPIM and PPM also use pulses with a much shorter pulse width. Hence, high power pulsed lasers (such as Q-Switched Lasers) can be used. Therefore, when higher power levels are being transmitted, this can help withstand the high attenuation. Plus any minor misalignment between the TX/RX would not be affect since the intensity of the laser beam away from the center will be higher than normal, hence increase the power received by the photodiode.

16. Pulse Time Modulation (cont’d)
Bandwidth and Power Efficiency Comparisons
Table Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland
For M = 4, L = 16. For PPM BW = 4Rb. For DPIM BW = 2.375Rb,

17. Conclusion Power Increased by DPIM @ the cost of increased BW.
Higher power means more power the high levels of attenuation and misalignment between TX/RX
Major FSO benefit: reliable link connection and/or increased distance between TX/RX for certain cities