Free Space Optics (FSO)

[Remo De Suza]

Hello Everyone.
I had posted my query some days ago but its very unfortunate that i did not get any replies. I am reposting it and i hope this time around i get some responses from you people.
Actually I have to design Simulation Of DWDM For RZ, NRZ and Coherent QPSK Modulation for free SPACE OPTICAL COMMUNICATION. In my work I analyze that:
Coherent QPSK Modulation is Best Among RZ AND NRZ for Free Space Optical Communication.
Either Increase in Transmitter Power of Aperture area of the Tx and Rx will significantly improve BER.
Compare to PIN diode, APD improves BER as Receiver Sensitivity is improved.
As we increase our aperture diameters Q factor increases but for a particular aperture diameter Q factor starts decreasing as we increase the link distance and bit rates.
Can anyone explain why this is happening?

Thanking you people in advance.

With Regards
Remo De’Suza

[prakash jat]

hi remo

1. coherent detection is better than direct detection so coherent qpsk is better than direct detection rz or nrz
2. definately increase power will improve in ber as well as increase in aperature area will increase more power collection so it will also improve ber.
3. apd will genrate 100 times more current than pin ( because of device characterstic)so definately it (apd) will have more increamnet in ber

[Remo De Suza]

Hello prakash jat. A big thank you for your concern.

Well here i would like to ask you that what is the maximum transmission power you have taken for your system. also can you tell me what distance you are transmitting wirelessly?

One more question , which channel model are you using for your system?
looking forward for you response. Thanking you.

With regards
Remo

[prakash jat]

hi remo
1. we should use class 1 type of laser so we can use < 10 mw power in case of 1550nm and < .22 mw in case of 850nm
2. distance is determined by system parameter ( ex. min. BER , power , etc)
3. gamma- gamma model ( include both weak and strong turbulence

[jyoti raina]

Hello remo Sir..
What is your exact topic for FSO i mean the research topic.. I am Also interested in going for FSO in future..
I think it is a good field to work in..Can you provide me some guidance in that regard.. I will appreciate that.

With Regards
Jyoti Raina

[Remo De Suza]

Hi Jyoti.
I am currently trying to implement an already implemented design. Just trying to increase the performance in other words just trying to increase the quality of service and other parameters by trying out different things. I tell you there is lot to be done in this field. It is still in its infancy stage. lot of research is being done in this field. I think it is a good choice to opt for.

with regards
Remo De’suza

[Dr. Dhiman Kakati]

Hi Remo De Suza, I want to mention one important point that we should not compare RZ and NRZ are bit encoding technique where bits are converted to respective electrical pulses (polar or non polar) it is not exactly a modulation scheme as we do not apply any other carrier here, on the other hand QPSK modulation will definite have some distinct advantages such as bit rate and spectrum efficiency, You can analyze RZ and NRZ encoding scheme inside the QPSK modulator and look for performance difference. Each of them has their specific advantages.

Regards,
Dhiman

[jyoti raina]

hello Dhiman sir..
Thanks for sharing it.. Iwas of the opinion that RZ and NRZ are modulation techniques because when we check parameters of our simulation it shows in modulation tab two options RZ and NRZ..that gave me the impression that both are modulation techniques.. Anywayz thanks for sharing it..
Indeed it was very helpful to me.

With Regards
Jyoti Raina

[Dr. Dhiman Kakati]

You are most welcome jyoti.

Regards,
Dhiman

[Dr. Dhiman Kakati]

Hi Jyoti,
Regarding your no. 36124, Here a few lines are showing but in my mail ID inbox something more is appearing. Let me know why it is happening. The following lines are showing in my Gmail mail inbox.

“”
jyoti raina wrote:

hello Dhiman sir..
Thanks for sharing it.. Iwas of the opinion that RZ and NRZ are modulation techniques because when we check parameters of our simulation it shows in modulation tab two options RZ and NRZ..that gave me the impression that both are modulation techniques.. Anywayz thanks for sharing it..
Indeed it was very helpful to me.
In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition. The pulses in NRZ have more energy than a return-to-zero (RZ) code, which also has an additional rest state beside the conditions for ones and zeros. NRZ is not inherently a self-clocking signal, so some additional synchronization technique must be used for avoiding bit slips; examples of such techniques are a run length limited constraint and a parallel synchronization signal.

For a given data signaling rate, i.e., bit rate, the NRZ code requires only half the baseband bandwidth required by the Manchester code (the passband bandwidth is the same). When used to represent data in an asynchronous communication scheme, the absence of a neutral state requires other mechanisms for bit synchronization when a separate clock signal is not available.

NRZ-Level itself is not a synchronous system but rather an encoding that can be used in either a synchronous or asynchronous transmission environment, that is, with or without an explicit clock signal involved. Because of this, it is not strictly necessary to discuss how the NRZ-Level encoding acts “on a clock edge” or “during a clock cycle” since all transitions happen in the given amount of time representing the actual or implied integral clock cycle. The real question is that of sampling—the high or low state will be received correctly provided the transmission line has stabilized for that bit when the physical line level is sampled at the receiving end.

However, it is helpful to see NRZ transitions as happening on the trailing (falling) clock edge in order to compare NRZ-Level to other encoding methods, such as the mentioned Manchester code, which requires clock edge information (is the XOR of the clock and NRZ, actually) see the difference between NRZ-Mark and NRZ-Inverted.

Contents [hide]
1 Unipolar non-return-to-zero level
2 Bipolar non-return-to-zero level
3 Non-return-to-zero space
4 Non-return-to-zero inverted
5 See also
6 References
Unipolar non-return-to-zero level[edit]
Main article: Unipolar encoding

Unipolar non-return-to-zero level
“One” is represented by a DC bias on the transmission line (conventionally positive), while “zero” is represented by the absence of bias – the line at 0 volts or grounded. For this reason it is also known as “on-off keying.” In clock language, a “one” transitions to or remains at a biased level on the trailing clock edge of the previous bit, while “zero” transitions to or remains at no bias on the trailing clock edge of the previous bit. Among the disadvantages of unipolar NRZ is that it allows for long series without change, which makes synchronization difficult – although this is not unique to the unipolar case. One solution is to not send bytes without transitions. More critically, and unique to unipolar NRZ, are issues related to the presence of a transmitted DC level – the power spectrum of the transmitted signal does not approach zero at zero frequency. This leads to two significant problems – first, the transmitted DC power leads to higher power losses than other encodings and second, the presence of a DC signal component requires that the transmission line be DC coupled.
With Regards
Jyoti Raina””

[SAHIL SINGH]

Hi Dhiman Kakati,

It is a very strange thing that you have come across, I don’t know what it really means.. May be Jyoti Raina might have modified her comment one or two times after posting and that might be the reason as there is somelog of the modificattion which you make in the optiwave forum.. Hope to hear from jyoti on this issue…

Regards
Sahil Singh

[Dr. Dhiman Kakati]

Hi Sahil This may be the issue you are correctly pointed out. anyways the answer 1st added by jyoti was also useful. but these contents may be from somewhere else Thats why she modified her answer. Thank you jyoti for providing so much of information.

Regards,
Dhiman

[jyoti raina]

Hello Dhiman Sir..
It was by mistake i posted it here..i was compiling a doc file simultaneously and instead to post it there it got posted here.. Sorry for that..
NO issues at all.

With Regards
Jyoti

[SAHIL SINGH]

Hi Dhiman kakati,

Well maybe i have pointed it out correctly… Well i agree with you that the the answer 1st added by jyoti raina was also indeed useful…. Thanks jyoti for the information

Regards,
Sahil Singh

[Ranjeet Kumar]

Hi Remo De Suza,
I will suggest you to use advanced modulation formats like: CSRZ, DRZ and MDRZ at transmitter end.
Because CS-RZ format has high tolerance to the mixed effect of self phase modulation(SPM)and group velocity dispersion(GVD)andhasnarrowerpedestalshapeof the opticalspectrumthantheconventionalRZformat.
IncaseofCSRZ transmitter,the NRZ optical signal after MZ modulator goes through phase modulator, The
duobinary precoder used here is composed of an
exclusive-orgatewithadelayed feedback path.DRZ
formats are very attractive,because their optical
modulation bandwidth can be compressed to the data
bit rateB,thatis,thehalf-bandwidthoftheNRZ
format2B. The generation of MDRZ signal is almost identical to the
DRZ signal,exceptthedelay-and-addcircuitisreplaced
by adelay-and-subtract circuit. In the duobinary signal
used earlier where thephase of bits‘1’s are modified
only after a bit‘0’appear where as in the modified
duobinary signal the phase is a lternated between 0 and p
for the bits‘1’.The phase of all the‘‘zero’’bits are kept
constant and a 180 degree 1 phase variation between all the
consecutive‘‘ones’’is introduced [5]. A l so optical signal
spectrum Fig.1(3b) shows that th ecarrier of the
duobinary signal has been suppressed.

[jyoti raina]

Hi Ranjeet..
Thanks for sharing some more information.. Indeed it is helpful.

[Remo De Suza]

Thank you All for your efforts and the concern you have shown.
The replies were all helpful and i am thankful for that to each and everyone of you.
It was rather a fruitful discussion.

With regards
Remo De’Suza

[SAHIL SINGH]

Hi Remo De Suza,

You are welcome and i am glad to know that you got your query resolved on the forum.. It no doubt was a fruitful discussion with a lot of useful information… All the best

Regards
Sahil Singh

[Manoj Kumar]

{REPLY DELETED BY ADMIN – DO NOT COPY/PASTE RESPONSES FROM OTHERS}

[Rajguru M. Mohan]

Hi Remo,

As FSO is line-of-sight technology approach uses invisible beams of light to provide optical bandwidth connections. It’s capable of sending up to 1.25 Gbps of data, voice, and video communications simultaneously through the air — enabling fiber-optic connectivity without requiring physical fiber-optic cable. It enables optical communications at the speed of light.
Originally developed by the military and NASA, FSO has been used for more than three decades in various forms to provide fast communication links in remote locations.
FSO technology is surprisingly simple. It’s based on connectivity between FSO-based optical wireless units, each consisting of an optical transceiver with a transmitter and a receiver to provide full-duplex (bi-directional) capability.
Each optical wireless unit uses an optical source, plus a lens or telescope that transmits light through the atmosphere to another lens receiving the information.
At this point, the receiving lens or telescope connects to a high-sensitivity receiver via optical fiber.
Thanks,

[Rajguru M. Mohan]

Hi,
here are some ADVANTAGES OF FSO SYSTEMS

No licensing required.
Installation cost is very low as compared to laying Fiber.
No sunk costs.
No capital overhangs.
Highly secure transmission possible.
High data rates, upto 2.5 Gbps at present and 10 Gbps in the near future.

Disadvantage of Fso Systems are:
Absorption and scattering can both occur whenever there is a lot of moisture in the air. Absorption of the signal causes a decrease in signal strength. Scattering does not cause a decrease in signal strength, but rather causes the signal to be sent off in different directions. This is an issue particularly over long distances.

Thanks,

[Hamza Ali Abbas Khan]

Hello All,
As far as your query is concerned I will suggest you to use advanced modulation formats like: CSRZ, DRZ and MDRZ at transmitter end because CS-RZ format has high tolerance to the mixed effect of self phase modulation(SPM)and group velocity dispersion(GVD) and has narrow erpedestal shape of the optical spectrum than the conventional RZ format.
Also in case of CSRZ transmitter,the NRZ optical signal after MZ modulator goes through phase modulator, The duobinary precoder used here is composed of an exclusive-or gate with a delayed feedback path. DRZ formats are very attractive,because their optical modulation bandwidth can be compressed to the data bit rate B,that’s,the half-bandwidth of the NRZ format2B. The generation of MDRZ signal is almost identical to the DRZ signal,except the delay and add circuit is replaced by a delay-and-subtract circuit. In the duobinary signal used earlier where the phase of bits‘1’s are modified only after a bit‘0’appear where as in the modified duobinary signal the phase is a alternated between 0 and p for the bits‘1’.The phase of all the‘‘zero’’bits are kept constant and a 180 degree 1 phase variation between all the consecutive ‘ones’ is introduced.
I hope you find it helpful.

Thanks

[Remo De Suza]

Thanks Rather for the information.

Regards

[Author]

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