# Exercise about designing Wimax telecommunications system

Discussion in 'Electronics Homework Help' started by PostMortem, Aug 21, 2012.

1. ### PostMortem

16
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Jul 27, 2012
Hello everyone
I have some questions about an exercise. The exercise is here: http://www.teiath.gr/userfiles/s.athinaios/documents/trapeza_thematwn/2011_12_xeim/feb_12_rh.pdf

It's in greek so i'm gonna try to translate it for you.

α. According to the following picture design a telecommunications system that offers Wimax services in 3,5 GHz to the regions A, B and C.
The specifications of the Wimax transmitter are Power(out)=1W, sensitivity=90dBw and bandwidth=40MHz.
β. Design and explain the radiation pattern of the antenna of the telecommunication centre. (ΤΚ in the picture is the telecommunications centre)
γ. The telecommunications centre is located on 37 49' 9'' N 22 39' 49'' E and it is being used as an telecommunications nodal point broadcasting sattelite programs of the HellasSat that is located on 39 E and is 35000km away from the telecommunications centre, through the Wimax system you designed. Calculate the specifications of the antenna of the nodal point (gateway) if the EIRP is 41 dBw.

I have calculated the specifications in question γ but i don't know how to design and explain the radiation pattern and where to put the reception antenna or what else to do in the design of the telecommunications system...Any help/suggestion would be greatly appreciated! Thank you for your time.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,481
2,830
Jan 21, 2010
I will refrain from saying that it's all Greek to me

I assume the idea is to have a single antenna to cover the clients, and a dish to communicate with the satellite.

The beam width is determined by the nearby customers. You need an antenna that has enough off axis signal for the nearby customers, yet also provides enough signal to the customers at the most distant site.

No specifications are given for the receiving equipment, so I assume they are standard and not mentioned.

My seat of the pants approach would be to determine the minimum gain required to satisfy the distant users, and then see how the nearby users fare. If they get far more than adequate signal I would reduce the beam width slightly in order to balance the signal for all users.

But I don't work in this industry.

I have no real idea of the dish design other than what I have said above. Presumably provide sufficient gain to ensure adequate signal strength, but considering the costs of large structures, don't make it too big

Perhaps you can tell us what you calculated. We have some people here who are far more into this stuff than I and maybe they can take a look at your calculations.

At the very least, your explanation may improve my understanding of the design process.

3. ### PostMortem

16
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Jul 27, 2012
Thank you! That was actually helpful. The calculations that i made include h1 and h2 which are the hights for the antennas (receiving and transmitting) , λ the wavelength for the given bandwidth, F1=square((X1*X2/X1+x2)*λ) where x1 is the distance from the telecommunications centre (TK) to the nearest client and X2 is the distance from the nearest client to the most distant client, FSL, Gain for the parabolic antenna using the formula G=17,8+20logd+20logf, the effective area using the formula Aeff=(λ^2/4π)*G .

I don't know where i have to use these: Pout, sensitivity and the EIRP. I researched the wimax on the internet and the users are supposed to have receiving antennas to their houses so I guess i don't need to put an antenna?? But maybe i have to figure out where to put a transmitter? I really don't know what i need to do for the exercise because our teacher sucks , he never properly solved an exercise like this..Also I don't know how to design the radiation pattern, I know that I need to find the maximum Gain and the angle at the -3dB point but i don't have a formula for it on my notes and i have not idea how to design the lobes..

Also any formulas for the beam width and the minimum gain you mentioned would be helpful

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,481
2,830
Jan 21, 2010
EIRP is (from memory) Equivalent Isotropic Radiated Power.

Essentially it tells you something about the gain of your antenna compared to some mythical antenna that radiates exactly equally in all directions.

For example, an antenna that radiated the power evenly in only one hemisphere would have a gain of 3db, so if your actual transmitter power was 5W, the EIRP would be 10W.

In some applications (maybe not this one) there are limits on EIRP. This means that if you increase the gain of the antenna beyond some point, you need to reduce the transmitter power. Alternatively, it allows the calculation of the transmitter power that will give a certain EIRP.

Again, this is not may area of speciality, so the information may not be helpful.

5. ### PostMortem

16
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Jul 27, 2012
Please anyone I really need some help with the exercise...