SOURCEFORGE » RALF » 2014 Dimensioning and budgeting of TM/TC link

h1. Simulations and results

From the method described previously, we have developed three Matlab programs enabling to compute or visualize the influence of various parameters according to what is assume known or not. 

h2. Computation of the MODCOD and (G/T) required:

Supposing that the specifications and frequency band are known (_Rb, R, f, B_ and _EIRP_ are known), we directly get all the parameters for the system design by following the method described previously.

_+f= 869,5 MHz    B= 250 kHz    PAR= 0.5 W    Rb=500 bits/s    R=50 km:+_

p=. !850M-MODCOD-GoT.png!

_+f= 5 GHz	  B= 500 kHz	EIRP= 1 W	Rb=500 bits/s	  R=50 km:+_

p=. !5G-MODCOD-GoT.png!

We can see from these two simulations that:

* The higher the Bandwidth, the lower the Modcod

* The higher the frequency, the higher the free space loss, and then the higher the (G/T) and EIRP requested to compensate these losses.  

Moreover, we can evaluate the requested gain of the antennas from the (G/T) values. In fact, we have been told by Mr Perrin that the receivers temperature was between 400K and 600K, depending on the quality receiver and the directivity of the antenna (looking toward the ground or not?).

Then, a (G/T) of -27.3 dB corresponds more or less to an isotropic antenna, and a (G/T) of -16.9 dB corresponds to a 9 dB gain antenna, which is easily feasible at the 5 GHz frequency.

h2. Influence of the bandwidth on the EIRP:

If the regulatory frequency for application is known, this tool enables to visualize the tradeoff between the EIRP and bandwidth to fulfil the specifications:

_+f= 869,5 MHz	  Rb=500 bits/s       R=50 km:+_

p=. !850M-BW_vs_EIRP.png!

_+f= 5 GHz          Rb=500 bits/s       R=50 km:+_

p=. !5G-BW_vs_EIRP.png!

These types of curves are especially helpful when it comes to a request of frequency allocation when you have to provide the required EIRP and bandwidth requested for the system. 

The increase of the requested EIRP at the transitions between the modulation comes from the fact that lower lower MODCOD might request higher Eb/N0. This has already been discussed in the part _"Evaluatioon of the required Eb/N0"_.

h2. Influence of the distance on the bite rate:

If the required system design is too constraining for a given specification, and then not achievable, we might lower our expectations and have a tradeoff between the desired bit rate and the range

_+f= 869,5 MHz	B= 250 kHz	PAR= 0.5 W+_

p=. !850M-range_vs_Bit_rate.png!

_+f= 5 GHz	B= 500kHz	EIRP= 1 W+_

p=. !5G-range_vs_Bit_rate.png!

We can notice the bite rate is dropping quickly with the range of transmission. Then, the video transmission can be lost if the aircraft moves just a little bit too far of the limit range.