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

h1. Constraints for the physical layer and RF equipment

From the given allocated frequency band, the following parameters are defined:

* f : Central frequency of the emitted signal

* B: Width of the allocated bandwidth

* EIRP: Maximum power that can be emitted in a given direction 

From the specifications, the following parameters are defined:

* Rb: Useful bit rate of the transmission

* R : Minimal distance for the transmission

The value of these parameters constrain the parameters of the physical layer and the RF equipment for the design of the system.

h2. Required bit rate

In order to define the bit rate required for the transmission of video streams we need some input parameters, provided in the mission statement:

> # aspect ratio of the video : AR=1,78 

> # quality of the encoded video L=720p

> # frame cadence FPS=12 fps

We used the following formula

p=. Bit rate = FPS*5,0692 * L ^1,391^ /(1000*AR) 

So 

p=. *Bit rate=322 Kbps*

However, we will use an adaptated encapsulation protocol to support the video transmission. Then, we have to take into account headers bits, and we decided to round the bit rate up to 500 kbps.

h2. Physical layer:

The study of the physical layer will be limited to the choice of the modulation, the coding and the shaping filter. We will consider a SRRC filter (Square Root Raised Cosine) for the shaping filter as it is commonly used in telecommunication systems for its good performances. 

Then, the parameters of the physical layer are:

* M : Modulation (_M=4 : QPSK, M=8 : 8PSK etc_)

* $\rho$ : Coding rate ($\rho <1$)

* $\alpha$ : roll-off of the SRRC filter

In fact all these parameters are linked through the spectral efficiency _T_ of the system, which is fixed by B and Rb:

p=. $\tau = \frac{R_{b}}{B}$ and $\tau =\frac{\log_{2}(M)\rho}{1+\alpha}$

Then, the parameters of the physical have to comply with the following relation:

p=.  $\frac{\log_{2}(M)\rho}{1+\alpha}>\frac{R_{b}}{B}$

h2. Link budget:

Here is the expression of the link budget:

p=. $(\frac{E_{b}}{N0})_{dB}=(\frac{G}{T})_{dB}+10\cdot log_{10}(EIRP\cdot (\frac{\lambda}{4\cdot \pi\cdot R})^{2}\cdot\frac{1}{k\cdot R_{b}\cdot L_{marg}})$

We can notice that all the parameters are already known, except:

* (G/T): Figure of merit of the receiver (ISAE antenna)

* _Lmarg_: Margin on the link budget to take into account all the perturbations (antenna depointing, atmosphere attenuation, interferences, non-ideal demodulator …)

_Lmarg_ being only linked to physical parameters, we don’t have any influence on it. Then, it has to be evaluated but it is not really a parameter of the design.