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

h1. Constraints and parameters for the system design

* 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*

As the link will allow video transmission, we will need an adapted encapsulation protocol, so we must take into account extra bits: we decide to define a bit rate of 500 kbps. 

   # Network constraints

> * Frame format

The application foreseen aims at transmitting multimedia content (video on the downlink), so we choose to use the MPEG-4 coding standard. The associated frame format MP4 is particularly adapted for the encapsulation of multimedia data type.

> * Protocol stack

   # Antenna choice

> * On board antenna

The antenna placed on the aircraft must be compliant with the flight constraints listed in the introduction. Indeed, we should maintain the communication link while the aircraft is performing a turn ( +/- 15° attitude), landing or taking off ( +/- 45° inclination). Therefore, he position of the antenna must be carefully determined in order to be at LOS. 

Another constraint is the embedded 

> * Ground station antenna

h2. Constraints for the physical layer and RF equipment:

(calcul Rb)

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

* f : Central frequency of the emitted signal

* B: Larger 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.

h3. 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:

T= cst  et  T=

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

T>cst

h3. Link budget:

Here is the expression of the link budget:

(link budget)

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.

Power amplifier

h2. Conclusion:

From these considerations, our aim will be to:

* Choose the modulation and the coding (according to the shaping filter)

* Compute the gain of the receiving antenna

* Propose some technical solution for the receiving antenna

We will also develop tools to visualize the influence of the bandwidth, EIRP, useful bit rate and distance on the system design.

The aircraft antenna will be considered able to fulfil the required antenna pattern, but we will not discuss about technical solutions for this antenna, as it can be really tricky.