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

h1. Constraints and parameters for the system design

   # 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

As seen before, getting an allocated frequency band is not an easy task. However, there are several more constraints on the system due to the specifications.

h2. Constraints for the choice of the antennas:

The type of antenna and associated performances are strongly linked to the considered central frequency. However, constraints are not the same for the aircraft antenna or for the ISAE antenna as we will describe below. 

h3. ISAE antenna:

Given the distance and the bit rate required to fulfil the specifications, a tracking antenna might be needed on the roof on the antenna. Nevertheless, as this solution is considerably increasing the system complexity, we will try to design a system with a fixed mounted antenna if possible.

In fact, the need (or not) for tracking will be determined by the required figure of merit (G/T) of the antenna (computed in part III).  

*As increasing the gain of the antenna decreases its coverage (theta 3 dB), if the required gain of the antenna leads to a coverage smaller than the aircraft action zone, a tracking antenna will be required. 

Aircraft action zone limited

Increase gain ->> increase G/T but decrease coverage. If no tradeoff can be found between the coverage and the gain, a tracking antenna has to be used

Rq: high gain easier to realize for high frequencies, T decreasing when G increasing*

h3. Aircraft antenna:

The connection between the aircraft and the ISAE building shall remain available for an inclination of 45° and an attitude of 15° at a distance of at least 50 km. 

As tracking antennas are not an option for the aircraft because of the complexity of integration, these availability requirements dictate a given radiation pattern for the aircraft antenna. 

*Graph pattern, idée antenne, mais masquage de l’avion …*

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.