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

h1. Methodology for the design

In order to fulfil the requirements, we have developed a methodology in three steps:

* Evaluation of the required Modulation and Coding (MOCOD)

* Estimation of the required Eb/N0 for this MODCOD

* Computation of the (G/T) of the ISAE receiver required to cope with this Eb/N0 

Then, from the (G/T) value, we can discuss about the type of antenna that can be used for the ISAE receiver according to the constraints developed in _"constraints for antennas selection"_.

h2. Modulation and Coding:

As developed in _"Constraints for the physical layer"_ , we have the following relation between the modulation, the coding and the roll-off:

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

For the sake of simplicity, we will fix the roll off to $\alpha=0.2$, which is a typical value enabling a good tradeoff between the spectral efficiency and the threshold detector performances (used in DVB-S2 for example).

Then, considering that _M_ is a power of 2 and $\rho<1$, we can estimate from the previous relation the minimal MODCOD required:

p=. $M = 2^{nextpow2(2^{\frac{(1+\alpha)*R_{b}}{B}})}$

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

+Conclusion:+ *After fixing the value of the roll-off (here $\alpha=0.2$), the MODCOD is directly conditioned by the values of _B_ and _Rb_.*

h2. Required Eb/N0:

We choose a coding technique based on LDPC codes as introduced in DVB-S2 standard. This is a powerful coding scheme allowing higher spectral efficiencies for a given Eb/N0 compare to basic modulations.

Here is a table extracted from DVB-S2 standard (ETSI EN 302 307 V1.2.1, table 13), corresponding to quasi-error free performances:

p=. !MODCOD_table.PNG!

Given the MODCOD previously determined, we can read in this table the required value of Es/N0. Then, Eb/N0 can be computed thanks to the relation:

p=. $(\frac{E_{b}}{N0})_{dB}=(\frac{E_{S}}{N0})_{dB}-10*log_{10}(\frac{\eta}{1+\alpha})$

With _$\eta$_ the value of the spectral efficiency corresponding to the given MODCOD (second row of the table above). In fact, this value of spectral efficiency does not take into account the roll-off of the shaping filter, so we correct it in order to get the real spectral efficiency of the system.

+Remarks:+

* The table specifies the ideal Es/N0. Margin for non-ideal demodulator will be taken into account in the computation of the link budget.

* We can notice that lower MODCODs sometimes require higher Eb/N0 (at transition between modulations). In fact, lower modulation are preferred if possible even if it requires higher Eb/N0. In fact, there are more robust to perturbations such as multipath or phase noise for instance. This has to be kept in mind in order to explain some curves plotted in the result part.

h2. Computation of (G/T):

From the expression of the link budget, we get:

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

With _Lmarg_ the only unknown which has to be evaluated in order to compute (G/T).

An accurate evaluation of the margin can be tricky and is not the aim of this project. Then we just roughly evaluated it:

* Depointing of the antennas: Ldep= 3dB + 3dB=6 dB

* Non-ideal demodulator: Ldem= 3 dB

* Atmosphere attenuation (clear sky conditions): Lprop= 1.5 dB

* Interferences: Lint= 1.5 dB

Then, we took _Lmarg= 12 dB_. However, the value of the margin can easily be modified for the simulations presented in _"Simulations and results"_  if needed.