Multi-spot beam satellites

Edit(2017/09): this article was written early in my PhD, with little knowledge of actual systems. It happens that the Uplink/Downlink frequency bands are not so linked together. I might update this article later, or publish a new one with more accurate assumptions.

[Image perdue : System view of a DVB-S2/RCS2 satellite. The Forward link uses DVB-S2, the return link the DVB-RCS2] (images/blog/dvbrcs2-sysview.png)

Let $\Delta F_{FL}$ , $\Delta F_{DL}$ and $\Delta F_{UL}$ be the Frequency bands for, respectively, the whole forward link (i.e. Gateway → Satellite → Users), downlink (Satellite → Users) and uplink (Gateway → Satellite).

In the simple scheme where a satellite covers a full continent with a single beam (traditional TV Broadcast model) we have this relation:

$\Delta F_{DL} = \Delta F_{UL} = \frac{1}{2} \Delta F_{FL}$

Multi-spot beam satellites allow to have different coverage beam, where the same frequencies can be used and not interfere with each other (the beams are sufficiently well focused and spaced).

[Image perdue : Ka-sat multi-spot beam coverage of europe] (images/blog/kasat.png)

In this case, a frequency can be used $k$ times, where k is the frequency reuse factor.

For example, in a 4-color distribution scheme, with 120 spots, there is a theoretical reuse factor of 20. In reality, the reuse factor is closer to 12 than 20, but that’s another topic.

In the case where the reuse factor is $k=2$ (meaning that a frequency is used twice over the whole coverage), taking the hypothesis of full use of the bandwidth over each spot beam, the necessary bandwidth in the uplink would be: $\Delta F_{UL} = 2\Delta F_{DL}$

Extending this to $k$ , we have: $\Delta F_{UL} = k\Delta F_{DL}$

Provided that the global bandwidth for this forward link is constant $\Delta F_{FL}=cst$ , we have the following relation of the uplink bandwidth according to the reuse factor: $\Delta F_{UL} = k(\Delta F_{FL} - \Delta F_{UL})$ $\Delta F_{UL} = \frac{k}{k+1}\Delta F_{FL}$

[Image perdue : Evolution of the uplink-to-forward bandwidth ratio according to the reuse factor k] (images/blog/ratio-ffr.png) [Image perdue : Bandwidth repartition with frequency reuse, without over-provisionning] (images/blog/bw-division.png)

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