More accurately the title should read “Estimating BUC or Transceiver power for Satellite Circuits” It’s not that we can’t quantify the exact power requirements, but absolute accuracy is a complicated calculation, and so teleports like ours have grown to use safe approximations.
The calculation of a circuits power requirement is done via something called a Link Budget, this is defined as the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system.
Back in the early days when we first owned a teleport, we had less confidence in our technical skills we used to send the details of every new remote site to PanAmSat and wait a few days for them to say, five watts or twenty watts at QPSK 3/4 or whatever.
Today, we can make our own calculation and we are accurate enough so that nobody suffers.
The basic parameters for a link budget calculation are the location of the distant site, their antenna size and circuit modulation. Then we add our own teleports parameters into the mix and we can calculate with confidence what power is needed for the circuit.
You can take two identical sites: 2.4m, 5 watt BUC and a Modem with LDPC2k. Each site may be in same country but there may be two completely different results – we could estimate that one site has a maximum transmit capacity of 384Kb whereas the maximum transmit for the other site may be 512Kb – but we have found that you cannot recommend based on MAX. Even though a BUC or transceiver may be sold as 5watts, the fine print says “Typical” in other words it can deliver slightly more power, or slightly less power.
Let me give you an example:
Customer A has a 3.8m with 10 watt BUC and is transmitting 2048kbps. Another customer in same country wants to TX 1760kbps so what do you suggest? You naturally suggest 3.8m with 10 watt BUC based on your experience…but, here is why you would NOT want to do that:
If the 1st customer has a 10 watt BUC that can TX 40db at max (10 watts = 39db) and the 10 watt BUC we sell to the 2nd customer can only TX 38.5db there is a 1.5db swing in available power – all within expected NORM. Note that 3db is double power which means double bandwidth – so 1.5db is a huge difference in effective output capacity! If customer 1 can do 2048kbps and customer 2 is 1.5db lower in available output power then customer 2 will only be able to transmit approximately 1536kbps at max.
Many of our clients are in West Africa and so our quick rule of thumb shows the theoretical maximum for two different antenna sizes, and although in some cases the maximum can be achieved, we never design according the maximum instead we quote the lower data rate as being the achievable number.
2.4 m with 5 watt BUC with Datum 508 TPC- LDPC 384kbps (max 512)
2.4 m with 10 watt BUC with Datum 508 TPC- LDPC 768kbps (max 1024)
3.8 m with 5 watt BUC with Datum 508 TPC- LDPC 768kbps (max 1024)
3.8 m with 10 watt BUC with Datum 508 TPC- LDPC 1536kbps (max 2048)