Previously, Link budget calculations for SATCOM have been presented for both downlink as well as uplink, where we discussed key parameter ‘SNR’ of any communication link. For details of SNR calculations, read here:
Next, extending our discussions on Link budget analysis, we explored another vital parameter associated with SATCOM link budget analysis that is called MODCOD (Modulation and Coding). This is important because every SATCOM link is designed for a particular MODCOD. For detailed discussion on MODCOD, read here: SATCOM Link Budget – MODCOD
Now, in this post we will explore two more important SATCOM link parameters : (a) Spectral efficiency (SE) and (b) Throughput or Data rate. Moreover, that are associated with SATCOM link analysis. And towards the end, we will conclude SATCOM link budget topic with a discussion on the inter-relationship of all these four parameters (SNR, MODCOD, SE, and Data rate).
Spectral efficiency and Throughput/Data-rate
Spectral efficiency of a communication link is defined in terms of how much useful information (excluding redundant bits used for FEC) can be transmitted over the channel in a given bandwidth. It is measured in the units of bits per second per Hz (bps/Hz). Note that channel throughput is calculated by multiplying SE with channel Bandwidth. There exists a theoretical limit on maximum throughput/data-rate of any communication link having a certain bandwidth and SNR as defined by Shannon channel capacity theorem (equations are presented below). However, practical spectral efficiency is always less than this limit.
where, C= Channel capacity or throughput or data rate in bps
SE = Spectral efficiency in bps/Hz
B= Available Channel Bandwidth in Hz
S/N = Signal-to-Noise ratio available at the input of receiver (not in dB)
Every satellite modem has a specified set of MODCODs along with associated spectral efficiency and SNR. So, depending upon the available SNR for a particular SATCOM link (calculated from link budget SNR calculations), the designer needs to select optimum MODCOD, where optimum means the MODCOD having the best spectral efficiency among all the available MODCODs. Note that more efficient MODCODs would require higher SNR available in the SATCOM link.
A theoretical Digital video broadcasting (DVB) modem is assumed and its MODCODs along with spectral efficiency (based on Shannon limit) are provided below for reference. The rightmost column provides the SNR required for error free communication. whereas code rates associated with each MODCOD is highlighted in red. It can be observed clearly that more efficient MODCOD requires better SNR available at modem input.
Assume that calculated SNR for a particular SATCOM downlink comes out to be 3.15 dB. Remember that, SNR calculations would require information regarding terminal G/T, Satellite transmitter EIRP and BW. For details, read here: Link budget calculations for SATCOM – Downlink
Now assume the modem supports MODCODs as tabulated above and look for the most efficient MODCOD with available SNR at modem input that is 3.15 dB. We can see that DPSK 1/4 is the most efficient MODCOD with a spectral efficiency of 0.75 bps/Hz. So, if the useable channel bandwidth is 5 MHz, data-rate would be 5MHz * 0.75 bps/Hz = 3.75 Mbps