SATCOM Link Budget – MODCOD

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:

• Link budget calculations for SATCOM – Downlink
• Link budget calculations for SATCOM – Uplink

Next, extending our discussions on Link budget analysis, we will explore a few more vital parameters that are associated with SATCOM link analysis. These parameters include MODCOD, Spectral efficiency, and Throughput (Data rate). Note that all these parameters are inter-related to each other and an optimum trade-off is performed to design a particular SATCOM link. This post is meant to explain MODCOD

Every satellite link is designed with a specific MODCOD (Modulation and Coding) scheme. Modulation means embedding/encoding digital information into analog carrier. So, the variation of analog/RF carrier according to digital/baseband information is called modulation. These variations are decoded at the receiver end to extract digital information from the modulated carrier. Most popular modulation scheme being used in SATCOM PSK (phase shift keying), where phase of the carrier is changed to represent digital data. One form of PSK is BPSK or 2-PSK that employs two carrier phases, 180 degrees apart, to represent 0 and 1. Another type is QPSK or 4-PSK that employs four different carrier phases, 90 degrees apart, to represent digital data (00, 01, 10, 11). Hence, it can transmit twice the data/bits as compared to BPSK within same bandwidth. However, to achieve the same Bit error rate (BER), double the transmit carrier power is required.

Coding refers to forward error correction (FEC), which is a scheme for reducing errors in wireless data transmission over noisy channels. This is implemented by embedding some extra bits in addition to useful information (or bits) so that receiver can analyse the code and correct for errors without requesting transmitter to re-transmit the corrupted bits. This way the communication system can improve bit-error-rate (BER) but at the cost of reduced throughput. Throughput (Data rate) is reduced because the extra bits that are being used to implement FEC do not carry any information. The FEC scheme for any wireless link is described by the code rate which is defined as the number of useful bits divided by the total number of bits (useful bits + redundant bits). For instance, a code rate of 4/5 means after every 4 useful bits (information bits), one redundant bit is sent for forward error correction.

Note that as the SNR increases, more efficient (spectrally efficient) MODCODs can be used because now few correction (redundant) bits are sufficient to achieve a specified bit-error-rate performance, and more useful information can be transmitted in a given bandwidth.

In next blogs, we will explore spectral efficiency and data rate of SATCOM link. Moreover, relationship between all these parameters will be described.

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