Symbol rate
In digital communications, the symbol rate is the gross bit rate, inclusive of channel coding overhead, divided by the number of bits transmitted in each symbol. Symbol rate is measured in symbols-per-second, hertz (Hz), or baud (Bd).
The term baud is synonymous with symbol rate, but is less frequently used today as it has in the past been commonly misused to mean bit rate or data rate.
The symbol rate of a signal can be directly measured using an oscilloscope (in the time domain, measuring the time between transitions) or a spectrometer (in the frequency domain, measuring the bandwidth).
There is no fixed relationship between the symbol rate and the bit rate. The simplest digital communication links (such as individual wires on a motherboard) typically have a symbol rate equal to the bit rate. Common communication links such as RS-232, EIA-485, USB, and FireWire typically have a symbol rate slightly higher than the data bit rate, due to the overhead of extra non-data symbols used for self-synchronizing code and error detection. Other communication links (such as communication by modem over telephone, and digital television) have a symbol rate much lower than the bit rate (they transmit many bits per symbol). Some communication links (such as GPS transmissions, CDMA cell phones, and other spread spectrum links) have a symbol rate much higher than the data rate (they transmit many symbols called "chips" per data bit).
Example
In digital television transmission the symbol rate calculation is:
symbol rate in symbols per second = (Data rate in bits per second * 204) / (188 * bits per symbol)
The 204 is the number of bytes in a packet including the 16 trailing Reed-Solomon error checking and correction bytes. The 188 is the number of data bytes (187 bytes) plus the leading packet sync byte (0x47).
The bits per symbol is the (modulation's power of 2)*(Forward Error Correction). So for example in 64-QAM modulation 64 = 26 so the bits per symbol is 6. The Forward Error Correction (FEC) is usually expressed as a fraction, i.e., 1/2, 3/4, etc. In the case of 3/4 FEC, for every 3 bits of data, you are sending out 4 bits, one of which is for error correction.
Example:
given bit rate = 18096263 Modulation type = 64-QAM FEC = 3/4
then