What Is a Symbol Rate?

Refers to the data transmission rate, which is related to the bit rate and channel parameters of the signal. The unit is MB / S. At present, the symbol rates of satellite TV digital decompressors such as "Nokia", "Philips", "Hyundai", "Tongzhou" and "Kyushu" commonly used in the market are between 6 and 30MB / S. From the satellite development trend in the world, the symbol rate of satellite TV is getting higher and higher. When the number of programs carried by a carrier signal is larger, this value is larger. ^[1]

Symbol rate (SR, Symbol Rate), also known as symbol rate, baud rate. in

Symbol rate calculation formula:

Symbol rate = data rate x RS code rate x convolutional code rate x modulation index

Known: RS code rate = 188/204; convolutional code rate = 1/2, 2 /, 3/4, 5/6, 7/8;

Modulation index = 1/2 QPSK modulation = 1/3 QPSK modulation = 1/4 16QAM modulation = 1/5 32QAM modulation = 1/6 640AM QPSK modulation

Example: QPsK modulation, convolutional code rate = 3/4:

Symbol rate = data rate x 188/204 x 3/4 x 1/2

If you know the data rate, you can find the symbol rate; on the contrary, you can get the data rate if you know the symbol rate.

The symbol rate is often difficult to distinguish from the bit rate. The symbol rate pays attention to the transmission efficiency per second; the bit rate pays attention to the transmission speed per second. "Bit rate" refers to the rate of information transmission per second, and the unit is bits / second (bits / s, b / s, bps). b represents the single digit of bit; ps refers to per second (/ s). In digital channels, the bit rate is the transmission rate of the digital signal. It is expressed by the number of significant bits of the binary code transmitted in a unit time. Units are bits per second (bit / s (bps)), kilobits per second (kbps), and megabits per second (Mbps). ^[2]

When setting the symbol rate, pay attention to the relationship between the set symbol rate and its corresponding effective code rate (bit rate). The effective output bit rate of the modulator should be greater than the total input bit rate, otherwise the modulator bit rate will overflow, and the receiving end image will have severe mosaics. The commonly used conversion formula for the effective output code rate and symbol rate of the modulator is

Output code rate = symbol rate X band utilization X 188/204

Among them, different constellations correspond to different frequency band utilization rates, that is, 16QAM corresponds to 4; 32QAM corresponds to 5; 64QAM corresponds to 6; 128QAM corresponds to 7; and 256QAM corresponds to 8. In this way, the effective output code rate of 64QAM is: 6.875 × 6 × 188/204 = 37.32 (Mbit / s). ^[3]