Symbol Rate In DVB Satellite TV Broadcasting
In Digital Video Broadcasting (DVB), the Symbol Rate (often called Baud Rate) is the number of symbol changes (waveform states) transmitted per second, measured in Baud (Bd) or Symbols per second (Sps). It determines how much bandwidth a signal occupies.
General Calculation Formula
The relationship between the Symbol Rate (R
s) and the Transport Stream Bitrate (Rb) is governed by the modulation efficiency and error correction overhead:
s) and the Transport Stream Bitrate (Rb) is governed by the modulation efficiency and error correction overhead:
Rs = Transport Stream Bitrate / Bits Per Symbol * Code Rate
Bits per Symbol:
Depends on the constellation (e.g., QPSK = 2, 8PSK = 3, 64QAM = 6).
Code Rate:
The FEC (Forward Error Correction) fraction (e.g., 3/4, 5/6).
Packet Overhead:
For DVB-S/C, a factor of 204/188 is often included to account for Reed-Solomon encoding.
Standard-Specific Symbol Rates
Typical symbol rates vary by transmission medium and standard.
DVB-S (Satellite):
Standard rates are often 27.5 MSps or 22 MSps. The physical symbol rate typically ranges from 1 to 45 MSps to fit standard 36 MHz satellite transponders.
DVB-S2/S2X:
Supports higher rates up to 100 MSps. In "Wideband" mode, it can support transponders up to 500 MHz.
DVB-C (Cable):
Commonly uses 6.875 MSps or 6.900 MSps to fit within an 8 MHz channel bandwidth while using 64QAM or 256QAM modulation.
DVB-T/T2 (Terrestrial):
Uses multi-carrier OFDM (Orthogonal Frequency Division Multiplexing). Instead of a single symbol rate, it uses thousands of sub-carriers (e.g., 8k or 32k mode). The total "equivalent" symbol rate is approximately 6.4 to 7.6 MSps for an 8 MHz channel.
Relationship to Bandwidth
The Occupied Bandwidth (BW) of a DVB signal is directly proportional to its symbol rate and the Roll-off Factor ( ɑ )
BW=Rs * (1+ ɑ)
DVB-S: Roll-off is fixed at 0.35.
DVB-S2/S2X: Introduces smaller roll-off factors (0.20, 0.25, 0.05) to pack more symbols into the same bandwidth.
Example: A 27.5 MSps signal with a 0.35 roll-off occupies 27.5 * 1.35 = 37.1 Mhz.
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