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Bit Error Rate (BER) and Roll Off Factor

Roll Off  Factor

W
hen a digital signal is modulated the output would be a waveform with the shape of sin(x)/x. Now this has an infinite bandwidth. So in order to be able to transmit this signal over satellite we need to filter it to reduce the occupied bandwidth. This is done by applying a "raised cosine roll-off filter". This filter is present both at the modulator output stage and at the input of the demodulator (I believe one says these filters are then matched). The steepness and the resulting occupied bandwidth can be set by the parameter roll-off factor. In DVB-S2 it can be 0.20 (20%); 0.25 (25%) or 0.35 (35%) where the 
Occupied bandwidth = symbol rate x (1 + roll-off factor).






Figure: Occupied bandwidth, Roll off factor and symbol rate 

Symbol rate, Bandwidth and Bit rate Calculation for Broadcast Engineering.

In satellite communication bandwidth (BW), symbol rate and bit rate are the most vital fact for proper signal transmission. There are many commercial software for BW, Symbol rate and bit rate calculation for TV signal transmission. But the equations of these parameter calculation are as follows.

BW= (1+Roll of factor)* Symbol rate
In general Roll of factor is 35%

Symbol Rate =BW / (1+Roll of factor)

and Bit Rate = Symbol rate * Modulation* FEC* (188/204)
                                    [when 188 is encoder packet length]
or Bit Rate = Symbol rate * Modulation* FEC* (204/188)
                                    [when 204 is encoder packet length]

Here,
FEC = Forward Error Correction. In general FEC is in the range from 1/2 to 7/8.
188/204 = constant

Unit
** BW= Hz, KHz, MHz, GHZ
** Symbol Rate= sps or baud, Ksps or Kbaud, Msps or Mbaud, Gsps or Gbaud
** Bit Rate= bps, Kbps, Mbps, Gbps

Baud: Unit of signalling speed
Baud rate: Number of symbols transmitted per second (s) 

Now b (bit) = s(symbol) * n (number of bit per-symbol) 


N.B.
# Uplink frequency - Downlink frequency = 2225 MHz  [for C band]

For Example,
Uplink frequency is 5964 MHz
Downlink frequency is 3739 MHz.

Now 5964 MHz - 3739 MHz = 2225 MHz.

# L band frequency for downlink = LNB local oscillator frequency - Downlink frequency.
                                                = 5150 - 3739 MHz
                                                = 1411 MHz
In general LNB local oscillator frequency is 5150 MHz, 5750 MHz and 5950 MHz.

# Center frequency for uplink is uplink frequency. Here the center frequency is 5964 MHz.


# L band frequency for uplink = Uplink frequency - LNA local oscillator frequency
                                                = 5964 MHz - 4900 MHz
                                                = 1064 MHz



Frequency range for some frequency band

# IF frequency range is from 70 MHz to 124 MHz

# L band frequency range is from 1 GHz to 2 GHz (more specifically from 950 MHz to 2100 MHz)

# C band frequency is from is from 4 GHz to 8 GHz

# Ku band frequency is from 12 GHz to 18 GHz

# Ka band frequency is from 26.5 GHz to 40 GHz.