‘Improved Roll Off Factor’ for DVB-S2 Carriers

Beginning with iDX Release 3.2, improvements to the iDirect carrier waveform allow reduced guard bands for DVB-S2 downstream carriers. In earlier releases, iDirect required a minimum guard band of 20% of the carrier symbol rate for both upstream and downstream carriers. For DVB-S2 downstream carriers, the required guard band now can be as small to 5% of the carrier symbol rate for some carriers. This improved guard band requirement was achieved by reducing the roll off factor of DVB-S2 downstream carriers.

Digital communication systems employ waveform pulse shaping using identical matched transmit and receive filters to limit the occupied bandwidth required by a carrier and to maximize received Signal to Noise Ratio (SNR). The roll off factor of a digital filter defines how much more bandwidth the filter occupies than that of an ideal “brick wall” filter which defines the theoretical minimum occupied bandwidth. This dictates the guard band requirement for the carrier.

A lower guard band between adjacent carriers makes it possible to fit a higher bit rate carrier into the same satellite bandwidth. Therefore, with a lower guard band requirement, a Network Operator can increase the bit rate of existing carriers without purchasing additional bandwidth.

The amount of required guard band between carriers can be expressed as the carrier spacing requirement. For example, if the required guard band is 20%, the channel spacing requirement is 1.2*Carrier Symbol Rate in Hz. Carrier Spacing can be configured in iBuilder for both upstream and downstream carriers. This field is used to document the total occupied bandwidth of the carriers. It represents the carrier bandwidth plus the guard band normalized by the symbol rate.

The Carrier Spacing configured in iBuilder is only enforced on multichannel receive line cards to prevent overlap when assigning upstream carriers to a specific line card. For all other carriers, including DVB-S2 downstream carriers, this field is informational only. Therefore, it is the responsibility of the Network Operator to ensure that DVB-S2 carriers do not interfere with adjacent carriers.

iBuilder allows Network Operators to select one of four values for Carrier Spacing when configuring a DVB-S2 downstream carrier:

  • 1.20 (Guard band = 20% of carrier symbol rate)
  • 1.15 (Guard band = 15% of carrier symbol rate)
  • 1.10 (Guard band = 10% of carrier symbol rate)
  • 1.05 (Guard band = 05% of carrier symbol rate)

Not all iDirect DVB-S2 carriers support all lower guard bands. For example, DVB-S2 carriers below 10 Msps require a guard band of a least 10%. Also, guard bands of 10% or 5% require a minimum MODCOD of QPSK-1/2. A table in the Technical Reference Guide documents all DVB-S2 guard band constraints with respect to minimum symbol rates and minimum MODCODs.

For more information on this topic, see the chapter “Carrier Occupied Bandwidth” of the Technical Reference Guide.

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