Would it be there a standard procedure and/or thumb rule for determining the rolloff factor of a modulated signal filter depending on a given type of modulation ?
For example, the DVB-S2 ( https://en.wikipedia.org/wiki/DVB-S2 ) allows for a rolloff factor of 0.20, 0.25 and 0.35 (inherited from DVB-S), however which metrics/equations where the basis for obtaining these values ?
Hello,
The choice of the roll-off factor Alpha (that belongs to [0,1]) is a trade-off between several parameters :
- the bandwidth of the baseband signal is equal to [-(1+ALpha)/2,(1+ALpha)/2] to determine the Spectrum shape of teh modulated signal.
- the complexity of realization of the digital filter square-root Nyquist filters. When Alpha is close to 0 then the transfert function of the Nyquist filter is an idealized brickwall filter that will need a great number of taps to be efficient (i.e. to get a small level of residual Intersymbol Interference). On the other hand, when Alpha is close to 1, the transfert function of the Nyquist filter is smoother and could be implemented in the digital domain whith few taps (for the same level of residual ISI).
- the eye opening is larger when the parameter Alpha is close to 1. As a consequence, the timing recovery loops are easier to design for higher values of Alpha.
Based on these 3 informations (bandwidth, complexity of realization for having a given residual ISI, timing recovery considérations), the system designer will select a vaue of teh parameter in between [0,1]. Usually the bandwidth consideration are of prime importance especially when several channels have to be be multiplexed in the frequency domain.
I hope this will help !
Pascal.
Mr. Scarlat and Mr. Murty
Thanks for the references !
Best Regards
I would add to Pascal's comment that the roll-off factor affects the peak-to-average (P2AV) ratio of the modulated signal (reduction of alpha results in increase of P2AV). This in turn affects other things such as transmit power amplifier efficiency, out-of-band emissions due to non-linear effects, etc.
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