SSB vs VSB | Difference between SSB and VSB modulation types
This page on SSB modulation vs VSB modulation describes difference between SSB and VSB modulation types.It provides link to DSBSC vs SSBSC.
Before we start our discussion, it is recommended to understand concept of DSB-SC vs SSB-SC which mentions difference between DSB-SC and SSB-SC modulation types. In DSB-SC signal, there is symmetry in band structure. Due to this fact even if one half is transmitted, the other half can be recovered at the receiver. Due to this, bandwidth and power of transmission is reduced by half. Depending upon which half of DSB-SC is transmitted there are two types of SSB modulation viz. LSB (Lower Side Band) modulation and USB (Upper Side Band) modulation. The same have been depicted in figure-2 below.
As shown in figure-1, DSB-SC modulator is used for SSB
signal generation. It is mathematically represented as below.
= m(t)*cos(Wc*t) +/- m^(t)*sin(Wc*t)
Original signal is retrieved using coherent demodulation of SSB signals. φSSB(t) is multiplied with cos(Wc*t) and passed through low pass filter to get back the original signal.
φSSB(t) * cos(Wc*t) = 1/2 m(t) + 1/2 cos(2*Wc*t) +/- 1/2 m^(t)*sin(2*Wc*t)
The demodulated signal is passed through an LPF to remove unwanted SSB.
Following are the drawbacks of SSB signal generation:
-Generation of SSB signal is difficult.
-Selective filtering is done to get back the original signal.
-Phase shifter should be of 90 degree exactly.
To overcome all these drawbacks of SSB modulation, VSB modulation is used. Hence VSB is considered as a compromise between SSB and DSB-SC.
In VSB modulation :
• One sideband is not rejected fully.
• One sideband is transmitted fully and a small part (i.e. vestige) of the other sideband is transmitted. Hence bandwidth here is BWv = B + v, where v is vestigial frequency band.
Figure-3 depicts VSB signal generation. Hi(w) is a filter which shapes the other sideband.
φVSB(w) = [ M(w-wc) + M(w+wc)]. Hi(w)
To recover the original baseband signal from the VSB signal, the VSB signal is multiplied with cos(Wc*t) and passed through an Low Pass Filter(LPF). Doing so, original signal is recovered.
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