Single SideBand Suppressed Carrier (SSB-SC) Modulation
As we know, the transmission bandwidth of standard AM as well as DSB-SC modulated wave is 2W Hz i.e., twice the message bandwidth W. Therefore, both these systems are bandwidth inefficient systems .
In both these systems, one half of the transmission bandwidth is occupied by the upper sideband (USB) and the other half is occupied by the lower sideband (LSB ) as shown in fig.1 .
However, the information contained in the USB is exactly identical to that carried by the LSB . So, by transmitting both the sidebands we are transmitting the same information twice .
Hence, we can transmit only one sideband (USB or LSB ) without any loss of information . So, it is possible to suppress the carrier and one sideband completely .
When only one sideband is transmitted, the modulation is called as single sideband modulation . It is also known as SSB or SSB-SC modulation .
Frequency Domain Description
Fig.1 (a) represents the spectrum X(f) of the message signal x(t) .
This spectrum is limited to the band :
The spectrum of DSB-SC wave which is obtained by taking the product of x(t) and c(t) is shown in fig.1 (b) . It contains the USB as well as LSB .
When only USB is transmitted by the SSB system, then the corresponding spectrum is as shown in fig.1 (c) . And when only LSB is transmitted, the frequency spectrum is as shown in fig. 1 (d) .
Thus, the essential function of SSB modulation is to translate the spectrum of the message signal to a new location in the frequency domain .
Transmission Bandwidth of SSB-SC
Since we are transmitting the frequencies only in the range (fc+ W) or ( fc – W), the transmission bandwidth for the SSB-SC will be :
Bandwidth B = (fc+ W) – fc = W Hz
Or B= fc – ( fc – W) =W Hz
This is exactly half the bandwidth of the DSB-SC modulated waves .
Advantages of SSB-SC Modulation Over DSB-FC
The advantages of SSB-SC over DSB-FC signal are :
- Less bandwidth requirements . This allow more number of signals to be transmitted in the same frequency range .
- Lots of power saving . This is due to the transmission of only one sideband component .At 100% modulation, the percent power saving is 83.33% .
- Reduced interference of noise . This is due to the reduced bandwidth . As the bandwidth increases, the amount of noise added to the signal will increase .
Disadvantages of SSB-SC Modulation
The disadvantage of SSB modulation are :
- The generation and reception of SSB signal is complicated ( which we will discuss later in our following post ).
- The SSB transmitter and receiver need to have an excellent frequency stability . A slight change in frequency will hamper the quality of transmitted and received signal . Therefore, SSB is not generally used for the transmission of good quality music. It is used for speech transmission .
Application of SSB
- SSB transmission is used in the applications where the power saving and low bandwidth requirements are important .
- The application areas are land and air mobile communication, telemetry, military communications, navigation and amateur radio . Many of these applications are point to point communication application .