ASK, FSK, and PSK

 

ASK or OFF ON Keying

Ask is a simple (less complex) Digital Modulation Scheme where we vary the modulation signal's amplitude or voltage by the message signal's amplitude or voltage. We select two levels (two different voltage levels) for transmitting modulated message signals for the exam. And for example, we mapped the signal as two-level "+5 Volt" (which is the upper level) and another level, "0 Volt," which is considered as the lower level. Whenever we need to transmit binary bit "1," then the transmitter transmits a signal of "+5 Volts," and when we need to send bit "0," then it transmits no power. But the receiver is intelligent enough to deflect whether you've sent binary bit "1" or "0" by deflecting with quipped filters that can distinguish strings of bits. It is possible by the switching capability of the filter with the particular period to determine each bit from a string of bits.  

 

 

 

(Get MATLAB Code)

Fig 1: Output of ASK, FSK, and PSK modulation using MATLAB for a data stream "1   1    0   0   1    0   1   0"

FSK

Like other modulation techniques, the message signal is modulated with the high-frequency carrier wave,e, and then two binary values are represented by two different frequencies. The two frequencies are near the carrier frequency. For example, 

We choose two carrier frequencies, f1 and f2, and f1 > f2. Then we modulate binary bit "1" with f1 and binary "0" with f2 frequency, which is a lower frequency than f1. Now, the modulated signal will look like that,

S₁(t) = A cos 2πf_c1t      for  binary 1   

And S₂(t) = A cos 2πf_c2t      for  binary 0

Here, fc1 is different from f1. As you know, when the signal goes thru the modulation process, the frequency of the modulated signal is different from the carrier signal by the message signal's frequency.

PSK

In PSK, here,e the carrier signal phase is with a modulated signal with the phase related to the last bit for binary "1," and binary "0" is sent with a signal with the same phase as the preceding one. For example, whenever we need to transmit binary "1", we change the signal's phase by 180 degree, but the phase remains the same when we transmit binary "0". PSK carrier is used as follows

s(t) = A cos (2πfct + π)    for binary 1

s(t) = A cos (2πfct)           for binary 0 

Which of the modulation techniques—ASK, FSK, or PSK—can achieve higher bit rates?

Among ASK, FSK, and PSK, PSK (Phase Shift Keying) can generally achieve higher bit rates.

Here's why:

PSK (Phase Shift Keying):

Phase Shift Keying, or PSK, uses various phase shifts to encode extra bits per symbol. For instance, QPSK (Quadrature Phase Shift Keying) doubles the data rate over binary PSK by representing two bits each signal.

The bit rate can be further increased by using higher-order PSK methods, such as 8-PSK and 16-PSK, which can encode even more bits per symbol.

FSK (Frequency Shift Keying):

Because FSK often requires a higher frequency separation to discriminate between different symbols, fewer bits can be broadcast in a given bandwidth, resulting in a lower bit rate than PSK.

Although there are higher-order FSK systems, PSK is more bandwidth-efficient than them.

ASK (Amplitude Shift Keying):

ASK's ability to successfully raise bit rates is limited by its lower bit rate efficiency and increased susceptibility to noise and interference.

Bit rates can be increased via higher-order ASK (such as QAM, which combines ASK and PSK), but pure ASK is typically less effective than PSK.

In conclusion, among these three modulation strategies, PSK has the potential to produce the highest bit rates, particularly when utilising higher-order modulation techniques.