Binary Amplitude Shift Keying | BASK | ON-OFF keying

In this lecture, we are going to learn about digital modulation techniques in communication systems. we will start with the very first technique called Binary Amplitude Shift keying. Then we will discuss the FSK, and PSK in subsequent chapters. So let’s discuss the BASK in detail.

Definition of ASK or On-OFF Keying

• Amplitude shift keying (ASK) or On-OFF keying (OOK) is the simplest digital modulation technique. In this method, there is only one unit energy carrier switched on or off depending upon the input binary sequence.

Expression and Waveforms for Binary Amplitude Shift Keying

• The ASK waveform may be represented as,

s(t)=\sqrt{2P_s} \cos (2\pi f_ct) (To transmit ‘1’) …… Eq.(1)

• To transmit symbol ‘0’, the signal s(t)=0 i.e., no signal is trasnmitted/ Signal s(t) contains some complete cycle or carrier frequency f_c.

• Hence, the ASK waveform looks like an ON-OFF of the signal. Therefore, it is also known as the ON-OFF keying (OOK). Figure (1) shows the ASK waveform.

Signal Space Diagram of Binary Amplitude Shift Keying

• The ASK waveform of the equation (1) for symbol ‘1’ can be represented as,

s(t)=\sqrt{P_sT_b} \sqrt{2/T_b} \cos (2\pi f_ct)=\sqrt{P_sT_b} \Phi_1(t) …… Eq.(2)

• This means that there is only one carrier function \Phi_1(t). The signal space diagram will have two points on \Phi_1(t). One will be at zero and other will be at \sqrt{P_sT_b}. Figure (2) shows this aspects.

• Thus, the distance between the two signal points is,

d=\sqrt{P_sT_b}=\sqrt{E_b} …… Eq.(3)

Generation of Amplitude Shift Keying

1. Description and Working Operation of ASK

• ASK signal may be generated by simply applying the incoming binary data and the sinusoidal carrier to the two inputs of a product modulator (i.e. balanced modulator). the resulting output will be the ASK waveform. This is shown in the figure below. Modulation causes a shift of the baseband signal spectrum.

2. Power Spectral Density (PSD)

• The ASK signal, which is basically the product of the binary sequence and the carrier signal, has a power spectral density (PSD) same as that of the baseband on-off signal but shifted in the frequency domain by \pm f_c. This is shown the below figure also. It may be noted that two impulses occur at \pm f_c.

3. Bandwidth of BASK

• The spectrum of the ASK signal shows that it has infinite bandwidth. However, for practical purposes, the bandwidth is often defined as the bandwidth of an ideal bandpass filter centered at f_c whose output contains about 95% of the total average power content of the ASK signal. It may be proved that according to this criterion the bandwidth of the ASK signal is approximately 3/T_b Hz. The bandwidth of the ASK signal can, however, be reduced by using smoothed versions of the pulse waveform instead of rectangular pulse waveforms.

Demodulation of ASK Signal | Coherent Detection

1. Working Operation

• The demodulation of binary ASK waveform can be achieved with the help of a coherent detector as shown in the figure. It consists of a product modulator which is followed by an integrator and a decision-making device.

• The incoming ASK signal is applied to one input of the product modulator. The other input of the product modulator is supplied with a sinusoidal carrier which is generated with the help of a local oscillator.

• The output of the product modulator goes to the input of the integrator. The integrator operates on the output of the multiplier for successive bit intervals and essentially performs a low-pass filtering action. The output of the integrator goes to the input of a decision-making device.

• Now, the decision-making device compares the output of the integrator with a present threshold. It marked a decision in favor of symbol 1 when the threshold is exceeded and in favor of symbol 0 otherwise. the coherent detection makes the use of linear operation.

• In this method, we have assumed that the local carrier is in perfect synchronization with the carriers used in the transmitter. this means that the frequency and phase of the locally generated carrier are the same as those of the carriers used in the transmitter.

2. Synchronization Requirement

• The following two forms of synchronization are required for the operation of a coherent detector.

1. Phase synchronization ensures that the carrier wave generated locally in the receiver is locked in phase with respect to one employed in the transmitter.
2. Timing synchronization enables proper timing of the decision-making operation in the receiver with respect to switching instants in the original binary data.

Non-coherent Detection of ASK

• In the binary ASK case, the transmitted signal is defined as,

s(t)=\sqrt{2P_s} \cos (2\pi f_ct)

• Binary ASk signal can also be demodulated non-coherently using envelop detector.

• This greatly simplifies the design consideration required in synchronous detection. Non-coherent detection schemes do not require a phase-coherent local oscillator. This method involves some form of rectification and low-pass filtering at the receiver.

• The block diagram of a non-coherent receiver for ASK signal has been shown in the figure below.

Advantages of Binary Amplitude Shift Keying

• The advantage of using BASK is its simplicity. It is easy to generate and detect. The circuitry of the ASK generator and detector is very easy.

Disadvantages of Binary Amplitude Shift Keying

• The drawback of the BASK is that it is very sensitive to noise, therefore, it finds limited application in data transmission. It is used at very low bit rates, up to 100 bits per sec.

Frequently Asked Question on ASK