Transmission of Binary Data by ASK
Whenever digital data needs to be transmitted over a communication channel, the information must first be converted into signals that the channel can carry. Two common transmission methods are:
- Baseband Transmission
- Bandpass Transmission
Baseband transmission sends digital pulses directly through a low-pass communication channel. Bandpass transmission first modulates the data onto a high-frequency carrier before transmission.
- Baseband → Direct pulse transmission.
- Bandpass → Carrier-based transmission.
Three Basic Modulation Techniques
Bandpass communication mainly uses three digital modulation techniques.
- Amplitude Shift Keying (ASK)
- Frequency Shift Keying (FSK)
- Phase Shift Keying (PSK)
Each technique modifies a different property of the sinusoidal carrier wave.
| Technique | Parameter Changed |
|---|---|
| ASK | Amplitude |
| FSK | Frequency |
| PSK | Phase |
1. Amplitude Shift Keying (ASK)
In ASK, binary information is represented by changing the amplitude of the carrier wave while keeping its frequency and phase constant.
| Feature | ASK | FSK | PSK |
|---|---|---|---|
| Amplitude | Changes | Constant | Constant |
| Frequency | Constant | Changes | Constant |
| Phase | Constant | Constant | Changes |
| Noise Immunity | Low | Medium | High |
| Typical Applications | RFID, Optical Links | Bluetooth, Paging Systems | Wi-Fi, GPS, Satellite, Cellular Networks |
✔ Baseband sends pulses directly over a low-pass channel.
✔ Bandpass sends data by modulating a carrier wave.
✔ ASK changes amplitude.
✔ FSK changes frequency.
✔ PSK changes phase.
✔ FSK and PSK are generally preferred because they are more robust against noise and nonlinear channel effects.
Amplitude Shift Keying (ASK) is a fundamental digital modulation technique in which the amplitude of a high-frequency sinusoidal carrier is varied in accordance with the digital information signal. In passband ASK, the modulated signal is centered around a carrier frequency, enabling efficient transmission over band-limited channels such as radio frequency (RF) links and optical communication systems.
In a binary ASK (BASK) system, the binary symbol '1' is represented by transmitting a carrier signal of amplitude Ac and frequency fc over the bit duration Tb, whereas the binary symbol '0' is represented by the absence of the carrier. Due to this characteristic, binary ASK is also referred to as On-Off Keying (OOK).
Baseband Signal: The input digital data represented as a unipolar or bipolar pulse train (e.g., 0 V for binary '0', +V for binary '1'). Its frequency components are centered around zero.
Passband Signal: The carrier amplitude is modulated according to the baseband signal. For BASK (OOK), the transmitted signal is: \[ s(t) = \begin{cases} A_c \cos(2 \pi f_c t), & \text{for binary '1'} \\ 0, & \text{for binary '0'} \end{cases} \] Here, \(A_c\) is the carrier amplitude and \(f_c\) is the carrier frequency.
The input binary sequence, represented as a unipolar pulse train, is multiplied with a sinusoidal carrier wave to produce the ASK signal.
Binary Data
Passband ASK Waveform
Binary 0 → Low or zero amplitude
Passband ASK Receiver (Coherent Detection):
At the receiver, coherent detection is commonly employed for demodulation. This technique requires the generation of a local carrier that is synchronized in both frequency and phase with the transmitted carrier.
The received signal is multiplied by the synchronized carrier and subsequently passed through a low-pass filter or integrator over the bit duration Tb.
- For binary '1': the integrator output is proportional to the signal energy, approximately Ac2Tb/2.
- For binary '0': the integrator output is ideally zero in the absence of noise.
A decision device (threshold detector) compares the output with a predefined threshold to determine the transmitted symbol.
The objective of the receiver is to reliably distinguish between the presence and absence of the carrier in the received passband signal.