Transmission of Binary Data by PSK
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.
Phase Shift Keying (PSK)
PSK keeps the amplitude and frequency constant while changing only the phase of the carrier.
Binary Data (1 0 0 1 1)
PSK Waveform
Binary 1 → 180°
Why FSK and PSK are Preferred
Wireless transmitters and satellite communication systems often contain nonlinear amplifiers. Amplitude distortion can affect ASK because information is stored in the signal amplitude. FSK and PSK maintain a nearly constant envelope, making them much more resistant to nonlinear distortion.
- ASK is simple but more sensitive to noise and amplitude distortion.
- FSK is more reliable than ASK.
- PSK offers the best performance and is widely used in modern communication systems.
Band-Pass Transmission of Digital Signals
Phase Shift Keying (PSK) is a digital modulation scheme in which information is transmitted by changing the phase of a carrier signal. Binary PSK (BPSK) is the simplest form of PSK, using two phases to represent binary '1' and '0'. Each input bit causes a phase shift of the carrier. The transmitted passband signal occupies a specific frequency band around the carrier frequency $f_c$, making it suitable for radio, optical, or other communication channels.
Baseband PSK: Binary data is mapped to voltage levels (e.g., +1 for '1' and -1 for '0')
before modulating the carrier. This signal is centered at zero frequency.
Passband PSK: The baseband signal modulates a high-frequency carrier $A_c \cos(2 \pi f_c t)$.
Binary '1' is represented by a 0° phase shift, and binary '0' by a 180° shift:
\[ x(t) = \begin{cases} A_c \cos(2 \pi f_c t), & \text{for binary 1} \\ A_c \cos(2 \pi f_c t + \pi) = -A_c \cos(2 \pi f_c t), & \text{for binary 0} \end{cases} \]
Passband PSK Receiver (Coherent Demodulation):
The received passband signal is multiplied by a synchronized local carrier and integrated over the bit duration $T_b$:
If '1' was transmitted:
\[
\int_0^{T_b} A_c \cos(2 \pi f_c t) \cdot A_c \cos(2 \pi f_c t) \, dt \approx \frac{A_c^2}{2} T_b
\]
If '0' was transmitted:
\[
\int_0^{T_b} (-A_c \cos(2 \pi f_c t)) \cdot A_c \cos(2 \pi f_c t) \, dt \approx -\frac{A_c^2}{2} T_b
\]
The output is then compared to a threshold $V_{th}$ to decide whether a '1' or '0' was sent.
Summary
Passband PSK modulates a carrier’s phase according to digital data. Coherent demodulation is used to recover the original signal. PSK is sensitive to phase noise and fading but robust in high SNR scenarios. Higher-order PSK increases data rate but requires better SNR. Performance can be optimized using error correction, adaptive modulation, and diversity techniques.
| 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.