BPSK & BFSK Modulation

Definition

BPSK (Binary Phase Shift Keying) and BFSK (Binary Frequency Shift Keying) are two fundamental digital modulation techniques used in digital communication systems to transmit binary data by changing a carrier wave parameter. In BPSK, the phase of the carrier is switched between two distinct values to represent binary 0 and 1. In BFSK, the frequency of the carrier is switched between two different frequencies to represent binary 0 and 1.

These modulation methods are widely taught in Unit 5: Digital Communication because they clearly show how digital bits can be mapped onto analog waveforms for transmission over a channel.

Main Content

1. BPSK (Binary Phase Shift Keying)

Meaning and principle

BPSK is the simplest form of phase shift keying. It uses two phases of the same carrier wave, usually separated by 180°. One phase represents binary 1, and the opposite phase represents binary 0.
If the carrier is $\cos(2\pi f_ct)$, then BPSK may transmit:

• Bit 1: $+A\cos(2\pi f_ct)$
• Bit 0: $-A\cos(2\pi f_ct)$
  Here, the negative sign means a phase reversal of 180°.

Signal representation and constellation

In a constellation diagram, BPSK has only two points on the in-phase axis: one at $+A$ and one at $-A$. This makes the system easy to understand and implement.
The separation between the two points is large, which gives BPSK excellent noise immunity compared to many other digital modulation schemes.

Example:
If the data sequence is 1 0 1 1 0, the transmitted BPSK carrier alternates between normal phase and inverted phase according to the bits.

2. BFSK (Binary Frequency Shift Keying)

Meaning and principle

BFSK is a digital modulation method in which the carrier frequency changes according to the input bit. One frequency is used for binary 1, and another frequency is used for binary 0. The amplitude generally remains constant.

Signal representation and frequency choices

If two frequencies are $f_1$ and $f_2$, then:

• Bit 1 may be represented by $A\cos(2\pi f_1 t)$
• Bit 0 may be represented by $A\cos(2\pi f_2 t)$
  The two frequencies are selected so that the receiver can distinguish them clearly.

Example:
For a given bit stream, when a 1 arrives, the transmitter sends a tone at $f_1$; when a 0 arrives, it sends a tone at $f_2$. This is why BFSK is often described as a frequency-based binary signaling method.

3. Comparison Between BPSK and BFSK

Parameter changed

• BPSK changes the phase of the carrier.
• BFSK changes the frequency of the carrier.

Performance and complexity

• BPSK generally offers better bandwidth efficiency and better error performance than BFSK in many cases.
• BFSK is often simpler to detect non-coherently, meaning exact carrier phase reference may not be required.

Simple view:

BPSK:
Bit 1 -> Carrier phase = 0°
Bit 0 -> Carrier phase = 180°

BFSK:
Bit 1 -> Frequency = f1
Bit 0 -> Frequency = f2

Waveform idea:

BPSK:
+ + + +    - - - -    + + + +
(phase flips)

BFSK:
~~~~~~~    ~~~~~~~~~~~~    ~~~~~~
(low f)      (high f)       (low f)

Working / Process

1. Binary data is generated

• The message is first available as bits, such as 1011001.
• Each bit must be mapped to a corresponding carrier modification.

2. Carrier wave is modulated

• In BPSK, the carrier’s phase is reversed depending on the bit value.
• In BFSK, the carrier’s frequency is switched between two preset values.

3. Signal is transmitted and detected at the receiver

• The modulated signal passes through the channel.
• The receiver identifies the phase or frequency change and reconstructs the original bit stream.
• In BPSK, coherent detection is often used.
• In BFSK, either coherent or non-coherent detection may be used depending on system design.

Advantages / Applications

Advantages of BPSK

• Very good noise performance and high reliability in noisy channels.
• Simple binary constellation makes it easier to analyze and implement.
• Good power efficiency, especially when compared with many higher-order digital modulation schemes.

Advantages of BFSK

• Easier to detect in some systems without needing exact phase synchronization.
• More robust in certain practical communication environments.
• Useful in systems where frequency-based signaling is convenient.

Applications

• Satellite and deep-space communication systems often use BPSK because of its efficiency and reliability.
• BFSK is used in low-speed data transmission, radio telemetry, and some wireless and paging systems.
• Both are important in modems, digital radios, and educational understanding of digital modulation principles.

Summary

• BPSK changes the phase of the carrier to send binary data.
• BFSK changes the frequency of the carrier to send binary data.
• Both are basic and important digital modulation techniques used to transmit bits over communication channels.
• Important terms to remember: carrier, phase, frequency, bit, modulation, constellation, coherent detection, non-coherent detection