Principle of BPSK

BPSK is the most straightforward and robust form of Phase-Shift Keying (PSK). The term "Binary" indicates that it uses only two distinct states to represent the two binary digits: '0' and '1'. This makes it a one-to-one encoding scheme, where one symbol corresponds to exactly one bit of information.

The core idea of BPSK is to use phase shifts of a constant-frequency carrier wave to represent data. To ensure maximum distinction between the states for '0' and '1', the phases are chosen to be as far apart as possible: 180 degrees $(\pi \text{ radians})$.

Encoding '0's and '1's

A 180° phase shift is essentially "flipping the wave upside down." This creates a clear, unambiguous difference between the two binary states. A common mapping is:

• Binary '1' is encoded with a phase of 0°. The carrier wave continues without any change.
• Binary '0' is encoded with a phase of 180°. The carrier wave is inverted at the start of the bit's duration.

The choice of mapping (e.g., '1' to $0^{\circ}$ and '0' to $180^{\circ}$, or vice versa) is arbitrary, as long as both the transmitter and receiver agree on the same convention.

The BPSK Constellation Diagram

The most effective way to visualize digital modulation schemes is with a constellation diagram. It plots the symbols on a two-dimensional map defined by In-phase (I) and Quadrature (Q) axes.

• Two Points: Since BPSK has only two symbols, its constellation consists of just two points.
• Constant Amplitude: Both symbols have the same amplitude (power), so they lie at the same distance from the origin (the center of the plot).
• Location on I-axis: As the phases used are 0° and 180°, both points lie on the horizontal I-axis. The symbol for a 0° phase (e.g., bit '1') is on the positive side, and the symbol for 180° (e.g., bit '0') is on the negative side. There is no Q-component.

Interactive BPSK – Constellation and Time Waveform

BPSK modulated signal

Constellation diagram

Robustness and Applications

Error Resilience

The greatest strength of BPSK is its high immunity to noise. On the constellation diagram, the distance between the two symbol points is maximized for a given signal power. Noise in the channel can shift the position of a received symbol, but for an error to occur, the noise must be strong enough to push the symbol past the decision boundary (the vertical Q-axis, exactly halfway between the points). A shift of more than $\pm 90^\circ$ in phase is required to cause an error, which is a relatively rare event in most channels. This makes BPSK highly robust, resulting in a low Bit Error Rate (BER).

Spectral Efficiency and Use Cases

The trade-off for this robustness is lower spectral efficiency. Since each symbol carries only one bit, BPSK is not suitable for high-speed data transmission. However, its reliability makes it the ideal choice for applications where data integrity is more critical than speed.

Common Applications include:

• Satellite Communication: Used in systems where signal power is limited and the channel is noisy.
• Deep-Space Probes: Communication with spacecraft like the Voyager probes relies on very robust, low-data-rate modulation like BPSK.
• RFID Tags: Simple RFID systems often use BPSK for its low implementation complexity and reliability.
• Basis for Complex Modulations: BPSK is a fundamental building block for more advanced schemes like QPSK, which can be viewed as two independent BPSK modulators.