BPSK vs QPSK: Key Differences in Modulation Techniques

This article compares Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) modulation techniques. We’ll cover the basics of BPSK and QPSK, highlighting the differences between various terminologies. These are digital modulation techniques, meaning the input data is digital (binary), and the output is a modulated analog spectrum, as visualized in the power spectral density plot. The other necessary input for modulating the digital bitstream is the analog RF carrier.

BPSK and QPSK have their own strengths and weaknesses, leading to their use in the same wireless systems for different purposes. We’ll compare BPSK and QPSK, examining differences in constellation diagrams, robustness, power spectral density (PSD), and other useful factors.

Fig. 1: BPSK Constellation Diagram

BPSK, short for Binary Phase Shift Keying, converts bitstreams into BPSK symbols.

Fig. 2: QPSK Constellation Diagram

QPSK, short for Quadrature Phase Shift Keying, converts bitstreams into QPSK symbols.

Often, the complex values above are multiplied by 1/sqrt(2) or 0.7071 to normalize the modulated waveform.

BPSK vs QPSK: Key Differences

Now that we understand the individual modulation types, let’s compare them. As shown in the tables above, BPSK represents binary inputs (1 and 0) by changing the carrier phase by 180 degrees. QPSK, however, represents two bits using a complex carrier symbol, with each symbol having a 90-degree phase shift relative to the others.

Robustness: BPSK is considered more robust than QPSK because it’s easier for the receiver to recover the original bits. After passing both BPSK and QPSK signals through a channel with noise, the BPSK demodulator only needs to distinguish between two decision points to retrieve the original binary information. The QPSK demodulator requires four decision points.

Coverage: BPSK allows for greater distance coverage from a base station (cellular or fixed) to mobile subscribers compared to QPSK.

Fig. 3: Power Spectral Density of BPSK vs. QPSK

Data Rate: QPSK has the advantage of offering double the data rate compared to BPSK. This is because QPSK supports two bits per carrier, while BPSK only supports one.

Figure 3 illustrates the difference in the power spectral density between BPSK and QPSK modulated spectra.

Combining the Best of Both Worlds

Advanced wireless systems like WLAN, WiMAX, and LTE leverage the advantages of both modulation schemes. For instance, BPSK is commonly used for preamble, pilot sequences, or beacon frames for channel synchronization. QPSK is employed for data transmission to achieve higher data rates.