BPSK vs QPSK: Key Differences in Modulation Techniques

bpsk
qpsk
modulation
digital communication
wireless communication

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.

BPSK constellation

Fig. 1: BPSK Constellation Diagram

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

InputOutput
11 (0 degree)
0-1 (180 degree w.r.t. reference)

QPSK constellation

Fig. 2: QPSK Constellation Diagram

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

Input1Input2Output
00-1-1*i
01-1+1*i
101-1*i
111+1*i

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.

QPSK BPSK Power Spectral Density

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.

BPSK: Advantages and Disadvantages

BPSK: Advantages and Disadvantages

Explore the advantages and disadvantages of Binary Phase Shift Keying (BPSK) modulation, including its robustness and power efficiency, alongside its limitations in data rate and bandwidth efficiency.

bpsk
modulation
data transmission
QPSK: Advantages and Disadvantages

QPSK: Advantages and Disadvantages

Explore the pros and cons of QPSK modulation, including bandwidth efficiency and receiver complexity. Learn about the benefits and drawbacks of QPSK.

modulation
digital communication
qpsk

GFSK Modulation: Advantages and Disadvantages

Explore the benefits and drawbacks of GFSK modulation, including its spectral efficiency, bandwidth usage, and limitations in noise sensitivity and constant envelope applications.

gfsk
modulation
spectral efficiency