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BPSK vs QPSK-Difference between BPSK and QPSK

This page on BPSK vs QPSK describes difference between BPSK and QPSK modulation techniques.It mentions link to basics of BPSK and QPSK along with difference between various terminologies.

These are digital modulation techniques where in data input is digital (i.e. in binary form) and output is modulated analog spectrum as shown below in power spectral density plot. Another input required for modulating the digital input bit stream is also analog RF carrier.

BPSK and QPSK modulations techniques have their own advantages and disadvantages and hence are used in the same wireless system for different purpose. On this page we will compare both BPSK vs QPSK modulations and also will go through difference between BPSK and QPSK in terms of many useful factors such as constellation diagram, robustness, power spectral density(PSD) etc.


BPSK constellation fig.1 BPSK constellation digram

The short form of Binary Phase Shift Keying is referred as BPSK. For step by step description of how bit streams get converted to BPSK symbols, refer BPSK modulation.

Input Output
1 1
0 (0 degree) -1 (180 degree w.r.t. reference)


QPSK constellation
fig.2 QPSK constellation diagram

The short form of Quadrature Phase Shift Keying is referred as QPSK. For step by step description of how bit streams get converted to QPSK symbols, refer QPSK modulation.

Input1 Input2 Output
0 0 -1-1*i
0 1 -1+1*i
1 0 1-1*i
1 1 1+1*i

Usually above complex values are multiplied with 1/sqrt(2) or 0.7071 in order to normalize the modulated waveform. After understandig the modulation types individually, now let us examine the difference between BPSK and QPSK.

As mentioned above in the table, BPSK represents binary input 1 and 0 w.r.t. change in carrier phase by 180 degree. While QPSK represents two bits using complex carrier symbol each having 90 degree shift with one another. BPSK is considered to be robust modulation scheme compare to the QPSK as it is easy in the receiver to receiver the original bits.

After passing both BPSK and QPSK through channel and noise, in the BPSK demodulator only two decision points are required to retrieve the original binary information. In QPSK demodulator four decision points are needed. With BPSK, higher distance coverage can be achieved from the base station cellular cell or fixed station to the mobile subscribers compare to QPSK.

QPSK BPSK Power Spectral Density
fig.3 Power spectral density of BPSK vs QPSK

QPSK has advantages of having double data rate compare to BPSK. This is due to support of two bits per carrier in QPSK compare to one bit per carrier in the case of BPSK.

Figure-2 mention difference between power spectral density of BPSK and QPSK modulated spectrum.

To take advantage of both the modulation schemes advance wireless systems such as WLAN, WiMAX, LTE utilize both for different channel or synchronization signals. For example BPSK is used for preamble or pilot sequences or beacon frame used for channel and other synchronization purpose. While QPSK is used for data transmission to provide higher data rate.

BPSK QPSK and other Modulation types links

Other than BPSK vs QPSK one can also refer following links which mentions difference between AM vs FM vs PM, ASK vs FSK vs PSK, about modulation basics and modulation types viz. BPSK, QPSK, QAM, 8PSK, DPSK, MSK, GMSK, C4FM vs CQPSK etc.
AM vs FM vs PM  ASK vs FSK vs PSK  what is modulation  MSK and GMSK modulation  8-PSK modulation  QPSK modulation  BPSK modulation  QAM modulation  DPSK Modulation  C4FM vs CQPSK

What is difference between

Refer QPSK vs OQPSK vs pi/4QPSK for difference between QPSK, OQPSK and pi/4QPSK modulation types
difference between OFDM and OFDMA
Difference between SC-FDMA and OFDM
Difference between SISO and MIMO
Difference between TDD and FDD

RF and Wireless Terminologies

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