QPSK- Quadrature Phase Shift Keying, Advantages, Disadvantages
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QPSK full form- Quadrature Phase Shift Keying
The Double Side Band Suppressed Carrier (DSBSC) modulation scheme, also known as Quadrature Phase Shift Keying (QPSK), is a variant of BPSK that sends two bits of digital data at a time, or bigits. Digital bits are changed into bit pairs rather than being converted into a stream of digital bits. In order to make room for the additional users, the data bit rate is reduced by half as a result.
QPSK Modulator
The bit-splitter, two multipliers with local oscillators, a 2-bit serial to parallel converter, and a summer circuit are all components of the QPSK Modulator. The block diagram for the same is provided below.
Even bits from the message signal, such as the second, fourth, and sixth bits, are separated from odd bits (i.e., 1st bit, 3rd bit, 5th bit, etc.) by the bits splitter at the modulator’s input before being multiplied by the same carrier to create odd BPSK (also known as PSKI) and even BPSK (also known as PSKQ). In any case, the PSKQ signal undergoes a 90° phase shift before being modulated. The following QPSK waveform for a two-bit input displays the modulated outcome for various binary input scenarios.
Quadrature Phase Shift Keying Demodulator
The Quadrature Phase Shift Keying Demodulator employs a 2-bit parallel to serial converter, two product demodulator circuits with local oscillators, two band pass filters, two integrator circuits, and two. The corresponding diagram is provided below.
The two BPSK signals are simultaneously demodulated by the two product detectors at the demodulator’s input. Here, the original data is used to recover the pair of bits. Following processing, these signals are sent to the parallel to serial converter.
Advantages of QPSK
- It provides low error probability
- QPSK provide very good noise immunity
- When compared to BPSK, QPSK requires only half the bandwidth for the same BER.
- Bandwidth is twice efficient is compared to BPSK modulation
- Carrier power almost remains constant because of OQPSK amplitude is not much
- It is a more effective use of the transmission channel’s available bandwidth.
Disadvantages of QPSK
- Due to the four states required to recover binary data information, QPSK is more complicated than BPSK receiver.
- Compared to other modulation types, QPSK is not a more energy-efficient modulation technique because it takes more energy to transmit two bits.