PSK (Phase-Shift Keying) – a digital modulation process also known as phase-shift keying. Keying means that the modulation process consists in switching periodical signal parameters, in this case, a phase. The data are represented using discrete changes in the phase of the carrier wave. The number of changes can be random and is usually equal to the power of 2 (used to differentiate between PSK modulation types, including BPSK and its DBPSK variant, and QPSK and its DQPSK variant).

Constellation diagram (phase-shift diagram) is a graphic representation of a digitally modulated signal consisting of the constellation points used to assess the quality of a transmitted signal.

Apart from usable data, the modulated and transmitted signal also includes interferences, noise, and disturbances. For the demodulator to correctly demodulate a usable signal bit, a symbol must be matched to a corresponding constellation point (bit). Otherwise, the received signal will be distorted.

BPSK (Binary Phase Shift Keying) – a coherent modulation technique that requires synchronization between the transmitter and the receiver using two phase values: –180° and +180°, encoded as a single bit in the binary code as a logical 0 or 1.

Fig. 2. BPSK constellation diagram

DBPSK (Differential Binary Phase Shift Keying) – a modulation similar to BPSK using two phase values: –180° and +180°; encoded in the binary code as a logical 0 or 1.

The difference between BPSK and DBPSK is in the transmitted information which in this case is not a phase (BPSK) but information on phase-shift of the carrier signal. DBPSK constellation diagram is identical to BPSK since both modulations are binary. A single constellation point (single digital/binary phase-shift value) is encoded in a single 0 or 1 bit.

BPSK and DBPSK modulation rates are equal at 1 Mbit/s.

QPSK modulation (Quaternary Phase Shift Keying) – a technique using four phase-shift values –45°, –135°, +45° or +135° (shift every 90°), expressed as two bits in the binary code: 00, 01, 11 or 10. The figure shows QPSK modulation constellation diagram including four constellation points. Each constellation point is encoded in two bits.

Fig. 3. QPSK constellation diagram

Quaternary modulations (QPSK/DQPSK) use two independent carrier signals and their constellation diagrams include twice the number of constellation points as the binary modulations (BPSK/DBPSK).

DQPSK modulation (Differential Quaternary Phase Shift Keying) – the main difference between QPSK and DQPSK is that the constellation points store information on the carrier signal phase-shift (not phase - as in QPSK). DQPSK constellation diagram is identical to QPSK constellation diagram since both modulations are quaternary. A single constellation point is encoded in two bits: 00, 01, 11 or 10.

DQPSK receiver compares the phase of a current signal with a previously received signal. If the phase does not shift, the signal is stored as a 00 point in the constellation diagram (logical zero). If the phase shifts, the signal is stored as 01, 11 or 10 depending on the phase-shift value.

Twice the number of QPSK/DQPSK constellation points can be interpreted as an ability to transmit twice the volume of data at the same time compared to BPSK/DBPSK since QPSK/DQPSK can use twice as many bits. QPSK and DQPSK modulation rates are equal at 2 Mbit/s.

BPSK and QPSK modulation techniques require synchronization between the transmitter and the receiver and are referred to as coherent techniques. Weather conditions, noise, interferences, and changes in the supply voltage can affect and interfere with the wanted signal. For signal transmission over radio frequency at large distances, maintaining the synchronization may prove impossible, and thus the coherent techniques are dedicated for short transmission paths.

The issue can be mitigated by using differential phase-shift keying (DPSK/DBPSK/DQPSK) - non-coherent techniques that do not require synchronization. In a non-coherent receiver, the phases of an incoming signal are compared with the phase of a previous signal. The process stores a single bit/bits value in the receiver - a function that can easily be implemented at the design stage. Non-coherent receivers are less complex and cheaper to manufacture.

Table 1. Comparison of BPSK, DBPSK, QPSK and DQPSK modulation parameters

Modulation parameter

BPSK

DBPSK

QPSK

DQPSK

Bits per symbol [bps*]

1

1

2

2

Detection method

coherent

non-coherent

coherent

non-coherent

Constellation points (number of phase-shift combinations)

2

2

4

4

Required bandwidth [%]

100

100

50

50

Bandwidth efficiency [bps*/Hz]

0.5

0.5

1

1

* bps – bits per symbol

A comparison of four modulation methods shows that DQPSK is the most effective and has the highest ratio of energy per bit to spectral noise density. In practice, the DQPSK modulated signal is less susceptible to noise, interferences, and errors. DQPSK does not require synchronization between the transmitter and the receiver, and since a single piece of information can be stored in two bits, it is more spectrally efficient than DBPSK.

Standard modulation techniques (BPSK, QPSK) does not require high SNR, however, higher bit rates are not available. Higher bit rates can only be achieved at higher SNRs.

Differential phase-shift keying is an improvement over a standard PSK modulation.

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