Briftly introduce Phase Modulation

Phase describes the measurement of signal waveform change, usually in degrees (angles), also called phase angle.When the signal waveform changes in a periodic manner, the waveform cycle is 360°. In the function y=Acos(ωx+φ), ωx+φ is called the phase.The real purpose of the phase shift is to modulate the digital bit stream on the radio frequency carrier by selectively phase shifting the continuous sine wave. The phase shifter changes the sine wave by a certain amount and the phase detector converts the phase of the signal into a voltage. In this way, the voltage would be transformed into a digital bit stream. BPSK and QPSK are the most popular in digital wireless communication.Subsequently, the receiver takes out the bit stream from the phase shift.

Binary phase-shift keying (BPSK)

BPSK (also sometimes called PRK, phase reversal keying, or 2PSK) is the simplest form of phase shift keying (PSK). It uses two phases which are separated by 180° and so can also be termed 2-PSK. It does not particularly matter exactly where the constellation points are positioned, and in this figure they are shown on the real axis, at 0° and 180°. Therefore, it handles the highest noise level or distortion before the demodulator reaches an incorrect decision. That makes it the most robust of all the PSKs. It is, however, only able to modulate at 1 bit/symbol (as seen in the figure) and so is unsuitable for high data-rate applications.

It has two phase shifts of 0° and 180°, and digital bits have two possibilities of 0 and 1, so BPSK can be used to modulate a digital bit in a sine wave. A phase shift of 0° represents 0, and a phase shift of 180° represents 1. This is a good method, but QPSK is even better.

Quadrature phase-shift keying (QPSK)

Sometimes this is known as quadriphase PSK, 4-PSK, or 4-QAM. (Although the root concepts of QPSK and 4-QAM are different, the resulting modulated radio waves are exactly the same.) QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize the bit error rate (BER) – sometimes misperceived as twice the BER of BPSK.The mathematical analysis shows that QPSK can be used either to double the data rate compared with a BPSK system while maintaining the same bandwidth of the signal, or to maintain the data-rate of BPSK but halving the bandwidth needed. In this latter case, the BER of QPSK is exactly the same as the BER of BPSK – and believing differently is a common confusion when considering or describing QPSK. The transmitted carrier can undergo numbers of phase changes.Given that radio communication channels are allocated by agencies such as the Federal Communications Commission giving a prescribed (maximum) bandwidth, the advantage of QPSK over BPSK becomes evident: QPSK transmits twice the data rate in a given bandwidth compared to BPSK - at the same BER. The engineering penalty that is paid is that QPSK transmitters and receivers are more complicated than the ones for BPSK. However, with modern electronics technology, the penalty in cost is very moderate.As with BPSK, there are phase ambiguity problems at the receiving end, and differentially encoded QPSK is often used in practice.

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