QAM is being used in optical fiber systems as bit rates increase QAM16 and QAM64 can be optically emulated with a 3-path interferometer. Arbitrarily high spectral efficiencies can be achieved with QAM by setting a suitable constellation size, limited only by the noise level and linearity of the communications channel. QAM is used extensively as a modulation scheme for digital telecommunication systems, such as in 802.11 Wi-Fi standards. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded as a special case of phase modulation. Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the transmitted signal is a constant, but its phase varies. Another key property is that the modulations are low-frequency/low-bandwidth waveforms compared to the carrier frequency, which is known as the narrowband assumption. At the receiver, the two waves can be coherently separated (demodulated) because of their orthogonality property. The transmitted signal is created by adding the two carrier waves together. The two carrier waves of the same frequency are out of phase with each other by 90°, a condition known as orthogonality or quadrature. It conveys two analog message signals, or two digital bit streams, by changing ( modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. Using four phases, the encoding in QPSK involves 2 bits in each symbol with grey coding to reduce the bit error rate (BER).Quadrature amplitude modulation ( QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. The QPSK diagram is constructed using four points over the constellation diagram, placed with equal spacing around the circle. There are various names of QPSK such as quaternary PSK, Quadriphase PSK, 4-PSK, or 4QAM. It is mainly used for satellite transmission of MPEG 2 video, cable modems, video conferencing, cellular phone systems, and other types of digital communication over Radio Frequency carrier. It can carry double information as standard PSK using the same bandwidth. Here quadrature is added to the standard PSK where 2 bits are modulated at one time by choosing one among the four probable carrier phase shifts (0, 90, 180, or 270 degrees). QPSK (Quadrature Phase-Shift Keying) is a kind of Phase Shift Keying.
The QAM modulations have various applications such as in optical fibre system when bit rates boosted, and QAM 16 and 64 can be optically emulated with a 3-path interferometer. To achieve more spectral efficiencies in QAM, the suitable constellation size is fixed and restricted by the linearity of the communication channels and the degree of the noise. QAM modulation scheme highly used in digital telecommunication systems. As the amplitude of the modulated carrier is consistent initiates the designing of the PSK modulators using QAM principles but not considered as QAM. In the case of digital QAM, a definite number of minimum of two phases and least of two amplitudes are employed. The modulated waves are merged and the consequent waveform is a composite form of both PSK (Phase Shift Keying) and ASK (Amplitude Shift Keying) techniques or PM (Phase Modulation) and AM (Amplitude Modulation) (in the analog case). There are two sinusoid carrier wave does not remain in phase with each other exhibiting the difference of 90° and therefore called quadrature carriers or quadrature components. In order to transmit two analog message signals/two digital bit streams, it modulates the amplitude of the two carrier waves with the help of amplitude shift keying (ASK). QAM (Quadrature Amplitude Modulation) is the combination of analog and digital modulation method. On the other hand, QAM is a group of ASK and PSK.īER is the percentage of the faulty bits as per the total number of transmitted, received and processed bits in a certain period of time which is equivalent to signal-to-noise ratio in an analog system.
The QPSK is very much similar to the PSK the only difference between PSK and QPSK is that in basic PSK the phase shift occurs in every 180° degrees while in QPSK the phase shift occurs in multiple of 90°. Among these modulation techniques, we are going to compare the two, QAM and QPSK. This emerged the development of the encoding or modulation techniques named as PSK, FSK, ASK, QPSK and QAM, to convert the digital data into analog signals. Modulation requires the alteration of any of the three characteristics (i.e., amplitude, frequency and phase) of the carrier wave.