Difference Between QAM and QPSK

The primitive difference between the QAM and QPSK is that the spectral width of QAM is narrower than the QPSK. Furthermore, the BER (Bit Error Rate) of the QAM is higher than the QPSK. Previously, to transfer the digital data we used analog transmission medium. Hence, we required a technology which can convert the digital data into analog signals, such as employed in the telephone networks. So, to perform such task the modem (modulator/demodulator) is used where the modulation and demodulation of the signal is carried out.

Modulation requires the alteration of any of the three characteristics (i.e., amplitude, frequency and phase) of the carrier wave. 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. Among these modulation techniques, we are going to compare the two, QAM and QPSK.

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°. On the other hand, QAM is a group of ASK and PSK.

BER 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.

Content: QAM Vs QPSK

1. 1. Comparison Chart
   2. Definition
   3. Key Differences
   4. Conclusion

Comparison Chart

Basis for comparison	QAM	QPSK
Stands for	Quadrature Amplitude Modulation	Quadrature Phase-Shift Keying
Spectral width	Narrow	Wide
Number of transmitted bits	Depends on its type	2 bits
Performance	Average	Better
Bit error rate	High	Low

Definition of QAM

QAM (Quadrature Amplitude Modulation) is the combination of analog and digital modulation method. 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).

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. 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).

In the case of digital QAM, a definite number of minimum of two phases and least of two amplitudes are employed. As the amplitude of the modulated carrier is consistent initiates the designing of the PSK modulators using QAM principles but not considered as QAM.

QAM modulation scheme highly used in digital telecommunication systems. 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. 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.

Definition of QPSK

QPSK (Quadrature Phase-Shift Keying) is a kind of Phase Shift Keying. 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). It can carry double information as standard PSK using the same bandwidth. 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.

There are various names of QPSK such as quaternary PSK, Quadriphase PSK, 4-PSK, or 4QAM. The QPSK diagram is constructed using four points over the constellation diagram, placed with equal spacing around the circle. Using four phases, the encoding in QPSK involves 2 bits in each symbol with grey coding to reduce the bit error rate (BER).

Conclusion

Both the modulation techniques QAM and QPSK are evaluated on the basis of the power efficiency, bit rate error, bandwidth efficiency and some other factors. However, in this particular case, the performance of QPSK is better than QAM in some aspects.