What are the advantages of QAM over ASK and PSK.

Subject Data Communication
NU Year Set: 2.(a) Marks: 5 Year: 2008

PSK and QAM modulation have two advantages over ASK:

*They are not as susceptible to noise.

*Each signal change can represent more than one bit


PSK Disadvantage

more complex signal detection / recovery process,

than in ASK and FSK

QAM advantage:

· data rate = 2 bits per bit-interval!

· higher data rate than in PSK (2 bits per bit interval), while bandwidth occupancy remains the same

 4-PSK can easily be extended to 8-PSK, i.e. n-PSK

• however, higher rate PSK schemes are limited by the ability of equipment to distinguish small differences in phase uses "two-dimensional" signaling

• original information stream is split into two sequences that consist of odd and even symbols

· PSK modulators are often designed using the QAM principle but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digital telecommunication systems. 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.

 Noise immunity of QAM is very high.

· QAM is best suitable for high bit rates.

· Low error probability.

· Baud rate is half the bit rate therefore more effective utilization of the available bandwidth of the transmission channel.

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