Question

to integrate (sum up) over each 'bit period'. 3. (9 marks) Give short answers and diagrams for the following. (a) Explain BASK and BPSK. (b) Describe QAM. In particular, explain how bytes are mapped to phase and amplitude in QAM.

Answer 1

a)BASK - Binary Amplitude Shift Keying, The amplitude of the sinusodial carrier signal is changed according to message level, while keeping the frequency and phase constant.

Sinusoidal carrier Band limited BASK output Bandpass BASK output Filter S1 Binary data (message)

BPSK-

Binary Phase Shift Keying, The phase of the sinusodial carrier signal is changed according to message level, while keeping the frequency and amplitude constant.

Sinusoidal carrier (0) BPSK S1 output Sinusoidal carrier (180) (nu Binary data (mossage) IMHHNH www WWW.WAN

b)QAM-Quadrature amplitude modulation (QAM) is a technique that mixes both amplitude and phase variations in a carrier at the same time.The length of the arrow called a phasor is the amplitude, and its direction or angle is the phase.If the receiver gets a signal that is at a phase angle of 135 degrees, it means that the 2-bit combination 01 is being transmitted.

voltage time (a) voltageA time (b)

Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by 90 degrees (i.e. sine and cosine) are modulated and combined. As a result of their 90° phase difference they are in quadrature and this gives rise to the name. Often one signal is called the In-phase or “I” signal, and the other is the quadrature or “Q” signal.

The resultant overall signal consisting of the combination of both I and Q carriers contains of both amplitude and phase variations. In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation.

In phase signal component Quadrature signal AAT component

Mapping diagrams

01 00 11 10 4-QAM 1 amplitude, 4 phases

QAM constellation diagram for 16QAM