Digitaltoanalog modulation Types of digitaltoanalog modulation Amplitude Shift

Amplitude Shift Keying (ASK) On/Off keying • • The amplitude of the carrier signal

Frequency Shift Keying • Frequency of the carrier is varied to represent digital data

Phase Shift Keying • • Phase of the carrier is varied to represent digital

4 -PSK (QPSK) COS SIN 45 +0. 707 135 -0. 707 +0. 707 225

QPSK TRANSMITTER a 1 (or) a 2=+√(E/2) (or)-√(E/2)

Offset QPSK waveforms • Presence of 180 degree phase shift in QPSK makes the

Pi/4 QPSK signaling ØMaximum phase angle change is limited to +135 (or) -135 ØCan

Possible values of Øk are +45, -45, +135, -135

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Digital-to-analog modulation Types of digital-to-analog modulation

Amplitude Shift Keying (ASK) On/Off keying • • The amplitude of the carrier signal is varied to represent binary 1 & 0. Frequency and phase remains the same. Highly susceptible to noise interference. Used up to 1200 bps on voice grade lines, and on optical fiber.

Frequency Shift Keying • Frequency of the carrier is varied to represent digital data (binary 0/1) • Peak amplitude and phase remain constant. • Avoid noise interference by looking at frequencies (change of a signal) and ignoring amplitudes. • Limitations of FSK is the physical capabilities of the carrier. • f 1 and f 2 equally offset by equal opposite amounts to the carrier freq. • In MFSK more than 2 freq are used, each signal element represents more than one bit

Phase Shift Keying • • Phase of the carrier is varied to represent digital data (binary 0 or 1) Amplitude and frequency remains constant. If phase 0 deg to represent 0, 180 deg to represent 1. (2 -PSK) PSK is not susceptible to noise degradation that affects ASK or bandwidth limitations of FSK

BPSK TRANSMITTER BPSK RECEIVER

Signal space diagram of BPSK

4 -PSK (QPSK) method

4 -PSK (QPSK)

4 -PSK (QPSK) COS SIN 45 +0. 707 135 -0. 707 +0. 707 225 -0. 707 315 +0. 707 -0. 707

4 -PSK (QPSK) SIGNAL SPACE DIAGRAM

4 -PSK (QPSK)

QPSK TRANSMITTER a 1 (or) a 2=+√(E/2) (or)-√(E/2)

QPSK RECEIVER

DERIVATION-PROBABILITY OF ERROR IN QPSK

Offset QPSK waveforms • Presence of 180 degree phase shift in QPSK makes the signal to loose constant envelope property. Non linear amplification of such signal causes unwanted side lobes and spectral widening. • In OQPSK maximum phase shift is limited to 90 degree. • Even and Odd bit streams are offset in their relative alignment by one bit period.

Offset QPSK waveforms

Offset QPSK Transmitter

Pi/4 QPSK signaling ØMaximum phase angle change is limited to +135 (or) -135 ØCan be noncoherently detected

Pi/4 QPSK phase shifts

Possible values of Øk are +45, -45, +135, -135

Pi/4 QPSK transmitter

Differential detection of pi/4 QPSK