DIGITAL MODULATION TECHNIQUES

Presented By: NIDHI BARANWAL MCA 3rd SEMESTER University of Allahabad

MODULATION SYSTEMS

WHAT IS MODULATION

• Modulation = Adding information to a carrier signal• The sine wave on which the characteristics of the information signal are modulated is called a carrier signal

CONTD.

ANALOG MODULATION

DIGITAL MODULATION• The move to digital modulation provides more information capacity, compatibility with digital data services, higher data security, better qualitycommunications, and quicker system availability.

• In digital communications, the modulating wave consists of binary data or an M-ary encoded version of it and the carrier is sinusoidal wave.

DIGITAL MODULATION TECHNIQUESDifferent Shift keying methods that are used in digital modulation techniques are:

HIERARCHY OF DIGITAL MODULATION TECHNIQUES

COHERENT AND NON COHERENT

In coherent modulation technique, process received signal with a local carrier of same frequency and phase.

In non coherent digital modulation technique,

there is no requirement of reference wave.Less complex receiver,but worse

performance.

AMPLITUDE SHIFT KEYING In ASK, the amplitude of the signal is changed in

response to information and all else is kept fixed.Bit 1 is transmitted by a signal of one particular amplitude.To transmit 0,we change the amplitude keeping the frequency constant.It is shown below.

FREQUENCY SHIFT KEYING In FSK,we change the frequency in

response to information,one particular frequency for a 1 and another frequency for a 0.

PHASE SHIFT KEYING In PSK,we change the phase of the sinusoidal carrier to

indicate information.Phase in this context is the starting angle at which the sinusoidal starts.To transmit 0,we shift the phase of the sinusoid by 180.phase shift represents the change in the state of the information.

QUADRATURE PHASE SHIFT KEYING

Quadrature Phase Shift Keying (QPSK) can be interpreted as two independent BPSK systems (one on the I-channel and one on Q-channel), and thus the same performance but twice the bandwidth (spectrum) efficiency.

QPSK has twice the bandwidth efficiency of BPSK since 2 bits are transmitted in a single modulation symbol

•The phase of the carrier takes on 1 of 4 equally spaced values, where each value of phase corresponds to a unique pair of message bits.The QPSK signal for this set of symbol states may be

M-ARY MODULATION TECHNIQUES

In binary data transmission, send only one of two possible signals during each bit interval Tb

In M-ary data transmission, send one of M possible signals during each signaling interval T

In almost all applications, M = 2n and T = nTb, where n is an integer

Each of the M signals is called a symbol These signals are generated by changing the amplitude, phase,

frequency, or combined forms of a carrier in M discrete steps.

Thus, we have: MASK MPSK MFSK MQAM

MPSK

DIFFERENTIAL MODULATION TECHNIQUES

DPSK is a non coherent form of phase shift keying which avoids the need for a coherent reference signal at the receiver.

ADVANTAGE:• Non coherent receivers are easy and cheap

to build, hence widely used in wireless communications.

• DPSK eliminates the need for a coherent reference signal at the receiver by combining two basic operations at the transmitter:

In DPSK, the phase shift is with reference to the previous bit transmitted rather than to some constant reference signal

Binary 0:signal burst with the same phase as the previous one

Binary 1:signal burst of opposite phase to the preceding one

In the transmitter, each symbol is modulated relative to the previous symbol and modulating signal, for instance in BPSK, 0 = no change, 1 = +1800

In the receiver, the current symbol is demodulated using the previous symbol as a reference. The previous symbol serves as an estimate of the channel. A no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous symbol.

METRICS FOR DIGITAL MODULATION

• Power Efficiency – Ability of a modulation technique to preserve the fidelity of the digital message at low power levels – Designer can increase noise immunity by increasing signal power – Power efficiency is a measure of how much signal power should be increased to achieve a particular BER for a given modulation scheme – Signal energy per bit / noise power spectral density• Bandwidth Efficiency – Ability to accomodate data within a limited bandwidth – Tradeoff between data rate and pulse width – Thruput data rate per hertz: R/B bps per Hz