On completion of this module, you will be able to:

• Understand Binary Frequency Shift Keying (BFSK)
• Explain generation and detection of BFSK system
• Understand phase shift keying (PSK)
• Understand mathematical models for communication channel
• Know the advantages & disadvantages of Digital Communication system
• Learn electromagnetic bands

Digital Modulation Techniques

• Modulation is defined as the process by which some characteristics of a carrier signal is varied in accordance with a modulating signal
• In digital communications, the modulating wave consists of binary data or an M-ary encoded version of it and the carrier is sinusoidal wave
• Shift keying methods which are used in digital modulation techniques are: -
  1. Amplitude Shift Keying (ASK)
  2. Frequency Shift Keying (FSK)
  3. Phase Shift Keying (PSK)
• Wave form representation of different types of digital modulation techniques is shown below:-


Fig. 3.1: Wave form Representation of different types of DMT.

Coherent modulation

• An estimate of the channel phase and attenuation is recovered
• It is then possible to reproduce the transmitted signal and demodulate in coherent detection
• Requires a replica carrier wave of the same frequency and phase at the receiver.
• The received signal and replica carrier are cross-correlated using information contained in their amplitudes and phases. Also known as synchronous modulation Applicable to : -
  1. Phase Shift Keying (PSK)
  2. Frequency Shift Keying (FSK)
  3. Amplitude Shift Keying (ASK)

Non-coherent modulation

• In non-coherent detection knowledge of the carrier waves phase is not required
• Non-coherent detection is less complex than coherent detection (easier to implement) but has worse performance
• Inferior error performance compared to coherent detection Applicable to :-
  1. Differential Phase Shift Keying (DPSK)
  2. Frequency Shift Keying (FSK)
  3. Amplitude Shift Keying (ASK)

Phase Shift Keying(PSK)

• PSK is a form of modulation which represents digital data as variations in the phase of a carrier wave
• It is usually imposed and measured with respect to a fixed carrier of known phase coherent PSK
• If the phase of the wave does not change, then the signal state remain the same (low or high)
• If the phase of the wave changes by 180 degrees, i.e., if the phase reverses, then the signal state changes (from low to high or from high to low)
• If two or more of the same logic level are received in secession the frequency will remain the same until the logic level changes


Fig. 3.2: Illustration of PSK Signal

• In binary phase shift keying (BPSK) the transmitted signal is a sinusoid of fixed amplitude.
• It has fixed phase, when data is at one level and the phase is different by 180 degrees, when the data is at other level.
• In BPSK the carrier phase is split between the digit 0 & 1


Fig. 3.3: BPSK Signal Waveform

Generation of BPSK Signal

• BPSK signal can be generated by applying carrier signal to the product modulator or balanced modulator
• The transmitter consists of a single arm supplied with its own basis function

• The input bit stream is coded in polar format (NRZ) with amplitude √E_b for digit '1' and - √E_b for digit '0'

Fig. 3.4: Generation of BPSK Signal

Detection of BPSK signal

• Received signal consists of the desired modulated signal s(t) and narrowband Gaussian noise w(t)


Fig. 3.5: Detection of BPSK Signal

• Detector consists of the product modulator, the integrator and a binary decision device, such as a comparator


• First term in r₁ is the desired term while the second term represents the effect of additive noise.
• Comparator will decide received signal as logic '1' if r₁ > '0' and logic '0' if r₁ < '0'.

Quadrature Phase Shift Keying (QPSK):

• QPSK used to double the data rate while maintaining the same band width as BPSK
• To reduce bit error rate by using gray coding
• More spectrally efficient, more complex receiver
• With four phases, QPSK can encode two bits per symbol
• Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same performance but twice the bandwidth efficiency
• Output waveform is sum of modulated, ± Cosine and ± Sine wave

Fig. 3.5: Quadrature Phase Shift Keying

Generation of QPSK:

Fig. : Generation of QPSK

• The binary wave is divided by means of a de-multiplexer in to two separate binary waves consisting of the odd and even numbered bits.
• The two binary waves (even and odd) are used to modulate a pair of orthogonal carriers f1and f2 . The result is a pair of binary PSK waves which may be detected independently due to the orthogonality of f1and f2 .
• Finally the two binary PSK waves are added to produce the desired QPSK wave.

Detection of QPSK:

Fig. : Detection of QPSK

• The QPSK receiver consists of a pair of correlator with a local generated pair of coherent reference signals f1and f2 .
• The correlator outputs x1(t) and x2(t) are each compared with a threshold of zero volt.
• Finally the two binary sequences at the in-phase and quadrature channel outputs are combined in a multiplexer to produce the original binary sequence.

Introduction to BASK

• In amplitude shift keying(ASK), the carrier wave amplitude is changed between discrete levels in accordance with the digital data.
• Two amplitudes are used to directly represent the data, either 0 or 1. In this case, the modulation is called binary amplitude shift keying (BASK).
• One binary digit is represented by the presence of a carrier and the other binary digit is represented by the absence of a carrier.
• Frequency and phase remains constant.

Fig. 3.1: Implementation of BASK.

Quadrature Amplitude Shift Keying(QASK)

• Quadrature Amplitude Shift Keying(QASK) also called Quadrature Amplitude Phase Shift Keying(QAPSK) or Quadrature Amplitude Modulation(QAM).
• Quadrature amplitude modulation is a combination of ASK and PSK.
• In QAM number of amplitudes shifts is fewer than the number of phase shifts
• It improves the noise immunity of a system by allowing the signal vector to differ in both phase and amplitude

Fig. 3.2: Waveform representation of 8- QAM for different bits

Introduction to FSK

Fig. 3.1: Representation of digital signal using FSK.

• Frequency Shift Keying (FSK) is a popular form of digital modulation technique.
• It is used in low-cost applications for transmitting data at moderate or low rate over wired as well as wireless channel.
• It is mainly used in higher frequency.
• The frequency of the carrier is changed as a function of the modulating signal (data), which is being transmitted.
• Amplitude remains unchanged.
• Two carrier frequency are used for binary frequency shift keying (BFSK) modulation.
• One frequency is called mark frequency (f1= binary 1) and other space frequency( f2= binary 0).
• In BFSK, the carrier frequency is shifted by the binary input data.
• Thus the input and output rates of changes are equal and therefore the bit rate is equal to baud rate.
• This method is less susceptible to errors than ASK.

Fig. 2: BFSK Signal.

Mathematical expression for BFSK signal

• If T_b indicates the duration of one information bit, the two time-limited signals can be expressed as :

• The BFSK uses two different carrier frequencies mark and space frequency

Basis function and signal vector

• For special relationship between the two frequencies one can also define two orthonormal basis functions as

Generation of BFSK Signal

• The incoming binary data sequence is applied to on-off level encoder
• The output of encoder is represented by √(E_b) volts for symbol '1' and 0 volts for symbol '0'
• For symbol '1', the upper channel is switched on with oscillator frequency f₁, while the oscillator with frequency f₂ in the lower channel is switched off
• For symbol '0', because of inverter the lower channel is switched on with oscillator frequency f₂
• These two frequencies are combined using an adder circuit and then transmitted
• The transmitted signal is nothing but required BFSK signal

Detection of BFSK Signal

• The detector consists of two correlators. The incoming noisy BFSK signal x(t) is common to both correlators
• The coherent reference signal φ₁ (t) and φ₂ (t) are supplied to upper and lower correlators respectively.
• The correlator outputs are then subtracted, one from the other and output y is compared with the threshold of zero volts
• If y > 0, the receiver decides in favour of symbol '1'. On the other hand, if y < 0, the receiver decides in favour of symbol '0'

Minimum Shift Keying(MSK)

• Minimum shift keying (MSK) is a special type of continuous phase-frequency shift keying(CPFSK) where the frequency spacing is minimum.
• This minimum spacing allows two FSK signals to be coherently orthogonal.
• MSK tries to shift the phase after one symbol to just half of FSK system. The transmitted signal is given by

s(t) =√( 2E_b ⁄ T_b)cos[2πf_ct+θ(t)]

where θ(t) is the phase of s(t)

• When the phase θ(t) is a continuous function of time, the modulated signal s(t) is also continuous at all times.