Digital Modulation Schemes | Communication System - Electronics and Communication Engineering (ECE) PDF Download

Digital Modulation Schemes

This is possible to transmit the analog signal i.e., speech, video etc, in digital format. Some digital modulation schemes are given below

• Digital Carrier Modulation: Commonly used digital modulation schemes are Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK).
• Amplitude Shift Keying (ASK): The amplitude of a high-frequency carrier is varied in accordance with digital data (0 or 1).

S(t) = A_c cos 2πf_ct; 0 ≤ t ≤ T_b

= 0; otherwise

Bandwidth = 2 × 1/T_b

= 2 × bit rate

• For digital input 1 amplitude level is high and for digital input 0 amplitude level is low.
• Signalling used is on-off signalling.

Demodulation of ASK

For binary digit 1, A_c cos 2π f_ct × A_c cos 2πf_ct = (A²_c /2)[1 + cos 4πf_ct]

Output of LPF = (A²_c /2)

For binary digit 0 output of LPF = 0

In ASK, probability of error (P_e) is high.

In ASK, SNR is less.

Example: Consider a simple ASK modulation scheme where two digital symbols, 0 and 1, are transmitted using two different amplitudes. When transmitting '0,' the amplitude is 0, and when transmitting '1,' the amplitude is 1.

When receiving the signal, a receiver can use a threshold detector to distinguish between the two amplitudes. If the received amplitude is greater than a certain threshold, it is decoded as '1,' and if it's below the threshold, it's decoded as '0.'

Phase Shift Keying (PSK)

In phase shift, keying phase of high-frequency carrier is varied in accordance with digital data 1 or 0.

• NRZ signalling is used.

S(t) = A_c cos 2πf_ct for bit 1

= – A_c cos 2pf_ct for bit 0

A frequency of the carrier must be a multiple of a bit rate.

T_b = n/f_c

F_c = nr_b

• In case of PSK, a probability of error is less.
• In case of PSK, SNR is high.
• Mainly used a technique in wireless transmission.

Example: PSK uses different phases of the carrier signal to represent digital symbols. Let's say '0' is represented by a phase of 0 degrees, and '1' is represented by a phase of 180 degrees.

The receiver uses a phase detector to measure the phase of the received signal. If the phase is 0 degrees, it's decoded as '0,' and if it's 180 degrees, it's decoded as '1.'

Frequency Shift Keying (FSK)

In frequency shift keying, a frequency of the carrier is varied in accordance with digital data (1 or 0).

For digital data 1 we use frequency f1 and for digital data 0 we use frequency f₂.

• NRZ signalling is used here
• VCO The schematic diagram of VCO is given below

Bandwidth = 2Δf + 2f_m

Bandwidth = f₁ + (1/T_b) – f₂ + (1/T_b)

= f₁ – f₂ + (2/T_b); f₁ – f₂ = 2Δf

Key Points

• In case of FSK, Pe is less but SNR is high.
• Multiplexing is difficult in FSK.

Example: In FSK, two digital symbols are transmitted by using two different carrier frequencies. For example, '0' might be transmitted at 1 kHz, and '1' at 2 kHz.

The receiver uses a frequency detector to determine which frequency is present in the received signal. If the detected frequency is 1 kHz, it's decoded as '0,' and if it's 2 kHz, it's decoded as '1.'

Differential Phase Shift Keying (DPSK)

In PSK it needs a complicated synchronising circuit at the receiver, this disadvantage of PSK is removed in DPSK.

A cos ω₀t = ± A cos ω₀t

Note: Advantage of DPSK over PSK is, DPSK does not require a coherent carrier for demodulation.

Comparison of Digital Modulation Schemes

Quadrature Amplitude Modulation (QAM)

In QAM, digital information is content in a both amplitude and phase of the signal. It is used in both digital modulation scheme and analog modulation scheme. Digital cable television and in cable modem applications QAM is used.

Example: QAM combines both amplitude and phase modulation. In a simple 4-QAM scheme, '00' is represented by a signal with a certain amplitude and phase, '01' by a different phase, '10' by another amplitude, and '11' by another amplitude and phase.

The receiver uses both amplitude and phase detectors to decode the received signal. It simultaneously measures both the amplitude and phase of the signal to determine which pair of bits (00, 01, 10, or 11) is transmitted.