Multiplexing and Digital Coding Techniques

What is Multiplexing?

Multiplexing is a technique in which several message signals are combined into a composite signal for transmission over a common channel. In order to transmit a number of these signals over the same channel, the signal must be kept apart so that they do not interfere with each other, and hence they can be separated easily at the receiver end.

Different Ways of Multiplexing

Digital radio has developed ways in which more than one conversation can be accommodated (multiplexed) inside the same physical RF channel. There are three common ways of achieving this

• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Divisional Multiple Access (CDMA)

1. Frequency Division Multiplexing

In FDMA, we divide the whole bandwidth of the channel into small segments and allot it to different users so that they can access the channel at the same time by using their allotted bandwidth.

2. Time Division Multiplexing

In TDMA, the whole time slot is divided among different users so that at a time only one user is accessing the channel.

Key Points

• The bandwidth requirement in TDMA and FDMA is almost the same for the same number of users.
• The TDMA system can be used to multiplex analog or digital signals, however, it is more suitable for digital signal multiplexing.
• The communication channel over which the TDMA signal is traveling should ideally have an infinite bandwidth in order to avoid signal distortion. Such channels are known as band-limited channels.

3. Code Division Multiplexing (CDMA)

• Instead of splitting the RF channel into sub-channels or time slots, each slot has a unique code. Unlike FDMA, the transmitted RF frequency is the same in each slot, and unlike TDMA, the slots are transmitted simultaneously. In the diagram, the channel is split into four code slots. Each slot is still capable of carrying a separate conversation because the receiver only reconstructs information sent from a transmitter with the same code.

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)
  • 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.
• The signaling used is on-off signaling.

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, the probability of error (P_e) is high.
• In ASK, SNR is less.

Phase Shift Keying (PSK)

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

• NRZ signaling is used.

S(t) = A_c cos 2πf_ct for bit 1

= – A_c cos 2pf_ct for bit 0

• The frequency of the carrier must be multiple of a bit rate.

T_b = n/f_c

F_c = nr_b

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

Frequency Shift Keying (FSK)

• In frequency shift keying, the 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 signaling 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 the case of FSK, P_e is less but SNR is high.
• Multiplexing is difficult in FSK.

Differential Phase Shift Keying (DPSK)

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

V_DPSK(t) = V(t)/V

A cos ω₀t = ± A cos ω₀t

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

Comparison of Digital Modulation Schemes

Digital modulation schemes are used to modulate a digital signal onto an analog carrier signal for efficient transmission over a communication channel. There are several types of digital modulation schemes that differ in their complexity, data rate, and susceptibility to noise and interference.

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