Learning Objectives:

1. Describe how time division multiplexing (TDM) can be used to improve the capacity of a PCM communications link, using TDM.

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Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.
The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from multiple senders. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.
If many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of TDM is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make optimum use of the available bandwidth. The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour to hour.

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Learning Objectives:

1. Describe the operation of Frequency Division Multiplexing.

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FDM is widely used in radio and television systems (e.g. broadcast radio and TV) and was widely used in multichannel telephony (now being superseded by digital techniques and TDM). The multichannel telephone system illustrates some important aspects and is considered below. For speech, a bandwidth of 3kHz is satisfactory.

• FDM is an analog multiplexing technique that combines analog signals into one medium by sending signals in several distinct frequency ranges over a single medium.
• The spectrum of each input signal is shifted to a distinct frequency range.
• Total available bandwidth of a medium is divided intoa series of non-overlapping frequency bands, each of which is used to carry a separate signal.
• Carrier frequencies separated so signals do not overlap (guard bands)
• Channel allocated even if no data
• e.g. radio and television broadcasting at different frequencies over the air at same time, cable television and analog telephone systems, etc.

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Advantages:

• Large cpacities
• Scalable

Disadvantages:

• Complexity

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Solved Example: 9197-01

An amplitude modulated signal has a carrier frequency of 10 kHz. The upper sideband is transmitted at 11 kHz. The bandwidth required for the AM signal to transmit is: (DFCCIL Executive S and T 2018)

A. 10 kHz

B. 2 kHz

C. 11 kHz

D. 1 kHz

Correct Answer: B

Solved Example: 9996-01

The maximum power efficiency of an AM modulator is: (GATE ECE 1992)

A. 25%

B. 50%

C. 33%

D. 100%

Efficiency of modulation can be given as \[\eta = \dfrac{P_S}{P_S + P_C} = \dfrac{\mu ^2}{2 + \mu ^2}\] $\mu$=1 is the optimum value \[\eta_{max} = \dfrac{1}{1 + 2} = 33\%\]

Correct Answer: C

Learning Objectives:

1. Analyse and draw graphs to illustrate pulse amplitude modulation (PAM) techniques.

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