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MULTIPLEXING

Definition : A technique where several users use the medium simultaneously without interference or with minimum interference.

Four techniques :- Space Division Multiplexing- Frequency Division Multiplexing- Time Division Multiplexing- Code division Multiplexing

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MULTIPLEXING

Space Division Multiplexing

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Space Division Multiplexing

Each user is allocated a space (si)

The circles show the interference rangeAs used in analog telephone system,

each user is provided with a pair of copper wires connecting to the exchange

It can not be used if several broadcasters want to use the same space simultaneously.

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Frequency Division Multiplexing

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Time Division Multiplexing

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Time Division Multiplexing

All senders use the same frequency but at different points in time

Guard spaces are time gaps between users

Precise synchronization is required between users

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TDM and FDM combined

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Code Division Multiplexing

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Modulation

Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency,

robustness Analog modulation

shifts center frequency of baseband signal up to the radio carrier

Motivation smaller antennas (e.g., /4) Frequency Division Multiplexing medium characteristics

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Modulation

Basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)

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Modulation and Demodulation

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Digital Modulation

Digital Data ‘1’ & ‘0’ is translated into an analog signal

The above is used when a computer generated digital data has to be transmitted over a telephone line using a Modem

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Digital Wireless Transmission

In digital wireless transmission, we have to use one of the following techniques to convert the signal to analog signal

- Amplitude shift keying

- Frequency shift keying

- Phase shift keying

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Amplitude Shift Keying

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Amplitude Shift Keying

The two binary bits ‘1’ and ‘0’ are represented by two different amplitudes.

Other drawbacks : Effects of multipath propagation, noise or path

loss largely influence the amplitude. In a wireless environment, it is difficult to

guarantee cont amplitude. Hence, this scheme is not used much in wireless.

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Amplitude Shift Keying

In a wired transmission using optical fibre, exact amplitudes can be achieved.

ASK is extensively used in wired fibre optic transmission.

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Amplitude Shift Keying

ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc .

It requires excessive bandwidth and is therefore a waste of power.

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Amplitude Shift Keying

More sophisticated encoding schemes have been developed which represent data in groups using additional amplitude levels. For instance, a four-level encoding scheme can represent two bits with each shift in amplitude; an eight-level scheme can represent three bits; and so on. These forms of amplitude-shift keying require a high signal-to-noise ratio for their recovery, as by their nature much of the signal is transmitted at reduced power.

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Frequency Shift Keying

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Frequency Shift Keying

A frequency f1 is assigned to ‘1’ and ‘f2’ is assigned to ‘0’

Use two oscillators with frequencies f1 and f2. Switch between the two.

At the demodulation stage, two band pass filters with frequencies f1 and f2 are used along with a comparator.

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Frequency Shift Keying - Example

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Minimum Frequency Shift Keying

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent frequency-shift keying. In MSK the difference between the higher and lower frequency is identical to half the bit rate. As a result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.

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Phase Shift Keying

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Phase Shift Keying

A 180 degree phase shift can be used as the bit changes from 1 to 0 or from

0 to 1. This scheme is also called binary shift keying.

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Phase Shift Keying

Instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used. .