What is multiplexing?


Multiplexing: Multiplexing technique combine signals from several sources. Multiplexing  is a process of sending multiple signals of information over a communications link at the same time in the form of a single, complex signal. Thus allows one channel to be used by multiple sources to send multiple messages.




  • Multiplexing techniques allow sharing a channel by keeping the transmitted signals from various sources separate so that they do not interfere with one another
  • This separation is accomplished by making the signals orthogonal to one another in the dimensions of frequency, time, code, space, etc.

Types of Multiplexing Techniques


  • Time division multiplexing (TDM)

  • Frequency division multiplexing (FDM)

  • Wavelength division multiplexing (WDM)

  • Code division multiplexing (CDM)

  • Space division multiplexing (SDM)

  • Orthogonal frequency division multiplexing (OFDM): a
  • variety of FDM

  • Polarization division multiplexing (PDM)


Frequency Domain Multiplexing (FDM)

Frequency-division multiplexing (FDM) is a technique by which the total bandwidth
available in a communication medium is divided into a series of non-overlapping
frequency sub-bands, each of which is used to carry a separate signal.

These sub-bands can be used independently with completely different information
streams, or used dependently in the case of information sent in a parallel stream.

This allows a single transmission medium such as the radio spectrum, a cable or optical
fiber to be shared by multiple separate signals.




The most natural example of frequency-division multiplexing is radio and television
broadcasting, in which multiple radio signals at different frequencies pass through the air
at the same time.




  • Available bandwidth of the common channel is divided into bands
  • Signals are separated in frequency domain
  • Requires guard bands to avoid adjacent channel interference
  • Requires filtering to minimize adjacent channel interference: costly



Time Division Multiplexing (TDM)

In TDM Instead of dividing the bandwidth, the entire bandwidth is used for each signal, but only for a small part of the time.






Transmission time is divided into time-slots and unique time slot(s) are allocated to each user. Different users can transmit or receive messages, one after the next in the same bandwidth but in different time slots: Orthogonal in time-domain.


TDM Advantages and Disadvantages


Advantages:

  1. Increases the transmission efficiency (i.e., better resource utilization)
  2. Permits the utilization of all the advantages of digital techniques: digital speech interpolation, source coding, channel coding, error correction, bit interleaving, etc.
  3. Suitable for asymmetric (i.e., unequal uplink and downlink data rate) data rate.
  4. Equipment is becoming increasingly cheaper


Disadvantages:

  1. Requires a significant amount of signal processing for synchronization as the transmission of all users must be exactly synchronized
  2. Requires guard times between time slots to reduce clock instabilities and transmission time delay


Problem – 1

5 channel each with 100 kHz bandwidth are to be multiplexed together.
What is the minimum bandwidth of the link if there is a need for a guard
band 10 kHz between the channel to prevent interference?


Solution:


For five channels, we need at least four guard bands.

This means that the required bandwidth is at least

5 x 100 + 4 x 10 = 540 KHz.




Problem – 2


Assume that a voice channel occupies a bandwidth of 4 KHz. We need to
combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to
32 KHz. Show the configuration using the frequency domain without the use of
guard bands.


Solution:


Shift (modulate) each of the three voice channels to a different bandwidth, as
shown in the following Figure –





Multiple Access

What is Multiple Access?

Multiple access is a system splits any allocated area into cells in which a mobile unit in every cell can communicate with a base station.

Multiple Access (MA) Techniques



Random access (contention methods): No station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send.

Controlled access: The stations consult one another to find which station has the right to send.  A station cannot send unless it has been authorized by other stations.

Channelization techniques: The available bandwidth of a link is shared in time,
frequency, or through code, between different stations. Usually, it is controlled by a
system administrator.


Channelization techniques


Various forms of channelization techniques:

  1. Frequency division multiple access (FDMA): e.g., 1G cellular system
  2. Time division multiple access (TDMA) : e.g., 2G GSM system
  3. Wavelength division multiple access (WDMA)
  4. Code division multiple access (CDMA): e.g., 2G CDMA, 3G UMTS system
  5. Orthogonal frequency division multiple access (OFDMA): e.g., LTE, WiMAX
  6. Space division multiple access (SDMA)

These techniques can be used in combination

Frequency-division multiple access (FDMA)


In FDMA, the available bandwidth of the common channel is divided into bands
that are separated by guard bands.



The best example of this is the cable television system.
The medium of a single co-axial cable is used to broadcast hundreds of channels of video / audio programming to homes. The co-axial cable has a useful bandwidth from 4 MHz to 1 GHz.  This bandwidth is divided up into 6 MHz wide channels. Initially, one TV station or channel used a single 6 MHz band.


Time-division multiple access (TDMA)


In TDMA, the bandwidth is just one channel that is timeshared between
different stations.



TDMA is used in the digital 2G cellular systems such as Global System for Mobile
Communications (GSM).TDMA was first used in satellite communication systems by Western Union in its Westar 3 communications satellite in 1979. It is now used extensively in satellite communications.

Code Division Multiple Access (CDMA)

In CDMA, one channel carries all transmissions simultaneously. In a CDMA system, each user is assigned a particular code, named as pseudo-noise (PN) code, which are ideally supposed to be unique for each user .This unique code enables the desired message to be extracted at the receiver. The transmissions from other users looks like interference.