Unit- 3
Multiplexing
is the process of transmitting several different signals or information streams via a single
carrier. The transmission of all these signals or streams takes place simultaneously by combining the several
signals into one common signal that will efficiently moves through the carrier bandwidth. Once the signal
reaches the destination point for one of the transmissions, that integrated signal re-assimilates into its original
form and is received.
Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a
single, complex signal and then recovering the separate signals at the receiving end. In analog transmission,
signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is
divided into sub channels of different frequency widths, each carrying a signal at the same time in parallel. In
digital transmission, signals are commonly multiplexed using time-division multiplexing (TDM), in which the
multiple signals are carried over the same channel in alternating time slots. In some optical fiber networks,
multiple signals are carried together as separate wavelengths of light in a multiplexed signal using dense
wavelength division multiplexing (DWDM).
Multiplexing is one of the common tools used today in just about every form of communications. A wide range
of telephony services, including online applications, are able to function with such a high degree of efficiency
because of the current technological advances this process has made possible. Optical networks also rely heavily
on multiplexing to carry voice and video transmissions along concurrent but separate wavelengths from a point
of origin to various points of determination. With an increasing range of communication functions taking place
across the Internet, it has become an effective tool that aids in everything from videoconferences and web
conferences to large data transmissions to even making a simple point-to-point telephone call.
Multiplexing is of following four types:
1. Frequency-division multiplexing (FDM) is an analog multiplexing technique that combines
analog signals. FDM is applied when the bandwidth of a link (in hertz) is greater than the combined
bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate
different carrier frequencies. These modulated signals are then combined into a single composite signal
that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to
accommodate the modulated signal. These bandwidth ranges are the channels through which the
various signals travel. Channels can be separated by strips of unused bandwidth i.e guard bands, to
prevent signals from overlapping. In addition, carrier frequencies must not interfere with the original
data frequencies.
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2. Wavelength-division multiplexing (WDM) is an analog multiplexing technique to combine
optical signals.WDM is designed to use the high data rate capability of fiber-optic cable. The optical fiber
data rate is higher than the data rate of metallic transmission cable. Using a fiber-optic cable for one
single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one.
WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve optical
signals transmitted through fiber-optic channels. The idea is the same: We are combining different
signals of different frequencies. The difference is that the frequencies are very high.
3. Space division multiplexing (SDM)
is nothing more than the provision of multiple fixed
bandwidth channels by multiple physical paths (i.e., pairs of wires or optical fibers). A good example of
SDM is the use of a 25-pair cable to carry the conversations of 25 individual users from the customer's
premises to the local telephone company’s central office location.
SDM is not the most efficient technique from the standpoint of outside plant resources, but it does play
a role in all carrier networks. A given copper or fiber facility has a finite capacity for information. When
that capacity is exhausted, SDM is the only alternative.
By some arguments, SDM is not a multiplexing scheme at all, since it does not support multiple
communication channels on a single medium. However, the concept is an important one because it
occurs both in the deployment of transmission facilities as well as the internal architecture of some
switches.
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4. Time Division Multiplexing (TDM) is a digital multiplexing technique for combining several
low-rate channels into one high-rate one.TDM is a digital process that allows several connections to
share the high bandwidth of a link Instead of sharing a portion of the bandwidth as in FDM, time is
shared. Each connection occupies a portion of time in the link. Digital data from different sources are
combined into one timeshared link. However, this does not mean that the sources cannot produce
analog data; analog data can be sampled, changed to digital data, and then multiplexed by using TDM.
Multiple Access
Multiple Access Techniques are ways to access a single channel by multiple users. They provide multiple access
to the channel. A “channel” refers to a system resource allocated to a given mobile user enabling the user to
establish communication with the network (other users). Based on the type of channel, we can use a particular
multiple access technique for communication.
The types of channel and the corresponding multiple access techniques are listed below:
Frequency Channels [FDMA - Frequency Division Multiple Access] - Frequency band divided into small
frequency channels and different channels are allocated to different users – like in FM radio. Multiple users can
transmit at the same time but on different frequency channels.
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This was the initial multiple-access technique for cellular systems in which each individual user is assigned a pair
of frequencies while making or receiving a call as shown in Figure 3.1. One frequency is used for downlink and
one pair for uplink. This is called frequency division duplexing (FDD). That allocated frequency pair is not used in
the same cell or adjacent cells during the call so as to reduce the co channel interference. Even though the user
may not be talking, the spectrum cannot be reassigned as long as a call is in place. Different users can use the
same frequency in the same cell except that they must transmit at different times.
The features of FDMA are as follows: The FDMA channel carries only one phone circuit at a time. If an FDMA
channel is not in use, then it sits idle and it cannot be used by other users to increase share capacity. After the
assignment of the voice channel the BS and the MS transmit simultaneously and continuously. The bandwidths
of FDMA systems are generally narrow i.e. FDMA is usually implemented in a narrow band system. The symbol
time is large compared to the average delay spread. The complexity of the FDMA mobile system is lower than
that of TDMA mobile systems. FDMA requires tight filtering to minimize the adjacent channel interference.
Fig: 3.1-The basic concept of FDMA
Time-slot Within Frequency Bands [TDMA - Time Division Multiple Access] – Each user is allowed to transmit
only in specified time-slots with a common frequency band. Multiple users can transmit at the same frequency
band at different times.
The features of TDMA include the following: TDMA shares a single carrier frequency with several users where
each user makes use of non overlapping time slots. The number of time slots per frame depends on several
factors such as modulation technique, available bandwidth etc. Data transmission in TDMA is not continuous but
occurs in bursts.
Fig: The basic concept of TDMA
Distinct Codes [CDMA - Code Division Multiple Access] – Users may transmit at the same time using the same
frequency band but using different codes so that we can decode to identify a particular user.
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In CDMA, the same bandwidth is occupied by all the users, however they are all assigned separate codes, which
differentiates them from each other. CDMA utilize a spread spectrum technique in which a spreading signal
(which is uncorrelated to the signal and has a large bandwidth) is used to spread the narrow band message
signal.
Fig: The basic concept of CDMA
Space Division Multiple Access
SDMA utilizes the spatial separation of the users in order to optimize the use of the frequency spectrum. A
primitive form of SDMA is when the same frequency is reused in different cells in a cellular wireless network.
The radiated power of each user is controlled by Space division multiple access. SDMA serves different users by
using spot beam antenna. These areas may be served by the same frequency or different frequencies. However
for limited co-channel interference it is required that the cells are sufficiently separated. This limits the number
of cells a region can be divided into and hence limits the frequency re-use factor. A more advanced approach
can further increase the capacity of the network. This technique would enable frequency re-use within the cell.
In a practical cellular environment it is improbable to have just one transmitter fall within the receiver beam
width. Therefore it becomes imperative to use other multiple access techniques in conjunction with SDMA.
When different areas are covered by the antenna beam, frequency can be re-used, in which case TDMA or
CDMA is employed, for different frequencies FDMA can be used.
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