Multiplexing and Spreading

Sometime called Muxing. Method by which multiple analog or digital signals

are combined into one signal over a shared medium. The aim is to share a scarce resource. E.g.in

telecommunications, several telephone calls may be carried using one wire.

The multiplexed signal is transmitted over a communication channel such as cable.

The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred.

Multiplexing

Figure : Dividing a link into channels

To reduce the number of electrical connections or lead.

To share the bandwidth between the users. To increase the capacity of channel. To increase the transmission speed. To make the signal secure. To make scalable. To make cost efficiency.

Need of Multiplexing

MULTIPLEXINGMULTIPLEXING

Whenever the bandwidth of a medium linking two devices Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques link can be shared. Multiplexing is the set of techniques that allows the (simultaneous) transmission of multiple that allows the (simultaneous) transmission of multiple signals across a single data link. As data and signals across a single data link. As data and telecommunications use increases, so does traffic.telecommunications use increases, so does traffic.

Frequency-Division Multiplexing (FDM) Wavelength-Division Multiplexing (WDM) Time-Division Multiplexing (TDM)

Synchronous Time-Division MultiplexingStatistical Time-Division Multiplexing

Different Multiplexing TechniquesDifferent Multiplexing Techniques

Figure : Categories of multiplexing

• 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 medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal.

Frequency Division Multiplexing (FDM)

Figure : Frequency-division multiplexing (FDM)

Frequency Division Multiplexing 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.

Frequency Division MultiplexingDiagram

Figure: FDM process

Figure : FDM demultiplexing example

FDM System

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. Assume there are no guard bands.

SolutionWe shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown in Figure below.

Example

Figure : FDM hierarchy

FDM Hierarchy (Analog Hierarchy)

S.N Designation Combination Number of Voice

Channel

Bandwidth

1 Voice Channel 1 Voice channel

1 4KHz

2 Group 12 Voice channel

12 48KHz

3 Super Group 5 Group 60 240KHz

4 Master Group 10 Super Group

600 2.52MHz

5 Super Master Group

6 Master Group

3600 17MHz

Advantages and Disadvantages of FDM

Advantages:•Doesn't need synchronization between transmitter and receiver.•Large number of signals (channels) can be transmitted simultaneously.•Demodulation process of FDM is easy.Disadvantages:•Communication channel must have large bandwidth.•FDM suffers from the problem of crosstalk.•Large numbers of filters and modulators are required.•Intermodulation distortion takes place.

Time Division Multiplexing (TDM)• TDM is a digital technology which uses time, instead of

space or frequency, to separate the different data streams.

• TDM involves sequencing groups of few bits or bytes from each individual input stream, one after the other and in such a way that they can be associated with the appropriate receiver.

Time Division Multiplexing

Figure :Time Division Multiplexing (TDM)

• At the transmitting end, input channels are sequentially sampled by a switch resulting in train of amplitude samples.

• The coder then sequentially converts each sample into a binary code using analogue to digital conversion techniques.

• In TDM, the channel/link is not divided on the basis of frequency but on the basis of time.

• The coder output is the string of binary digits representing channel 1, channel 2, and so on.

• These channels are combined with framing bits for MUX/DEMUX synchronization.

• Two types of TDM • Synchronous Time Division Multiplexing• Asynchronous Time Division Multiplexing

TDM System

Synchronous Time Division Multiplexing In synchronous TDM data flow of each input connection is

divided into units and each input occupies one output time slot. In synchronous TDM no. of slots in each frame are equal to

number of input lines. Buffering is not done, frame is sent after a particular interval of

time whether someone has data to send or not. Slots in synchronous TDM carry data only and there is no need

of addressing. Synchronization and pre assigned relationships between input and outputs that serve as an address.

Synchronous bits are used at the beginning of each frame. Maximum bandwidth utilization if all inputs have data to send. In synchronous TDM demultiplexer at receiver end decomposes

each frame, discards framing bits and extracts data unit in turn. This extracted data unit from frame is then passed to destination device.

Advantages and disadvantages of Synchronous TDM

Advantages: An order is maintained.No addressing information.

Disadvantages:High bitrate is required.If no input signal is present at one channel since a fixed time slot is assigned to each channel, that time slot for that channel does not carry any information and there is wastage of bandwidth.

Statistical Time Division Multiplexing In statistical TDM slots are allotted dynamically.i.e. input line

is given slots in output frame if and only if it has data to send. In Statistical TDM, no. of slots in each frame are less than the

number of input lines. Buffering is done and only those inputs are given slots in output

frame whose buffer contains data to send. Slots in statistical TDM contain both data and address of the

destination. No synchronization bits are used. The capacity of link is normally is less than the sum of the

capacity of each channel. In statistical TDM de-multiplexer at receiving end decomposes

each frame by checking local address of each data unit. This extracted data unit from frame is then passed to destination device.

Advantages and Disadvantages of Asynchronous/statistical TDM

Advantages:Communication link of low capacity is used.The problem of crosstalk is not severe.Full available channel bandwidth can be utilized for each channel.Intermodulation distortion is absent.

Disadvantages:Frames have different sizes.Requires buffers.Address information is needed.

Figure : Digital hierarchy- TDM Hierarchy

Table :DS and T line rates

Figure : T-1 line for multiplexing telephone lines

Figure : T-1 frame structure

Figure: E1 and T1 Framing Structure

Table : E line rates

Wavelength Division Multiplexing (WDM)• WDM is a technology which multiplexes a number of

optical carrier signals onto a single optical fiber by using different wavelengths of laser light.

• This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.

• WDM is commonly applied to an optical carrier, which is typically described by its wavelength, whereas FDM typically applies to a radio carrier which is more often described by frequency.

• WDM systems are divided into three different wavelength patterns, normal (WDM), Coarse (CWDM) and dense (DWDM).

• Normal WDM uses two normal wavelengths 1310 to 1550nm on one fiber.

• Coarse WDM provides up to 16 channels across multiple transmission windows of silica fibers.

• Dense wavelength division multiplexing (DWDM) uses the 1530-1565nm transmission window but with denser channel spacing.

Figure : Prisms in wavelength-division multiplexing and demultiplexing

WDM Operation Same general architecture as other FDM Number of sources generating laser beams at different

frequencies Multiplexer consolidates sources for transmission over single

fibre. Optical amplifiers amplify all wavelengths

Typically tens of km apart Demux separates channels at the destination Mostly 1550nm wavelength range

Why WDM? Capacity upgrade of existing fiber networks.

(without adding fibers) Transparency: Each optical channel can carry any

transmission format. (different asynchronous bit rates, analog or digital)

Scalability– Buy and install equipment for additional demand as needed.

Wavelength routing and switching: Wavelength is used as another dimension to time and space.

TDM Vs WDM

Ex: SONET

WDM advantages:Advantages:•It has greater transmission capacity.•Duplex transmission.•Simultaneous transmission of various signals.•Easy system expansion.•Lower Cost.•Faster access to new channels.•Higher security.Disadvantages:•Signals can not be very close. • Light wave carrying WDM are limited to 2-point circuit.

•Cost of system increases with addition of optical components. •Inefficiency in BW utilization, difficulty in wavelength tuning, difficulty in cascaded topology.

Spread Spectrum• In spread spectrum, we combine signals from different sources to

fit into a larger bandwidth.• In wireless applications, stations must be able to share the

medium without interception by an eavesdropper and without being subject to jamming from a malicious intruder.

• To achieve these goals, spread spectrum techniques add redundancy; they spread the original spectrum needed for each station.

• Spread spectrum achieves its goals through two principle:• The bandwidth allocated to each station needs to be larger

than what is needed. This allows redundancy.• The expanding process occurs after the signal is created by

the source. i.e. B to Bss

• After the signal is created by the source, the spreading process uses a spreading code and spreads the bandwidth.

• Two techniques to spread the bandwidth: • Frequency hopping spread spectrum (FHSS)• Direct sequence spread spectrum (DSSS)

• The frequency hopping spread spectrum technique uses M different carrier frequencies that are modulated by the source signal.

• At one moment, the signal modulates one carrier frequency; at the next moment, the signal modulates another carrier frequency.

• Although the modulation is done using one carrier frequency at a time, M frequencies are used in the long run. The bandwidth occupied by a source after spreading is BFHSS>>B.

Direct sequence spread spectrum

Spreads the baseband data by directly multiplying the baseband data pulses with a pseudo-noise sequence produced by a pseudo-noise generator

Each bit in original signal is represented by multiple bits in the transmitted signal, using spreading code

Combine information stream with the spreading code bit stream using an exclusive-OR gate

A single pulse or symbol of the PN waveform is called a chip

Direct Sequence Spread Spectrum Transmitter

Direct Sequence Spread Spectrum Transmitter

Direct Sequence Spread Spectrum Using BPSK Example

Advantages of Spread Spectrum

Reduced crosstalk interference Better voice quality/data integrity and less static

noise. Lowered susceptibility to multipath fading. Inherent security. Longer operating distance. Hard to detect. Harder to jam.

Frequency hopped spread spectrum

Involves a periodic change of transmission frequency (i.e. the signal is broadcast over a random series of radio frequencies, hopping from frequency to frequency at fixed intervals)

A number of channels are allocated for the FH signal The transmitter operates in one channel at a time for a

fixed interval During this interval, some number of bits is

transmitted using some encoding techniques. The set of possible carrier frequencies is called hopset

Frequency Hopping Example

Frequency Hopping Spread Spectrum System (Transmitter)

Frequency Hopping Spread Spectrum System (Receiver)

FH SS contd..

The BW of a channel used in the hopset is called the instantaneous bandwidth

The BW of the spectrum over which the hopping occurs is called the total hopping bandwidth

FH may be classified as fast or slow Fast FH occurs if there is more than one frequency

hop during each transmitted symbol Slow FH occurs if one or more symbols are

transmitted in the time interval between frequency hop

Advantages

Inherent interference rejection capability Since all users share same spectrum ,it eliminate

frequency planning Each user is assigned a unique PN code , the receiver

can separate each user based on their codes even though they occupy the same spectrum at all times

Uniform energy over a very large BW, only small portion of spectrum will undergo fading

Can be used for hiding and encrypting signals

Multiple AccessMultiple Access: Simultaneous private use of a transmission

medium by multiple, independent users.

Transmission

Medium

Advantages of Multiple Access• Increased capacity: serve more users• Reduced capital requirements since fewer

media can carry the traffic• Decreased per-user expense

FrequencyTime

Power

FrequencyTime

Power

FrequencyTime

Power

FDMA

TDMA

CDMA

FDMA (Frequency Division Multiplex Access)each user on a different frequencya channel is a frequency

TDMA (Time Division Multiplex Access)each user on a different window period in time (“time slot”)a channel is a specific time slot on a specific frequency

CDMA (Code Division Multiplex Access)each user uses the same frequency all the time, but mixed with different distinguishing code patternsa channel is a unique set of code patterns

Code Division Multiplexing Access (CDMA) CDMA is a channel access method used by various

radio communication technologies. CDMA is an example of multiple access, where

several transmitters can send information simultaneously over a single communication channel.

This allows several users to share a band of frequencies without undue interference between the users.

CDMA employs spread spectrum technology and a special coding scheme.

CDMA is used as the access method in many mobile phone standards, UMTS, the 3G standard used by GSM carriers W-CDMA, GPS,

ORIGINATING SITE DESTINATION

SpreadingSequence

SpreadingSequence

InputData

RecoveredData

Spread Data Stream

Any data bit stream can be combined with a spreading sequence

The resulting signal can be de-spread and the data stream recovered if the original spreading sequence is available and properly synchronized

After de-spreading, the original data stream is recovered intact

CDMA- Code Division Multiple Access

CDMACDMA

Each user’s signal is a continuous unique code pattern buried

within shared signal, mixed with other user’s code pattern. If a

user’s code pattern is known, the presence or absence of their

signal can be detected, thus conveying information

All CDMA users occupy the same frequency at the same time.

- Time and Frequency are not used as discriminators

CDMA interference comes mainly from nearby users

CDMA operators by using CODING to discriminate between

users

Each user is a small voice in a roaring crowd – but with a

uniquely recoverable code

Advantages

Disadvantages

- Easy to Voice Encryption through the PN Sequence using spread

spectrum

- Large Capacity

- High Frequency Utilization - Frequency Reusing Factor is 1

-Better Voice Quality : Less Fading - Easy to trace the MS’s Location by

using the GPS

-Flexible transfer may be used. Mobile base stations can switch without

changing operator. Two base stations receive mobile signal and the mobile

receives signals from the two base stations. (soft handoff)

- Difficult to Control the Power of MS and BTS

- Failing the Power Control affect all voice call in one cell

- Receiver is complex for PN Sequence Acquiring and Tracing