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Mobile Communication

By: Dr. Manzoor H. UnarBSc Technology & E-commerce

Module Title:Mobile CommunicationModule Code:IM3013Chapter-4:Modulation and Access Techniques

Access Techniques or Multiple Access TechniquesModule: Level-3W12-Ch-4-Lec: 21

Operating environments Mobile Communication Overview

Figure 1: An illustration of the future mobile and fixed communication environments

Module Title:Mobile CommunicationModule Code:IM3013Chapter-4:Modulation and Access Techniques

Access Techniques or Multiple Access TechniquesModule: Level-3W12-Ch-4-Lec: 21

4.9 Modulation

In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch.

Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.

ModulationW12-Ch-4-Lec: 21

4.9 Modulation

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for "Modulator-Demodulator").

Figure 4.11 shows an illustration of modulation with the horse is the carrier signal and the rider is the message signal.ModulationW12-Ch-4-Lec: 21

Figure 4.11: Example An illustration of modulation concept.

ModulationW12-Ch-4-Lec: 21Air SpaceHours = CarrierRider = Signal

4.9.1 Introduction to modulation (or Signal modulation)

Modulation modifies a signal to make it suitable for transmission over the selected transmission system.

Definition -1: Modulation

The principle behind modulation is that a carrier wave, normally at a fixed frequency, is modified by the signal that is to transmit across a medium. This can be achieved by applying various modulation techniques.

Definition: DemodulationAt the receiving end of the system, demodulation is the technique by which the original signal is extracted from the carrier wave. This method allows the use of different carrier frequencies to carry different information channels. Each carrier frequency is capable of carrying one or more information channels, depending on the system. .ModulationW12-Ch-4-Lec: 21


Definition -2: DemodulationAt the receiving end of the system, demodulation is the technique by which the original signal is extracted from the carrier wave.

This method allows the use of different carrier frequencies to carry different information channels. Each carrier frequency is capable of carrying one or more information channels, depending on the system.ModulationW12-Ch-4-Lec: 21


For example, in GSM, each carrier frequency can carry up to eight voice channels (using a single carrier frequency, but dividing the time into eight regularly repeating time slots).

Modulation would be used to carry: Analogue signals Digital signals The carrier frequency to be modulated is often referred to as the un-modulated carrier, and can be represented as a sine wave at a specific frequency. ModulationW12-Ch-4-Lec: 21


As the carrier is modulated with the analogue or digital signal, the frequency spreads out to cover frequencies on either side of the specified carrier frequency.

In general, this spreading depends on how efficient the modulation system actually is (systems vary markedly) and how much information is to be carried. . ModulationW12-Ch-4-Lec: 21


The amount of frequency spectrum needed to carry the whole (modulated) signal is referred to as the bandwidth.

Bandwidth required increases with:

Increasing information Less-efficient modulation schemes

Modulation techniques require alteration of one of the major characteristics of the carrier waveform. Modulation techniques are:

Amplitude modulation (AM) Frequency modulation (FM) Phase modulation (PM) Commercial radio stations often use AM or FM.



When used to modulate digital signals, the: AmplitudeFrequency, or Phase

would take discrete values to represent the digital information.



In this case, the modulation would be termed digital modulation:

Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK)

GSM radio:For example, would use a form of FSK, whereas UMTS would use a form of PSK.


4.9.2 Examples of modulation schemes


4.9.2.1 Amplitude modulation (AM)(Analogue)

In amplitude modulation, the amplitude of a high frequency carrier signal is varied in accordance to the instantaneous amplitude of the modulating message signal.

Figure 4.12 shows a sinusoidal modulating signal and the corresponding AM signal.


Figure 4.12: (a) A sinusoidal modulating signal and (b) the corresponding AM signal.

ModulationW12-Ch-4-Lec: 214.9.2.1 Amplitude modulation (AM)(Analogue)

4.9.2.2 Frequency modulation (FM) (Analogue)

The use of frequency modulation (FM) within the world of telecommunications is very common. Perhaps the best-known use is that within broadcast radio services.

FM modulates the frequency of the carrier based upon the level of the modulating signal (Figure 4.13).

Better signal-to-noise ratio than AM High-quality audio transmission Unaffected by signal-level variations ModulationW12-Ch-4-Lec: 21


ModulationW12-Ch-4-Lec: 21Figure 4.13: Frequency modulation - The frequency of the carrier is modulated.


The main advantage of FM is its ability to be unaffected by signal-level fluctuations.

The original signal can be decoded however much the broadcast signal varies in level and will continue to do so down to relatively low levels. The signal-to-noise ratio is also improved with the use of FM.

The process of modulating and demodulating within FM systems is more complex than that of amplitude modulation (AM) and consequently more costly.



However, FM has the advantage of being able to deliver much higher quality transmission.

Figure 4.14 Highlights some of the more important characteristics of the AM and the FM modulation techniques.


Figure 4.14: The two most popular analogue modulation techniques are AM and FM.)ModulationW12-Ch-4-Lec: 21


In Figure 4.14 sound coming into the microphone at an AM radio station create continuous voltage variations, which are then superimposed on the stations regularly spaced sinusoidal carrier waves.

The AM modulation technique creates localised amplitude variations in the carrier waves actually broadcast to the stations for the radio broadcast listeners.

While, for FM broadcasts, the voltage variations coming from the microphone create localised variations in frequency of the carrier waves.


4.9.2.3 Example: AM, FM and PAM

Key words:

Analogue modulating signal Sinusoidal carrierModulated waveform

Analogue modulating signalFigure 4.15 (a) depicts a portion of an analogue modulating signal (See Part-a).

Amplitude modulation (AM)(See next slide)



Amplitude modulation (AM)Figure 4.15 (b) depicts the corresponding modulated waveform obtained by varying the amplitude of a sinusoidal carrier wave (See Part-b).

This is the familiar amplitude modulation (AM) used for radio broadcasting and other applications.

A message may also be impressed on a sinusoidal carrier by frequency modulation (FM) or phase modulation (PM).

Note:All methods for sinusoidal carrier modulation are grouped under the heading of continuous-wave (CW) modulation.



Pulse modulation (PM)

Pulse modulation has a periodic train of short pulses as the carrier wave. Figure 4.15 (c) shows a waveform with pulse amplitude modulation (PAM).

Notice that this PAM wave consists of short samples extracted from the analogue signal at the top of the figure.

Sampling is an important signal-processing technique and, subject to certain conditions, it's possible to reconstruct an entire waveform from periodic samples (See Figure 4.18). ModulationW12-Ch-4-Lec: 21


ModulationW12-Ch-4-Lec: 21Figure 4.15: (a) Modulating signal, (b) Sinusoidal carrier with amplitude modulation, (c) Pulse-train carrier with amplitude modulation.

4.9.2.4 Pulse code modulation

Because digital transmission and switching systems are used within modern telecommunications networks, there is a requirement to convert any analogue signals that require transmission over the network into a digital form.

Conversely, at the receiving end, the digital signals must be reconverted back to their original, analogue form.

Pulse code modulation (PCM) is the process of initially digitizing the analogue signal to enable it to be transferred effectively through the network (See Figure 4.16).

It modifies a signal to make it suitable for transmission over the selected transmission system, and hence the term "modulation" is used to describe it. ModulationW12-Ch-4-Lec: 21


However, PCM only digitizes the signal, and a second modulation technique would be required to transfer the newly digitized signal over the chosen transmission system.


4.9.2.4 Pulse code modulation ModulationW12-Ch-4-Lec: 21Figure 4.16: The need for pulse code modulation.


Once a signal is digitized by PCM, it can be transferred over different transmission systems (and the corresponding modulation techniques) within the network, without having to convert back to analogue form as the signal is passed from transmission system to transmission system.

In fact, the signal usually (although not always) stays in PCM format until final conversion back to analogue ready for presentation to the user.



The devices that perform the analogue-to-digital (A-D) and digital-to-analogue (D-A) conversions are known as codecs (coder / decoder) and are often located within street cabinets or in the local telephone exchange.

There are essentially three processes involved within the production of a PCM signal (See Figure 4.17):


4.9.2.4 Pulse code modulation ModulationW12-Ch-4-Lec: 21Figure 4.17: Pulse code modulation.

4.9.2.4 Pulse code modulation ModulationW12-Ch-4-Lec: 21Analogue to digitalconversion of the signalscarries three (3) stages

Take a signalTake samples of the signalQuantisation (a Processes)Coding


Sampling

The first stage of the PCM process is known as sampling. Here the analogue waveform is measured at regular intervals.

This frequency, at which the measurements are taken, is known as the sampling rate.

The standard sampling rate employed for the A-D conversion of the voice within PCM systems is 8 kHz, or 8000 times per second.



2. QuantizationAnalogue signals have an infinite number of discrete values, between zero and the peak level of the signal, to represent the amplitude.

For transmission on a digital network, however, the number of values that represents the amplitude of the signal must be defined.

In PCM systems, once the samples of the source analogue system have been taken, they must be rounded to the nearest value.



2. Quantization (Cont.)

In standard PCM, we use 256 values (or levels). This number was carefully chosen to provide adequate voice quality, but a reasonably low bandwidth (to allow relatively more channels to be carried over the transmission equipment).

The levels are arranged to enable both quiet and loud sounds to be distinguished evenly (i.e., the levels are not linear).



3. CodingBecause we use 256 levels, we need 8 bits to represent each level, and the conversion between the level and the 8-bit representation is performed by the coder.

There are two main PCM coding formats for this coding: ModulationW12-Ch-4-Lec: 21


With 8000 samples per second (each requiring 8 bits to represent the sampled level), this means that each channel will require 64 kbps to represent the voice or data.

There will usually be a channel required in each direction (duplex).

Each PCM channel requires 64 kbps ModulationW12-Ch-4-Lec: 21


PCM was originally designed to digitize telephone-quality speech.

Data can be carried within the PCM channels, as long as the information is initially presented as voice-band tones.

This is the case with modem (modulator/demodulator) tones generated by a computer data card in the case of a dial-up data connection, or for facsimile (fax) tones.

4.9.2.4.1 Example: Pulse code modulation (PCM)Figure 4.18 and shows the construction of the pulse code modulation.ModulationW12-Ch-4-Lec: 21

4.9.2.4.1 Example: Pulse code modulation (PCM)

ModulationW12-Ch-4-Lec: 21Figure 4.18: An analogue message signal is regularly sampled. Quantisation levels are indicated. For each sample the quantised value is given and its binary representation is indicated.

4.9.2.4.1 Example: Pulse code modulation (PCM)Figure 4.19 shows the representation of the PCM from the message signal. ModulationW12-Ch-4-Lec: 21Figure 4.19: (a) Pulse representation of the binary numbers used to code the samples in Figure 4.18 (b) Representation by voltage levels rather than pulses.

4.9.2.5 Modulation benefits and applications

The primary purpose of modulation in any communication system is to generate a modulated signal suited to the characteristics of the transmission channel.

Actually, there are several practical benefits and applications of modulation, such as:

Modulation for efficient transmissionModulation to overcome hardware limitationsModulation to reduce noise and interferenceModulation for frequency assignmentModulation for multiplexingModulationW12-Ch-4-Lec: 21

4.10 Modulation for multiplexing Multiplexing is the process of combining several signals for simultaneous transmission on one channel.

4.10.1 Multiplexing systems

When several communication channels are needed between the same two points, significant economies may be realized by sending all the messages on one transmission facility - a process called multiplexing.

Applications of multiplexing range from the vital, if prosaic, telephone network, to the glamour of FM stereo systems.



There are many multiplexing techniques used for radio communication, such as:

Time-division multiplexing (TDM)Frequency-division multiplexing (FDM)Code-division multiple access (CDMA)Time-division multiplexing (TDM)



Time-division multiplexing (TDM)

Time-division multiplexing (TDM) uses pulse modulation to put samples of different signals in non-overlapping time slots.

Back in Fig. 1.2-1 C, for instance, the gaps between pulses could be filled with samples from other signals.

A switching circuit at the destination then separates the samples for signal reconstruction. Applications of multiplexing include FM stereophonic broadcasting, cable TV, and long-distance telephone.



II.Frequency-division multiplexing (FDM)

Frequency-division multiplexing (FDM) uses CW modulation to put each signal on a different carrier frequency, and a bank of filters separates the signals at the destination.

A variation of multiplexing is multiple access (MA).

Whereas multiplexing involves a fixed assignment of the common communications resource (such as frequency spectrum) at the local level, MA involves the remote sharing of the resource. ModulationW12-Ch-4-Lec: 21


II.Frequency-division multiplexing (FDM)

For example:Code-division multiple access (CDMA) assigns a unique code to each digital cellular user, and the individual transmissions are separated by correlation between the codes of the desired transmitting and receiving parties.

Since CDMA allows different users to share the same frequency band simultaneously, it provides another way of increasing communication efficiency. ModulationW12-Ch-4-Lec: 21

4.10.2 Time-division multiplexing technique

A sampled waveform is off most of the time, leaving the time between samples available for other purposes.

In particular, sample values from several different signals can be interlaced into a single waveform. This is the principle of time-division multiplexing (TDM).

The simplified TDM system in Figure 4.20 demonstrates the essential features of TDM.

Several input signals are pre-filtered by the bank of input low pass filters (LPFs) and sampled sequentially. ModulationW12-Ch-4-Lec: 21

4.10.2 Time-division multiplexing technique

The rotating sampling switch or commutator at transmitter extracts one sample from each input per revolution.

Hence, its output is a PAM waveform that contains the individual samples periodically interlaced time.

A similar rotary switch at the receiver, called a de-commutator or distributor separates the samples and distributes them to another bank of LPFs for reconstruction of the individual messages.

The Figure 4.20 shows mechanical switching to generate multiplexed PAM. But almost all practical TDM systems employ electronic switching [Carlson-B-2002].


4.9.2.4.1 Example: Pulse code modulation (PCM)

ModulationW12-Ch-4-Lec: 21Figure 4.20: An example of TDM system, (a) Block diagram, (b) Waveforms.

4.10.2 Time-division multiplexing technique Figure 4.21 shows a popular synchronisation technique devoted one time slot per frame.

ModulationW12-Ch-4-Lec: 21Figure 4.21: TDM synchronisation markers.

4.10.3 Frequency-division multiplexing technique

The commercial AM or FM broadcast bands are everyday examples of FDMA, where several broadcasters can transmit simultaneously in the same band, but slightly different frequencies Example: FDMA satellite systems

The Intelsat global network adds a third dimension to long-distance communication.

Since a particular satellite links several ground stations in different countries, various access methods have been devised for international telephony. ModulationW12-Ch-4-Lec: 21


Example: FDMA satellite systems (Cont.)

FDMA

One scheme, known as frequency-division multiple access (FDMA).

FDMA assigns a fixed number of voice channels between pairs of ground stations.

These channels are grouped with standard FDM hardware, and relayed through the satellite using FM carrier modulation.



Example: FDMA satellite systems (Cont.)

FDMA

ExampleAs an example, suppose a satellite over the Atlantic Ocean serves ground stations in the:

United StatesBrazil, and France



Example: FDMA satellite systems (Cont.): - FDMA: ExampleFurther suppose that:

36 channels (three groups) are assigned to the US-France route, and 24 channels (two groups) to the US-Brazil route

Figure 4.22 shows the arrangement of the US transmitter and the receivers in: Brazil, and France

Not shown are the French and Brazilian transmitters and the US receiver needed for two-way conversations. ModulationW12-Ch-4-Lec: 21


Example: FDMA satellite systems (Cont.): - FDMA: Example

Additional transmitters and receivers at slightly different carrier frequencies would provide a Brazil-France route.

The FDMA scheme creates at the satellite a composite FDM signal assembled with the FM signals from all ground stations.

The satellite equipment consists of a bank of transponders. Each transponder has 36-MHz bandwidth accommodating 336 to 900 voice channels, depending on the ground-pair assignments.

More details and other access schemes can be found in the literature [Carlson-B-2002]. ModulationW12-Ch-4-Lec: 21


ModulationW12-Ch-4-Lec: 21Figure 4.22: An example of a simplified FDMA satellite system.

4.10.4 Coding methods and benefits

As described earlier, modulation is used as a signal-processing operation for effective transmission.

While, coding is a symbol-processing operation for improved communication when the information is digital or can be approximated in the form of discrete symbols.

Both coding and modulation may be necessary for reliable long-distance digital transmission.


4.10.5 Access schemes

Multiple access requirementsA cellular system employs a multiple access technique to control the allocation of the network resources.

The purposes of a multiple access technique are:

To provide each user with unique access to the shared resource: the spectrum.To minimise the impact of other users acting as interferers.To provide efficient use of the spectrum available.To support flexible allocation of resources (for a variety of services).


4.10.5.1 Access schemes: OverviewFor radio systems there are two resources:

Time, andFrequency

4.10.5.1.1 Time division multiple access (TDMA)Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA).

4.10.5.1.2 Frequency division multiple access (FDMA)Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA).


4.10.5.1 Access schemes: Overview

4.10.5.1.3 Code division multiple access (CDMA)

In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time.

CDMA uses codes to identify connections.

Figure 4.23 shows illustrations of TDMA, FDMA and CDMA.



ModulationW12-Ch-4-Lec: 21Figure 4.23: Example: Illustrations of multiple access schemes.



CodingCDMA uses unique spreading codes to spread the base-band data before transmission.

The signal is transmitted in a channel, which is below noise level.

The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow band-pass filter.

Unwanted signals will not be despread and will not pass through the filter. ModulationW12-Ch-4-Lec: 21



CodingCodes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the base-band data. The rate of a spreading code is referred to as chip rate rather than bit rate.

4.10.5.1.4Wideband Code Division Multiple Access (WCDMA)http://www.iec.org/online/tutorials/wcdma/

4.11 SummaryModulationW12-Ch-4-Lec: 21

End of Lecture Chapter-5: Mobile communication Systems design techniques

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