wireless and cellular communications-unit four

8/6/2019 Wireless and Cellular Communications-unit Four

1/77

UNIT IV

Modulations and Signal Processing


2/77

Outline

4.1 Digital and Analog Modulations Techniques4.2 Amplitude Modulation

4.3 Angle Modulation

4.4 Frequency vs. Amplitude Modulations4.5 Advantages of Digital Modulation

4.6 Performance of a modulation scheme

4.7 Equalization, Diversity, and Channel Coding4.8 Speech Coding


3/77

4.1 Digital and Analog Modulations

Techniques

Modulation is the process of encoding

information from a message source in a

manner suitable for transmission

It generally involves translating a baseband

message signal (called the source) to a

bandpass signal at frequencies that are very

high when compared to the baseband

frequency


4/77

The bandpass signal is called the modulated

signal and the baseband message signal iscalled the modulating signal

Modulation may be done by varying the

amplitude, phase, or frequency of a high

frequency carrier in accordance with the

amplitude of the message signal

Demodulation is the process of extracting the

baseband message from the carrier so that itmay be processed and interpreted by the

intended receiver (also called the sink)


5/77

The ultimate goal of a modulation technique isto transport the message signal through aradio channel with the best possible qualitywhile occupying the least amount of radiospectrum

The three basic modulation schemes areAmplitude Modulation(AM), FrequencyModulation(FM) , and Phase Modulation(PM)

FM and PM belong to Angle Modulation

For digital modulation digital data (0 and 1) istranslated into an analog signal (basebandsignal)


6/77

Digital modulation is required if digital data

has to be transmitted over a medium that onlyallows for analog transmission

Best example for wired networks is the old

analog telephone system: to connect a

computer to this system a modem is needed

The modem then performs the translation of

digital data into analog signals and vice versa

Digital transmission is used, for example, in

wired local area networks or within a

computer


7/77

In wireless networks digital transmission cannot

be used directly

Here, the binary bit-stream has to be translated

into an analog signal rst

The three basic methods for this translation are

Amplitude Shift Keying (ASK), Frequency ShiftKeying (FSK), and Phase Shift Keying (PSK)

Apart from the translation of digital data into

analog signals, wireless transmission requires

an additional modulation, an analog

modulation that shifts the center frequency of

the baseband signal generated by the digital

modulation up to the radio carrier


8/77

For example, digital modulation translates a

1Mbit/s bit-stream into a baseband signal with

a bandwidth of 1 MHz

There are several reasons why this baseband

signal cannot be directly transmitted in a

wireless system

1. Antennas: For the 1 MHz signal we

dont need an antenna some hundred

meters high practically, With 1 GHz,antennas a few centimetres in length

can be used


9/77

2. Frequency division multiplexing: Using

only baseband transmission, FDM couldnot be applied and Analog modulationshifts the baseband signals to differentcarrier frequencies

3. Medium characteristics: Path-loss,penetration of obstacles, reflection,scattering, and diffraction depend heavilyon the wavelength of the signal and

Depending on the application, the rightcarrier frequency with the desiredcharacteristics has to be chose


10/77

Modulation in a transmitter

Demodulation and data reconstruction in a receiver


11/77

Amplitude shift keying

Amplitude Shift Keying (ASK): is the most

simple digital modulation scheme

The two binary values, 1 and 0, are

represented by two different amplitudes

ASK illustrated by a figure in the next slide,

this simple scheme only requires low

bandwidth, but is very susceptible tointerference

Effects like multipath propagation, noise, or

path loss heavily influence the amplitude


12/77

In a wireless environment, a constant

amplitude cannot be guaranteed, so ASK istypically not used for wireless radio

transmission

ASK can also be applied to wireless infrared

transmission, using a directed beam or diffuse

light

Amplitude shift keying (ASK)


13/77

Frequency shift keying

A modulation scheme often used for wireless

transmission is frequency shift keying (FSK)

The simplest form of FSK, also called binary

FSK (BFSK), assigns one frequency f1 to thebinary 1 and another frequency f2 to the

binary 0

A very simple way to implement FSK is toswitch between two oscillators, one with the

frequency f1 and the other with f2, depending

on the input


14/77

To avoid sudden changes in phase, special

frequency modulators with continuous phase

modulation (CPM) can be used

Sudden changes in phase cause high

frequencies, which is an undesired side-effect

Frequency shift keying (FSK)


15/77

Phase shift keying Phase shift keying (PSK) uses shifts in the

phase of a signal to represent data

The figure in the next slide shows a phase shift

of 180or as the 0 follows the 1 (vice versa)

This simple scheme, shifting the phase by

180each time the value of data changes, is

also called binary PSK (BPSK)

A simple implementation of a BPSK modulatorcould multiply a frequency f with +1 if the

binary data is 1 and with 1 if the binary data

is 0


16/77

To receive the signal correctly, the receiver

must synchronize in frequency and phase withthe transmitter. This can be done using a

Phase Lock Loop (PLL)

Compared to FSK, PSK is more resistant tointerference, but receiver and transmitter are

also more complex

Phase shift keying (PSK)


17/77

4.2 Amplitude Modulation

In amplitude modulation, the amplitude of a highfrequency carrier signal is varied in accordance to

the instantaneous amplitude of the modulating

message signal

If Accos(2fct)is the carrier signal and m (t) is the

modulating message signal, the AM signal can be

represented as

SAM(t)=Ac[1+m(t) cos(2fct)] The modulation index k of an AM signal is defined

as the ratio of the peak message signal amplitude

to the peak carrier amplitude


18/77

For a sinusoidal modulating signal m(t)=(Am/Ac)cos (2fmt) , the modulation index is given by

Where Am is the modulated and Ac carrier signals

The modulation index is often expressed as a

percentage, and is called percentage modulation For instance , if Am = 0.5 Ac and the signal is said

to be 50% modulated the correspondingsinusoidal modulating and AM signalrepresented by figures in the next slide

A percentage of modulation greater than 100%will distort the message signal if detected by anenvelope detector


19/77

A sinusoidal modulating signal

AM signal with modulation index 0.5


20/77

AM signal may be equivalently expressed as

SAM (t) = Re { g(t) exp (j2fct) }

where g (t) is the complex envelope of the AM

signal given by

g(t)=Ac{1+m(t)}

The spectrum of an AM signal(function of f)

can be shown to be

where () is the unit impulse function, and

M(f) is the message signal spectrum


21/77


22/77

The AM spectrum consists of an impulse atthe carrier frequency, and two sidebands

which replicate the message spectrum The sidebands above and below the carrier

frequency are called the upper and lowersidebands, respectively

The bandwidth of an AM signal is equal to

where fm is the maximum frequency containedin the modulating message signal


23/77

The total power in an AM signal can be shownto be

where * represents the average value

If the modulating signal is the

above equation may be simplified as

where is the power in the carriersignal , is the power in themodulating signal m(t), and k is themodulation index


24/77

Example:

A zero mean sinusoidal message is applied to atransmitter that radiates an AM signal with 10

kW power. Compute the carrier power if the

modulation index is 0.6. What percentage of

the total power is in the carrier?


25/77

4.3 Angle Modulation

The two most important classes of angle

modulation being frequency modulation and

phasemodulation

Angle modulation varies a sinusoidal carriersignal in such a way that the angle of the

carrier is varied according to the amplitude of

the modulating baseband signal

In this method, the amplitude of the carrier

wave is kept constant (this is why FM is called

constant envelope)


26/77

There are a number of ways in which the

phase (t) of a carrier signal may be varied in

accordance with the baseband signal

Frequency modulation (FM) is a form of angle

modulation in which the instantaneous

frequency of the carrier signal is varied

linearly with the baseband message signal

m(t), as shown in equation


27/77

where Ac is the amplitude of the carrier, fc is

the carrier frequency, and kf is the frequency

deviation constant (measured in units of

Hz/volt)

If the modulating signal is a sinusoid of

amplitude Am , and frequency fm , then the FMsignal may be expressed as

f=kfAm

Where f is the peak frequency deviation of

the transmitter


28/77

For sinusoidal modulating signal with

modulating frequency fm

The frequency modulation index f defines

the relationship between the message

amplitude and the bandwidth of the

transmitted signal, and is given by

Where Am is the peak value of the modulatingsignal, f is the peak frequency deviation of

the transmitter and W is the maximum

bandwidth of the modulating signal


29/77

If the modulating signal is a low pass signal, as

is usually the case, then W is equal to thehighest frequency component fmax =fm present

in the modulating signal, hence

Example

A sinusoidal modulating signal,

m(t) = 4cos24 x 103t, is applied to an FM

modulator that has a frequency deviation

constant gain of 10 kHz/V. Compute

(a) The peak frequency deviation, and

(b) The modulation index


30/77

Phase modulation (PM) is a form of angle

modulation in which the angle (t) of the

carrier signal m(t) is varied linearly with the

baseband message signal as shown in

equation

Where k

is the phase deviation constant

(measured in units of radians/volt)

From the above equations, it is clear that anFM signal can be regarded as a PM signal in

which the modulating wave is integrated

before modulation


31/77

Note that an FM signal can be generated by

first integrating m (t) and then using the result

as an input to a phase modulator, conversely a

PM wave can be generated by first

differentiating m(t) and then using the result

as the input to a frequency modulator

The phase modulation index p , is given by

Where is the peak phase deviation of the

transmitter


32/77

4.4 Frequency Modulation vs.

Amplitude Modulation

Frequency modulation (FM) is the mostpopular analog modulation technique used inmobile radio systems

In FM, the amplitude of the modulatedcarrier signal is kept constant while itsfrequency is varied by the modulatingmessage signal

Thus, FM signals have all their information inthe phase or frequency of the carrier


33/77

This provides a nonlinear and very rapid

improvement in reception quality once acertain minimum received signal level, calledthe FM threshold, is achieved

In amplitude modulation (AM) schemes, there

is a linear relationship between the quality ofthe received signal and the power of thereceived signal since AM signals superimposethe exact relative amplitudes of the

modulating signal onto the carrier Thus, AM signals have all their information in

the amplitude of the carrier


34/77

FM offers many advantages over amplitude

modulation (AM), which makes it a betterchoice for many mobile radio applications

Frequency modulation has better noise

immunity when compared to amplitude

modulation

Since signals are represented as frequency

variations rather than amplitude variations,

FM signals are less susceptible toatmospheric and impulse noise, which tend

to cause rapid fluctuations in the amplitude of

the received radio signal


35/77

Also, message amplitude variations do not

carry information in FM, so burst noise does

not affect FM system performance as much as

AM systems, provided that the FM received

signal is above the FM threshold

Unlike AM, in an FM system, the modulationindex, and hence bandwidth occupancy, can

be varied to obtain greater signal-to-noise

performance Under certain conditions, the FM signal-to-

noise ratio improves 6 dB for each doubling of

bandwidth occupancy


36/77

This ability of an FM system to trade

bandwidth for SNR is perhaps the most

important reason for its superiority over AM However, AM signals are able to occupy less

bandwidth as compared to FM signals, since

the transmission system is linear An FM is a constant envelope signal, due to

the fact that the envelope of the carrier does

not change with changes in the modulating

signal

Hence the transmitted power of an FM signal

is constant regardless of the amplitude of the

messa e si nal


37/77

The issue of amplifier efficiency is extremely

important when designing portable subscriberterminals since the battery life of the portable

is tied to the power amplifier efficiency

The capture effect is a direct result of the

rapid nonlinear improvement in received

quality for an increase in received power

If two signals in the same frequency band are

available at an FM receiver, the one appearingat the higher received signal level is accepted

and demodulated, while the weaker one is

rejected


38/77

This inherent ability to pick up the strongest

signal and reject the rest makes FM systems

very resistant to co-channel interference and

provides excellent subjective received quality

In AM systems, on the other hand, all of the

interferers are received at once and must bediscriminated after the demodulation process


39/77

4.5 Advantages of Digital

Modulation Modern mobile communication systems use

digital modulation techniques

Advancements in very large-scale integration

(VLSI) and digital signal processing (DSP)technology have made digital modulation

more cost effective than analog transmission

systems

Digital modulation offers many advantages

over analog modulation


40/77

Some advantages include greater noise

immunity and robustness to channelimpairments, easier multiplexing of various

forms of information (e.g., voice, data, and

video), and greater security

Digital transmissions accommodate digital

error-control codes which detect and/or

correct transmission errors, and support

complex signal conditioning and processingtechniques such as source coding, encryption,

and equalization to improve the performance

of the overall communication link


41/77


42/77

In digital wireless communication systems, the

modulating signal (e.g., the message) may berepresented as a time sequence of symbols or

pulses, where each symbol has m finite states

Each symbol represents n bits of information,

where n = log2m bits/symbol


43/77

4.6 Performance of a modulation

scheme

The performance of a modulation scheme is

often measured in terms of its bit error rate,

power efficiency and bandwidth efficiency

Power efficiency describes the ability of a

modulation technique to preserve the fidelity

of the digital message at low power levels

In a digital communication system, in order to

increase noise immunity, it is necessary to

increase the signal power


44/77

However, the amount by which the signal

power should be increased to obtain a certainlevel of fidelity(i.e., an acceptable bit error

probability) depends on the particular type of

modulation employed

The power efficiency, p , (sometimes called

energy efficiency) of a digital modulation

scheme is a measure of how favourably this

trade-off between fidelity and signal power ismade


45/77

It is often expressed as the ratio of the signal

energy per bit to noise power spectral density

(Eb/N0) required at the receiver input for a

certain probability of error (say 10-5)

Bandwidth efficiency describes the ability of a

modulation scheme to accommodate datawithin a limited bandwidth

In general, increasing the data rate implies

decreasing the pulse width of a digital symbol,which increases the bandwidth of the signal


46/77

Bandwidth efficiency reflects how efficiently

the allocated bandwidth is utilized and isdefined as the ratio of the throughput data

rate per Hertz in a given bandwidth

If R is the data rate in bits per second, and B is

the bandwidth occupied by the modulated

1W signal, then bandwidth efficiency B is

expressed as


47/77

The system capacity of a digital mobile

communication system is directly related tothe bandwidth efficiency of the modulation

scheme, since a modulation with a greater

value ofB will transmit more data in a given

spectrum allocation

There is a fundamental upper bound on

achievable bandwidth efficiency


48/77

Shannon's channel coding theorem states that

for an arbitrarily small probability of error,

the maximum possible bandwidth efficiency is

limited by the noise in the channel, and is

given by the channel capacity formula

where C is the channel capacity (in bps), B is

the RF bandwidth, and S/N is the signal-to-noise ratio


49/77


50/77


51/77

If the modulation bandwidth exceeds the

coherence bandwidth of the radio channel, ISI

occurs and modulation pulses are spread intime

An equalizer within a receiver compensates

for the average range of expected channelamplitude and delay characteristics

Equalizers must be adaptive since the channel

is generally unknown and time varying

Diversity is another technique used to

compensate for fading channel impairments,

and is usually implemented by using two or

more receivin antennas


52/77

As with an equalizer, the quality of a mobile

communications link is improved without

increasing the transmitted power orbandwidth

While equalization is used to counter the

effects of time dispersion (ISI), diversity

usually employed to reduce the depth and

duration of the fades experienced by a

receiver in a flat fading (narrowband) channel

One of the best techniques to mitigate the

effects of fading is diversity combining of

independently fading signal paths


53/77

Diversity combining exploits the fact that

independent signal paths have a low

probability of experiencing deep fadessimultaneously

Thus, the idea behind diversity is to send the

same data over independent fading paths These independent paths are combined in

such a way that the fading of the resultant

signal is reduced For example, consider a system with two

antennas at either the transmitter or receiver

that experience independent fading


54/77

If the antennas are spaced sufficiently far

apart, it is unlikely that they both experiencedeep fades at the same time

By selecting the antenna with the strongest

signal, a technique known as selection

combining, we obtain a much better signal

than if we had just one antenna

Diversity techniques can be employed at both

base station and mobile receivers


55/77


56/77

CDMA systems often use a RAKE receiver,

which provides link improvement through

time diversity

By coding is meant the purposeful

introduction of additional bits in a digital

message stream to allow correction and/ordetection of bits in the message stream that

may have been received in error

Channel coding improves mobilecommunication link performance by adding

redundant data bits in the transmitted

message


57/77

At the baseband portion of the transmitter, a

channel coder maps a digital message

sequence into another specific code sequence

containing a greater number of bits than

originally contained in the message

The coded message is then modulated fortransmission in the wireless channel

Channel coding is used by the receiver to

detect or correct some (or all) of the errorsintroduced by the channel in a particular

sequence of message bits


58/77

Because decoding is performed after the

demodulation portion of the receiver, coding

can be considered to be a post detectiontechnique

The added coding bits lowers the raw data

transmission rate through the channel

(expands the occupied bandwidth for a

particular message data rate)

There are two general types of channel codes:

block codes and convolutional codes


59/77

Channel coding is treated independently from

the type of modulation used

Although this has changed recently with theuse of trellis coded modulation schemes that

combine coding and modulation to achieve

large coding gains without any bandwidthexpansion

The three techniques of equalization, are used

to improve radio link performance (to

minimize the instantaneous bit error rate), but

the approach, cost, complexity, and

effectiveness of each technique varies widely

in practical wireless communication systems


60/77

RAKE Receiver

In CDMA spread spectrum systems, the chip

rate is typically much greater than the flat

fading bandwidth of the channel

Whereas conventional modulation techniques

require an equalizer to undo the inter symbolinterference between adjacent symbols,

CDMA spreading codes are designed to

provide very low correlation betweensuccessive chips

Propagation delay spread in the radio channel

merely provides multiple versions of the


61/77

If these multipath components are delayed in

time by more than a chip duration, they

appear like uncorrelated noise at a CDMAreceiver, and equalization is not required

A RAKE receiver attempts to collect the time-

shifted versions of the original signal byproviding a separate correlation receiver for

each of the multipath signals

CDMA systems often use a RAKE receiver,which provides link improvement through

time diversity


62/77

The RAKE(M branch or M-finger) receiver, shown in the

Figure, is essentially a diversity receiver designed

specifically for CDMA, where the diversity is provided

by the fact that the multipath components are

practically uncorrelated from one another when their

relative propagation delays exceed a chip period


63/77

A RAKE receiver utilizes multiple correlators to

separately detect the M strongest multipath

components

The outputs of each correlator are weighted

to provide a better estimate of the

transmitted signal than is provided by a singlecomponent

Demodulation and bit decisions are then

based on the weighted out puts of the M

correlators

A weighting network is used to provide a

linear combination of the correlator output for


64/77

The outputs of the M correlators are denoted

as Z1, Z2 ,... and ZM, They are weighted by

1,2,M respectively

The overall signal Z' is given by

The weighting coefficients, am, are normalized

to the output signal power of the correlator in

such a way that the coefficients sum to unity,as show below


65/77

4.8 Speech Coding

Speech coders determines the quality of the

recovered speech and the capacity of the

system

Service providers are continuously met with

the challenge of accommodating more users

within a limited allocated bandwidth

Low bit-rate speech coding offers a way tomeet this challenge


66/77


67/77

Speech coders differ widely in their

approaches to achieving signal compression

Based on the means by which they achieve

compression, speech coders are broadly

classified into two categories: Waveform

Coders and Vocoders Waveform coders essentially strive to

reproduce the time waveform of the speech

signal as closely as possible They are designed to be source independent

and can hence code equally well a variety of

signals


68/77

Examples of waveform coders include Pulse

Code Modulation (PCM), Differential Pulse

Code Modulation (DPCM), AdaptiveDifferential Pulse Code Modulation (ADPCM),

Delta Modulation (DM), Continuously Variable

Slope Delta Modulation (CVSDM), andAdaptive Predictive Coding (APC)

Vocoders on the other hand achieve very high

economy in transmission bit rate and are in

general more complex

They are based on using a priori knowledge

about the signal to be coded(Signal specific)


69/77

Characteristics of Speech Signals

1. The non-uniform probability distribution ofspeech amplitude

2. The nonzero autocorrelation between

successive speech samples3. The non-flat nature of the speech spectra,

the existence of voiced and Unvoiced

segments in speech4. The quasi-periodicity of voiced speech signals

are the properties that are most often

utilized in coder design


70/77

The most basic property of speech waveforms

that is exploited by all speech coders is that

they are band limited Probability Density Function (pdf): The non-

uniform probability density function of speech

amplitudes is perhaps the next most exploitedproperty of speech

The pdf of a speech signal is characterized by a

very high probability of near-zero amplitudes,

a significant probability of very high

amplitudes, and a monotonically decreasing

function of amplitudes between these

extremes


71/77

The exact distribution depends on the input

bandwidth and recording conditions

The two-sided exponential (Laplacian)

function given in the equation below provides

a good approximation to the long-term pdfof

telephone quality speech signals

This pdf shows a distinct peak at zero

Short-time pdfs of speech segments are also

single-peaked functions and are usually

approximated as a Gaussian distribution


72/77

Autocorrelation Function (ACF): Another very

useful property of speech signals is that there

exists much correlation between adjacent

samples of a segment of speech

This implies that in every sample of speech,

there is a large component that is easilypredicted from the value of the previous

samples with a small random error

All differential and predictive coding schemesare based on exploiting this property

It gives a quantitative measure of the


73/77

It gives a quantitative measure of the

closeness or similarity between samples of a

speech signal as a function of their time

separation and mathematically defined as:

where x (k) represents the kth speech sample

The autocorrelation function is often

normalized to the variance of the speech

signal and hence is constrained to have values

in the range {-1,1} with C (0) = 1 signals have

an adjacent sample correlation, C(1) , as high

as 0.85 to 0.9


74/77

Power Spectral Density Function (PSD):The

non-flat characteristic of the power spectral

density of speech makes it possible to obtainsignificant compression by coding speech in

the frequency domain

The non-flat nature of the PSD is basically afrequency domain manifestation of the

nonzero autocorrelation property

Typical long-term averaged PSD's of speechshow that high frequency components

contribute very little to the total speech

energy

Thi i di h di h l i


75/77

This indicates that coding speech separately in

different frequency bands can lead to

significant coding gain A qualitative measure of the theoretical

maximum coding gain that can be obtained by

exploiting the non-flat characteristics of thespeech spectra is given by the Spectral

Flatness Measure (SFM)

The SFM is defined as the ratio of the

arithmetic to geometric mean of the samples

of the PSD taken at uniform intervals in

frequency


76/77

Mathematically,

where, Sk is the kth frequency sample of the

PSD of the speech signal

Typically, speech signals have a long-term

SFM value of 8 and a short-time SFM value

varying widely between 2 and 500


77/77

wireless and cellular communications-unit four

Documents