Transcript Wireless systems 27 th february

Wireless Systems
Source coding and Channel coding
Block Diagram of a Digital System
Digital Transmission
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What is Digital Transmission?
A computer
network is designed to send
information from one point to another.
This information needs to be converted either to
digital signal or analog signal for transmission.
Why Digital Signals are better than Analog
Signals?
Digital to Digital Conversion
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In previous lectures you have come to know what
is the difference between a data and signal.
We said that data can be either digital or analog.
Here we will see how we can represent digital
data by using digital signals.
The conversion involves three techniques: Line
Coding, Block Coding and Scrambling.
Line Coding is always needed, Block Coding and
scrambling may not be needed.
Line Coding
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Line Coding is the process of converting Digital Data
to Digital Signals.
We assume data can be in the form of text, numbers,
graphical images, audio or video are stored in the
computer memory as sequences of bits.
You know computer is a Digital Device so data is
always in the form of binary (0,1)
Line Coding converts a sequence of bits to a Digital
Signal.
At the Sender Digital data are encoded into a Digital
Signal. So at the receiver you need to decode the
signal to retrieve the digital data.
Characteristics
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Before discussing different line coding schemes,
we will address their common characteristic and
their common characteristic will be?
Signal Element Versus Data Element
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Let us distinguish between a signal element and a
data element.
In data communications, our goal is to send data
elements.
A data element is the smallest entity that can
represent a piece of information: this is the bit.
In digital data communications, a signal element
carries data elements.
A signal element is the shortest unit (time wise)
of a digital signal.
Signal Element Versus Data Element
(Continued)
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In other words, data elements are what we need
to send: signal elements are what we can send.
Data elements are being carried; signal elements
are the carrier.
We define a ratio r which is the number of data
elements carried by each signal element. Figure
on the next page will highlight several situations
with different values of r.
Signal Element Verses Data Element
Explanation
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In part a of the figure, one data element is carried
by one signal element so r=1.
In part b of the figure, we need 2 signal elements
to carry each data element (r=1/2).
We will see later why extra signal element is
needed to guarantee synchronization.
In part c of the figure a signal element carries 2
data elements so r=2 and finally r= 4/3.
For every line coding schemes we discuss, will give
the value of r.
Explanation (Continued)
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An analogy may help here. Suppose each data
element is a person who need to be carried from
one place to another.
We can think of a signal element as a vehicle that
can carry people.
When r=1 means each person is driving a vehicle.
Where r>1 means that more persons are traveling
in the vehicle.
Do you have a clear understanding now? It is not
difficult. Easiest way to tackle it is keep things
simple and straightforward.
Data Rate Versus Signal Rate
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The data rate defines the number of data elements
(bits) sent in 1 second.
The unit is bits per second (bps)
The signal rate is the number of signal elements sent
in 1 second.The unit is baud.
The data rate is sometimes called the bit rate, and the
signal rate is sometimes called a pulse rate, the
modulation rate or the baud rate.
Our goal in data communication is to increase the
data rate while decreasing the signal rate. Increasing
the data rate will increase the speed of transmission,
decreasing the signal rate decreases the bandwidth
requirement.
Discussion (Continued)
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We now need to consider the relationship
between the data rate and signal rate.
The relationship, of course depends on the value
of r.
It also depends on the data pattern.
If we have the pattern of all 1’s and 0’s the signal
rate may be different from a data pattern of
alternating 0’s and 1’s.
To establish a mathematical formula we need to
define three cases.The worst, best and average.
The worst case is when we need max signal rate
and the best case is when we need the minimum.
Relationship
signal rate
between
data
rate
and
The formula is s= c x N x 1/ r
 Where N is the data rate, bps c is the case factor
that varies for each case, s is the number of signal
elements, and r is the previously defined factor.
Example:
 A signal is carrying data in which one data
element is encoded as one signal element. If the
bit rate is 100kbps, what is the average value of
the baud rate if c is between 0 and 1.
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Line Coding Schemes
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We can roughly divide the line coding schemes
into 5 broad categories.
Unipolar Scheme
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In a unipolar scheme, all the signal levels are on
one side of the time axis, either above or below.
NRZ
(Non-Return-to-Zero)
Traditionally
a
unipolar scheme was designed as non return to
zero scheme in which the positive voltages define
bit 1 and the zero voltages define bit 0.
It is called NRZ because the signal doesn’t return
to zero at the middle of the bit.
Polar Schemes
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In polar schemes the voltages are on both sides of
the time axis. For example the voltage level for 0
can be positive and the voltage level for 1 can be
negative.
NRZ, In polar NRZ we use two level of voltage
amplitude.
We can have 2 versions of polar NRZ: NRZ-L and
NRZ-I
Return to Zero
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It
describes
a
line
code
used
in
telecommunication signals in which the signal
drops (returns to zero) between each pulse.
This takes place even a number of consecutive 0’s
and 1’s occurs in a signal.
The signal is self clocking. This means that a
separate clock doesn’t needed to be sent along
side the signal, but suffers from twice the
bandwidth to achieve the same data rate as
compared to NRZ Format.
RZ (Figure)
Introduction
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Why do we need channel coding and error control
for radio communication?
You all should be aware of the fact that the
medium is not noise less.
Also in the case of satellite communication you
have limited transmitting power in forward
channels (downlink)
In wireless communications, message go through
the Noise medium between BS and MS and
reflection, refraction, diffraction and scattering
effects the quality of the signal.
Introduction (Continued)
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So any phenomenon that can enhance correct
reception of radio signals is always welcome.
Channel coding add redundancy information to
the original information at the transmitter side.
It follows some logical relation with the original
information.
After reception the receiver receives the encoded
data, possibility with the degree of degradation.
At the receiver end the original information can
be extracted from the relation between the logical
and actual information.
Introduction (Continued)
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Introduction to this redundancy will cause
consumption of more bandwidth.
However it offers benefits of recovering from higher
bit error rates.
In other words the channel coding allows signal
transmission power and useful bandwidth as higher
degree of redundancy can tolerate a larger number of
errors.
However yet it is a fact that in wireless
communications the traffic consists of compressed
data example audio/ video in digital form, that makes
them very sensitive to transmission errors.
Definition of Channel Coding
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The channel coding can be defined as “the process
of coding discrete digital information in a form
suitable for transmission, with an emphasis on
enhanced reliability”
Channel coding is applied to ensure adequacy of
transmission quality bit error rate and frame
error rate.
Introduction
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One of the most important control messages sent
to MS is its readiness to send information to the
BS.
In return the BS advises the MS which traffic or
information channel is to be used exclusively by
that MS for actual information.
Such channel allocation is done for the duration of
a call from the MS, and such an assignment is
done dynamically as needed so that the wireless
resources can be used effectively and efficiently.
Introduction (Continued)
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In wireless environment, a BS needs a radio
connection between a BS and all the MSs in their
transmission range.
Since wireless communication is categorized by a
wide propagation, there is a need to address the
issue of simultaneous multiple access by
numerous users in the transmission range.
User can also receive signals transmitted by other
users in the system.
In fact many users access the traffic channels
when the uplink path from MS to BS is to be
established.
Introduction (Continued)
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Therefore it is important for users to distinguish
among different signals.
To accommodate the number of users, many
traffic channels needs to be made available.
In principle there are three basic ways to have
many channels within an allocated bandwidth,
they are;
1. Frequency addressed by FDMA
2.Time addressed by TDMA
3. Code addressed by CDMA
Concepts
Divisions
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and
Models
for
Multiple
There are many MSs allocated in the range/
coverage of a BS.
A MS must distinguish which signal is meant for
itself among many signals being transmitted by
other users or BSs.
The BS must be able to recognize the signal sent
by a particular user.
In other words a wireless cellular system, each MS
not only can distinguish a signal from a serving BS
but also can discriminate signals from the
adjacent BS.
Concepts and Models for Multiple
Divisions (Continued)
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Therefore a multiple access technique is
important in mobile cellular system.
Multiple Access techniques are based on
orthogonalization of signals.
A radio signal can be presented as a function of
frequency, time or code as;
s(f, t, c)= s(f, t) c(t)
f is the function of frequency and t is the function
of time.
when c(t)= 1
s(f, t, c)= s(f, t)
Concepts and Models for Multiple
Divisions (Continued)
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If a system employs different carrier frequencies
to transmit the signal for each user it is called
FDMA System.
If a system uses distinct time slots to transmit the
signal from different users, it is a TDMA system.
If a system users different codes to transmit the
signal for each user, it is a CDMA system.
Let us write to signals Si and Sj both transmitted
in the cell space., the orthogonality conditions can
be given by using a general mathematical mode,
and we formally consider them as;
Explanation
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In wireless communication, frequency bands are
limited and one has to utilize them at the same time.
That will allow multiple users to share radio channels
simultaneously.
To provide simultaneously two way communications
duplex communications a forward and reverse channel
are necessary.
Two types of duplex systems are utilized: FDD and
TDD.
FDMA mainly use FDD and TDMA use TDD.
A number of channels can be used to transfer data in a
much higher rate and the technique is called OFDM
FDMA
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The Orthogonality condition of the two signals in
FDMA is given by;
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There is no overlapping frequency in frequency
domain F and the two signals don’t interfere with
each other.
FDMA (Continued)
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FDMA is a multiple access technique that has
been widely adopted in Analog system for a
portable and wireless mobile telephones.
The BS dynamically assigns a different carrier
frequency to each active user (MS).
A frequency synthesizer is used to adjust and
maintain the transmission and reception
frequencies.
FDMA (Continued)
FDMA (Continued)
TDMA
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The orthogonality condition of TDMA is given as;
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Because it is based on the division of time, one can
easily determine that there is no over lapping in
the time axis.
TDMA (Continued)
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So TDMA splits a single carrier wave into several
time slots and distributes slots among multiple
users.
The communication channels essentially consists
of many units i.e. time slots over a time cycle,
which makes it possible for one frequency to be
efficiently utilized by multiple users.
Given that each utilize a different time slot.
This system is widely used in the field of Digital
Portable and automobile telephone and mobile
satellite communications systems.
TDMA (Continued)
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A TDMA can use two modes FDD and TDD.
TDMA (Continued)
TDMA (Continued)