EC302 Digital communications_module5_Part 3
Summary
TLDRThis lecture delves into the critical role of synchronization in digital communication systems, emphasizing the necessity of aligning transmitter and receiver clocks to ensure accurate data transfer. It explores various synchronization techniques, including carrier synchronization for phase and frequency recovery, symbol synchronization which involves transmitting a clock signal with data, and frame synchronization that uses flags to mark the beginning and end of data packets. The lecture highlights the advantages of coherent detection over noncoherent methods, particularly in noise performance and bandwidth efficiency, and discusses the challenges posed by unsynchronized clocks, such as incorrect symbol decoding and data loss.
Takeaways
- 📡 The lecture discusses the significance of synchronization in digital communication, focusing on carrier, symbol, and frame synchronization techniques.
- 🌐 Spread spectrum techniques and PN0 noise sequences were covered in the previous lecture, setting the stage for this discussion on synchronization.
- 📶 Synchronization is crucial for coherent detection in digital communication systems, where the receiver needs to recreate the carrier signal to correctly detect or decode the transmitted information.
- 🔄 In non-coherent detection, one or more characteristics of the sample functions must be estimated, which is less efficient than coherent detection in terms of noise performance and bandwidth.
- 🕰️ Synchronization ensures that the transmitter and receiver clocks are aligned, which is vital for the correct sampling and decoding of the transmitted symbols.
- 🔗 Asynchronous communication is used when each symbol is independent and does not rely on previous symbols, whereas synchronous communication treats a set of symbols as a single entity.
- 🛠️ The need for synchronization arises from potential clock discrepancies between the sender and receiver, which can lead to incorrect sampling and symbol decoding if not addressed.
- 🔄 Carrier synchronization involves techniques like the squaring loop and Costas loop to recover the carrier's frequency and phase for coherent detection.
- 📈 Symbol synchronization can be achieved by transmitting the clock signal along with the data, which is then extracted at the receiver, minimizing the time required for clock recovery.
- 📋 Frame synchronization is essential in modern digital communication systems that transmit data in frames or packets, using flags to indicate the start and end of a frame for proper data decoding.
Q & A
What is the main topic of the lecture?
-The main topic of the lecture is synchronization in digital communication systems, including the need for synchronization and the types of synchronization techniques such as carrier synchronization, symbol synchronization, and frame synchronization.
Why is synchronization important in digital communication?
-Synchronization is important because it ensures that the transmitted data is correctly recovered at the receiver. Without proper synchronization, noise and distortions in the channel can lead to incorrect decoding of symbols, resulting in data loss and inefficient communication.
What are the two types of detection methods mentioned in the lecture?
-The two types of detection methods mentioned are coherent detection and noncoherent detection. Coherent detection uses all possible sample functions, while noncoherent detection estimates one or more characteristics of the sample functions.
What is the difference between synchronous and asynchronous communication?
-In synchronous communication, the transmitter and receiver clocks are closely synchronized, allowing for easier detection of signals. Asynchronous communication, on the other hand, treats each element of the transmission independently, without synchronization of transmitter and receiver clocks.
What is the purpose of a start bit and a stop bit in asynchronous transmission?
-In asynchronous transmission, a start bit and a stop bit are used to indicate the beginning and end of data transmission. These bits help the receiver to identify when the actual data starts and ends, as the clock information is not transmitted with the data.
What is carrier synchronization and how is it achieved?
-Carrier synchronization is the process of estimating and recovering the carrier frequency and phase at the receiver. It is achieved through methods like the squaring loop, where the received signal is squared and filtered to recover the carrier, or the Costas loop, which involves phase discrimination and a phase-locked loop (PLL).
What is symbol synchronization and how does it work?
-Symbol synchronization involves the extraction of the clock signal from the received signal to sample the data at the correct instants. It is achieved by transmitting the clock along with the data signal and then using filtering techniques at the receiver to extract the clock for data recovery.
What is frame synchronization and why is it necessary?
-Frame synchronization is the process of identifying the start and end of a data frame in packet-based communication systems. It is necessary to ensure that the data is correctly demodulated and decoded, preventing data loss and ensuring efficient communication.
How does the lack of synchronization between transmitter and receiver clocks affect data transmission?
-The lack of synchronization between transmitter and receiver clocks can lead to incorrect sampling instances, causing the receiver to sample the signal at the wrong time. This can result in incorrect decoding of symbols and an increase in the number of bits received, as illustrated in the lecture with an example where an 8-bit transmission resulted in 9 bits received due to clock speed differences.
What are the two methods discussed for carrier synchronization in the lecture?
-The two methods discussed for carrier synchronization are the squaring loop method, where the received signal is raised to the power of M and then filtered and phase-locked, and the Costas loop method, which involves multiplying the received signal with a carrier and using phase discrimination to recover the carrier.
Outlines
📡 Introduction to Synchronization in Digital Communications
This paragraph introduces the topic of synchronization in digital communication systems. It explains the necessity of synchronization for the proper functioning of spread spectrum techniques and the role of PN0 noise sequences. The speaker outlines the agenda for the lecture, which includes discussing the need for synchronization, the types of synchronization techniques such as carrier, symbol, and frame synchronization, and the differences between coherent and noncoherent detection methods. The importance of synchronization in improving noise performance and bandwidth efficiency is highlighted.
🕰️ Synchronization Challenges in Digital Data Transfer
The second paragraph delves into the challenges of synchronizing sender and receiver clocks during the serial transfer of digital data. It explains how a lack of synchronization can lead to incorrect sampling instances, resulting in the wrong decoding of symbols. An example is provided to illustrate the consequences of a receiver's clock running faster than the transmitter's clock, leading to an incorrect bit sequence at the receiver. The paragraph emphasizes the need for synchronization to ensure accurate data transmission and reception.
🔄 Methods of Synchronization: Carrier, Symbol, and Frame
This paragraph discusses the different types of synchronization techniques used in digital communication systems. It explains carrier synchronization, which involves the recovery of carrier frequency and phase, and mentions two methods: the squaring loop and the Costas loop. Symbol synchronization is then described, where the clock is transmitted along with the data signal, allowing the receiver to extract the clock and recover the original information. Lastly, frame synchronization is introduced, which is necessary for modern digital communication systems that transmit data in frames or packets, using start and end flags to identify frame boundaries.
📶 Carrier Synchronization Techniques: Squaring Loop and Costas Loop
The fourth paragraph focuses on the technical details of carrier synchronization, specifically the squaring loop and Costas loop methods. It describes the squaring loop, where the received signal is raised to a power, filtered, and then used in a phase-locked loop (PLL) to recover the carrier signal. The Costas loop is also explained, where the received signal is split and multiplied with a carrier, and the phase difference is used to adjust the voltage-controlled oscillator (VCO) for synchronization. The paragraph provides a detailed technical overview of how these methods ensure accurate carrier recovery.
📜 Symbol and Frame Synchronization in Digital Communication
The final paragraph discusses symbol synchronization, where the clock is transmitted with the data signal, and the receiver extracts the clock for accurate data recovery. It also covers frame synchronization, which is crucial for modern digital communication systems that use packet transmission. The use of start and end flags (flags) to mark the beginning and end of a frame is explained, ensuring that the receiver can correctly identify and decode the transmitted data. The paragraph concludes by summarizing the importance of synchronization in maintaining efficient and accurate data transmission.
Mindmap
Keywords
💡Synchronization
💡Spread Spectrum
💡Coherent Detection
💡Noncoherent Detection
💡Carrier Synchronization
💡Symbol Synchronization
💡Frame Synchronization
💡PN Sequences
💡Modulation
💡Asynchronous Communication
💡Synchronous Transmission
Highlights
Introduction to the need for spread spectrum techniques and the role of PN0 noise sequences.
Explaining the concept of synchronization in digital communication systems.
Differentiating between coherent and noncoherent detection methods.
The importance of synchronization for noise performance and bandwidth efficiency.
Definition and implications of synchronous and asynchronous communication.
Challenges posed by unsynchronized transmitter and receiver clocks.
Example illustrating the impact of clock desynchronization on data transmission.
Classification of synchronization techniques: carrier, symbol, and frame synchronization.
Carrier synchronization methods: squaring loop and Costas loop.
Explanation of the squaring loop method for carrier synchronization.
Costas loop method for carrier synchronization and its components.
Symbol synchronization and its role in multiplexing the clock with data.
Techniques for extracting the clock from the modulated waveform.
Frame synchronization and the use of flags to mark the start and end of data frames.
Importance of frame synchronization in modern digital communication systems.
Summary of the necessity and types of synchronization in digital communications.
Transcripts
hello everyone welcome to the second
lecture on EC 3:02 digital
communications module files and the
previous video I had covered about the
need for spread spectrum technique and
the importance of pn0 noise sequences in
spread spectrum in this video I will be
covering the topic of synchronization
the need for synchronization and the
types of synchronization techniques
which would cover carrier
synchronization symbol synchronization
and strain synchronization so let's
quickly begin in a digital communication
system data is transmitted from one
location to another by mapping the
sequences paratus by coding the
sequences into symbols be modulated we
use a proper technique of modulation to
transmit the signal depending on the
conditions for the environment and the
power transmitted power and the
bandwidth availability now once the
signal is transmitted at the receiver
these symbols have to be recovered by
some means so as it is propagated noise
may be added in the channel and the
signal may be distorted so we have to
overcome such distortions and then
detect the signals back in its original
form now in the receiver or at the
receiver we have two different ways of
for detection possible one is coherent
detection the other one is in
noncoherent detection now coherent
detection is where all the possible
sample functions are known to us
whereas in on coherent detection one or
more characteristics of the sample
functions are to be estimated now if you
look at coherent or detection or a
coherent receiver a very important or a
very commonly used technique is
synchronization
that is the samples whichever we have
received are again processed to get the
reference signals for the correlator
that is that nothing but a carrier
signal is being recreated to detect our
or decode it correctly we say
synchronisation baseless rivers are more
advantages over non-coherent ones in
terms of noise performance and bandwidth
efficiency if we say a synchronous
communication what does it mean when
there is no synchronization of
transmitter and receiver clocks when we
say synchronization it is all about the
transmitter and receiver clocks the
clock frequency so when there is no
synchronization of transmitter and
receiver clocks we call it as a
synchronous transmission this mode is
used when each element of the
transmission I think I have not made a
mistake I was saying about asynchronous
communication when there is no
synchronization of transmitter and
receiver we call it as a synchronous
mode of communication this mode is used
when each element of the transmission
should be treated independently that is
they are all each symbol is independent
of feature that they don't depend on the
previous symbols are the successful
symbols
what about synchronous communication
where there is a close synchronization
of transmitter and receiver we call such
a mode of transmission have synchronous
transmission that is the complete block
or the complete symbols or the set of
symbols which we have received is
considered to be one single entity of
transmission so for a synchronous
transmission in the receiver since they
are not synchronous or since the clocks
are Marta synchronized
we'll have to find out some other way or
means to actually detect the signals
correctly when they're synchronized
it becomes easier for us because we know
when the carrier carrier or our
frequencies or the carrier signals time
periods starts and ends right so it's
possible for us to actually detect it
but whereas in a synchronous mode things
that they are not synchronized we need
to find another alternative source to
actually detect these signals signals
correctly
what does it need for synchronizing if
we look at the actual need for
synchronization there are several points
which make it clear why it is essential
for the serial transfer of data of
digital data the one major problem we
would say is the sender and receiver
clocks may not run synchronous so and so
the sampling instance the instant at
which we sample the signal will vary
will shift from the beginning of a
signal if suppose we have toasting
sample it at the beginning of the signal
and then the signals or the clocks are
not synchronized what happens is the
samples may be taken and at the end of
the signal which is completely wrong and
we don't get the symbols decoded
correctly so let's let's take an example
of some sequences of which is trans to
transmitter and what happens at the
receiver if they're not synchronized
okay so let's take an example of but
let's imagine a communication system the
two equipments where or where two
equipments are being transmitted
transmitting information and their
clocks are there's a major difference in
the speed of their system clocks let's
say the receivers clock was running at
around 12.5% ahead of time that is 12.5%
ahead of time the receiver is receiver
clock is 12.5% ahead of the transmitter
clock in such a scenario what happens
that means there's no synchronization
between the transmitter
Randy receiver rockabilly considered
we'll take an example the first one is a
transmitted signal and the second one
the we are going to draw is the receive
sing no cut considering this condition
that is the receiver clock is 12.5%
times faster than the transmitter clock
so clock frequencies faster which means
the time period is going to be smaller
yeah
so here over here what happens is this
signal sings are not synchronized what
happens is they're coded this way that
is this time period or this time axis is
not maintained see it exits and then you
see over here you get this kind of a
signal which does not follow the time
period for off the transmitted signal so
you get something of this the amplitude
is going to be the same there's no much
variation in the amplitude let's not
take that into consideration enough they
can't sit talking about the time periods
or detecting it in proper time intervals
now suppose this is the way you have
decoded it because of this condition now
what happens over here if you read this
out over here it is 1 0 its 1 ok 0 0 0
this is 0 so in this time period
actually this isn't far not of delay
yeah so we hear this is going to be 0
and this is going to be 1 if we check
this out our coded value 0 what we
transmitted what was 1 0 1 0 0 0 0 1 ok
what you've received is 1 0 1 0 0 0 0 1
2 3 4 5 zeros and a one here let's not
take this value the last bit is 1 so
check out these two sequences it's a
or is it descent you can see there is
one extra bit right so though we have
transmitted how many bits there eight
bits in the receiver we have to be coded
to be nine bits due to this error or
this rate of difference in get speeds
off the clock of transmitter and
receiver now whatever
8% 8-bit is sent but in the receiver we
are we have obtained nine bits when we
decoded it yes this happens because it
doesn't the receiver clock does not
follow the time period of the
transmitter block so this is the problem
of if the transmitter receiver is not
synchronized now if you consider the
different types of synchronization how
are they actually classified see when
covariant detection is used we we need
to know both the frequency and the phase
of the carrier only then can be detected
correctly right so the estimation of
carrier phase and frequency is called
carrier recovery or carrier
synchronization that is if we are able
to correctly obtain the frequency and
phase of the carrier or the method used
to actually find the frequency and phase
of the carrier is perfect when scarier
synchronization and to perform again the
second one if we have to perform the
modulation in case of a synchronous mode
what happens we don't have this carrier
clock synchronized sight so we need to
find an alternative method so that
alternative method is to use suppose we
have some set of data I need to transmit
hello yeah so I need to transmit hello
and what I do is I am using an a'
synchronous mode of transmission what I
do is I actually insert a dark bit and a
stop bit
this is a start bit and this is the stop
bit the reason why I am inserting the
start bit and stop it are for the
receiver to know when
since we don't know the time interval or
the rate at which that it is being
modulated we have not aware of the
carrier at all so we insert a start and
a stop bit at the transmission side so
that the receiver can actually identify
when does this actual data start and
when is it going to end so one major
point you have to keep in mind see if
you don't use a same set of information
for start and stop bits the reason
because we cannot to the receiver cannot
identify if this is a stop bit or a
start bit if we use the same set of
values
so whatever start value value they use
for start it should be the opposite of
what we use for stop so both are
different so that we can identify if
it's a start bit or a stop it so this
method is used for s think Rinna's mode
of transmission next this type of
detection is the third-best symbol
synchronization and the third one we
have a frame or chip synchronization
frame synchronization is also called
shift synchronization where in modern
digital we consider the modern
communication system modern digital
communication systems then we transmit
the data in the form of frames or
packets so since we transmitted in the
form of packets of frames we need to
synchronize them when there's a start of
a frame and when doesn't end
so this synchronization technique is
referred to as frame synchronization or
chip synchronization so in so 3 times
carrier synchronization where the clock
frequency and phase are to be recovered
second is a simple synchronization where
you have to be transmitted bits to know
where when and where to sample the
signal signal and third one is the frame
synchronization so let's quickly move on
to the carrier the first one which is
carrier synchronization carrier
synchronization we have two methods one
was empty power method empty power loop
okay and the second one is cost as loop
cost as low
maybe I'm not aware of those names but
when I start explaining you what these
are you feel that you have already
covered it previously in few techniques
of detection and transmitting BPSK where
you could not use a coherent detection
we had gone for a non coherent one and
castas also we had gone through PLL so
cute and all that so so I'm sure or
you'll be familiar with these pumps and
when I start explaining you the first
concept is empower loop this is the
block diagram for our carrier
synchronization but you can see the SIP
signal is raised to its mPower if it is
safe ms2 then it's called a squaring
device so it's raised to a power of M
and then which is passed to a bandpass
filter and here if you say this what is
this all about if you crawl you have a
close look this is a mixer you have a
low-pass filter and you have a VCO
voltage control oscillator this is
nothing but your phase locked loop right
so here whatever signal is being
received it's raised with M power and
here we actually lock it to a particular
carrier frequency and then if things we
have raised it to its empower we need to
get back the signal so what we do is we
give it to a frequency divider divide by
M here are you get the reference signal
this reference signal is used at the as
carrier to detect recover the original
information so this is a k-11 one of the
methods of carrier synchronization where
empower this method is called empower
loop where it signal is raised to its
empower and then it is filtered off and
is given to a PLL where it the phase is
locked and then it's possible frequency
divided where phase and the frequency is
decided of the carrier so reference
signal is used as a carrier signal so
this is one of the men this is the
second method
castas loop who sells loop we can see
that feather such as a signal is being
split into two parts and each separately
multiplied with a carrier
oh yeah the scheduled signal is taken
from this output of the VCO signal
multiplied with the carrier so this
carrier is one nine minus ninety that is
90 degree out of phase compared to this
carrier and the output is here there's a
phase to termination or face
discrimination the two signals
difference in those two signals is
identified and given correspondingly a
voltage is being supplied over here so
this is the second method that's
Acosta's loop for carrier
synchronization I and I am sure this
figure as well is familiar to you you
learn to turn some of the detection
methods already next we move on to the
tower same next one symbol
synchronization symbol synchronization
here the simple synchronization our
technique what happens is the clock has
been transmitted along with the data
bearing signal in multiplexed form and
then at the receiver the clock is
extracted by prompted filtering of the
modulated waveform now this it a
minimizes the time required for carrier
all clock recovery because in the
previous series the carrier was not
transmitted along with the signal
it was extracted out from there is a
receive signal so there is in symbol
synchronization what we do is we send
the carrier also along with the data
signal but here the negative or the
drawback was a fraction of the
transmitted power this all has to be
allocated for transmitting the clock we
cannot use it user entire powers for
transmitting the data when as a part of
set has to be allocated to transmit the
clock to explain simple symbol
synchronization let me consider a few
waveforms which will help you get a
clear picture on this concept
consider this waveform we have this bit
stream given as 1 0 0 1 1 1 0 & 1 and
this is the top part and let's consider
we have used bipolar encoding scheme so
here if you see whenever there is one
and the clock goes high you get the
output
1 and then 0 here it goes 0 so and
whenever the clock goes I it goes to 0
and it comes back sorry it goes to minus
1 and comes back to 0 similar way we can
quote the sass I hope it's clear
whenever the clock OSA in the value of 0
it goes to minus 1 and comes back it is
returned to 0 form so it's coming back
to 0 and here again the clock is there
and it's high so it repeats this way no
way it is negative and away its positive
so this is how you get the coded signal
now send this coded signal this is a
signal which is being transmitted right
so from this signal what we do is we
perform some filtering technique and we
extract the clock pulse that is let's
say we have extracted the clock clock
pulse to be the same so we have the
clock pulses loving extracted this way
yes so how do you this this is being
extracted or the flam the received
signal using some filtering techniques
and other using the same time period so
you can see whenever it goes high or low
you get a clock pulse right so whenever
it goes high or low you get a clock
pulse so this is being extracted two
different techniques of filtering and
all that now from this clock pulse how
do we recover they received coming sorry
the bitstream
which is transmitted so you can see over
here there's a clock pulse which is
fatter and it going low over here but
just from this actually we take this
value
we'll get it this way something similar
to those transmitted midstream but it is
little shifted because this is the time
axis actually and so you have a gap over
here this small gap yeah so it is we are
receiving the signal but still due to
some disturbances caused in the channel
we are getting a shifted little not too
much still we can recover or get back
this signal properly so this is the
method of single symbol synchronization
where you extract the clock is being
filtered out filled clock is also
transmitted along with the signal so
which is being filtered out and then run
the clock we have obtained the bitstream
so this method is called symbol
synchronization the next one that's
frame synchronization frame
synchronization and now as I told you in
modern digital communication systems we
already start on the data transfer is
done in the form of packets so for that
for transmitted in the form of packets
we also have gain we have to synchronize
them right so we use some frames or bits
at the start and end to actually detect
the start and end of the frame we call
them
we call those start and end bits as
flags yeah
so there are fertilized Flags do you
have a start flag and the f9 flag now
these start and end flags are assigned
to both the ends so that the start flag
indicates the start of a frame and n
flag indicates the end of the frame no
this is how the receiver actually
identifies the frame or actually detects
and the demodulates
or decodes the message being received so
we need to ensure that they are
synchronized properly otherwise we it
may then do the loss of data and in
efficient communication or transmission
may take place so this is all about
synchronization topic we have covered
three types in detail and and the need
for synchronization
have been discussed I hope the video is
clear to you thank you for watching and
we'll see in the next video soon
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