EC302 Digital communications_module5_Part 3

Summary

This 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

00:00

📡 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.

05:03

🕰️ 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.

10:04

🔄 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.

15:06

📶 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.

20:08

📜 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.

Keywords

💡Synchronization

Synchronization in the context of digital communications refers to the process of aligning the receiver's clock with the transmitter's clock to accurately detect and decode the transmitted data. It is crucial for maintaining the integrity of the signal and ensuring that the receiver samples the signal at the correct moments. In the video, synchronization is discussed as a fundamental requirement for coherent detection, where the receiver must recreate the carrier signal to correctly decode the information.

💡Spread Spectrum

Spread Spectrum is a technique used in digital communications to spread the signal over a wider frequency range than the minimum required bandwidth. This technique helps in reducing interference and improving signal reliability. The video script mentions the need for spread spectrum techniques and the importance of PN (Pseudorandom Noise) sequences in achieving this spread.

💡Coherent Detection

Coherent Detection is a method of signal detection where the receiver has knowledge of the carrier signal's phase and frequency, allowing for a more accurate and noise-resistant signal recovery. The video explains that coherent detection is possible when synchronization is achieved, as it enables the receiver to recreate the carrier signal for accurate decoding.

💡Noncoherent Detection

Noncoherent Detection is a detection method used when the receiver does not have precise knowledge of the carrier signal's phase and frequency. This method is less susceptible to noise and phase variations but is generally less efficient than coherent detection. The video contrasts coherent and noncoherent detection, highlighting the need for synchronization in coherent systems.

💡Carrier Synchronization

Carrier Synchronization is the process of estimating and aligning the carrier frequency and phase at the receiver to match those of the transmitter. This is essential for coherent detection. The video discusses two methods for carrier synchronization: the squaring loop and the Costas loop, both aiming to recreate the carrier signal for accurate signal decoding.

💡Symbol Synchronization

Symbol Synchronization involves aligning the receiver's sampling instants with the symbol transitions in the transmitted signal. This ensures that the receiver samples the signal at the correct points to recover the data. The video explains that symbol synchronization can be achieved by transmitting a clock signal along with the data, which is then extracted at the receiver.

💡Frame Synchronization

Frame Synchronization is the process of identifying the start and end of data frames in packet-based communication systems. This is achieved by using unique flags or markers that indicate the boundaries of the frames. The video script mentions that frame synchronization is necessary to prevent data loss and ensure efficient communication.

💡PN Sequences

PN (Pseudorandom Noise) Sequences are sequences of bits that appear random but are actually generated deterministically. They are used in spread spectrum communication to spread the signal over a wide frequency range. The video script touches upon the importance of PN sequences in spread spectrum, which is a technique discussed in the previous lecture.

💡Modulation

Modulation is the process of varying a carrier signal to represent the information-bearing signal. In digital communications, this involves converting digital data into a form suitable for transmission over a physical medium. The video script refers to modulation as a step in the transmission process where the data is prepared for sending over the channel.

💡Asynchronous Communication

Asynchronous Communication is a mode of data transmission where data elements are sent independently, without a continuous clock signal to synchronize the sender and receiver. The video script contrasts asynchronous with synchronous communication, explaining that in asynchronous systems, each symbol is independent and does not rely on the timing of previous symbols.

💡Synchronous Transmission

Synchronous Transmission is a mode of data transmission where the transmitter and receiver clocks are closely synchronized, allowing for the transmission of data blocks as a single entity. The video script discusses synchronous transmission as a method that requires precise clock synchronization to ensure that the data blocks are correctly received and decoded.

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