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The goal of symbol-timing recovery is to sample message signals at the receiver for best performance. After the in-phase and quadrature signals pass through a matched filter, a delay-locked loop attempts to find the peaks in the output waveforms.


This receiver exercise introduces the primary components of a QPSK receiver with specific focus on symbol-timing recovery.In a receiver, the received signal is first coherently demodulated and low-pass filtered (see Digital Receivers: Carrier Recovery for QPSK ) to recover the message signals (in-phase and quadrature channels). The next step for the receiver is tosample the message signals at the symbol rate and decide which symbols were sent. Although the symbol rate is typicallyknown to the receiver, the receiver does not know when to sample the signal for the best noise performance. Theobjective of the symbol-timing recovery loop is to find the best time to sample the received signal.

illustrates the digital receiver system. The transmitted signal coherently demodulated with both a sineand cosine, then low-pass filtered to remove the double-frequency terms, yielding the recovered in-phase andquadrature signals, s I n and s Q n . These operations are explained in Digital Receivers: Carrier Recovery for QPSK . The remaining operations are explained in this module. Both branches are fed through a matched filter and re-sampled at the symbol rate. The matched filter is simply an FIR filter with an impulse responsematched to the transmitted pulse. It aids in timing recovery and helps suppress the effects of noise.

Digital receiver system

If we consider the square wave shown in as a potential recovered in-phase (or quadrature) signal ( i.e. , we sent the data

    +1 -1 +1 -1
) then sampling at any point other than the symbol transitions will result in the correct data.

Clean BPSK waveform.
Noisy BPSK waveform.

However, in the presence of noise, the received waveform may look like that shown in . In this case, sampling at any point other than the symboltransitions does not guarantee a correct data decision. By averaging over the symbol duration we can obtain a betterestimate of the true data bit being sent ( +1 or -1 ). The best averaging filter is the matched filter, which has the impulse response u n u n T symb , where u n is the unit step function, for the simple rectangular pulse shape used in Digital Transmitter: Introduction to Quadrature Phase-ShiftKeying .

For digital communications schemes involving different pulse shapes, theform of the matched filter will be different. Refer to the listed references for more information on symbol timing andmatched filters for different symbol waveforms.
and show the result of passing both the clean and noisy signal through the matchedfilter.

Averaging filter output for clean input.
Averaging filter output for noisy input.

Note that in both cases the output of the matched filter has peaks where the matched filter exactly lines up with thesymbol, and a positive peak indicates a +1 was sent; likewise, a negative peak indicates a -1 was sent. Although there is still some noise in second figure, the peaks are relatively easy to distinguishand yield considerably more accurate estimation of the data ( +1 or -1 ) than we could get by sampling the original noisy signal in .

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Ece 320 - spring 2003. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10096/1.2
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