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Interim Specification 95 (IS-95) describes a Direct-Sequence Spread-Spectrum (DS/SS) cellular system that uses a Rake receiver to provide path diversity for mitigating the effects of frequency-selective fading. The Rake receiver searches through the different multipath delays for code correlation and thus recovers delayed signals that are then optimally combined with the output of other independent correlators.

Figure 1 show the power profiles associated with the five chip transmissions of the code sequence 1 0 1 1 1. Each abscissa shows three components arriving with delays τ 1 size 12{τ rSub { size 8{1} } } {} , τ 2 size 12{τ rSub { size 8{2} } } {} , and τ 3 size 12{τ rSub { size 8{3} } } {} . Assume that the intervals between the transmission times t i size 12{t rSub { size 8{i} } } {} and the intervals between the delay times τ i size 12{τ rSub { size 8{i} } } {} are each one chip in duration. The component arriving at the receiver at time t 4 size 12{t rSub { size 8{ - 4} } } {} , with delay τ 3 size 12{τ rSub { size 8{3} } } {} , is time-coincident with two others, namely the components arriving at times t 3 size 12{t rSub { size 8{ - 3} } } {} and t 2 size 12{t rSub { size 8{ - 2} } } {} with delays τ 2 size 12{τ rSub { size 8{2} } } {} and τ 1 size 12{τ rSub { size 8{1} } } {} respectively. Since in this example the delayed components are separated by at least one chip time, they can be resolved.

At the receiver, there must be a sounding device dedicated to estimating the τ i size 12{τ rSub { size 8{i} } } {} delay times. Note that the fading rate in mobile radio system is relatively slow (in the order of milliseconds) or the channel coherence time large compared to the chip time duration ( T 0 > T ch size 12{T rSub { size 8{0} }>T rSub { size 8{ ital "ch"} } } {} ). Hence, the changes in τ i size 12{τ rSub { size 8{i} } } {} occur slowly enough that the receiver can readily adapt to them.

Once the τ i size 12{τ rSub { size 8{i} } } {} delays are estimated, a separate correlator is dedicated to recovering each resolvable multipath component. In this example, there would be three such dedicated correlators, each one processing a delayed version of the same chip sequence 1 0 1 1 1. Each correlator receives chips with power profiles represented by the sequence of components shown along a diagonal line. For simplicity, the chips are all shown as positive signaling elements. In reality, these chips form a pseudonoise (PN) sequence, which of course contains both positive and negative pulses. Each correlator attempts to correlate these arriving chips with the same appropriately synchronized PN code. At the end of a symbol interval (typically there may be hundreds or thousands of chips per symbol), the outputs of the correlators are coherently combined, and a symbol detection is made.

The interference-suppression capability of DS/SS systems stems from the fact that a code sequence arriving at the receiver time-shifted by merely one chip will have very low correlation to the particular PN code with which the sequence is correlated. Therefore, any code chips that are delayed by one or more chip times will be suppressed by the correlator. The delayed chips only contribute to raising the interference level (correlation sidelobes).

The mitigation provided by the Rake receiver can be termed path diversity, since it allows the energy of a chip that arrives via multiple paths to be combined coherently. Without the Rake receiver, this energy would be transparent and therefore lost to the DS/SS receiver.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
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
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
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.
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Source:  OpenStax, Principles of digital communications. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10805/1.1
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