<< Chapter < Page Chapter >> Page >

The revolution in data communications technology can be dated from the invention of automatic and adaptive channelequalization in the late 1960s.

—Gitlin, Hayes, and Weinstein, Data Communication Principles , 1992

When all is well in the receiver, there is no interaction between successive symbols; each symbol arrivesand is decoded independently of all others. But when symbols interact, when the waveformof one symbol corrupts the value of a nearby symbol, then the received signal becomes distorted. It isdifficult to decipher the message from such a received signal. This impairment is called “intersymbol interference” and was discussed inChapter  [link] in terms of non-Nyquist pulse shapes overlapping in time. This chapter considersanother source of interference between symbols that is caused by multipath reflections(or frequency-selective dispersion) in the channel.

When there is no intersymbol interference (from a multipath channel, from imperfectpulse shaping, or from imperfect timing), the impulse response of thesystem from the source to the recovered message has a single nonzero term. The amplitude ofthis single “spike” depends on the transmission losses, and the delayis determined by the transmission time. When there is intersymbol interference caused by a multipath channel,this single spike is “scattered,” duplicated once for each path in the channel.The number of nonzero terms in the impulse response increases. The channel can be modeled as afinite-impulse-response, linear filter C , and the delay spread is the total time interval during which reflections with significant energy arrive.The idea of the equalizer is to build(another) filter in the receiver that counteracts the effect of the channel. In essence, the equalizermust “unscatter” the impulse response. This can be stated as the goal of designing the equalizer E so that the impulse response of the combined channel and equalizer C E has a single spike. This can be cast as an optimization problem,and can be solved using techniques familiar from Chapters [link] , [link] , and [link] .

The transmission path may also be corrupted by additive interferences such as those caused byother users. These noise components are usually presumed to beuncorrelated with the source sequence and they may be broadband or narrowband, in band or out of bandrelative to the bandlimited spectrum of the source signal. Like the multipath channel interference,they cannot be known to the system designer in advance. The second job of the equalizer is to reject such additivenarrowband interferers by designing appropriate linear notch filters “on-the-fly” based on the received signal.At the same time, it is important that the equalizer not unduly enhance the broadband noise.

The baseband linear (digital) equalizer is intended to (automatically) cancel unwanted effects of the channel and to cancel certain kinds of additive interferences.
The baseband linear (digital) equalizer is intended to (automatically) cancel unwanted effects of the channel and tocancel certain kinds of additive interferences.

The signal path of a baseband digital communication system is shown in [link] , which emphasizes the role of the equalizer in trying to counteract theeffects of the multipath channel and the additive interference. As in previous chapters, all of the inner parts ofthe system are assumed to operate precisely: thus, the upconversion and downconversion,the timing recovery, and the carrier synchronization (all those parts of the receiver that are not shown in [link] ) are assumed to be flawless and unchanging.Modelling the channel as a time-invariant FIR filter, the next section focuses on the taskof selecting the coefficients in the block labelled “linear digital equalizer,” with the goalof removing the intersymbol interference and attenuating the additive interferences.These coefficients are to be chosen based on the sampled received signal sequenceand (possibly) knowledge of a prearranged “training sequence.” While the channel may actually be time varying, thevariations are often much slower than the data rate, and the channel can be viewed as (effectively) time invariant oversmall time scales.

Questions & Answers

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
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
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.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Software receiver design. OpenStax CNX. Aug 13, 2013 Download for free at http://cnx.org/content/col11510/1.3
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Software receiver design' conversation and receive update notifications?