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#### ekant

##### Newbie level 2
1) Do negative frequency exist in real life ?
2) define ISI (inter symbol interference)?
3) define ICI (interchannel interference) ?

and many more questions are yet to come !!!

cheers !!
ekant

#### ekant

##### Newbie level 2

4) Define Interleaver ?
5) Define Crosstalk ?
6) what do you mean by Equalizer ?

This are some questions on which i generally get confused , so do please help me out !!

#### Communications_Engineer

try wikipedia if you don't get any response, which you might not, as they are very simple questions.

During my time as a member, I've observed that members are more interested to answer technical queries rather than answer basic questions whose answers are easily available on google.

Just a suggestion

#### Tamas888

##### Junior Member level 3
The channel ISI free if: H(z)=1 on every frequency.. Interference can be because of multipath because each symbol can be faster than the before symbol... Am I right?
crosstalk can be for example in case of UTP cable. it means you can hear in each cable what you sent on another cable.. therefore you have to “furl” the cables...
Equalizer need when you have multipath channel, it is important for example if your channel isn’t ISI free.. your algorithm must use learning sets and you can for example to minimize the error function which is the difference between the input and the got signal...
and so on....

#### Ahmed Alaa

##### Full Member level 5

1) No, negative frequencies are just a mathematical artifact. Because a complex exponential is a phasor that rotates in a certain direction with a certain velocity has real and imaginary projections, we eliminate the imaginary projections by adding another phasor rotating in the OPPOSITE direction with the same velocity. This opposite direction is the negative frequency.
Because no imaginary projections in real life, then no negative frequencies as well !!
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2) ISI is the presence of contributions of all other symbols in a certain symbol. That is because all channels have low pass nature and won't carry sharp pulses that start and end strictly instantaneously. A dispersive channel is either dispersive because of its nature (the material of a cable) which treats every frequency with a diff wave velocity, or because we are obligated to use LPF at the transmitter because the medium is shared (like the case of wireless communications).

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3) ICI is a problem that is specific for multi-carrier systems, which results from the loss of Orthogonality between the sub carriers.This happens when the FFT is considered over duration where the subcarrier is non integer number of cycles. We mitigate this using cyclic prefix.

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4) Interleaving is the scattering of contiguous elements in a data packet into different packet, so that if we lost a whole packet, the loss is in smaller elements that belong to different packets, when we apply de-interleaving, we recover packets that are partially corrupted but not totally lost.

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5) Cross talk is the undesired coupling between two neighboring systems, each systems creates an undesired interference on the system in its neighborhood (ex. two cables).

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6) An Equalizer is a filter that mitigates ISI, it simulates the inverse transfer function of the channel in order to retrieve a better version of the signal. Digital equalizer are usually tapped-line filters the taps' gain may be adapted through estimating the channel and recognize the varying channel's transfer function.

Hope this helps !

#### vegetablevn

##### Newbie level 4

Ahmed Alaa said:
1) No, negative frequencies are just a mathematical artifact. Because a complex exponential is a phasor that rotates in a certain direction with a certain velocity has real and imaginary projections, we eliminate the imaginary projections by adding another phasor rotating in the OPPOSITE direction with the same velocity. This opposite direction is the negative frequency.
Because no imaginary projections in real life, then no negative frequencies as well !!
---------------------------------------

2) ISI is the presence of contributions of all other symbols in a certain symbol. That is because all channels have low pass nature and won't carry sharp pulses that start and end strictly instantaneously. A dispersive channel is either dispersive because of its nature (the material of a cable) which treats every frequency with a diff wave velocity, or because we are obligated to use LPF at the transmitter because the medium is shared (like the case of wireless communications).

----------------------------------------

3) ICI is a problem that is specific for multi-carrier systems, which results from the loss of Orthogonality between the sub carriers.This happens when the FFT is considered over duration where the subcarrier is non integer number of cycles. We mitigate this using cyclic prefix.

-----------------------------------------

4) Interleaving is the scattering of contiguous elements in a data packet into different packet, so that if we lost a whole packet, the loss is in smaller elements that belong to different packets, when we apply de-interleaving, we recover packets that are partially corrupted but not totally lost.

-----------------------------------------

5) Cross talk is the undesired coupling between two neighboring systems, each systems creates an undesired interference on the system in its neighborhood (ex. two cables).

-----------------------------------------

6) An Equalizer is a filter that mitigates ISI, it simulates the inverse transfer function of the channel in order to retrieve a better version of the signal. Digital equalizer are usually tapped-line filters the taps' gain may be adapted through estimating the channel and recognize the varying channel's transfer function.

Hope this helps !

Hi

You can see same topic at the side bar of this site. You can find out some thing same your questions.

Rgs

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