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ISI in time domain is similar to aliasin

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  • #55606 Reply

    Hi Folks,
    I was reading the Intersymbol interference(ISI) definition and I’m wondering if ISI (defined in time domain) is similar to what happens if we undersample in frequency domain which is called Aliasing distortion, or folding distortion(Thanks in advance

    #55607 Reply


    I will try to explain intersymbol interference

    After analog signal is converted into pulses, these pulses needed to be shaped (pulse shaping). In theory pulses are rectengular, this shaping is done so make pulses similar to theory. Then we have now symbols. If one pulse effects(because of multipath transmission. etc) the consecutive one this will be ISI and to fix this equalizer is used.

    i will be appreciated someone can add something or correct me if i am wrong.
    And if someone explain working principal of equalizer

    #55608 Reply

    I was going to ask how did the equalizer work … until I read the end of your msg :))

    Thanks a lot for your explanation of ISI.


    #55609 Reply

    RF is effected in Environment by a number of ways.
    1.Phase Shifting.
    2. Frequency shifting and
    3. Timing shifting
    Equalizer is used to cater for these shifts to ensure correct decoding of symbols. so we decode the signal right by sampling it at the right moment.
    Equilization is usually of two broad types blind and assisted / aided.
    In GSM as far as I know its the asisted equilization in which the transmitter transmits a known sequence of bits in a fixed part of the burst. The receiver already knows the bit sequence and its location and tunes its equilizer waits to recover the desired signal. then assuming that the environmental effect on these bits would be similar to that on the rest of the signal it applies the same weights to the other signal.
    PIX could confirm that in Alcatel it is called TSC training sequence code and is taken as BCC (if I remember correctly).
    It is transmitted in the middle of a normal burst between the flag bits.

    #55610 Reply

    As far as the question of Aliasing it is slightly different from ISI.
    ISI is when two consequetive symbols overlap and the receiver might decode the signal wrongly.
    An d Aliasing is when the signal is sampled usually below the Nyquest criterion and this results in incorrect decoding of the signal. Like the blades of Halicopter seem to rotate slowly in the direction opposite to their actuall. this is because our eye’s sample at a lower rate that required to perceive the movement of the halicopter blades

    #55611 Reply

    Thanks Mania

    for ur detailed explanation,

    and in nokia as well TSC number is taken from bcc of bsic part.

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