Pulse shaping

Discussion in 'Wireless & RF Design' started by jmsa7, Mar 17, 2010.

  1. jmsa7

    Thread Starter New Member

    Mar 17, 2010
    Hello everybody,

    I would like to understand why is pulse shaping used in wireless communications. I dont understand the point. Wikipedia says:

    "Typically pulse shaping occurs after line coding and before modulation."

    Then it says:

    "Transmitting a signal at high modulation rate through a band-limited channel can create intersymbol interference"

    So if pulse shaping is done before modulation, why do they say "high modulation rate".

    And I believe that the rectangular (I presume bits of data) pulses are not transmitted over the wireless channel. What is transmitted is a carrier that is modulated (QPSK) by the data bits. So what the point of pulse shaping? The resulting modulated signal will be the same either the data bits are squared, or shaped... isnt it?

    Please can someone give me some light about this topic?


  2. skeptic

    Active Member

    Mar 7, 2010
    A pure sine wave contains only the fundamental of its frequency. As a pulse deviates from a sine wave to a squarewave or rectangular pulse, it contains more and more harmonics. If you modulate a carrier with with a square 5 kHz pulse train, you will have many harmonics of that 5 kHz signal causing interference to adjacent channels. To prevent that, the pulse is shaped so it more closely resembles a sinewave, eliminating the harmonics and hopefully adjacent channel interference.

    If the receive bandwidth is narrower than the data rate it will try to convert the pulse train into a sinewave. Then consecutive symbols will blend into each other until it is difficult to distinguish between them.
    Last edited: Mar 22, 2010