OFDM or Multicarriers modulation with pulse shaping in Matlab

Discussion in 'Wireless & RF Design' started by _Speedbird, Dec 12, 2011.

  1. _Speedbird

    Thread Starter New Member

    Dec 12, 2011
    Hi ,

    I've coded in Matlab an OFDMA downlink transmission scheme (OFDM modulation for sure) using QAM for each user, after that i wanted to implement a clipping function to reduce PAPR (PEAK to Average Power Ratio) until here everything gone fine, i fellowed the clipping function by implementing a pulse shaping filtring to reduce ISI (Inter symbol Interference) & to reduce too clipping's effect wich affected the BER (Bit Error rate) at the receiver, i choose a Nyquist filtering, an RRC filter (Raised Cosin Filter), when it comes to apply this filter to my OFDMA system by using the matlab function rcosflt() i got some weird results (BER very high > 50%).

    I have doubts how to apply the filter, i explain, i applied the filter at rate of OFDM symbols not at the rate of the QAM symbols inside each OFDM symbol, i don't know if i'm doing it wrong?

    I tried a rectangular pulse shaping (rectpulse()) it works on OFDM symbols with good results however this method is hard to implement in practice (infinite bandwidth) so not recommended as in practice a signal exist only for a duration & has a limited bandwidth.

    I need help from specialist.My questions:

    - How do we apply Nyquist filtering in OFDM signal case & in Matlab?

    - How the filter parameters are chosen in OFDM case?

    - the RRC should be applied to the OFDM symbol or to QAM symbols themself in the OFDM symbol?

    If you want i will post you my Matlab program filtering part.

    Thanks in advance.