Frequency Domain Equalisation & Multicarrier Modulation

Frequency Domain Equalisation & Multicarrier Modulation

1. COFDM

   • Modulates onto 64 sub-carriers
     • Makes it into 64 slower comms systems
     • Multicarrier Modulation
       • take a wideband channel and divide it up into multiple narrowband channels
   • Orthogonality
     • Individual sub-bands can be separated without the need for individual sub-band filters
     • Each subcarrier must maintain an integer number of cycles in the receive FFT window
     • Modulation scheme can be the same or varied between sub-carriers

2. OFDM

   • Modulates data symbols onto N frequency parallel sub-carriers
   • Orthogonal by choosing their frequency spacing to be equal to the reciprocal of the OFDM symbo period
   • Increased OFDM symbol period makes the system more robust to delay spread and noise effects
   • Each sub-channel is designed to use b/w that is less than the correlation b/w i.e sub-channels are narrowband
   • Ideal sub-band transmissions suffer from uncorrelated fast fading
     • Enables Error Bursts to randomized
       • Leading to effective FEC coding
   • TRANMISSION
     • Pre-IFFT
       • Data is supplied in the form of an i/p PDU train
       • Data is scrambled
       • FEC is normally applied
       • Coded data is interleaved
         • Reduces impact of error bursts
       • Interleaved data is mapped to produce modulated freq. domain OFDM vector
     • OFDM Transmitter
       • Based on the sum of N modulated sub-carriers
       • Required OFDM time-domain waveform is obtained by transforming the mod. freq. vector using an N-point IFFT

3. Guard Interval

   • Other names
     • Cylic Extension
     • Cyclic Prefix
   • Protects the OFDM time-domain signal from orthogonality loss
     • Due to channel & bandlimiting filter delay spread
     • Maintain orthogonality
       • Maintain an INTEGER number of cycles in the FFT window
   • Length of GI
     • A fixed fraction of the OFDM symbol period
     • Must be sufficient to cover all of the significant delay taps
     • In practice, often set at around twice the delay spread
       • Provide an additional margin for symbol timing error in the receiver
     • OFDM symbol period has been extended with a cyclic prefix
       • Copy part of the end of the unextended symbol period
         • Append to the beginning of the symbol
       • Cycle now includes INTEGER cycles per subcarrier within the FFT window
         • Ensures that cyclic ext is longer than the worst case delay spread
     • Math G.I
       • IFFT & FFT processes represent a unitary transform pair
       • Resulting domain matrix is diagonalised
   • ICI
     • To View
       • Only transmit energy on the 1st carrier
     • No ICI in figures 1 - 3
       • Energy level on all unmodulated carriers is zero
     • Figure 4 delay spread exceeds GI
       • Shows Energy Leakage
         • Result in interference to and from the other subcarriers
           • Causes merging of symbols from different subcarriers & prevents reliable detection

4. OFDM Reception

   • Receiver signal passes through a Low Noise Amplifier (LNA)
     • Each entry in the received freq domain has been distorted by the freq selective channel
       • Apply FDE
         • Zero-forcing
           • Used to compute each FDE weight as the inverse of the corresponding channel est.
           • Requires CSI (channel state information)
             • Division of received by known transmitted symbol

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