OFDM transmitter side, advantages and disadvantages

OFDM transmitting side

OFDM sends a high-rate data stream as many low-rate streams on closely spaced orthogonal subcarriers (spacing Δf = 1/T_s). The transmitter does this with an IFFT.

bits->[FEC +     ]->[QAM/PSK]->[S/P]->[  IFFT  ]
      [interleave]  [mapper ]         [N-point ]
                                          |
RF <-[Up-conv]<-[DAC]<-[P/S]<-[Add CP]<---+
     [+ PA   ]

1. Channel coding and interleaving: adds redundancy (e.g. convolutional code) and spreads bits across subcarriers so one deep fade does not wipe out consecutive bits.
2. Mapping: bits are grouped and mapped to complex symbols (BPSK, QPSK, 16-QAM, 64-QAM).
3. Serial-to-parallel: N symbols are placed on N subcarriers; pilot tones and null (guard) carriers are added.
4. IFFT: computes x(n) = (1/N) Σ_{k=0}^{N−1} X_k e^(j2πkn/N), i.e. modulates every subcarrier at once and produces one time-domain OFDM symbol.
5. Parallel-to-serial and cyclic prefix: the last N_cp samples are copied to the front. If the CP is longer than the maximum delay spread, ISI is absorbed and orthogonality is kept.
6. DAC, filter, up-converter and power amplifier: send the signal at RF.

Advantages

• Strong resistance to multipath fading and ISI (long symbols + CP).
• Simple one-tap equalization per subcarrier instead of a long time-domain equalizer.
• High spectral efficiency because subcarrier spectra overlap.
• Efficient implementation with FFT/IFFT.
• Flexible: bit loading and different modulation per subcarrier; easy to extend to OFDMA and MIMO.
• Robust against narrowband interference (only a few subcarriers affected).

Disadvantages

• High peak-to-average power ratio (PAPR), needing linear, less efficient power amplifiers.
• Very sensitive to carrier frequency offset and Doppler, which destroy orthogonality (inter-carrier interference).
• Cyclic prefix and pilots waste some bandwidth and power.
• Needs accurate time and frequency synchronization.