OFDM Orthogonal Frequency Division Multiplexing Channel multipath Amplitude

OFDM (Orthogonal Frequency Division Multiplexing)

Channel multipath Amplitude Channel Impulse Response Tx Rx Channel Frequency Response time

Received data with channel multipath

Deep channel fading A few frequencies are completely faded Whole message can be destroyed even if few frequencies of the message are lost

Revisit the transmitted spectrum -F F

Increase the symbol duration Decreases the bandwidth of the signal -F F

Modified signal passed through the channel Data is unaffected since the fading frequencies do not overlap with data frequencies

Coping with fading • Decrease bandwidth so that data frequencies do not overlap with fading frequencies • This helped eliminate the effect of fading • Disadvantage: This would waste a lot of available bandwidth • Can we do better to achieve throughput proportional to the channel quality, without wasting any bandwidth

Coping with fading The message is chunked into groups of sub-messages -F F

Coping with fading -F F x x

Coping with fading -F x + x x x + + F

-F x x F + + + Sub-messages are loaded on different frequencies

-F F

OFDM (Orthogonal frequency division multiplexing) transmission I = Real part x + IFFT Q = Imag part x

OFDM performance under deep fading

OFDM reception x x OFDM receives messages proportional to the number of good frequencies (not faded), instead of losing out the whole message

OFDM vs • Robust to deep fading • Very efficient, achieves capacity limits, used widely in LTE/Wi. Fi • Robust to synchronization errors • Requires FFT/IFFT power intensive • High variation in signal amplitude – needs better h/w Conventional • Complete loss of performance under deep fading • Cannot reach maximum capacity • High synchronization overhead • Suitable for low power/batteryless communication • Low variation in signal amplitude