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OFDM (Orthogonal Frequency Division Multiplexing)

OFDM (Orthogonal Frequency Division Multiplexing). What is OFDM. Assume we need to send data at a speed of R symbols/sec. We break the data sequence into N (an integer, say, 48) sub-sequences. The data rate of each sub-sequence will be R/N, much slower than the original sequence.

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OFDM (Orthogonal Frequency Division Multiplexing)

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  1. OFDM (Orthogonal Frequency Division Multiplexing)

  2. What is OFDM • Assume we need to send data at a speed of R symbols/sec. • We break the data sequence into N (an integer, say, 48) sub-sequences. The data rate of each sub-sequence will be R/N, much slower than the original sequence. • N carriers are used, each having a different frequency and each sending one sub-sequence. • At the receiver end, the N sub-sequences are put together to get the original data sequence.

  3. Advantages of OFDM • Robust against multipath interference because: • In each sub-sequence the symbols are N times longer than the original symbols. • The longer the symbol, the weaker the multipath interference (the signals representing the same symbol but coming from multiple paths will be close enough compared with the width of the symbol so they don’t interfere with each other).

  4. Frequency selective fading • To battle the frequency selective fading (signals at certain frequencies might be much weaker than that of other frequencies), error-control coding can be used in each subchannel. • If the signal for a particular subchannel(s) is weak, the transmitting power of that subchannel can be increased to compensate for the fading.

  5. Drawbacks of OFDM • Complexity: you need to put together N signals to rebuild the original one.

  6. Example: OFDM in 802.11a (p.109) • 64 subchannels are used, among which 48 are used for data transmission, the remaining 16 are for other purposes. • The symbol rate of each channel is 250 kilo symbols per second (ksps). • The actual data rate for the user is 48X250 ksps = 12 Msps. • The overall bandwidth is 20 MHz.

  7. SOFDMA (Scalable Orthogonal Frequency Division Multiple Access) • Used in 802.16e (WiMax for mobile users) • The same OFDM technique will be used, but each user may only get a part of the spectrum, depending on the application the user is running. TDMA is also used. • In 802.16-2004 (WiMax for fixed users) OFDM is used and a user get all the available spectrum. Users are separated by TDMA.

  8. Assigning sub-channels

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