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IV. Orthogonal Frequency Division Multiplexing (OFDM)

IV. Orthogonal Frequency Division Multiplexing (OFDM). Mitigation of Fading. Frequency Equalization: Divide by received signal Y[i] by H[i] for all sub-carriers Requires channel estimation For low values of H[i] equalization results in noise amplification Precoding

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IV. Orthogonal Frequency Division Multiplexing (OFDM)

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  1. IV. Orthogonal Frequency Division Multiplexing (OFDM)

  2. Mitigation of Fading • Frequency Equalization: • Divide by received signal Y[i] by H[i] for all sub-carriers • Requires channel estimation • For low values of H[i] equalization results in noise amplification • Precoding • Divide transmitted signal X[i] by H[i] for all sub-carriers • Requires channel estimation knowledge at transmitter • Does not result in any noise amplification at the receiver • For low values of H[i] excessively high transmission power might be needed at the transmitter

  3. Adaptive Loading Problem Definition: N Sub-carriers, Maximum Transmission Power P How to divide the available power P over sub-carriers so as to maximize the total rate Problem Formulation:

  4. 16 8 4 2 1 1/2 1/4 1/8 1/16 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Adaptive Loading Solution: In this region a increase in power corresponds to minimal increase in rate Intuitively in order to maximum the total rate the following partial differentiation equation should be satisfied where C is some constant Ri SNRi In this region a small increase in power corresponds to a significant increase in rate

  5. 16 8 4 2 1 1/2 1/4 1/8 1/16 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Adaptive Loading Solution: In this region a increase in power corresponds to minimal increase in rate Intuitively in order to maximum the total rate the following partial differentiation equation should be satisfied Ri SNRi In this region a small increase in power corresponds to a significant increase in rate

  6. 16 8 4 2 1 1/2 1/4 1/8 1/16 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Adaptive Loading Solution: In this region a increase in power corresponds to minimal increase in rate Intuitively in order to maximum the total rate the following partial differentiation equation should be satisfied Ri Constant SNRi In this region a small increase in power corresponds to a significant increase in rate Bandwidth for all sub-carriers is usually equal

  7. 16 8 4 2 1 1/2 1/4 1/8 1/16 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Adaptive Loading Solution: In this region a increase in power corresponds to minimal increase in rate Water-Filling Algorithm Ri K SNRi In this region a small increase in power corresponds to a significant increase in rate 0 1 N-1

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