Iv orthogonal frequency division multiplexing ofdm
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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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Iv orthogonal frequency division multiplexing ofdm

IV. Orthogonal Frequency Division Multiplexing (OFDM)


Mitigation of fading

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


Adaptive loading

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:


Adaptive loading1

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


Adaptive loading2

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


Adaptive loading3

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


Adaptive loading4

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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