# Ofdma for Lte

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LTE FDD vs. TDD

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LTE-FDD FRAME

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LTE-TDD FRAME

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Comparison between FDD-LTE and TDD-LTE

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Comparison between FDD-LTE and TDD-LTE

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OFDMA

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What is Orthogonal signal

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FDM

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OFDM

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Working of OFDM (Example)

Suppose we are having following bits for transmission1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1,1,-1,-1,-1,1,1

Convert this high speed serial stream to low speed parallel stream.

C1 C2 C3 C4

1 1 -1 -1

1 1 1 -1

1 -1 -1 -1

-1 1 -1 -1

-1 1 1 -1

-1 -1 1 1

These low speed streams will be transmitted on different sub-carriers

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Modulated signal for C1 Modulated signal for C2

Modulated signal for C3 Modulated signal for C4

Each signal will be modulated using BPSK.

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Final OFDMA wave, sum of all signals.

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OFDM Block Diagram

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OFDMA

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OFDMA

OFDM can be easily used as multiple access technology, namedOFDMA.

Total spectrum space is divided into sub-channels(SC).

One SC is a sub-set of available channels. It contains more thanone sub-carrier.

Users may occupy more than one SC depending upon user QOS

profile.

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Subset is also called as OFDMA traffic

channel.

user1

user2

user3

user4

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Sub-channel implementation techniques

In first technique, sub channels are grouped in same frequency range.

In second technique, sub channels are spread over whole spectrum

Second technique is more immune to narrow band fading as it willEffect fraction of sub channels.

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OFDMA Illustration

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Frequency Re-use

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OFDM and OFDMA illustration

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Cyclic Prefix

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In OFDM, multipath causes loss of orthogonality .Delayed paths

cause overlap between symbols

Cyclic Prefix (CP) insertion helps maintain orthogonality

Reduces efficiency (or Usable Symbol time, Tu)CP is repetition of modulation symbol

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Cyclic Prefix

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How OFDMA saves bandwidth

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Synchronization Tight Synchronization between users are required forFFT in receiver

Pilot signals are used for synchronizations Co-channel interference

Dealing with this is more complex in OFDM than inCDMA

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Peak to average power ratio (PAPR)

The large amplitude variation increases in-band noise and increasesthe BER when the signal has to go through amplifier nonlinearities.

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Why SC-FDMA?

Low Peak to average ratio.

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SC-FDMA Signal generation

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SC-FDMA Generation

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10/4/2013 ALUMS TEC 31

THANK YOU

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