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LTE FDD vs. TDD
-Yatin Jadhav
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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
Dynamic channel allocation with advancedcoordination among adjacent base stations
Disadvantages
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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.
Carries all advantages of OFDMA such as low ISI, multipath fading.
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SC-FDMA Signal generation
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SC-FDMA Generation
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THANK YOU