ofdm-05WirelessMAN-OFDM PHY - MdKhairulAnam.ppt

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IEEE 802.16 2004 WirelessMAN-OFDM PHY

Md. Khairul Anam – 040805017P

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WirelessMAN-OFDM PHYBased on OFDM modulationNLOS operationFreq band below 11 GHz

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Topics covered

Channel Coding Transmission convergence (TC) sub layer Frame Structure

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Channel Coding

WHY?

Sender Receiver

TEA

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Steps in Channel Coding

1. Randomization2. Forward error correction coding3. Interleaving

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Randomization

Distributes 0 and 1 bits into each burst of Data

Performed on both downlink and uplinkPadding is added at end of Transmission

block

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Randomization cont.

PRBS(Pseudo Random Binary Sequence)

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Randomization cont.

Each data byte to be transmitted shall enter sequentially into the randomizer, MSB first

Preambles are not randomizedDownlink: reinitialized at start of each

frame with SEQ: 100101010000000Shall not be reset at the start of burst #1,

shall be initialized for subsequent bursts

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Randomization cont.

Randomizer initialization vector

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Forward error correction (FEC) CodeWhy FEC?

Retransmissions is costly or impossible Back Channel may not available

What is FEC? It is a error control system

How it works? Sender adds redundant data (error correction

code) to its messages This allows the receiver to detect and correct

errors

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FEC example

Triplet received Interpreted as 000 0001 0010 0011 0100 1101 1110 1111 1

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Concatenated Reed Solomon - Convolutional Code

RS (N=255,K=239,T=8)Code Generator Polynomial

Field Generator Polynomial

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Convolutional Code

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Concatenated Reed Solomon - Convolutional Code

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Block Turbo Code (optional)

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Block Turbo Code (optional) cont.

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Interleaving

Interleaving is a way to arrange data in a non-contiguous way in order to increase performance

It protects from Burst Error

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Consider transmission without interleaving:

Original transmitted sentence: ThisIsAnExampleOfInterleavingReceived sentence with a burst error: ThisIs______pleOfInterleaving We find that the term "AnExample" is lost or unintelligible.

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Consider transmission with interleaving: Transmitted sentence:

ThisIsAnExampleOfInterleaving... Error-free transmission:

TIEpfeaghsxlIrv.iAaenli.snmOten. Received sentence with a burst error:

TIEpfe______Irv.iAaenli.snmOten. Received sentence after deinterleaving:

T_isI_AnE_amp_eOfInterle_vin_...

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Interleaver is defined by two step permutation

1. Adjacent coded bits are mapped onto non adjacent subcarriers

1. Adjacent coded bits are mapped alternately onto less or more significant bits of the constellation

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Modulation

After interleaving, Data bits are entered serially to Constellation mapper

Per-allocation adaptive modulation and coding shall be supported in downlink

Uplink shall support different modulation scheme for each SS

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Pilot Modulation Pilot subcarriers shall be inserted into each

data burst Value of K is derived from Wk

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Preamble structure and Modulation

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Transmission Convergence Sublayer

Enable on per Burst basis for both uplink and downlink

At SS initialization, TC sublayer capablity is negotiated between the BS and SS

Optional mechanism for the OFDM PHY

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Frame Structure

Point to Multi Point (PMP)Mesh

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PMP

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Thank You