IEEE802'16d Simulator WirelessMANOFDMPHY layer

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IEEE802.16dSimulatorWirelessMAN-OFDM-PHY layer
Rev-s3 24 Sept 2004

• Mohamad Charafeddine

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Introduction

• The Matlabs Simulink simulation work is for the
  broadband wireless standard IEEE802.16d and
  subsequently for the IEEE802.16-2004 once it is
  published
• Currently the mandatory channel coding scheme is
  used (Reed-Solomon/ convolutional code), 16-QAM
  modulation, RS(64,48,8), CC rate2/3
• A state machine can be implemented on top of the
  current model to take into account adaptive rate
  modulation.

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IEEE802.16d, WirelessMAN-OFDM 256-FFT, 16-QAM
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Randomization

• PRBS generator 1 x14 x15
• On DL the scrambler is re-initialized at start
  of each frame with the vector 100101010000000

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Concatenated Reed-Solomon / convolutional code
(RS-CC)
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Reed-Solomon coding

• Tail byte appended at end of the scrambled data
• For the 16-QAM mode selected, RS(64,48,8) is
  used, with
• A primitive GF polynomial of p(x) x8 x4
  x3 x2 1
• And a code generator polynomial of g(x)(x?0)
  (x?1) (x?2).. (x?2T-1), ?02Hex

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Convolutional Coding Puncturing

• Convolutional coding with a native rate of ½
• Constraint length of 7
• Generated with the following 2 generator
  polynomials
• G1171oct1111001 For X
• G2133oct1011011 For Y
• Puncturing of 4/3 X1Y1Y2 thus overall CC rate
  of ½4/3 2/3

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2-steps Interleaver (I)

• Define
• Ncpc the number of coded bits per carrier (i.e.
  2,4,6 for QPSK, 16QAM, 64QAM respectively)
• s Ncpc/2
• k the index of the coded bit before the first
  implementation
• m the index after first permutation
• j the index after the second permutation
• --------------------------------------------------
  --------------------------
• 1st permutation for carriers
• m (Ncbps/16)mod(k,16)floor(k/16)
• 2nd permutation for bits constellation mapping
  on carriers
• jsfloor(m/s)mod((mNcbps-floor(16m/Ncbps)),s)

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2-steps Interleaver (II)
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16-QAM modulation

• 16-QAM modulation
• Gray mapped
• Normalized constellation average power

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OFDM Transmitter DataPilots, Zero Padding,
Shaping, Cyclic Prefix (I)

• 192 Data carriers
• 8 Pilot carries
• 0 DC carrier
• 55 zero carriers added

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OFDM Transmitter Pilots (II)

• Pilots BPSK modulated
• Uses a PRBS generator of x11 x9 1
• Initialized in DL with the vector 1 1 1 1 1 1
  1 1 1 1 1
• Why cant we always transmit all 1s on the
  pilots without the need of the PRBS
  generator?

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OFDM Transmitter Zero Padding (II)
Freq axis 0 ? Fs

• 55 zeros carriers are padded.
• They will take the guard bands role.
• Reshaping is done to ensure the spectrum falls
  off on both sides when plotted from Fs/2 to Fs/2

Freq axis -Fs/2 ? Fs/2
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OFDM Transmitter IFFT Cyclic Prefix (III)
After IFFTbefore CP

• Tg/Tb ratio of ¼ used. Thus CP is 1/fourth the
  length of data time
• Tg/Tb ratios specified by the protocol are ¼,
  1/8, 1/16, 1/32. Question it does not specify
  when to use them.

After CP
Received Spectrum after the AWGN channel
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Receiver side
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OFDM receiver

• Reciprocal work of the OFDM transmitter
• Remove Cyclic Prefix (assuming synchronization)
• Perform the FFT
• Remove the zero padding and reorder
• Separate the data carriers from the pilot carriers

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Received signal

• AWGN channel is used
• If a Rayleigh channel is to be used, the
  receiver side would need a channel equalization
  section with the usage of the pilots in the
  channel estimation (currently pilots are ignored
  after being removed from the received OFDM
  frame).

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QAM demodulation 2-stepsDe-interleaving
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Viterbi decoding

• Using a trellis generated from the polynomial of
  constraint length 7, and the polynomials
  G1171oct, and G2133oct.
• Traceback depth specified to be equal to 34.
• It can be possible if the future to use soft
  decisions, LLRs, in decoding.

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Reed Solomon decoding

• Some work is needed to compensate for the delay
  introduced by the Viterbi decoder.
• Reed Solomon decoding is done, and tail byte
  removed.
• Num of corrected is also outputted, with a -1
  when the errors exceeding the max allowed bit
  errors in this case equals to T 8.

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De-randomization

• Same operation as in the Randomization
• XORing with the same PRBS generator will
  retrieve back the data

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Bit Error Rate

• Bit Error Rate is displayed, along with,
• Number of bits in error, and
• The total number of bits compared with the
  original stream of raw data.

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Generated BER curve for the IEEE 802.16d,
256-FFT, 16QAM, RS(64,48,8) CC rate of 2/3
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BER curves over AWGN channel incremental effects
of channel coding