2 Introduction The Matlab’s Simulink simulation work is for the broadband wireless standard IEEE802.16d [and subsequently for the IEEE once it is published] Currently the mandatory channel coding scheme is used (Reed-Solomon/ convolutional code), 16-QAM modulation, RS(64,48,8), CC rate=2/3 A state machine can be implemented on top of the current model to take into account adaptive rate modulation.
3 IEEE802.16d, WirelessMAN-OFDM 256-FFT, 16-QAM
4 Randomization PRBS generator: 1 + x 14 + x 15 On DL: the scrambler is re-initialized at start of each frame with the vector:
6 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) = x 8 + x 4 + x 3 + x And a code generator polynomial of: g(x)=(x+λ 0 ) (x+λ 1 ) (x+λ 2 ).. (x+λ 2T-1 ), λ=02 Hex
7 Convolutional Coding & Puncturing Convolutional coding with a native rate of ½ Constraint length of 7 Generated with the following 2 generator polynomials: G1=171oct= For X G2=133oct= For Y Puncturing of 4/3: X 1 Y 1 Y 2 ; thus overall CC rate of ½*4/3 = 2/3
8 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 st permutation [for carriers] m = (Ncbps/16)*mod(k,16)+floor(k/16); 2nd permutation [for bits constellation mapping on carriers] j=s*floor(m/s)+mod((m+Ncbps-floor(16*m/Ncbps)),s);
9 2-steps Interleaver (II)
10 16-QAM modulation Gray mapped Normalized constellation average power
11 OFDM Transmitter: Data&Pilots, Zero Padding, Shaping, Cyclic Prefix (I) 192 Data carriers 8 Pilot carries 0 DC carrier 55 zero carriers added
12 OFDM Transmitter: Pilots (II) Pilots BPSK modulated Uses a PRBS generator of x 11 + x Initialized in DL with the vector: [ ] Why can’t we always transmit all 1s on the pilots without the need of the PRBS generator? Sample of a DL sequence: Corresponding sample of an UL sequence: PRBS for pilot modulation
13 OFDM Transmitter: Zero Padding (II) 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: 0 → Fs Freq axis: -Fs/2 → Fs/2
14 OFDM Transmitter: IFFT & Cyclic Prefix (III) After IFFT before CP After CP Received Spectrum after the AWGN channel T g /T b 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.
15 Receiver side
16 OFDM receiver Reciprocal work of the OFDM transmitter: 1.Remove Cyclic Prefix (assuming synchronization) 2.Perform the FFT 3.Remove the zero padding and reorder 4.Separate the data carriers from the pilot carriers
17 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).
18 QAM demodulation & 2-stepsDe-interleaving
19 Viterbi decoding Using a trellis generated from the polynomial of constraint length 7, and the polynomials G 1 =171 oct, and G 2 =133 oct. Traceback depth specified to be equal to 34. It can be possible if the future to use soft decisions, LLRs, in decoding.
20 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.
21 De-randomization Same operation as in the Randomization XORing with the same PRBS generator will retrieve back the data
22 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.
23 Generated BER curve for the IEEE d, 256-FFT, 16QAM, RS(64,48,8) & CC rate of 2/3
24 BER curves over AWGN channel; incremental effects of channel coding