WLAN a Spec. (Physical Layer) 2005/04/ /4/28. WLAN Group 1

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1 WLAN a Spec. (Physical Layer) 2005/4/ /04/28 1

2 802.11a PHY SPEC. for the 5GHz band Introduction The radio frequency LAN system is initially aimed for the , GHz, & GHz unlicensed national information infrastructure (U-NII) band. The support of transmitting & receiving at data rates of 6, 12, 24 Mbit/s is mandatory. (9, 18, 36, 48, 54Mbit/s) The system uses 52 subcarriers that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16- quadrature amplitude modulation (QAM), or 64-QAM. Forward error correction coding (convolutional coding) is used with a coding rate of 1/2, 2/3, or 3/4 2005/4/28 2

3 RATE-dependent parameters 2005/4/28 3

4 Timing related parameters 2005/4/28 4

5 PLCP sublayer PLCP Header RATE 4 bits Reserved 1 bit LENGTH 12bits Parity 1 bit Tail 6 bits SERVICE 16 bits PSDU Tail 6 bits Pad Bits Coded/OFDM (BPSK, r=1/2) Coded/OFDM (RATE is indicated in SIGNAL) PLCP Preamble 12 Symbols SIGNAL One OFDM Symbol DATA Variable Number of OFDM Symbols 2005/4/28 5

6 PLCP preamble (SYNC) RATE 4 bits Reserved 1 bit LENGTH Parity 12bits 1 bit PLCP Header Tail 6 bits SERVICE 16 bits PSDU Tail 6 bits Pad Bits Coded/OFDM (BPSK, r=1/2) Coded/OFDM (RATE is indicated in SIGNAL) PLCP Preamble 12 Symbols SIGNAL One OFDM Symbol DATA Variable Number of OFDM Symbols 2005/4/28 6

7 PLCP preamble (SYNC) It consists of 10 short training symbols & 2 long training symbols 2005/4/28 7

8 PLCP preamble (SYNC) In the PLCP (Physical Layer Convergence Procedure) preamble field Composed of 10 repetitions of a shorting training sequence,used for AGC (Automatic Gain Control) convergence Diversity selection Timing acquisition Coarse frequency acquisition in the receiver Two repetitions of a long training sequence,used for Channel estimation Fine frequency acquisition in the receiver 2005/4/28 8

9 Short training symbol A short OFDM training symbol consists of 12 subcarriers, which are modulated by the elements of the sequence S, given by S 26,26 = 13 6 {0, 0, 1 + j, 0, 0, 0, 1 j, 0, 0, 0, 1 + j, 0, 0, 0, 1 j, 0, 0, 0, j, 0, 0, 0, 1 + j, 0, 0, 0, 0, 0, 0, 0, 1 j, 0, 0, 0, 1 j, , 0, 0, 1 + j, 0, 0, 0, 1 + j, 0, 0, 0, 1 + j, 0, 0, 0, 1 + j, 0, 0} The multiplication by a factor of 13 6 is in order to normalize the average power of the resulting OFDM symbol, which utilizes 12 out of 52 subcarriers. The fact that only spectral lines of S -26:26 with indices that are a multiple of 4 have nonzero amplitude results in a periodicity of 0.8 us. 2005/4/

10 Short training symbol The signal shall be generated according to the following equation: r SHORT ( t) = w TSHORT ( t) N ST k= N 2 ST S k 2 exp( j2πk F t) 2005/4/28 10

11 Long training symbol A long training symbol consists of 53 subcarriers (including a zero value at dc), which are modulated by the elements of the sequence L, given by L 26,26 = {1,1, 1, 1,1,1, 1,1, 1,1,1,1,1,1,1, 1, 1,1,1, 1,1, 1,1,1,1,1, 0, 1, 1, 1,1,1, 1,1, 1,1, 1, 1, 1, 1, 1,1,1, 1, 1,1, 1,1, 1,1,1,1,1} A long OFDM training symbol shall be generated according to the following equation: N ST 2 r ( t) = w ( t) L exp( j2 π k ( t T )) LONG TLONG k F GI 2 k= N 2 ST where TGI 2 = 1.6µ s 2005/4/28 11

12 Long training symbol Two period of the long sequence are transmitted for improved channel estimation accuracy, yielding T LONG = = 8µs The sections of short repetitions and long repetitions shall be concatenated to form the preamble r PREAMBLE ( t) = rshort ( t) + rlong ( t TSHORT ) 2005/4/28 12

13 Signal field (SIGNAL) RATE 4 bits Reserved 1 bit LENGTH 12bits Parity 1 bit Tail 6 bits SERVICE 16 bits PSDU Tail 6 bits Pad Bits PLCP Header Coded/OFDM (BPSK, r=1/2) Coded/OFDM (RATE is indicated in SIGNAL) PLCP Preamble 12 Symbols SIGNAL One OFDM Symbol DATA Variable Number of OFDM Symbols 2005/4/28 13

14 Signal field (SIGNAL) 2005/4/28 14

15 Signal field (SIGNAL) Bit 4 : shall be reserved for future use Bit 17: shall be positive parity (even parity) bit for bit 0~16 Bit 18~23 : all 6 bits shall be set to zero Data rate 2005/4/28 15

16 Signal field (SIGNAL) The SIGNAL field contains the RATE & the LENGTH field of the TXVECTOR The RATE field conveys information about the type of modulation & the coding rate as used in the rest of the packet The LENGTH field indicates the number of octets in the PSDU The encoding procedure, which includes convolutional encoding, interleaving, modulation mapping processes, pilot insertion, & OFDM modulation as used for a transmission of data at a 6 Mbit/s rate 2005/4/28 16

17 Block diagram (SIGNAL) Convolutional encoder Interleaver Modulator Pilot insrtion IFFT GI insertion Window function 2005/4/28 17

18 Data field (DATA) PLCP Header RATE 4 bits Reserved 1 bit LENGTH 12bits Parity 1 bit Tail 6 bits SERVICE 16 bits PSDU Tail 6 bits Pad Bits Coded/OFDM (BPSK, r=1/2) Coded/OFDM (RATE is indicated in SIGNAL) PLCP Preamble 12 Symbols SIGNAL One OFDM Symbol DATA Variable Number of OFDM Symbols 2005/4/28 18

19 Service field The 0 ~ 6 bits are set to zeros and are used to synchronize the descrambler in the receiver 2005/4/28 19

20 Tail bit field The tail bit field shall be six bits of 0, required to return the convolutional encoder to the zero state. This procedure improves the error probability of the convolutional decoder, which relies on future bits when decoding and which may not be available past the end of the message. 2005/4/28 20

21 Pad bits The number of bits in the DATA field shall be a multiple of NCBPS (the number of coded bits in an OFDM symbol) To achieve that, the length of the message is extended so that it becomes a multiple of NDBPS (the number of data bits per OFDM symbol) 2005/4/28 21

22 Pad bits N SYM = Ceiling (( LENGTH + 6)/N DBPS ) N DATA = N SYM N DBPS N PAD = N DATA ( LENGTH + 6) where N SYM The number of OFDM symbols N DATA : The number of bits in the DATA field N PAD : The number of pad bits LENGTH : The length of the PSDU 2005/4/28 22

23 Block diagram Scrambler Convolutional encoder Puncture Interleaver Modulator Pilot insertion IFFT GI insertion Window function 2005/4/28 23

24 Scrambler and descrambler Data in X 7 X 6 X 5 X 4 X 3 X 2 X 1 Descramble dataout 2005/4/28 24

25 PLCP DATA scrambler and descrambler The frame scrambler uses the generator polynomial S(x) is S(x) = x 7 + x The 127-bit sequence generated repeatedly by the scrambler shall be (leftmost used first), ,when the all ones initial state is used. 2005/4/28 25

26 Convolutional encoder The DATA field shall be coded with a convolutional encoder of coding rate R = 1/2 Output dataa Input data Tb Tb Tb Tb Tb Tb Output datab 2005/4/28 26

27 Higher code rate: 2/3, or 3/4 Convolutional encoder Puncturing is a procedure for omitting some of the encoded bits in the transmitter (thus reducing the number of transmitted bits and increasing the coding rate) Increasing the BW efficiency Increasing the bit error rate (BER) 2005/4/28 27

28 Puncturing 2005/4/28 28

29 Data interleaving In order to avoid the presence of deep fade The interleaver is defined by a two-step permutation The permutation ensures that adjacent coded bits are mapped on to nonadjacent subcarriers We shall denote by k the index of the coded bit before the first permutation Permutation i shall be the index after the first and before the second permutation Second permutationj shall be the index after the second permutation 2005/4/28 29

30 Data interleaving first permutation i ( N /16)( mod16) + floor(k/16) k = 0,1,...,N 1 = k CBPS CBPS second permutation ( i / s) + ( i + N floor( 16 i / N )) j = s floor i = 0,1,..., s = max N CBPS 1 ( N / 2,1 ) BPSC CBPS CBPS mods 2005/4/28 30

31 Data interleaving Block interleaver Write bits Read bits 2005/4/28 31

32 Modulator 2005/4/28 32

33 Modulator 2005/4/28 33

34 Pilot subcarriers In each OFDM symbol, four of the subcarriers are dedicated to pilot signals in order to make the coherent detection robust against frequency offsets and phase noise. These pilot signals shall be put in subcarriers 21, 7, 7 and 21. The pilots shall be BPSK modulated by a pseudo binary sequence to prevent the generation of spectral lines. d 5 d 0 d 4 P -21 d 17 P -7 d 18 d 23 DC d 24 d 29 P 7 d 30 d 42 P 21 d 43 d Subcarrier number 2005/4/28 34

35 Pilot subcarriers The polarity of the pilot subcarriers is controlled by the Pn sequence ( i.e. the output sequence of the scrambler ) Replacing all 1 s with -1 and all 0 s with 1 Each sequence element is used for one OFDM symbol In the sequence of the pilot polarity, the first element, p 0, multiplies the pilot subcarriers of the SIGNAL symbol, the others p n are used for the DATA symbols 2005/4/28 35

36 Pilot subcarriers /4/28 36

37 Polarity of the pilot subcarriers Pilot subcarriers 2005/4/28 37

38 ( ) IFFT An OFDMsymbol, r t,is defined as NSD: DATA, n r DATA, n N SD 1 N ST / 2 = TSYM k, n F GI n 1 k = 0 k = N ST / 2 () t w () t d exp( j2πm ( k) ( t T )) + p p exp( j2π ( t T )) + k F GI NSD: the number of modulated data symbols NST: the number of pilot symbols 2005/4/28 38

39 IFFT The common way to implement the inverse Fourier transform is by an Inverse Fast Fourier Transform (IFFT) algorithm An example, a 64-point IFFT is used, the coefficient 1~26 are mapped to the same numbered IFFT inputs -26~-1 are copied into the IFFT inputs 38~63 The rest of the input are set to zero 2005/4/28 39

40 Guard Interval Shifting the time by TGUARD creates the cyclic prefix used in OFDM to avoid ISI from the previous frame Three kinds of TGUARD are defined For the short training sequence (0 s) For the long training sequence (TGI2 = 1.6 s) For the data OFDM symbols (TGI = 0.8 s) Guard interval Useful symbol 2005/4/28 40

41 Windowing function T:Time-windowing function duration TFFT:IFFT/FFT period TGI : GI duration w t () t 2 π sin 2 = 2 π sin 2 ( t / T ) 1 TR ( 0.5 ( t T )/ T ) TR ( TTR / 2 < t < TTR / 2) ( TTR / 2 < t < T TTR / 2) ( T T / 2 < t < T + T / 2) T TR : Transition time is the transition time between two consecutive periods of FFT (about 100ns), smooth the transition is required in order to reduce the spectral sidelobes of the transmitted waveform 2005/4/28 41 TR TR

Mohammad Hossein Manshaei 1393

Mohammad Hossein Manshaei 1393 Mohammad Hossein Manshaei manshaei@gmail.com 1393 1 PLCP format, Data Rates, OFDM, Modulations, 2 IEEE 802.11a: Transmit and Receive Procedure 802.11a Modulations BPSK Performance Analysis Convolutional