射频、高频 通信教程解压后12GB 数字通信系统和WLAN 课程教程 数字通信系统和WLAN 课程ppt1 060215 WLAN

1、WLAN 802.11a/b/g,Tung-Chou Chen,2,Outlines,IEEE Std. 802.11a-1999 IEEE Std. 802.11b-1999 IEEE Std. 802.11g-2003 System Performance Hardware Implementation,IEEE Std 802.11aOFDM PHY specification for 5GHz Band,System Overview Channel Coding Interleaving Subcarrier Modulation OFDM Modulation Frame Form

2、at,4,System Overview,Transceiver block diagram for the OFDM PHY,5,System Overview,Transceiver block diagram for the OFDM PHY,6,System Overview,Timing-Related Parameters,7,System Overview,Information Data Rate,8,System Overview,Rate-Dependent Parameters,9,Channel Coding,Punctured Convolutional Coding

3、 Coding Rate = 1/2, 2/3, or 3/4,10,Channel Coding,Convolutional Encoder (K=7) Generator Polynomials g0 = 1338 g1 = 1718,11,Channel Coding,Punctured Coding (R = 2/3,12,Channel Coding,Punctured Coding (R = 3/4,13,Interleaving,Two-Step Permutation,14,Interleaving,Rule First Permutation i = (NCBPS/16)(k

4、 mod 16) + floor(k/16), k = 0, 1, ., NCBPS 1 Second Permutation j = s floor(i/s) + i + NCBPS floor(16 i/ NCBPS) mod s, i = 0, 1, ., NCBPS 1 s = max(NBPSC/2 , 1,15,Interleaving,First Permutation i = (NCBPS/16)(k mod 16) + floor(k/16), k = 0, 1, ., NCBPS 1,Input,Output,3 subcarrier symbols,16 bits,16,

5、Interleaving,First Permutation Example: 16-QAM,17,Interleaving,First Permutation Example: 64-QAM,18,Interleaving,Second Permutation Example: 16-QAM,19,Interleaving,Second Permutation Example: 64-QAM,20,Deinterleaving,Rule First Permutation i = s floor(j/s) + j + floor(16 j/ NCBPS) mod s, j = 0, 1, .

6、, NCBPS 1 s = max(NBPSC/2 , 1) Second Permutation k = 16 i (NCBPS 1 )floor(16 i / NCBPS), i = 0, 1, ., NCBPS 1,21,Deinterleaving,First Permutation Example: 16QAM,22,Deinterleaving,First Permutation Example: 64QAM,23,Deinterleaving,Second Permutation i = s floor(j/s) + j + floor(16 j/ NCBPS) mod s,24

7、,Subcarrier Modulation,Modulation Type BPSK, QPSK, 16-QAM, or 64-QAM,25,Subcarrier Modulation,BPSK Encoding Table Constellation,26,Subcarrier Modulation,QPSK Encoding Table Constellation,27,Subcarrier Modulation,16-QAM Encoding Table Constellation,28,Subcarrier Modulation,64-QAM Encoding Table,29,Su

8、bcarrier Modulation,64-QAM Constellation,30,Subcarrier Modulation Mapping,Output Value d = (I + j Q) KMOD KMOD: normalization factor,31,OFDM Modulation,64-Point DFT/IDFT + 16-Point Cyclic Prefix,32,OFDM Modulation,Block Diagram,33,OFDM Modulation,Inputs and Outputs of IDFT Pilot Subcarriers: # -21,

9、-7, 7, 21 Data Subcarriers: # 1 6, 8 20, 22 26, -26 -22, -20 -8, -6 -1,34,OFDM Modulation,Pilot Subcarriers,35,OFDM Modulation,Polarity of the Pilot Subcarriers,36,OFDM Demodulation,Block Diagram,37,Frame Format,PLCP Frame Format The PPDU includes the OFDM PLCP preamble, OFDM PLCP header, PSDU, tail

10、 bits, and pad bits,38,Frame Format,PLCP Preamble (SYNC field) The PLCP preamble field is used for synchronization. It consists of 10 short training symbols and two long training symbols,39,Frame Format,PLCP Preamble (SYNC field) Short Training Symbol A short OFDM training symbol consists of 12 subc

11、arriers, which are modulated by the elements of the sequence S: S-26, 26 = sqrt(13/6) 0, 0, 1+j, 0, 0, 0, -1-j, 0, 0, 0, 1+j, 0, 0, 0, -1-j, 0, 0, 0, -1-j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, -1-j, 0, 0, 0, -1-j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0 The fact that only spectral

12、 lines of S-26:26 with indices that are a multiple of 4 have nonzero amplitude results in a periodicity of TFFT/4 = 0.8 ms. The interval TSHORT is equal to ten 0.8 ms periods (i.e., 8 ms,40,Frame Format,PLCP Preamble (SYNC field) Short Training Symbol Frequency Domain,41,Frame Format,PLCP Preamble (

13、SYNC field) Short Training Symbol Time Domain (one period of IFFT of the short sequences,42,Frame Format,PLCP Preamble (SYNC field) Long Training Symbol A long OFDM training symbol consists of 53 subcarriers (including a zero value at dc), which are modulated by the elements of the sequence L: L-26,

14、 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, 1,43,Frame Format,SIGNAL Field The SIGNAL field consists the RATE and LENGTH fields. The encoding of the SIGNAL singl

15、e OFDM symbol includes convolutional encoding (R = 1/2), interleaving, modulation mapping (BPSK), pilot insertion, and OFDM modulation. The contents of the SIGNAL field are not scrambled,44,Frame Format,SIGNAL Field Contents of the SIGNAL Field,45,Frame Format,SIGNAL Field Example for SIGNAL Field D

16、ata Rate: 36Mbps 100 octets PSDU,46,Frame Format,DATA Field The DATA field contains the SERVICE field, the PSDU, the TAIL bits, and the PAD bits. All bits in the DATA field are scrambled,47,Frame Format,SERVICE Field The bits from 0 - 6 of the SERVICE field are set to zeros and are used to synchroni

17、ze the descrambler in the receiver. The remaining 9 bits (7 - 15) of the SERVICE field shall be reserved for future use. All reserved bits shall be set to zero,48,Frame Format,DATA Field TAIL Bit Field The PPDU tail bit field shall be six bits of “0”, which are required to return the convolutional e

18、ncoder to the “zero state”. This procedure improves the error probability of the convolutional decoder. The PLCP tail bit field shall be produced by replacing six scrambled “zero” bits following the message end with six nonscrambled “zero” bits,49,Frame Format,DATA Field PAD Bits The number of bits

19、in the DATA field shall be a multiple of NCBPS (48, 96, 192, or 288 bits). 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. At least 6 bits are appended to the message, in order to accommodate the TAIL bits. The n

20、umber of pad bits, NPAD NSYM = Ceiling(16 + 8 LENGTH + 6)/ NDBPS) NDATA = NSYM NDBPS NPAD = NDATA (16 + 8 LENGTH + 6,50,Frame Format,PCLP DATA Scrambler and Descrambler The DATA field shall be scrambled with a length-127 frame-synchronous scrambler. When transmitting, the initial state of the scramb

21、ler will be set to a pseudo random non-zero state. The seven LSBs of the SERVICE field will be set to all zeros prior to scrambling to enable estimation of the initial state of the scrambler in the receiver,IEEE Std 802.11bHigh Rate, DSSS PHY Specification,System Overview DSSS (1Mbps/2Mbps) CCK (5.5

22、Mbps/11Mbps) PBCC (5.5Mbps/11Mbps) Frame Format,52,System Overview,Overview of IEEE 802.11 Standards,53,System Overview,High-Rate Extension Modes HR/DSSS/CCK HR/DSSS/PBCC (optional) HR/DSSS/short (optional) HR/DSSS/PBCC/short (optional,54,System Overview,DSSS (1Mbps/2Mbps) System Block Diagram,55,Sy

23、stem Overview,CCK (5.5Mbps/11Mbps) System Block Diagram,56,System Overview,DSSS/CCK,57,System Overview,PBCC (5.5Mbps/11Mbps) System Block Diagram,58,DSSS,Barker Sequence +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 Correlation,59,DSSS,DSSS/DBPSK (1 Mbps,60,DSSS,DSSS/DQPSK (2 Mbps,61,CCK,CCK Modulation

24、 For the CCK modulation modes, the spreading code length is 8 and is based on complementary codes. The chipping rate is 11 Mchip/s. The symbol duration shall be exactly 8 complex chips long. CCK codeword c = c0, c1, c2, c3, c4, c5, c6, c7: CCK modulation is a form of the generalized Hadamard transfo

25、rm encoding,62,CCK,CCK Modulation for 5.5Mbps The data bits d0 and d1 encode 1 based on DQPSK. j2 = (d2 p) + p/2, j3 = 0, and j4 = d3 p,63,CCK,CCK Modulation for 5.5Mbps The data bits d0 and d1 encode 1 based on DQPSK. The data dibits (d2, d3), (d4, d5), and (d6, d7) encode j2, j3, and j4, respectiv

26、ely, based on QPSK,64,CCK,CCK Modulator,65,PBCC,Convolutional Encoder,66,Frame Format,Long PLCP PPDU Format PLCP Preamble PLCP Header PSDU,67,Frame Format,Short PLCP PPDU Format PLCP Preamble PLCP Header PSDU,68,Frame Format,Long PLCP Preamble SYNC field The SYNC field shall consist of 128 bits of s

27、crambled “1” bits. This field is provided so the receiver can performthe necessary synchronization operations. The initial state of the scrambler (seed) shall be 1101100,69,Frame Format,Long PLCP Preamble SFD (Start Frame Delimiter) field The SFD shall be provided to indicate the start of PHY-depend

28、ent parameters within the PLCP preamble. The SFD shall be a 16-bit field, 1111 0011 1010 0000, where the rightmost bit shall be transmitted first in time,70,Frame Format,PLCP Header SIGNAL field The 8-bit SIGNAL field indicates to the PHY the modulation that shall be used for transmission (and recep

29、tion) of the PSDU. The data rate shall be equal to the SIGNAL field value multiplied by 100 kbit/s. The High Rate PHY supports four mandatory rates given by the following 8-bit words, which represent the rate in units of 100 kbit/s, where the lsb shall be transmitted first in time: a) X0A (msb to ls

30、b) for 1 Mbit/s b) X14 (msb to lsb) for 2 Mbit/s c) X37 (msb to lsb) for 5.5 Mbit/s d) X6E (msb to lsb) for 11 Mbit/s,71,Frame Format,PLCP Header SERVICE field Bit 7 shall be used to supplement the LENGTH field Bit 3 shall be used to indicate whether the modulation method is CCK or PBCC Bit 2 shall

31、be used to indicate that the transmit frequency and symbol clocks are derived from the same oscillator The SERVICE field shall be transmitted b0 first in time,72,Frame Format,PLCP Header LENGTH field The PLCP length field shall be an unsigned 16-bit integer that indicates the number of microseconds

32、required to transmit the PSDU. LENGTHx = (number of octets + P) 8) / R LENGTH = Ceiling (LENGTH) R is the data rate in Mbit/s; P = 0 for CCK and P = 1 for PBCC Ceiling (X) returns the smallest integer value greater than or equal to X If (R = 11) and (LENGTHLENGTH) 8/11), then LengthExtension = 1 els

33、e LengthExtension = 0,73,Frame Format,PLCP Header LENGTH field Example of LENGTH calculations for CCK Example of LENGTH calculations for PBCC,74,Frame Format,PLCP Header CRC field The SIGNAL, SERVICE, and LENGTH fields shall be protected with a CCITT CRC-16 frame check sequence (FCS,75,Frame Format,

34、PLCP Header Example The SIGNAL, SERVICE, and LENGTH fields for a DBPSK signal with a PPDU length of 192 ms (24 octets) would be given by the following: 0101 0000 0000 0000 0000 0011 0000 0000 The ones complement FCS for these protected PLCP preamble bits would be the following: 0101 1011 0101 0111,7

35、6,Data Scrambler and Descrambler,Data Scrambler Data Descrambler,IEEE Std 802.11g,System Overview ERP-DSSS/CCK (1, 2, 5.5, and 11 Mbps) ERP-OFDM (6, 9, 12, 18, 24, 36, 48, and 54 Mbps) ERP-PBCC (5.5, 11, 22, and 33 Mbps) DSSS-OFDM (6, 9, 12, 18, 14, 36, 48, and 54 Mbps) Frame Format,78,System Overvi

36、ew,ERP-DSSS/CCK (1, 2, 5.5, and 11 Mbps) ERP-OFDM (6, 9, 12, 18, 24, 36, 48, and 54 Mbps) ERP-PBCC (5.5, 11, 22, and 33 Mbps) DSSS-OFDM (6, 9, 12, 18, 14, 36, 48, and 54 Mbps,79,System Overview,802.11a/b/g,80,System Overview,ERP-DSSS/CCK DSSS CCK,81,System Overview,ERP-PBCC 5.5 Mbps: R-1/2 BCC P = 0

37、 for CCK and P = 1 for PBCC Ceiling (X) returns the smallest integer value greater than or equal to X The Length Extension bits are provided to resolve the ambiguity in the number of octets that is described by an integer number of microseconds for any data rate over 8 Mb/s,89,Frame Format,PPDU Form

38、at for ERP-DSSS/CCK and ERP-PBCC PLCP Header LENGTH field Example of LENGTH calculations for ERP-PBCC-22,90,Frame Format,PPDU Format for DSSS-OFDM,91,Frame Format,PPDU Format for DSSS-OFDM LENGTH field The length field calculation in terms of data packet length is as follows: LENGTH = PSDUsyncOFDM +

39、 PSDUSignalOFDM + 4 Ceiling(PLCPServiceBits + 8 (NumberOfOctets) + PadBits) / NDBPS) + SignalExtension PSDUsyncOFDM is 8 ms (OFDM long training symbols), PSDUSignalOFDM is 4 ms, SignalExtension is 6 ms,92,Frame Format,PPDU Format for DSSS-OFDM PSDU,93,Frame Format,PPDU Format for ERP-OFDM The PPDU f

40、ormat is the same as 802.11a described in clause 17,94,References,References IEEE Std.802.11, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, June 1997. IEEE Std.802.11a, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 1: High-speed Physical Layer in the 5 GHz band, Sep.