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802.11a and 802.11g OFDM PHY Specification

802.11a and 802.11g OFDM PHY Specification. Contents. OFDM Specifications (PMD) PLCP Frame Format and Procedures. MAC Layer. PHY Layer. 2.4 GHz FHSS 1 Mbps 2 Mbps. 2.4 GHz DSSS 1 Mbps 2 Mbps. Infrared IR 1 Mbps 2 Mbps. 2.4 GHz DSSS 5.5 Mbps 11 Mbps. 5 GHz OFDM 6, 9, 12,

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802.11a and 802.11g OFDM PHY Specification

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  1. 802.11a and 802.11gOFDM PHY Specification 1

  2. Contents • OFDM Specifications (PMD) • PLCP Frame Format and Procedures 2

  3. MAC Layer PHY Layer 2.4 GHz FHSS 1 Mbps 2 Mbps 2.4 GHz DSSS 1 Mbps 2 Mbps Infrared IR 1 Mbps 2 Mbps 2.4 GHz DSSS 5.5 Mbps 11 Mbps 5 GHz OFDM 6, 9, 12, 18, 24, 36, 48, 54 Mbps IEEE 802.11b IEEE 802.11a IEEE 802.11 Where does OFDM fit in the standard? 3

  4. OFDM Specifications 4

  5. Overview of OFDM Specifications • Operates in 5.15-5.25, 5.25-5.35 and 5.725-5.825 GHz U-NII bands • Multiple data rates between 6 and 54 Mbps • BPSK, QPSK or QAM modulation depending on data rate • Convolutional (Forward Error Correcting) encoding at 1/2, 2/3 or 3/4 rate • 52 subcarriers used for each transmission • Total Occupied bandwidth: 16.6 MHz (= 20 MHz*52/64) • OFDM symbol duration of 4 s (fixed symbol rate of 0.25 Msps) 5

  6. Modulation • 802.11a uses Quadrature Amplitude Modulation (QAM), in addition to BPSK and QPSK. • QAM uses amplitude modulation in both in-phase (real) and quadrature (imaginary) components simultaneously • QAM is highly spectrally efficient • However, it is more susceptible to noise than other modulation schemes (e.g. m-PSK) 6

  7. QAM - Constellations • The way data is encoded can be represented in a constellation diagram. e.g. 16-QAM which encodes 4 bits per symbol: • OFDM uses 16-QAM and 64-QAM as well as BPSK and QPSK 7

  8. Multiple Carriers • OFDM uses 52 carriers: • 48 data carriers • 4 pilot carriers used to make reception more robust against frequency offset and noise • Carriers are spaced 312.5 kHz apart • The 48 data carrier channels can be generated using an Inverse Fast Fourier Transform (IFFT) • Total bandwidth = (52 x 312.5 kHz) + 312.5 kHz = 16.6 MHz 8

  9. Data Rates used in OFDM 9

  10. Spectrum use for OFDM 10

  11. Structure and PLCP Frame Format 11

  12. PLCP Sublayer PLME PHY Layer PMD Sublayer Structure • Like the DSSS and FHSS PHYs, the OFDM PHY is made up of the following elements: • PMD: Physical Medium Dependent • PLCP: Physical Layer Convergence Procedure • PLME: Physical Layer Management Entity 12

  13. Scrambling, FEC, Interleaving • Before being transmitted, data undergoes the following processes: • Scrambling: randomizes data, removes long strings of 1’s or 0’s (this makes clock recovery easier); • Convolutional Encoder: adds redundancy to reduce the bit error rate • Interleaving: in conjunction with the encoder, reduces the bit error rate by separating bits containing common information 13

  14. Scrambling Process • At the start of transmission, the scrambler is set to a psuedo-random state • A row of seven zeros in a known place in the header allows the receiver to synchronize its descrambler • Bits are passed serially through the scrambler: • The descrambling process uses the same procedure 14

  15. Convolutional Encoder • The encoder adds redundancy to the data to allow bit errors to be corrected • The basic process results in a bit rate twice that of the original data • Lower encoder output rates are achieved by ‘puncturing’ (i.e. removing) certain bits before transmission 15

  16. Puncturing • See Figure 115 on page 18 of the standard. 16

  17. Interleaving • After the encoding process, coded bits that are close together are closely related. • Interleaving ensures that adjacent bits are separated: • adjacent bits are not mapped onto adjacent subcarriers • adjacent bits are mapped alternately onto less and more significant bits of the constellation to avoid long runs of low reliability (LSB) • Bits are read into a grid row by row, and read out by columns 17

  18. PLCP Frame Format (1) 18

  19. PLCP Frame Format (2) • Rate: 6, 9, 12, 18, 24, 36, 48 or 54 Mbps • Reserved: Reserved for future use • Length: Number of bytes in the PSDU • Parity: Even parity bit for Rate, Reserved and Length fields • Tail: 6 zero bits to enable decoding of the Rate and Length fields immediately after the reception of the tail bits • The above fields shall be transmitted at 6 Mbps (BPSK, R=1/2) and shall not be scrambled 19

  20. PLCP Frame Format (3) • Service: Bits 0 to 6 are used to synchronize the descrambler. Bits 7 to 15 are reserved for future use • Tail: 6 zero bits to return the convolutional encoder to the zero state • Pad bits: Appended zeros for the purpose of making the number of bits in the Data field a multiple of the number of data bits per OFDM symbol • Note that all bits in the above fields shall be scrambled and transmitted at the desired data rate 20

  21. PLCP Preamble • PLCP preamble: It consists of • 10 short OFDM training symbols consisting of 12 modulated subcarriers and used for signal detection, diversity selection, timing acquisition and coarse frequency acquisition • 2 long identical OFDM training symbols consisting of 52 modulated subcarriers and used for channel estimation and fine frequency acquisition, preceded by a guard interval (GI) 21

  22. Transmit and Receive Procedures 22

  23. Transmit Procedure 23

  24. Transmit State Machine 24

  25. Receive Procedure 25

  26. Receive State Machine 26

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