802.11 DSSS and 802.11b PHY Specifications

1. 802.11 DSSS and 802.11b PHY Specifications 1

2. Contents • Direct Sequence Specifications (PMD) • Structure • Physical Layer Convergence Procedure (PLCP) • Procedures: Transmit, Receive, Clear Channel Assessment 2

3. MAC 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 DSSS fit in the standard? • 802.11 specifies 1 & 2 Mbps • 802.11b specifies additional 5.5 & 11 Mbps rates PHY Layer 3

4. Direct Sequence Specifications(PMD) 4

5. 802.11 DSSS Overview • 2.4 GHz ISM band • 1 and 2 Mbps data rate (DBPSK and DQPSK modulation) • Symbol rate of 1 Msps • Chipping rate of 11 MHz with 11-chip Barker sequence 5

6. 802.11b Overview • 2.4 GHz ISM band • 5.5 and 11 Mbps data rate • Symbol rate of 1.375 Msps • Optional shorter PLCP preamble mode • Optional Channel Agility mode 6

7. 01 - /2 11 -  00 - 0 0 - 0 1 -  10 - 3/2 QPSK BPSK ‘Base’ Modulation: 802.11 • 802.11 DSSS uses DBPSK (1 Mbps) and DQPSK (2 Mbps) at a rate of 1Msps (symbols per second): • In Phase Shift Keying (PSK), data is encoded in the phase of the carrier • In Differential PSK (DxPSK) the difference between the previous and the current symbol is encoded in the phase 7

8. Spreading Modulation: 802.11 • The modulated signal is then spread by a further modulation • This uses the Barker sequence which we have already seen: • This sequence repeats every symbol • The chip rate is 11 Mchip/s • The resulting bandwidth is 1 Msps x 11 chips/symbol  11 MHz 8

9. Modulation: 802.11b • Symbols are transmitted at a rate of 1.375 Msps • Each symbol encodes 4 bits (5.5Mbps) or 8 bits (11 Mbps) • There are 8 chips per symbol • Resultant bandwidth is approx. 11 MHz 9

10. 802.11b Chip codes • The 802.11b chip codes are based on Complementary Code Keying • One 8-bit code word is generated per symbol • The code words are generated as follows: • From the 4 (d0-d3) or 8 (d0-d7) bits for that symbol, four complex phases 1, 2, 3, 4, are calculated: • 1 encodes (d0, d1) using DQPSK relative to 1 of the previous symbol • 2, 3, 4 encode the remaining bits, using algebraic relationships (5.5 Mbps) or QPSK (11 Mbps) • The code symbols c0-c7 are calculated by the following addition: 10

11. Code Word calculation • e.g. c3 = - e j(1 + 4) 11

12. 802.11 DSSS Spectrum • Both 802.11 and 802.11b generate signals with a bandwidth of approx. 11MHz. • 11 channels are defined for N. America, with center frequencies at (2407 + 5n) MHz 1  n  11 • For non-interfering coexistence, systems must use center frequencies separated by 30 MHz 12

13. Frequency Hopping DSSS • Channel Agility is an optional enhancement to 802.11b which implements Frequency Hopping of a DSSS signal • This has two benefits: • Overcomes narrowband interference • Allows coexistence of DSSS and FHSS systems • Two different hop sets are defined: • Non-overlapping (contains 3 channels) • Overlapping (contains 6 channels), for interoperability with FHSS systems 13

14. Non-overlapping Hop Set • Because of the wide bandwidth of the signal, there are only 3 non-overlapping channels within the ISM band • There are two hopping patterns: • {1, 6, 11} • {1, 11, 6} 14

15. Overlapping Hopping Channels • For N. America, the six channels used in the overlapping hop set are 1, 3, 5, 7, 9, 11 15

16. Hop patterns – Overlapping channels • The hop patterns are based on the 79-hop sequences used in FHSS • The transmitter selects the channel with the nearest center frequency to the frequency that would be used were the transmitter using the 802.11 FHSS PHY. • e.g. the beginning of the base FHSS sequence is {0, 23, 62, 8, … } corresponding to center frequencies (in GHz) {2.402, 2.425, 2.464, 2.408, … } • The DSSS system uses the following channels {1, 3, 11, 1, … } corresponding to center frequencies (in GHz) {2.412, 2.422, 2.462, 2.412} • If the DSSS center frequency is further than 5MHz from the corresponding FHSS frequency, only the 1 or 2 Mbps rates may be used (for example at the 1st hop above) 16

17. DSSS PHY specification • Slot time (MAC layer) 20 s • SIFS time (MAC layer) 10 s • Operating temperature range 0 °C to 40 °C (type 1) • Maximum output power • 1000 mW USA • 100 mW Europe • 10 mW/MHz Japan • Minimum transmitted power 1 mW • Receiver sensitivity -80 dBm @ 0.08 FER (1024 bytes) • Rx adjacent channel rejection > 35 dB @ 30 MHz (or 25 MHz) separation between channels 17

18. Structure and PLCP 18

19. PLCP Sublayer PLME PHY Layer PMD Sublayer Structure of the DSSS PHY • The structure of the DSSS PHY is the same as the FHSS PHY • PMD: Physical Medium Dependent • PLCP: Physical Layer Convergence Procedure • PLME: Physical Layer Management Entity 19

20. PLCP Sublayer • The PLCP sublayer in the DSSS PHY performs the same functions as the PLCP in the FHSS PHY: • Indicate to the receiver the start and end of a MAC frame • Indicate the data rate (modulation) used • Allow receiver to acquire bit-synchronization (since PHY is not synchronous) • Carry out functions which are PMD-dependent but which act only on certain parts of the frame (e.g. scrambling) • There are two PLCP frame formats: • Long frame format • Short frame format (optional – 802.11b only): reduces framing overhead 20

21. DSSS PLCP preamble(18 octets) DSSS PLCP header(6 octets) PSDU 1 Mbps transmission 1 or 2 Mbps transmission Synchronization (128 bits) Start Frame Delimiter (16 bits) Signal (8 bits) Service (8 bits) Length (16 bits) CRC (16 bits) PLCP Long Frame Format (1) 21

22. PLCP Long Frame format (2) • Synchronization: 128 scrambled 1 bits; used for signal detection, antenna selection, frequency offset compensation, and synchronization • Start Frame Delimiter: 1111 0011 1010 0000; used for frame timing • Signal: Data rate used to encode the PSDU; the data rate is equal to the signal field value multiplied by 100 kbit/s; • Service: Reserved for future use; X’00’ signifies 802.11 compliant 22

23. PLCP Frame format (3) • Length: The number of microseconds required to transmit the PSDU; used for the end of frame detection • CRC: CCITT CRC-16 FCS; used to protect Signal, Service and Length fields • All bits transmitted by the DSSS PHY shall be scrambled; The purpose is to whiten the spectrum and to limit DC offset changes 23

24. 802.11b Frame Format • The 802.11b standard PLCP is different to the low-rate (802.11) standard in the following way: • Service: 3 out of 8 bits have been defined: • To resolve ambiguity in the number of octets described by a length in integer microseconds for any rate over 8 Mbit/s • To indicate if the optional PBCC mode is being used • To indicate that the transmit frequency and chip clocks are locked (i.e derived from the same oscillator); this is highly recommended for optimum performance • An optional shorter PLCP frame format is defined 24

25. DSSS PLCP preamble(9 octets) DSSS PLCP header(6 octets) PSDU 1 Mbps transmission 2 Mbps transmission 2, 5.5 or 11 Mbps transmission Synchronization (56 bits) Start Frame Delimiter (16 bits) Signal (8 bits) Service (8 bits) Length (16 bits) CRC (16 bits) 96 s Shorter PLCP Frame (Optional) 25

26. Shorter PLCP Frame Fields • Synchronization: 56 scrambled 0 bits; used for signal detection, antenna selection, frequency offset compensation, and synchronization • Start Frame Delimiter: 0000 0101 1100 1111; used for frame timing • Signal: PSDU’s data rate indication; there are 3 mandatory rates: • X’14’ for 2 Mbps • X’37’ for 5.5 Mbps • X’6E’ for 11 Mbps • Other fields are the same as the long field format 26

27. Interactions between PLCP, PMD and PHY 27

28. PHY-SAP Primitives 28

29. PMD SAP Primitives 29

30. PHY Layer Functions Clear Channel Assessment Transmit Receive 30

31. Clear Channel Assessment (1) • CCA reports the state of the medium (idle/busy) and is used to initiate the frame reception and to avoid transmitting when the channel is busy • Different methods: • CCA mode 1: Energy above threshold • CCA mode 2: Carrier sense only [802.11] • CCA mode 3: Carrier sense with energy above threshold [802.11] • CCA mode 4: Carrier sense only [802.11b] • CCA mode 5: Carrier sense with energy above threshold [802.11b] • CCA mode 4 and 5 are respectively equivalent to CCA mode 2 and 3 but for high rate PHY signal detected (802.11b) 31

32. Clear Channel Assessment (2) • Energy detection threshold is a function of Tx power • Tx power > 100 mW: -80 dBm (-76 dBm for 802.11b) • Tx power > 50 mW: -76 dBm (-73 dBm for 802.11b) • Tx power  50 mW: -70 dBm • The CCA detection time is set to 15 s • The backoff slot time is set to 20 s. The CCA should thus detect a signal which started up to 15 s before the end of the slot • If a correct PLCP header is received, CCA should be busy for the full (intended) duration of the frame as indicated by the PLCP Length field, even if a loss of carrier sense occurs in the middle of the reception 32

33. PLCP Transmit procedure 33

34. * * * * Transmit State Machine *802.11b only 34

35. PLCP Receive procedure 35

36. *802.11b only * Receive State Machine 36