1. 802.11b PHY 83180 Wireless LANs Page 1 of 23 IEEE 802.11b WLAN Physical Layer Svetozar Broussev 16-Feb-2005

2. 802.11b PHY 83180 Wireless LANs Page 2 of 23 Outline Introduction Direct Sequence Spread Spectrum basics Modulation schemes used in IEEE 802.11b –DBPSK and DQPSK –CCK (Complementary Code Keying) –PBCC (Packet Binary Convolutional Coding) Data unit format (long and short) Other specifics –Output Power requirements –Frequency allocation

3. 802.11b PHY 83180 Wireless LANs Page 3 of 23 Introduction IEEE 802.11b standard is an extension to the 802.11 standard concerning the Direct Sequence Spread Spectrum (DSSS) physical layer. The goal of 802.11b is to provide higher data rate in addition to 1Mb/s and 2Mb/s, without increasing the occupied channel bandwidth. To achieve higher rate, 8-chip complementary code keying (CCK) modulation scheme is employed. –Optional short preamble code provides higher data throughput. –Optional mode replacing the CCK modulation with PBCC modulation is specified. Both standards (.11 and.11b) can co-exist in practice. 802.11b has higher data rate, higher complexity and lower BER performance compared to 802.11 standard.

4. 802.11b PHY 83180 Wireless LANs Page 4 of 23 DSSS basics The signal symbol is spread with a sequence in time domain The result of the spreading is a signal with wider bandwidth and smaller amplitude Less power density (sometimes could be even smaller than the noise level).

5. 802.11b PHY 83180 Wireless LANs Page 5 of 23 DSSS basics (2) Barker sequence is selected for IEEE 802.11 standard + - + + - + + + - - - This is the minimal sequence allowed by FCC. Properties of Barker code: –It has good autocorrelation properties –Provides coding gain of 10.4 dB –Robust against interferers and noise (10dB suppression) –Robust against time delay spread (echoes removal) Correlation of the Barker code with the receiver signal from the previous slide Correlation with echoes

6. 802.11b PHY 83180 Wireless LANs Page 6 of 23 Modulation schemes DBPSK and DQPSK is used for low bit rate transmission (1Mb/s and 2Mb/s, specified by IEEE 802.11). These modulations have to be supported in 802.11b, as well as other modification of the standard. Convolutional Code Keying (CCK) for data rate of 5.5 Mb/s and 11Mb/s. Packet Binary Convolutional Coding (PBCC) is an optional modulation scheme for the 802.11b standard. It achieves the same data rate of 5.5 Mb/s and 11Mb/s.

7. 802.11b PHY 83180 Wireless LANs Page 7 of 23 DBPSK and DQPSK Tables 1 and 2 present the encoding tables used for DBPSK and DQPSK. 1Mb/s and 2Mb/s are supported bit rates in 802.11 and 802.11b standard. Non-coherent detection can be used. DBPSK achieves the same bit error rate at lower SNR compared to DQPSK. Table 1. 1Mb/s DBPSK encoding table Table 2. 2Mb/s DQPSK encoding table

8. 802.11b PHY 83180 Wireless LANs Page 8 of 23 CCK Modulation CCK is M-ary Orthogonal Keying modulation, where M unique (nearly orthogonal) code words are chosen for transmission 8 chips = 1 symbol (11 for 802.11) CCK uses one vector from a set of 64 complex (QPSK) vectors -> modulates 6 bits. Two more bits are sent by QPSK modulating the whole symbol, resulting in total 8bits/symbol. => 11Mb/s For 5.5 Mb/s transmission, CCK picks up one from a set of 4 complex vector (2 bits). The other two bits are sent by the QPSK symbol. Better performance is expected, because the distance the 4 vectors can be maximized (1) – formula for deriving the CCK code words for 5.5 and 11Mb/s (4 th and 7 th chips are rotated by  to optimize the correlation properties and to minimize the dc offsets in the code) (1)

9. 802.11b PHY 83180 Wireless LANs Page 9 of 23 CCK 5.5 Mb/s modulation 8 complex chips (c0-c7) form one symbol, 11Mchips/s d0 and d1 encode  1 based on QPSK modulation d2 and d3 encode the chips using the (1) and (2) 8 chips transmit 4bits with rate 11Mchip/s -> 4x(11/8)=5.5Mb/s Table 3. 5.5Mb/s CCK encoding table (2)

10. 802.11b PHY 83180 Wireless LANs Page 10 of 23 CCK 11 Mb/s modulation d0 and d1 encode  1 based on DQPSK modulation The data bits (d2, d3), (d4, d5) and (d6, d7) encode  2,  3 and  4 respectively. The table is binary (not Grey) coded. 8 chips transmit 8bits with rate 11Mchip/s -> 8x(11/8)=11Mb/s Table 4. QPSK encoding table

11. 802.11b PHY 83180 Wireless LANs Page 11 of 23 PBCC modulation (optional) The packet binary convolutional coding (PBCC) is optional modulation scheme for 802.11b. Bit from the SERVICE field selects which modulation scheme is in use (CCK or PBCC). Incoming data is encoded with a binary convolutional code. The output of the BCC is encoded jointly onto I and Q channels. Mapping the constellation points depends on the cover sequence, which is 256bits pseudo-random sequence, known in the receiver.

12. 802.11b PHY 83180 Wireless LANs Page 12 of 23 PBCC modulation (2) The encoder block diagram is shown above, (3) is the generator matrix. The encoder should be in state zero at the beginning of each data unit frame. The encoder should be placed in known state at the end of each PPDU in order to increase the reliability of the last transmitted bits. One octet zeros are added at the end of the data stream, and they are removed in the receiver side. (3)

13. 802.11b PHY 83180 Wireless LANs Page 13 of 23 PHY protocol data unit formats Together with the actual data, the receiver needs additional information for correct reception. This service information includes the modulation in use, length of the transmission, data rate and other important parameters. The service information is transmitted first, before the actual data. Correct reception of this information is crucial for the whole transmission, thus the most robust modulation against noise is used i.e. DBSPK (1Mb/s) Two data units formats are defined in IEEE 802.11b – long and short.  Long data unit format – the same format as defined in 802.11. Assure compatibility between the two systems.  Short data unit format – optional format, included in order to increase the actual throughput of the system and to take advantage of the higher rate modulation.

14. 802.11b PHY 83180 Wireless LANs Page 14 of 23 Long data unit format The message is divided in three parts – Preamble, Header and Data. The Preamble and Header structure are the same as the ones in 802.11, however some specific for 802.11b information is included in the header. The data (PSDU) is transmitted at higher rate

15. 802.11b PHY 83180 Wireless LANs Page 15 of 23 Field description PLCP Preamble – 144bits transmitted at 1Mb/s (144  s) –Synchronization SYNC field – 128bits –Start Frame Delimiter (SFD) – 16bits PLCP Header – 48bits transmitted at 1Mb/s (48  s) –SIGNAL – 8bits –SERVICE – 8bits –LENGTH – 16bits –CRC – 16bits PSDU – actual data, transmitted at 2, 5.5, or 11Mb/s depending on the information in the SIGNAL field

16. 802.11b PHY 83180 Wireless LANs Page 16 of 23 Field description (2) SYNC – Synchronization field, contains 128 scrambled one’s. This field is provided to help the receiver with the synchronization. The initial state of the scrambler is [1101100]. The same SYNC is used in 802.11 standard. SFD – The Start Frame Delimiter indicates the start of PHY dependent parameters within the PLCP preamble. SFD shall be [1111 0011 1010 0000]. SIGNAL – 8 bits that indicate the modulation that will be used for transmission. By definition, the data rate is equal to SIGNAL field value multiplied by 100kb/s (for example X’0A’ correspond to 1Mb/s). SERVICE field (not in use in 802.11, reserved -> 0x00): – ‘0’ – not locked, ‘1’ – locked to the same oscillator; – ‘0’ – CCK modulation; ‘1’ – PBCC modulation; – length extension bit; the other bits are reserved for future use

17. 802.11b PHY 83180 Wireless LANs Page 17 of 23 Field description (3) LENGTH – 16bit integer that indicates the number of  s required to transmit the data. The maximum possible duration is 65536  s, which corresponds to ~720kb. Bit from the SERVICE field is used to remove the uncertainty when the transmission rate is above 8Mb/s. PLCP CRC (CCITT CRC-16) field – Cyclic Redundancy Code that protects SIGNAL, SERVICE and LENGTH fields. Since those fields are very important for the entire transmission, the redundancy code gives an opportunity for possible error detection. The frame check sequence calculations are done prior the data scrambling. PSDU – this is the actual data to be transmitted. The data rate (2, 5.5 or 11Mb/s) and the modulation are determined based on the information in the SIGNAL and SERVICE field

18. 802.11b PHY 83180 Wireless LANs Page 18 of 23 Short data format The short format is optional, the goal is to increase the system throughput Short Preamble; The header is transmitted with 2Mb/s; Same PSDU 192  s in the long format

19. 802.11b PHY 83180 Wireless LANs Page 19 of 23 Short data format (2) short SYNC – 56bits, scrambled zero’s, 1Mb/s. Initial state of the scrambler is [001 1011]. short SFD – 16bit [0x05CF] (reverse SFD from the long format). SFD is transmitted with 1Mb/s. short PLCP Header – the header is the same as for the long format, but it is transmitted with 2Mb/s. The price to be paid is the increased system complexity Systems using short PPDU format should support as well the long format -> complex realization. BENEFIT of the short format: 96  s corresponds to 1056 bits at transmission rate 11Mbit/s

20. 802.11b PHY 83180 Wireless LANs Page 20 of 23 Data scrambler and descrambler The Scrambler and descrambler are self- synchronized. No prior knowledge is needed. Scrambler (descrambler) polynomial is G(z)=z -7 +z -4 +1 ALL bits transmitted by DSSS PHY are scrambled The purpose of the scrambling is whitening the spectrum and minimizing the DC offsets scrambler descrambler

21. 802.11b PHY 83180 Wireless LANs Page 21 of 23 Output Power requirements Transmit Spectrum Mask Maximum output power for IEEE 802.11

22. 802.11b PHY 83180 Wireless LANs Page 22 of 23 Frequency allocation The chip rate is 11Mchips/s => the occupied bandwidth is 22 MHz. The frequency range is divided in three non-overlapping channels. The frequency allocation depends on the country and the geographic region. North America non-overlapping channel selection European non-overlapping channel selection

23. 802.11b PHY 83180 Wireless LANs Page 23 of 23 References [1]. T. Cooklev, “Wireless Communication Standards, A Study of 802.11, 802.15, and 802.16”, IEEE Press, 2004. [2]. “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, IEEE Std, 1999. [3]. “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band”, IEEE Std 802.11b-1999. [4]. “Tutorial: The 802.11 standard”, SAME conference, 2002. [5]. Jan Boer, “Direct Sequence Spread Spectrum Physical Layer Specification IEEE 802.11”, doc. IEEE P802.11- 96/49E.