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

802.11b PHY83180 Wireless LANs Page 1 of 23

IEEE 802.11bWLAN Physical Layer

Svetozar Broussev

16-Feb-2005


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


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.


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).


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


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.


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


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 (4th and 7th chips are rotated by to optimize the correlation properties and to minimize the dc offsets in the code)

1 2 3 4 1 3 4 1 2 4( ) ( ) ( ){ , , ,j j jc e e e },,,, 1213132141 )()()()( eeeee jjjj

(1)


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

2/)xd2(2 ππ 03 πxd34

Table 3. 5.5Mb/s CCK encoding table

(2)


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


PBCC modulation (optional)

• The packet binary convolutional coding (PBCC) is optional modulation scheme for 802.11b. Bit <b3> 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.


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)


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.


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


Field description• PLCP Preamble – 144bits transmitted at 1Mb/s (144s)

– Synchronization SYNC field – 128bits – Start Frame Delimiter (SFD) – 16bits

• PLCP Header – 48bits transmitted at 1Mb/s (48s)– 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


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):– <b2> ‘0’ – not locked, ‘1’ – locked to the same oscillator;

– <b3> ‘0’ – CCK modulation; ‘1’ – PBCC modulation;

– <b7> length extension bit; the other bits are reserved for future use


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 <7> 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


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

192s in the long format


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: 96s corresponds to 1056 bits at transmission rate 11Mbit/s


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


Output Power requirements

Transmit Spectrum Mask

Maximum output power for IEEE 802.11


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


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.

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