CCK is a variation on M-ary Orthogonal Keying modulation, which uses I/Q modulation architecture with complex symbol structures. CCK allows the 802.11b for multi-channel operation in the 2.4 GHz band using the existing 802.11 DSSS channel structure scheme. The spreading employs the same chipping rate and spectrum shape as the 802.11 Barker’s code word. Spreading functions, allows three non-interfering channels in the 2.4 to 2.483 GHz band.
CCK used in 802.11b is an M-ary Orthogonal Keying modulation where one of M unique (nearly orthogonal) signal codewords is chosen for transmission. The spread function for CCK in 802.11b is chosen from a set of M nearly orthogonal vectors by the data word. CCK uses one vector from a set of 64 complex (QPSK) vectors for the symbol and thereby modulates 6 bits (one of 64) on each 8 chips spreading code symbol. Two more bits are sent by QPSK modulating the whole code symbol. This results in modulating 8 bits onto each symbol. In the 802.11b, the formula that defines the CCK codewords has 4 phase terms. The first of them modulates all of the chips and this is used for the QPSK rotation of the whole code vector. The second modulates every odd chip, the third modulates every odd pair of chips and the forth modulates every odd quad of chips.
Walsh functions used for the M-ary Bi-Orthogonal keying (MBOK) modulation are the most well known orthogonal BPSK vector set. To transmit enough bits per symbol, the MBOK modulation is used independently on the I and Q channels of the waveform effectively doubling the data rate. CCK on the other hand uses a complex set of Walsh/Hadamard functions known as Complementary Codes.
Walsh/Hadamard properties are similar to Walsh functions but are complex, that is, more than two phase, while still being nearly orthogonal. With complex code symbols, it is not possible to independently transmit simultaneous code symbols without suffering amplitude modulation. Since the set of complementary codes is more extensive, however, we have a larger set of nearly orthogonal codes to pick from and can get the same number of bits transmitted per symbol without simultaneous transmission of symbols.
The multi-path performance of CCK used in the 802.11b is better than MBOK due to the lack of cross rail interference. For MBOK, there are 8 BPSK chips that have a maximum vector space of 256 code words of which it is possible to find sets of 8 that are orthogonal. Two independent BPSK vector sets are selected for the orthogonal I and Q channels which modulate 3 bits on each. In the 802.11b standard two additional bits are used to BPSK modulate each of the spreading code vectors. For CCK, there are 65536 possible code words, and sets of 64 that are nearly orthogonal. This is because it really takes 16 bits to define each code vector. To get a half data rate version, a subset of 4 of the 64 vectors having superior coding distance is used.
CCK that is used in the 802.11b standard suffers less from multi-path distortion in the form of cross coupling (of I and Q channel information) than MBOK. The information in CCK is encoded directly onto complex chips, which cannot be cross-couple corrupted by multi-path since each channel finger has an Ae jq distortion. A single channel path gain-scales and phase-rotates the signal. A gain scale and phase rotation of a complex chip still maintains I/Q orthogonal. This superior encoding technique avoids the corruption resulting from encoding half the information on the I-channel and the other half on the Q-channel, as in MBOK, which easily cross-couple corrupts with the multipath’s Ae jq phase rotation.
For 1 Mbps in the 802.11b the signal is modulated BPSK by one bit per symbol and then spread by BPSK modulating with the 11 chip Barker code at 11 Mcps. For 2 Mbps in the 802.11b the signal is QPSK modulated by two bits per symbol and then BPSK spread as before. In the 802.11b for the 5.5 Mbps CCK mode, the incoming data is grouped into 4 bits nibbles where 2 of those bits select the spreading function out of the set of 4 while the remaining 2 bits QPSK modulate the symbol. The spreading sequence then DQPSK modulates the carrier by driving the I and Q modulators. To make 11 Mbps CCK modulation, the input data is grouped into 2 bits and 6 bits. The 6 bits are used to select one of 64 complex vectors of 8 chip length for the symbol and the other 2 bits DQPSK modulate the entire symbol. The chipping rate is maintained at 11 Mcps for all modes.
The signal acquisition scheme for 802.11 uses a specific preamble and header using the 1 Mbps modulation and has provision for sending the payload at different rates. The packet frame structure and protocol of 802.11 is much like 802.3 Ethernet, however it must operate wirelessly in a harsh RF environment. This means that the signal levels may become corrupted and subject to multi-path. Signal acquisition and synchronization of the preamble and header are critical. In the 802.11b the preamble and header consists of six fields. They are: Preamble, SFD, Signal (rate), Service, Length and CRC. The header takes 48 bits, and the total length of the acquisition sequence is 192 ms. The preamble and header is modulated using the 1 Mbps modulation rate and is scrambled with a self-synchronizing scrambler. The high rate scheme will use this acquisition sequence, which already has a rate field that can be programmed for 1, 2, 5.5 or 11 Mbps.
The 802.11 packet transmission protocol is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). This differs from “wired” Ethernet, which uses collision detection. Radios can’t detect collisions, therefore they use collision avoidance using a listen before talk and random back off deferral mechanism. Since all stations use the same acquisition sequence at the lowest basic rate, all stations can see the traffic and process the signals at the appropriate rate. If legacy 1 and 2 Mbps stations receive the packet header, but are not capable of processing the higher rate, they can still defer the medium based on knowing that an 802.11 signal has been sensed and knowing the length of time it will be on the air.
To insure that the modulation has the same bandwidth as the existing 802.11 DS modulation, the chipping rate is kept at 11 Mcps while the symbol rate is increased to 1.375 MSps. This accounts for the shorter symbols and makes the overall bit rate 11 Mbps. This approach makes system interoperability with the 802.11 preamble and header much easier. The spread rate remains constant and only the data rate changes and the spectrum of the CCK waveform is same as the legacy 802.11 waveform.
CCK allows interoperability with 802.11 DSSS. CCK is an M-ary Orthogonal Keying modulation that uses one vector from a set of 64 complex (QPSK) vectors for the symbol and thereby modulates 6 bits (one of 64) on each 8 chips spreading code symbol. Two more bits are sent by QPSK modulating the whole code symbol. This results in modulating 8 bits onto each symbol CCK of the 802.11b codewords has 4 phase terms. Information encoded directly onto complex chips which cannot be cross-couple corrupted by multi-path. CCK uses a complex set of Walsh/Hadamard functions known as Complementary Codes.