Complementary code keying
Encyclopedia
Complementary Code Keying (CCK) is a modulation
scheme used with wireless network
s (WLANs) that employ the IEEE 802.11b specification. In 1999, CCK was adopted to supplement the Barker code in wireless digital networks to achieve data rate higher than 2 Mbit/s at the expense of shorter distance. This is due to the shorter chipping sequence in CCK (8 bits versus 11 bits in Barker code) that means less spreading to obtain higher data rate but more susceptible to narrowband interference resulting in shorter radio transmission range. Beside shorter chipping sequence, CCK also has more chipping sequences to encode more bits (4 chipping sequences at 5.5 Mbit/s and 64 chipping sequences at 11 Mbps) increasing the data rate even further. The Barker code, however, only has a single chipping sequence.
The complementary codes first discussed by Golay were pairs of binary complementary codes and he noted that when the elements of a code of length N were either [-1 or 1] it followed immediately from their definition that the sum of their respective autocorrelation sequences was zero at all points except for the zero shift where it is equal to K*N. (K being the number of code words in the set).
CCK is a variation and improvement on, M-ary Orthogonal Keying and utilises ‘polyphase complementary codes’. They were developed by Lucent Technologies and Harris Semiconductor and were adopted by the 802.11 working group in 1998. CCK is the form of modulation utilised when 802.11b operates at either 5.5 or 11 Mbit/s. CCK was selected over competing modulation techniques as it utilised approximately the same bandwidth and could utilise the same preamble and header as pre-existing 1 and 2 Mbit/s wireless networks and thus facilitated interoperability.
Polyphase complementary codes, first proposed by Sivaswamy, 1978, are codes where each element is a complex number of unit magnitude and arbitrary phase, or more specifically for 802.11b is one of [1,-1, j,-j].
Networks using the 802.11g specification employ CCK when operating at 802.11b speeds.
, where each chip is a complex QPSK bit-pair at a chip rate of 11Mchip/s. In 5.5 Mbit/s and 11 Mbit/s modes respectively 4 and 8 bits are modulated onto the eight chips of the symbol c0,...,c7, where
and
are determined by the bits being modulated.
In other words, the phase change
is applied to every chip, 
is applied to all even code chips (starting with 
), 
is applied to the first two of every four chips, and 
is applied to the first four of the eight chips. Therefore, it can also be viewed as a form of generalized Hadamard transform
encoding.
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
scheme used with wireless network
Wireless network
Wireless network refers to any type of computer network that is not connected by cables of any kind. It is a method by which homes, telecommunications networks and enterprise installations avoid the costly process of introducing cables into a building, or as a connection between various equipment...
s (WLANs) that employ the IEEE 802.11b specification. In 1999, CCK was adopted to supplement the Barker code in wireless digital networks to achieve data rate higher than 2 Mbit/s at the expense of shorter distance. This is due to the shorter chipping sequence in CCK (8 bits versus 11 bits in Barker code) that means less spreading to obtain higher data rate but more susceptible to narrowband interference resulting in shorter radio transmission range. Beside shorter chipping sequence, CCK also has more chipping sequences to encode more bits (4 chipping sequences at 5.5 Mbit/s and 64 chipping sequences at 11 Mbps) increasing the data rate even further. The Barker code, however, only has a single chipping sequence.
The complementary codes first discussed by Golay were pairs of binary complementary codes and he noted that when the elements of a code of length N were either [-1 or 1] it followed immediately from their definition that the sum of their respective autocorrelation sequences was zero at all points except for the zero shift where it is equal to K*N. (K being the number of code words in the set).
CCK is a variation and improvement on, M-ary Orthogonal Keying and utilises ‘polyphase complementary codes’. They were developed by Lucent Technologies and Harris Semiconductor and were adopted by the 802.11 working group in 1998. CCK is the form of modulation utilised when 802.11b operates at either 5.5 or 11 Mbit/s. CCK was selected over competing modulation techniques as it utilised approximately the same bandwidth and could utilise the same preamble and header as pre-existing 1 and 2 Mbit/s wireless networks and thus facilitated interoperability.
Polyphase complementary codes, first proposed by Sivaswamy, 1978, are codes where each element is a complex number of unit magnitude and arbitrary phase, or more specifically for 802.11b is one of [1,-1, j,-j].
Networks using the 802.11g specification employ CCK when operating at 802.11b speeds.
Mathematical Description
The CCK modulation used by 802.11b transmits data in symbols of eight chipsDirect-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...
, where each chip is a complex QPSK bit-pair at a chip rate of 11Mchip/s. In 5.5 Mbit/s and 11 Mbit/s modes respectively 4 and 8 bits are modulated onto the eight chips of the symbol c0,...,c7, where
and
are determined by the bits being modulated.In other words, the phase change
is applied to every chip, is applied to all even code chips (starting with ), is applied to the first two of every four chips, and is applied to the first four of the eight chips. Therefore, it can also be viewed as a form of generalized Hadamard transformHadamard transform
The Hadamard transform is an example of a generalized class of Fourier transforms...
encoding.