CCK (Complementary Code Keying)

Modulation scheme for 5.5 & 11 Mbps data rates in 2.4 GHz band.

Complementary codes comprise a pair of equal finite length sequences having the property that the number of pairs of like elements with any given separation in one series is equal to the no. of pairs of unlike elements with the same separation in the other.

 

 

 

 

Given a pair of complementary sequences with a_i & b_i elements, where I = 1,2,…n, the respective autocorrelative series are given by

Ideally, the two sequences {a_i} & {b_j} are complementary if

c_j + dj = 0 j =/= 0

c_j + dj = 2n j == 0

 

IEEE 802.11 CCK codes are polyphase complementary codes. A polyphase complementary code is a sequence having complementary properties, the elements of which have phase parameters. The code set defined in the IEEE 802.11 high rate draft is standard is a complex complementary code set i.e. it’s elements are from complex set {1,-1,j, -j}..

 

The IEEE CCK has code length  of 8 & chipping rate of 11 Mchips/s. The 8 complex chips comprise a single symbol. Symbol rate is therefore 1.375 MS/s.

The 8 bit CCK code words are derived from following formula-

 

 

An Example:

Lets say the input is a data stream d6,d5,…d0= 1 0 1 1 0 1 0 1

The complex CCK codeword will be, as per the formula & tables above, given by-

 

The CCK modulation has been employed in the 802.11 because of following reasons-

It has periodic auto-correlative vector sum is zero everywhere except at the zero shift.

These codes also possess good Euclidean distance properties for yielding low bit error rates in multipath environments.

Occupy same BW as DBPSK & DQPSK to deliver 11 Mbps

 

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