CCK (complementary code keying)

Complementary code keying (CCK) is a modulation scheme used in wireless networks such as Wi-Fi. CCK is an improvement on the original direct sequence spread spectrum (DSSS) modulation used in the 802.11b standard. CCK is used to provide higher data rates and improve the reliability of wireless communications.

CCK works by modulating the amplitude and phase of a carrier signal using a set of complementary codes. These complementary codes are sequences of +1s and -1s that are designed to cancel out each other's interference when added together. The complementary codes are chosen to have specific mathematical properties that allow them to be added and subtracted without interfering with each other.

In CCK, the complementary codes are used to modulate data onto the carrier signal. The data is first encoded using a convolutional code to add redundancy and improve error correction capabilities. The encoded data is then mapped onto the complementary codes using a mapping table. The mapping table determines which complementary codes to use for each symbol in the encoded data.

CCK supports data rates of up to 11 Mbps in the 2.4 GHz band. It uses a 4-bit symbol encoding scheme, which allows for 16 different complementary code combinations. Each symbol represents two bits of data, so a 4-symbol sequence can transmit 8 bits of data. CCK uses a combination of modulation techniques to achieve the high data rates. These include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), and differential quadrature phase-shift keying (DQPSK).

BPSK is used to transmit data at 1 Mbps. BPSK modulates the carrier signal by changing its phase by 180 degrees to represent a binary 1 or 0. QPSK is used to transmit data at 2 Mbps. QPSK modulates the carrier signal by changing its phase by 90 degrees to represent two bits of data. DQPSK is used to transmit data at 5.5 Mbps and 11 Mbps. DQPSK is similar to QPSK, but it uses differential encoding to improve the reliability of the transmission.

CCK uses a spreading code to spread the signal over a wider frequency range. The spreading code is a pseudo-random sequence of bits that is multiplied with the data signal to spread it over a wider frequency range. The spreading code is unique for each transmission, which makes it difficult for an eavesdropper to intercept the signal.

CCK also uses a technique called interleaving to improve the reliability of the transmission. Interleaving rearranges the order of the data symbols before transmission so that if a burst of errors occurs, they are spread out over multiple symbols rather than being concentrated in a single block. This makes it easier for the error correction code to correct the errors.

CCK provides several advantages over the original DSSS modulation scheme. It provides higher data rates, improved reliability, and better resistance to interference. CCK is also backward compatible with DSSS, which allows it to work with existing 802.11b networks.

In conclusion, complementary code keying (CCK) is a modulation scheme used in wireless networks such as Wi-Fi. CCK uses a set of complementary codes to modulate data onto a carrier signal. It supports data rates of up to 11 Mbps and uses a combination of modulation techniques to achieve these rates. CCK also uses spreading codes and interleaving to improve the reliability of the transmission. Overall, CCK provides significant improvements over the original DSSS modulation scheme and is widely used in wireless networks today.