Chipping Codes One of the desirable mathematical properties of PN codes is that it enables the receiver's PN code generator to very rapidly synchronise with the PN code in the received signal. This synchronisation is the first step in the de-spreading process. Fast synchronisation requires that the position of the code word can be quickly identified in a received signal, and this is achieved as a result of the low auto-correlation property of the Barker codes. Another benefit of low auto-correlation is that the receiver will reject signals that are delayed by more than one chip period. This helps to make the data link robust against multipath interference, which will be discussed in the Section "RF Signal Propagation and Losses, p. 112". A second key property of chipping codes that is important in applications where interference between multiple transmitters must be avoided, for example in mobile telephony, is low cross-correlation. This property reduces the chance that a correlator using one PN code will experience interference from a signal using a different code (i.e. that it will incorrectly decode a noisy signal that was encoded using a different chipping code). Ideally codes in use in this type of multiple access application should have zero cross-correlation, a property of the orthogonal codes used in CDMA (Section "Code Division Multiple Access, p. 94"). Code orthogonality for multiple access control is not required for wireless networking applications, such as IEEE 802.11 networks, as these standards use alternative methods to avoid conflict between overlapping transmitted signals from multiple users, which are described in the Section "Wireless Multiplexing and Multiple Access Techniques, p. 87". Complementary Code Keying An alternative to using a single chipping code to spread every bit in the input data stream is to use a set of spreading codes and to select one code from the set depending on the values of a group of input data bits. This scheme is known as complementary code keying (CCK). CCK was pro