Editing Channel access method (section)

===Code-division multiple access and spread spectrum multiple access===
The [[code-division multiple access]] (CDMA) scheme is based on [[spread spectrum]], meaning that a wider radio channel bandwidth is used than the data rate of individual bit streams requires, and several message signals are transferred simultaneously over the same carrier frequency, utilizing different spreading codes. Per the [[Shannon–Hartley theorem]], the wide bandwidth makes it possible to send with a [[signal-to-noise ratio]] of much less than 1 (less than 0&nbsp;dB), meaning that the transmission power can be reduced to a level below the level of the noise and [[co-channel interference]] from other message signals sharing the same frequency range.

One form is [[direct-sequence CDMA]] (DS-CDMA), based on [[direct-sequence spread spectrum]] (DSSS), used for example in [[3G]] cell phone systems. Each information bit (or each symbol) is represented by a long code sequence of several pulses, called chips. The sequence is the spreading code, and each message signal (for example each phone call) uses a different spreading code.

Another form is [[frequency-hopping CDMA]] (FH-CDMA), based on [[frequency-hopping spread spectrum]] (FHSS), where the channel frequency is changed rapidly according to a sequence that constitutes the spreading code. As an example, the [[Bluetooth]] communication system is based on a combination of frequency-hopping and either CSMA/CA statistical time-division multiplexing communication (for [[data communication]] applications) or TDMA (for audio transmission). All nodes belonging to the same user (to the same [[piconet]]) use the same frequency hopping sequence synchronously, meaning that they send on the same frequency channel, but CDMA/CA or TDMA is used to avoid collisions within the virtual personal area network (VPAN). Frequency-hopping is used by Bluetooth to reduce the cross-talk and collision probability between nodes in different VPANs.

Other techniques include OFDMA and [[multi-carrier code-division multiple access]] (MC-CDMA).