Code-division multiple access: Difference between revisions

Revision as of 11:40, 15 November 2021

Revision as of 19:44, 27 November 2021

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It can be also used as a channel or medium access technology, like [[ALOHA]] for example or as a permanent pilot/signalling channel to allow users to synchronize their local oscillators to a common system frequency, thereby also estimating the channel parameters permanently.

In these schemes, the message is modulated on a longer spreading sequence, consisting of several chips (0es and 1es). Due to their very advantegous auto- and crosscorrelation characteristics, these spreading sequences arehave also been used for radar applications sincefor many decades, where they are called Barker-Codes (with a very short sequence length of typically 8 to 32).

For space-based communication applications, CDMA has been used for many decades due to the large path loss and Doppler shift caused by satellite motion. Due to these effects, in those applications neither FDMA nor TDMA is typically used as a single modulation. CDMA is often used with BPSK in its simplest form, but can be combined with any modulation scheme like (in advanced cases) QAM or OFDM, which typically makes it very robust and efficient (and equipping them with accurate ranging capabilities, which is difficult without CDMA). Other schemes use subcarriers based on binary offset carrier (BOC), which is inspired by Manchester codes and enable a larger gap between the virtual center frequency and the subcarriers, which is not the case for OFDM subcarriers. Many carriers (such as [[AT&T Corporation|AT&T]] and [[Verizon (mobile network)|Verizon]]) will shut down 3G CDMA networks in 2022.<ref>{{Cite web|last=Johnson|first=Allison|date=2021-03-30|title=Verizon will shutter its 3G CDMA by the end of 2022|url=https://www.theverge.com/2021/3/30/22358628/verizon-3g-cdma-shutdown-date-2022|access-date=2021-10-09|website=The Verge|language=en}}</ref>