Code-division multiple access: Difference between revisions

Revision as of 17:39, 13 February 2022

Revision as of 19:18, 5 April 2022

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Since each user generates MAI, controlling the signal strength is an important issue with CDMA transmitters. A CDM (synchronous CDMA), TDMA, or FDMA receiver can in theory completely reject arbitrarily strong signals using different codes, time slots or frequency channels due to the orthogonality of these systems. This is not true for asynchronous CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they will overwhelm it. This leads to a general requirement in any asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power-control scheme to tightly control each mobile's transmit power.

In 2019, schemes to precisely estimate the required length of the codes in dependence of Doppler and delay characteristics have been developed.<ref>{{Cite journal|last=Enneking, Antreich, Appel, Almeida|date=2019|title=Pure Pilot Signals: How short can we choose GNSS spreading codes?|url=https://www.researchgate.net/publication/331065214|journal=Ion GNSS Itm 2019<!---->|series=Proceedings of the 2019 International Technical Meeting of the Institute of Navigation|pages=925–935|doi=10.33012/2019.16737|isbn=978-0-936406-21-3|s2cid=86666944}}</ref> Soon after, machine learning based techniques that generate sequences of a desired length and spreading properties have been published as well. These are highly competitive with the classic Gold and Welch sequences. These are not generated by linear-feedback-shift-registers, but have to be stored in lookup tables.

===Advantages of asynchronous CDMA over other techniques===