Code-division multiple access: Difference between revisions

Revision as of 03:02, 9 December 2021

Revision as of 11:03, 23 December 2021

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* The [[Qualcomm]] standard [[IS-95]], marketed as cdmaOne.

* The Qualcomm standard [[IS-2000]], known as CDMA2000, is used by several mobile phone companies, including the [[Globalstar]] network.{{refn|group=nb|Globalstar uses elements of CDMA, [[time-division multiple access|TDMA]] and [[FDMA]] combining with satellite multiple beam antennas.<ref>M. Mazzella, M. Cohen, D. Rouffet, M. Louie and K. S. Gilhousen, "Multiple access techniques and spectrum utilisation of the GLOBALSTAR mobile satellite system," Fourth IEE Conference on Telecommunications 1993, Manchester, UK, 1993, pp. 306-311.</ref>}}

* The [[UMTS]] 3G mobile phone standard, which uses [[W-CDMA]].{{refn|group=nb|The UMTS networks and other CDMA based systems are also known as a kind of ''interference-limited'' systems.<ref>{{cite book|editor-last1=Holma|editor-first1=H.|editor-last2=Toskala|editor-first2=A.|date=2007|url=https://books.google.com/books?id=7m-MnwW_o7AC&q=lte+umts+are+interference+limited&pg=PT439|title=WCDMA for UMTS: HSPA Evolution and LTE|publisher=[[Wiley (publisher)|John Wiley & Sons]]|isbn=9781119991908}}</ref><ref>{{cite book|editor-last1=Laiho|editor-first1=J.|editor-last2=Wacker|editor-first2=A.|editor-last3=Novosad|editor-first3=T.|date=2002|url=https://books.google.com/books?id=9RE32TlXZBQC&q=is+umts+interference+limited+systems&pg=PA303|title=Radio Network Planning and Optimisation for UMTS (Vol. 2)|___location=New York|publisher=[[Wiley (publisher)|John Wiley & Sons]]|page=303|isbn=9780470031391}}</ref> This relates to the properties of the CDMA technology: all users operate in the same frequency range that impacts [[Signal-to-interference-plus-noise ratio|SINR]] and, hence, reduces coverage and capacity.<ref name="Walke-Seidenberg-Althoff_2003">{{cite book |title=UMTS: The Fundamentals |author-first1=Bernhard H. |author-last1=Walke |author-link1=Bernhard Walke |author-first2=Peter |author-last2=Seidenberg |author-first3=Marc Peter |author-last3=Althoff |translator-first=Hedwig Jourdan |translator-last=von Schmoeger |date=March 2003 |orig-date=2001 |publisher=[[John Wiley & Sons, Ltd]] |edition=First English |isbn=0-470-84557-0 |pages=18-1918–19 |url=https://books.google.com/books?id=KRlUvPWeTYQC&pg=PA18}} (NB. Based on the 2001 German edition.)</ref>}}

* CDMA has been used in the '''OmniTRACS''' satellite system for transportation [[logistics]].

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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/profile/Felix-Antreich/publication/331065214_Pure_Pilot_Signals_How_Short_can_we_Choose_GNSS_Spreading_Codes/links/5eb33c7592851cbf7fada2d0/Pure-Pilot-Signals-How-Short-can-we-Choose-GNSS-Spreading-Codes.pdf331065214|journal=IONIon GNSS ITMItm 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===