Content deleted Content added
Tags: Mobile edit Mobile web edit |
Citation bot (talk | contribs) Removed URL that duplicated identifier. Removed access-date with no URL. Removed parameters. | Use this bot. Report bugs. | Suggested by Headbomb | Linked from Wikipedia:WikiProject_Academic_Journals/Journals_cited_by_Wikipedia/Sandbox | #UCB_webform_linked 451/1032 |
||
| (151 intermediate revisions by 86 users not shown) | |||
Line 1:
{{Short description|Channel access method used by various radio communication technologies}}
{{About|a channel access method|the mobile phone technology referred to as CDMA|cdmaOne|and|CDMA2000}}
{{Multiplex_techniques}}
{{Confused|Carrier-sense multiple access}}
'''Code-division multiple access''' ('''CDMA''') is a [[channel access method]] used by various [[radio]] communication technologies. CDMA is an example of [[channel access method|multiple access]], where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see [[bandwidth (signal processing)|bandwidth]]). To permit this without undue interference between the users, CDMA employs [[spread spectrum]] technology and a special coding scheme (where each transmitter is assigned a code).<ref name=
CDMA optimizes the use of available bandwidth as it transmits over the entire frequency range and does not limit the user's frequency range.
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 [[Chip_(CDMA)|chips]] (0s and 1s). Due to their very advantageous auto- and crosscorrelation characteristics, these spreading sequences have also been used for radar applications for many decades, where they are called [[Barker code]]s (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. CDMA is often used with [[binary phase-shift keying]] (BPSK) in its simplest form, but can be combined with any modulation scheme like (in advanced cases) [[quadrature amplitude modulation]] (QAM) or [[orthogonal frequency-division multiplexing]] (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 modulation]] (BOC modulation), which is inspired by [[Manchester code]]s and enable a larger gap between the virtual center frequency and the subcarriers, which is not the case for OFDM subcarriers.
==History==
The technology of code-division multiple access channels has long been known.
===United States===
In the US, one of the earliest descriptions of CDMA can be found in the summary report of Project Hartwell on "The Security of Overseas Transport", which was a summer research project carried out at the [[Massachusetts Institute of Technology]] from June to August 1950.<ref name=Scholtz_1982May_IEEE>{{cite journal| author=Robert A. Scholtz| date=May 1982| title=The Origins of Spread-Spectrum Communications| journal=[[IEEE Transactions on Communications]]| volume=30| issue=5| pages=822–854| doi=10.1109/TCOM.1982.1095547}}</ref> Further research in the context of [[Radio jamming|jamming]] and [[Electronic counter-countermeasure|anti-jamming]] was carried out in 1952 at [[MIT_Lincoln_Laboratory|Lincoln Lab]].<ref name=Price_Shannon_19820728>{{cite web| author=Robert Price| date=28 July 1982| url=https://ethw.org/Oral-History:Claude_E._Shannon| title=Oral-History: Claude E. Shannon| publisher=[[Engineering and Technology History Wiki]]| access-date=30 January 2022}}</ref>
===Soviet Union===
In the [[Soviet Union]] (USSR), the first work devoted to this subject was published in 1935 by [[Dmitry Vasiliyevich Ageev|Dmitry Ageev]].<ref>{{cite journal|last=Ageev|first=D. V.|title=Bases of the Theory of Linear Selection. Code Demultiplexing|journal=Proceedings of the Leningrad Experimental Institute of Communication|year=1935|pages=3–35}}</ref> It was shown that through the use of linear methods, there are three types of signal separation: frequency, time and compensatory.{{clarify|date=August 2020|reason=No such thing.}} The technology of CDMA was used in 1957, when the young military radio engineer [[Leonid Kupriyanovich]] in Moscow made an experimental model of a wearable automatic mobile phone, called LK-1 by him, with a base station.<ref>{{Cite patent | country = Soviet Union
| number = 115494 | title = Устройства вызова и коммутации каналов радиотелефонной связи (Devices for calling and switching radio communication channels) | pubdate = 1957-11-04 | inventor = Куприянович (Leonid Kupriyanovich) | url = https://patents.su/7-115494-ustrojjstva-vyzova-i-kommutacii-kanalov-radiotelefonnojj-svyazi.html}}</ref> LK-1 has a weight of 3 kg, 20–30 km operating distance, and 20–30 hours of battery life.<ref>''[[Nauka i Zhizn]]'' 8, 1957, p. 49.</ref><ref>''Yuniy technik'' 7, 1957, p. 43–44.</ref> The base station, as described by the author, could serve several customers. In 1958, Kupriyanovich made the new experimental "pocket" model of mobile phone. This phone weighed 0.5 kg. To serve more customers, Kupriyanovich proposed the device, which he called "correlator."<ref>''Nauka i Zhizn'' 10, 1958, p. 66.</ref><ref>''[[Tekhnika Molodezhi]]'' 2, 1959, p. 18–19.</ref> In 1958, the USSR also started the development of the "[[Altai (mobile telephone system)|Altai]]" national civil mobile phone service for cars, based on the Soviet MRT-1327 standard. The phone system weighed {{convert|11|kg|abbr=on}}. It was placed in the trunk of the vehicles of high-ranking officials and used a standard handset in the passenger compartment. The main developers of the Altai system were VNIIS (Voronezh Science Research Institute of Communications) and GSPI (State Specialized Project Institute). In 1963 this service started in Moscow, and in 1970 Altai service was used in 30 USSR cities.<ref>{{cite web|url=http://englishrussia.com/2006/09/18/first-russian-mobile-phone/|title=First Russian Mobile Phone|date=September 18, 2006}}</ref>
== Uses ==
[[File:Au CDMA 1X WIN W31SAII gravelly silver expansion.jpg|thumb|A CDMA2000 [[mobile phone]]]]
* Synchronous CDM (code-division 'multiplexing', an early generation of CDMA) was implemented in the [[Global Positioning System]] (GPS). This predates and is distinct from its use in [[mobile phone]]s.
* 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=18–19 |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]].
==Steps in CDMA modulation=
CDMA is a spread-spectrum multiple-access technique. A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a [[Pseudorandom binary sequence|pseudo-random code]] in the time ___domain that has a narrow [[ambiguity function]] in the frequency ___domain, unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is combined by bitwise [[XOR#Bitwise operation|XOR]] (exclusive OR) with the faster code. The figure shows how a spread-spectrum signal is generated. The data signal with pulse duration of <math>T_b</math> (symbol period) is XORed with the code signal with pulse duration of <math>T_c</math> (chip period). (Note: [[Bandwidth (signal processing)|bandwidth]] is proportional to <math>1/T</math>, where <math>T</math> = bit time.) Therefore, the bandwidth of the data signal is <math>1/T_b</math> and the bandwidth of the spread spectrum signal is <math>1/T_c</math>. Since <math>T_c</math> is much smaller than <math>T_b</math>, the bandwidth of the spread-spectrum signal is much larger than the bandwidth of the original signal. The ratio <math>T_b/T_c</math> is called the spreading factor or processing gain and determines to a certain extent the upper limit of the total number of users supported simultaneously by a base station.<ref name="ref 1"/><ref name="ref 2"/>
[[File:Generation of CDMA.svg|thumb|500px|center|Generation of a CDMA signal]]
Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance occurs when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by [[Cross-correlation|correlating]] the received signal with the locally generated code of the desired user. If the signal matches the desired user's code, then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal, the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as [[cross-correlation]]. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference.<ref name="ref 3">{{cite book| title= Digital Communications: Fundamentals and Applications, 2nd ed.| year=2014|last1=Sklar|first1=Bernard|last2=Ray|first2=Pabitra K.}}</ref><ref name="ref 4">{{cite book| title=Wireless Communications, 2nd ed.| year=2010|last1=Molisch|first1=Andreas}}</ref>
An analogy to the problem of multiple access is a room (channel) in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but other languages are perceived as [[noise]] and rejected. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate.
In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).
==Code-division multiplexing (synchronous CDMA)==
The digital modulation method is analogous to those used in simple radio transceivers. In the analog case, a low-frequency data signal is time-multiplied with a high-frequency pure sine-wave carrier and transmitted. This is effectively a frequency convolution ([[Wiener–Khinchin theorem]]) of the two signals, resulting in a carrier with narrow sidebands. In the digital case, the sinusoidal carrier is replaced by [[Walsh function]]s. These are binary square waves that form a complete orthonormal set. The data signal is also binary and the time multiplication is achieved with a simple XOR function. This is usually a [[Gilbert cell]] mixer in the circuitry.
Synchronous CDMA exploits mathematical properties of [[orthogonality]] between [[coordinate vector|vectors]] representing the data strings. For example, the binary string ''1011'' is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their [[dot product]], by summing the products of their respective components (for example, if '''u''' = (''a'', ''b'') and '''v''' = (''c'', ''d''), then their dot product '''u'''·'''v''' = ''ac'' + ''bd''). If the dot product is zero, the two vectors are said to be ''orthogonal'' to each other. Some properties of the dot product aid understanding of how [[W-CDMA]] works. If vectors '''a''' and '''b''' are orthogonal, then <math>\mathbf{a}\cdot\mathbf{b} = 0</math> and:
:<math>
\mathbf{a}\cdot(\mathbf{a} + \mathbf{b}) = \|\mathbf{a}\|^2,\ \text{since}\ \mathbf{a}\cdot\mathbf{a} + \mathbf{a}\cdot\mathbf{b} = \|\mathbf{a}\|^2 + 0,
Line 53 ⟶ 67:
[[Image:Cdma orthogonal signals.png|thumb|right|An example of 4 mutually orthogonal digital signals]]
Start with a set of vectors that are mutually [[orthogonality|orthogonal]]. (Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from [[Walsh matrix|Walsh matrices]].) An example of orthogonal functions is shown in the adjacent picture
Each user is associated with a different code, say '''v'''. A 1 bit is represented by transmitting a positive code '''v''', and a 0 bit is represented by a negative code '''−v'''. For example, if '''v''' = (''v''<sub>0</sub>, ''v''<sub>1</sub>) = (1, −1) and the data that the user wishes to transmit is (1, 0, 1, 1), then the transmitted symbols would be
Line 115 ⟶ 129:
|}
Further, after decoding, all values greater than 0 are interpreted as 1, while all values less than zero are interpreted as 0. For example, after decoding, data0 is (2, −2, 2, 2), but the receiver interprets this as (1, 0, 1, 1). Values of exactly 0
Assume signal0 = (1, −1, −1, 1, 1, −1, 1, −1) is transmitted alone. The following table shows the decode at the receiver:
Line 145 ⟶ 159:
|}
When the receiver attempts to decode the signal using sender1's code, the data is all zeros
==Asynchronous CDMA==
{{See also|Direct-sequence spread spectrum|
When mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, a different approach is required. Since it is not mathematically possible to create signature sequences that are both orthogonal for arbitrarily random starting points and which make full use of the code space, unique "pseudo-random" or "pseudo-noise"
All forms of CDMA use the [[spread-spectrum]] [[
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 conference|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|book-title=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===
Line 160 ⟶ 176:
In theory CDMA, TDMA and FDMA have exactly the same spectral efficiency, but, in practice, each has its own challenges – power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.
TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct time slot and do not cause interference. Since this cannot be perfectly controlled in a mobile environment, each time slot must have a guard time, which reduces the probability that users will interfere, but decreases the spectral efficiency.
Similarly, FDMA systems must use a guard band between adjacent channels, due to the unpredictable [[Doppler effect|Doppler shift]] of the signal spectrum because of user mobility. The guard bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum. ====Flexible allocation of resources====
Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.e. allocation of
In other words, asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (synchronous CDMA), TDMA, and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of [[orthogonal]] codes, time slots or frequency channels that can be assigned to individual transmitters. For instance, if there are ''N'' time slots in a TDMA system and 2''N'' users that talk half of the time, then half of the time there will be more than ''N'' users needing to use more than ''N'' time slots. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal-code, time-slot or frequency-channel resources. By comparison, asynchronous CDMA transmitters simply send when they have something to say and go off the air when they
===Spread-spectrum characteristics of CDMA===
Most modulation schemes try to minimize the bandwidth of this signal
CDMA can also effectively reject narrow-band interference. Since narrow-band interference affects only a small portion of the spread-spectrum signal, it can easily be removed through notch filtering without much loss of information. [[Convolution encoding]] and [[forward error correction#Interleaving|interleaving]] can be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fading. Since the spread-spectrum signal occupies a large bandwidth, only a small portion of this will undergo fading due to multipath at any given time. Like the narrow-band interference, this will result in only a small loss of data and can be overcome.
Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random codes will have poor correlation with the original pseudo-random code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.
Some CDMA devices use a [[rake receiver]], which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlation tuned to the path delay of the strongest signal.<ref name="ref 1"/><ref name="ref 2"/>
Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems, frequency planning is an important consideration. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.<ref name
Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft hand-offs. Soft hand-offs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the hand-off is complete. This is different from hard hand-offs utilized in other cellular systems. In a hard-hand-off situation, as the mobile telephone approaches a hand-off, signal strength may vary abruptly. In contrast, CDMA systems use the soft hand-off, which is undetectable and provides a more reliable and higher-quality signal.<ref name
==Collaborative CDMA==
| first = Indu L.
| last = Shakya
| year = 2011
| title = High User Capacity Collaborative CDMA
| publisher = IET Communications
}}</ref> has been investigated for the uplink that exploits the differences between users' fading channel signatures to increase the user capacity well beyond the spreading length in the MAI-limited environment. The authors show that it is possible to achieve this increase at a low complexity and high [[bit error rate]] performance in flat fading channels, which is a major research challenge for overloaded CDMA systems. In this approach, instead of using one sequence per user as in conventional CDMA, the authors group a small number of users to share the same spreading sequence and enable group spreading and despreading operations. The new collaborative multi-user receiver consists of two stages: group multi-user detection (MUD) stage to suppress the MAI between the groups and a low-complexity maximum-likelihood detection stage to recover jointly the co-spread users' data using minimal Euclidean-distance measure and users' channel-gain coefficients. An enhanced CDMA version known as interleave-division multiple access (IDMA) uses the orthogonal interleaving as the only means of user separation in place of signature sequence used in CDMA system.
==See also==
{{Div col|colwidth=25em}}
* [[CDMA spectral efficiency]]
* [[CDMA2000]]
* [[Comparison of mobile phone standards]]
* [[cdmaOne]]
* [[Orthogonal variable spreading factor]] (OVSF), an implementation of CDMA
* [[
* [[Quadrature-division multiple access]] (QDMA), an implementation of CDMA
* [[Rise over thermal]]
* [[Spread spectrum]]
* [[W-CDMA
{{Div col end}}
==
{{Reflist|group=nb}}
==References==
{{Reflist|30em}}
==Further reading==
* Papathanassiou, A., Salkintzis, A. K., & Mathiopoulos, P. T. (2001). [https://www.researchgate.net/publication/3344159_A_comparison_study_of_the_uplink_performance_of_W-CDMA_and_OFDM_for_mobile_multimedia_communications_via_LEO_satellites "A comparison study of the uplink performance of W-CDMA and OFDM for mobile multimedia communications via LEO satellites"]. ''IEEE Personal Communications'', 8(3), 35–43.
==External links==
{{Commons category|CDMA}}
* [http://video.ias.edu/goresky-lecture-4-13 Talk at Princeton Institute for Advanced Study on Solomon Golomb's work on pseudorandom sequences]
{{cdma|state=uncollapsed}}
{{Channel access methods}}
{{Authority control}}
{{DEFAULTSORT:Code Division Multiple Access}}
| |||