Code division multiple access refers to the use of codeSequence correlationRealized multiple access communication[1]。The basic idea of code division multiple access is to distinguish addresses by different address codes.Each is equipped with a different address codecarrier(The same carrier) is modulated by both baseband digital signal and address code.
When receiving, only the receiver that knows its assigned address code can demodulate the corresponding baseband signal, while other receivers cannot demodulate the signal because of different address codes.The division is realized and recognized according to the different code structure.
Pseudo random code (PN code) is generally selected as the address code.Since the symbol width of PN code is much smaller than that of PCM signal (usually an integer multiple), the spectrum of the signal with pseudo-random code is much larger than that of the original baseband signal. Therefore, code division multiple access is also called spread spectrum multiple access[2]。
Code division multiple access (CDMA) refers to the use of different pseudo-random codes to distinguish base stations.The technology that all base stations use the same frequency and transmit information at the same time.
Because the pseudo-random code is superimposed when the signal is transmitted.The spectrum of the signal is greatly widened.The communication system using this technology is also called spread spectrum communication system.It is an advanced wireless spread spectrum communication technology in the process of digital mobile communication in recent years.It can meet the market's high requirements for mobile communication capacity and quality.
It has high spectrum utilization.Good voice quality, strong confidentiality, low call drop rate, small electromagnetic radiation, large capacity, wide coverage, etc.[3]
Code division multiple access (CDMA) is that each transmitter modulates its transmitted signal with different and mutually orthogonal address codes.At the receiving end, using the orthogonality of the code type, the corresponding signal is selected from the mixed signals through address recognition (correlation detection).[4]
Figure 1 CDMA
The characteristics of CDMA are that all users in the network use the same carrier, occupy the same bandwidth, and each user can send or receive signals at the same time.
In the code division multiple access communication system, the signals transmitted by each user share the entire frequency band, and the transmission time is arbitrary. The transmission signals of each user may overlap each other in time and frequency.Therefore, it is impossible to separate signals by using traditional filters or gates, so that the signals sent by a user can only be correctly received by the correlation detector through the matching receiver.[4]
As shown in Figure 1.
Spread spectrum principle
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The spread spectrum principle is shown in Figure 2 below.It can be seen from Figure 2 that the transmitting end is the information code to be transmitted
After coding, the pseudo-random code
Spread spectrum modulation is performed, and then RF modulation is performed. The output signal is:
Where:
The rate (chip/s) of is
，
The rate (bit/s) of
。usually
Much greater than
Therefore, the bandwidth of modulated spread spectrum signal mainly depends on
Bandwidth.
Figure 2 Spread Spectrum Principle Block Diagram
After the signal is transmitted wirelessly, it will be interfered by noise and other signals.Therefore, the signals received by the receiving end contain interference signals in addition to useful signals.That is:
Where
Is the sum of noise and interference signal.
The signal received by the receiver is demodulated by the coherent carrier.
After wide-band (bandwidth is about chip rate) filtering, we can get:
And will
And local pseudo-random code
Multiplication, that is, despreading processing.because
And originating
The codes are identical, so the output signal
After passing the baseband filter, the bandwidth of the baseband filter is signal
The bandwidth of is far less than that of the broadband filter before despreading, but it is still a broadband signal. After baseband filtering, only a small part of the noise power is left.After processing, its signal power remains unchanged.So the output signal to noise ratio of the despreading is much larger than the input signal to noise ratio of the despreading.After the decoder, the original signal will be restored[5]。As shown in Figure 3.
Figure 3 Time slot structure of IS-136 digital cellular system
Figure 4 Spread spectrum signal receiving and despreading block diagram
Code Division Multiple Access
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The technical principle of CDMA is based on the spread spectrum technology, that is, the information data with a certain signal bandwidth to be transmitted is modulated with a high-speed pseudo-random code sequence (PN) whose bandwidth is far greater than the signal bandwidth, so that the bandwidth of the original data signal is expanded, and then modulated by the carrier and sent out;The receiver uses exactly the same pseudo-random code to perform correlation processing on the received broadband signal, and converts the broadband signal into the narrowband signal of the original information data, that is, despreading, to achieve information communication.CDMA code division multiple access technology fully meets the requirements of modern mobile communication networks for large capacity, high quality, integrated services, soft handover, etc., and is being favored by more and more operators and users.
Spread spectrum technology mainly includesDirect sequence spread spectrum technology, frequency hopping (FH) spread spectrum technology and time hopping (TH) spread spectrum technology, among which direct sequence spread spectrum technology and frequency hopping spread spectrum technology are mostly used. In addition, hybrid systems composed of these common basic spread spectrum technologies are often used[6]。
Code Division Multiple Access Multiplexing
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Code division multiplexing access (also referred to as CDMA) is another method of sharing channels. Each user can use the same frequency band to communicate at the same time.In a CDMA system, the sender modulates the signal to be sent with an incoherent, mutually orthogonal (quasi orthogonal) address, and the receiver selects the corresponding signal from the mixed signal through the address using the orthogonality of the code type.[7]
CDMA was originally used for military communication, because the signals sent by this system have strong anti-interference ability, and its spectrum is similar to white noise, which is not easy to be found by the enemy.
With the progress of technology, the price and volume of CDMA devices have dropped significantly, so they are now widely used in civilian mobile communications, especially inWireless LANMedium.CDMA can improve the voice quality of communication and the reliability of data transmission, reduce the impact of interference on communication, increase the capacity of the communication system (4-5 times the capacity of the system using GSM), and reduce the average transmission power of mobile phones.[7]
Hybrid code division multiple access
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There are various forms of hybrid code division multiple access, such as FDMA and DS.CDMA mixing, TDMA and DS-CDMA mixing (TD/DMA), TDMA and frequency hopping mixing (TDMA/FH), FH-CDMA and DS-CDMA mixing (DS/FH-CDMA), etc.[6]
In the hybrid system K of FDMA and DS.CDMA, a broadband CDMA channel is divided into several narrowband DS.CDMA channel.The processing gain rate of narrowband DS-CDMA is lower than that of broadband DS-CDMA.In this system, the allocated narrowband CDMA frequency bands must be continuous, and each user can use different frequency bands.Each user can also occupy multiple narrowband DS-CDMA bands at the same time.[6]
In the TD/CDMA system, it introduces DS-CDMA in each time slot of TDMA, so that each time slot can transmit the information of multiple users at the same time.The number of DS.CDMA users and spread spectrum gain per time slot are generally much smaller than those of the system directly using DS.CDMA.For example, in the frame structure of the European Mobile Communication System Standard (GSM), each time slot is expanded by 16 times, and the information of 8 users is transmitted at the same time. The receiver can use the joint detection method to detect the information of 8 users at the same time.TD/CDMA has the advantage of reducing multiple access interference and reducing the complexity of the receiver.[6]
In the TDMA/FH system, the carrier frequency of each TDMA timeslot changes randomly.The operating frequency is changed once per frame.This technology has been applied in GSM system, which can effectively overcome serious co channel interference and multipath fading.[6]
In DS/FH-CDMA, the center frequency of DS-CDMA changes randomly according to the PN sequence.Because the center frequency of each user is different, the near far effect in DS-CDMA can be overcome.However, it is relatively difficult to realize the frequency hopping synchronization of the base station.[8]
Code division multiple access communication
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Code division multiple access systemAssign each user a multiple access code.It is required that the autocorrelation characteristics of these codes are sharp, and the peak value of the cross-correlation characteristics is as small as possible, so as to accurately identify and extract useful information.At the same time, the interference between users can be minimized.[9]
CDMA system has the following characteristics:
All users can share the entire frequency band resources asynchronously, that is, the time when different user symbols send signals is not required to be synchronized;
Large system capacity;
The channel data rate is very high.[9]
There are two kinds of spread spectrum signals commonly used in CDMA spread spectrum communication: frequency hopping signal and direct sequence spread spectrum signal.The corresponding multiple access modes are frequency hopping code division multiple access and direct sequence code division multiple access.[9]
CDMA technology
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CDMA code division multiple access is a wireless communication technology developed from the branch spread spectrum communication technology of digital technology.The principle of CDMA technology is based on the spread spectrum technology, that is, the information data with a certain signal bandwidth that needs to be transmitted is modulated with a high speed pseudo-random code whose bandwidth is far greater than the signal bandwidth, so that the bandwidth of the original data signal is expanded, and then modulated by the carrier and sent.The receiver uses the same pseudo-random code to correlate the received bandwidth signal, so that the broadband signal is replaced by the narrowband signal of the original information data, that is, despreading, to achieve information communication.[10]
There are many ways to classify mobile communication systems.For example, it can be divided into analog and digital according to the signal nature;According to modulation mode, it can be divided into frequency modulation, phase modulation and amplitude modulation; according to multiple access connection mode, it can be divided into frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA).GSM used by China Unicom and China MobileMobile telephone networkIt is a combination of FDMA and TDMA.GSM has more advantages than analog mobile phones, but its spectral efficiency is only three times that of analog systems, and its capacity is limited: its voice quality is inferior to that of wired phones;The maximum speed of TDMA terminal access can only reach 9.6 Kb/s:TDMA systemThere is no soft handover function, so it is easy to drop the line and affect the call.Therefore, TDMA is not the best wireless accessCDMA Multiple Access TechnologyIt is more suitable for large capacity, high quality, integrated services, soft handover, etc. required by modern mobile communication networks, and is popular with "large operators and users".[10]
CDMA technology originated fromthe Second World WarDuring the war, CDMA technology was developed to prevent the enemy from interfering with each other's communication. During the war, CDMA was mainly used in militaryAnti-jamming communication, which was later updated by Qualcomm into a commercial cellular telecommunications technology.In 1995, the first CDMA commercial system was officially put into operation, and was tested in practice, and was rapidly promoted and applied in North America, South America and Asia.Many countries and regions in the world, including Hong Kong, Japan, South Korea and the United States, have established CDMA commercial networks.In the United States and Japan, CDMA is listed as the main mobile communication technology in China.[10]
Related issues
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⑴ In terms of cell planning, although CDMA does not need frequency planning, its cell planning is not very easy.Since all BTSs use the same frequency, there is interference between them. If the cell planning is not done well, the voice quality will be directly affected and the system capacity will be discounted. Therefore, you should be careful in the design of station spacing and antenna height.[5]
⑵ Secondly, on the issue of standards, CDMA standards are not very perfect.Many standards are still in the process of research and trial development.For example, interface A, some manufacturers provide IS-634 version 0, and some support IS-634 version.Others use Is － 634/TSB － 80.Therefore, it is difficult for system operators to choose a unified A interface.[5]
⑶ Reduction of system capacity due to power control error[5]。
Characteristics of CDMA mobile communication system
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CDMA communication system does not divide into channels or time slots. The channels for transmitting information are distinguished by using different code types. It has the following characteristics:
(1) Large communication capacity.According to theoretical analysis, CDMADigital cellular mobile communication systemIts capacity is 20 times that of analog cellular communication system or 4 times that of GSM digital cellular communication system.[11]
(2) It has soft capacity characteristics.All users of the CDMA digital cellular mobile communication system share a wireless channel, and the distinction of user signals depends only on the difference of all code types. Therefore, when the cellular system is fully loaded, adding a few additional users will only cause a slight decline in voice quality (or a slight reduction in signal-to-noise ratio), without blocking.In FDMA cellular communication system or TDMACellular mobile communication systemWhen all channels or timeslots are occupied, it is impossible to add even one user.This characteristic of CDMA system makes a "soft" relationship between the system capacity and the number of users.[11]
(3) With soft switching function.At the initial stage of handover, the mobile phone in the CDMA cellular mobile communication system is served by the base station in the original cell and the base station in the new cell at the same time for the mobile station in the cross zone. The original base station does not interrupt the connection between the mobile station and the mobile station until a reliable communication is established between the mobile station and the new base station,The soft handover function of CDMA cellular mobile communication system can ensure the reliability of mobile station handover.[11]
Figure 5 CDMA receiver
(4) CDMA cellular mobile communication system is based on spread spectrum technology, so it has the characteristics of anti-jamming, anti multipath fading, and strong confidentiality.[11]As shown in Figure 5.
IS-95 system
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In addition to IS-136 system, the second generation cellular system developed in North America also has IS-95 system based on CDMA technology.The IS-95 system connects users in a wireless cell to the same frequency channel and distinguishes them with different characteristic codes.The pseudo-random code assigned to each user (or pseudonoise code, because of its autocorrelation characteristics similar to white noise) has excellent autocorrelation and cross-correlation performance (large autocorrelation coefficient, small correlation number).These code sequences, which are much faster than the user signal rate, transform the user signal into a broadband signal.At the transmitting end, the signals of each user are transmitted on a common frequency band;At the receiving end, the signals received by each user include the signals of other users in addition to the useful signals of the user.These signals are transformed into the original narrowband signals by the receiver using the same code sequence of the user as the transmitter, using the autocorrelation characteristics. The broadband signals of other users are still broadband signals because they are uncorrelated. After baseband filtering, the useful signals with high despreading output signal-to-noise ratio can be obtained[5]。
Code division multiple access modulation
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There are many address codes and modulation modes used. At present, there are three basic CDMA modes:
① Pseudo random code (PN code) direct multiple access mode, also known as direct sequence (DS) mode, uses high-speed address code -- pseudo-random code to carry out phase shift keying modulation on the carrier, and uses the address code generated locally that is consistent with the transmitter to conduct despreading modulation when receiving;
② Time frequency code code division multiple access mode, also known as frequency hopping (FH) mode, uses address code to carry out frequency shift keying modulation on the carrier;
③ Low density convolutional code code division multiple access mode, also known as time hopping (TH) mode, first encodes the information code with convolutional code to achieve low density ratio, then adds the PN code as the address code with the low-density information code sequence, and then performs pulse amplitude modulation, that is, "1" sends the carrier, "0" does not send, thus forming time hopping.[12]
In addition to the three basic code division multiple access modes, the combination of the three basic modes can also be used.[12]
Advantages of Code Division Multiple Access
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① It has strong anti-interference ability, because the interference signal that has no correlation with the local code cannot enter the receiver, which is only equivalent to Gaussian noise.[12]
② It is easy to keep secret and has good confidentiality performance.[12]
③ Multiple access mode is flexible and belongs to random multiple access mode.When the number of stations communicating simultaneously (sharing a carrier frequency) decreases, the communication quality automatically improves.When the number of stations increases, it can also communicate, but the communication quality is poor.[12]
④ It has strong anti fading ability and high communication quality. For example, only 3dB of fading reserve is required in ground mobile communication, while FM mode requires 10-20dB of reserve.[12]
⑤ Because GOD codes and other symbols can be used to form a large number of code groups, different address codes can be given to different mobile users in mobile communication, making it easy to gate control. Because the same frequency is used in each cell, the mobile station does not need to change channels when moving, nor does it need to extract synchronization in the network. It can be selected randomly, and only needs to allocate channels through coding,Spectrum management is easy to implement.[12]
Disadvantages of Code Division Multiple Access
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As CDMA belongs to the spread spectrum system, it needs to occupy a very wide frequency band (far more than the information bandwidth). Compared with narrowband systems, it has low spectrum utilization. However, from the perspective of serving multiple users (multiple access), it makes up for this shortcoming.In addition, it is difficult to select a sufficient number of available address codes, and it also takes some time to capture and synchronize the address codes when receiving.[12]
Code division multiple access is applicable to military satellite communication systems and small capacity user systems, as well as point to multipoint communication systems such as ground tactical communication, naval shore station and ship station communication, and can also be used for land mobile communication systems.[12]
