The CDMA (Code Division MulTIple Access) technology, is mulTI-sites connecTIon technology based on the wide frequency communicaTIons. The CDMA multi-sites technology completely adaptes to the high request of modern mobile communications, such as large capacity, high grade, the comprehensive service, the soft cut etc, and it will be the most important develop direction.
This article carries on a description of the CDMA communications system essential elements, pivotal technology and the characteristic, mainly introduces the address choice, the wide frequency system characteristic, the power controlling, the RAKE receiving, the CDMA diversity receiving, the soft cut and so on, it especially disscusses the channel structure and parameter of the Q-CDMA digital cellular mobile communications system wireless channel. Then it carries on a introduction to the simulation tool -SystemView. On this foundation, it has simulated the baseband system of CDMA downlink, the uplink Access Channel and the downlink Traffic Channel, making use of Systemview.It has designed the concrete communications system model.In the model design process, it has given a specific explanation and analysis to the goal of model designning, the concrete structure composition, the simulation flow as well as the simulation results.
KEY WORDS: CDMA, SystemView Simulation, uplink, downlink
table of Contents
Chapter 1 Introduction 1
1.1 Background and significance of communication system simulation 1
1.2 Development Overview of CDMA Communication System 1
1.3 The project requirements and the main work arrangement of this article 4
Chapter 2 Selection of CDMA basic theory and development tools 5
2.1 Introduction to CDMA communication system 5
2.1.1 Basic principles of CDMA communication 5
2.1.2 Technical characteristics of CDMA 6
2.1.3 Selection of address code 7
2.2 Key technologies of CDMA communication system 8
2.3 Theoretical basis of the IS-95 CDMA system wireless link 11
2.3.1 Downlink 11
2.3.2 Uplink 13
2.4 Selection of development tools 14
2.5 Brief introduction of Systemview 15
Chapter 3 Simulation Research on CDMA (IS-95) Downlink Traffic Channel 17
3.1 Downlink traffic channel structure 17
3.2 Downlink business channel simulation scheme design and module parameter setting 18
3.3 System debugging and simulation result analysis 20
Chapter 4 Simulation Research on CDMA (IS-95) Uplink Access Channel 23
4.1 Introduction to Uplink Access Channels 23
4.2 Uplink access channel simulation scheme design and module parameter setting 24
4.3 Analysis of system debugging and simulation results 27
Chapter 5 Simulation Research of IS-95 CDMA Downlink Baseband System 30
5.1 Structure of the downlink baseband system 30
5.2 Downlink baseband system simulation scheme design and module parameter setting 31
5.2.1 Sending section 31
5.2.2 Receiving part 32
5.3 System debugging and analysis of simulation results 34
Chapter 6 Conclusion 37
6.1 Summary of project work 37
6.2 Problems and solutions in the design process 38
CDMA (Code Division Multiple Access) technology is a multiple access connection technology based on spread spectrum communication. CDMA multiple access technology is fully suitable for high-performance requirements such as large capacity, high quality, integrated services, and soft handover of modern mobile communication networks. It is an important development direction of modern communication technology.
This article describes the basic principles, characteristics and key technologies of CDMA cellular communication systems in detail. It mainly introduces address code selection, spread spectrum system characteristics, power control, RAKE reception, CDMA diversity reception, soft handover, etc., especially for Q-CDMA The structure, parameters and signal design of the wireless channel of the digital cellular mobile communication system are discussed in detail, and the simulation tool SystemView is introduced. On this basis, the powerful simulation function of SystemView is used to simulate the CDMA downlink baseband system, uplink access channel and downlink service channel establishment module respectively, and a specific communication system model is designed. In the process of model design, specific and detailed explanation and analysis are given for the purpose of model design, specific structural composition, simulation process and simulation results.
Keywords: CDMA, SystemView simulation, uplink, downlink
Chapter One Introduction
1.1 Background and Significance of Communication System Simulation In recent decades, the scale and complexity of communication systems have grown at an unprecedented rate, making analysis and design of communication systems cost more time, manpower, and material resources. Existing communication systems are very complicated, mainly reflected in the complex composition of the system, the complicated connections between the modules in the system, and the influence of the external environment on the system is difficult to grasp. This makes system analysis and designers researching the system by mathematical analysis only. The conclusions drawn are often far away from the reality. Sometimes they are limited by the development level of modern mathematics, and even mathematical analysis is impossible.
In this case, there are two options, one is to make the actual system, and the other is to use computer simulation to simulate this system. Obviously, the former is high risk, high cost and long cycle. In contrast, the low-risk, low-cost, high-speed advantages unique to computer simulation will certainly be valued.
Computer simulation of a communication system refers to system analysis and design of mathematical models based on the physical meaning of the modules of the communication system, designers then compile simulation programs based on these models, and use computers to reproduce the operating state of the system to study and analyze system characteristics . The main task of the simulation program of the communication system is to process the "waveform" passed between the modules in the system and analyze the data obtained by the simulation. System analysts need to have a deeper understanding of the various modules (such as modulators and demodulators) that make up the communication system and the relationship between the modules. However, there is a major disadvantage of using computer simulation, that is, its calculation will be quite large. Sometimes even if the fastest computer is used today to execute the simulation program, the running time of the program will be an astronomical number. The amount of calculation is related to the complexity of the system, the choice of simulation model and the simulation method. Obviously, the finer the simulation model, the greater the amount of potential computation .
Through computer simulation, we can get a deeper understanding of the performance of the CDMA system, and provide a feasible solution for the final implementation of the CDMA system with better performance in hardware. There are many kinds of communication system simulation tools today, and MATLAB, SystemView, Simulink, etc. are more commonly used. SystemView is a modern communication system design, analysis and simulation test tool that is more suitable for the modeling methods of physical models and mathematical models. In CDMA systems It is often used in simulation .
1.2 Overview of the development of CDMA communication systems Since the emergence of cellular communication in the late 1970s, the mobile communication industry around the world has experienced rapid development, and the technology of cellular communication itself has also made great progress. Mobile communication networks have started to simulate cellular The network has developed into a digital cellular network. In terms of multiple access technology, the first-generation analog cellular network used the frequency division multiple access (FDMA) method, which was used in the early 1980s; the time division multiple access (TDMA) system was developed in the late 1980s; after the 1990s, The TDMA digital cellular network represented by GSM has been widely used at home and abroad. In the second half of the 1990s, on the basis of frequency division multiple access (FDMA) and time division multiple access (TDMA) digital cellular networks, code division multiple access (CDMA) cellular network systems, including narrowband and broadband systems, gradually emerged [3 ].
Advocates led by Qualcomm (Qualcomm) of the United States have proposed a system implementation scheme using CDMA technology in cellular mobile communication systems. Through theoretical analysis and continuous field experiments, they proved that this cellular system can fully meet the standards proposed by CTIA (American Cellular Communications Industry Association). The system not only has a large capacity, but also has outstanding advantages such as soft capacity and soft handover. It is considered to be a flexible and advantageous technology for obtaining large capacity and high quality in a mobile communication environment. Since 1998, CDMA commercial systems based on IS-95 have been used in Hong Kong, China, South Korea and other regions and countries, and users have received good feedback. At the 18th meeting of TU TG8 / 1 held in Helsinki, Finland on November 5, 1999, three types (TDMA, CDMA-FDD (frequency division multiplexing), CDMA-TDD (time division multiplexing)) were finally determined. 5 technologies as the basis of the third generation mobile communication, among which WCDMA (Wideband Code Division Multiple Access) in Europe, CDMA 2000 in the United States and TD-SCDMA (Time Division-Sync Code Division Multiple Access) in China are the 3 mainstream standards of 3G .
In 1995, after the first CDMA commercial system was in operation, many theoretical advantages of CDMA technology were tested in practice, and they were rapidly promoted and applied in North America, South America, and Asia. Many countries and regions around the world, including Hong Kong, China, South Korea, Japan, and the United States have established CDMA commercial networks. In the United States and Japan, CDMA has become the main domestic mobile communication technology. In the United States, 7 out of 10 mobile communication operating companies choose CDMA. As of April this year, 60% of South Korea's population has become CDMA users. In order to adapt to the rapid development of China's mobile communications market, in April 1999, the State Council approved China Unicom to take charge of the construction, operation and management of China's CDMA network. In September 2000, the National Development Planning Commission and the Ministry of Information Industry issued the "Notice on Matters Related to the Start of CDMA Mobile Communication Network Construction", and China Unicom's CDMA network construction plan was officially launched, thus kicking off the CDMA network construction.
In the second half of 1995, the former Ministry of Posts and Telecommunications and the army decided to use the 800MHz frequency, selected four cities of Beijing, Shanghai, Xi'an, and Guangzhou to establish an CDMA experimental network based on IS-95.
At the end of 1995, the world's first CDMA system based on the IS-95 standard was put into commercial use in Hong Kong. At the end of 1997, four 133CDMA commercial experimental networks in Beijing, Shanghai, Xi'an, and Guangzhou operated by the Great Wall Company of Telecommunications were successively announced, and roaming between networks was realized.
On February 11, 2000, the CDMA mobile switching system independently developed by ZTE successfully docked with Ericsson's CDMA base station system, and basically completed the effectiveness test; CDMA BSS products were launched to the market in the second half of 2000. In January 2001, China Unicom signed a CDMA intellectual property framework agreement with Qualcomm on behalf of the country.
In March 2001, ZTE successfully demonstrated the integrated transmission of voice, data and image services using its independently developed CDMA2000 -1X mobile communication system.This is the first CDMA mobile communication system in China to realize data and image services. It reaches 144K, marking the success of the broadbandization of the domestic CDMA mobile communication system.
On the morning of August 29, 2001, the Ministry of Information Industry held a special meeting to announce the decision of the State Planning Commission. 19 domestic enterprises were approved to be qualified to produce CDMA terminal products. In the afternoon, Hisense Group held a press conference to launch China's first CDMA color screen mobile phone. In addition to green functions such as low radiation, high voice quality, and low power consumption, the phone also has a 256-color color LCD display and a unique 16-harmonic function.
In November 2001, the Fujian Unicom CDMA intelligent network project undertaken by Huawei was opened, and the first prepaid business phone of Unicom's CDMA network was opened. Subsequently, CDMA intelligent networks in Liaoning and Heilongjiang provinces were also opened at the same time. China Telecom has made certain achievements in the field of CDMA2000. The Shenzhen branch opened 2G narrow-band CDMA in 2002. The systems and terminals used are exactly the same as those of Unicom's CDMA network that has not yet been assigned. Unicom will begin to smooth the existing network around 2003. The transition to upgrade to CDMA2000 1x network, which makes mature 3G primary novel services (such as video services, VOD on demand, etc.) already mature in South Korea and Japan and other countries will be introduced in the near future.
So far, some cities have established cdma2000 1X networks, or are transitioning from IS-95 to cdma2000 1X. The total number of CDMA users nationwide has exceeded 7 million by the end of 2002, and by May 2004, more than 20 million.
After the third generation mobile communication (3G) Chinese standard TD-SCDMA became China's communication industry standard on January 20, 2006, in early May 2007, the Ministry of Information Industry promulgated WCDMA proposed by Europe and CDMA2000 proposed by the United States as our communication Industry Standard. This means that China's 3G marketization process has made a substantial breakthrough. TD-SCDMA has always been called the "Chinese standard" of 3G, and WCDMA and CDMA2000 were proposed by Europe and the United States, respectively. China's listing of WCDMA and CDMA2000 as standards in the communications industry means that the Chinese government is fulfilling its commitment to "technical neutrality" to provide a more open and fully competitive market for various communications technologies. It is believed that CDMA will play an increasingly important role in China's future mobile communications market.
In Asia Pacific and North America, the commercialization trend of CDMA technology is most obvious. After 1995, many countries and regions such as South Korea, Japan, Singapore, Australia, Thailand, India, and New Zealand have established CDMA networks. As of the end of December 1999, the total number of CDMA users in the Asia-Pacific region had reached 28 million, and North America had reached 16.5 million. In the United States, seven of the top ten cellular mobile companies use IS-95 CDMA cellular networks, accounting for 70% of the total population. In Europe, where GSM is dominant, CDMA has also received widespread attention from operators. By the end of April 2001, the CDMA network had been put into operation in 35 countries and regions around the world, and the total number of users reached 90 million. South Korea embarked on the development of CDMA technology in 1994. In January 1996, it was the first commercial service for CDMA mobile phones in the world; it began to develop the IMT-2000 test system in 1997. By August 2000, CDMA mobile phone users exceeded 15 million, and the market share The rate was 58%; the CDMA2000 1X test system was successfully developed in September 2000; the CDMA2000 1X commercial service was first launched in the world in October 2000. At present, South Korea has achieved success in the operation of CDMA. Domestic companies have developed their own production system equipment, which has led to the development of the national industry and has a place in the world CDMA stage.
1.3 Subject requirements and main work arrangements of this article This design requirement analyzes the principles, characteristics, and key technologies of the code division multiple access system, that is, the CDMA communication system, discusses the parameters, system structure, and signal design of the CDMA communication system, focusing on mastering channel coding , Address code selection, spreading code characteristics and power control. The SystemView software package is used to simulate the wireless interface of the CDMA system, mainly through the simulation of the CDMA downlink baseband system, the uplink access channel and the downlink traffic channel establishment module. Perform simulation parameter setting and analysis of simulation results.
SystemView is a complete visual development environment for dynamic system design, analysis and simulation . This design scheme uses SystemView software to carry out the simulation of each channel separately by using the CDMA expansion library that comes with the software. By constructing a block diagram of the communication system, a brief introduction to the simulation process, then the module design and parameter configuration, and finally debugging and results analysis.
The thesis is divided into six chapters. The first chapter introduces the CDMA communication system simulation background, requirements and significance, the generation of CDMA technology, and puts forward the main research content and program introduction of this thesis.
The second chapter first introduces the basic principles and main features of the CDMA communication system. Next, the key technologies and link components of the CDMA communication system are described in detail. Finally, the selection of development tools and the introduction of SystemView are discussed.
The third, fourth, and fifth chapters simulate each channel separately. Through the design of the simulation scheme, the configuration of parameters, and the commissioning operation, the results of the simulation operation are analyzed.
Chapter 6 summarizes the work done in this article as a conclusion, and discusses the problems and solutions in the design process and the harvest of this design.
Chapter 2 Selection of CDMA basic theory and development tools
2.1 Introduction of CDMA communication system
2.1.1 Basic principles of CDMA communication
CDMA (Code Division Multiple Ac2cess) technology is a multiple access connection technology based on spread spectrum communication. CDMA multiple access technology is fully suitable for high-performance requirements of modern mobile communication networks such as large capacity, high quality, integrated services, soft handover, and international roaming. With the continuous improvement of CDMA technology and the resolution of some key technologies, the third generation mobile communication system (IMT-2000) based on CDMA multiple access technology has become an important development direction of modern communication technology .
CDMA technology distinguishes different users with different orthogonal code sequences, so it is called "code division multiple access" technology. It is a multiple access connection technology based on spread spectrum communication, that is, a high-speed pseudo-random sequence (PN code) with a bandwidth much larger than that of the data signal. Modulate the data signal to be transmitted (spread spectrum), so that the bandwidth of the original data signal is expanded, and then modulated by the carrier and sent out. The receiver uses the exact same pseudo-random PN code to perform correlation processing on the received broadband signal, and converts the broadband signal into a narrow-band signal (de-spread) of the original information data to realize data information communication. The same pseudo-random PN code generated by the receiver must be completely synchronized with the pseudo-random PN code contained in the received signal. Therefore, before transmitting the information signal, a special PN code sequence must be generated (when the sequence is interfered, the receiver still has a high recognition rate), which is used for synchronization, and communication starts after synchronization is established. In a mobile communication system, many mobile stations communicate with other mobile stations through a certain base station at the same time, and the base station distinguishes different mobile stations through multiple access technology.
The technical characteristics of CDMA in mobile communication are: the signals transmitted by each site in the communication network occupy the same bandwidth, the transmission time is arbitrary, and each signal is distinguished by the structural (quasi) orthogonality (code pattern). The basic modulation method is spectrum broadening modulation, and the transmitted modulation signal spectrum is much larger than the information spectrum. Its anti-interference ability is strong. First, the non-spreading interference signal enters the receiver and is multiplied by the spreading code of this station. The interference power is dispersed on the spread spectrum, and the interference power falling on the effective bandwidth is greatly reduced. Secondly, other non-local spreading codes (even the same series of spreading codes) interfere with the receiver, after the relevant acceptance, the output is minimal or no output, only the local spreading code solution completely synchronized to the PN sequence Only output after expansion. Third, when spread spectrum modulation makes the signal bandwidth much larger than the relevant bandwidth, the effect of selective fading caused by multipath is greatly reduced.
2.1.2 Technical characteristics of CDMA In mobile communication systems, the basic types of multiple access methods currently mainly include FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access).
FDMA is to subdivide the limited frequency band into multiple carriers. For example, divide the bandwidth of 890MHz ï½ž 915MHz according to every 25kHz carrier frequency interval, so that we can get 890.0125MHz, 890.0375MHz, 890.0605MHz ... 1000 carriers. In FDMA, one carrier can only be used by one user at a time. At present, FDMA is mainly used in analog communication systems.
TDMA also subdivides the bandwidth of the system, and at the same time divides each carrier by time slot, which is used by multiple users. For example, GSM (Global System for Mobile Communications) divides carrier waves at 200 KHz frequency intervals, and each carrier is divided into 8 time slots for 8 users. Now our country mainly adopts TDMA GSM system.
In the CDMA system, the signals used by different users to transmit information are not distinguished by different frequencies or different time slots, but by different and unrelated orthogonal codes. From the perspective of frequency domain or time domain, multiple CDMA signals overlap each other. The correlator for the receiver can select a signal in which a predetermined code pattern is used from a plurality of CDMA signals, and a signal using other code patterns cannot be demodulated.
Compared with FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access), CDMA has the following unique advantages.
(1) Large system capacity and high connection rate. In theory, the system capacity of the CDMA mobile network is 20 times larger than that of the analog network and more than 5 times larger than that of GSM. The operation of the CDMA system that has been opened proves that its spectrum utilization rate is about 10 times that of the analog system and about 3 times that of the GSM system. The "processing gain" parameter in the CDMA system is much higher than other systems, plus the CDMA signal occupies the entire frequency band, which is almost 7 times the efficiency of the ordinary narrow-band modulation, so in general, for the same bandwidth, the CDMA system is the capacity of the GSM system 4-5 times, the network congestion is greatly reduced, and the connection rate is naturally high.
(2) Flexible configuration of system capacity. FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) both have fixed channel allocations that limit system capacity, while CDMA (Code Division Multiple Access) is a broadband transmission that can effectively avoid bandwidth limitations. Although an increase in the number of users is equivalent to an increase in background noise, which will cause a drop in voice quality, there is no limit to the number of users, and operators can consider the trade-off between capacity and voice quality. The same frequency can be used repeatedly in multiple cells. When the number of simultaneous communication sites is reduced, the communication quality is automatically improved, and multiple cells can be automatically balanced according to the amount of traffic and interference.
(3) High voice quality and better system performance quality. The vocoder can dynamically adjust the data transmission rate, and select different levels of transmission according to the appropriate threshold. At the same time, the threshold value can be changed according to the change of background noise. In this way, even in the case of large background noise, you can get better call quality.
(4) Not easy to drop calls. The base station is the guarantee of the mobile phone call. When the user moves to the edge of the coverage of the base station, the base station should automatically "switch" to allow the call to continue, otherwise the call will be dropped. The base station coverage during CDMA system switching is "single coverage, one pair of coverage and one separate coverage", and it is automatically switched to the neighboring relatively free base station, that is, when it is confirmed that the mobile phone has moved to another base station's separate coverage area, Only disconnected from the original base station, so as to ensure that the mobile phone will not be dropped. The CDMA system uses soft handover technology, "connect before disconnect", which completely overcomes the shortcomings of hard handover that is easy to drop calls, and ensures the stability of the call quality during the handover process.
(5) Simple frequency planning. In the CDMA system, users are distinguished by different, unique and specific pseudo-random sequence codes, so different CDMA carriers can be used in adjacent cells. Flexible network planning and simple expansion.
(6) The wireless transmission power is small. Because the CDMA system uses very accurate power control technology and variable rate vocoder, base station equipment and mobile phones and future portable personal communicators can communicate normally with very little transmission power. The transmission power of CDMA mobile phones is usually only 0.6mW, which is nearly a hundred times different from the transmission power of mobile phones of other standards. This means that the service life of mobile phone batteries is extended, smaller batteries can be used, and also, the harm of mobile phone radio waves to the human body is greatly reduced.
(7) The CDMA mobile communication network is a combination of several technologies such as spread spectrum, multiple access, cellular networking and frequency multiplexing.It is a collaborative technology that includes three-dimensional signal processing in the frequency domain, time domain and code domain. . Therefore, it has strong anti-interference, can overcome the selective decay caused by multi-path propagation, improve transmission performance, and has good confidentiality; its ability to overcome co-channel interference is also strong, making all sectors, inter-sector and inter-cell Can be multiplexed with the same frequency to improve spectrum efficiency.
(8) The cost of network construction decreased. Due to the large capacity and high frequency utilization of the CDMA system, it can accommodate more users in a certain frequency band. Due to the characteristics of CDMA technology, under the condition of the same coverage area, the CDMA system needs to build more than 80% fewer base stations than the GSM system, which greatly reduces the cost of network construction .
2.1.3 Selection of address codes In the CDMA cellular system, in the selection of address codes, three kinds of codes are adopted .
One is a PN code of length 215, which is obtained by adding a "0" after outputting 14 consecutive "0" s of m sequence of length 215-1. It is used to distinguish the signals of different base stations and does not maintain synchronization with the base station, but the phase shift of the PN code sequence used by different base stations is different. The phase shift of each base station's PN code can only be an integer multiple of 64, so 512 values â€‹â€‹can be used by different base stations. Use different phases of the same sequence as address codes to facilitate searching and synchronization.
The other is a pseudo-random PN sequence with a length of 242-1. It is used for signal security in the downlink, and it is used to distinguish a different mobile station in the uplink. Such a long code is conducive to the security of the signal, and at the same time the base station knows the long code and phase of a particular mobile station, so there is no need to search and acquire it. Uplink channels are distinguished by a long PN code with a period of 242-1, and a common mask associated with the mobile station is used to generate a long code for different users' access channels. Long code PNA and long code provide code division physical channels for access logical channels and uplink service logical channels, respectively. The maximum number of access channels that can be set is n = 32, and the corresponding physical channel is PNAn (n = 1, 2, ..., 32). The maximum settable uplink traffic channel is m = 64, and the corresponding physical channel is PNTm (m = 1, 2, ..., 64). PNAn and PNTm are determined by the 42-bit mask. The uplink includes only two logical channels, namely the access channel and the traffic channel. Their total number of channels (n + m) is equal to 64.
In addition, the CDMA cellular system divides the downlink physical channel into 64 logical channels, that is, a pilot channel and a synchronization channel (which can be changed to a traffic channel if necessary, because the mobile station does not need to monitor the synchronization channel after obtaining synchronization ), 1-7 paging channels (can be changed to traffic channels if necessary) and 55 downlink traffic channels (up to 63). The dividing method is to use Walsh sequence to modulate the signal on the PN sequence. The Walsh sequence generated by the Walsh function is 64 chips. The orthogonal signal has a total of 64 Walsh sequence patterns, denoted as W0, W1, W2, ... W63, which can provide 64 code division channels. The corresponding relationship between logical channels and code division physical channels are: pilot channel W0, synchronization channel W32, paging channel W1-W7 and downlink service information W8-W31, W33-W63. In the service channel, contains service data and Power control sub-channel. The former transmits user information and accompanying signaling information. Due to the orthogonality of Walsh sequences, the signals of different channels are orthogonal, and different mobile station users are distinguished at the same time. Adjacent base stations can use the same Walsh sequence, which may not be orthogonal, but can be distinguished by PN short codes. Since 512 64chips long Walsh sequences are exactly equal to the length of the PN sequence, in the uplink, Walsh sequences are used to perform orthogonal code multi-ary modulation on the signal to improve the quality of the communication link.
2.2 Key technologies of CDMA communication system
1. Power control technology
CDMA uses the same frequency at the same time, and only distinguishes channels by different code words. There are shadows, multipath fading and distance loss effects in the mobile radio environment. The location of cellular mobile stations in the cell is random and often changes, so the path loss varies greatly, especially in multi-cell cellular DS / CDMA systems. , All cells use the same frequency, although in theory, the address codes assigned by different users are orthogonal, but in fact it is difficult to be guaranteed, resulting in the mutual interference of various channels, which will inevitably cause serious multiple access interference , Near-far effect and corner effect.
The CDMA system is to reduce the transmission power, reduce interference, and increase capacity under the premise of ensuring quality. It is a self-interference limiting system that does not require transmit power margin. Power control is a key technology in CDMA. Without good power control, the system cannot achieve the desired goal and cannot form a qualified product. The power control of the CDMA system is divided into downlink power control (that is, controlling the base station transmit power) and reverse power control (that is, controlling the mobile station transmit power), in which reverse power control is particularly important. This is because the channel condition of the reverse link is relatively bad. To ensure the system capacity and communication quality, to overcome the problems of fading and to solve the near-far effect, to a large extent, all rely on reverse power control. Reverse power control includes three types of reverse power control: split-loop power control, closed-loop power control, and outer-loop power control .
2. Spread spectrum coding technology
CDMA assigns a unique code sequence (spreading code) to each user and uses it to encode information-bearing signals. The receiver that knows the user of the code sequence decodes the received signal and restores the original data, because the cross-correlation between the user code sequence and other user code sequences is very small. Because the bandwidth of the code sequence is much larger than the bandwidth of the signal carrying the information, the coding process expands the frequency spectrum of the signal, so it is also called spread spectrum modulation, and the resulting signal is also called a spread spectrum signal. CDMA is also usually characterized by spread spectrum multiple access (SSMA). Multiple access capability is given to CDMA for the spread of the transmitted signal spectrum. Therefore, it is very important to understand the generation and performance of spread spectrum signals. Spread spectrum modulation technology must meet two basic requirements:
(1) The bandwidth of the transmitted signal must be much greater than the bandwidth of the information.
(2) The bandwidth of the generated RF signal has nothing to do with the transmitted information.
The receiver uses the same spreading code to perform correlation operations with the received signal to recover the original information it carries. Since the spread spectrum signal expands the frequency spectrum of the signal, it has a series of properties that are different from narrowband signals:
â— Multi-access capability â— Anti-multipath interference capability â— With privacy performance â— Anti-human interference capability â— Low load probability performance â— With narrow-band interference capability
CDMA can be divided into direct sequence spread spectrum (DS) \ frequency hopping spread spectrum (FH) time hopping spread spectrum (TH) and composite spread spectrum according to the different spread spectrum modulation methods it uses, as shown in Figure 2.1.
Figure 2.1 Schematic diagram of CDMA spread spectrum modulation method The composition of the direct sequence spread spectrum (DS-SS) transmitter and receiver is shown in Figure 2.2.
Figure 2.2 Direct sequence spread spectrum (DS-SS) transmitter and receiver structure diagram
3. Soft handover
Due to its unique technical characteristics and the use of RAKE receiver technology, the CDMA system allows mobile stations to maintain communication with two or more cell base stations at the same time, which greatly improves the performance of handover, and only realizes soft handover and softer handover. Distinctive characteristics and technological advancement.
In the IS-95CDMA system, each mobile station has a RAKE receiver with three fingers, which can simultaneously communicate with two or more cell base stations. While communicating with base station A, the mobile station continuously monitors the pilot signal strength of neighboring cells (such as base stations B and C). Whenever the strength of one of the pilots exceeds a predetermined threshold Tadd (such as base station B), it immediately notifies The system commands the base station B to establish communication with the mobile station. At this time, there are signals from two base stations on the downlink. On the reverse link, the mobile switching center (or base station) selects it based on which base station receives the stronger signal, and the time for releasing the weak signal depends on Tdrop and other parameters of Tdrop. When a mobile station in a CDMA system communicates on a service channel, four types of handovers occur: soft handover, softer handover, hard handover, and CDMA to analog handover.
The performance improvement brought by the soft handover comes at the cost of increasing the system complexity, which is mainly manifested in that the mobile station must receive signals from different base stations, which requires a complex RAKE receiver; the base station must maintain communication with it for each The mobile station provides channels, including both the mobile station that is about to switch out of the cell and the mobile station that is switching into the cell; the base station must provide each mobile station that maintains communication with it and is in the inter-cell handover state to the MSC The link is used for reverse link diversity combining to achieve seamless handover, and these mobile stations are not necessarily under the power control of the base station, that is, they do not necessarily belong to the base station. Therefore, the seamless coverage of the system can be achieved by switching, providing high-quality services. In practice, according to the specific requirements of the system, various factors such as system burden, spatial service distribution and wireless propagation environment should be considered comprehensively, and a reasonable and effective switching scheme should be designed .
4. Diversity reception Diversity reception technology is to use two or more different methods to receive the same signal to reduce the impact of attenuation, is an effective anti-fading measures. The basic idea is to divide the received signal into multiple independent independent signals, and then combine these signals with different energy according to different rules. Diversity receiving technology can be divided into macrodiversity (macroscopic) and microscopic (microscopic) diversity according to the purpose. According to the signal transmission mode, it can be divided into explicit diversity and implicit diversity. Explicit diversity refers to the transmission method that constitutes the obvious diversity signal, which refers to the diversity of receiving signals using multiple antennas. Implicit diversity refers to the transmission method in which the diversity function is included in the transmission signal. At the receiving end, signal processing technology is used to achieve diversity. It includes interleaving coding technology and frequency hopping technology. Diversity receiving methods mainly include spatial diversity, frequency diversity, polarization diversity and angle diversity.
Spatial diversity: Maintaining sufficient spacing between the two receiving antennas at the receiving end can reduce the correlation between the two received signals.Spatial diversity uses this principle to set up two antennas that maintain a certain distance from each other and connect their respective receiving Machine, and then synthesize the signals of each receiver. Spatial diversity reception can improve the received signal, and can effectively improve the fast fading and smooth channel fading phenomena, thereby greatly reducing the bit error rate of digital signals.
Frequency diversity: At the sending end, using two carrier frequencies that are not in the same relevant bandwidth, the transmitter transmits the same information at the same time; at the receiving end, using two receivers corresponding to different frequencies to receive the two carrying the same information The signal is synthesized after demodulation. Because two carrier frequencies are used, the spectrum utilization rate is reduced.
Polarization diversity: The base station has two different polarization antennas to transmit the same information at the same time, and two different polarization antennas corresponding to the mobile station simultaneously receive two polarization components Ex and Ey containing the same information, using the difference between Ex and Ey Uncorrelated, synthesize it. Since the transmitter power is divided into two antennas, this method halves the transmitter power.
Angle diversity: Two directional antennas are used at the receiving end to point to different directions, so that they receive signals that are independent of each other but contain the same information at different angles, and the received signals are synthesized.è¿™ç§æ–¹æ³•ç”¨äºŽç§»åŠ¨å°æ¯”ç”¨äºŽåŸºç«™å°æ›´åŠ æœ‰æ•ˆ,ä½†è¿™ç§æ–¹æ³•åªé€‚ç”¨äºŽ10GHz æˆ–æ›´é«˜é¢‘çŽ‡ä¸Šã€‚
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Full color LED Display, with the red, green, blue color in one dot, which enabled the display
to show a dynamic imagine with rich colors, high saturation and high resolution.
The LED Mirror Screen usually apply for physical sports, advertisement, bank, stock exchange, station, port,
marketplace, telecommunication, department, school, monitor, restaurant and entertainments.
Features:The pixel is small, the quality is exquisite and realistic, the color is pure and thick;The flatness is good and the picture is uniform and pleasing to the eye;High grayscale and high refresh, the studio video effect is realistic;Safety performance structure, enhanced design is not easy to deform;Large-angle imaging display technology to meet the wide range Angle to view unbiased color;The LED Light Box can be designed and maintained on the front and back, easy and fast and easy to maintain
Led Poster With Ad Player,Smart Led Ad Player,Led Poster Display,Led Ad Player
Shenzhen Bako Vision Technology Co., Ltd. , http://www.rentalleddisplays.com