Simulation of code division multiple access system

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

Summary II
ABSTRACT III
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
Acknowledgements 39
References 40

Summary

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

ABSTRACT

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 [1].
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 [2].
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 [4].
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 [5]. 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 [6].
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 ~ 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 [7].
2.1.3 Selection of address codes In the CDMA cellular system, in the selection of address codes, three kinds of codes are adopted [8].
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 ​​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 [9].
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:
● Multi-access capability ● Anti-multipath interference capability ● With privacy performance ● Anti-human interference capability ● Low load probability performance ● 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 [10].
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.这种方法用于移动台比用于基站台更加有效,但这种方法只适用于10GHz 或更高频率上。
2.3 IS-95 CDMA系统无线链路的理论基础
2.3.1 下行链路下行链路采用频分、扩频码分、正交信号多址技术。
频分区域:可间隔1.25MHZ多载波工作,将不同频率的载波指配给不同区域。
码分区域:用一种PN码,依PN码的相位(偏移)不同区分不同的基站站址。
码分信道:用正交信号区分信道。
用户识别:以用户掩码和长PN码对用户话音信号帧的数据加扰。
下行链路的64个信道是由正交的Walsh函数来实现码分的。而每个基站的下行链路信号由短码PN(215)来识别的。短码PN序列规定有64个偏移,每一个偏移为512chips。
下行链路的信道结构包括导频信道、同步信道、寻呼信道和下行链路业务信道。这64个下行链路信道的源信息分别被各信道对应的码片速率为1.2288Mc/s的64元Walsh函数扩展后,经I/Q支路分别被码片速率为1.2288Mc/s的短码PN序列(215)进行四相扩频,然后进行QPSK调制。
在下行信道中,基站台要在导频信道不断地发送导频信号。它是未经调制不包含信息的扩频信号,主要用于基站覆盖区内移动台的同步捕获。同步信道的信息用于移动台建立系统的同步,其信息速率为1.2kb/s。寻呼信道以固定的寻呼速率9.6 kb/s或4.8 kb/s传送信息。在下行业务信道中,基站台是以变速率传送信息的,信息速率可以是9.6 kb/s、4.8 kb/s、2.4kb/s、1.2 kb/s,每帧数据的传送率可以不同。尽管是变信息速率传送,但由于码元重复,使用重复后的传输速率保持恒定,为19.2 kb/s。
下行链路业务信道的信源分别为172/80/40/16b/帧(每帧20ms)。根据用户讲话激活程度的不同选取不同的速率。当用户不讲话时,速率最低,移动台的发射功率也最小。速率调整的目的是减少相互干扰,增大系统容量。由于是多种传输速率的信源,当不同数据速率时利用重传次数的不同来保证较之前的编码比特率为19.2kb/s。
同时,寻呼信道和下行业务信道的数据扰码,长码掩盖生成码片速率为1.2288 Mc/s的序列,没64chips对应1个符号,则有码速率为1.2288/64=19.2ks/s的扰码。寻呼信道的扰码由寻呼长码掩盖生成,而下行业务信道的扰码有用户长码掩盖生成。
下行链路信道参数:
下行链路信道除引导信道不传输数据外,其余信道的参数分别如下表2.1-2.3所列,在下行链路中,基站台要在导频信道不断的发送导频信号,它是未经调制、不包含信息的扩频信号,主要用于基站覆盖去内移动台的同步捕获。同步信道的信息用于移动台建立系统的同步,其信息速率为1.2kb/s。寻呼信道以固定的信息速率9600b/s或4800b/s传送信息。在下行业务信道,基站台是以变信息速率传送信息的,尽管是变速率传送,但由于码符号的重复,使得传送的调制符号速率保持恒定,为19.2kb/s【11】。
表2.1 同步信道参数数据速率(b/s) 1200
PN子码速率(Mc/s) 1.2288
卷积码编码率1/2
码元重复后出现次数2
调制码元速率(b/s) 4800
每调制码元的子码数256
每比特的子码数1024
表2.2 寻呼信道参数数据速率(b/s) 9600 4800
PN子码速率(Mc/s) 1.2288 1.2288
卷积码编码率1/2 1/2
码元重复后出现次数1 2
调制码元速率(b/s) 19200 19200
每调制码元的子码数64 64
每比特的子码数256 256

表2.3 下行链路业务信道参数数据速率(b/s) 9600 4800 2400 1200
PN子码速率(Mc/s) 1.2288 1.2288 1.2288 1.2288
卷积码编码率1/2 1/2 1/2 1/2
码元重复后出现次数1 2 4 8
调制码元速率(b/s) 19200 19200 19200 19200
每调制码元的子码数64 64 64 64
每比特的子码数128 256 512 1024

2.3.2 上行链路上行链路采用与下行链路相同的频分、扩频码分多址技术。
频分区域:采用与下行链路相对应的频率码分区域:采用与下行链路同相位的PN码码分信道:用不同的长PN码进行码分信道,以识别接入信道和业务信道用户识别:以用户掩码和长PN码对用户话音信号帧的数据加扰,以识别用户上行链路CDMA信道有上行接入信道和上行业务信道组成,其中上行链路接入信道的数据传输速率固定为4.8kb/s,由长码序列来识别不同的接入信道,上行链路业务信道的数据传输速率9.6/4.8/2.4/1.2kb/s可变,由用户长码来识别不同的业务信道,上行链路的数据传输帧长为20ms。
上行链路接入信道的信源帧结构88b/帧,即数据速率为88/20=4.4kb/s。每帧附加供译码用的8位尾比特,则传输速率变成96/20=4.8kb/s。经过编码率为R=1/3的FEC编码后,传输速率为4.8kb/s×3=14.4kS/s.经过二重传后传输速率为28.8kS/s.经分组交织处理后的传输速率不变。交织后的比特流每6位符号为一组,在正交调制器被64元Walsh函数调制,即每6位换成1位持续时间的Walsh函数序列,则其输出信号的Walsh函数符号的传输速率为28.8/6=4.8kS/s。正交调制器输出的Walsh函数符号速率为4.8kS/s,而Walsh序列的码片速率为4.8*64=307.2kc/s。调制器输出的序列被长码PN序列所掩盖,该PN序列码片速率为1.2288Mc/s。然后I/Q支路分别被码片速率为1.2288Mc/s的短码PN扩展即QPSK调制。
上行链路信道参数:
表2.4 信道参数数据速率(b/s) 9600 4800 2400 1200
PN子码速率(Mc/s) 1.2288 1.2288 1.2288 1.2288
卷积码编码率1/3 1/3 1/3 1/3
传输占空比100 50 25 12.5
码元速率(S/s) 28800 28800 28800 28800
Walsh调制的码元数6 6 6 6
Walsh函数符号速率(S/s) 4800 4800 4800 4800
Walsh子码速率(kc/s) 307.2 307.2 307.2 307.2
调制码元宽度208.33 208.33 208.33 208.33
每码元的PN子码数42.67 42.67 42.67 42.67
每调制码元的PN字码数256 256 256 256
每Walsh子码的PN字码数4 4 4 4

表2.5 接入信道参数数据速率(b/s) 4800
PN子码速率(Mc/s) 1.2288
卷积码编码率1/3
码元重复后出现次数2
传输占空比100
码元速率(S/s) 28800
Walsh调制的码元数6
Walsh函数符号速率(S/s) 4800
Walsh子码速率(kc/s) 307.2
调制码元宽度208.33
每码元的PN子码数42.67
每调制码元的PN字码数256
每Walsh子码的PN字码数4
2.4 开发工具的选择仿真是指通过建立系统的模型来部分或全部地仿真实际的系统,并且对系统模型进行实验研究,以替代实际系统的研究。国外不少公司推出了许多优秀的仿真软件,其中比较著名的有:HugesAircraft Company开发的SSITD软件(System Simulation in TimeDomain) ,Cadence公司的SPW仿真软件包(Signal Process Worksystem),Synopsys公司的COSSAP仿真软件包和美国Elanix公司推出的基于PC机Windows平台的SystemView动态系统仿真软件。其中.SystemView动态系统仿真软件以其方便、直观、形象的过程构建系统,提供丰富的部件资源,强大的分析功能和可视化开放的体系结构,已逐渐被电子工程师、系统开发/设计人员所认可,并作为各种通信、控制及其它系统的分析、设计和仿真平台以及通信系统综合实验平台。
SystemView是一个完整的动态系统设计、分析和仿真的可视化开发环境。它可以构造各种复杂的模拟、数字、数模混合及多速率系统,可用于各种线性、非线性控制系统的设计和仿真。
其专业库中的IS-95 库、3G 库、Tu rboCode Library 库等更充分显示了SystemV iew 用于第三代移动通信系统设计仿真的强大和优越。基于SystemView,的上述优点,我们把SystemView动态仿真软件作为码分多址系统仿真的首选仿真软件。利用IS-95 (CDMA /PCS) 库和其他专业库的功能模块, 对CDMA ( IS-95A )通信系统进行仿真, 从而充分展示利用SystemView设计的优越性, 并为以后进一步研究CDMA 提供良好的仿真平台。
2.5 Systemview的简单介绍美国Ellanix公司系统仿真软件SystemView是一个完整的动态系统设计、仿真和分析的综合性可视化软件。是一个很好的信号及系统分析、设计、研究平台。它运行于Windows操作系统有非常友好的界面,用户只需用鼠标就能完成各种复杂的应用处理,用户还可以通过界面和对话窗口对功能模块参数进行定义。如定义仿真的起始时间和结束时问,以及系统的抽样频率等。使用SystemView能迅速建立和修改系统。对系统进行仿真、分析和处理,并能利用系统提供的开发工具迅速地建立动态系统的精确模型【12】。
SystemView包含基本库和通信、DSP、逻辑、射频/模拟、用户代码等专业库。
基本库是SyMemView仿真的基本构造模型。基本库中包括:信号源、子系统加法器、子系统输入输出端口、算子、函数、乘法器及观察窗等共8组基本器件。
通信库:包括了在设计和仿真现代通信系统中可能用到的各种模块。它使在一台PC上仿真一个完整的通信系统成为可能。该库中包括各种纠错码编码/解码器、基带信号脉冲成型器、调制器/解调器、各种信道模型以及数据恢复等模块。
DSP库:包含大量的DSP芯片的算法模式仿真和DSP函数,主要有加法器、乘法器、除法器、反向器、先进先出缓冲器、离散的Hadamard变换、混合的Radix FFT变换、FIR和IIR滤波器等逻辑库:包括了在设计和仿真数字电路系统中可能用到的各种模块。主要有与、或、非门、缓冲器,触发器、寄存器、计数器、多路调制的多路输出选择器、多谐振荡器,数模转换器等。
射频/模拟库:包括了在设计和仿真高频或模拟电路系统中可能用到的各种模块,主要有运算放大器、双平衡混频器、整流电路、限幅器、高低通滤波器锁相环、PID调节器等。
用户代码库:可以让设计者建立自己习惯的SystemView图标库,这些图标库可以使用c语言编写并且插入提供的模板,并自动地集成到SystemView中,象内库一样使用。
Also. SystemView还提供了与Matlab的接口,能很方便地实现与Matlab的交互式数据传送与仿真。 In short. System View提供了先进快速的设计,仿真环境。不仅能设计开发创建子系统,而且能方便地建立大的复杂系统。

第三章CDMA(IS-95)下行链路业务信道的仿真研究
3.1 下行链路业务信道结构下行链路业务信道结构如图3.1所示,下行链路业务信道工作在9600/4800/2400/1200b/s的数据速率下,根据用户讲话速度的不同选取不同的数据速率。业务信道的数据在每帧末尾含有编码器尾比特,另外在9.6kb/s 和4.8kb/s的数据中都含有帧质量指示比特,以帮助接收端判定数据速率和误帧率。因此,实际上下行链路业务信道的信息比特率是8.6/4.0/2.0/0.8kb/s 【13】。
在下行业务信道结构中主要包含了帧质量标记、编码器尾码、卷积编码器、符号重复、块交织器以及抽样器等。在下行链路业务信道中,数据在传输之前赞经过编码率为1 / 2 ,约束长度为9的卷积编码。编码后,如果数据速率低于9 600 b/s,在分组交织以前都要重复,使各种信息速率均变成相同的调制码元速率,即19200个调制码元每秒。重复之后要进行分组交织。 下行链路业务信道所用的交织跨度等于20ms ,相当于码元速率为19200 S / s时的384 个调制码元宽度。交织器组成的阵列是24 行×l6 列(即384 个码元)。交织后的教据要进行数据扰乱。扰码器把交织器和按用户编址的伪随机序列PN 长码进行模2 相加。这种时钟为l . 2288MHz,长码经分频后,码元速率变为19200S/ s ,因而送入模2 加法器进行数据扰乱的是每个子码中的第一个子码在起作用。下行链路业务信道数据掩码使用长码的公开掩码与上行业务信道相同。在下行链路业务数据扰码以后,功率控制比特插入到业务数据流中。
为了使下行链路传输的各信道之间具有正交性,在下行CDMA 信道中传输的所有信号都要用64 元的Walsh 函数进行直序扩频正交调制。这是采用BPSK 调制的扩频。 64个函数标志64个码分信道,使下行链路中的码分信道相互正交。互不串扰。在QPSK 调制前,还须使用I和Q 正交序列对数据流作四相扩频调制。然后,经过基带滤波,并按照QPSK 方式进行发送载波调制。
图3.1 CDMA 下行链路业务信道结构图
3.2 下行链路业务信道仿真方案设计及模块参数设置根据IS-95 下行业务信道原理及其结构, 用SystemView 中的模块进行架构, 系统仿真组成如图3.2所示。系统采用了CDMA /PCS 库中的TRFCCH 信道模块, 即下行业务信道, 并与通过用其他库中的模块构成的下行业务信道进行信号输出比较, 以此进行IS-95下行业务信道的仿真。在仿真开始之前,系统的抽样频率设为5MHz。主要组成模块的设置说明如下:
(1) 信号源(t4, 此标号为仿真图上对应模块上的数字标号)
这部分采用了伪随机序列PN Seq 模块, 把信号幅度设为1, 电平数设为2, 频率设为8.6kHz, 作为下行业务信道信息。
(2) 帧质量标记(t18)
采用帧质量检测编码器FrameQ 模块, 作用是在20ms的数据帧后面加入CRC 校验功能的编码,这里把数据速率设为8.6kb/s, 为的是在加入12比特/20m s 校验比特和8比特/20ms的编码器尾比特后, 数据速率变为9.6kb/s。
(3) 卷积编码器(t0)
采用卷积编码器Cnv Coder 模块, 对输入的码元卷积编码, 把输入比特数n, 信号位k , 约束长度l分别设为2, 1, 9, 从而使卷积码的码率为1/ 2。
(4) 码元重复(t23)
采用符号中继器SYMRPT 模块, 作用是对经过卷积编码后, 在分组交织以前的各码元进行重复。因为在下行业务信道中, 只要速率低于9.6kb/s , 在分组交织前码元都要重复, 从而使各种信息速率变成相同的调制码元速率, 即19.2kb /s。这里在卷积编码后是19.2kb/s, 因此把重复因子设为1。
(5) 块交织器(t1)
采用交织器Intlvr 模块, 把数据速率设为19.2kb /s, 因为是下行业务信道, 所以使用24 行3 16列单元作为交织长度。
(6) 长码扰码生成(t15+ t16+ t17)
采用脉冲串PlusTrain 模块、长PN 码LongPn模块和采样器Sample 模块构成数据扰码, 作用是把交织器输出码元和用户的长码进行模2加。脉冲串的频率设为1. 2288MHz, 幅度设为1, 根据正向业务信道数据掩码所使用长码的公开掩码中M 41 到M 32 要置成” 1100011000″ [1] , 所以长码PN 码中的MaskM 32 to M 39 设为十进制的24,MaskM 40toM 41 设为十进制的3, 同时采样器(t17) 的采样频率设为19.2kHz。
图3.2 下行链路业务信道仿真组成
(7) 复用(t27+ t29)
采用一个功率控制位PWR , 功率控制位为800b/s 的数据流, 这里由一个伪随机序列PN Seq(t28) 产生800b /s 的数据流。同时采用一个符号中继模块SYMRPT (t27) , 把重复因子设为64, 负责码元重复, 使码元速率达到1.2288M b/s。
(8) Walsh 函数生成(t1+ t3+ t12)
采用一个脉冲串PlusTrain 模块、Walsh 函数发生器模块和采样器Sample 模块构成Walsh 函数生成器。脉冲串的幅度设为1, 频率设为1.2288MHz,Walsh 函数发生器中Order N 设为64,Row K 设为55, 为了使正向传输的各信道之间具有正交性, 下行业务信道中的所有信号都要用64 阵列的Walsh 码进行正交调制[1]。采样器的采样频率设为1.2288MHz。
(9) I、Q 信道引导PN 序列(t11+ t5+ t21)
采样脉冲串Plu s Train 模块和采样器Sample模块以及I 通道PN 扩展模块、Q 通道PN 扩展模块分别构成I 信道引导PN 序列和Q 信道引导PN 序列。采样脉冲的幅度设为1, 频率设为1.2288MHz,采样器采样频率设为1.2288MHz。I、Q 通道PN 扩展模块保持原定设置不变。
(10) 调制(t33+ t34+ t19)
利用阶跃函数Step Fct 模块、采样模块以及复数旋转模块CxRotate 对经过基带滤波器的I、Q 信号进行调制。把阶跃函数的幅度设成0, 采样频率设为4.9152e+ 6Hz, 复数旋转中的相位增益设成0, 相位偏移设为30。
(11) 输出显示(t30+ t31+ t38+ t42)
利用分析Analysis 模块, 查看I、Q 信号输出, 并进行对比, 在CustmSin Name 中输入I_ data 和Q_ data, 这样在观察输出信号时, 在对应的波形图上方看见对应的信号标记。
(12) 下行业务信道利用SystemView 自带的下行业务信道模块,即TRFCCH 信道模块, 利用这个模块和组建的下行业务信道进行信号输出对比。
3.3 系统的调试及仿真结果分析运行该系统,将信号经过单个图标13和组建的前向业务信道输出的结果比较,可以看出两个信号输出基本完全吻合,如图3.3和3.4。

图3.3 信号经各图标组建的信道输出波形图3.4 信号经单个图标13的输出波形

为了更好看出两个前向业务信道的仿真误差, 把对应的信号输出进行波形覆盖,在同一坐标系中叠加, 如图3.5所示。从信号叠加输出图中, 可以发现在对应的时间上, 信号幅度差值很小, 基本上可以忽略不计。
图3.5 两个信道输出信号叠加输出

因此,由图标18到图标22组成的下行业务信道与单个图标13的功能相同,符合IS-95CDMA标准的下行链路业务信道模型。

第四章CDMA(IS-95)上行链路接入信道的仿真研究
4.1 上行链路接入信道介绍上行链路接入信道是一个随机接入信道,供网内移动台随机占用。移动台在此信道发起呼叫及传送应答信。 每个接入信道对应下行链路中的一个寻呼信道,但每个寻呼信道可对应多个接入信道。移动台通过接入信道向基站登记,发起呼叫,响应基站发来的呼叫等。当呼叫时,在移动台没有转入业务信道以前,移动台通过接入信道向基站传送控制信息(信令)。当需要时,接入信道可以变成业务信道。用于传输用户业务数据。所传输的数据经过与用户号码所对应的长伪随机码的变换序列调制后再传输,以使通信保密。
在一个CDMA信道中,最多可有32 个接入信道,最少可能是0个。每个接入信道用不同的接入信道长伪随机码序列加以识别。上行链路接入信道的结构图如图3.6所示,上行链路接入信道以固定的4.8 kb/s 速率传输。在其传输过程中没有随机化选通门的参与,因而两个重复的码符号均被发送。接入信道的信息帧首先在每帧末尾加入8位,称为编码器尾比特。用于把卷积编码器复位到规定的状态,卷积编码编码率为1/3 ,约束长度为9。卷积编译码的初始状态应为全0 。以后每输入1 个数据符号则产生3个编码符号。在每个20ms帧结束时,由编码器尾比特将其初始化为全0状态。接入信道的数据速率为4800b/s,因此,在分组交织前码元重复1次,两个重复的码元都要发送。码元重复后要进行分组交织。分组交织的跨度为20ms。交织器组成的阵列是32行x18列(即576 个单元)。输入码元(包括重复单元)按顺序逐列从左到右写入交织器,输出码元则按行从上到下从交织器读出。交织后进行64进制Walsh函数正交调制。之后,用长码进行直接序列扩频调制。长码的各个PN子码是用一个42位的掩码和序列产生器的42 位状态矢量进行模2加产生的,只要改变掩码,产生的PN子码的相位则随之改变,产生每个用户特定的掩码,并对应一个特定的PN 码相位。在进行直接序列扩频以后,使用I 和Q正交序列作四相扩频调制,加入基站特征,使用户信号的相位充分地随机化。这一对I 和Q正交序列称为引导PN 序列,即正交PN序列对。上行链路信道四相扩频使用的都是固定零偏置的PN 序列对。经PN 序列对扩频生成的正交信道序列最后进行OQPSK 调制。 Q支路的序列经延迟106.901ns后,I路和Q路序列送到基带滤波器限带并滤波。最后按QPSK 的方式进行发送载波调制。
图3.6 上行链路接入信道结构

4.2 上行链路接入信道仿真方案设计及模块参数设置根据IS-95 上行链路接入信道原理及其结构, 用SystemView 中的模块进行仿真, 系统仿真组成如图3.7所示。系统采用了CDMA /PCS 库中的AccessCH信道模块, 即上行链路接入信道, 并与通过用其他库中的模块构成的上行链路接入信道进行信号输出比较, 以此进行IS-95上行链路接入信道的仿真。在仿真开始之前,系统的抽样频率设为5MHz,采样点数为5000个。
图中图标l 是AccessCH信道模块其功能也可由CDMA库、通信库以及一些相关图标组成的系统来完成。图中以伪随机序列发生器图标0作为系统的信息源。它产生的序列分为两路,分别经过由图标3、4 到图标33 等组成的信号通路和图标1。为了降低系统的最高信号频率以提高仿真效率,两路信道均未进行载波调制,而主要针对前面所述各基带信号处理步骤进行仿真。
图标0产生的系统输入信源的帧结构为88b/帧,即数据速率为88/20=4.4kb/s。经帧质量校验器图标3加入编码器尾比特后,每帧附加供译码用的8 位尾比特。再经过编码率R=l/3 的卷积编码器图标4 和码元重复器图标23 使之加倍,码速率为28.8kb/s。经交织器图标22进行

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