What is a CDMA mobile phone?

Code Division Multiple Access (code division multiple access)

CDMA is the abbreviation of Code Division Multiple Access, which is a brand-new and mature wireless communication technology developed on spread spectrum communication technology, a branch of digital technology. The principle of CDMA technology is based on spread spectrum technology, that is, the information data to be transmitted with a certain signal bandwidth is modulated by a high-speed pseudo-random code with a bandwidth much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded and then transmitted by carrier modulation. The receiver uses the same pseudo-random code to correlate with the received bandwidth signal, and changes the broadband signal into the narrowband signal of the original information data, that is, despreading, thus realizing information communication.

There are many classification methods for mobile communication systems. For example, according to the nature of the signal, it can be divided into analog and digital; According to modulation methods, it can be divided into frequency modulation, phase modulation and amplitude modulation; According to the multiple access connection mode, it can be divided into frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). At present, the GSM mobile phone network used by China Unicom and China Mobile adopts the combination of FDMA and TDMA. GSM has great advantages over analog mobile phones, but its spectrum efficiency is only three times that of analog systems, and its capacity is limited. It is also difficult to reach the level of wired telephone in voice quality; The maximum access rate of TDMA terminal can only reach 9.6 kbit/s; TDMA system has no soft handover function, so it is easy to drop calls and affect the quality of service. Therefore, TDMA is not the best wireless access for modern cellular mobile communication, while CDMA multiple access technology is completely suitable for large capacity, high quality, integrated services and soft handover required by modern mobile communication networks, and is being favored by more and more operators and users.

The emergence of CDMA technology stems from the human demand for higher quality wireless communication. During the Second World War, CDMA technology was studied and developed for the needs of war. Its original intention is to prevent the enemy from interfering with their own communications. It was widely used in military anti-jamming communication during the war, and was later updated as a commercial cellular telecommunication technology by Qualcomm. 1995, after the first commercial CDMA system was put into operation, many theoretical advantages of CDMA technology were tested in practice, thus it was rapidly popularized and applied in North America, South America and Asia. Many countries and regions in the world, including China, Hongkong, South Korea, Japan and the United States, have established commercial CDMA networks. In the United States and Japan, CDMA has become the main mobile communication technology in China. In the United States, 7 out of 65,438+00 mobile communication operators choose CDMA. By April this year, 60% of South Korea's population had become CDMA users. CDMA technology played an important role in the 28th Olympic Games held in Australia.

The standardization of CDMA technology has gone through several stages. IS-95 is the first released standard among cdmaONE series standards, and the first CDMA standard that is really widely used in the world is IS-95A, which supports 8K coded voice services. Subsequently, TSB74 standard of 13K speech encoder came out respectively, which supported STD-008 standard of CDMA PCS system of 1.9GHz, and the voice service quality of 13K coding was very close to that of wired telephone. With the increasing demand for data services in mobile communication, Qualcomm announced in February 1998 that the IS-95B standard will be used in CDMA infrastructure platform. IS-95B can provide CDMA system performance, increase the data flow of users' mobile communication devices, and provide support for 64kbps data services. Since then, cdma2000 has become the transition standard of narrowband cdma system to the third generation system. At the initial stage of standard research, cdma2000 put forward the development strategies of 1X and 3X, but subsequent research shows that the enhancement technologies of 1X and 1X represent the future development direction.

The standardization of CDMA technology promotes its worldwide application. At present, CDMA technology has been widely used in the United States, South Korea, Japan and other countries. In some European countries, some operators have also built CDMA networks. According to the statistics of CDG (World CDMA Development Group), the number of CDMA users at the end of 1996 was only 1 10,000; By March of 1998, it had rapidly increased to100000; By September, 1999, the number of users had exceeded 40 million. At the beginning of 2000, the total number of CDMA mobile phone users in the world exceeded 50 million, and the number of users increased by 1 18% within one year. CDG said that at present, Asia has become the main driving force for the growth of CDMA market, and the number of CDMA users in Asia has increased by 88% compared with a year ago, reaching 28 million. The growth rate of the United States is as high as 143%, reaching16.5 million, but the absolute number of users is lower than that of Asia. In the Asia-Pacific region, China, Hongkong, Japan, South Korea, Australia, Thailand, India, the Philippines, New Zealand, Bangladesh and other countries and regions have established CDMA commercial networks with more than 26,543,800 users. The third growth rate is in Central America and South America, with 5 million CDMA users. CDG also said that Chinese mainland is the region with the greatest growth potential in the global CDMA market in the future, and it is estimated that the number of users in Chinese mainland market will reach 40 million in 2003.

CDMA is the development direction of mobile communication technology. In the 2G stage, CDMA enhanced IS95A and GSM are the same generation products in technical system, providing roughly the same services. However, CDMA technology has its unique features, such as good call quality, less dropped calls, low radiation, health and environmental protection. At the 2.5G stage, CDMA2000 1X RTT is obviously different from GPRS in technology. In terms of transmission rate, 1X RTT is higher than GPRS, and in terms of new service bearer, 1X RTT is more mature than GPRS, which can provide more new services with medium and high rates. Transition from 2.5G to 3G and from CDMA2000 1. X to CDMA20003. X is smoother than GPRS to WCDMA.

CDMA is the abbreviation of code division multiple access, which is developed on the spread spectrum communication technology, a branch of digital technology. CDMA is a mobile communication technology designed for large capacity, high quality, integrated services, soft handover and international roaming required by modern mobile communication networks.

The principle of CDMA technology is based on spread spectrum technology, that is, the information data to be transmitted with a certain signal bandwidth is modulated by a high-speed pseudo-random code with a bandwidth much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded and then transmitted by carrier modulation. The receiver uses the same pseudo-random code to correlate with the received bandwidth signal, and changes the broadband signal into the narrowband signal of the original information data, that is, despreading, thus realizing information communication.

CDMA mobile communication network is a combination of spread spectrum, multiple access, cellular networking and frequency reuse, which includes the coordination of three-dimensional signal processing in frequency domain, time domain and code domain, so it has the characteristics of good anti-interference, anti-multipath fading, high security, multiple cells can reuse the same frequency, and the capacity and quality can be weighed. These characteristics make CDMA have greater advantages than other systems.

(1) Large system capacity

Theoretically, under the condition of using the same frequency resources, the capacity of CDMA mobile network is 20 times larger than that of analog network, 10 times larger than that of analog network in practical use, and 4-5 times larger than that of GSM.

(2) Flexible configuration of system capacity

In CDMA system, the increase of users is equivalent to the increase of background noise, which leads to the decline of voice quality. But there is no limit to the number of users, and operators can compromise between capacity and voice quality. In addition, multiple cells can be automatically balanced according to traffic and interference.

This characteristic is related to the mechanism of CDMA. CDMA is a self-jamming system, and all mobile users occupy the same bandwidth and frequency. For example, think of bandwidth as a big house, and everyone will enter the only big house. If they use completely different languages, they can clearly hear the voices of their peers, with only some other people's conversations interfering. Here, you can think of the air in the room as a broadband carrier and treat different languages as codes. We can keep adding users until the whole background noise limits us. If we can control the signal strength of users, we can accommodate more users while maintaining high-quality calls.

(3) The call quality is better.

The channel structure of TDMA can only support a 4Kb speech encoder at most, but it cannot support a speech encoder above 8Kb. The structure of CDMA can support 13kb speech coder. Therefore, better call quality can be provided. The vocoder of CDMA system can dynamically adjust the data transmission rate and select different levels for transmission according to the appropriate threshold. At the same time, the threshold changes according to the change of background noise, so that even in the case of large background noise, better call quality can be obtained. In addition, TDMA adopts hard handoff, so users can obviously feel the interruption of the call, especially in cities with dense users and dense base stations, because there will be 2 to 4 handovers every minute in such areas. However, the phenomenon of "dropped call" in CDMA system is obviously reduced. CDMA system adopts soft handover technology, "connect first and then disconnect", which completely overcomes the shortcoming that hard handover is easy to drop calls.

(4) the frequency planning is simple

Users are distinguished by different sequence codes, so adjacent cells can use different CDMA carriers, and the network planning is flexible and the expansion is simple.

(5) The network construction cost is low.

CDMA technology simplifies the planning of the whole system by using the same frequency in all parts of each cell, and reduces the number of required stations without reducing the traffic, thus reducing the deployment and operation costs. CDMA network has large coverage, high system capacity and few base stations, which reduces the network construction cost.

CDMA digital mobile technology is different from GSM digital mobile system. Analog technology is called the first generation of mobile phone technology, GSM is the second generation, CDMA belongs to the second and a half generation of mobile communication technology, which is more advanced than GSM.

Added some content.

CDMA is the abbreviation of code division multiple access technology, which is an advanced wireless spread spectrum communication technology in the process of digital mobile communication in recent years. It can meet the high requirements of the market for the capacity and quality of mobile communication, and has the characteristics of high spectrum utilization rate, good voice quality, strong confidentiality, low dropped call rate, small electromagnetic radiation, large capacity and wide coverage, which can greatly reduce investment and operating costs.

CDMA was first introduced by Qualcomm in the United States. In recent years, influenced by many factors such as technology and market, it has developed rapidly. At present, the global users have exceeded 50 million, and China has also opened CDMA telephone networks in Beijing, Shanghai and other cities.

Technical persistence of CDMA

1.CDMA is a spread spectrum communication with the following characteristics:

(1) has strong anti-interference ability. This is the basic feature of spread spectrum communication, which is incomparable to all communication methods.

(2) Broadband transmission with strong anti-fading ability.

(3) Because of the broadband transmission, the power of the useful signal transmitted in the channel is much lower than that of the interference signal, so the signal seems to be hidden in the noise; That is, the power density is relatively low, which is beneficial to signal hiding.

(4) Using the correlation of spreading codes to obtain user information has strong anti-interception ability.

2. In the spread spectrum CDMA communication system, it has some new features due to the adoption of new key technologies:

(1) adopts various diversity modes. In addition to the traditional spatial diversity. Because broadband transmission plays the role of frequency diversity, and the base station and mobile station adopt RAKE receiving technology, which is equivalent to time diversity.

(2) Adopt voice activation technology and sectorization technology. Because the capacity of CDMA system is directly related to interference, using voice activation and sectorization technology can reduce interference and increase the capacity of the whole system.

(3) Soft handover assisted by mobile station. It can realize seamless handover, ensure the continuity of calls and reduce the possibility of dropped calls. The mobile station in the handover area can reduce its own transmission power by receiving signals from multiple base stations in diversity, thus reducing the interference to surrounding base stations, which is conducive to improving the capacity and coverage of the reverse link.

(4) Using power control technology, the collimated emission power is reduced.

(5) It has soft capacity characteristics. The number of available channels can be increased by increasing the frame error rate during peak traffic hours. When the load of neighboring cells becomes heavier and heavier, the heavily loaded cell can reduce the transmission power of the pilot, so that the edge users of the cell switch to neighboring cells due to insufficient pilot strength, thus sharing the burden.

(6) Good compatibility. Because CDMA has a large bandwidth, the power is distributed in a wide spectrum, the power density is low, and the interference to narrowband analog systems is small, so both can be stored. That is, good compatibility.

(7)COMA has high frequency utilization rate and does not need frequency planning, which is also one of the characteristics of CDMA.

(8) Effective OCELP speech coding by 8)CDMA. Speech coding technology is an important subject in digital communication. OCELP quantizes the difference signal by using the code table vector, and generates a signal with variable output rate according to the degree of voice activation. This five-horse coding method is considered to be the most efficient coding technology at present, which greatly improves the capacity of the system on the premise of ensuring good voice quality. This vocoder has two rate sequences: 8 kilobits per second and 13 kilobits per second. The 8 kbit/sec sequence can be changed from 1.2 kbit/sec to 9.6 kbit/sec, and the 13 kbit/sec sequence can be changed from 1.8 kbit/sec to 14.4 kbit/sec .. Recently, an 8 kbit/sevr has been developed.

Problems existing in CDMA

(1) On the issue of cell planning, although CDMA does not need frequency planning, its cell planning is not easy. Because all base stations use the same frequency, there is interference between them. If the cell planning is not done well, it will directly affect the voice quality and reduce the system capacity, so we should be careful in the design of station distance and antenna height.

(2) Secondly, the standard of CDMA is not perfect. Many standards are still under study. For example, interface A, at present, some manufacturers provide IS-634 version 0, and some support IS-634 version. Others use is-634/TSB-80. Therefore, it is difficult for system operators to choose a unified A interface.

(3) The system capacity decreases due to power control error.

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