Why are there so few varieties of cdma mobile phones? Most people don't wear Bluetooth?

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.

China Internet questionnaire Sina free mailbox rose to 30 trillion.

Get value-added gifts from Wangpu and enter Yi Meier Online Building.

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 uses 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.