1. CDMA
Code Division Multiple Access (CDMA) 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 with a certain signal bandwidth to be transmitted 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 it is modulated by the carrier and sent out. The receiver uses exactly the same pseudo-random code to make correlation processing with the received bandwidth signal, and changes the broadband signal into the narrowband signal of the original information data, that is, despreading, so as to realize information communication. CDMA refers to a spread spectrum multiple access digital communication technology, which can be used in any protocol of the second and third generation wireless communication by establishing a channel through a unique code sequence. CDMA is a multi-channel mode, and multi-channel signals only occupy one channel, which greatly improves the bandwidth utilization rate. It is applied to 8MHz and 1.9GHz ultra-high frequency (UHF) mobile phone systems. CDMA uses analog-to-digital conversion (ADC) with spread spectrum technology, and the input audio is first digitized into binary elements. The frequency of the transmitted signal is coded according to the specified type, so only the receiver with consistent frequency response coding can intercept the signal. Because there are countless kinds of frequency sequence codes, it is difficult to repeat, which enhances the confidentiality. The CDMA channel width is nominally 1.23MHz, and the soft handover scheme is used in the network to minimize the signal interruption during the mobile phone call. The combined application of digital and spread spectrum technology makes the number of signals per unit bandwidth double than that in analog mode, CDMA is compatible with other cellular technology, and realizes national roaming. The CMDAOne standard, originally used only in American cellular phones, only provides transmission speeds of 14.4Kbps for single channel and 115Kbps for eight channels. The speed of CDMA2 and broadband CDMA has doubled.
CDMA mobile communication network is a combination of spread spectrum, multiple access, cellular networking and frequency reuse, which includes the cooperation of three-dimensional signal processing in frequency domain, time domain and code domain, so it has the properties of good anti-interference, anti-multipath fading, high security, repeated use of the same frequency in multiple cells, and trade-off between capacity and quality. These properties make CDMA have great advantages over other systems.
(1) Large system capacity
Theoretically, the capacity of CDMA mobile network is 2 times larger than that of analog network, 1 times larger than that of analog network in actual use and 4-5 times larger than that of GSM.
(2) Flexible configuration of system capacity
In CDMA system, the increase of the number of users is equivalent to the increase of background noise, resulting in the decline of voice quality. However, 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 < P > and interference.
this feature is related to the mechanism of CDMA. CDMA is a self-disturbing system, all mobile users occupy the same bandwidth and frequency. For example, imagine the bandwidth as a big house, and all people will enter the only big house. If they use completely different languages, they can clearly hear their peers' voices with only some interference from other people's conversations. Here, the air in the room can be imagined as a broadband carrier, and different languages are regarded as codes, which can continuously increase users until the whole background noise is limited. If we can control the signal strength of users, we can accommodate more users while maintaining high-quality calls.
(3) Better call quality
The channel structure of p>TDMA can only support 4Kb speech encoders at most, but it cannot support speech encoders above 8Kb. The structure of CDMA can support a 13kb speech encoder. Therefore, better call quality can be provided. The vocoder of CDMA system can dynamically adjust the data transmission rate, and select different levels to transmit according to the appropriate threshold. At the same time, the threshold value 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 a hard handover method, and 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 call.
(4) Simple frequency planning
Users are distinguished by different sequence codes, so different CDMA carriers can be used in adjacent cells, and the network planning is flexible and the expansion is simple.
(5) Low network construction cost
CDMA technology simplifies the planning of the whole system by using the same frequency in each part 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 the well-known GSM digital mobile system. Analog technology is called the first generation mobile phone technology, GSM is the second generation, CDMA belongs to the second generation and a half technology of mobile communication, which is more advanced than GSM.
2. WCDMA
WCDMA mainly originated from the early third-generation wireless research activities in Europe and Japan, and the great success of GSM had a great impact on the standardization of the third-generation system in Europe. In 1988, Europe launched the RACEⅠ (Research on Advanced Communication Technology in Europe) program, which lasted until June 1992, and it represented the beginning of the third generation wireless research activities. During 1992-1995, the RACEⅡ II program started in Europe. ACTS (Advanced Communication Technology and Service) was established at the end of 1995, and proposed FRAMES (Future Wireless Broadband Multiple Access System) scheme for UMTS (universal mobile telecommunications system). In these early studies, various access technologies, including TDMA, CDMA and OFDM, were tested and evaluated. It laid a technical foundation for WCDMA.
as a new technology, WCDMA is also derived from the practice and application of CDMA technology. WCDMA is rapidly sweeping the world and has occupied 8% of the wireless market. By 213, the number of WCDMA users worldwide had exceeded 3.6 billion, and 156 operators in 17 countries had commercialized 3GWCDMA services.
WCDMA-FDD has the advantages of high chip rate, effective use of frequency selective diversity and spatial receiving and transmitting diversity, which can solve the multipath problem and fading problem. Turbo channel coding and decoding is adopted to provide high data transmission rate, FDD system can provide wide-area full coverage, and downlink base stations are differentiated by a unique cell search method without strict synchronization between base stations. The continuous pilot technology can support high-speed mobile terminals. Compared with the second generation mobile communication system, WCDMA has the technical advantages of larger system capacity, better voice quality, higher spectrum efficiency, faster data rate, stronger anti-fading ability, better anti-multipath, and can be applied to mobile terminals up to 5km/h, and it can smoothly transition from GSM system, ensuring the investment of operators and providing a good technical foundation for 3G operation.
3. TD-SCDMA
TD-SCDMA is the abbreviation of English Time Division-Synchronous Code Division Multiple Access (time division-synchronous code division multiple access), the third generation mobile communication standard (3G for short) proposed by China, and one of the three 3G standards approved by ITU. TD-SCDMA is an international wireless communication standard which is mainly based on China's intellectual property rights and is widely accepted and recognized internationally. It is an important milestone in the history of China's telecommunications. Compared with the other two main 3G standards, CDMA2 and WCDMA, it started late and its technology is not mature enough. According to the statistics of Nomura Securities, by the end of 214, the accumulated investment in TD-SCDMA network construction exceeded 188 billion yuan. Coupled with the terminal subsidies and marketing resources invested by China Mobile, it is conservatively estimated that the investment will far exceed 2 billion yuan.
features: unique advantages in spectrum utilization and flexibility in business support.
Advantages: China has its own 3G technology, which is supported by the government.