The key technology of CDMA mobile communication network
1. Power control technology
Power control technology is the core technology of CDMA system. CDMA system is a self-jamming system, all mobile users occupy the same bandwidth and frequency, and the problem of "near-far utility" is particularly prominent. The purpose of CDMA power control is to overcome the "near-far utility", so that the system can maintain high-quality communication without interfering with other users. Power control can be divided into forward power control and reverse power control, and reverse power control can be divided into open-loop power control in which only mobile stations participate and closed-loop power control in which mobile stations and base stations simultaneously participate.
(l) reverse open-loop power control. It is that the mobile station adjusts the transmission power of the mobile station according to the change of the power received in the cell, so that all the signals sent by the mobile station have the same power at the base station. It is mainly used to compensate the effects of shadows and turns, so it has a large dynamic range. According to IS-95 standard, it should at least reach a dynamic range of plus or minus 32dB.
(2) reverse closed-loop power control. The design goal of closed-loop power control is to make the base station quickly correct the open-loop power estimation of the mobile station, so as to keep the optimal transmission power of the mobile station. (3) Forward power control. In the forward power control, the base station adjusts the transmission power of each mobile station according to the measurement results, with the purpose of allocating smaller forward link power to the mobile stations with small path fading and allocating larger forward link power to those mobile stations far away from the base station and with high bit error rate.
2. pn code technology
the selection of pn code directly affects the capacity, anti-interference ability, access and switching speed of CDMA system. CDMA channels are distinguished by PN codes, so PN codes are required to have good autocorrelation, weak cross-correlation, simple implementation and coding scheme. At present, the CDMA system uses a basic PN sequence-M sequence as the address code, and uses its different phases to distinguish different users.
3. RAKE receiving technology
The mobile communication channel is a multipath fading channel. Rake receiving technology is to receive the signals of each channel separately for demodulation, and then add the output to achieve the purpose of enhancing the receiving effect. Here, multipath signals are not only a disadvantage, but also an available advantage in CDMA systems.
4. Soft handover technology
Connecting first and then disconnecting is called soft handover. CDMA system works on the same frequency and bandwidth, so soft handover technology is much more convenient and easier to implement than TDMA system.
5. Speech coding technology
At present, there are two main kinds of speech coding in CDMA system, namely code excited linear prediction (CELP)8kbit/s and 13bit/s.. The voice coding of 8kbit/s reaches the voice level of 13bit/s of GSM system or even better. 13bit/s voice coding has reached the level of wired long-distance voice. CELP adopts the same principle as pulse-excited linear predictive coding, except that the pulse position and amplitude are replaced by a vector code table.
Difference among FDMA, TDMA and CDMA:
Frequency division multiple access (FDMA) is a frequency-modulated multiple access technology. Traffic channels are allocated to different users in different frequency bands. Such as TACS system, AMPS system, etc.
time division multiple access (TDMA) is a time division multiple access technology. Traffic channels are allocated to different users at different times. Such as GSM, DAMPS, etc.
CDMA (code division multiple access) is a code division multiple access technology using spread spectrum. All users obtain service channels at the same time and in the same frequency band according to different codes.
at present, the main multiple access methods of digital mobile communication network are TDMA and TDMA system (GSM, DAMPS). The spectrum efficiency is about three times that of the analog system, and the capacity is limited; 13k bit/s coding is also difficult to reach the level of wired telephone in voice quality; TDMA system has high service comprehensive ability, which can synthesize data and voice, but the terminal access rate is limited (the highest is 9.6kbit/s). TDMA system has no soft handover function, so it is easy to drop calls and affect the quality of service; The international roaming protocol of TDMA system needs to be further improved and developed. Therefore, TDMA is not the best wireless access for modern cellular mobile communication, while CDMA code division multiple access technology is completely suitable for large capacity, high quality, integrated services, soft handover, international roaming and so on required by modern mobile communication networks.
the principle of CDMA code division multiple access 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 wave 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.
characteristics of CDMA cellular mobile communication network
compared with FDMA and TDMA, CDMA has many unique advantages, some of which are inherent in spread spectrum communication systems, and the other part is brought by technologies such as soft handover and power control. CDMA mobile communication network is a combination of spread spectrum, multiple access, cellular networking, frequency reuse and other technologies, which includes the cooperation of three-dimensional signal processing in frequency domain, time domain and code domain. Therefore, it has the properties of good anti-interference, anti-multipath fading, high security, repeated use of the same frequency in multiple cells, required carrier-to-interference ratio (C/I) less than L, and trade-off between capacity and quality. These properties make CDMA have a very important advantage over other systems. For operators, the large capacity and large coverage of CDMA system can not only improve the spectrum utilization rate and greatly save frequency resources, but also reduce the number of base stations, reduce the construction cost and speed up the construction progress. And can smoothly transition to the third generation mobile communication in a more economical way, which is an effective way to solve the capacity demand of the mobile communication market in this century quickly, well and economically.
1. CDMA has high spectrum utilization rate and large capacity
The inherent code division spread spectrum technology of CDMA system and advanced power control and voice activation technology make the capacity much larger than that of GSM system. According to cdmaOne technology of IS-95A/B protocol, it can provide at least three times the current capacity of GSM network. After implementing cdma2 1X RTT, the capacity it can achieve will be 6 times that of GSM.
the direct benefits of high spectrum utilization and large capacity are: 1) saving valuable spectrum resources greatly; 2) meeting the needs of high traffic density in cities and reducing network congestion; 3) reducing the number of base stations, making network expansion simple and easy to implement.
2. The coverage of CDMA is large
The wireless link budget of CDMA is 3~6dB more than that of GSM. Under normal circumstances, the radius of CDMA cell can reach 6 kilometers, and after adopting special technical means, the radius can reach more than 2 kilometers. At present, there is a base station with a coverage radius of more than 2 kilometers in Telstra's CDMA network. The maximum radius of GSM system base station shall not exceed 35 kilometers.
the direct advantage brought by the expansion of coverage is that the number of base stations is reduced and the location of base stations is easy. More importantly, the reduction in the number of base stations will greatly reduce the investment in network supporting telecommunications facilities such as computer rooms, power supply and transmission, and speed up the construction. It is also suitable for remote towns and rural areas with low telephone traffic and wide coverage.
3. The voice quality of CDMA is good, and the confidentiality is strong
Because CDMA uses pseudo-random sequence PN for spreading/despreading, the processing gain is up to 21dB in CDMA 8K EVRC voice coding, and the voice quality is quite good. According to Telstra's test, its voice quality is not only obviously superior to GSM and analog, but also comparable to that of fixed-line telephone, and it is also superior to fixed-line telephone in strong background noise environment. This is also confirmed on the commercial websites of Hutchison in Hong Kong and DDI/IDO in Japan. Moreover, the spread spectrum communication technology adopted by CDMA makes the communication have natural confidentiality, and the probability of its message being intercepted on the air channel is almost zero.
4. The dropped call rate of CDMA is low
Statistics show that handover failure is the main cause of dropped calls. 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. The soft handover technology of CDMA makes the success rate of handover much higher than that of analog hard handover cellular systems.
5. Data services can be provided
In terms of data communication, the cost of CDMA transmission per bit is lower than that of GSM, so it is more suitable as an access means for wireless high-speed packet data (such as 144Kbps) services, which provides better technical conditions for the integration of mobile/wireless and Internet. Relevant statistics show that the growth rate of global mobile voice services is slowing down, and the growth trend of mobile data services is rapid. IS-95 CDMA just caters to this development trend.
6. CDMA mobile phones meet the requirements of environmental protection
The transmission power of CDMA mobile phones is small, which has little impact on some medical equipment. In Australia, about 4, hearing aid users have become potential users of CDMA. At the same time, low emission power not only has little radiation to human body, but also has the reputation of "green mobile phone", and can prolong the standby and talk time of mobile phone battery. Moreover, with the increase of the number of base stations, the transmission power will become smaller and smaller, and the advantages in this respect will become more and more obvious.
7. Low cost of CDMA network
The advantages of CDMA system, such as large coverage, small number of base stations, large capacity, high spectrum utilization and simple frequency planning, can greatly reduce the construction cost and operation cost of the system.