Principle of CDMA: CDMA 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 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.
I. Characteristics of CDMA cellular mobile communication network 1. In terms of system capacity, CDMA mobile network is 20 times larger than analog network. Actually, it is 10 times larger than analog network and 4-5 times larger than GSM. 2. Flexible configuration of system capacity: This 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, we think of bandwidth as a big house. Everyone will enter the only big house. If they use completely different languages, they can clearly hear the voices of their peers and will only be disturbed by other people's conversations. Here, you can think of the air in the house 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. Good call quality: The voice quality of CDMA system is very high, and the vocoder 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, 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. Simple frequency planning users are distinguished by different sequence codes, so adjacent cells can use different CDMA carriers, and the network planning is flexible and easy to expand. 5. Using power control and variable rate vocoder to prolong the service life of mobile phone battery. 6. The cost of network construction has been reduced.
GPRS (General Packet Radio Service) is a Chinese general packet radio service, which is a new packet data carrying service developed on the existing GSM system. The fundamental difference between GPRS and GSM is that GSM is a circuit-switched system, while GPRS is a packet-switched system. GPRS is especially suitable for intermittent, sudden or frequent, small data transmission and occasional large data transmission. We can understand GPRS as a higher level of GSM.
CDMA is a mobile communication technology in parallel with GSM, which is the English abbreviation of code division multiple access digital wireless technology. It has the advantages of high frequency utilization rate and low power consumption. Like GSM, CDMA has the second, second and third generation technologies. CDMA, which China Unicom will launch in the second half of this year, is a 2.5-generation technology. CDMA is considered as the first choice of the third generation mobile communication technology, and the current standards include WCDMA, CDMA2000 and TD-SCDMA.
In the past, the technical development trend of China mobile communication industry was the transition from the second generation mobile communication to the third generation technical standard through 2.5 generations. Because there are three mainstream technical standards for the third generation mobile communication, the choice of technical upgrading and industrial technology development standards for mobile communication operators will determine the direction of industrial technology development.
Table15 Comparison of GSM, GPRS and CDMA technologies
GSM (second generation)
GPRS(2.5G)
Code Division Multiple Access (code division multiple access)
full name
Global System for Mobile Communications is one of the major cellular systems in the world.
General wireless packet service is a wireless packet switching technology based on GSM system, which belongs to the transition technology of 2.5G
Code Division Multiple Access (CDMA) is a digital mobile phone system with a frequency range of 900- 1800 MHz, which is designed according to the American standard (IS-95).
superiority
The adopted countries and regions are wider, cross-regional roaming can be realized, and the background billing system is easier to support.
Wireless internet access, GSM backward compatibility; Data can be sent and received in groups, which reduces the cost and expense.
The system capacity is 4-5 times larger than that of GSM; Flexible configuration, CDMA is a self-jamming system, all mobile users occupy the same bandwidth and frequency, and the call quality is good, and its threshold value can be changed according to the change of background noise, which overcomes the shortcoming that hard handover is easy to drop calls. The network planning is simple and flexible, and the expansion is simple; Network construction cost is low.
disadvantaged
Insufficient capacity or spectrum utilization; The voice quality is low and it is easy to drop the line; It can not support high-speed data and multimedia services, and it is difficult to meet the growing market demand such as Internet wireless access.
It's a transitional technology, and commercialization is under pressure of cost recovery, so the actual internet access speed is slow.
The 3G standard has not been unified, which brings difficulties to the R&D and production of equipment suppliers.
prospect
GSM technology is fully mature, its development potential is close to the limit, and it is on the verge of product elimination.
Many operators all over the world have opened GPRS commercial system, trial commercial system or experimental system, including BT Cellnet in Britain, T-Mobile in Germany, Smartone in China, TSL in Taiwan Province Province of China, Cellcom in Malaysia, Optus in Australia and operators in France, Spain, Portugal, Finland, Czech Republic and other countries and regions.
The technical advantages of CDMA are obvious. China Unicom has started the construction of CDMA network, and the current enhanced CDMA IS95 will smoothly transition to the third generation CDMA2000 through CDMA1X.
Source: CCID, 2002.
Table 16 3G standard technology comparison
W-CDMA
CDMA2000
TD-SCDMA
Formula designer
WCDMA standard is mainly proposed by ETSI in Europe.
CDMA2000 is an RTT scheme proposed by TIA TR45.5 of the United States to ITU, and its core is the broadband cdmaOne technology jointly proposed by Qualcomm, Lucent, Motorola and Nortel.
It was put forward by China on behalf of the Institute of Telecommunications Science and Technology of the Ministry of Information Industry (Datang Telecom Group). In May 2000, it was officially listed as one of the wireless transmission standards of the third generation mobile communication in the world at the ITU wireless conference.
superiority
It has advantages in long-distance communication and other fields; Before GSM goes to WCDMA, it has to go through GPRS and EDGE to increase the bandwidth. This year, GPRS has been tested all over the world. Because the drafter of WCDMA standard-3GPP is also the maker of GSM standard, and the huge GSM network is all over the world, developing 3G and WCDMA on this basis will take the lead.
Priority should be given to scalability and clear growth and upgrading routes when designing. In just three years of commercial operation, the number of users has reached 30 million, which is the fastest growth rate in the world. Low cost is also one of the reasons why many communication service providers decide to adopt CDMA2000, which can make use of existing equipment and greatly reduce the recent investment; Obtaining the support of the US government is an important bargaining chip in the Sino-US negotiations.
It has unique technical and economic advantages. Using TDD mode, the transmitter and receiver are in the same frequency band, which can save the frequency band; The spectrum utilization rate is very high, which can solve the problem of tight frequency resources in high population density areas, and has outstanding advantages in asymmetric mobile data such as Internet browsing and multimedia services such as video on demand. The base station of the system adopts smart antenna, which has small radiation and good call quality, and reduces the system cost. It has good compatibility with the existing GSM system and can solve the problem of smooth transition from GSM to the third generation.
condition
At present, the world's major telecom manufacturers Ericsson, Nokia, Lucent, Motorola, Nortel, Alcatel, Siemens, NEC, Fuji and China's communication equipment manufacturers Huawei and ZTE all regard WCDMA system as the most important 3G system, and actively participate in the formulation of WCDMA standards and specifications. The participation of Japanese companies has accelerated the maturity and stability of WCDMA system equipment. At present, major operators in Europe and Asia have chosen WCDMA as their 3G operating system.
CDMA2000 relies on Global Positioning System (GPS) to achieve synchronization, which on the one hand makes it difficult to synchronize indoor base stations, and on the other hand, because GPS is a military technology controlled by the United States, it is easily controlled by the United States. Therefore, some countries are unwilling to fully support CDMA2000 as a global unified standard. The United States, the birthplace of CDMA2000, makes operators in other countries lack confidence in the development of the third generation. A few years ago, the American government auctioned the frequency resources of the third generation mobile communication. It is impossible for the government to recover this frequency resource and send it to operators as 3G frequency. This is a very embarrassing thing for the United States on the third generation issue. At present, in the United States, a variety of different mobile communication technology standards coexist. Even if the third generation technology in the United States adopts CDMA2000, it is impossible to unify to this standard very uniformly.
At present, the maturity of TD-SCDMA is relatively lagging behind, and there are not many equipment vendors that carry out R&D and production under this technical standard. Any kind of advanced technology can only show its due value if it is put into business and has real products for consumers to use. On February 25th, 2000, Datang Telecom disclosed some key technologies of TD-SCDMA standard to domestic operators and manufacturers. This means that from now on, Chinese communication enterprises can participate in the formulation of this international standard, which is conducive to the overall breakthrough of the third-generation mobile communication and the formation of the third-generation mobile communication industry cluster in TD-SCDMA. On September 22nd, 2000, seven Chinese and foreign telecom giants, including China Mobile, China Unicom, Datang Telecom, Huawei Technologies, Motorola, Nortel Networks and Siemens, launched the TD-SCDMA Technology Forum in Beijing to promote the development and application of TD-SCDMA technology in the world.
Source: CCID, 2002.
Table 17 Selection and Prediction of 3G Standards for Operators
operator
third generation
W-CDMA
CDMA2000
TD-SCDMA
China Mobile
Based on the existing GSM network, GPRS service can be carried out nationwide, which is an effective way to transition to the third generation W-CDMA through GSM network. At present, most mobile communication systems in China are based on GSM technology. China Mobile's networks are all GSM systems, and China Mobile has the largest GSM network in the world, covering 20 18 counties and cities, 330 cities, and covering more than 98% of township cities. International roaming covers 72 countries and regions, with 138 operators. At the beginning of July, 20001year, its GPRS service was put into commercial trial in 25 cities in China 16 provinces and cities, with a capacity of 400,000 households, and was finally delivered to W-CDMA.
Because this standard is the first international technical standard put forward by China, China Mobile will consider certain ethnic factors when choosing it, and may have corresponding support in policy.
China Unicom
At present, the CDMA network owned by Unicom is mainly the mobile communication network operated by the former Great Wall Telecom Company, and 200 1 and 1 took over. This makes the capacity of China Unicom CDMA mobile communication network expand to 750,000, and the number of users increase to 550,000. The total capacity of the first phase of CDMA project is 1, 5 1.5 thousand, covering more than 200 important cities and all areas above the county level in one year, and transitioning to the third generation CDMA2000 through 2.5 generation CDMA1X.
Because this standard is the first international technical standard put forward by China, China Unicom will consider certain ethnic factors when choosing it, and its policies may be supported accordingly.
Emerging mobile operators
WCDMA covers the whole world. The acceptance and participation of many operators and manufacturers is conducive to technology optimization and continuous cost reduction. At the same time, the consistency of standards is also more conducive to achieving global roaming. However, the support for its voice service needs to be verified and the terminal products need to be commercialized.
It can provide high-quality voice services and advanced 3G data services, and can be upgraded to 1x EV in the future to provide broadband data services required by IMT2000.
China is the main initiator of TD-SCDMA standard. TD-SCDMA, as the first international technical standard put forward by China, is of great significance to China's technological progress. However, the standard is not complete, so it cannot be tested on a large scale, and the commercialization gap is obvious.
Brief introduction of CDMA technology
1. In the international proposal of the third generation mobile communication radio interface, W-CDMA and cdma2000 are the most concerned. Except for chip rate, synchronization mode and pilot mode, these two wideband CDMA schemes have little difference in power, soft handover and other basic technologies.
The cdma2000- 1X introduced in this lecture belongs to one of the cdma2000 series schemes. In view of the numerous and confusing terms related to CDMA, even people in the industry are often confused. It is helpful for readers to make a brief and clear description of this series of noun abbreviations first.
1, IS-95A and is-95b-the second generation CDMA technical standards ▲ is-95a-are narrowband CDMA(N-CDMA) standards officially promulgated by TIA of the United States in 1995.
▲ IS-95B is a further development of IS-95A, and the standard is formulated in 1998. The main purpose is to meet the needs of higher bit rate services. The maximum bit rate that IS-95B can theoretically provide is 1 15kbit/s, but it can only be realized at 64kbit/s. IS-95A and IS-95B have a series of standards, which are collectively called IS-95. 2 and cdma one The second generation cdma technical standard cdma one is
3.cdam2000 and IS-2000-the third generation cdma technology ▲cdma2000 is a proposal of the third generation mobile communication air interface standard put forward by the United States to ITU, a technical system scheme for the evolution of IS-95 standard to the third generation, and a broadband CDMA technology. The highest indoor data rate of cdma2000 is above 2Mbit/s, 384kbit/s in walking environment and 144kbit/s in vehicle environment.
▲IS-2000 is the general name of the formal standard adopting cdma2000 technology. IS-2000 series standards have six parts, which define various interfaces between mobile stations and base station systems.
4. CDMA 2000-1x/cdma2000-3x/cdma2000-1ev ▲ cdma2000-1x originally refers to the first stage of cdma2000 (the rate is higher than IS-95, but lower than 2Mbit/s), which can support 3000.
▲cdma2000-3X Some people call it the second stage of cdma2000, but it is not accurate. The main difference between cdma2000- 1X and cdma2000- 1X is that the forward cdma channel adopts 3-carrier mode, while CDMA 2000- 1x adopts single-carrier mode. Therefore, its advantage is that it can provide higher data rate, but it also occupies wider spectrum resources. For a long time, operators may not consider cdma2000-3X, but cdma2000- 1Xev. ▲ CDMA 2000- 1xev is an enhanced system that further improves the speed on the basis of CDMA 2000- 1x, and adopts high-rate data (. It can provide data services above 2Mit/s at 1.25MHz (the same bandwidth as cdma2000- 1X), which is the edge technology of CDMA 2000-1x. 3GPP has started to formulate the technical standard of cdma2000- 1XEV, in which the technology in Qualcomm is called HDR, and the technology jointly developed by Motorola and Nokia is called 1XTREME, and LAS-CDMA in China also belongs to this category.
Different from GSM, GPRS evolved from GSM is the second generation semi-finished product, while CDMA has no second generation semi-finished product.
IS-95 is the second generation, and IS-2000 (including cdma2000- 1X, cdma2000-3X, cdma2000- 1XEV, etc. ) is the third generation product. Of course, there are obvious differences in business performance and functions among various series of products.
Second, the characteristics of cdma2000 series 1, cdma2000 series third generation mobile communication system mainly pursues higher bit rate and better spectrum efficiency.
The final formal standard of cdma2000 was passed in March 2000. The following table summarizes the main technical features of cdma2000 series.
Bandwidth (MHz)1.25 3.75 7.515 Wireless interface comes from IS-95 network structure comes from IS-4 1 service evolution comes from IS-95 maximum user bit rate (bit/s) 307.2k/kloc-0.
The pilot mode is -95 (using common pilot mode and multiplexing with service code).
Analysis of the above table shows that compared with cdma one, cdma2000 has the following technical characteristics: ● The channel bandwidth is diverse. The forward link supports multicarrier (MC) and direct spread spectrum (DS). The reverse link only supports direct spread spectrum.
When the multi-carrier mode is adopted, various radio frequency bandwidths can be supported, that is, the radio frequency bandwidth can be N X 1.25MHz, where N = 1, 3, 5, 9 or 12. At present, the technology only supports the first two, namely1.25mhz (cdma2000-1x).
● Wireless resources can be used more effectively.
● Smooth transition from cdma one to cdma2000 system can be realized.
● Core network protocols can use IS-4 1, GSM-MAP and IP backbone network standards.
● Forward transmit diversity.
● Fast forward power control.
● Use Turbo code.
● Auxiliary pilot channel.
● Flexible frame length.
● Reverse link coherent demodulation.
● A longer interleaver can be selected.
2. CDMA 2000-1x CDMA 2000-1x (the first stage) adopts the spreading rate of SR 1, that is to say, both the forward channel and the reverse channel adopt the single-carrier direct sequence spreading method, and the chip rate is1.2288mbit/s/s. Operators can deploy cdma2000- 1X system with the same carrier on some cdma one which needs high-speed data services, thus reducing the investment of users and operators.
Because cdma2000- 1X adopts new technologies such as reverse coherent demodulation, fast forward power control, transmit diversity and Turbo coding, its capacity has been greatly improved compared with IS-95. Under the same conditions, the capacity of ordinary voice service is roughly twice that of IS-95 system.
3.cdma2000-3X cdma2000-3X is a cdma2000 system with spreading rate SR3 (denoted as 3X). Its technical feature is that the forward channel adopts three-carrier multi-carrier modulation mode, each carrier adopts direct sequence spread spectrum of 1.2288Mbit/s, and the reverse channel adopts direct spread spectrum with chip rate of 3.6864Mbit/s, so the channel bandwidth of cdma2000-3X is 3.75MHz, and the maximum user bit rate is1.0368mbit.
4. High-speed data rate HDR(cdma2000- 1XEV)
This is an enhanced 3G system based on CDMA 2000-1X. It can share the original system resources with CDMA 2000- 1x * * except the signal processing part of the base station and the user's handheld terminal. The bandwidth of 1.25MHz can reach 2.4Mbit/s (even higher than cdma2000-3X), and the data service of 153.6kbit/s can also be provided on the reverse link, which supports high-speed packet service well and is suitable for mobile IP. This lecture mainly introduces cdma2000-1x.