At present, the 3G standards accepted by ITU mainly include the following three types: WCDMA, CDMA2000 and TD-SCDMA. CDMA is the abbreviation of Code Division MulTIple Access (Code Division Multiple Access) and the technical foundation of the third generation mobile communication system. The first-generation mobile communication system adopted the analog modulation method of frequency division multiple access (FDMA). The main disadvantages of this system are the low spectrum utilization rate and the interference of voice services by signaling. The second-generation mobile communication system mainly uses time division multiple access (TDMA) digital modulation to increase system capacity, and uses independent channels to transmit signaling, which greatly improves system performance. However, TDMA system capacity is still limited, and handover Performance is still not perfect. The CDMA system shows huge development potential due to its simple frequency planning, large system capacity, high frequency reuse factor, strong anti-multipath capability, good communication quality, soft capacity, and soft handover.
The competition between China Mobile and China Unicom in the mobile communication market is becoming increasingly fierce, and the field of competition has evolved from the original voice business to value-added services. With the continuous development of mobile value-added services, moving towards 3G (3rd GeneraTIon, third-generation mobile communications) is an inevitable choice for the two major mobile operators. Compared with the previous two generation systems, the main feature of the third generation mobile communication system is that it can provide a variety of mobile multimedia services. Its transmission rate supports 144kb / s in high-speed mobile environments and 384kb / s in slow-moving mobile environments. , Support 2Mb / s in static state. Its design goal is to provide greater system capacity and better communication quality than the second-generation system, and to better achieve seamless roaming and provide users, including voice, data and multimedia, etc., on a global scale. At the same time, we must also consider the good compatibility with the existing second-generation system.
1. WCDMA
The full name is Wideband CDMA, which is a 3G technical specification developed based on the GSM network. It is a broadband CDMA technology proposed in Europe. It is basically the same as the broadband CDMA technology proposed in Japan and is currently being further integrated. This standard proposes the evolution strategy of GSM (2G) -GPRS-EDGE-WCDMA (3G). GPRS is the abbreviation of General Packet Radio Service, and EDGE is the abbreviation of Enhanced Data rate for GSM EvoluTIon. These two technologies are called 2.5 generation mobile communication technologies. China Mobile is currently adopting this plan to transition to 3G, and has upgraded the original GSM network to a GPRS network.
2. CDMA2000
CDMA2000 is a wideband CDMA technology developed from narrowband CDMA (CDMA IS95) technology, mainly promoted by the United States. The standard proposes an evolution strategy from CDMA IS95 (2G) to CDMA20001x to CDMA20003x (3G). CDMA20001x is known as 2.5 generation mobile communication technology. The main difference between CDMA20003x and CDMA20001x is the application of multi-carrier technology, and the bandwidth is improved by using three carriers. At present, China Unicom is adopting this plan to transition to 3G, and has built a CDMA IS95 network.
3. TD-SCDMA
The full name is TIme Division-Synchronous CDMA (Time Division Synchronous CDMA), which is a 3G standard proposed by China's Datang Telecom Company. The standard proposes a transition to 3G without going through the 2.5-generation intermediate link, which is very suitable for GSM system to 3G upgrade. But at present, Datang Telecom has not launched commercial products based on this standard.
Comparison of three technical standards
WCDMA, CDMA2000 and TD-SCDMA are all broadband CDMA technologies. Broadband CDMA further expands the standard CDMA concept, spreading the signal over a relatively wider frequency band, thereby reducing the propagation problems caused by multipath and attenuation, has greater capacity, and can use different bandwidths according to different needs. It has strong anti-fading ability and anti-interference ability, supports multiple simultaneous calls or data transmission, and is compatible with existing equipment. WCDMA, CDMA2000 and TD-SCDMA can all provide a 2Mbit / s data transmission rate in a static state, but some key technologies of the three still have large differences and performances are also different.
1. Duplex mode
Both WCDMA and CDMA2000 use FDD (Frequency Division Digital Duplex) mode, and TD-SCDMA uses TDD (Time Division Digital Duplex) mode. FDD is a duplex mode that uses two separate symmetrical frequency bands for uplink (transmit) and downlink (receive) transmissions. It requires paired frequencies. Frequency is used to distinguish between upstream and downstream, and it can be fully utilized for symmetric services (such as voice). Uplink and downlink spectrum, but for asymmetric packet-switched data services (such as the Internet), due to low upstream load, spectrum utilization is greatly reduced. TDD is a duplex mode that uses the same frequency band for upstream and downstream transmissions. It distinguishes between upstream and downstream according to time and switches. The time slot of the physical layer is divided into upstream and downstream parts. There is no need for paired frequencies. Link services share the same channel and can be unevenly distributed, especially suitable for asymmetric packet-switched data services (such as the Internet). TDD has a high spectrum utilization rate and low cost. However, due to the multi-slot discontinuous transmission method, the ratio of the peak power to the average power of the base station is high, which results in a large base station power consumption and a small base station coverage radius. The performance of anti-fading and anti-Doppler frequency shift is poor, and the communication ability is poor when the mobile phone is in a state of high-speed movement. WCDMA and CDMA2000 can support the normal communication of mobile terminals at a speed of about 500 kilometers per hour, while TD-SCDMA can only support the normal communication of mobile terminals at a speed of about 120 kilometers per hour. TD-SCDMA is at a disadvantage in high-speed mobile environments such as highways and railways.
2. Chip rate and carrier bandwidth
WCDMA (FDD-DS) uses direct sequence spread spectrum, and its chip rate is 3.84 Mchip / s. The difference between CDMA20001x and CDMA20003x is the number of carriers. CDMA20001x is a single carrier with a chip rate of 1.2288 Mchip / s, CDMA20003x is a triple carrier with a chip rate of 1.2288 × 3 = 3.6864 Mchip / s. The chip rate of TD-SCDMA is 1.28Mchip / s. The high chip rate can effectively use frequency selective diversity and spatial receive and transmit diversity, which can effectively solve the multipath problem and fading problem. WCDMA has the most advantage in this regard.
In terms of carrier bandwidth, WCDMA uses direct sequence spread spectrum technology with a carrier bandwidth of 5 MHz. CDMA20001x uses a carrier bandwidth of 1.25MHz, and CDMA20003x uses a combination of three 1.25MHz carriers to form a 3.75MHz carrier bandwidth. TD-SCDMA uses a three-carrier design, each carrier has a bandwidth of 1.6M. The higher the carrier bandwidth, the more users it supports, and the less likely it is that network congestion will occur during communication. In this respect, WCDMA has obvious advantages.
The TD-SCDMA system only uses the chip rate of 1.28Mchip / s and adopts the TDD duplex mode, so it only needs to occupy a single 1.6M bandwidth to transmit 2Mbit / s data services. While WCDMA and CDMA2000 need to transmit 2Mbit / s data services, they need two symmetrical bandwidths, which are used as uplink and downlink frequency bands, respectively. Therefore, TD-SCDMA has the highest utilization rate of frequency resources.
3. Smart antenna technology
Smart antenna technology is the key technology adopted by TD-SCDMA. Datang Telecom has applied for a patent. Currently, WCDMA and CDMA2000 have not adopted this technology. A smart antenna is a two-way antenna installed at the site of a base station. The directionality is obtained through a set of fixed antenna units with programmable electronic phase relationships, and the directionality of each link between the base station and the mobile station can be obtained simultaneously. The high efficiency of the TD-SCDMA smart antenna is based on the symmetry of the uplink and downlink wireless paths (the wireless environment and transmission conditions are the same). Smart antennas can also reduce interference between and within cells. These characteristics of smart antennas can significantly improve the spectrum efficiency of mobile communication systems.
4. Handover technology
Both WCDMA and CDMA2000 use cross-region "soft handover" technology, that is, when the mobile phone is moving or the traffic of the base station that is currently communicating with the mobile phone is busy so that the mobile phone needs to communicate with a new base station, the communication with the original base station is not interrupted Instead, it connects to the new base station first and then disconnects from the original base station. This is the classic CDMA technology. "Soft handover" is relative to "hard handover". Both FDMA and TDMA systems use "hard handover" technology, which interrupts the connection with the original base station first, and then connects with the new base station, so it is easy to cause dropped calls. Because soft handover connects two base stations at the same time, it occupies a large amount of channel resources. TD-SCDMA uses the "relay switching" technology of over-the-zone. The smart antenna can roughly locate the user's position and distance. The base station and the base station controller can determine whether the user moves to another location based on the user's position and distance information. If a neighboring area of ​​a base station enters the handover area, the base station controller notifies the other base station to prepare for handover to achieve the purpose of relay handover. Relay handover is an improved hard handover technology, which can improve the success rate of handover, and can reduce the occupation time of adjacent base station channel resources during handover compared with soft handover.
During the handover process, coordinated operation between the two base stations is required. WCDMA does not require synchronization between base stations and completes the soft handover through the timing difference report between the two base stations. Both CDMA2000 and TD-SCDMA require strict synchronization between base stations. Therefore, GPS (Global Positioning System) and other devices must be used to determine the location of the mobile phone and calculate the distance to the two base stations. Because GPS relies on satellites, the deployment of CDMA2000 and TD-SCDMA networks will be subject to some restrictions, and WCDMA networks are easier to deploy in many environments. Base stations can be installed even in places where GPS signals cannot reach, such as subways Seamless overlay. And GPS is an American system. If the mobile communication system is built on the basis of reliable GPS operation, it will be subject to the US GPS policy and has certain risks.
5. Compatibility with the second generation system
WCDMA is a transition from the GSM network. Although the GSM core network can be retained, the WCDMA access network must be re-established, and it is impossible to reuse the GSM base station. CDMA20003x transitions from CDMA IS95 and CDMA20001x, and can retain the original CDMA IS95 equipment. The construction of TD-SCDMA system only needs to add TD-SCDMA equipment on the existing GSM network. Among the three technical standards, WCDMA costs the most in the upgrade process.
3G strategy for mobile operators
Currently, 73 WCDMA operating licenses and 13 CDMA2000 operating licenses have been issued worldwide. China's 3G license has not yet been issued, and the technical standards that operators such as China Mobile and China Unicom will adopt have not yet been determined. Not long ago, the Ministry of Information Industry has planned the frequency of use of WCDMA, CDMA2000, and TD-SCDMA, indicating that these three standards will be adopted in China.
2.5G technology was derived between 2G and 3G. 2.5G technology breaks through the restriction of 2G circuit switching technology on data transmission rate, and introduces packet switching technology, so that the data transmission rate has a qualitative breakthrough. It is a transition technology between 2G and 3G. At present, China Mobile has built a 2.5-generation GPRS network and is developing in the direction of WCDMA. After China Unicom developed the GSM network, it suddenly turned to the CDMA IS95 network, which is developing in the direction of CDMA2000. Although CDMA2000 saved investment in the process of upgrading, China Unicom developed a CDMA IS95 network from a GSM network, and its network cost is also quite large. Due to the late start of China Unicom's CDMA network construction, the 2.5-generation CDMA20001x network has not yet been built, and it is at a disadvantage in competing with China Mobile's 2.5-generation business. On October 1, this year, China Mobile officially launched the MMS service (MMS, multimedia information service) based on the 2.5 generation network. This service can load multimedia information such as voice, image, and video in mobile phone short messages, and can reach about 40Kbit using GPRS network. The transmission speed of / s opened the prelude to the era of mobile multimedia, and mobile phones with new functions such as color screen polyphonic built-in digital cameras immediately became popular in the market. In response to China Mobile's MMS service, Guangdong Unicom recently launched the color e service, but China Unicom's CDMA IS95 network can only provide 14.4Kbit / s transmission speed based on circuit switching, which forms a bottleneck for the transmission of multimedia information. The rapid development of the 2.5 generation CDMA20001x network has become a top priority for China Unicom.
Whether China's TD-SCDMA with independent intellectual property rights can occupy a place in the 3G technical standard hegemony has attracted much attention. TD-SCDMA can effectively save spectrum resources and can achieve cheap upgrade from GSM system, but its communication quality is worse than WCDMA and CDMA2000. After all, whether it is possible to save spectrum resources and investment costs is only a matter of concern to governments and operators. As a user, communication quality is always the first choice. Under the fierce competition in China's mobile communications market, satisfying the needs of users has always been the goal pursued by operators. In the future, TD-SCDMA may be applied in the low-end 3G market. At present, TD-SCDMA technology has not been adopted by foreign operators. If only China adopts this standard in the future, it will pose new problems for international roaming. Datang Telecom has not yet launched a mature product based on TD-SCDMA technology, and its R & D progress lags behind WCDMA and CDMA2000. But not long ago we were happy to see the establishment of the "TD-SCDMA Industry Alliance". Datang Telecom, Southern High-Tech, Huali, Huawei, Lenovo, ZTE, China Electronics, China Putian and other 8 companies formed the first core of the alliance. The technology has a strong technical force to support the commercialization. TD-SCDMA is China's first standard in the field of mobile communications, and its emergence is a zero breakthrough in China's 100-year telecommunications history. We are happy to see that TD-SCDMA can mature.
At present, the second-generation mobile communication system, whether it is GSM or CDMA IS95, can already provide basically satisfactory voice quality and communication stability, but its data transmission rate is low, so the third-generation mobile communication system is the most attractive The place is not the improvement of voice quality and communication stability, but the substantial increase of data transmission rate, which will greatly promote the development of mobile multimedia services. However, the main use of mobile phones is, after all, not other value-added services. Whether 3G's huge investment can create benefits is still unknown. The current 2.5G business development status can provide some help for our 3G strategy.
Since the introduction of China Mobile's GPRS, the more successful MMS service is a multimedia service based on GPRS bandwidth. The number of users who directly use GPRS mobile phones and computers to connect to the Internet has been small. After all, there are only a few people with mobile Internet needs. At present, 2.5 generation GPRS or CDMA20001x can already provide a data transmission rate of about 40Kbit / s, which can basically meet the bandwidth requirements for the transmission of multimedia information such as sound, images, and short videos. The main purpose of mobile Internet access is to send and receive E-Mail and other official business with very urgent time requirements, rather than downloading video and other entertainment activities, and the current bandwidth can basically meet. The theoretical transmission rate of GPRS or CDMA20001x is about 150kbit / s. With the continuous upgrade of 2.5G networks in the future, its actual transmission rate will gradually approach this value, which can provide stronger support for mobile multimedia and mobile Internet services.
The 3G network can have a data transmission rate of 2Mbit / s when the mobile phone is stationary. In terms of multimedia services, the advantage of 3G over 2.5G is that it can provide more colorful video information; as far as mobile Internet access is concerned, the speed of mobile Internet access can basically reach the level of the current wired broadband network. But can the greatly increased bandwidth increase enough business volume to make 3G profitable? In terms of multimedia applications, mobile phones can be used for digital video recording and the video can be quickly sent to other mobile phones, which can be used in important gatherings such as reporter interviews and wedding banquets. This is a profit point for 3G, but the number of users is very small after all. In terms of mobile Internet access, you can use mobile phones to download videos online or watch online movies, online TV live broadcasts, etc. However, due to the rapid popularity of cable broadband networks, such users are few. Moreover, the cost of mobile communication is much higher than that of wired communication, and its tariff is naturally not low, and the price will also become an unfavorable factor restricting the development of 3G services.
At present, mobile operators need to focus on how to build and further optimize 2.5G networks to provide better support for mobile multimedia and mobile Internet services. This is, after all, a project with little investment and great benefits. The development of 3G is the general trend, but the maturity of the potential market should be used as the basis for starting, and it must not fall into the blind comparison between countries or between operators and operators.
Considering the above factors, China currently does not have the market conditions to develop 3G. The rest of the world has adopted a very cautious attitude towards the development of 3G. Japan, which has developed rapidly as WCDMA, has postponed its 3G development plan. Vodafone Group of the United Kingdom announced that the 3G service originally planned to be launched in Germany this fall will be postponed for about 6 months, while terminating the 3G network infrastructure being carried out in the United Kingdom and other parts of Europe. Orange, a subsidiary of France Telecom, is negotiating with Swedish officials to postpone the 3G service in Sweden. Telefonica of Spain and Sonera Telecom of Finland announced their suspension of the provision of 3G services to Germany, Italy, Austria and Switzerland. Six German suppliers of universal mobile communication systems have been forced to postpone the commercial operation of 3G. In Hong Kong, China, operators who originally expected to fully develop 3G this year and next will also postpone their time to 2005 or 2006.
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