What is TD-SCDMA?
TD-SCDMA means Time Division Synchronous Code Division Multiple Access (TD-SCDMA) in Chinese, which is also a technical standard for wireless communications, and it was first proposed by China and based on this Radio Transmission Technology (RTT), in cooperation with the international community, the TD-SCDMA standard was completed. SCDMA standard and became a member of CDMA TDD standard, which is a pioneering move in China's mobile communication industry and China's contribution to the development of third-generation mobile communication. In the competition with the 3G standards proposed by Europe and the United States respectively, the TD-SCDMA proposed by China has officially become one of the
global 3G standards, which signifies that China has entered the world's leading position in the field of mobile communications. The main technology of the program is centralized in the hands of Datang, and it is designed with reference to the time-domain mode of TDD (time-division duplexing) in unpaired frequency bands.
The TDD mode is realized based on a periodically repeating TDMA frame structure in the time domain of the radio channel. This frame structure is subdivided into time slots. In TDD mode, flexible switching between uplink and downlink can be easily realized. The outstanding advantage of this mode is that the allocation of time slots between uplink and downlink can be changed by a flexible switching point to meet different service requirements. In this way, all 3G symmetric and asymmetric services can be realized using TD-SCDMA by flexibly changing the switching point between uplink and downlink. The appropriate TD-SCDMA time-domain operation model can solve the problem of uplink/downlink resource allocation for all symmetric and asymmetric services and any hybrid services by itself.
The TD-SCDMA radio transmission scheme flexibly integrates the basic transmission methods of FDMA, TDMA and CDMA. By combining it with joint detection, it performs exceptionally well in terms of transmission capacity. Capacity can be further increased by introducing smart antennas. With its directionality, the smart antenna reduces interference caused by inter-cell frequency multiplexing and provides higher call volumes through higher frequency multiplexing rates. Based on a high degree of service flexibility, TD-SCDMA wireless networks can be connected to switched networks via a Radio Network Controller (RNC), as defined for circuit- and packet-switched services in three generations of mobile communications. In the final version, the plan is for TD-SCDMA wireless networks to be directly connected to the INTERNET.
The advanced mobile radio system presented by TD-SCDMA is designed for both symmetric and asymmetric 3G services in all wireless environments, and operates on unpaired RF spectrum. time-domain adaptive resource allocation in the direction of TD-SCDMA transmissions achieves optimal utilization of spectrum allocations independently of symmetric service load relationships. optimal utilization of spectrum allocation. Therefore, TD-SCDMA can support all 3G services such as voice and Internet at rates ranging from 8kbps to 2Mbps through optimal adaptive resource allocation and optimal spectral efficiency.
TD-SCDMA is a TDD mode, which has its own characteristics in the application range: First, the mobile speed of the terminal is limited by the existing DSP computing speed can only do 240km/h; second, the base station coverage radius of 15km or less when the spectrum utilization and system capacity can be the best, in the user capacity is not very large area, the maximum base station coverage up to 30-4km. Therefore, TD-SCDMA is suitable for use in urban and suburban areas, and these two shortcomings do not affect the actual use in urban and suburban areas. In urban and suburban areas, vehicle speed is generally less than 200km/h, urban and suburban population density is high, due to capacity reasons, cell radius is generally within 15km. In rural areas and large areas of full coverage, the WCDMA FDD method is also appropriate, so TDD and FDD modes are complementary to each other.
TD-SCDMA's role in 3GPP international standardization
As we all know, TD-SCDMA is a third-generation mobile communication technology promoted and being developed by Chinese and European companies***, which is particularly suitable for the Chinese market's demand for third-generation mobile communication services. Currently, this technology has been formally adopted by the International Telecommunication Union (ITU) as a member of the IMT-2000 family of international standards for third-generation mobile communications, and is recognized as a technology capable of fully supporting third-generation services. This technology has attracted a lot of attention, and at the same time, because IMT-2000 contains TD-SCDMA technology, therefore, within the Third Generation Partnership Project (3GPP) is stepping up the work of standards convergence, in order to promote the development of third-generation mobile communication standards, which makes the TD-SCDMA even further by the attention of the international community.
In fact, people began to study the third-generation mobile communication system very early. in January 1998, the European standardization organization - European Telecommunications Standards Institute Special Mobile Group (ETSI SMG) adopted a proposal on the air interface of the third-generation mobile communication system, which was named Global The UMTS land radio access (UTRA) includes two modes, frequency division duplex (FDD) and time division duplex (TDD).
The technology used for the former is WCDMA, and the technology used for the latter is TD-CDMA.
While the UMTS standard was being developed in Europe, Japan was also conducting extensive research on third-generation mobile communication systems. Japan's standardization organization, the Association of the Radio Industry and Trade (ARIB), likewise chose WCDMA technology, meaning that the Japanese and European proposals for the FDD model were virtually identical. The T1 standardization organization in North America is also developing extremely similar concepts.
At the same time, the Chinese Academy of Telecommunications Technology (CATT) of the Ministry of Information Industry, Siemens, and the China Wireless Telecommunication Standards Committee (CWTS) are stepping up the development of TD-SCDMA technology in TDD mode.
In order to establish a truly global third-generation mobile communications standard, in December 1998, the Third Generation Partnership Project organization (3GPP, http://www.3gpp.org) was established. This organization consists of various national and regional telecom standardization organizations, including ETSI in Europe, T1 in the US, ARIB in Japan, TTA in Korea, CWTS in China, etc. 3GPP coordinated very well the proposals made by different standardization organizations from different parts of the world and worked hard to establish a unified standard for third-generation mobile communications. This standard, which we still call UTRA, is a third-generation mobile communications standard based on the GSM core network and includes both FDD and TDD modes.
In contrast, the Third Generation Partnership Project 2 organization (3GPP2, http://www.3gpp.org/) is developing a third-generation mobile wireless standard called cdma2000. This standard is based on the IS-95 CDMA network.
Third-generation mobile communications cannot be developed without the support of operators. in June 1999, the major international operators in the Operator Harmonization Group (OHG) proposed a harmonized global third-generation mobile communications (G3G) concept, which has been accepted by the 3GPP and 3GPP2. The harmonized G3G concept is a single standard with the following three modes of operation:
* Direct Sequence Spread Spectrum CDMA (CDMA-DS), based on the UTRA FDD mode standardized by 3GPP;
* Multi-Carrier CDMA (CDMA-MC), based on the FDD mode of cdma2000, standardized by 3GPP2;
* TDD (CDMA TDD), based on the UTRA TDD mode standardized by 3GPP.
By working with the producer community, the Operator Coordination Organization (OCO) will try to achieve convergence of all CDMA-based recommendations by making radio parameters as consistent as possible and defining a common protocol stack. This will simplify the implementation of multimode terminals and enable access to existing GSM MAPs and the ANSI-41 core network. OHG's recommendations were taken into account in the first version of the 3GPP specification in 1999, which was finalized at the end of 1999.
A new harmonization effort has been initiated in 3GPP to integrate TD-SCDMA into UTRA. As a first step, TD-CDMA and TD-SCDMA are referred to as 3.84 Mcps TDD and 1.28 Mcps TDD, respectively, based on the difference in code slice rates. These are being worked on in a number of working groups within the 3GPP Technical Specification Group. These include:
* WG1 (Physical Layer)
* WG2 (Protocol Layer, MAS and RLC)
* WG3 (Interfaces, IuB and IuR)
* WG4 (RF Requirements and Test Specification)
Engineers from Europe, China, and South Korea have contributed to this. The goal of this standardization effort was to incorporate TD-SCDMA as part of the UTRA version 4 standard (Release 2000). These specifications were elaborated in 3GPP and 3GPP2 and became part of the ITU's IMT-2000 recommendations.
In order to further develop these standards, all members and participants of 3GPP meet regularly to exchange views and propose new ideas. Based on presentations and suggestions from experts, new features and improvements are extended to existing specifications after the 3GPP organization has reached agreement on particular issues.
The 3GPP working groups expect to complete the Year 2000 version of the standard in early 2001. For a market that is looking forward to a three-generation standard, three sub-standards -- CDMA-DS (UTRA FDD), 3.84Mcps TDD, and 1.28Mcps TDD (TD-SCDMA) -- will gradually mature.
An overview of TD-SCDMA's technical characteristics
TD-SCDMA was proposed later than other standards, which brings certain challenges to its product maturity, but on the other hand, TD-SCDMA has absorbed the most advanced technologies in the field of mobile communications since the 1990s, and to a certain extent represents the direction of development of the technology, with foresight and a strong latecomer's advantage. TD-SCDMA has a forward-looking and strong latecomer's advantage. Compared with other 3G standards, TD-SCDMA system and its technology have the following outstanding advantages:
High Spectrum Efficiency
TD-SCDMA system comprehensively adopts advanced technologies such as joint detection, smart antenna and uplink synchronization, and the multi-access and multipath interference within the system has been greatly mitigated, which effectively improves the utilization rate of the frequency spectrum, and then improves the whole system's capacity of the whole system.
Specifically, joint detection and uplink synchronization can greatly reduce interference within a cell, while smart antennas can effectively suppress interference between and within cells. In addition, joint detection and smart antennas are also extremely useful in mitigating the more pronounced multipath interference on the 2G band. Therefore, this feature of the TD-SCDMA system determines that it will be ideally suited to provide high-capacity network solutions in the early stages of 3G network construction.
Multi-carrier support
For the TD-SCDMA system, its capacity is mainly limited by code resources.TD-SCDMA supports multi-carrier, and inter-carrier switching is easy to realize. Because TD-SCDMA is a time-division system, the cell phone can scan other frequencies while controlling the channel, without any hardware to easily realize the inter-carrier switching, and can guarantee a high success rate. In addition, through the multi-carrier can eliminate the guide frequency pollution and burst guide frequency, thus reducing the call drop rate. Because the TD system can arrange the neighboring cell's guide frequency in different carriers, so as to reduce the guide frequency pollution. As we all know, CDMA system is the most problematic area of the guide frequency pollution, TD has a unique advantage in this regard. In addition, TD also has a great advantage in indoor coverage.
No breathing effect and soft switching
The phenomenon of coverage radius shrinking as the number of users increases is known as the breathing effect. CDMA is a self-interfering system, and when the number of users increases significantly, the self-interference generated by the users increases exponentially, so the breathing effect is an inherent defect of CDMA systems in general.
Another manifestation of the breathing effect is that changes in the number of subscribers for each service result in changes in the coverage radius of all services, which can cause significant problems for network planning and network optimization.
TD-SCDMA is a system that combines CDMA, FDMA, and TDMA, which suppresses the main interference of the system through low-bandwidth FDMA and TDMA, so that factors that generate the breathing effect are significantly reduced. The factor of breathing effect is significantly reduced;
Since TD-SCDMA adopts CDMA technology in each time slot to increase capacity, the only cause of breathing effect is self-interference among multiple users in a single time slot, and since TD-SCDMA can only support up to eight 12.2k voice users in a single time slot, the number of users is small, which makes self-interference among users relatively small.
Meanwhile, this part of self-interference is further suppressed by joint detection and smart antenna technology, so TD-SCDMA is no longer an interference-limited system but a code channel-limited system, and the coverage radius does not change with the increase of the number of users, i.e., there is no breathing effect.
Network Flexibility
Flexible Spectrum Utilization, Abundant Frequency Resources
TD-SCDMA adopts a time-division duplex mode, which allows it to provide 3G services at a rate of up to 2Mbps by occupying a bandwidth of only 1.6MHz on one carrier, making the requirements for frequency allocation much simpler and more flexible. In the future, the use of spectrum resources will be relatively complex in the case of multiple mobile operators***, and the TD-SCDMA system greatly improves the flexibility of spectrum resource utilization.
The Chinese government has allocated a 155MHz frequency band for TDD, which compares with the symmetrical 90MHz frequency band for FDD uplink and downlink***. The advantage of TDD in terms of frequency resources has made it easy for TDD to expand its network capacity and subsequent development.
In addition to China, the 3G spectrum planning of countries around the world include TDD bands, and the 3G licenses of Japanese and European operators already include TDD bands, which provides an opportunity for TD-SCDMA to enter the international market in the future. This provides the necessary conditions for the international application of TD-SCDMA technology and international roaming.
Easy implementation of networking with GSM
From the system point of view, TD-SCDMA and GSM are both time division multiplexing systems, which allow flexible measurement control and switching between systems. From the terminal point of view, TD-SCDMA and GSM switching easier to introduce the current single-mode cell phone, TD-SCDMA/GSM dual-mode cell phone cost lower than the cost of WCDMA/GSM. Currently, Spreadtrum, T3G and other chip vendors support TD-SCDMA/GSM dual-mode cell phone solutions.
Flexible and efficient asymmetric data services
The adoption of TDD technology is the fundamental difference between the TD-SCDMA system and the other two mainstream 3G standards, FDD. The uplink and downlink transition points in the subframe of the TD-SCDMA system can be flexibly set up, so that uplink and downlink resources can be symmetrically divided from 3:3 to 3:3, based on the distribution of the data volume of the different services on the uplink and downlink. According to the distribution of data volume on the uplink and downlink of different bearer services, the uplink and downlink resources can be adjusted from the symmetric allocation of 3:3 to the asymmetric allocation of 1:5.
In the future 3G diversified business applications, asymmetric data services will occupy more and more proportion, and most of the services are typically characterized by asymmetric service volume in uplink and downlink.
The FDD system, due to its fixed symmetric occupancy of uplink and downlink frequencies, will result in a waste of spectrum resources when carrying asymmetric services. On the other hand, TD-SCDMA system can flexibly schedule the system uplink and downlink resources by configuring the position of switching point, so as to maximize the utilization rate of system resources. Therefore, TD-SCDMA system is more suitable for future 3G asymmetric data services and Internet services.
In summary, TD-SCDMA alone has the benefits of simple network planning and low construction and maintenance costs. And TD-SCDMA has the characteristics of asymmetric data service transmission, making it even more incomparable advantages of other technologies.
TD-SCDMA system relay switching technology
Cross-area switching plays an important role in cellular mobile communication systems. In the early Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) mobile communication systems, the use of "hard switching technology", which makes the system in the switching process of about 300ms loss of information, while occupying more channel resources.
The CDMAIS-95 wireless communication system developed by Qualcomm uses "soft switching technology", which does not lose information and does not interrupt communication, but also increases the capacity of the CDMA system. However, soft-switching technology only solves the problem of terminal switching between cells or sectors using the same carrier frequency, but for base stations with different carriers, the FDDCDMA system can only use hard-switching. Moreover, each terminal in the switching process has to receive information from two or three base stations at the same time, and send the corresponding information to these base stations in the reverse link, which occupies more communication equipment and channels, resulting in a waste of system resources.
In TD-SCDMA, a new method of over-the-horizon switching, called relay switching, is used, which is unique in that it uses a smart antenna to obtain the bearing of the user's terminal (DOA), and synchronized CD?MA technology. The unique feature of TD-SCDMA is that it uses a smart antenna to obtain the bearing of the user terminal (DOA) and synchronized CD?MA technology to obtain the distance between the user terminal and the base station. If these two pieces of information are combined, the base station can determine the specific location of the user terminal, thus laying the foundation for relay switching. Relay switching does not lose information, does not interrupt communication, saving channel resources.
It is because the TD-SCDMA system uses a smart antenna and the use of two base stations to locate the terminal, the terminal has the function of accurate positioning, so it can realize more effective switching across the region, the so-called "relay switching". In the process of relay switching, the base stations of the two cells between the same frequency cells will receive the signal of the same terminal and locate it, and report the locating result of determining the possible switching area to the base station controller to complete the switching to the target base station, which overcomes the disadvantage of wasting channel resources in "soft switching". Relay switching not only has the above-mentioned "soft switching" function, but also can be used between TD-SCDMA base stations with different carrier frequencies, and can even be used between TD-SCDMA systems and base stations of other mobile communication systems (e.g. GSM, CDMAIS-95, etc.). -95, etc.), to realize the ideal trans-area switching without loss of information and interruption of communication. Under normal circumstances, "relay switching" and "soft switching" compared to the system capacity can be more than doubled.
TD-SCDMA system smart antenna technology
Smart antenna basic concept
In recent years, smart antenna technology has become one of the most attractive technologies in mobile communications. Smart antenna adopts Space Division Multiple Access (SD?MA) technology, which uses the difference of signals in the transmission direction to distinguish the signals of the same frequency or the same time slot and the same code channel, and maximize the use of limited channel resources. Compared with the non-directional antenna, the antenna gain of its uplink and downlink is greatly improved, which reduces the transmit power level, improves the signal-to-noise ratio, and effectively overcomes the effect of channel transmission fading. At the same time, due to the antenna flap directly pointing to the user, reducing the interference with other users in the district, as well as with neighboring districts between the users, but also reduces the multipath effect of the mobile communication channel. cdma system is a power constrained system, the application of the smart antenna achieves the two major purposes of improving the antenna gain and reducing the interference of the system, which significantly expands the capacity of the system and improves the utilization of the spectrum.
Smart antenna in essence is to use the spatial orthogonality of multiple antenna units, that is, the space division multiple access multiplexing (SDMA) function, to improve the system capacity and spectrum utilization. In this way, the TD-SCDMA system makes full use of the technical advantages of the four multiple access modes, CDMA, TDMA, FD?MA and SDMA, to optimize system performance.
The core of a smart antenna lies in the digital signal processing part, which makes the antenna array generate a directional beam pointing at the user according to certain criteria, and automatically adjusts the coefficients to realize the required spatial filtering. The two key issues to be solved by the smart antenna are recognizing the direction of the signal and realizing the digital assignment.
Operating principle of smart antenna
The smart antenna of TD-SCDMA uses a loop antenna array, which consists of eight identical antenna elements evenly distributed on a circle with radius R. The function of the smart antenna is defined by the antenna array. The function of the smart antenna is accomplished by the antenna array and the baseband digital signal processing part*** connected to it. The smart antenna has the same radiation pattern in the elevation direction as each antenna element. The directional graph in the azimuth direction is controlled by the baseband processor, which can generate multiple beams at the same time, which are arbitrarily fitted in a 360° range according to the distribution of communication users. In order to eliminate interference, the beam assignment can also set a zero point where there is interference, and the antenna radiation level at this zero point is about 40 dB lower than the maximum radiation direction.The smart antenna used in TD-SCDMA has a gain of 9 dB (for receiving) and 18 dB (for transmitting) greater than that of the non-directional, single-vibrator antenna, when N = 8, respectively. With a gain of 8dB per oscillator, the antenna has a maximum receive gain of 17dB and a maximum transmit gain of 26dB. Because the transmit gain of the base station smart antenna is much larger than the receive gain, it is ideal for transmitting data such as asymmetric IP and downloading larger service information.
The main function of smart antenna
According to the above basic principle, in CDMA system (no matter TDD or FDD mode), the use of smart antenna and beam fouling technology can greatly improve the performance of the communication system in many ways, in a nutshell, the main features are: improve the sensitivity of the base station receiver, increase the equivalent transmit power of the base station transmitter, reduce system interference, and increase the performance of the CDMA system, which can improve the sensitivity of the base station receiver and increase the equivalent transmit power of the base station transmitter, and reduce system interference. interference, increasing the capacity of the CDMA system, improving cell coverage, and reducing the cost of wireless base stations.
Due to the use of smart antennas, the application of beam fouling technology significantly improves the receiving sensitivity and equivalent transmit power of the base station, which can greatly reduce the interference within the system and the interference between neighboring cells, thus expanding the capacity of the system more than doubled; at the same time, it can also be used to make the business density of the urban area and the suburban areas of the number of base stations required to reduce. In sparsely populated rural areas, wireless coverage is doubled, which means that the number of BTSs in the covered area is reduced to 1/4 of what it would normally be.The increase in antenna gain also reduces the linear output power of the high-frequency power amplifier (HPA). This is because the cost of the HPA accounts for a major portion of the cost of the transceiver. Therefore, the adoption of smart antennas will significantly reduce operating costs and improve system economics.
What are the advantages of TD-SCDMA networks?
Compared with WCDMA and CDMA2000 networks, TD-SCDMA network is a perfect combination of TDD and CDMA, TDMA technology, there are good technical advantages: the first advantage, high spectrum utilization, only one 1.6M bandwidth can be communicated; the second advantage, TD-SCDMA adopts smart antenna, software antenna, and software antenna. SCDMA adopts a large number of advanced technologies such as smart antenna, software radio, etc., which can improve the system capacity; the third advantage, TD-SCDMA is more suitable for transmitting asymmetric Internet services. From the point of view of global frequency division, countries have reserved frequency bands for TDD, in this sense, only TD-SCDMA is possible to realize global roaming.