TD-SCDMA monograph This article was published in June 2003.
Open a Smooth Air Passage ——TD-SCDMA Wireless Transmission Technology and System Features
As long as China adopts the third generation communication technology, it is inevitable to bring TDMA and SCDMA into the overall system framework. Now the government's support makes TD-SCDMA gain more confidence, so judging from the current situation, how long TD-SCDMA transmission technology can be applied and how effective it is will determine where operators finally put it.
As a TDD mode technology, TD-SCDMA is more suitable for the business environment with asymmetric uplink and downlink than FDD, and it is a new technology that combines multi-slot TDMA, direct spread spectrum CDMA and synchronous CDMA technologies. At the same time, the air interface technology recommended by TD-SCDMA standard, as one of the most advanced transmission technologies in the industry, can be easily integrated with other technologies, such as smart antenna technology, synchronous CDMA technology and software radio technology. Among them, smart antenna technology effectively takes advantage of TDD uplink and downlink working at the same frequency, which can greatly increase the system capacity, reduce the transmission power, and better overcome the multipath attenuation problem encountered in wireless propagation.
The biggest advantage of good compatibility is that it can achieve the leap to 3G in various ways, thus avoiding many patent problems in FDD CDMA technology. At the same time, TD-SCDMA also uses joint detection, software radio, relay switching and other technologies to greatly improve the overall performance of the system, thus gaining more advantages in controlling the total cost of hardware manufacturing investment.
Radio frequency channel
TD-SCDMA system will work in the frequency band defined by ITU. After spread spectrum, the bandwidth of each carrier is 1.6MHz, the chip rate is about l. 3542 MHz/s, and 200kHz is reserved as the step size of frequency synthesizer.
System code channel
Each radio frequency channel includes 10 time slots, and the average length of the time slots after removing the guard time slots is 478us, and each time slot includes 16 Walsh differential code channels, which share the same radio frequency channel by using direct spread spectrum technology.
Physical channels (Mux0d, Muxd, muxiu, Xu Mu, Muxlu, etc. ) can be used as a resource unit and assigned to any user. The protection time slots of uplink and downlink services can guarantee the communication range of 20 kilometers between the mobile phone and the base station, and there are 8 chip protection time slots between each time slot unit to prevent the overlap between different time slots.
After dynamically allocating code channels, data services up to 2048Kbps can be supported, but at least one code channel should be used for uplink access.
Synchronous code division multiple access
This is a very important technology in TD-SCDMA technology, which means that all users' pseudo-random codes are synchronized when they arrive at the base station. Due to the synchronous orthogonality between pseudo-random codes, the system can effectively eliminate the inter-symbol interference and expand the system capacity. At present, the capacity of similar systems in TD-SCDMA will be at least four times that of other two CDMA standards.
When 3G mobile terminals (mobile phones, etc. ), it will receive the strongest signal from the base station, then get the reception synchronization and get the relevant information from the common control physical channel. After the reception synchronization is established, the mobile phone user directly registers in the air, and the base station searches for the power redundancy and synchronization of transmission by receiving the registration information, and puts the power control and synchronization offset information into the downlink common control physical channel for transmission. During the whole response period, the mobile phone will adjust its transmission power and transmission time to establish initial synchronization.
The maintenance of synchronization will depend on the null or Sync 2 sequence in each uplink time slot (except uplink access frames). Only when the Walsh number assigned to a code channel in this time slot matches the current frame number will Sync 2 get power transmission. Although other mobile phones are in the same time slot, due to the different Walsh codes, their Sync 2 will turn to an empty state without any power transmission. This design can make the base station receive the Sync 2 sequence with less interference, thus keeping the synchronization between the mobile phone and the base station. In the downlink frame, synchronization offset and power control information are sent to the mobile phone for closed-loop power control and synchronization control.
Smart antenna technology
The test of TD-SCDMA smart antenna technology has been completed, and this wireless technology has also been adopted in the communication system scheme.
Smart antenna consists of loop antenna array and corresponding transmitting and receiving units, which are controlled by corresponding algorithms. Unlike the traditional omni-directional antenna, which only generates one beam, the smart antenna system can give multiple beamforming, each lobe corresponds to a specific mobile phone user, and the beam can also dynamically track the user.
In terms of reception, this technology allows spatial selective reception, which not only increases the reception sensitivity, but also minimizes the interference of mobile phones in different locations on the * * * code channel, so as to increase the overall capacity of the network.
Smart antenna adopts bidirectional beamforming, which increases the capacity of CDMA system and reduces the transmission power requirements of base station while eliminating interference. Even if a single antenna unit is damaged, the system operation will not be greatly affected.
Baton handover
Different from the hard handover and soft handover adopted by the other two technologies at present, TD-SCDMA adopts a brand-new handover technology named "relay handover".
Relay switching is based on the combination of synchronous code division multiple access technology and smart antenna. How to accurately locate mobile users in mobile systems has always been a topic of concern to users. TD-SCDMA system can get the user's position by carefully measuring the chip period in the antenna array and the synchronous code division multiple access technology. Then, with the help of the mobile phone, the servo base station will switch the mobile phone to the base station with better signal according to the surrounding air propagation and signal quality.
In this way, the technology can also dynamically optimize the capacity allocation of the whole base station network, and can also realize the handover between different systems.
Software radio
In TD-SCDMA system, DSP (Digital Signal Processing Technology) will replace the conventional mode and complete many wireless transmission functions originally realized by RF, baseband analog circuits and ASIC. These functions mainly include intelligent RF beamforming, on-board RF correction, carrier recovery and timing adjustment.
The main advantages of adopting software radio technology are: it can flexibly complete the functions originally completed by hardware through software and reduce the network burden; It has advantages in repeatability and accuracy, low error rate and high fault tolerance; Unlike hardware, it is not easy to age and is more sensitive to the environment; Realize complex hardware functions with less software cost and reduce the total investment.
In terms of system application, TD-SCDMA system follows the requirements of the third generation mobile communication system of ITU. Compared with the second generation mobile communication system, TD-SCDMA system not only greatly improves the capacity and spectrum utilization, but also provides packet-based data services in addition to traditional voice services. In addition, in terms of operational flexibility, TD-SCDMA can also be fully compatible with GSM networks.
Spectrum utilization ratio
Spectrum utilization is one of ITU's main requirements for 3G applications. In the current 2G system, the CDMA technology of IS-95 has the highest spectrum utilization rate.
Since CDMA itself is a self-jamming system, if the problem of inter-symbol interference and the distance between pseudo-random codes can be solved, the system capacity will be greatly improved. TD-SCDMA technology can provide 4 ~ 5 times the capacity of IS-95CDMA system through the orthogonality between spreading codes and smart antenna technology. This conclusion has been confirmed by the field test of wireless local loop of SCDMA technology.
As the largest 3G network system, the capacity of TD-SCDMA system is 20 times that of GSM and 4 times that of other 3G standards. Because of CDMA technology, system deployment does not need frequency planning. At the same time, due to the TDD working mode, TD-SCDMA does not need paired frequency sources like the third generation mobile communication system based on FDD, so it is more flexible in frequency utilization.
Multimedia service
The communication system under TD-SCDMA standard will not only provide basic voice communication services, but also provide digital and packet video services. Although the adopted mode is that all users share the same frequency resource, combined with smart antenna, power can be dynamically allocated to different users according to the quality of service and requirements, and the interference can be guaranteed not to exceed the upper limit.
The communication resources in TD-SCDMA system are allocated to each user by resource units determined by Walsh code channels and time slots. Each user can get one resource unit, or a single user can occupy multiple resource units. Different service code channels of the same user are combined to form a multimedia service. This enables users to obtain voice communication services as well as data communication, such as web browsing and email. For 2Mbps service (indoor data transmission), more than 90% of the code channels are allocated to users, and at the same time, some voice communication services can be synchronized. 2Mbps data service is aimed at indoor application environment, but in low-speed environment and high-speed environment, the data transmission rate will drop to 384Kbps and 144Kbps respectively.
In addition to the advantages of wireless technology, on the networking level, because the 3G mobile communication partnership alliance has considered how to deal with the investment of the second generation network when formulating the third generation standard, TD-SCDMA system can get the maximum application only if it is as close as possible to the physical layer of the third generation standard formulated by 3GPP until it is consistent with it, so as to participate in the transition from 2G to 3G and seize more market opportunities. For China, to build a 3G communication system and communication network, we must consider the existing number of more than 200 million users, and for the fixed operators who are about to obtain mobile licenses, we must also consider the network compatibility. Just want to know more friends, and add QQ36325 1355 overtime instructions.