Third-Generation Mobile Communications System IMT-2000 Various systems were developed and used around the world for first-generation analog mobile communications, and three systems now coexist for second-generation digital communications (PDC in Japan, GSM in Europe and TIA standards in the US). The global standard for third-generation wireless communications has been determined by the International Telecommunication Union (ITU) under the integrated name of IMT-2000. IMT-2000 realizes mobile communications systems that offer high quality equivalent to that of fixed networks under global standard radio interfaces and can provide a wide range of services. In addition to making it possible to easily communicate with anybody, anywhere and anytime on a global scale, it also permits high-speed, large-volume data communications and image transmissions. NTT DoCoMo has been active in research and development activities relating to IMT-2000, and the W-CDMA (wideband code-division multiple access) radio interface that NTT DoCoMo has been promoting is included in IMT-2000. Now in 2001, NTT DoCoMo has launched FOMA, which is the first service in the world based on IMT-2000. FOMA realizes clearer and more comfortable communications environments than ever before and provides a new mobile environment where voice, still images and video are freely handled through the introduction of new technologies. These include large-volume communications using broad frequency bands and intelligent networks that can select the optimal communications rates and paths according to the type and volume of information being transmitted. Concept of IMT-2000 IMT-2000 Mobile Communications Network Mobile communications in the 21st century will enter an era of mobile multimedia service and universal mobility. Various technological developments are being carried forward towards this goal, including the development of an advanced intelligent network that will integrate different communications systems to establish a sophisticated mobile communications network that will realize these services. As part of these efforts, NTT DoCoMo is working towards expanding its new "IMN" intelligent mobile communications network. In addition, we are aiming at structuring our third-generation mobile communications system (IMT-2000) as a global standard that is capable of handling communications requirements ranging from low-speed (e.g., email) up to high-speed (e.g., video-on-demand) communications. The technologies to achieve this include the W-CDMA system for radio transmissions, and the ATM system for wired transmissions. Because ATM carries out communications by transmitting data in fixed-length cells, a single transmission path can handle multiple communications speeds simultaneously. By combining ATM with W-CDMA, a network that can flexibly meet any speed requirements in communications -- ranging from voice to IP communications such as the Internet -- can be structured. ATM and W-CDMA Technologies in IMT-2000 Network ATM Switching System Under the third-generation IMT-2000 services, high-speed and large-capacity multimedia data will be delivered through the network and a wide variety of network services such as global roaming will be provided at the same time. Supporting these IMT-2000 services on the NTT DoCoMo core network is the ATM switching system. ATM (Asynchronous Transfer Mode) divides data into fixed-length cells that can be transmitted at hardware speed. A logical channel number is assigned to the cell head, and low-speed services such as voice communications to high-speed services such as moving pictures can be efficiently transmitted through multiplexing. With its advanced service menu, the IMT-2000 network is required to provide not only mobility management for individual terminals but also various service control functions. And with capabilities such as QoS (Quality of Service) in which the desired communications quality is individually established for each communications session, the ATM switching system provides for a sophisticated and economic network configuration. NTT DoCoMo's IMT-2000 Core Network Using ATM W-CDMA Technology Supporting Third-Generation Mobile Communications System CDMA can efficiently utilize the limited frequency resources available and accommodate as many users as possible. This is achieved by sharing frequencies on the basis of spectrum-spread codes assigned to each user, rather than through dividing broadband channels by frequency or time. Among the various systems, DoCoMo is promoting the introduction of W-CDMA that can transmit high-quality moving images in addition to voice and fax, and also allows for connections to the Internet. As it offers many benefits such as high transmission quality and needs little power for transmissions, W-CDMA is the most suitable technology to meet the objectives of third-generation mobile communications -- namely, multimedia, personal and intelligent systems. By combining W-CDMA with the asynchronous transfer mode, which is the most effective technology to process text and image information in addition to voice on an integrated basis, it is possible to structure a mobile, multimedia integrated communications system that enables high-speed, flexible and efficient transmissions. Targets of Third Generation Communications Systems Characteristics of W-CDMA Amplitude fluctuates widely under the influence of terminal movement. While large fluctuations in average received power are encountered in the case of a bandwidth in which the spread of the frequency spectrum is narrow, such fluctuations become smaller as the bandwidth becomes wider. As the W-CDMA system utilizes this characteristic, it can improve the efficiency of frequency utilization in addition to achieving high communications quality. Moreover, W-CDMA can economically realize a mobile communications environment that supports various multimedia transmissions from low to high speeds, such as data, images and moving images in addition to voice. W-CDMA realizes a multi-band system, in which efficiency is enhanced by dual-spread codes, and asynchronous inter-cell system, coherent RAKE reception, SIR-based adaptive transmission power control and orthogonal-variable, multi-rate transmissions. In the future, moreover, using both adaptive antenna array diversity and interference cancelers will increase subscriber accommodation capacity. W-CDMA Wireless Techniques Packet Technology of W-CDMA One of several currently used packet transmission methods is common/dedicated channel switching, which provides a selection between common and dedicated channels depending on traffic characteristics for both the uplink and downlink. In low-traffic situations, the mobile terminal uses a common channel that is shared by multiple mobile terminals, whereas the DPCH (Dedicated Physical Channel) system allocates a dedicated channel to each mobile terminal in high-traffic situations. Another downlink packet transmission method is the DSCH (Downlink Shared Channel), which permits efficient transmissions by enabling radio resources to be shared by multiple terminals and by permitting the use of transmission power control. Discussions regarding higher information rates and greater efficiency for packet transmissions are still under way in the 3GPP (3rd-Generation Partnership Project). A Scheme for packet transmission on Common channels/Dedicated channels DSCH (Downlink Shared Channel) Multi-Band In order to achieve a maximum transmission speed of 2 Mbps, which is one of the objectives of the third-generation IMT-2000 mobile communications system, frequency resources must be flexibly utilized to provide the optimal transmission conditions depending on the service. Accordingly, W-CDMA includes four basic bandwidths (1.25 MHz, 5 MHz, 10 MHz, 20 MHz) and spread bandwidths -- namely, spectrum-spread codes with different "chip rates" assigned in accordance with the transmission rate (Note). (Note) While subsequent ARIB and ETSI studies on basic 5-MHz bandwidth radio systems also included the 10-MHz and 20-MHz bandwidths, the current standard is set at 5MHz only as 2-Mbps transmissions are fully possible even with a 5-MHz carrier. This also enabled detailed specifications to be completed at an early stage. W-CDMA frequency bandwidths and chip rates (as of September 1999) Frequency bandwidth (MHz) 5 Chip rate (Mchip/s) 3.84 Dual-Spread Codes and Inter-cell Asynchronous The W-CDMA system adopts a dual-spread code arrangement, which uses an almost infinite number of spectrum-spread codes, generated by the combination of symbol-length "spread codes" and multiple-symbol-length "scrambling codes." This makes it possible to freely carry out channel arrangement for each cell. While GPS synchronization of chip levels is normally required between cells in systems in which each cell uses the same scrambling code and changes the code phase, it is not necessary to adopt inter-cell synchronization in this system. As a result, usage both indoors -- where it is difficult to apply GPS -- and outdoors can be seamlessly supported. In the W-CDMA system, channels can be designated through a combination of the spread and scrambling codes. As different scrambling codes for each cell are used in the downlink from the base station to the portable unit, the same set of spectrum-spread codes can be used for channel assignment between different cells. In the case of the uplink, the switchover in spread codes at hand-over becomes unnecessary as different scrambling codes can be used for each user, and orthogonal-spread codes can be used for orthogonal multi-rate transmissions. Inter-Cell Asynchronous System SIR-Based Adaptive Transmission Power Control One of the problems under CDMA is that carrier waves of the same frequency are employed for all users. This means that, when portable units transmit radio waves at the same transmission power regardless of the distance from the base station, radio waves from nearby mobile units are so strong that signals from more distant portable units cannot be separated. In order to resolve this so-called "near/far problem," adaptive transmission power control in which each portable terminal reduces its transmission power to the required minimum becomes essential. Adaptive transmission power control is also effective in suppressing interference to low levels, thereby contributing to increasing the subscriber capacity of the system. In the W-CDMA system, adaptive transmission power control is based on the signal-to-interference ratio, or SIR measurement value. SIR is measured by the RAKE synthesis of inverse-spread signals at the base station, and a command is issued to the portable unit to reduce the transmission power whenever the measurement value is larger than the target value. In the opposite case, a command to increase the transmission power is issued. The portable terminals receiving such commands control transmission power accordingly. By realizing this operation at the extremely high speed of once every 0.625 ms, transmission-side power control can eliminate fading-induced fluctuations in the reception level and also make it possible to minimize transmission power. Orthogonal Variance Multi-Rate Transmission Orthogonal variance multi-rate transmission is used to carry out variable rate transmissions while maintaining orthogonality between channels in the same propagation path. Under the W-CDMA system, multi-code transmissions using multiple orthogonal-code channels is used for high-speed transmissions that exceed the maximum transmission rate per code channel. This method can multiplex code channels without interference to the number of orthogonal-spread codes, since all the spectrum-spread code channels in each path are orthogonalized. Using multiple paths decreases the number of code channels that can be multiplexed due to the cross-correlation between paths, but overall the transmission characteristics are improved from the case of using random codes. Multi-Rate Transmissions Using Multi-Rate Codes Multi-Stage Interference Canceler An interference canceler facilitates increased system capacity in wideband CDMA by reducing the cross-correlation between spread codes that occurs when multiple users with different spread codes communicate on the same carrier wave frequency. Three methods are used to achieve interference cancellation: orthogonalizng filters, decorrelators, and multi-stage interference cancelers. The orthogonalizing filter, which is used in single-user reception systems, orthogonalizes the tap coefficient of the matched filter against the spread codes of interference signals. While the configuration is simple, it is difficult to apply to a multi-path environment using scrambling codes. The decorrelator, which is employed in multi-user reception systems, is a method to reduce cross-correlation by calculating the inverse of the cross-correlation matrix between spread codes. While channel predictions are not necessary, the processing involved is complex. The multi-stage interference canceler is also used in multi-user reception systems. It is a method to improve the signal-to-interference ratio by generating interference replicas of other users and subtracting these values from the received signals. As stages are repeated, the precision of channel predictions can be improved. Realizing Interference Canceling System Classified by type Characteristics Orthogonal matched filter Single user reception system Easy Configuration, but difficult to use in multi-path environments Decorrelator Multi-user reception system Complicated processing to make channel estimation unnecessary Multi-stage Interference canceler Multi-user reception system Possible to raise the estimated level of channel precision as the stages are layered
发表于 2009-6-20 21:57:36