Next-generation Mobile Telecommunications System

Transcription

1 Next-generation Mobile Telecommunications System Wideband CDMA System Evaluation 238 Next-generation Mobile Telecommunications System Wideband CDMA System Evaluation Toshiro Suzuki Nobukazu Doi, Ph.D. Tetsuhiko Hirata OVERVIEW: Activities are progressing for international standardization of the next-generation mobile communications system (IMT-2000). This new system is intended to substantially improve the quality of voice communications as well as to implement multimedia communications including Internet services. Hitachi developed a base station, mobile station and mobile switching center-simulator for a mobile telecommunications system, using the wideband CDMA system proposed for the international standard by Japan. Based on these results, Hitachi manufactured and delivered to KDD Corporation a test bed following the system specifications presented by KDD. KDD conducted field trials of this system, in the central part of Tokyo in November 1998 and confirmed that high-quality voice communications and image communications by video telephone could be implemented (including soft handover operation). Hitachi conducted a system test for itself and confirmed that the test bed could connect to the mobile station provided by NTT Mobile Communications Network and that the outdoor service area conformed well with the simulation. INTRODUCTION RECENTLY, the telecommunications market has changed dramatically. Specifically, there has been a rapid shift around the world from voice communications to non-voice communications, such as Internet services, and from wired network to wireless networks. These innovative changes have resulted in more terminals being owned by individuals (personalization) and have brought on new needs for multimedia communications that combine voice, images, and the Internet. Conventional mobile communications allocated each user only a very narrow band channel for voice communication. These new needs, however, will require a radical review of the concept of frequency utilization and radio communications technology. In view of these societal needs, the International Telecommunication Union (ITU) has been working on international standardization of the next-generation mobile telecommunications system, which is referred to as IMT-2000 (International Mobile Telecommunications-2000) 1). The goal of the IMT-2000 standard is twofold: to implement a seamless communications environment, covering everything from voice communications to up to 2 Mbit/s high-speed multimedia communications, for use indoors (in offices), outdoors, and in vehicles, and to implement a mobile telecommunications system that can be used around the world. IMT-2000 is expected to use frequencies around the 2-GHz band. The most difficult technical requirements for IMT is that the system implemented must be able to simultaneously handle voice signals compressed to approximately 8 kbit/s and image signals up to 2 Mbit/ s. The digital mobile radio systems that are widely used now, such as PDC (personal digital cellular) and GSM (global system for mobile communications), employ TDMA (time division multiple access) technology to multiplex the radio channels arranged in time slots. However, CDMA (code division multiple access) technology is more suitable than TDMA for multiplexing radio channels operating at different bit rates and using different spreading codes. The handling of data at up to 2 Mbit/s requires the adoption of wideband CDMA with a wide spreading bandwidth over 5 MHz. European countries, the United States, Japan, and other countries have each proposed their own wideband CDMA systems candidates for IMT ) Based on the wideband CDMA draft that Japan proposed, Hitachi recently developed a base station, mobile station, and mobile switching center-simulator. Using the results of this development, Hitachi manufactured and delivered to KDD a test bed following the specifications that KDD Corporation

2 Hitachi Review Vol. 48 (1999), No presented. KDD installed the test bed in Shibuya and Meguro (in metropolitan Tokyo) and started a field test in November ). The test bed was open to the public in December 1998 so that high-quality voice communications, high-speed image communications, and soft handover between base stations installed on building rooftops and the mobile equipment mounted on vehicles could be verified. In addition, Hitachi participated in an open demonstration test performed by NTT Mobile Communications Network, Inc. in collaboration with both Japanese and foreign telecommunication operators and manufacturers. Hitachi confirmed that its base station, which uses the same system specifications that KDD s test bed uses, successfully connects to the mobile equipment provided by NTT Mobile Communications Network, Inc. Hitachi also started an outdoor system test of its own equipment in the suburbs of Yokohama in January 1999, and confirmed that the measured service area conforms well with the results of a simulation conducted based on topographical data. Hitachi will identify the technical issues remaining to be resolved for its wideband CDMA system through more system tests to contribute to the promotion of international standardization. It will also identify problems with system construction and administration to gain know-how for implementing the system. CONFIGURATION OF WIDEBAND CDMA TEST BED SYSTEM The IS-95, a CDMA mobile communications system developed in the United States, has been put in commercial service in the US, Korea, Japan, and other countries. Because it uses a comparatively narrow spreading bandwidth (1.25 MHz), the IS-95 system has some limitations for transmission of high-speed multimedia signals, faster than 100 kbit/s. The radio system Hitachi developed for the wideband CDMA test bed is a direct spread system using the MHz spreading bandwidth (see Table 1). It enables transmission of continuous digital signals of up to 128 kbit/s and packet data signals at 384 kbit/s. These specifications are based on the Specification of Air-Interface for 3G Mobile System prepared by ARIB (Association of Radio Industries and Businesses) in July ). After that the specifications proposed by ARIB were improved through intensive discussions, they were submitted by Japan under the title Proposal for Candidate Radio Transmission Technology on IMT-2000: W-CDMA to ITU. The equipment specifications of Hitachi products were prepared according to ARIB s original specifications. System specifications were based on the test bed specifications proposed by KDD. The specifications for the open demonstration test presented by NTT Mobile Communications Networks, Inc. were also taken into account. The test bed delivered to KDD included features for synchronous/asynchronous operation among base stations so that advantages and disadvantages to the system performance of changing the synchronization setting could be evaluated. As illustrated in Fig. 1, the present test bed is composed of two kinds of mobile stations (MS): one for voice and one for data communications, two kinds TABLE 1. Wideband CDMA Test Bed Radio System Specifications Item Specification Remarks Radio frequencies Transmission frequency band Chip rate Channel bit rate Channel coding Spreading code Detection system Base station synchronization Uplink: 1,920-1,940 MHz Downlink: 2,110-2,130 MHz FDD system with 190 MHz spacing in frequency band 5 MHz Mchip/s Cicuit switching: 32/64/128 kbit/s Packet switching: 384 kbit/s Transmitting side: Convolutional (R=1/2, 1/3, K=9) Receiving side: Soft-decision Viterbi decoding Long code: Gold code Short code: Orthogonal code Synchronous detection by time multiplexing individual pilot Switchable synchronous/asynchronous Within the frequencies specified by ITU for IMT-2000 Uplink and downlink communications Reed-Solomon coding added to data communications

3 Next-generation Mobile Telecommunications System Wideband CDMA System Evaluation 240 MS BS wireless connection antennas MS : mobile station BS : base station MSC-SIM: mobile switching center-simulation (MS for voice communications) (BS for three-sector antenna) Telephone set MS BS MSC-SIM ISDN terminal ISDN terminal (MS for data communications) (BS for omni antenna) Fig. 1 Wideband CDMA Test Bed System Configuration. of base stations (BS); one for three-sector and one for omni antennas, and the mobile switching centersimulator (MSC-SIM) for implementing the base station control and switching functions. The voice communication mobile station is equipped with a handset and a ten-key pad for performing normal telephone call originating and terminating operations. The data communication mobile station is not equipped with a handset, but it can be connected to an ISDN terminal having a terminal adapter and video telephone via an ISDN S-interface. The base station has a maximum average transmission output power of 5.2 Watts (for 14 code multiplexing) per antenna. Because of the demands for high linearity and power efficiency of the transmission amplifier, the common amplifier for the digital cellular base station using the self-adjusting feed-forward method was modified and used. 5) In addition to inter-cell soft handover capability, the base station with the three-sector antenna has a feature for softer handover synthesis between sectors. The base station with the omni antenna can mount an interference canceler. The mobile switching centersimulator (MSC-SIM) carries out soft handover by integrating signals received from each base station, while establishing switching connections between the mobile stations and between the mobile station and the fixed terminal. Each station or unit can be connected to an administrative terminal that sets parameters, monitors the operation status, and processes in real time various information such as transmission power, receiving signal level, signal-tointerference ratio (SIR), delay profile, bit error rate, and frame error rate. CONTROL TECHNOLOGY FOR WIDEBAND CDMA Wireless Communication Protocol The test bed s software carries out call processing and system control as the radio link layer 3 functions. Generally, the control protocol for mobile communications requires certain system capabilities such as call connection service control, mobility management, and radio access link control. To implement these system capabilities, the following routines were developed for the test bed. Call control (CC) Generates and transfers messages concerned with call/connection control according to the user s request. CC basically conforms to the ITU-T Q.2931 standard. Terminal association control (TAC) Generates and transfers messages concerned with controlling the association between the mobile station and the mobile switching center-simulator, including,

4 Hitachi Review Vol. 48 (1999), No for example, authentication of the unique identifier of a mobile station. Mobility management (MM) Generates and transfers messages concerned with mobility control including location registration at power turning-on based on the result of cell search by a mobile station, and location update during mobile station movement. The mobility management depends partially on each radio system. A new protocol for the wideband CDMA system has been developed for the present test bed. Radio resource control (RRC) Generates and transfers messages concerned with radio resource control including radio bearer control, radio resource management, handover activation, and target value management at transmission power control. The test bed performs the control inherent to CDMA, which is not done by the conventional PDC or other systems. The test bed does not incorporate a redundant configuration such as duplex control, or a feature for self-recovery from an abnormal condition or congestion, but basic features required for usual mobile communication system control are implemented in the above control routines. Cell Search System The most important technical requirement for the mobile communications system employing CDMA technology is the implementation of a rapid and robust cell search feature. Cell search is a feature for searching for a radio channel to secure communications with an appropriate base station when the mobile station s power is turned on, or when the mobile station moves into a new base station area. Generally, the CDMA system cannot enable a mobile station to communicate with a base station without making the mobile station recognize the inherent spreading code of that base station beforehand. However, the mobile station cannot know the spreading code used by the base station until it starts communicating with that base station. The W- CDMA system proposed by Japan employs a threestage cell search system to resolve this problem. 6) This test bed uses a similar method. Specifically, the base station constantly transmits the perch channel separately from the common physical channel and the individual physical channel directed to each mobile station, as listed in Table 2. The perch channel is doubly spread by two spreading codes, the spreading code common to all base stations and a spreading code unique to each base station. Because it can store the common spreading code in advance, the mobile station performs demodulation using the code to find the existence of a new base station and communication timing, and receives the spreading code information of that base station. After that, using the particular spreading code, the mobile station recognizes detailed information about the base station and obtains appropriate communication parameters. The mobile station receiver must continuously perform the following operations on a real-time basis, using its hardware: First stage: It must use the common spreading code to identify the existence of a new base station and the radio wave receiving condition, transmission timing, and so forth. Second stage: It must determine to which group the unique spreading code of the new station belongs. Third stage: It must determine which spreading code in the group identified in the second stage TABLE 2. Wideband CDMA Test Bed Channel Structure Physical channel Logical channel Uplink Downlink Transferred information Perch channel BCCH Notice each mobile station of timing reference, system information, communication quality, etc. Call up mobile station Common control channel PCH FACH RACH PCCH PRCH SDCCH SDCCH Initial access control Packet communication control Location registration, call setup Individual physical channel DTCH DTCH ACCH ACCH UPCH UPCH General user data Control during communication User packat data

5 Next-generation Mobile Telecommunications System Wideband CDMA System Evaluation 242 corresponds to the unique code of the new base station, and secure a communication channel with the base station. Upon securing a communication channel with the base station, the mobile station uses layer 3 software to monitor the transmission level from the base station and the communication quality in the cell area. If the quality is degraded, the mobile station immediately starts processing to secure a communication channel with another base station. Because the W-CDMA system requires complex processing for carrying out cell search, it must provide a receiver solely for the perch channel in addition to the regular receiver for the individual physical channel. W-CDMA employs a configuration in which software performs parallel processing to enable rapid cell search at any time, even during talking. Soft Handover System Handover is a feature for maintaining continuous communications when a talking mobile station enters another base station area. The conventional mobile radio system prevents interference by using different frequencies between adjoining base stations, while the CDMA system prevents interference by using different codes, not different frequencies, between adjoining base stations. Thus, the CDMA system enables its mobile station to receive radio waves from multiple stations simultaneously by using the radio frequency section of the receiver in common and only providing parallel demodulators. The conventional system was unable to avoid a momentary signal loss when switching receiving frequencies, while the CDMA system can switch base stations without signal interruption. This is referred to as soft handover. It is necessary to provide a feature for simultaneously communicating with multiple base stations to execute a soft handover. The present test bed is configured for communication with up to three base stations at any time. Rapid cell search and rapid and robust chip synchronization are quite important for smooth execution of the soft handover feature. The mobile station is often in a location that is difficult to be reached by either base station, making communication unstable when the execution of a soft handover is required. Thus, the system developed has been given double or triple fault processing functions, including retransmission upon error and retry start, for dealing with unstable transmission of the control signals for handover. SIGNAL PROCESSING TECHNOLOGY FOR WIDEBAND CDMA Channel Structure Table 2 lists the radio channels installed on the present test bed 7). Individual physical channels of 32 k symbol/s 256 k symbol/s are supported for voice signals (8 kbit/s coding by ITU-T G.729 system), unrestricted digital signals (128 kbit/s max.), and highspeed packet communication (384 kbit/s max.). Hitachi s packet reservation technology is employed for high-speed packet communications 8). Overview of Wideband CDMA Signal Processing The CDMA system separates and extracts delayed waves reflected by buildings and hills from the signals on other channels and noises through digital signal processing, which is referred to rake receiving, in addition to the signals received directly from the transmitter antenna. The system combines all of these signals and utilizes them, so the interference in radio communications and performance due to noise degradation can be reduced extensively. When communicating at a high chip rate, the system separates and identifies complex delayed waves far more accurately than the conventional system by fully employing advanced digital signal processing technology. However, the system also needs digital signal processing hardware that is much more complex than existing digital mobile radio systems. This hardware restores signals correctly by conducting correlation analyses of the spreading code and the received signals and performs highly accurate phase synchronization. These features are carried out by a custom signal processing LSI for DSPs (digital signal processors) installed in the base and mobile stations. The receiver LSI developed by Hitachi yields a performance of about 100,000 MIPS (million instructions per second) per chip 9). The LSI integrates 103,000 gates within general-purpose gate arrays using 0.5-µm technology. Table 3 lists the technical data on the receiver LSI. In addition to this LSI, the present test bed utilizes a variety of the latest digital signal processing techniques such as error correction, high-quality voice coding (standardized by ITU-T G.729), and Reed- Solomon encoding to efficiently correct bit errors of high-speed data transmission. These processing techniques are carried out by using a general-purpose DSP or FPGA (field programmable gate array). New signal processing systems have also been created, such

6 Hitachi Review Vol. 48 (1999), No TABLE 3. Technical Data on Baseband Signal Processing LSI Item Despreading system Detection system Diversity system Number of antenna branches Number of rake fingers Path search range Power supply voltage Power consumption Clock frequency Number of logical gates LSI technology Specification Quadruple over-sampling matched filter (± 8 chips) Pilot weighted interpolation synchronous detection Inter-antenna and rake maximum ratio synthesis 2 branches 4 fingers/antenna branch ± 128 chips 3.3 V 0.55 Watts (standard) 20 MHz 103,000 gates 0.5 µm CMOS gate array as a high-speed chip synchronization system, which is indispensable for securing stable operation at the occurrence of complex delayed waves. Fig. 2 shows a configuration of the signal processing section in the base station. The hatched portion in the diagram is integrated in the receiver LSI. The diagram shows a base station with a multisector antenna configuration. Diversity reception is performed by using two antennas. In a multisector antenna configuration, the number of radio sections and individual physical channel modem sections increases as the number of antennas increases. The three-sector configuration ensures highly accurate reception because a maximum of 24 signal paths, including direct and delayed paths, can be combined in the intersector synthesis through two-branch antenna diversity per sector, and four-path rake synthesis per branch is performed. SYSTEM DEMONSTRATION TEST The Experiment Working Group (EWG) of ARIB s IMT-2000 Study Committee is working to establish a more enhanced standard. System evaluation test using the completed test bed equipment was conducted under generic adjustment of the Working Group. KDD started an evaluation test of the present test bed system in Tokyo s Shibuya ward and Meguro ward in November The test was conducted by performing high quality voice communication with the ITU-T G.729 system and voice/image multimedia communication with the ITU-T H.261 system through dialed originating and terminating call processing. The test verified that satisfactory communications, including soft handover operation, could be performed. Hitachi participated in the open demonstration test performed by NTT Mobile Communications Network, Inc. in collaboration with Japanese and foreign telecommunication operators and manufacturers for standardizing the next-generation mobile communications system. In this test, Hitachi conducted an indoor connection test of the mobile station provided by NTT Mobile Communications Network and Common control channel modem Packet control channel modem Base station control section RF section Despreading SIR measurement and TPC Rake synthesis Intersector HD Viterbi decoding HW-IF To MSC-SM Modulation and spreading Convolution coding 3-sector Individual physical channel MODEM Fig. 2 Base Station Configuration (Three-Sector). The BS has dedicated RF sections for each sector antenna and separated channel modems for each individual physical channel.

7 Next-generation Mobile Telecommunications System Wideband CDMA System Evaluation 244 Fig. 3 Multipurpose Radio Measurement Vehicle. Diversity antenna, and built-in power generator are installed. Results of Area Verification Test in Totsuka Yabecho Yazawa Kumisawa Totsukacho Kanaicho Kanaicho park Tayacho Railway Kashio River Yokohama Totsukaku Maiokacho Yoshidacho Kamikuratacho Meijigakuin University Kuratacho Naganumacho Hongodai 1 km Measured Service Area Simulated Service Area Base station Fig. 4 Results of Area Verification Test in Totsuka. Compare simulated service area with actually measured service area. N Hitachi s base station, adopting system specifications that were identical to KDD s test bed, to confirm that basic connections could be established. Hitachi also installed antennas on the rooftop of Hitachi s Communication Technology Center building in Totsuka, Yokohama, and started a system demonstration test on Hitachi s own products in an outdoor environment in January The demonstration test system was composed of a base station with a threesector antenna configuration, two base stations with omni antenna configuration, a mobile switching centersimulator, and six mobile stations (two of which were for data communications). The antenna of the threesector base station was a polarization diversity antenna, which could accommodate a total of nine antenna modules (one-sector transmitter antenna, and twosector receiver antennas for three sectors) in one housing. 10) Each antenna was installed on the rooftop of the Totsuka building (approximately 25 meters above the ground). The mobile unit was installed in a multipurpose radio instrumentation vehicle (Fig. 3). Fig. 4 illustrates an example of the base station propagation area in and around Totsuka measured by the radio instrumentation vehicle. The figure shows a map on which the results of the area simulation calculated from a propagation model and the measured service area have been plotted. 11) The Communication Technology Center building is located in a small/ medium residential area of Totsuka. It is an environment with low buildings and houses, a river running from north to south, and hills on the east and west sides, forming a service area which is elongated in the north-to-south direction. The diagram shows that the communication area actually measured conforms well with the simulation. Using the experimental system, Hitachi conducted field tests on area verification, bit error rate (BER) characteristics, delay profile, diversity characteristics, handover characteristics, interstation interference characteristics, and high speed movement characteristics. These field tests provided information on the usefulness of the wideband CDMA system, clarified issues concerned with its implementation, and provided implementational know-how. They also contributed to the activities of the international standardization organizations, including ARIB. CONCLUSIONS Based on the draft specifications prepared by ARIB, which became the basis of the W-CDMA system Japan proposed to ITU, Hitachi developed a base station, mobile station, and mobile switching center-simulator using wideband CDMA technology. Using the results of this development, Hitachi then manufactured a test bed of mobile communications system based on KDD s system specifications for IMT-2000 and delivered it to KDD. The test bed system was

8 Hitachi Review Vol. 48 (1999), No composed of mobile stations for voice and high-speed data communications, omni and three-sector base stations, and mobile switching center-simulator. KDD conducted a system test in central Tokyo in November 1998, that verified satisfactory implementation of highquality voice communications (equivalent to fixed telephones) using the ITU-T G.729 voice coding system and image communications using the ITU-T H.261 video telephone. The test also verified satisfactory soft handover operation. Hitachi participated in an open demonstration test performed by NTT Mobile Communications Network, Inc., in collaboration with Japanese and foreign telecommunication operators and manufacturers. Hitachi then confirmed in an indoor connection test that the mobile station provided by NTT Mobile Communications Network could connect to Hitachi s base station, which uses the same system specifications as KDD s test bed. Hitachi also confirmed that the actual system coverage area conformed well with the simulation in an outdoor test of Hitachi-made equipment in Totsuka, Yokohama. The authors will try to identify the characteristics of and the technical issues with the wideband CDMA, and to collect know-how in system construction through the use of the present test bed, thereby contributing to the activities for international standardization of IMT-2000, while aiming at developing a more enhanced commercial system at an early date. ACKNOWLEDGMENT The authors wish to express their gratitude to the people at KDD and KDD Research Institute concerned with awarding Hitachi the order for the wideband CDMA test bed development and for their technical support. REFERENCES (1) Recommendation ITU-R M.687-2, International Mobile Telecommunications 2000 (IMT-2000), (2) A. Sasaki and K. Yamamoto, The Current Situation of IMT Standardization, The J. of The IEICE, Vol. 82, No. 2, (Feb. 1999), pp (3) S. Onoe and Y. Takeuchi, Field Trials of Wideband CDMA, The J. of The IEICE, Vol. 82, No. 2, (Feb. 1999), pp (4) ARIB, Recent Study on Candidate Radio Transmission Technology for IMT-2000, ITU-R Toronto Workshop, Sept (5) K. Takenaga, Common Amplifier for Digital Cellular Base Station, Kokusaidenki-Gihou No. 23, (March 1996). p. 35. (6) F. Adachi, M. Sawahashi, and H. Suda, Wideband DS- CDMA for Next-Generation Mobile Communications Systems, IEEE Communications Magazine, Vol. 36, No. 9, (Sept. 1998), pp (7) S. Okasaka, Y. Furuya, and F. Watanabe, A Radio Transmission System for IMT-2000, The J. of The IEICE, Vol. 82, No. 2, (Feb. 1999), pp (8) R. Esmailzadeh, N. Doi, H. Masui, Y. Ohgoshi, and T. Yano, Spread Spectrum Slot Reservation Multiple Access, IEEE VTC, Vol. 3, (April 1996), pp (9) T. Uta, M. Suzuki, S. Hanaoka, S. Masuda, and N. Doi, Gate Array Design for Wide-band CDMA Receiver, Proc. of the 1998 IEICE General Conference, B-5-132, (March 1998), p (10) N. Kuga, H. Kikuchi, X. Zhang, Y. Miyane, Antennas for Next Generation Wideband CDMA Communication Systems," The Hitachi Densen, No.18, (January 1999), pp (11) M. Hata, Empirical Formula for Propagation Loss in Land Mobile Radio Service, IEEE Trans. VT, Vol.VT-29, No.3 (1980), pp ABOUT THE AUTHORS Toshiro Suzuki Joined Hitachi, Ltd. in 1972, and now works at the CDMA System Business Operation in Telecommunications System Group. He is currently engaged in the development of next-generation mobile telecommunication system including wideband CDMA system. Mr. Suzuki is a member of the IEEE and the Institute of Electronics, Information and Communication Engineers of Japan, and can be reached by at tsuzuki@cm.tcd.hitachi.co.jp Nobukazu Doi Joined Hitachi, Ltd. in 1983, and now works at the Communication Systems Research Department at the Central Research Laboratory. He is currently engaged in the development of IMT-2000 system. Dr. Doi is a member of the IEEE and the Institute of Electronics, Information and Communication Engineers of Japan, and can be reached by at n_doi@crl.hitachi.co.jp Tetsuhiko Hirata Joined Hitachi, Ltd. in 1984, and now works at the Network Technology Development Center at the Systems Development Laboratory. He is currently engaged in the research and development of mobile IP networks and CDMA networks for IMT Mr. Hirata is a member of the Institute of Electroncis, Information and Communication Engineers of Japan, and can be reached by at t-hirata@sdl.hitachi.co.jp