3.5GHz Fixed Wireless Access System

Andlocal Multipoint Distribute Service (LMDS)

1 Technical Features of 3.5 GHz Fixed Wireless Access System

1.1 Overview

         3.5 GHz fixed wireless access system is a point-to-multipoint wireless technology providing broadband services. It抯 suitable for small and medium-sized enterprise users and corporate users. It can transmit services transparently. At the base station end, the interfaces with the network is T1/E1, 100Base-T, OC-3, or V5 with the backbone network; at the user end, interfaces are E1, FR, N×64kbit/s, POTS, 10Base-T, ISDN, etc. which can provide the users with Internet access, local users' data exchange, voice services and VOD services. The users can choose equipments based on circuits, IP or ATM. The available bandwidth for 3.5 GHz fixed wireless access system is 2×31.5 MHz. The system needs quasi-view-distance transmission, supports multi-sector networking and supports QPSK, 16QAM and 64QAM modulation.

1.2 Technical Features of 3.5 GHz Fixed Wireless Access Systems

          Since 3.5 GHz fixed wireless access system does not have uniform air interface standard, its technical features vary with different equipments. Since the positions of system equipments and the user station are fixed, there is no such problem as roaming and incompatibility of interfaces basically will not affect the function of equipments. The service capacity and RF indicators provided by the system should conform to China抯 standard.

1.2.1 System Composition

         As indicated in Figure1, 3.5 GHz fixed wireless access system is mainly composed of three parts: base stations, end user stations and network management system.

        Base station is also called central station. It converges services and signaling data of the center RF station and provides interfaces to the network side. The central station is mainly responsible for covering far-end stations and providing interfaces to the central control station.

        The end user station is in the users?location. It can serve one or more user end equipments. Its RF receiving and sending equipments can be either separated or integrated and usually include an indoor unit and an outdoor unit. Small-bore outdoor directional antenna is usually adopted. Service interfaces at the user end provide interfaces for various user services and also have the function of multiplexing/demultiplexing.

        With the central station and the end station, the network management system fulfill the functions of configuration between the central station and the far-end station, performance, failure and security management and billing through such means as in-band, out-band and serial port. The network management system of the current 3.5 GHz fixed wireless access system basically adopts SNMP protocol. Generally speaking, all the equipment vendors can guarantee the following network management function, but different network management systems are incompatible with each other.

        Although the network management system of the current 3.5 GHz fixed wireless access system is yet to be improved, eg. some problems still exist with the billing and compatibility, the normal equipment operation and service promotion and not affected.

1.2.2 Wireless Feature

        According to Technology Requirements for Access Net-work-3.5 GHz Fixed Wireless Access issued by the Radio Regulation Bureau under the Ministry of Information Industry (MII), the frequency bands of FDD fixed wirele

       * The end station emission frequency band: 3 399.5 MHz～3 431.0 MHz;
        * The central station emission frequency band: 3 499.5 MHz～3 531.0 MHz;
        * Interval between receiving and emission carrier frequencies: 100 MHz.

         There are four wave canal allocation scenarios. The signaling channel interval is 1.75 MHz, 3.5 MHz, 7 MHz and 14 MHz respectively. Multi-address modes like FDMA, TDMA and CDMA can be applied. As for modulation, BPSK, QPSK, 4QAM, 16QAM and 64QAM are applicable. The typical coverage of the base station is 10km and the maximum coverage is more than 20km. View-distance transmission is necessary between the base station and the user station. There can be 1～24 sectors due to the deployment of the difference between frequency-division and polarization.

        When cellular coverage is adopted, the more usable frequencies are, the easier the network plan is.

        3.5G wireless access system generally has simple power control function which can adjust the emission power of the base station and the end station. The power control mechanism varies with different vendors. The system also has simple wireless link encryption function which can satisfy the demands of common users and enterprises. The encryption system also varies with different vendors.

         As for the capacity of the system, currently the maximum is 8Mbit/s/sector/3.5MHz carrier frequency and the minimum is 1.3Mbit/s/sector/carrier frequency.

         The total capacity of a base station = Mbit/s/sector/carrier frequency×no. of sectors/base station×no. of carrier frequencies/sector.

        4 sectors and 8 sectors are typical and the maximum is 24 sectors. The more the sectors are, the larger the capacity that a base station can provide is. If there are 8 sectors, the maximum capacity provided by a carrier frequency of a base station can be up to 8×8=64Mbit/s. Since the bandwidth provided is limited, the number of users supported by each sector is limited. The number varies with different services from 2 to 10. So when there are more users, there should be more sectors.

         The unit of frequency spectrum utilization ration is bit/s/Hz. In other words, it is calculated with sector and the net load provided by each Hz frequency. The frequency utilization rate is relevant to the modulation mode and the coding system and varies with different equipments.

1.2.3 Factors Affecting Performance-to-Price Ratio

        The following factors are expected to affect the perfor-mance-to-price ratio: system capacity, coverage, the price of single-sector equipment and the price of the end station. Other factors might also be included, such as the reliability of the equipment, the convenience of installation and maintenance and services.

         As the 3.5GHz fixed wireless access system has not commercially launched in China, there are some uncertainties concerning the price. The current price for base station equipment is about 250 000 RMB per carrier frequency per sector; the price for user station is around 20,000 RMB.

1.3 Service Features of 3.5 GHz Fixed Wireless Access System

1.3.1 Categories of Services

         The services provided by 3.5 GHz fixed wireless access system include connection-oriented voice/data services and wireless connection-oriented IP services.

        Connection-oriented services:

        * ISDN 2B+D or 30B+D;
        * Circuit bearer services lower than E1, eg. wireless DDN such as 64 kbit/s and N×64 kbit/s;
        * E1 digital circuit bearer services and digital circuit bearer services higher than E1.
Wireless connection-oriented services:
        * IP-based real-time services such as IP telephony;
        * Internet access (WWW browsing, E-mail, high-speed file transmission, etc.)
        * LAN interconnection;
        * VPN.

        Network-side interfaces provided by equipment vendors include E1, V.35/X.21, ATM, ETH10/100, etc.; interfaces provided by user station include E1, 100Base-T, 10Base-T, ATM, ISDN, Frame Relay, etc.

1.3.2 Applications

         3.5 GHz fixed wireless system is mainly suitable for small and medium-sized enterprises and cells including buildings, factories, department stores, shops, branches of banks, private enterprises, apartments, schools and intelligent cells with certain individual demand.

         It is especially worth noting that since the inter-station transmission distance of 3.5 GHz fixed wireless access system is relatively long, the link is less affected by rain attenuation, thus is suitable for the inter-station transmission between the base station controller and the base station of PHS and mobile communication systems. In those areas in shortage of enterprise transmission resources, the system is undoubtedly an economic means for providing mobile communication system transmission.

2 Technical Features of LMDS

2.1 Overview

        LMDS ( Local Multipoint Distribute Service) is a new broadband wireless access technology and was selected as one of the ten booming telecommunications technologies by US telecommunications industry in 1998. By employing high-capacity point-to-multipoint microwave transmission, it can provide two-way voice, data and video services. The user access rate is from N×64 kbit/s to 2 Mbit/s (or even as high as 155 Mbbi/s) and it is highly reliable. Thus it is called a 搘ireless optical fiber?technology.

        LMDS system works in frequency bands ranging from 20 to 40 GHz. Currently the most widely used are 24/26/28 GHz and 38 GHz. There is no uniform LMDS technology standard up to now. Operators around the world choose different technologies in consideration of their specific conditions.

        Radio Regulation Bureau under MII published Notice About 26 GHz FDD Local Multipoint Distribute Service (LMDS) Frequency Planning (Trial) in November 2001, deciding to allocate part of the frequencies between 24.45 GHz and 27 GHz for FDD LMDS use (trial).

         In China, the FDD LMDS system works in frequency band ranging from 24 450 to 27 000 MHz, of which the following frequency band has been assigned:

       * The central station emission frequency band: 24 507-25 515 MHz;
        * The end station emission frequency band: 25 757-26 765 MHz;
        * Interval between receiving and emission frequency bands in the same wave canal is 1250 MHz.

         LMDS has four wave canal allocation scenarios in the frequency band and the basic signaling channel interval is: 3.5 MHz, 7 MHz, 14 MHz and 28 MHz. Basic signaling channels might be combined in accordance with demands of different services.

2.2 System Features

2.2.1 System Composition

        LMDS is generally based on wireless ATM technology and is seamlessly integrated with ATM backbone network. As indicated in Figure2, LMDS system is composed of three parts: the base station (or the central station), the user end equipment (or the far-end station) and the network management system.

         LMDS system is connected to the basic backbone network through the interface module of the base station. The basic backbone network is enabled by optical fiber or microwave transmission network, including ATM switch, IP switch or the core switch platform of IP+ATM, and the interconnection modules with Internet and PSTN.

        The base station equipment includes the interface module connected with the basic backbone network, modulation-demodulation module (IDU indoor unit) and microwave receiving-sending module (ODU outdoor unit). The base station can support omnidirectional antenna and sector antenna. Microwave receiving-sending module is usually installed at the top of a building or a tower. The interfaces between the base station and the basic backbone network are often STM-1, E1, etc.

         User end equipment includes the outdoor unit of ODU (including directional antenna and microwave receiving-sending equipment) and the indoor unit of IDU (including modula-tion-demodulation module and network interface module). The outdoor directional antenna is like single wave beam parabolic microwave reflector used to receive satellite live broadcast signals. Interfaces used by the user end equipment are often POTS, 10Base-T, Frame Relay, E1, etc. which can support many kinds of applications.

        The network management system has such functions as warning, fault diagnosis, system configuration, system performance analysis and security management. There is no uniform standard among vendors, which results in some problems for operation and maintenance.

2.2.2 LMDS Equipment

        Both FDD and TDD are applicable for LMDS. Each coin has two sides. Since there is no uniform standard, different vendors have different preferences. TDD uses a shared bandwidth for both uplink and downlink communications. Sending and receiving are not at the same time. The bandwidth assigned to each direction is flexible. When necessary, even all the bandwidth can be used for sending or receiving. FDD divides the bandwidth into two relatively fixed parts: the uplink frequency band and the downlink frequency band, and there is a protective bandwidth between the uplink and the downlink. The capacities of the uplink and downlink are fixed.

        Users choose LMDS mainly because it can transmit data services and can provide high-speed Internet access. The uplink and downlink data traffic changes with time. In other words, users need more bandwidth only at certain time. So compared with FDD, TDD is more efficient in utilize frequency band. But technically speaking, FDD puts lower demands on equipments and network plan since the bandwidth assignment is fixed. In regards of services, FDD is more suitable for voice transmission and TDD for data services. Now data services are increasing much faster that voice services. In the future, LMDS services will mainly be data services. So adopting TDD is the natural tendency. But at present, there are more FDD equipments and they are relatively mature. And what is assigned to LMDS by Radio Regulation Bureau is also FDD.

         The LMDS system mainly adopts FDMA or TDMA. It depends on the service features, service strategies and market conditions. FDMA is suitable for those users with large traffic and continuous usage; while TDMA is suitable for users with accidental services.

        LMDS can adopt many modulation modes, mainly characterized as PSK and QAM.

        At present in LMDS system, QPSK is often the choice of TDMA which cannot adopt 64QAM yet. While in FDMA system, 64QAM can be adopted, so FDMA boasts higher data rate. But 64QAM which can provide high-speed data transmission in LMDS system is only suitable for transmission over short distance because 64QAM requires higher signal level and the signal level will decay with the extension of distance.

        Following is a major equipment's data bandwidth capacities under different modulation modes (see Table1).

        Note: In the text below, QPSK includes various enhanced QPSK modulation mode, such as C-QPSK and D-QPSK.

        LMDS can integrate multi-protocols into one convenient 搘ireless optical fiber?and use the embedded ATM protocol. Thus it can build an independent 搗irtual circuit?connection and distribute bandwidth according to different needs at different time, which will lower time delay to its minimum.

LMDS requires view-distance transmission. The coverage of the base station depends on the environment (mainly the rainfall), frequency band and the availability requirement of the user. The coverage radius is usually 3～5km. LMDS has automatic power control function. Currently, LMDS often has 90o sectors.

2.2.3 Technical Features

         LMDS works in Ka wave band. It can provide wide data bandwidth and it has inborn advantages of wireless access, such as networking flexibility, short construction cycle, low maintenance fee and lower construction cost when the cost for equipment drops. Yet high frequency band has some inborn disadvantages. To be specific, LMDS has the following technical features:

         LMDS is called 搘ireless optical fiber? It boasts large system capacity and can satisfy bandwidth demand of broadband access. It can provide such services as voice, data, conference TV and VOD. Generally speaking, LMDS equipments can support all voice and data transmission standards, such as ATM/IP.

         Interfaces can satisfy demands on different occasions. Current LMDS equipments produced can provide a series of network interfaces, such as E1,FR, 10Base-T and ATM.

         The market access cost is rather low. The Capex at the initial stage is low as the major part of the cost for wireless network is the cost of the user station which will incur when the user residence equipment is installed. Thus the network service provider can measure the investment plan, which means the operator will invest only when a profitable user signs a contract. This enables the operator to avoid spending too much on facilities at the beginning. What抯 more, LMDS will avoid the cost of building pipes and that of laborious municipal engineering works. It enables competitive service providers to provide services directly for users.

         Services and coverage with relatively low cost allow more flexibility for system operators to provide customized services. Overlap coverage between cellulars is possible. As the coverage of each cellular can be divided into many sectors, it抯 easier to provide customized services in neighboring cells, to better adapt to the growth of traffic, and to determine the growth of equipment cost according to the end users?demands.

         With relatively low cost for maintenance, management and operation, and a small number of equipments, and without mass pipes which is necessary for wire access, operation and maintenance becomes more convenient.

         High frequency band transmission requires view-distance transmission. This has brought some inconvenience and difficulty to networking, especially when the municipal engineering works are developing very fast, the buildings in cities are ever changing, the routes formally planned are blocked by high buildings, and the adjustment of network layout becomes a necessity.

         Rain attenuation will affect the transmission distance and the availability. While planning the networking, the operators should take into consideration users?demand for availability, transmission distance, investment cost, different frequency bands used by different systems (26GHz, 38 GHz), and different LMDS coverage in accordance with different demands for availability.

2.3 LMDS Service Features

2.3.1 Service Categories

        The broadband feature of LMDS and the diversity of its interfaces (with a varie ty of necessary added equipments) make it capable of providing any service. LMDS can provide users with voice, data and video services at the same time and it can also provide bearer services such as transmission between cellular systems or PCS/PCN base stations. Its services can be categorized as follows:

         Basic voice/telephony services (POTS). LMDS can provide quality voice services, and it can provide standard interfaces such as RJ-11.

         Data services including low-speed data services, medium-speed data services and high-speed data services. To be specific:

       * Leased data line services;
        * Internet access/LAN interconnection services;
        * High-speed data broadcasting services;
        * Image service: LMDS can support high-speed digital image services.

2.3.2 Applications

         LMDS services are mainly applied for:
        * Business/cell access;
        * Mobile communication base station transmission.

3 Comprehensive Comparison

        Both 3.5 GHz and LMDS fall into the category of fixed wireless access technology. Please refer to Table2 for the comparison between they two.

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Biography:

　　Dou Li was graduated in 1989 from Beijing University of Posts and Telecommunications with bachelor's degree. He had worked in the wireless section at the institute of design under the ministry of post and telecommunication having served as an engineer, senior engineer and office director since 1989. Since 2000, he had worked with China Telecom Group as deputy section chief of wireless and mobile section of technological department, and in 2002 he joined China Netcom Group Co. serving as the section chief of wireless technological section of technological department.

Li Song-lin was graduated from Huazhong University of Science and Technology with bachelor's degree in 1991 and graduated in 1999 from Beijing University of Aeronautic and Astronautic Science and Technology with master's degree.
He once worked at No. 4057 Plant under the Ministry of Electronics and telecom programming research institute under the Ministry of Information Industry. Now he is working at wireless section of technological department of China Netcom Group.

Wang Jing-yu was graduated from computer department of Nankai University in 1992 with bachelor's degree and was awarded master's degree in telecom and electronic engineering at the Research Institute of Telecom Science and Technology under the Ministry of Post and Telecommunication. He once worked at Datang Telecom Group engaged in the exchange sub-system of IS-95-CDMA, research and development of base station sub-system and research and development work of TD-SCDMA Standard. He is now working at the wireless section of Technological Department of China Netcom Group.