Introduction
Global integration and fast-growing business activity in conjunction with remote multisite operations have increased the need for high-speed information exchange. In many places around the world, the existing infrastructure is not able to cope with such demand for high-speed communications. Wireless systems, with their fast deployment, have proven to be reliable transmission media at very reasonable costs. Fixed broadband wireless access (BWA) is a communication system that provides digital two-way voice, data, Internet, and video services, making use of a point-to-multipoint topology. The BWA low-frequency radio systems addressed in this article are in the 3.5 GHz and 10.5 GHz frequency bands. The BWA market targets wireless multimedia services to small offices/home offices (SOHOs), small and medium-sized businesses, and residences. Currently licensed bands for 3.5 GHz BWA systems are available in South America, Asia, Europe, and Canada. The 10.5 GHz band is used in Central and South America as well as Asia, where expanding business development is occurring. The fixed wireless market for broadband megabit per second transmission rates is growing for providing an easily deployable low-cost solution, compared with existing cable and digital subscriber line (xDSL) technologies for dense and suburban environments.
This article describes the BWA network system, the radio architecture, and the BWA planning and deployment issues for 3.5 and 10.5 GHz systems. Table 1 summarizes the system characteristics for each frequency range according to various International Telecommunication Union -- Radiocommunication Standardization Sector (ITU-R) drafts, EN 301 021, IEEE 802.16, and other national regulations. A maximum of 35 Mb/s capacity is achievable for 64-quadrature amplitude modulation (QAM) over 7 MHz channel bandwidth. Coverage ranges for line-of-sight links are given for 99.99 percent availability.
The BWA System Network
A BWA system comprises at least one base station (BS) and one or more subscriber remote station(s) (RS). The BS and RS consist of an outdoor unit (ODU), which includes the radio transceiver and antenna, and an indoor unit (IDU) for modem, communication, and network management (Fig. 1). The two units interface at an intermediate frequency (IF); optionally, the RS ODU and IDU can be integrated. The BS assigns the radio channel to each RS independently, according to the policies of the media access control (MAC) air interface. Time in the upstream channel is usually slotted, providing for time-division multiple access (TDMA), whereas on the downstream channel a continuous time-division multiplexing (TDM) scheme is used. Each RS can deliver voice and data using common interfaces, such as plain old telephony service (POTS), Ethernet, video, and E1/T1. Depending on the type of service required by the client, remote stations can provide access to a 10/100Base-T local area network (LAN) for data access and voice over IP (VoIP) services; to a LAN and up to 8 POTS targeted to small businesses; or to a LAN and an E1/T1 channel connected to a private branch exchange (PBX) for small and medium enterprises.
The BS grooms the voice and data channels of several carriers and provides connection to a backbone network (i.e., IP or asynchronous transfer mode, ATM) or transport equipment via the STM1/OC-3c (155.52 Mb/s) high-capacity fiber link. The ATM network gives access to the public switched telephone network (PSTN) gateway through competitive local exchange carriers (CLECs) using V5.2/GR.303 standards, or to an edge router for accessing the Internet data network through Internet service providers (ISPs). The ATM network interface is also connected to the network management system via Simple Network Management Protocol (SNMP) for performing tasks such as statistics and billing, database control, network setup, and signaling alarms for radio failures. Configuration of the radio network link is made possible through a Web browser http link via TCP/IP.
Each BS has a certain available bandwidth per carrier that can be fully or partially allocated to a single RS either for a certain period of time (i.e., variable bit rate,VBR, or best effort), or permanently (i.e., constant bit rate, CBR). BWA systems are envisioned to work with a TDMA rather than code-division multiple access (CDMA) scheme in order to counteract propagation issues. Also, for non-line-of-sight (NLOS) environments, BWA systems with a single carrier with frequency domain equalizer and decision feedback equalizer (FD-DFE) or orthogonal frequency-division multiplexing (OFDM) technologies are applicable [1]. Small and medium-sized businesses require fast and dynamic capacity allocation for data and voice packet-switched traffic. This TDMA access scheme can be applied to either frequency-division duplexing (FDD) or time-division duplexing (TDD) [2]. Both duplexing schemes have intrinsic advantages and disadvantages, so the optimum scheme to be applied depends on deployment-specific characteristics (i.e., bandwidth availability, Tx-to-Rx spacing, frequency congestion, and traffic usage). Targeting the business market, Harris ClearBurst™ MB products are designed for FDD. In symmetric two-way data traffic, FDD allows continuous downstream and upstream traffic on both low- and high-band channels. Moreover, it has full flexibility for instantaneous capacity allocation, dynamically set through the MAC channel assignment.
The Radio Frequency System
RF subsystems consist of the base station and remote station ODUs. This section will provide a global understanding of the different RF technology employed for high-performance low-cost radio design. In addition to meeting all the functional, performance, regulatory, mechanical, and environmental requirements, the radio system must achieve most of the following criteria:
- Cost effectiveness
- Be maintenance-free
- Be easily upgradable
- Quick installation
- Attractive appearance
- Flexibility
- Scalability
An example of a BWA radio system is shown in Fig. 2: a base station ODU, part of the ClearBurst™ MB product. Its radio enclosure contains two sets of identical transceivers with high-power amplifiers and RF diplexers for redundancy. A dual flat panel antenna is directly integrated with the enclosure. A single coaxial cable is used to connect to the indoor base station router unit. The base station radio units can be mounted on either a pole, a tower, or a wall mount. The remote station ODU is an unprotected unit, where a single transceiver with a medium-power amplifier is used. The enclosure is directly connected to the flat panel antenna. In addition, an alignment indication connector is also provided for antenna installation and alignment with the base station.
An ODU radio consists of transmitter and receiver circuits, frequency sources, a diplexer connected to the antenna, and a cable interface to connect to the indoor modem unit. Moreover, a minimum of "intelligence" is required in the radio to control the power level throughout the transceiver. Development of software-controlled radios is presently underway, but the issue of cost-effectiveness remains. Typically for small businesses or residential markets, cost is the main factor that comes into play; hence, simpler design by limiting radio "intelligence" may translate into less demanding requirements for the radio processor. Software-controlled radios present many advantages, such as reducing hardware complexity, but it is up to the design engineers to compromise among the high performance, low cost, and flexibility of the product.
A low-cost low-performance radio solution appropriate for the high-volume residential market is shown in Fig. 3 as a "dumb" transceiver. This architecture uses a minimal number of hardware components, integrated with or without software control capabilities. Following the RF diplexer, the receive (Rx) path includes a low-noise amplifier, bandpass filters (BPFs) for image-reject and channel select filtering, a downconverter mixer, and an open loop gain to allow a wide input dynamic range. The transmitter (Tx) consists mainly of an upconverter associated with some filtering and a power amplifier (PA). The local oscillator (LO) may provide for fixed or variable frequency to the mixers. A fixed LO would give a variable IF; hence, by using a wider BPF bandwidth, the receiver would not be immune to interference. Adding a microcontroller to the radio provides control of the phase locked loop (PLL) for the transceiver synthesizer and can put the PA into mute mode. Single up/downconversion stages further reduce the overall cost, but at the expense of lower radio performance. Two separate IF cables simplify the interfacing.
An "intelligent" transceiver involves more digital and software-controlled circuitry, and hence higher cost. Figure 4 shows a transceiver block diagram which includes closed loop gain control, cable, and fade margin compensation on the transmit and receive paths; that is, power detection circuits on Rx IF, Tx chain, and PA. The transmitter mutes on a synthesizer out-of-lock alarm in order to avoid transmitting undesirable frequencies, and also on no received signal. The microcontroller provides for the receive signal strength indicator (RSSI) level for antenna alignment, and for control and monitor channels. A single cable is used for all input and output IFs, the telemetry signal, and the DC biasing from the IDU. Software control also allows for calibrated radios, which results in no gain variation or frequency shifting of the signal with respect to temperature variation. Technology advancement in the past few years in the RF integrated circuit market allows for greater chip integration using commercial off-the-shelf (COTS) devices and simplified hardware board level design [3]. This architecture achieves better performance, especially for higher-modulation schemes, and therefore is suitable for higher-capacity radios targeting the business market.
The modulation scheme chosen for the radio system depends on several product definition factors, such as required channel size, upstream and downstream data rates, transmit output power, minimum carrier-to-noise ratio (C/N), system availability, and coverage. Table 2 gives the characteristics for quadrature phase shift keying (QPSK) and QAM signals typically used for BWA systems for 7 MHz channel bandwidth. Higher-modulation schemes provide higher data rates at the expense of better C/N requirements and smaller coverage radii for the same availability, adding to the hardware complexity. For the 64-QAM 7 MHz channel bandwidth signal typically used on the 3.5 GHz system, a maximum of 35 Mb/s is achieved.
A system can require symmetric or asymmetric capacity depending on its specific application. For a symmetric capacity system, upstream and downstream traffic are equivalent, whereas for an asymmetric system the downstream link usually requires more capacity. Hence, higher-level modulations with higher capacity are better suited to downstream transmissions. Using n-QAM modulations for downstream transmission becomes advantageous, whereas QPSK can be used in the upstream direction. Since lower-level modulations perform better in more constrained environments, they can be not only used in burst, low-power, low-capacity, or upstream transmissions, but also adjusted dynamically in link fading conditions.
Radio Transmission System and Deployment
The maximum cell size for the service area is related to the desired availability level. At 3.5 GHz and 10.5 GHz, the average cell radius for line-of-sight (LOS) 99.99 percent availability is 19 km and 8 km, respectively. Principal factors affecting cell radius and availability include the rain region, the antenna and its height, foliage loss, modulation, Tx power, Rx sensitivity, and sectorization. These effects are generally related to the service area, such as dense urban, suburban, and low-density. As an aid to determining these parameters, a powerful point-to-multipoint RF transmission engineering tool is used to estimate the maximum distance between the BS and RS, while maintaining the desired link performance and availability in a single or multihub environment. Taken into account are the margins required to combat multipath fading, rainfall attenuation, and interference. The effect of the rainfall attenuation is negligible at 3.5 GHz but noticeable at 10.5 GHz.
The base station hub is divided into a number of sectors to accommodate all received signals and cumulative traffic from the remote stations. The number of cell sectors affects the cost per cell and complicates cell planning, but also increases the capacity of the system. Each BS unit typically serves 1000 and 100 remote stations at 3.5 and 10.5 GHz, respectively. The deployment consists of a four-sector/90° or six-sector/60° cell configuration. The antenna panel can be assembled for horizontal or vertical polarization for reduced interference.
Conclusions
Growing demand for fast information exchange to support business activities requires the implementation of low-cost, easily deployable communications networks. Fixed low-frequency BWA radio systems at 3.5 and 10.5 GHz were presented as an attractive solution. System architecture was presented from a signal processing and radio frequency perspective. Architecture compromises were discussed, enabling the use of cost-effective solutions that meet quality and performance requirements.
Acknowledgment
This article is based on our previously published material from WAS '2000 organized by DELSON GROUP.
References
[1] IEEE 802.16a, Draft documents for Sub 11 BWA, IEEE802.16.3c-58 and 59.
[2] J. Klein, "TDD vs. FDD: The Drive for Effective Bandwidth Management," RF Design, Aug. 1999, pp. 36–55.
[3] M. Danesh, N. Hassaïne, and F. Concilio, "New Transceiver Design Approaches for Digital Microwave Radios," 2000 IEEE Radio and Wireless Conf., Denver, CO, Sept. 10–13 2000, pp. 39–42.
Biographies
Mina Danesh [S'93–M'99] received her B.Eng. from Concordia University, Montreal, Canada, and an M.A.Sc. from the University of Toronto, Canada, in 1996 and 1999, respectively, all in electrical engineering. In 1999 she joined Harris Corporation, Microwave Communications Division, Montreal, Canada, as an RF design engineer. She is currently involved in the design of radio transceivers for broadband wireless access products. Her current research interests cover wireless communications, RF/microwave, and millimeter-wave MIC and MMIC design.
Juan-Carlos Zuniga [M] received his B. Eng. degree in electrical engineering from the National University of Mexico (UNAM) in 1995, and his M.Sc. and DIC in communications and signal processing from the Imperial College of Science, Technology and Medicine, University of London, in 1998. He worked at Kb/TEL Telecommunications in Mexico City, Mexico, then at Nortel Networks in Harlow, England, in the Broadband Satellite Networks department. He joined Harris Corporation, Microwave Communications Division, Montreal, Canada, as a system design engineer in the Broadband Wireless Access department in 1998. He is presently a senior systems engineer and a member of the IEEE 802.16 Working Group on Broadband Wireless Access Standards, and his research interests are in high-speed mobile and fixed wireless networks.
Fabio Concilio received his B. Eng. in electrical engineering in 1975 followed by post-graduatie courses in microelectronics and yelecommunications in 1978 from Escola Politécnica, University of São Paulo, Brazil. He first joined Philips Transmission Division in Brazil working in the Advanced Development Group in the microwave field, then as a senior engineer at the development laboratory at Telefunken, returning to Philips where he was responsible for the microwave group developing high-capacity microwave digital radios. He moved to Harris Corporation, Microwave Communications Division as a senior engineer in 1992 and currently manages the RF Design department responsible for the development of point-to-point and point-to-multipoint radio units.
