Building Broadband Wireless

Demand for telecommunications bandwidth to support the explosive growth in personal Internet access, telecommuting, interconnection of corporate computing networks and video entertainment is outpacing landline infrastructure build-out. At the same time, commercial wireless technologies are maturing and becoming increasingly accepted as credible communications alternatives.

The demands of the broadband wireless market and the emerging high-frequency point-to-multipoint technologies will require expertise from two types of engineering design disciplines: high-frequency fixed point-to-point microwave and lower frequency wide-area cellular and PCS. Given the availability of wireline technologies and other competing wireless providers, broadband wireless operators need to differentiate their services quickly to attract and retain prospective customers. By using appropriate engineering principles, algorithms, processes and data, you can successfully build, deploy and manage a superior network cost-effectively.

Point-to-Multipoint Technology Several technology options provide potential solutions for broadband wireless operators. Historically, spectrum allocations have focused operators' choices into one of two technology groupings: 1) wide area, where cell sites are used to propagate radio signals across a general coverage area, or 2) fixed point-to-point, where the radio beam is focused between two fixed points.

Suppliers of both technologies are pushing solutions for broadband wireless applications. For wide-area deployments, spectrum options include the current cellular and PCS bands as well as potential new allocations in the 3GHz range. Cellular and PCS suppliers are tuning their standard equipment to provide higher bandwidth rates in the near term and are working on third-generation standards to fully meet the expected demand. For operators in the higher millimeter bands, initial deployments are using point-to-point microwave equipment.

As a third option, point-to-multipoint technology can be viewed as a middle ground between the point-to-point and wide-area deployments. It provides the benefits of high bandwidth through fixed directed antennas, yet optimizes the required equipment and physical site requirements. You can derive much of the solution set from the core principles used in deploying point-to-point microwave and wide-area networks.

The Flexible Deployment Opportunity The broadband wireless market is emerging as a way to satisfy the needs of small and medium businesses, home offices, multi-unit dwellings, office buildings and telecommuters. In heavy urban centers, broadband wireless also may satisfy transitional or backup service requirements as well as transport technology needs. Residential access to Internet and video also is viewed as a high potential market.

New service providers will need to distinguish themselves clearly from current wire-line and mobile operators. One way to do this is to shift the network-to-customer paradigm. Historically, telecommunications providers' first step was to build a network. Only when they had deployed a significant portion of the network did they market a list of standard service offeri ngs.

Point-to-multipoint wireless technologies allow you to turn this concept around. Customers truly can come first and, based on a business' telecommunications requirements, the local loop network can be built incrementally. The local connection also can be dropped easily and refocused as customers move. This flexibility to deploy new network connections quickly gives wireless providers a marketing advantage over the wireline competition. Not only can the connection itself be deployed rapidly, but also it can be tailored to the customer requirements for capacity and reliability.

Design Requirements Designing a point-to-multipoint network requires engineering expertise in both traditional RF planning and focused point-to-point link planning to achieve an optimal network design. The design of a point-to-multipoint wireless local loop system begins with detailed network planning based on the requirements of each individual receiver site. Because fixed wireless networks can be built incrementally, you don't need a large footprint at design start, unlike the design of mobile implementations. This may simplify the effort initially but will grow increasingly complex as the network and customer base expand.

The RF Network The typical broadband wireless network configuration uses radio communications for the local loop portion. Generally, the radio sub-network is comprised of multiple hubs and hundreds of subscriber locations. The hub location serves a dual purpose: 1) interface with the network's backbone and 2) relay information to and from the subscriber. The carrier's backbone network provides large capacity transport such as a fiber cable ring linking the hubs and providing connections to the switches and network controllers.

The links between the hub and subscriber locations are accomplished by point-to-point or point-to-multipoint radio transmitters. For many new operators, these transmitters operate at microwave frequencies. Each point-to-point microwave link has a dedicated pencil-beam antenna at the hub and at the subscriber location. As the network evolves, a hub location will be expanded to physically support a cluster of point-to-point links. In the point-to-multipoint scenario, the hub uses multiple broad-beam antennas or sectors. Each sector is capable of communicating with several subscribers' pencil-beam antennas.

Radio Network Planning Criteria for selecting and engineering hub sites for high-frequency, fixed-wireless deployments differs significantly from the criteria used to design lower frequency wide-area mobility networks. For the higher bands, engineering reliable broadband wireless networks depends on rainfall rates and line-of-sight calculations. Rain absorption is a key element in predicting the performance of a high-frequency microwave radio transmission accurately. Similarly, clear line-of-sight between the hub and subscriber is essential at these frequencies. Climate, rainfall rate, antenna type and transmitter power will dictate the distance between the hub and the subscriber locations.

Propagation prediction models are used to assess signal-strength loss from obstacles and climatic variations. Well-established propagation models such as the International Telecommunications Union's Radio branch (ITU-R) recommendations are applicable for high microwave because of the dependency on line-of-sight. In the United States, the Crane rain model also is used widely. These propagation models, applied to both point-to-point and point-to-multipoint systems, can predict the link budget and link availability, which are used to assess the system's performance.

Traditional non-line-of-sight microwave models, established by the ITU-R and the National Institute of Standards TechNote 101, can be modified to handle interference prediction at the higher frequency ranges. A modified wide-area model such as COST231 or Bertoni-Xia can be used for urban environments after they are adjusted for higher frequencies and rain attenuation. Currently, COST231 is used for cellular and PCS propagation operating in the 900MHz and 1.9GHz bands, respectively. At these lower bands, rain attenuation is less pronounced, and the models focus on median losses and diffraction over obstacles such as terrain and buildings.

An operator's ability to predict system performance by selecting the appropriate propagation models and engineering techniques will minimize the need for additional measurements and delays, speeding time to market.

Fixed Subscriber Characteristics Although the frequency range selected has a significant effect on the engineering algorithms used, broadband wireless deployments also are driven by the defined service application. Rather than portable or vehicle-mounted mobile handsets, subscriber equipment typically will include building-mounted directional antenna systems. Furthermore, you can exploit the equipment location and height, in addition to the usage of directed pencil-beam antennas for improved performance and interference prediction.

Frequency planning is an essential engineering process in any wireless network. The goal is to avoid and to control interference between radio waves.

Interference assessment in broadband wireless networks, both point-to-point and point-to-multipoint topologies, is less complex than in a wide-area, point-to-multipoint network. In broadband wireless networks, subscriber and the limited propagation range of high microwave frequencies simplify computational algorithms. The analysis becomes more complex as the network grows and needs to handle localized congestion, dense hub sites and aggressive subscriber growth.

The incremental growth of a broadband wireless network does not allow for up-front comprehensive frequency allocation of the entire network or region. The goal is to minimize changes on implemented network segments. This can be accomplished through proper frequency plans that accommodate expansion.

Usually, the principal interference-avoidance measure is to maintain accurate site and frequency data. Accurate data will lead to reduced performance and interference prediction errors, allowing for more efficient frequency assignment.

Accuracy of prediction algorithms and propagation models has a large impact on system planning and growth. Over-prediction of interference at a hub site can limit the number of frequencies operating simultaneously, leading to reduced spectral efficiency.

Accurate Engineering is Critical Broadband wireless solutions appear particularly attractive, enabling rapid deployment, incremental capital investment more closely tied to the number of paying subscribers, low operating cost and equipment mobility. In the United States, recent spectrum allocations and the upcoming LMDS auction are providing sufficient radio frequencies to make broadband wireless a viable technology choice for many new alternate access providers.