Flexibility key to NLOS fixed wireless deployments

Successfully deploying a fixed broadband wireless (FBW) network depends on two key factors: flexibility in planning radio frequency (RF) network layouts and line-of-sight (LOS)/non-line-of-sight (NLOS) conditions. Because LOS conditions require an unobstructed view between the transmitter and receiver, carriers face technical and business challenges when planning and deploying LOS-based FBW networks; in fact, the first generation of FBW systems failed because of these challenges.

Conversely, NLOS product solutions that allow layout customization provide enhanced RF planning opportunities with low up-front capital costs and short time-to-market. Deploying to ten North American markets, Netro Corp.'s engineers found that the keys to successful large-scale NLOS FBW installations were variable cell layouts, compact channelization, frequency reuse, and providing RF designers with flexible tools. As the industry matures, we believe these same factors will remain dominant.

RF network planning and design is one of the most important elements of deploying a carrier-class FBW network. Variable cell sizes allow RF network designers to plan around available spectrum, market density, physical environment, and business priorities to target revenue-producing customers.

Super-cells provide service over large areas with coverage from 20 to 30 kilometers depending on antenna heights and the RF environment. Super-cells may be deployed quickly using part of the available spectrum to establish market coverage with low initial capital outlay.

Mini-cells provide service over densely-populated areas of 1 to 5 kilometers. These cells offer significantly more capacity than super-cells using the same spectrum resources, but require more careful planning for frequency reuse, inter-cell interference, and spectral efficiency.

Integrating mini-cells for areas with high capacity requirements and super-cells for the rest of a market is an ideal solution for frequency planning and reuse, maximum capacity, and minimal frequency retunes.

Operators also require cost-effective geographic data to design predictable RF coverage for large-scale deployments over multiple markets. High-resolution 1-meter data maps are necessary input for LOS coverage, but can be expensive, require extensive computer processing resources, and are not available in many markets. NLOS systems leverage the cellular industry's commercially-available 30-meter data maps with much lower financial cost and better results. First generation FBW systems using 1-meter map data had limited coverage. In contrast, planning Netro's NLOS Angel system using 30-meter map data yielded 95 percent probability of coverage.

Compact channelization allows operators to use available spectrum efficiently. RF network designers can maximize system capacity with high frequency reuse and apply system coverage to subscriber density cost-effectively. For example, an operator with 7 MHz of spectrum cannot use a radio with a 14-MHz channel. But a radio with smaller 1.75-MHz channels could provide four frequencies within that 7-MHz range and increase capacity with frequency reuse. That same 14-MHz channel might provide more capacity than low subscriber density areas need. Compact channelization enables operators to deploy just one smaller channel. Angel reached 2 million households with more than 90,000 subscriber lines using as little as a 5-Mhz pair.

The complexities of FBW RF planning require robust design tools. Current commercially-available prediction programs were developed primarily for cellular and personal communication system (PCS) designs and use omni-directional receive antenna data for propagation analysis. The RF design team deploying Angel needed planning tools that could support directional customer premises equipment (CPE) antennas for coverage, capacity, and co-channel interference analysis in NLOS conditions, and that produced coverage plots which could be integrated with sales, marketing, and provisioning systems. Faced with these requirements, Netro developed a suite of FBW RF design tools that combined commercially-available geographic information system (GIS) and RF propagation platforms with a specialized FBW network design engine.

The physical environment presents challenges for signal transmission in any system. Shadowing and RF dead zones from man-made and natural obstructions interfere with signals in LOS systems. Zoning laws, limited installation locations, and tower heights all contribute to potentially reduced coverage footprints. Deploying around these challenges increases time-to-market and capital and operating expenses.

In a NLOS system, a signal travels around obstructions via multiple reflections and reaches the receiver through many paths. A product that merely increases power to penetrate obstructions is not NLOS technology because it still relies on a strong direct signal without using energy present in indirect signals. True NLOS systems make use of signals received from a direct path, multiple reflected paths, scattered energy, and diffracted propagation.

Signal quality is based on strength, stability, propagation, fade margin, and probability of coverage. These factors are, expressed as a percentage representing the statistical probability that customers with a predicted coverage footprint will have sufficient signal quality.

Because direct line-of-sight isn't necessary between base stations and customer equipment, NLOS installation is cheaper and easier, base station towers can be shorter, the system adapts to its changing environment, and the installation success rate can be assured to be better than 95 percent.

Sorting out the diverse multipath signals to form a robust radio channel requires a variety of sophisticated air interface techniques to account for different delay spreads, path loss, attenuation, and stability relative to the direct path.

By virtue of its symbol rate and use of a cyclic prefix, the OFDM waveform eliminates intersymbol interference (ISI) without the complexities of equalization. Also, because the OFDM waveform is composed of multiple narrowband orthogonal carriers, selective fading is localized to a subset of carriers. The ability to overcome delay spread, multipath distortion, and ISI enables high data throughput.

Adaptive modulation adjusts signal modulation depending on the signal-to-noise condition of the air interface. Most FBW systems adjust modulation for all subscribers in a sector simultaneously; more advanced systems dynamically adjust modulation for each subscriber independently to overcome fading. Subscriber-by-subscriber adaptive modulation uses high and low modulation levels to increase system capacity where necessary while maintaining overall link quality and stability.

Based on field deployments of more than 500 base stations in North America, we found that to meet capacity and link quality requirements, FBW equipment needed to support adaptive modulation of ranges from quaternary phase shift keying (QPSK) to 64 quadrature amplitude modulation (QAM).

Dynamic power control is imperative in multiple-cell deployments to mitigate co-channel interference and increase overall system quality and capacity. The optimal implementation of this concept uses subscriber-by-subscriber closed loop power control in both the downlink and uplink.

Diversity schemes take advantage of multipath propagation and reflections that occur in NLOS conditions. Diversity technology in both the transmitter and receiver use maximal combining ratio algorithms to improve the propagation link budget. Technologies such as smart antennas or beamforming can increase capacity, but are cost-prohibitive for carriers to deploy due to increased equipment costs, zoning, tower structural loading, and complex installation. For these reasons, these technologies have not been widely adopted in the cellular industry and remain to be proven for FBW.

The wireless industry is increasingly adopting NLOS technologies that use OFDM by incorporating them into standards such as IEEE 802.1, Fourth Generation (4G) cellular, and European Telecommunications Standards Institute (ETSI) Broadband Radio Access Networks (BRAN). Successful FBW systems will support these standards with spectrally-efficient compact channelization and robust RF planning tools.