RF net scales broadband to local area

Local multipoint distribution service (LMDS) is primarily aimed at providing fast network access for buildings within a radius of several kilometers using wireless technology. Typical system setup involves parabolic antennas mounted on roofs and thus operating in an outdoor environment. As a result, transmission is subject to atmospheric conditions. Carrier frequencies range from 10 to 40 GHz, depending on local spectrum allocation and climate conditions.

A number of modulation schemes are available as a compromise mainly between signal points and receiver structure complexity, so that receiver sensitivity becomes a function of the modulation scheme. The choice of modulation scheme also affects the system capacity in terms of data rate and number of subscriber sites.

The modulation choice also affects channel spacing. For example, a 4-QAM scheme will double the channel spacing of 16-QAM, directly affecting frequency reuse and, hence, the cost-effectiveness of the deployment.

In geographic regions where frequent heavy rainfalls greatly affect radio transmission, a carrier frequency range of 15 to 30 GHz is usually undesirable. That's because the typical size of raindrops varies between 1 and 2 millimeters-a size that corresponds to the wavelengths of these frequencies, making for severe signal-attenuation problems. As a result, lower frequencies of around 10 GHz are preferred in tropical regions.

Raindrops have depolarization effects on radio waves as the waves scatter. Such effects deviate the wave components so that signals with vertical polarization suffer less attenuation than those with horizontal polarization.

The selection of frequencies of around 10 GHz raises the issue of multipath propagation. Multipath generally is not a significant problem with high frequencies due to their shorter path lengths and the narrow beam width of the antennas used. But it can be a factor at lower frequencies.

Careful planning
Wireless broadband access networks normally require line-of-sight transmission, especially at higher frequencies. Careful planning in antenna setup to ensure line-of-sight reduces the impact of multipath interference when a frequency of 10 GHz is used.

Mobile network access is being provided now by wireless local-area networks (WLANs) using portable access units. Although this solution is widely used throughout the world in public areas such as school campuses and office environments, its main drawback is the very limited coverage of roughly a 100-meter radius, thus requiring that a large number of intermediate devices be attached to the wired network infrastructure.

Using existing LMDS infrastructures, we have designed a system that provides network access to a small area-within several kilometers-using radio links. This system offers broadband wireless access to fixed (stationary) nodes in conjunction with wired backbone networks, based on an architecture similar to that of a cellular system, where omnidirectional basestations provide the coverage.

Similar methodology has been deployed as wireless local loops (WLLs), often for countries without a comprehensive telephone network. WLL is aimed at providing wireless network access without relying on legacy wired infrastructures such as the plain old telephone system (POTS). Since WLL is primarily designed as an alternative for providing telephone service, its supported data speed is very limited, although the technology can be made capable of supporting data rates similar to those of the integrated services digital network (ISDN).

Since LMDS provides point-to-multipoint wireless access services to subscribers within a geographical area far greater than that of WLANs, it is drawing much interest as a potential way to extend services to portable devices. With its promise of higher bandwidth for more subscribers at higher data rates than other wireless distribution technologies, adapting the existing LMDS infrastructure to provide network access to mobile subscribers would be of great interest to consumers eager to get online faster. A conceptual model of such a system provides wider coverage than WLANs and higher data rates than WLLs. But several factors are keeping this out of reach.

The type of antenna used is one consideration, especially when the system is adapted to provide access for mobile subscribers. Typically, an antenna with a narrow beam width of approximately 2 degrees to 3 degrees and gain of 40 dBi, and which maintains a line-of-sight to the basestation antenna, is used. At present, LMDS receivers use antennas of around 0.3 to 0.6 meter in size, because they are roof-mounted and not intended to be mobile. Antennas of this size are far too large for any mobile devices. With additional circuitry needed to form an integral part

of the wireless access model for consumer equipment, mobility of subscriber devices is very limited. Size and weight of the access modem must therefore be taken into account in system development. Cell overlap control is another consideration in such deployment.

Distinct advantages
Although LMDS has distinct advantages over other means of wireless access in terms of network capacity-number of subscribers supported and data rate-it iscurrently impractical to adapt this system for mobile devices. Rudimentary problems associated with its deployment are mainly due to the line-of-sight requirement and the practicality of carrying a too-hefty mobile device. The main challenge is to make the receiver small and light enough for mobile users to tote, and cheap enough to make.

Field measurements have been conducted to prove the viability of this approach using a 0.3-meter antenna at 26 GHz. To simplify the receiver structure while balancing performance, 4-QAM turns out to be the optimal choice as the modulation scheme for providing wireless network access to mobile subscribers.

This article will be presented at the ICCE show in Los Angeles, in a paper called "Modulation Schemes for Broadband Wireless Internet Access."