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21-10-2009, 08:55 PM

Local Multipoint Distribution Service (LMDS)


Local Multipoint Distribution Service (LMDS), or Local Multipoint Communication Systems (LMCS), is a broadband wireless point-to-multipoint communication system operating above 20 GHz that can be used to provide digital two-way voice, data, Internet, and video services. The term "Local" indicates that the signals range limit. "Multipoint" indicates a broadcast signal from the subscribers; the term "distribution" defines the wide range of data that can be transmitted, data ranging anywhere from voice, or video to Internet and video traffic. It provides high capacity point to multipoint data access that is less investment intensive. Services using LMDS technology include high-speed Internet access, real-time multimedia file transfer, remote access to corporate local area networks, interactive video, video-on-demand, video conferencing, and telephony among other potential applications. In the United States LMDS uses 1.3 GHz of RF spectrum to transmit voice, video and fast data to and from homes and businesses. With current LMDS technology, this roughly translates to a 1 Gbps digital data pipeline. Canada already has 3 GHz of spectrum set aside for LMDS and is actively setting up systems around the country. Many other developing countries see this technology as a way to bypass the expensive implementation of cable or fiber optics into the twenty-first century.
Point-to-point fixed wireless network has been commonly deployed to offer high-speed dedicated links between high-density nodes in a network. More recent advances in a point-to-multipoint technology offer service providers a method of providing high capacity local access that is less capital intensive than wire line solution, faster to deploy than wire line, and able to offer a combination of applications. Moreover, as large part of a wireless network's cost is not incurred until the Customer Premise Equipment (CPE) is installed, the network service operator can time capital expenditures to coincide with the signing of new customers. LMDS provides an effective last-mile solution for the incumbent service provider and can be used by competitive service providers to deliver services directly to end-users.
The main benefits of LMDS are listed below: 1 Lower entry and deployment costs 2 Ease and speed of deployment (systems can be deployed rapidly with minimal disruption to the community and environment) 3 Fast realization of revenue (as a result of rapid deployment) 4 Demand based build out (scalable architecture employing open industry standards ensuring services and coverage areas can be easily expanded as customer demand warrants) 5 Cost, shift from fixed to variable components. (For wireline systems most of the capital investment is in the infrastructure, while with LMDS a greater percentage of investment is shifted to CPE) 6 No stranded capital when customers churn. 7 Cost-effective network maintenance, management, and operating costs.
Technical Basics
Past communication technologies focused their attention lower in the RF spectrum because low frequency signals with enough power could be sent long distances and penetrate buildings. Such is the case with television and radio. LMDS, however, uses low powered, high frequency (25 -31 GHz) signals over a short distance. LMDS systems are cellular because they send these very high frequency signals over short line-of-sight distances. These cells are typically spaced 4-5 kilometers (2.5 - 3.1 miles) apart. LMDS cell layout determines the cost of building transmitters and the number of households covered. Direct line-of-sight between the transmitter and receiver is a necessity. Reflectors and/or repeaters can spray a strong signal into shadow areas to allow for more coverage. Various isolation techniques can be used to prevent interference between signals. Cell size is also influenced by the amount of local rainfall. Because LMDS signals are microwaves, they are attenuated by water and lose strength. To correct this, LMDS operators can either increase the power of their transmissions when it rains in an attempt to ensure a strong signal reaches its destination, or they can reduce their cell size. Leaves, trees and branches can also cause signal loss, but overlapping cells and roof-mounted antennas generally overcome the problem. LMDS system operators offer different services and have different legacy systems, financial partners, and business strategies. As a result, the system architecture used will differ between all system operators. The most common architectural type uses co-sited, base-station equipment. The indoor digital equipment connects to the network infrastructure, and the outdoor microwave equipment mounted on the rooftop is housed at the same location Typically, the radio frequency (RF) planning for these networks uses multiple sector microwave systems, in which transmit- and receive-sector antennas provide service over a 90-, 45-, 30-, 22.5-, or 15-degree beam width. The idealized circular coverage area around the cell site is divided into 4, 8, 12, 16, or 24 sectors.
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