About LMDS

LMDS in use at Virginia Tech

LMDS hub deployed at Virginia Tech

Local Multipoint Distribution Services (LMDS) is a fixed broadband wireless access system using a range of frequencies around 27.5 GHz to 31.3 GHz. The technology allows for two-way digital communication for voice, digital video, and high speed data communication. There are two blocks of spectrum, A and B blocks, which differ by the amount and location of spectrum. The A block spectrum is a total of 1150 MHz as compared to the B block at 150 MHz. See the LMDS bandplan (pdf) to compare the two licenses.

Broadband wireless LMDS can provide data rates of tens or hundreds of megabits per second with no wires, no rights-of-way, no digging, quick and easy to deploy.

The bandwidth of the A block of LMDS is more than twice the total bandwidth of AM/FM radio, VHF/UHF TV, and cellular telephone combined. Using LMDS, transmission speeds of several Gigabits per second are possible along line of sight distances of several miles.

LMDS, as a licensed service, has advantages over unlicensed systems as it is less likely to receive radio interference from others and offers far greater bandwidth. LMDS may be deployed in point-to-multipoint (PMP) or point-to-point (PTP) configurations. PMP uses a central "hub" connected to the backbone network that feeds "remote" locations within range of that cell. LMDS PMP systems may have a range of several miles. A PTP system connects two locations and can operate over a longer distance. The short range of LMDS makes it ideal for providing dense, high speed, cell like coverage to an area. With planning, the spectrum may be reused over and over in non-contiguous areas.

LMDS has a number of challenges. It requires line of sight between a transmitter and receiver. There must be no obstructions between hub and remote. This requires careful planning and to reach an obstructed site, it may require a repeater to retransmit the signal over a building, hilltop, or other obstacle. LMDS signal strength is greatly reduced by the presence of heavy rainfall. Rainfall of several inches per hour may cause signal dropouts. Outages due to heavy rain are uncommon in many areas and typically brief, so it may not be a significant issue.

An advantage of LMDS, in addition to a licensed, high-bandwidth service, is that it can be deployed to provide a communication link without potentially high expense and right-of-way issues associated with laying fiber or cable.


Blacksburg Deployment

The first rural LMDS network was located on the Virginia Tech campus that extended into the Town of Blacksburg. The network was deployed in May 1999 and operated for several years. It was a joint effort of Communications Network Services of Virginia Tech (CNS) with support from Virginia Tech's Center for Wireless Telecommunications (CWT), and WavTrace, the equipment manufacturer.

The Blacksburg network was a point-to-multipoint (PMP) system using ClearBurst time division duplex (TDD) equipment donated by WavTrace, who was acquired by Harris Corporation. A central LMDS "hub" with three sectors fed 7 off-campus sites. The system met diverse customer requirements including telephone, Internet/Intranet, and digital video services. The hub was located on Slusher Tower, a 12 story dormitory located near the center of campus. It connected into the campus backbone network over ATM OC3 and SONET via a fiber optic link. Remote sites served 3 apartment complexes, 2 office buildings, a satellite teleport, and a local telecommunications service provider. One office building served a student run TV Studio and administrative offices.

For LMDS research and performance monitoring purposes, the hub and selected remote outdoor sites were equipped to gather data on time, temperature, rainfall, wind speed, and wind direction. The data was correlated with radio link performance, including signal strength and bit error rate.

In January 2001, Harris Corporation and Virginia Tech received an award from the Wireless Communication Association for demonstrating how a university and private sector partnership can advance the rollout of a new technology. After 2001, the telecommunications industry suffered a major recession that resulted in most companies that were developing LMDS equipment going out of business, or changing their product direction. Eventually, Harris Corporation stopped development of the ClearBurst LMDS equipment and the Blacksburg deployment had to be terminated.