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LMDS (Local Multipoint Distribution Service)

Source: "High Technology Careers Magazine; May 1999

"LMDS: Moving Communications Beyond Tin Cans and String"

The blazing fast 1999 model 450 MHz computer sitting on your desk is connected to an even faster multi-Gbps fiber optic communications backbone. Unfortunately, the connection between the two leaves much to be desired.

While overloaded routers and Internet servers account for much of today’s network congestion, the bottleneck within the "last mile" to the customer’s office or home remains the most challenging to remedy. The answer may lie in the new Local Multipoint Distribution Service (LMDS), one of the least publicized yet perhaps most powerful emerging communications technologies.

Not Yet in the Public Eye

Digital Broadcast Satellites, Switched Broadband, Hybrid Fiber/Coax, and other forms of wireless cable have all received more attention than LMDS. However, several technologies have matured in the last few years that work to make LMDS even more feasible. Gallium Arsenide (GaAS) integrated circuits, digital signal processors, video compression techniques, and advanced modulation systems have all made significant improvements in cost and performance. A major rewrite of telecommunication regulations has removed many of the barriers preventing companies from entering new businesses. The demand for bandwidth, which was once limited to large corporations, governments, and universities, is now rising at the consumer level thanks to the Internet. These factors have combined to create a need for a technology that has access to a huge amount of bandwidth and can be deployed for low up-front costs.

Proponents of LMDS say the technology can easily satisfy these demands and relieve bottlenecks by providing high-speed, highly reliable connections from the workstation or LAN to the high-speed backbone. The wireless delivery of LMDS, combined with the significant amount of spectrum allocated for its use, promises to allow for the delivery of high-quality telecommunications services such as multichannel video, high-speed Internet service, and local telephone service at prices far below those of incumbent providers.

LMDS is considered a proven technology, because it’s been tested by the U.S. military and corporate pioneers such as, formerly CellularVision USA, in New York. offers high-speed TV and Internet access in New York City under a special commercial license.

Basically, LMDS is a wireless service transmitting fixed, broadband microwave signals (actually millimeter-wave signals) in the 28 GHz band of the spectrum within small cells roughly three miles in diameter. LMDS’s gigantic appeal lies in its ability to offer a wide range of one-way and two-way voice, video, and data service transmission capabilities with a capacity many times larger than any current wireless or non-wireless service.

Because of its multipurpose applications, LMDS has the potential to become a major competitor to local exchange and cable television services.

Greed for Speed

What currently passes as broadband speed pales to insignificance when compared to the speeds promised by LMDS. Hewlett-Packard predicts throughputs as fast as 1.5 gbps downstream, with upstream rates as high as 200 mbps.

That’s enough bandwidth to transmit 8,000 high-density color photographs per second, provide high-speed Internet access at 100 times current modem rates, or carry over 200 video channels simultaneously. The residents in most homes in a neighborhood will be able to watch separate digital movies, teleconference, and surf the Internet at high speed all at the same time.

LMDS will provide customers with telephone service, multichannel video programming, video communications, and two-way data services. While LMDS isn’t linked to any one technology, it is, however, a very large data pipe.

According to Ihor Nakonecznyj, senior manager of product marketing at Nortel (Northern Telecom) Broadband Wireless Access, LMDS differs from the ordinary data transport systems in the way a train differs from a pipeline. "Both are transport systems, but a pipeline can transport only one product from one place to another. A train, on the other hand, can transport many different products over the same infrastructure. LMDS, implemented with a multi-service protocol such as asynchronous transfer mode (ATM), can transport, among others, voice, Internet, Ethernet, video, computer files, and transaction data."

It is the multipoint radio technology, combined with the appropriate protocol, access method, and speed, that gives LMDS the potential to transform society.

Among the overwhelming advantages of LMDS is reliability. “As a transport system, LMDS can be engineered to provide 99.999 percent availability, rivaling that of the best fiber backbones,” said Nakonecznyj.

The "Negroponte Flip"

Just as television and radio are becoming wired, telephones and computers are becoming wireless, a paradigm shift now called the "Negroponte Flip," first articulated by Nicolas Negroponte, director of the Massachusetts Institute of Technology’s eminent Media Lab.

In the past, communication technologies exploited the lower end of the radio frequency spectrum because, when boosted with enough power, low-frequency signals can be transmitted long distances and even penetrate buildings, as is the case with television and radio signals. LMDS, on the other hand, uses low-powered, high-frequency signals (the Ka band lies above the UHF band and below the far infrared region) over short distances.

Because of this short distance, LMDS systems are configured in stationary, line-of-sight cells. These cells are typically spaced on a three-mile radius, with a single hub transceiver in the center communicating at Gigabits per second with special devices affixed to residences and businesses in the cell. 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 or repeaters can spray a strong signal into shadow areas to allow for more coverage. Various isolation techniques are used to prevent interference between signals. Tests have determined that a single transmitter would reach only slightly more than 60 percent of the homes in a cell. With overlapping cells and repeaters, however, that number jumps to almost 85 percent of the homes.

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 that 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 that problem.

The sheer size of the LMDS spectrum and the expectation that it will be lightly regulated are other attractive aspects of LMDS. And since LMDS can be used for two-way transmission through the use of low-powered residential transceivers, it is seen as a way to provide interactive services without the installation and maintenance expense encountered on fiber or coax lines.

Talking from Both Sides

LMDS talks out of both sides of its connection mouth simultaneously. This concurrent, two-way, wireless microwave transmission of mixed video, audio, and data is possible thanks to an invention by electrical engineers at the Microwave Laboratory at the Illinois Institute of Technology (IIT).

The IIT invention permits transmission of multiple mediums within one microwave system, all handled simultaneously. The new IIT system uses the same principle as radio, where there are many stations, containing a spectrum of information, broadcasting at the same time.

There are numerous directions in which the new IIT technology can be applied. Among the applications made possible are the simultaneous transmission of video conferencing, movies on demand, home shopping, high-definition television (with as many as 500 channels), common carrier telephone service, and various satellite communication applications.

"Advantages of this system include large capacity, fast deployment, and low-cost maintenance," said principal researcher Thomas Wong, director of IIT’s Microwave Laboratory. Each signal requires its own carrier frequency. A device’s bandwidth is the range of carrier frequencies within which it can operate. Wong’s invention allows a single device to send and receive multiple signals with a higher carrier density than conventional designs. It is particularly effective in and above the Ka band (26.5 to 40 GHz) of the microwave spectrum used by LMDS.

Wong’s work resulted from an effort to develop equipment for LMDS. Wong has been granted a patent (U.S. patent No. 5701591) on the design of a unique system that uses circular and elliptical polarizations (as opposed to vertical and horizontal polarizations) in the transmission and reception of signals, which makes his multifunction communication system particularly well suited for urban environments. Polarization is the orientation of the electric field of a horizontally propagating electromagnetic wave.

When It Rains, It Distorts

There has been concern whether wireless communication can maintain data integrity during sudden changes in weather. The concern is with data drop-out or distortion every time it rains too hard or there’s a crack of thunder.

Interference in the LMDS millimeter-wave signals results from physical objects, overlapping signals, and weather. Absorption of microwaves by water molecules in raindrops accounts for most of the signal attenuation in the open space between the transmitter and the receiver, causing microwaves to lose signal strength, a phenomenon engineers call "rain fade."

"It is certainly true that rain has an effect on millimeter-wave propagation," Wong said. "To overcome this attenuation, one needs to put in reserved power in the transmission system. This is already practiced in deployed systems." For weather systems similar to Chicago, for instance, a 1-Watt per carrier transmitted power can provide 360-degree coverage for a radius of 5 kilometers when the antennas and receivers used provide reasonable performance.

Wong’s system uses a different twist on polarization diversity to increase the capacity of a communications link. “In C-band satellites, for example, the use of vertical and horizontal polarizations in signal transmission can double information capacity,” Wong said. “At millimeter-wave bands, however, rainfall depolarizes linearly polarized [vertical and horizontal] signals, rendering the use of such polarization diversity inefficient. My invention makes use of circular/elliptical polarizations and receiver designs to eliminate the depolarization of effects of the propagation medium.”

There are numerous avenues for further research activity in this new technical area. “As far as the deployment of systems utilizing the current design we have developed, manufacturing capacity needs to be geared up,” said Wong. “Current practice is to build the head-end and relay stations in the United States and [the] subscriber units off shore.”

Wong developed his invention under a collaborative agreement between IIT and Telecommunications Equipment Corp., Palantine, Ill., which is now in the process of developing commercial communications devices that incorporate Wong’s multifunction microwave technology.

Business Prospects

With 1.3 GHz of spectrum, LMDS can provide a pipeline for an enormous amount of data. Homeowners currently pay about $30 per month for CATV, but businesses regularly pay over $1,000 per month for a high-speed T1 (1.544 Mbps) line from phone companies. Using only the 850 MHz unrestricted bandwidth, along with a modulation scheme such as quadrature phased shift keying (QPSK), well over 100 T1 equivalent lines can be provided in a cell without splitting cells into separate sectors. Even at half the price charged by a phone company, 100 leased T1 lines would generate $50,000 in revenue per month in a cell. By using horizontal and vertical polarized sectors in a cell, LMDS providers will be able to re-use bandwidth and multiply the number of T1 equivalents available.

A typical commercial LMDS application can potentially provide a staggering downlink throughput of 51.84 to 155.52 Mbps and a return link of 1.544 Mbps (T1). This capacity translates into phenomenal potential to provide the "full-service network" packages of integrated voice, video, and high-speed data services. Actual service-carrying capacity depends on how much spectrum is allocated to video versus voice and data applications.

Assuming that one GHz of spectrum is available, an all-video system could provide up to 288 channels of digital broadcast quality television, plus on-demand video services. Fast data, Internet access, PCS backhaul, local loop bypass, digital video, digital radio, work at home, and telemedicine are all possible. In fact, they are all possible within the same cell.

Hubs and cell sites must be established early on for LMDS, but once they are completed, new costs are incurred only as additional customers sign on. The largest fixed expense associated with building out LMDS cells will probably be the cost of subscriber equipment, not the transmission and infrastructure equipment itself. By contrast, when installing wire-line networks, the majority of the costs are incurred before the first paying customer is even turned on.

Doug Page writes about science and technology from Redondo Beach, California.

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