Chapter 18: MMDS and LMDS

 MMDS

           Multichannel Multipoint Distribution Service (MMDS) is a broadcasting and communications service that operates in the ultra-high-frequency (UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television. However, it also has applications in telephone/fax and data communications. In MMDS, a medium-power transmitter is located with an omnidirectional broadcast antenna at or near the highest topographical point in the intended coverage area. The workable radius can reach up to 70 miles in flat terrain (significantly less in hilly or mountainous areas). Each subscriber is equipped with a small antenna, along with a converter that can be placed next to, or on top of, a conventional TV set. There is a monthly fee, similar to that for satellite TV service. MMDS frequencies provide precise, clear, and wide−ranging signal coverage. Customers are protected from interference from other users when the provider uses the licensed frequencies.

System Configuration

       The wireless system consists of head−end equipment (satellite signal reception equipment, radio transmitter, other broadcast equipment, and transmission antenna) and reception equipment at each subscriber location

A typical configuration of an MMDS system

Advantages and disadvantages of  MMDS                    
                    • Propagation over long distances up to 100 km. with single tower
                    • Less attenuation due to rain, foliage
                    • RF component costs lower at 2.5 GHz
                    • Equipment readily available today
                    • Limited capacity without sectorization, cellularization which adds  complexity and cost
                    • Interference issues with other MMDS and ITFS licensees
                    • Large upstream bandwidth in MMDS band requires careful  planning, filtering etc.
                    • Cellularization later on may require retuning the entire network

Key Elements of MMDS system

   1. The Headend

                Equipment such as signal processors, demodulators and Satellite Receivers to generate input baseband video and audio signals. It is an optional Encoding System to scramble some channels or an addressable control system to control the decoders at the subscriber’s premises.

   2. The Transmitter

                The Transmitter converts the broadband signal provided by the modulators to the transmit microwave frequency (2500 to 2586 MHz) and amplifies the resulting microwave signal to the power level desired for transmission.

   3. The Transmitting Antenna

                The Transmitting Antenna system includes the cables or waveguide connecting the transmitter to the antenna, as well as the antenna itself and, if required, the pressurization system for the antenna.
   4. The Subscriber Equipment

                The Subscriber Equipment consists of an outdoor unit (an integrated antenna + down-converter), which converts the received microwave signal to frequencies in the 220 to 408 MHz range, which is suitable for feeding standard TV sets. The outdoor unit is connected through a coaxial cable to the subscriber’s home wiring or directly to the TV set.

LMDS

           Local Multipoint Distribution Service (LMDS) is an ideal solution for bringing high-bandwidth services to homes and offices within the last-mile—an area where cable or optical fibre may not be convenient or economical. Having architectural similarities with cellular networks, LMDS is a fixed (non-mobile) point-to-multipoint wireless access technology that typically operates in the 28 GHz band and offers Line-of-Sight (LoS) coverage up to 3-5 km. Depending on the local licensing regulations in a country, such broadband wireless systems may operate anywhere from 2 to 42 GHz. Though data transfer rates for LMDS can reach 1.5 to 2 Gbps, in reality it is designed to deliver data at speeds between 64 Kbps to 155 Mbps a more realistic downstream average being around 38 Mbps.

               At such speeds, LMDS may be the key to bringing multimedia data, supporting voice connections, the Internet, videoconferencing, interactive gaming, video streaming and other high-speed data applications to millions of customers worldwide over the air.

            As with other wireless networks, LMDS technology offers the advantage that it can be deployed quickly and relatively inexpensively. New market entrants who do not have an existing network like incumbent's copper wires or fibres - can rapidly build an advanced wireless network and start competing. LMDS is also attractive to incumbent operators who need to complement or expand existing networks.

How LMDS works and its limitation

             Sending digital signals of the required complexity at 28 GHz is made possible by recent advances in technologies such as Digital Signal Processors, advanced modulation systems and Gallium Arsenide (GaAs) integrated circuits, which are cheaper and function much better than silicon ones at these high frequencies. Unlike a cellular mobile phone network, in which a user can move one cell to another, the transceiver of an LMDS customer has a fixed location and remains within the same cell. Normally the customers' antennas are located on rooftops, to get a good LoS to the hub transceiver.

             Like in other microwave applications, LMDS cell size too is limited by rain fade,Also, walls, hills and even leafy trees block, reflect and distort the signal, creating significant shadow areas for a single transmitter. Some operators may serve a cell with several transmitters to increase coverage; most prefer one transmitter per cell, sited to target as many users as possible.

Advantages and disadvantages of LMDS

                • Very large bandwidth available for data, IP telephony,  video conferencing services

                • Large capacity

                • Higher RF component costs 

                • Small cell size, 2-8 Km.

                • Does not cover entire metropolitan area of a large city without adding  many cells at high cost

SOURCES:
http://searchnetworking.techtarget.com/definition/Multichannel-Multipoint-Distribution-Service

http://www.rficsolutions.com/publishedpapers/Broadbandwireless.pdf
http://www.cableaml.com/ing_mmds_system_description.html

http://www.angelfire.com/nd/ramdinchacha/DEC00.html

Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf