MMDS AND LMDS

           Multichannel Multipoint Distribution Service (MMDS) is another form of a wireless communication system. It is known as the wireless cable. It is used in television broadcasting and voice communications. It operates on the Ultra High Frequency band (UHF) from 2.1 GHz to 2.7 GHz.

           MMDS uses Omni-directional antenna for broadcasting. The MMDS belongs to the medium power level. The radius of the transmitter can reach up to 70 miles in flat terrain, without mountains. MMDS has a service monthly fee. MMDS are immune to snow and fog. The MMDS originally started broadcasting at 33 analog video channels. When the analog channels where converted to digital, the 33 analog channels became 99 digital channels.

Here is a figure of a typical MMDS arrangement source: AMD

            MMDS system consists of head-end equipment and reception equipment. Head-end equipment’s are the radio transmitters and other broadcast equipment and also transmission antenna. The reception equipment is the antenna, frequency conversion device, and set-top device.  The typical range of a transmitting antenna can reach up to 35 miles, depending on the broadcast power. Usually transmitters broadcast in 1 to 100 watts range. One of the disadvantages of MMDS is that it transmits on line of sight so it is not applicable in areas that have mountains. MMDS antennas are designed to receive signals in horizontal or vertical polarized signals or even both at the subscribers’ location.  

Advantages of using MMDS:

1.    It has chunks of under-utilized spectrum that will become increasingly valuable and flexible.

2.    System implementation, which is little more than putting an installed transmitter on a high tower and a small receiving antenna on the costumers balcony or roof, is quick and inexpensive.

3.    MMDS services have been around for 30 years. There is a wealth of experience regarding the use and distribution of the services.

Internet Access is one of the applications of MMDS. This application differs from the one way communication or half duplex used in television broadcasting.

           MMDS consist of key elements. These are head-end, transmit antenna, transmission line and channel combiners. The head end functions as the distributor of the signals. It distributes the signal to as many as subscribers possible. With a good elevation and clear line of sight to the service area, provides real dividends. Community antennas are used by the CATV companies which then delivers the signal over coax cables.

            The Transmit antenna can handle a bandwidth of 200 to over 300 MHz and depending on the number of channels and their spacing. Transmit antenna requires wide bandwidth together with downward tilt and horizontal radiation patterns to focus the signal in a service are or in the subscribers area.

             In MMDS the transmission line is the free space path because it uses radio waves to transmit data. Major head-end site operates typically on 50 to 100 watts in their transmitters but often only 50 percent of this signal reach the antenna of the receiver after passing through channel combiners and transmission feeders.

             Channel combiners functions as a special filter that is used to combine the outputs of the transmitters to the transmission feeder and antenna. Combiners are designed critical to ensure that they are reliable, stable with temperature, have low return loss and provide low pass band loss.

            Local Distribution Service (LMDS) is a broadband wireless point to point specification utilizing microwave communications. It operates on FCC licensed frequencies.

Sources: 

• Broadband Telecommunications Handbook by Regis J. Bates.