MMDS

Multichannel Multipoint Distribution Service (MMDS), is a wireless telecommunications technology, used for general-purpose broadband networking or, more commonly, as an alternative method of cable television programming reception. MMDS has been around since the 1970s and is a well−tested wireless technology, which has been used for TV signal transmission for more than 30 years. The service is delivered using terrestrial−based radio transmitters located at the highest location in a metropolitan area. Each subscriber receives the MMDS signal with an exclusive, small, digital receiver placed at your location with line of sight to the transmitters. MMDS channels come in 6 MHz chunks and run on licensed and unlicensed channels. Each channel can reach transfer rates as high as 27 Mbps (over unlicensed channels: 99 MHz, 2.4 GHz, and 5.7 to 5.8 GHz) or 1 Gbps (over licensed channels). Typically, a block of 200 MHz is allocated to a licensed carrier in an area.

The MMDS band was separated into 33 6 MHz "channels" which were auctioned off like other bands. The concept was to allow entities to own several channels and multiplex several television, radio, and later Internet data onto each channel using digital technology. Just like with Digital Cable channels, each channel is capable of 30.34 Mbit/s with 64QAM modulation, and 42.88 Mbit/s with 256QAM modulation. Due to forward error correction and other overhead, actual throughput is around 27 Mbit/s for 64QAM and 38 Mbit/s for 256QAM.

MMDS is a broadcasting and communications service that operates in the ultra−high−frequency portion of the radio spectrum between 2.1 and 2.7 GHz. In MMDS, a medium−power transmitter is located with an omni−directional broadcast antenna at or near the highest topographical point in the intended coverage area. 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.

Limited Frequency Spectrum

The limited number of channels available in the lower radio frequency (RF) bands characterizes MMDS networks. Only 200 MHz of spectrum (between 2.5 GHz and 2.7 GHz) is allocated for MMDS use. This constraint reduces the effective number of channels in a single MMDS system. For TV signals using 6 MHz of bandwidth, 33 channels can be fit into the spectrum. The FCC allowed for digital transmission utilizing Code Division Multiple Access (CDMA), quadrature phase shift keying (QPSK), vestigial sideband (VSB), and Quadrature Amplitude Modulation (QAM) schemes yielding up to five bits per hertz (one gigabit per second total raw capacity for the band), and return transmission from multiple sites within a 35 mile radius protected service area.

Here's how a wireless cable system works:

1.      The cable studio, along with the head−end, receives programming from a variety of sources. Each source is assigned a channel number, processed to improve quality, encoded, and then sent to a transmitter. The signal is broadcast in the super−high−frequency (SHF) range. Using an omni−directional transmit pattern, the signal reaches subscribers located up to 50 KM from the antenna, depending on the terrain and transmit power.

2.      Wireless cable signals are received by the subscriber's small rooftop antenna, decoded (pay TV), and down−converted to standard TV channels on the subscriber's TV set.

3.      One of the two systems are normally used for multiple−dwellings (condo, apartment, and so on) to receive and distribute wireless TV.

a.       The building management pays for all units to receive the programming from a single communal antenna. This agreed fee is usually based on the number of potential viewers.

b.      In other buildings, a single community antenna is installed with each tenant subscribing separately and billed separately by the cable company.

4.      In all cases, deposits are paid by subscribers that cover receiver system costs, much like cable subscribers.

Advantages of Using MMDS

The following list includes some advantages of using MMDS:

·         It has chunks of under−utilized spectrum that will become increasingly valuable and flexible.

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

·    Because MMDS services have been around for 30 years, there is a wealth of experience regarding the use and distribution of the services.

Internet Access

            The hottest application for MMDS is Internet access; this differs from MMDS' original application of one−way "wireless cable" service to deliver television programming. This application never proved popular, and most license holders are now concentrating on data service.  This is an Ethernet connection to a wireless modem. Alternatively, some vendors provide a wireless modem card for their routers. A cable runs from the modem to a radio, which connects to the antenna. The radio and antenna can be combined in one compact unit. This antenna is mounted directly on your building or on a pole and points at the service provider's tower. Future versions of the technology will omit the line−of−sight requirement.

The MMDS architecture key elements

Key Elements

The Head−End

The key elements in optimizing transmitted signal levels are the selection of the head−end site and the transmitting antenna, transmission feeders, channel combiners, channel diplexers, and transmitters. The head−end's task is to distribute the signal to as many subscribers as possible. Choosing a site with good elevation and a clear line of sight to the service area provides real dividends. This is how the CATV companies do it with their community antenna, which then delivers the signal over coax cables.

The Transmit Antenna

The bandwidth allocated to MMDS operators can vary from 200 to over 300 MHz, depending on the number of channels and their spacing. Wide bandwidth is a requirement of MMDS antennas together with downward tilt and horizontal radiation patterns to concentrate on the signal in the service area.

The Transmission Line

This is another critical component that can have a substantial effect on system losses. Major head−end sites typically use 50 or 100 watt transmitters, yet often only 50 percent of this power reaches the antenna after passing through channel combiners and transmission feeders. Waveguides from the antenna to the radio equipment vary to reduce loss and add gain.

Channel Combiners

MMDS sites normally transmit a number of channels. Special filters (channel combiners) are used to combine the outputs of the transmitters to the transmission feeder and antenna. The design of these combiners is critical to ensure they are stable with temperature, have low return loss, and provide low pass band loss.

LMDS

Local Multipoint Distribution Service (LMDS) is generally considered a broadband fixed service LMDS is a broadband wireless access technology originally designed for digital television transmission (DTV). It was conceived as a fixed wireless, point-to-multipoint technology for utilization in the last mile. LMDS commonly operates on microwave frequencies across the 26 GHz and 29 GHz bands. In the United States, frequencies from 31.0 through 31.3 GHz are also considered LMDS frequencies. Although LMDS may render any kind of communications service consistent with Commission rules, one current use is in competitive local exchange carrier (CLEC) service providing voice and data connectivity to business customers. LMDS uses a cellular infrastructure, with multiple base stations and small customer transceivers able to return communications.

The local loop is prone to problems.

What About the Bandwidth?

The bandwidth necessary for each of these services listed previously changes the rules considerably. In many of the WLL providers' backbone, there is not enough bandwidth to support the number of users and the higher−speed services. For this reason, the marriage of the providers may occur sooner than expected. If a cellular provider joins forces with a WLL microwave supplier, then the bandwidth for the fixed needs at the door is assured while the cellular provider handles the demands of the roaming user. These combinations and permutations can be very complicated as the number of providers expands and the services they offer shift in any direction. The interesting point will be to see how the total market plays out with an expectation that approximately five to seven providers will dominate, and the rest will be absorbed or fail.

LMDS, as its name implies, is a broadband wireless technology that is used to deliver the multiple service offerings in a localized area. The services possible with LMDS include the following:

·         Voice dial−up services

·         Data

·         Internet access

·         Video

Just as the network providers were getting used to the battlegrounds between the ILECs and the new providers, RF spectrum was freed up around the world to support access and bypass services. Typically, the services operate in the RF spectrum above 25 GHz, depending on the licenses and spectrum controlled by the regulatory bodies. This offering operates as a point−to−point, broadband wireless access method, which can provide two−way services. Because LMDS operates in the higher frequencies, the radio signals are limited to approximately five miles of point−to−point service. This makes it somewhat like a cellular operation in the way the carriers lay out their operations and cells. An architectural concept for the LMDS operation is shown in Figure 18−5 from the perspective of the supplier to the user. This figure uses some of the premises that the service is constrained to a localized area.

Typical LMDS service areas Source: LMDS Org.

Propagation Issues

Like any radio−based system, the issue of propagation is always a concern. Like the analog cellular networks of the past, there are several factors that contribute to the quality of the signal. Many operators have to consider that at the higher frequencies (over 25 GHz), rain fade will be a critical factor. The higher the frequency, the more susceptible to rain fade than lower frequencies. One CLEC chose to use all 31 GHz radio equipment in their infrastructure to get to the customer's door. Other issues have a bearing on the design and layout of the system such as the following:

· Distance

· RF Interference (RFI)

· Electromagnetic Interference (EMI)

· Multipath fade

· Frequency reuse

            In each case, the individual carrier will have to assess the overall system design specifications to meet the needs of the consumer, either residential or business. No one solution is going to satisfy all systems providers or consumers. The constant shift in network architecture will be required in a fine−tuning approach to provide the necessary quality.

http://en.wikipedia.org/wiki/Multichannel_Multipoint_Distribution_Service

http://en.wikipedia.org/wiki/Local_Multipoint_Distribution_Service

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