MMDS and LMDS

Multichannel Multipoint Distribution Service (MMDS):

Multichannel Multipoint Distribution Service (MMDS), also known wireless cable, is another wireless broadband technology for Internet access. 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.

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.

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 (TV). However, it also has applications in telephone/fax and data communications. MMDS frequencies provide precise, clear, and wide−ranging signal coverage.

The MMDS wireless spectrum originally consisted of 33 analog video channels, which were 6 MHz wide. The evolution of video technology into digital capacities enables the carriers to convert these 33 analog MMDS channels into 99 digital, 10 Mbps data streams, enabling full Ethernet connectivity. Therefore, a carrier with a normal operation can have as much as 1 Gbps of capacity at a single transmitter providing adequate capacities for most applications. This capacity is also readily expandable by using a sector cell concept (see the analog cellular chapter to get a handle on sectors), which reuses the same frequency many times. The combination of super cells and sectors enable the carrier to reuse the same frequency many times by building multiple cell sites. When enough customers sign on and as their bandwidth demands grow, the growth in traffic can be handled expeditiously through a new cell or a new sector.

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.

System Configuration:

The typical configuration of an MMDS system

Signals for MMDS broadcast at the transmitter site originate from a variety of sources, just like at cable head−ends. Satellite, terrestrial, and cable delivered programs, in addition to local baseband services, comprise the material to be delivered over MMDS. All satellite−delivered baseband formats are remodulated and subsequently up−converted to microwave frequencies. Terrestrially delivered signals are usually passed through a heterodyne processor prior to up−conversion to the desired MMDS frequencies. Repeater stations can be used to direct MMDS signals to blocked areas. The typical range of a transmitting antenna can reach up to 35 miles, depending on the broadcast power. Transmitters usually operate in the 1 to 100 watt range. MMDS is a line−of−sight service, so it does not work well around mountains, but it will work in rural areas, where copper lines are not available.

How a wireless cable system works:

• The cable studio, along with the head−end, receives programming from a variety of sources (see the following sect ion) .  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.
• 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.
• One of the two systems are normally used for multiple−dwellings (condo, apartment, and so on) to receive and distribute wireless TV.

1. 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.
2. I n other buildings, a single community antenna is installed with each tenant subscribing separately and billed separately by the cable company

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

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.

Key Elements:

The key elements of an MMDS system consist of the following pieces.

• 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.
• 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.
• 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.
• 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.

Local Multipoint Distribution Service (LMDS):

                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

LMDS operates in the higher frequencies, the radio signals are limited to approximately five miles of point−to−point service.

Architectural concept for the LMDS operation

Modulation and Access Techniques:

                The modulation and access method falls into two primary categories, FDMA and TDMA. Each of these techniques differs but also creates other sub modulation capabilities. For the broadband LMDS services, the system is usually separated into both phase and amplitude modulation of the RF. Phase−shift keying (PSK) and amplitude modulation combinations have been successfully used to achieve high rates of multiplexing and carrying capacities.

Summary of modulation techniques available for LMDS in FDMA

Two−Way Service:

The TDMA and FDMA modulation techniques on the LMDS network allow for the bidirectional flow between the carrier and the end user. In many cases, a different upstream is required than the downstream.