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.
