INTRODUCTION TO MULTICHANNEL
MULTIPOINT DISTRIBUTION SERVICE.
MMDS (Multichannel Multipoint
Distribution Service) is a Multichannel
broadcast service that operates in the 2.0 to 2.9 GHz.
Frequency range. It’s designed to be a POINT to MULTIPOINT broadcast
service that is capable of delivering multiple channels of
television programming for digital or analogue mode together with
internet access, telephone and data transfer services to individual
receive sites. Channels containing video, audio and data transmitter
from a central site to individual residences, multiple dwelling
units, and business locations. MMDS systems, which can be configured
to offer just television are commonly referred to as “ Wireless
DOES MMDS SYSTEM WORK?
The architecture of a typical
wireless cable system can be divided into two basic nodes:
transmitting sites and receiving sites. Transmitting sites are the
main or “HUB” node of MMDS system.
Programming information is
generally delivered to an MMDS system from satellites, it can also
be generated locally or come from pre-recorded video tapes, these
signals are fed into discrete transmitters where they are modulated
up converted and amplified. Transmitter powers range 1 to 200 watts
per channel and 15 to 100 watts average power per digital channel,
individual microwave channels are then multiplexed or combined
together and are passed to a transmitting antenna via low loss
coaxial cable or waveguide. A centrally located transmitting antenna
radiates the MMDS signal to the desired receiving sites, generally
in an omni directional (360 degrees) pattern.
MMDS systems usually transmit
on channels in the frequencies band of 2000-2900 MHz (S-band) They
use standard broadcast TV modulation techniques (NTSC/PAL/SECAM) in
analogue mode and (QAM/QPSK) modulation in digital signals
transmitted in the 2000-2900 MHz frequency range require a clear,
line-of-sight path from the transmitting antenna to each between the
transmit site and the receive site can deteriorate signal coverage.
The MMDS signals are received
with a small antenna. At micro wave frequencies these lightweight
antenna exhibit high gain with helps compensate for propagation
losses a block down converter converts the incoming MMDS signals
from the micro wave band to a predetermined set of output channels
typically in the CATV frequency range. The output of the down
converter can be fed into a set-top, converter/decoder or directly
to MATV system for distribution to individual apartments.
Most wireless cable systems use
some sort of scrambling technique for signal security and other
Scrambling for MMDS systems
works much the same as scrambling in a wired cable system signals
are scrambled at transmit site and each receive site incorporates a
Present generation scrambling
systems allow the operator to offer services such as premium
channels, pay per view (PPV), and messaging capabilities. Wireless
cable scrambling systems also feature set-top addressability,
program tiring levels (basic, premium channels, and PPV), and on
screen channel identification.
IS MMDS SO POPULAR?
MMDS systems have been
increasing in popularity for several reasons. First, since the
television signals are transmitting from a central location, the
service area is immediately available for subscriber installations
upon completion of the transmit site construction.
Construction of the MMDS
transmit site can generally be accomplished in a few weeks to a few
months whereas a conventional cable TV system may require in excess
of three years. A conventional cable TV system requires a dedicated
coaxial “wire” to be connected to each subscriber.
Another advantage of an MMDS
system is the picture quality. A properly designed MMDS system will
deliver clear, distortion free pictures to each subscriber’s home.
Since the signals do not have to pass through many kilometers of
coaxial cable and numerous amplifier chains, the noise associated
with typical cable systems is not introduced to MMDS signals. The
overall result is better picture quality and greater subscriber
The third advantage of an MMDS
system is the reduced capital expenditure. The majority of the
capital invested in an MMDS system is goes directly into the
subscriber’s receive equipment. A minimal capital investment is
required to install a transmit station. Therefore, as soon as a
subscriber is installed, revenue is immediately generated.
The fourth advantage of an MMDS
system is greater reliability. Since conventional cable signals are
delivered through many chains of line amplifiers, there is a greater
chance for service interruptions (amplifiers are pole and strand
mounted). MMDS electronic equipment is generally co-located,
yielding much easier system maintenance. MMDS transmit sites are
located in climate controlled equipment shelters for maximum
performance and greater system reliability.
WHAT KIND OF COVERAGE AREA CAN BE EXPECTED FROM AN MMDS SYSTEM?
The term “coverage” should be
defined as it relates to the service range of an MMDS system since
wireless cable systems transmit at micro wave frequencies (2.0-2.9
GHz), all receive sites must have a clear “line-of-sight” path to
the transmit antennas. Therefore, coverage area of an MMDS system is
limited to the line-of-sight radius from the transmit site.
Several factors influence the
maximum service range of the terrain and other obstructions on the
path between the transmitting antenna and receiving antenna, the
size (gain) of the receive antenna, the internet level of “noise” in
the down converter, and the minimum picture quality deemed necessary
for the subscriber installations.
Typically, a 10 watt per
channel wireless cable system would provide service to a 20 km.
radius using adequate sized receiving antennas and 50dB C/N. this
assumes that receiving antennas are high enough to clear all
obstruction (trees, buildings, terrain. Etc.) Wireless cable systems
using 100 watts per channel have the potential to reach customers as
far away as 70 km based on a 50dB C/N. in most locations,
obstructions typically prevent service at this distance, and however
in some parts of the world this range of this service can be