AN INTRODUCTION TO MULTICHANNEL
MULTIPOINT DISTRIBUTION SERVICE

What is MMDS ?

MMDS (Multichannel Multipoint Distribution Service) is a Multichannel broadcast service that operates in the 2.0 to 2.7 GHz frequency range. It is designed to be a POINT to MULTIPOINT broadcast service that is capable of delivering multiple channels of television programming in digital or analogue mode together with Internet access, telephone and Data transfer services to individual receive sites. Channels containing video, audio, and data are transmitted from a central site to individual residences, multiple dwelling units, and business locations (see Figure 1). MMDS systems, which can be configured to offer just television, are commonly referred to as "Wireless Cable Television".

How Does An MMDS System Work?

Figure 1 depicts a typical wireless cable TV system. 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 an MMDS system (see Figure 2). 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, upconverted, and amplified. Transmitter powers range from 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 omnidirectional (360 degrees) pattern.

MMDS systems usually transmit on channels in the frequency band of 2000-2700 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 2000-2700 MHz frequency range require a clear, line-of-sight path from the transmitting antenna to each receiving antenna. Obstructions such as buildings, trees, or mountains that lie in the path between the transmit site and the receive site can deteriorate signal coverage.

Figure 3 depicts a typical MMDS receiving site. The MMDS signals are received with a small antenna. At microwave frequencies, these lightweight antennas exhibit high gain which helps compensate for propagation losses. A block downconverter converts the incoming MMDS signals from the microwave band to a predetermined set of output channels typically in the CATV frequency range. The output of the downconverter can be fed into a set-top converter/decoder or directly to a MATV system for distribution to individual apartments.

Most wireless cable systems use some sort of scrambling technique for signal security and other features. 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 set-top decoder/converter.

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 tiering levels (basic, premium channels, and PPV), and on screen channel identification.

Why 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 satisfaction.
The third advantage of an MMDS system is the reduced capital expenditure. The majority of the capital invested in an MMDS system 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. Most electronic equipment used in cable distribution is subject to harsh environmental conditions (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 microwave frequencies (2.0-2.7 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 a wireless cable system. These include the power radiated, the nature of the terrain and other obstructions on the path between the transmitting antenna and receiving antenna, the size (gain) of the receive antenna, the inherent level of "noise" in the downconverter, and the minimum picture quality deemed necessary for 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 a 50 dB C/N. This assumes that receiving antennas are high enough to clear all obstructions (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 50 dB C/N. In most locations, obstructions typically prevent service at this distance, however in some parts of the world this range of service can be exceeded. Figure 5 shows the relationship of transmit power to coverage area.

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