Multichannel Multipoint Distribution System
Multichannel multipoint distribution service (MMDS) is a new technology for wireless access - particularly useful for the Internet (see Figure 22.19).
Figure 22.19 Typical MMDS system for digital video and wireless Internet.
MMDS signals have longer wavelengths than that of LMDS and can travel farther without losing significant power. MMDS signals do not get blocked easily by objects and are less susceptible than LMDS to rain absorption. Repeater stations can be used to redirect MMDS signals.
With MMDS, a transmitting tower placed at a high elevation can reach customers within a 35-mile radius who have receiving dishes on the side or roof of the building. Several service providers consider MMDS a technology they can use to reach local customers without negotiating access agreements with regional operating companies. LMDS, in contrast to MMDS, covers a smaller radius (only up to 5 miles) and is more expensive to deploy.
MMDS has a narrow spectrum allocation (2.5 to 2.7 GHz in the United States); hence, it has a slower data rate compared to LMDS. The typical data rates of MMDS are 0.5 to 3 Mbps. The access schemes in MMDS are FDMA, TDMA, OFDM, or CDMA. Most of the MMDS are line-of- sight systems, but a non-LOS system is possible.
The network topology for MMDS can be either point-to-point or point-to-multipoint. Transmission power used in MMDS is usually in the 1 to-100-watt range, which is substantially below the transmission power requirements of VHF and UHF terrestrial broadcasting stations.
MMDS-favored cell architecture is a single, large microcell. While multicell deployments have been implemented, they are generally not efficient. The reason for this is twofold. The 2.5-GHz frequency band requires either large antennas, which are not well-received consumer client receivers, or smaller antennas with a very broad beam. Often smaller, low-cost antennas are used. The consequence is that multipath is induced, due to reflections of the signal associated with broad antenna beams. This in turn requires that the access method provide signifi cant immunity to the effects of multipath.
In the single cell MMDS deployment, the available bandwidth is limited to the frequency band licensed, which is equal to or less than 200 MHz total. Using this limited bandwidth for two-way, interactive Internet access is certainly feasible, but does not produce broadband access to any reasonably sized population base. However, the use of data casting, which exploits the multiplying effect of data multicast and broadcast, can provide a cost-effective and easily deployed model. The MMDS model is dramatically affected by the combination of bandwidth limitations, propagation characteristics, and the resulting impact of preferred modulation type. Table 22.1 provides a comparison of LMDS and MMDS.
Table 22.1 Comparison of LMDS versus MMDS.