Web ProForums
Wireless Broadband Modems
PDF of this tutorial3. Architecture of a Wireless Downstream System
Engineering of a wireless system is complicated by several factors:
- The physical wireless-transmitter location may be different from the Internet head-end location.
- LOS transmission is required.
- The signal power falls with distance.
- Different fixed transmission frequencies are used according to license availability.
- Multipath distortion
- A receiving antenna is required on the roof, the side of the building, or sometimes in a window.
- Two-way operation is limited by available licenses, bandwidth, and Federal Communications Commission (FCC) regulation.
Some of these requirements interact as described below:
- If one had complete freedom, the transmitter would be at a convenient site for an Internet service connection yet have LOS transmission to customers. These two requirements conflict about half the time, so the transmitter and the Internet head end cannot be colocated.
- The site for the wireless antenna and the transmitter is usually chosen to provide LOS television to a metropolitan area or several rural communities. The frequency bands were licensed for this wide-area coverage—they are not personal communications service (PCS) cells. As a result, the transmitter site may be physically inaccessible on a mountain.
- The nearest point of connection to the Internet is often downtown. This is often the best place for the Internet head-end maintenance.
As can be seen from Figure 3, the multipoint multichannel distribution service (MMDS) transmitter may not cover the whole area. A low-power booster may be needed to fill in areas where LOS does not exist.
Figure 3. MMDS Transmission Configuration: Distant Transmitter
with Internet Head End Downtown Co-Located with Point of Presence (POP)
