Multipoint Distributive Systems
Local Multipoint Distributive System (LMDS)

A LMDS (Local Multipoint Distributive System) is a broadband wireless access technology originally designed for Digital Television Transmission. It has been conceived as a fixed wireless, point-to-multipoint technology for utilization in the last mile. The LMDS commonly operates on microwave frequencies across 26 GHz and 29 GHz bands. In the United States, frequencies from 31.0 to 31.3 GHz are also considered.

The LMDS systems are based on fixed wireless group access of fixed terminal stations, abbreviated as SAS (Subscriber Access System), to BS through radio interface. The SASs are equipped with directional antennas oriented to the BS.

Network structure of LMDS system

Throughput capacity and reliable distance of the link depends on common radio link constraints and used modulation method - either PSK or AM (Amplitude Modulation). Distance is typically limited to approximately 2 or 3 km due to rain fade attenuation constraints. Deployment links of up to 8 km from the BS are possible under specific circumstances such as point-to-point systems that can reach slightly farther distances due to increased antenna gain.

Although some operators in Europe use LMDS systems to provide access services among different networks, LMDS is more commonly used for high-capacity links for interconnection of backhauls of networks such as GSM, UMTS, WiMAX or Wi-Fi.

Number of sectors of the BS and their allocation is dependent on number of access places and on total data capacity routed to specific BS. Physical distribution of sectors is not naturally unchangeable and its modification is interconnected with increasing utilization of network. Basic allocation of territory and its coverage is implemented by two channels (A and B) as dual frequency plan.

We use the dual frequency plan, because we have serious problems with reciprocal interference among sectors. There are several techniques how to reduce reciprocal interference. The first condition is a utilization of direction quality of the BS’s antennas serving specific sector. The second condition is that SAS's parabolic antennas with narrow direction are aimed to the serving BS. Other way, how to reduce interference is allocation of different channels for each sector of the BS or the modification of wave polarization of radio signal in particular sectors to horizontal or to vertical wave.