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Local Multipoint Distribution System (LMDS)
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PDF of this tutorial7. Network Planning
Cell Design
When planning the cell sites for an LMDS network, it is important to take the following attributes into consideration:
- subscriber penetration—Distribution system performance is measured by subscriber penetration—the percentage of subscribers having sufficient signal level to achieve excellent service quality.
- quality of service (QoS)—QoS can be affected by several factors, including transmission path obstruction, cell overlap (15 percent is normal), and system redundancy.
- link budget—Link budget is used to estimate the maximum distance that a subscriber can be located from a cell site while still achieving acceptable service reliability. The budget accounts for all system gains and losses through various types of equipment. The link budget analyzes several network parameters, including carrier-to-noise ratios (CNRs); carrier-to-composite triple beat ratios; and self-repeat site interference (C/I) and link fade margins. In some cases, the microwave equipment is channelized to support a single carrier. Other systems offer broadband multichannel capability in which multiple carriers can be supported through a single transmitter.
- cell-size selection—The maximum cell size for the service area is related to the desired reliability level obtained from the link budget. Cell size can vary within a coverage area due to the type of antenna, its height, and signal loss. These effects are generally related to the coverage area service type such as urban, suburban, or low-density coverage. Cell-size selection affects the total capital cost for the required coverage area.
- capital-cost model—The capital-cost model is used to estimate the network capital requirements. The required model encompasses design considerations such as link budget, cell size, cell overlap, number of cells, traffic capacity, number of sectors, capital cost per cell, and total capital cost.
Frequency Reuse Optimization
The following techniques are used to optimize the frequency reuse of LMDS networks:
- minimization of multipathing and cross-polarization by using highly directional antennas and by positioning them as high as possible
- maximization of the directivity of the cell antennas by sectoring the distribution system; the cell-site microwave equipment is generally configured with multiple sectors, antennas, transmitters, and receivers. A typical configuration is a four-sector cell site using 90-degree beamwidth antennas to provide services to the subscriber environment. Each of these sectorized antennas (transmitters and receivers) can support the full bandwidth of the allocated spectrum.
- maximization of the isolation between the adjacent sectors through polarization; horizontal (H) and vertical (V) polarization can be employed throughout the system in an alternate pattern between the sectors, as shown in Figure 6. The H and V polarization is also reused throughout the system.
Figure 6. Horizontal and Vertical Polarization Reuse
