và phát sóng dịch vụ tại các khu vực nông thôn. Bởi vì các kích thước tế bào lớn, các hệ thống MMDS không thực hiện tốt thông tin liên lạc hai chiều tích hợp một kênh trở lại. LMDS hệ thống làm việc với tần số cao hơn phổ tần số lớn hơn là có sẵn trong các hệ thống MMDS. | 40 WIRELESS LOCAL AREA NETWORKS and broadcast services in rural areas. Because of the large cell size MMDS systems do not perform well for bidirectional communication that integrates a return channel. The LMDS systems work with higher frequencies where a larger frequency spectrum is available than that in the MMDS systems. The coverage for LMDS systems involves smaller cells of up to 5 km radius and requires repeaters to be placed in a Line Of Sight LOS configuration. This local coverage with a large available bandwidth makes LMDS systems suitable for interactive multimedia services distribution. Broadband wireless access is based on the Two-Layer Network TLN concept in which subscribers are grouped into microcells which are embedded into a macrocell. The microcells coverage uses local repeaters operating at GHz fed by a BS through 40 GHz links. OFDM modulation is used to allow the reception with plug-free receivers located inside the buildings. A 40 GHz band fixed receiver provides a rooftop antenna in LOS with the transmitting antenna. This LMDS system provides an integrated wireless return channel. The LMDS architecture uses co-sited BS equipment. The indoor digital equipment connects to the network infrastructure and the outdoor microwave equipment mounted on the rooftop is housed at the same location. The Radio Frequency RF planning uses multiple sector microwave systems where the cell site coverage is divided into 4 8 12 16 or 24 sectors. The user accesses the network through Hybrid Fiber Radio HFR Radio To The Building RTTB and Radio To The Curb RTtC . In HFR a Radio Frequency Unit RFU carries out signal down conversion from RF frequency to the intermediate frequency. The signal feeds the Radio Termination RT of each user through a bus link. In RTTB architecture the signal feeds the user Network Termination NT through point-to-point cable links. In RTTC the RFU is placed in a common outdoor unit and is shared among several buildings. In high-population .
