TABLE OF CONTENTS Some of the key technologies used in WiMax systems include orthogonal frequency division multiplexing, frequency reuse, adaptive modulation, diversity transmission and adaptive antennas.
Orthogonal frequency division multiplexing is a process of transmitting several high speed communication channels through a communication using separate sub-carriers (frequencies) for each communication channel. The use of OFDM reduces the effects of multi-path and delay spread, which is especially important for lower frequencies and near line of sight (NLOS) transmission.
Multi-path propagation is the transmission of a radio signal over two or more paths from a transmitter to a receiver. Multi-path transmission can cause changes in the received signal level as delayed signals can either add or subtract from the received signal level. Multi-path is not usually a challenge on systems that use higher frequencies as these systems tend to use highly directional (high-gain) antennas for direct line of sight transmission.
Multi-path propagation is frequency dependent. This means that the multiple paths that radio signals travel will vary depending on its frequency.
Figure 1.14 shows how a transmitted signal may travel through multiple paths before reaching its destination. In this example, the same signal is reflected off an office building where it is received by the subscriber device. This diagram shows that the reflected signal is delayed (travels a longer path) and it subtracts from the direct signal resulting in a dead spot (fade) at the receiver. This example also shows that multi-path propagation is sensitive to frequency and that distortion...
