LOS versus NLOS

Line-of-sight (LOS) is a condition where a signal travels over the air directly from a wireless transmitter to a wireless receiver without passing an obstruction. LOS is an ideal condition for a wireless transmission because the propagation challenge only comes from weather or atmospheric parameters and the characteristic of its operating frequency. In LOS environment, signal can reach longer distance with better signal strength and higher throughput.

Conversely, non-line-of-sight (NLOS) is a condition where a signal from a wireless transmitter passes several obstructions before arriving at a wireless receiver. The signal may be reflected, refracted, diffracted, absorbed or scattered. These create multiple signals that will arrive at a receiver at different times, from different paths, and with different strength. Consequently, wireless systems developed for NLOS environment have to incorporate a number of techniques to overcome this problem and that make the systems more complex than those for LOS. But NLOS capable systems simplify network planning and site acquisition.

Picture. Multipath in NLOS environment
Signal travels thru multiple paths. Multiple reflected signals hit a receiver.

WiMAX offers operators the solution to address many applications with its outstanding performance in NLOS environment. WiMAX is built on the robust OFDM/OFDMA physical layer (PHY) which can work with larger delay spread typical in NLOS environment. It can easily overcome frequency selective fading by equalizing at sub-carrier level. And its long symbol period (due to its use of parallel sub-carriers) will eliminate the ISI (inter symbol interference).

A number of advanced techniques such as subchannelization, adaptive modulation and coding (AMC), AAS and MIMO are also incorporated into WiMAX to make it even perform better in NLOS environment. Subchannelization allows WiMAX to concentrate transmit power on sub-channel(s), increasing the reach of a signal. AMC in both downlink and uplink enables WiMAX to adjust modulation and code rate dynamically based on each sub-channel condition and data rate requirement. AAS can focus the WiMAX Base Station sector beam to the direction of a user device. And MIMO can improve signal strength and throughput significantly with every additional antenna at the BS and the SS. Those techniques can collectively improve WiMAX performance and coverage in areas with difficult terrain (hilly), many obstructions (buildings and trees), or inside a building.