Praise for Fundamentals of WiMAX

3.2 The Broadband Wireless Channel: Pathloss and Shadowing 73Sidebar 3.2 Large PathLoss and Increased CapacityBefore continuing, it should be noted that, somewhat counterintuitively,severe pathloss environments often are desirable in a multiuser wireless network,such as WiMAX. Why? Since many users are attempting to simultaneouslyaccess the network, both the uplink and the downlink generallybecome interference limited, which means that increasing the transmitpower of all users at once will not increase the overall network throughput.Instead, a lower interference level is preferable. In a cellular system withbase stations, most of the interfering transmitters are farther away than thedesired transmitter. Thus, their interference power will be attenuated moreseverely by a large path loss exponent than the desired signal. As noted inthe Sidebar 3.1, a large pathloss exponent can be caused in part by a highercarrier frequency. Example 3.1 will be instructive.Example 3.1Consider a user in the downlink of a cellular system, where the desiredbase station is at a distance of 500 meters, and numerous nearby interferingbase stations are transmitting at the same power level. If threeinterfering base stations are at a distance of 1 km, three at a distance of2 km, and ten at a distance of 4 km, use the empirical pathloss formula tofind the signal-to-interference ratio (SIR)—the noise is neglected—whenα =3 and when α =5.SolutionForα =3, the desired received power isP3 −3= PPd (0.5) ,rd , t o o(3.8)and the interference power isP3 −3 −3 −3= PPd ⎡⎣3(1) + 3(2) + 10(4) ⎤⎦.rI , t o o(3.9)The SIR expressions compute toPrd,SIR( α = 3) = = 28.25 = 14.5 dB,PrI ,SIR( α = 5) = 99.3 = 20 dB,(3.9)demonstrating that the overall system performance can be substantiallyimproved when the pathloss is in fact large. These calculations can beviewed as an upper bound, where the SINR γ < SIR , owing to the additionof noise. This means that as the pathloss worsens, microcells grow