Wimax standards and Security The Wimax

FIGURE 5.5
Throughput measured at customer locations as a function of distance to base station with ten point moving average and logarithmic fit.

5. 
   Throughput Measurements
   

Having now characterized RF levels, we focus on the parameter of most interest: data throughput. Throughput is affected by distance, shadowing, and interferences. The parameter of importance is the signal to noise ratio (SNR); it can be estimated from RSSI and ambient noise measurements or can usually be reported in some form by the RF equipment. The SNR has a direct impact on the modulation used by the link^∗ and therefore on the throughput of that link. That throughput is graphed as a function of distance in Figure 5.5. In fact, modulation and throughput change from time to time. It may be important to study the statistical distribution of the resulting throughput, as in Figures 5.6 and 5.7. These figures show the probability of reaching a certain throughput, over the population of fixed location under test. These plots may be compared to plots representing fixed modulations and controlled fading environments described in Section 5.3.1. Fading statistics in suburban areas shows close correlation with SUI models 3 and 5, and throughput density
functions near those of 16QAM 3/4 in such fading environments [38].
Finally, we report on the standard deviation of measured signal strength. In most cellular trials mobile data is collected, which makes it impossible to quantify variations over long periods of time for a given location. In a popu- lation of fixed location, however, a measured standard deviation over a long


^∗ The details of that correlation are far from simple and depend greatly on the suppliers’ implementation choices.


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