In terms of UMTS or LTE in dense urban areas with a multitude of cells there is another phenomenon called "pilot pollution" that interferes with the reception:
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“Pilot pollution” (or “no dominant server”) describes a situation where power transmitted from many different cells appear in a location, but
none is significantly better than others. As a result, the composite signal level is high, but the SINR from any single cell is poor because the total
interference is too high. The result is poor RF performance even with a high overall signal level.
Figure 3 shows one common cause of the “no dominant server” problem. The cells located nearby have unfavorable RF propagation conditions;
but the cells located far away have favorable RF propagation conditions. The net result is: many servers appear in a location but no server is
offering a strong enough signal. Without soft handoff, the terminal can only treat received power from one cell as “signal;” power from all other
cells is treated as “interference.” Thus, the “no dominant server” problem means high interference and poor SINR:
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