The 3G network might be divided up in hierarchical fashion:
•Macro cell - the area of largest coverage, e.g., an entire city.
•Micro cell - the area of intermediate coverage, e.g., a city centre.
•Pico cell - the area of smallest coverage, e.g., a "hot spot" in a hotel or airport.
Why is there this sub-division of regions? It is because smaller regions (shorter ranges) allow higher user density and faster transmission rates. This is why they are called "hot spots".
TDD mode does not allow long range transmission (the delays incurred would cause interference between the uplink and the downlink). For this reason, TDD mode can only be used in environments where the propagation delay is small (pico cells). As was explained in the previous section on symmetric transmission vs. asymmetric transmission, TDD mode is highly efficient for transmission of internet data in pico cells.
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