Nil.St Wakefield St Albert Ln Figure 13.8 Smaller cells underlay a macrocell to cover a high subscriber density area (Rundle Mall in Adelaide) Figure 13.9 An Ericsson and Philips small cell design 'zero site' integrated into an electricity pole. Reproduced by permission of Ericsson AB [9] When a small cell underlays an existing macrocell service area, the signals from the two base stations may interfere with each other and network planning needs to find ways to mini- mise intra-system interference. In systems where the carrier frequency is reused such as GSM (Global System for Mobile communications), the overlapping configuration uses different carrier frequencies. In 3G and 4G standards, dynamic resource allocation techniques are used to minimise mutual interference. Picocell Another small cell configuration with a coverage area smaller than that of a microcell is a pico- cell. The difference from the micro-cell however is not only in size: picocell base stations are usually installed in isolated, confined areas. This can be in an auditorium, inside an aeroplane, or Network Topologies, Design and Convergence on an underground train platform. A picocell base station capacity is also usually smaller than that of a microcell as the expected traffic is quite low. Accordingly, the transmission powers and antenna sizes are also smaller. Picocells are designed to fill in gaps left by macro- and microcell configurations. As they are generally well isolated, frequency planning is not a significant issue: low transmit powers will further ensure minimal interference to its surroundings. Again, connections to the core network can be via a macro- or microcell base station, or through dedicated fixed links. The connection may itself be wireless, fibre or cable. At times the picocell base station may act as a subscriber to the overlaying cell structure's larger cell, which relays traffic to the core network. Other wireless connections may be fixed wireless, or even satellite links as in the case of aeroplane picocells. In prac