anded over if the quality of the link drops below a 2 or 3 transceivers in a typical BTS. certain threshold. 48.9.3.3 Discontinuous transmission To reduce the effects of interference, PCN mobiles and base stations 48.10 Microcells usually support discontinuous transmission (DTX). This involves turning off the radio transmitter when speech is not present (which The term microcell is often used to describe very small cells, may be up to 50% of the time), using a Voice Activity Detection particularly as used in PCN networks. The definition of a microcell (VAD) algorithm in the speech coder. 'Comfort noise' is inserted by is open to interpretation, but will generally apply to any of the the receiving end to mask the silences that arise. This reduction in following types of cell: interference is particularly important in the PCN environment of small cells and high traffic density. It also has the desirable advant- 1. Cells with antennas positioned below rooftop or clutter height. age of increasing the battery of the MS. 2. Cells with antennas inside a building (sometimes referred to as a picocell). 3. Cells covering a specific, restricted area with a typical range of 48.9.3.4 Discontinuous reception less than 400m. The MS need not have its receiver continuously turned on, provided There are a number of significant problems associated with micro- it wakes up periodically to look for paging requests or short mess- cells which PCN manufacturers and operators have taken care to ages. This 'sleep mode' can improve the MS battery life quite minimise: significantly. 1. Range. Because of the low range, large numbers of microcells are needed to provide area coverage. 48.9.3.5 Dynamic power control Handover. Handover between microcells can be difficult to achieve, due to the short elapsed time required to move from A further technique for reducing interference is to allow the mobile one to another. Also, problems arise due to the 'street corner station to use only just enough RF power to maintain acceptable effect', where turni