of the drive testing path considers the site and its adjacent best serving areas. The aim of this optimisation solution is not to review all the network's engineering but only sites having problems. Statistical filtering (bad timing advance, high DCR, high interference outgoing handover percentage, traffic decrease, etc.) is then done to select the specific sites to visit. A traffic map of the area is also important to prepare in order to visualise the traffic distribution between sites. Before going on a site survey, the site history is considered (all operations that previously took place on the site). Radio Network Optimisation Site Survey Visiting a site on air is a good opportunity to update the planning database and obtain newer panoramic pictures. A complete audit of the site is done (re-checking coordinates, azimuths, heights, tilts, antenna types, etc.). Urban changes affect the cellular network in the sense that a new building, for example, recently built in front of an existing antenna generates a signal propagation obstruction. Statistically speaking, this is translated into a traffic loss. The planners therefore check antenna horizontal and vertical spacing. Site Re-engineering Proposal The first reflex of the planning engineer when proposing a re-engineering solution is trisectorisation. In fact, with network expansion, in order to use the frequency spectrum efficiently, the network should be well designed to constitute better motifs (the Subsection on Frequency Planning Review deals in more detail with the motifs notion). Sectors of adjacent sites follow the same directions (0°, 120° and 240°, for example) to make it easy to use the manual and automatic frequency allocation. A re-engineering solution consists either of downtilting/uptilting, re-azimuthing or changing the antenna's height, location or type. Such operations have an impact on signal propagation. They make it larger or narrower and at the end improve coverage or reduce interference. Mastering propagation mechanisms in different types o