Lecture 1. Introduction to Wireless Communications 1

Cellular systems provide two-way voice and data communication with regional, national, or international coverage. Cellular systems were initially designed for mobile terminals inside vehicles with antennas mounted on the vehicle roof. Today these systems have evolved to support lightweight handled mobile terminals operating inside and outside buildings at both pedestrian and vehicle speeds. The basic premise behind cellular systems design is frequency reuse, which exploits the fact that signal power falls off with distance to reuse the same frequency spectrum at spatially separated locations. Specifically, the coverage area of a cellular system is divided into nonoverlappping cells, where some set of channels is assigned to each cell. This same channel set is used in another cell some distance away. Operation within a cell is controlled by a centralized base station, as described in more detail below. The interference caused by users in different cells operating on the same channel set is called intercell interference. The spatial separation of cells that reuse the same channel set, the reuse distance, should be as small as possible so that frequencies are reused as often as possible, thereby maximizing spectral efficiency. However, as the reuse distance decreases, intercell interference increases owing to the smaller propagation distance between interfering cells. Since intercell interference must remain below a given threshold for acceptable systems performance, reuse distance cannot be reduced below some minimum value. In practice it is quite difficult to determine this minimum value, since both the transmitting and interfering signals experience random power variations due to the characteristics of wireless signal propagation. In order to determine the best reuse distance and base station placement, an accurate model is needed. Initial cellular system designs were mainly driven by the high cost of base stations. For this reason, early cellular systems used a relatively small number of cells to cover an entire city or region. The cell base stations were placed on tall buildings or mountains and transmitted