 no harm to humans in the neighborhood. This is an EIRP limit, SO antenna gains do not give an improvement for the sending side. With limited given transmit power level, when using higher © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks transmission rates we get lower energy per bit, and the Shannon bound dictates that there will be more bit errors. Whether we use forward error correction (FEC) or automatic repeat request (ARQ), there is no way to increase the effective net bit rate with a single antenna. Radio propagation. Above ultrahigh frequencies (e.g., 2 GHz), there is a much higher vulnerability to bad non-line-of-sight conditions, but this is a common scenario in densely populated urban areas due to buildings, cars, and other obstructions. In effect, the path loss is higher between BS and mobile. Nonconstant rate offer. The maximum data rate offered by a BS depends on the distance of the mobile from it. The closer the UT is to the base station, the higher the received SINR (signal-to-interference-plus-noise ratio) value will be. A high SINR allows the highest modulation and coding scheme (PhyMode), which offers the highest data rate. But, with increasing dis- tance, weaker PhyModes must be used. At its extreme near the cell border, the maximum possible offered data rate is one order of magnitude lower. If we consider many UTs distributed equally over the area, the problem looks worse because, if some terminals operate at the lowest PhyMode, they require a 10-times bigger share of the spectrum than the same number of terminals operating at the highest PhyMode. This means the average cell capacity is more than proportionally determined by the maximum possible rate at the outer regions. Figure 8.1 gives an idea of this problem. Actual Available Capacity vs. Requested Capacity #users at distance Available Requested by user Distance d Cell border Figure 8.1 The single-hop problem: with a constant user density, the number of users increases with the distance d, so the cell capacity