tions (MSs) moving through the cell. A cell is the area of responsibility of the fixed station, a cell site. It usually is pictured as a hexagon in shape, although its propagation profile is more like a circle with the fixed station in its center. Cell radii vary from 1 km (0.6 mi) in heavily built-up urban areas to 30 km (19 mi) or somewhat more in rural areas. 18.3.2.1 Path Loss or Transmission Loss. We recall the free-space loss (FSL) formula in Section 9.2.3. It simply stated that FSL was a function of the square of the distance and the square of the frequency plus a constant. It is a very useful formula if the strict rules of obstacle clearance are obeyed. Unfortunately, in the cellular situation, it is impossible to obey these rules. Then to what extent must this free-space loss formula be modified by the proximity of the earth and by the effects of trees, buildings, and hills in, or close to, the transmission path? There have been a number of models that have been developed that are used as a basis for the calculation of transmission loss, several assumptions are made: That we will always use the same frequency band, often 800 or 900 MHz. Thus it is common to drop the frequency term (the 20 log F term) in the FSL formula and include a constant that covers the frequency term. If we wish to use the model for another band, say 1800 MHz, a scaling factor is added. That we will add a term to compensate for the usual great variance between the cell site antenna height when compared to the mobile (or hand-held) antenna height. We often call this the height-gain function, and it tends to give us an advantage. It is often expressed as -20 log(hThR), where hT is the height of the transmit antenna (cell site) and hr is the height of the receive antenna (on the mobile platform). These are comparative heights. Commonly, the mobile platform antenna height is taken as 6 ft or 3 m. That there is a catch-all term for the remainder of the losses, which in some references is expressed as B (in dB). That at least three models