Public Comment

Cell Antennas: Science vs. Emotion

By Richard Perlman
Thursday June 26, 2008 - 09:43:00 AM

I have been following the recent debate about cellular system antenna sites in Berkeley. Part of the debate seems to center around the perceived difference between the way the flat and hillside sections of Berkeley are treated, with the implication that the supposedly wealthier hills get more favorable treatment than the presumably poorer flatlands. 

I cannot comment on the demographics associated with the geography of Berkeley, but I can shed some light on the science behind how cellular radio systems work. My hope is that if the basic way these systems work is better understood, there might be a little more intellect and a little less emotion involved in this debate. 

The fundamental problem in designing mobile telephone (i.e. radio) systems is that radio frequency spectrum is quite limited. If mobile phone systems still served wide coverage areas, the capacity (simultaneous conversations) would be very limited, maybe a few hundred conversations in the entire Bay Area at the same time. 

Cellular radio systems are given their name because they consist of one or more “cells” through which users travel. Because the size of each cell is carefully controlled, and because at the frequency on which these systems operate radio transmission is limited to very short line-of-sight distances, it is possible to have two or more users use the same frequency if there is enough distance between them. Thus, only a small number of the channels available to an operator are used in any single cell. It is a complex science to determine the distance between cells in which the same channels can be reused. As a user travels between cells, their conversation is “handed-off” to new cell sites and the channel assignment is automatically changed. All of this is transparent to the user. 

In areas where usage is light it is possible to have a small number of fairly large cells. However, as usage grows, it becomes necessary to create more smaller cells. In order to limit the distance the radio waves will travel, and avoid interference, cellular system antennas are generally located at very low elevations, typically roof-top height. The antennas are also tuned to aim the radio waves downward so they do not stray far beyond the cell limits. It also possible to aim the radio waves to better serve odd shaped cells, like along freeways, shore lines, etc. 

The point here is that mobile phone companies need more cell sites in those locations where system usage is highest, typically along major communications corridors and areas where large numbers of people congregate. Because most people today still use their cell phone as a mobile device (though more and more people are giving up their landline) usage in areas that are principally residential with no significant shopping districts, like the Berkeley hills will have lower cellular traffic density. Also, hillside locations make poor sites for cellular antenna sites because they are at too high an elevation increasing the chance of interference. 

In fact, to serve hillside locations, cell sites in flat areas are “tuned” to provide coverage towards the hills. This generally results in poorer coverage in the hills than in the flat lands. 

The information I have provided has been simplified greatly, but still presents an accurate picture of the way cellular radio systems work and why they are designed the way they are. This does not mean that there are not reasons to object to creation of new cell sites. But, it does mean that the reason for the unequal distribution of cell sites is far more likely to be based on science and meeting customer demand than on vague suggestions of social/political/racial conspiracies. 


Richard Perlman is a Berkeley resident.