ts, in the received signal. Co-channel interference-Although frequencies are not to be reused in adja- cent cells, if a clear path exists, a transmission on the same channel in a non- adjacent cell might reach a user in another part of the service area and cause interference. Wireless systems operating in unlicensed spectrum are also sub- ject to interference that may come from other wireless devices or networks. Mobile Phones and Health Issues Health issues related to mobile phone use are of legitimate concern. One important find- ing is that mobile phones may cause the blood-brain barrier-a network of tiny capillar- ies surrounding the brain that protects the brain from toxins-to break down. Another finding is that mobile phone radiation has shown effects on blood proteins. While there is no conclusive proof, many studies point to the need to continue research. You can watch an interesting video on the radiation effects of mobile phones and find summaries of many of the research studies conducted over the years at www.globalchange.com/ radiationtv.html. Interference and noise-Interference and noise are by-products of precipi- tation in the air, metals in the environment, and a variety of other anoma- lies. Error correction techniques are needed to fix these problems. Foliage-Foliage can be a source of interference because the water in leaves absorbs radio signals. Chapter 13 Wireless Communications Basics Weather effects-Weather can cause interference, particularly in the higher frequencies, where each radio wave is smaller than a drop of rain. Environmental obstacles-Radio signals cannot penetrate various materi- als, SO walls, desks, buildings, hills, vehicles, and other environmental obstacles can affect the performance of radio. Range and electrical power-Range and electrical power are consider- ations because more radio power is required to increase the range or to compensate for poor path quality. Antennas An antenna is a device through which radio frequency (RF) energy is coupled from the transmitter to 