The main themes of this book are to explore evaluation methods for quantifying the mutual effects of interference on the performance of wireless networks and to investigate system-level solutions for their coexistence in the same environment.

The coexistence of wireless communication systems operating in the same environment has become a “hot” topic in recent years as more systems are choosing to use the unlicensed bands and forfeiting the need to purchase spectrum.

There are two specified unlicensed bands for the operation of wireless systems, namely:

1. the industrial scientific and medical (ISM) band that includes the 900 MHz, 2.4 GHz, and 5.8 GHz frequencies;

2. the unlicensed national information infrastructure (UNII) band that includes the 5.2 GHz band. This band was opened in 1997 in the United States in order to expand broadband access opportunities.

Few rules apply in the unlicensed bands such as the ISM band. For example, the rules defined in the Federal Communications Commission Title 47 of the Code for Federal Regulations Part 15 relate to the total radiated power and the use of the spread spectrum and frequency hopping modulations. It is commonly understood that all users of the unlicensed bands can equally affect the quality and the usefulness of this spectrum. Thus, the major downside of the unlicensed band is that frequencies must be shared and potential interference tolerated.

We distinguish between several types of users in these unlicensed bands.