When cellular phones were first introduced, they only had one function?placing and receiving phone calls. Since then, phones have shrunk, dropped in price, and increased in functionality. Convenience and new applications warrant the addition of new, short- to medium-range wireless technologies:
- Bluetooth: for the main purpose of conducting phone calls using wireless headsets and Bluetooth-enabled cars, as well as wireless stereophonic headsets for playing songs stored in the handset internal storage
- Wi-Fi: for wireless Internet connection in hotspots, as well as placing Voice over Internet Protocol phone calls over the much cheaper Wi-Fi infrastructure
- Ultra-Wideband (UWB): for fast wireless upload of stored pictures from the camera phone to a PC and fast wireless download of songs to an MP3-enabled phone
- Mobile DTV: for viewing premium-content live TV programs over the high-quality, reasonably sized color LCD display
- GPS: for regulatory requirements (such as E911) and navigation purposes
- FM radio: for listening to streaming audio content
These technologies are in different stages of integration into handsets. Bluetooth is the most pervasive technology so far. But soon, phones will feature two, three, and more of these technologies.
Integrating multiple radios into a handset poses many challenges, primarily ensuring coexistence of the technologies. Radios aren't perfect. While tuned to transmit at a 2.6-GHz frequency, transmitters will transmit some power (although much lower than the main transmit frequency) out of that band due to imperfections inherent in the radio technology. Receivers will receive transmissions outside of the frequency they are tuned to, for the same reason, and will be affected by it.
In some cases, multiple radio technologies use the same frequencies. For example, Bluetooth and Wi-Fi use the same 2.4-GHz ISM (industrial, scientific, medical) band. These multiple technologies need to know how to share this spectrum effectively and efficiently, enabling both to operate as intended without any ?penalty? to the overall operation of the cellular phone.
At one time or another, we have all seen the mutual interference caused by cordless phones and Wi-Fi networks, as well as interference caused by different radios located within a few feet of each other. This problem increases exponentially when radios are located not within feet of each other, but within millimeters of each other.
The interference created by radios located 3 ft from each other increases by approximately 40 dB when they are moved to 10 mm from each other. The ?spectral protection masks? required by regulatory agencies to ensure minimal interference between devices located close to one another (a few feet) is almost useless when those radios are co-located within the same handset.
A multidisciplinary art that should not be treated lightly solves the problem. It involves hardware modifications (in both analog and digital components of the radio), protocol modifications, software stack modifications, and smart antenna sharing. But most of all, it requires system-level understanding of the operation of all the radios in a way that lets designers solve these problems.
The coexistence issue of multiple radios in a handset is lurking. When handsets including multiple technologies appear on the market, the ability of technology providers to solve the co-existence problem will be the main differentiator of ?non-cellular wireless? manufacturers.