July 27, 2007 4:00 AM PDT
FAQ: The 411 on radio frequency interference
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The cell phone interference issue seems to be a bigger problem for people using certain carrier networks. Why?
It's true, customers on AT&T/Cingular, T-Mobile and the old Nextel networks experience this problem more frequently than those on Verizon Wireless and Sprint networks. The reason is that AT&T/Cingular, T-Mobile and Nextel use cell phone technologies that use a radio channel access method known as TDMA (time division multiple access).
Networks for AT&T/Cingular and T-Mobile are built on GSM (Global System for Mobile Communications), while Nextel uses iDEN (Integrated Digital Enhanced Network). These TDMA-based technologies allow several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium or radio frequency channel while using only the part of its bandwidth they require.
Because these networks operate in a "time division" fashion their radio frequency transmitters are turned on and off at fast rates. And this can often be picked up by nearby devices.
Verizon and Sprint's network use a technology called CDMA (code division multiple access). It does not use TDMA for sharing channels. CDMA transmitters are transmitting signals almost constantly, so they don't cause the interference buzz.
GSM and iDEN are 2G technologies. Will this problem still occur as mobile operators migrate to 3G technology?
It shouldn't be as prevalent. AT&T/Cingular has built its 3G network using WCDMA, which is based on CDMA technology. So new 3G phones on AT&T's network should not have as many interference issues.
Is there anything consumers can do to reduce this problem?
Unfortunately, the answer is no. The best solution is to turn off cell phones when you're near a speaker or some other device that is amplifying the cell phone signal. Or you can try to stay far enough away from speakers and other electrical equipment if they're turned on. For example, if you're on a conference call using a speakerphone don't put your phone on the table next to the speaker.
Hospitals for years have banned cell phones, but can cell phones really interfere with medical equipment?
Hospitals use sensitive equipment such as ventilators and ECG (electrocardiography) monitors for patient care. And just like TVs or speakers, some of this equipment is susceptible to electromagnetic interference.
But some hospitals are starting to lift the ban on cell phones as newer digital cell phone technologies and better shielding on hospital equipment have decreased the potential for interference. Plus the ban is nearly impossible to enforce.
Still, some hospitals have kept the policy in place, mainly to keep noise levels down so that patients aren't disturbed by people gabbing on their cell phones.
Do cell phones interfere with communications on airplanes?
Experts have debated for years whether it is safe to use cell phones on airplanes. Most of the evidence suggesting that it interferes with aviation equipment is anecdotal. But last year, researchers at Carnegie Mellon University concluded that cell phones can disrupt normal operation of key cockpit instruments, especially Global Positioning System receivers, which are increasingly vital for safe landings, according to Bill Strauss, an expert in aircraft electromagnetic compatibility at the Naval Air Warfare Center in Patuxent River, Md., and one of the researchers who conducted the study. Strauss said risks are caused by radio emissions from cellular calls that are higher than previously believed.
Is that the only reason why the FCC and Federal Aviation Administration have banned cell phone use on planes?
There is another reason why the FCC isn't keen on allowing people to chat on cell phones while flying. The problem is that when cell phones are used in flight they are traveling rapidly over hundreds if not thousands of cell phone towers. As the plane flies over these towers, the cell phones inside the plane are connecting and disconnecting from various cell towers much faster than was intended. The rapid signal hand-off from tower to tower of hundreds or thousands of cell phones flying overhead could disrupt service on the ground, affecting millions of cellular customers.
What about other consumer electronic devices such as iPods and laptops? Why do those need to be turned off too?
Actually, the Carnegie Mellon study also found that other electronic devices such as laptops and handheld games can send out potentially harmful signals that interfere with aviation equipment.
The bottom line is that RF interference is a fact of life, says Craig Mathias, a principal analyst for Farpoint Group.
"Most people have just learned to deal with it," he said. "The alternative is to live without wireless, and who would be willing to give up their cell phone?"
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