Ever notice that during a sporting event, or parade, or local emergency—when we rely on our cellphones the most—calls are often dropped and text messages go undelivered? That’s because there are a limited number of radio frequencies for cellphone users in any one area. When a big event occurs, lines are flooded and networks can’t handle all the calls.
One IEEE graduate student member believes she has a solution to this problem by tapping into unused frequencies from nearby television and radio broadcast channels. Mai Hassan’s study “Cooperative Beamforming for Cognitive Radio Systems With Asynchronous Interference to Primary User” was recently published in the IEEE Transactions on Wireless Communications Journal. It proposes a cooperative beamforming method—a signal-processing technique that makes use of available signal bands in an area. Both cell towers and cellphones may borrow signals from broadcast channels nearby. She further tweaked this method so that cellphone users can collectively share antenna signals for stronger reception, turning the challenge of crowded lines into an opportunity. Hassan is a Ph.D. candidate at the University of British Columbia, in Vancouver.
“Every cellphone is assigned its own radio frequency so that callers do not interfere with each other,” she says. “The answer to dropped calls is to widen the spectrum of available frequencies.” Hassan turned to the large number of frequencies from radio and television networks that go unused. And by tweaking a smartphone antenna, she was able to develop a low-cost answer to avoid channel overload.
To design her system, Hassan explored cognitive radio (CR) technology, an intelligent radio that can be programmed and configured to use the best wireless channels in its vicinity. CR can tap into spectrum bands, often referred to as spectrum holes and slots, to find service when cellphone frequencies are all in use.
For cooperative beamforming to work, the CR technology must be set up from the cell towers used with the mobile devices. When frequencies from a broadcast channel are available near the tower, the CR will use them for the cellphones. Each broadcast channel provides 6 megahertz of bandwidth, while each cellphone requires just 200 kilohertz. This means that each channel could potentially provide service to an additional 30 cellphone users.
“Cellphone antennas can already detect different frequency channels for transmission,” Hassan says. “This method will now allow them to have more channel options to carry out a call.”
But Hassan faced two challenges in using the beamforming method. By sharing frequencies with television and radio networks, for one thing, it’s crucial that those networks not be interrupted by an influx of cellphone users borrowing their channels when they’re being used. The other concern is that cellphones would require multiple, and pricey, antennas to detect the frequencies in an area.
To deal with this, Hassan developed what she calls a leakage beamforming (LBF) method to stop asynchronous interference, which can interrupt the broadcast network. This type of interference occurs because calls are placed at different times from different locations, hence the term “asynchronous,” notes Hassan. LBF helps solve this problem by maximizing the signal power within the cellphones while placing a limit on how much interference the CR system will put on the broadcast channels. The threshold limit will have to be regulated in order not to disturb these channels.
“By knowing the location of the broadcast network, which the antennas can locate when accessing a frequency channel, we can limit interference by modifying the direction of the transmission from cell antennas away from the broadcaster,” Hassan says.
She was able to change the direction of transmission by aligning signals that cancel out one another—known as destructive interference. This blocks transmission and cancels access in regions where radio or television signals are available so that the broadcast networks are not interrupted. Hassan was also able to produce the opposite effect—called constructive interference—by aligning signals that enhance reception to help a call reach its destination.
She concluded that the more cost-effective way is not to place multiple antennas in each phone but instead to develop a single antenna that cooperates with other cellphone antennas in the CR network. Together, the cellphone users would collectively obtain better network access and ultimately accomplish the same objective as could several antennas in a single phone.
For Hassan’s method to become a reality, cellphones would require software-defined radio. Such a communication scheme relies on software that could replace some of the phones’ hardware, she notes, and essentially make phones less expensive.
Hassan believes her method could be adopted on a wide scale because all it requires is a simple algorithm. The technology already exists, she says, and no extra hardware is needed. In fact, some hardware could even be eliminated.
She is currently testing her system in her own home using a network that picks up unused frequencies from her television and radio. So far her experiments have been successful, and she plans to expand her system with additional channels and cellphone users.