At this point, the question arises as to how functions and channels of mobile devices can be changed without violating regulatory rules. For example, the Federal Communications Commission currently authorizes each piece of equipment for a type of use and a specific frequency channel. Both national and international regulatory bodies will need to co-operate and resolve this issue before frequency- and service-agile SDRs can become widespread.
SDRs can also work with smart antennas on combatting interference, poor connection quality, and limited system capacity. The antennas are arrays of devices capable of sending and receiving energy in precisely controlled, changeable directions. They form beams that receive (or radiate) energy only in particular directions, providing gain in those directions and attenuation at other angles. Placed at a cell tower base station, they can track the signals of several users while suppressing other interfering signals. In effect, they create multiple noninterfering channels where there had previously been just one, enlarging the traffic-carrying capacity of the cell.
Because it is difficult to build an antenna–smart or otherwise–that provides high gain over a broad frequency range, SDR designers have tended to focus on concatenating multiple narrowband antennas, thus leading to multiband SDR systems. The number of antennas required to cover a specified part of the RF spectrum depends on how well amplitude and phase errors across the individual bands can be compensated for, which in turn depends in part on how wide those bands are. In the future, advanced RF techniques may allow the development of reconfigurable multiband antennas.
Dynamic software download
Having made the case for SDR, it remains to explain how, exactly, a radio’s software can be changed. Upgrading application features by software download over the Internet is now widely accepted, even though it can be a slow and frustrating experience when performed over a dial-up modem. The concept can be extended to the wireless realm by having subscribers download complete radio standards, or even custom applications over the air whenever it suits them. The download process can be made simple and entirely transparent to the user by having the service provider control the complete download process.
To arrive at this happy state of affairs, an open application programming interface needs to be defined to provide a mechanism through which different vendors can develop compatible software and hardware interfaces. To that end, object-oriented technologies like the Common Object Request Broker Architecture (Corba) and Java can be used to support the download mechanism.
But downloading software is hardly without problems. For example, it is not clear how alterations in equipment functionality will affect the equipment’s FCC certification. It is also crucial to verify the integrity of software download to protect users against fraud.
Interested parties
SDR research is being pursued not only in industry and the military, but also in academia. At The Georgia Institute of Technology, through funding provided by the state of Georgia’s economic development program known as the Yamacraw Initiative, researchers are developing a high-data-rate wireless system using SDR. The project has two main objectives:
- To develop fully adaptive mobile wireless communications. This will include the coordination of changes in the application, network, access, and physical layers of the network as it changes from one wireless channel or standard to another.
- To develop a high-performance wireless system. This will include the use of adaptive antennas and coding algorithms to combat the effects of channel variation in time and space.
The SDR testbed includes wideband RF front-ends, ADCs, DACs, DSPs, and other commercial off-the-shelf systems. It serves as a functional prototype for the implementation, test, and integration of cutting-edge research developed by faculty members.
