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Smart antennas arrive for in-vehicle systems

Posted: 16 Dec 2004 ?? ?Print Version ?Bookmark and Share

Keywords:suv? minivan? antenna? satellite? smart-antenna?

One of the fastest-growing automotive accessories is the backseat video system. There were an estimated 1 million automotive multimedia units installed in 2002, and, according to a Time survey in June 2003, more than one-third of all new minivans and SUVs are now sold with a video screen for games or DVDs. However, the killer application would be live entertainment, such as satellite TV, giving consumers the same video experience in their vehicles as at homenever missing the big game, able to check the financial markets and keeping up to the minute with the news.

Two main factors have limited the application of in-motion satellite TV in vehicles: cost and size. Ideally, the cost of such a system should be less than $1,000, and the antenna system should be small enough to fit into the car roof, unseen either inside or outside the vehicle.

Standard satellite TV dish antennas are too big and bulky to be practical for SUVs and minivans. Such systems also require steering of the antenna to keep it pointed in the direction of the satellite while the vehicle is in motion. Thus, a large radome is needed to cover the antenna, which is only practical for large vehicles or boats.

To bring down the size, the antenna can be a flat plate that is mechanically rotated and tilted to steer the antenna beam. This reduces the size of the antenna system to a height of several inches, with tilting about the center of the plate. The required width (about 30 inches) is less than that of standard vehicles. Such a system is currently commercially available from KVH, for example. The system is still too tall for embedding into the roof of a vehicle, however, and it requires an engineering-challenging (especially for the rigors of a vehicle-mounted system), 2D steering mechanism.

To reduce the height further, the plate can be split into multiple narrow plates, each one mechanically tilted. Some systems using this technique also increase the distance between the plates as the plates are tilted, so plates do not partially block one another, even at low elevation angles. Although this reduces the height of the antenna system, it again does not allow for embedding the antenna into the roof of a vehicle and uses an even more-complicated mechanical-steering mechanism.

To keep the height of the antenna to a minimum, a fully electronic flat smart antennathat is, a phased-array antenna having a large number of antenna elementscan be used. Antenna aiming in the azimuth and elevation directions is achieved by passing the received signal from each antenna element through a phase shifter. The phase shifter rotates the phases of the signals received from all antenna elements to a common phase before they are combined. While such antennas can be implemented with a very low profile and fit into the roof of a vehicle, the large number of microwave processing elements such as amplifiers and phase shifters (at least 1,000 antenna elements are required for the same performance as a dish antenna) results in high implementation cost, preventing mass, volume commercial use.

To keep the height low enough and the cost reasonable, a hybrid approach can be used. The antenna is mechanically rotated in azimuth but electronically steered in elevation. The antenna itself is a planar antenna consisting of parallel slotted waveguides. Each waveguide contains multiple radiating slots, with the signals received by these slots combined in the waveguide to produce output signals with the two polarizations used by satellite TV systems in the U.S.-right-hand circular polarization out of one end of the waveguide and left-hand circular polarization out of the other end. Thus, only about 30 received signals need to be co-phased and combined to form a narrow beam in the direction of the satellite, vs. more than 1,000 with an electronically steerable antenna, greatly reducing the cost.

This phased-array approach of co-phasing the received signals can be further improved upon by using smart-antenna technology. That is because the phased-array approach has a few issues. First, it provides the best performance only under ideal conditions. That is, it assumes that all the waveguides have the same gain, as well as the same phase shift between waveguides for a given angle of arrival. This is typically not exactly true for every antenna, though, and the parameters will vary with temperature, frequency and so on. Although calibration of the waveguides in each antenna can partially compensate for these effects, this can be a costly addition to the antenna. Furthermore, the arriving signal may not be a narrow beam because of atmospheric conditions, trees, reflections and the like, and these cannot be compensated for by calibration. This last set of issues occurs even in a fully mechanical system.

A roof-mounted hybrid antenna that uses planar and smart-antenna techniques can bring satellite TV to moving vehicles.

With smart-antenna technology, the multiple received signals are weighed both in phase and amplitude and are combined using weights that are adaptively and continuously generated to maximize system performance. Since the phase and amplitude of the received signals are adjusted independently, the above impairments are automatically mitigated. These techniques are well-known from military and commercial wireless systems, and can readily be applied to satellite TV receivers, even using the same low-cost ICs. Thus, smart-antenna technology can significantly improve the performance of satellite TV systems, overcoming various impairments, while allowing for integration to reduce cost. Further cost reduction can be achieved with additional integration of the receiver electronics.

The hybrid approach allows for a low-cost, low-profile solution that can be embedded into, or mounted discretely on, the roof of the vehicle. One such system was shown at the "Consumer Electronics Show" earlier this year. Although the antenna cover was clear plastic, in practice the cover would be the same color as the roof and thus unseen from the outside. The headliner would be lowered by only about an inch. Also, although such a setup means the luggage rack could not be used while the satellite TV signal was being received, the antenna could alternatively be incorporated with a moonroof. In other words, the satellite antenna would be movable to an open position beneath a transport plate of the moonroof or sunroof system and into a closed position beneath the vehicle roof.

Finally, in addition to TV, satellite access to the Internet is desirable. In the Ku band, however, the required size of the transmit antenna for a suitable narrow beam is twice that needed for receive and is therefore too wide for standard vehicles. But at Ka band, the antenna is only half the size of the Ku-band antenna and therefore is feasible for standard vehicles.

Smart-antenna technology makes true in-vehicle satellite TV feasible. By using this technology, a lower-profile and low-cost satellite TV antenna system is made possible. With a low-profile antenna, the door is opened for live, in-motion TV antennas to be either installed in the roof of cars or vans, or to be sold as a low-profile add-on piece. Smart-antenna technology can also provide a signal that is stronger than older, fully mechanically steered systems.

- Jack Winters

Chief Scientist

Motia Inc.

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