03.01.2008 12:00 PM
Originally featured on
Smart antennas

The analog cutoff is now less than one year away, and with that change will come new reception issues for terrestrial broadcast viewers. Although the FCC has sought to replicate analog service in its digital channel allocation plan, most broadcasters will have a digital channel assignment different from their analog one. Inherently, this means that the RF field conditions at the viewer's location will be considerably different from those of the analog service. Smart antennas offer a convenient way to minimize the impact on the viewer.

Because terrestrial television receivers must potentially receive signals from various locations, a fixed antenna cannot provide optimum reception across the available channels. In addition, community antennas using the same transmitting site may cause receivers to experience different multipath reception conditions across different channels. While indoor antenna re-aiming may not affect an analog viewer — or the viewer simply tolerated a lower SNR on some channels — re-aiming an antenna for optimum digital service could be quite burdensome.

Electronically steerable smart antennas that automatically optimize the preferred signal direction for each particular broadcast emission were developed years ago for military applications and are increasingly being used in cellular telephone base stations. This optimization can take into account various signal quality factors, such as signal strength, multipath energy and BER.

Digital signal reception varies widely

Terrestrial television reception is subject to many transmission path impairments, including multipath interference, where delayed echos of the transmitted signal can arrive at the antenna because of reflections off large objects in the receiving space. Moderate to severe multipath can lead to an increase in BER, which could compromise video and audio or, in the worst case, result in no reception at all.

While this situation can often be remedied by physically re-aiming the receiving antenna, this adjustment may not be ideal for all received stations because of their different transmission powers, frequencies and locations.

These difficulties are compounded because of the cliff effect,wherein the BER increases catastrophically below a certain C/N ratio or D/U interference ratio. As such, antenna adjustment can be problematic under many reception conditions. The situation is equally inconvenient with outdoor antennas (requiring a rotator) or indoor ones (requiring frequent trips to the television).

It is now practical to use this same technology for consumer digital television reception. By providing an automatic mechanism to adjust the antenna, the direction and gain (amplification) of the antenna can be electronically changed, with no need for user intervention or physical adjustment of the antenna. This type of antenna functions by changing the relative gain and phase (delay) of the internal elements. While offering a high degree of optimization for both signal capture and interference rejection, this kind of adaptive antenna is somewhat complex and hence expensive to implement.

Practical smart antennas

An alternate type of smart antenna is the so-called switched beam antenna system. In this system, multiple fixed elements within the antenna are selectively used so that a primary receiving direction is favored. At the same time, strong sources of multipath can be negated. An optimization algorithm can perform a trade-off between the two factors. The user simply plugs the antenna in to a suitably equipped DTV receiver or converter box, and the receiver automatically adjusts the antenna for optimal reception of each DTV station.

One example of such a system is shown schematically in Figure 1. The optimization algorithm is typically executed by the CPU in the receiving device and is done once during initial setup. In addition to selecting different azimuth directions, units can operate with different levels of RF amplification. This is useful in areas of high signal strength to avoid overload of the receiver front end, which could otherwise result in high intermodulation distortion.

Selecting an antenna direction and gain setting for optimum signal reception involves assessing the signal quality over the operating extent of the antenna. Various parameters of the received signal can be evaluated and weighed, including signal strength, mean squared error of the channel equalizer, spectral flatness and unwanted interference.

Depending on the system architecture, this optimization process can be tightly integrated with the demodulator or implemented separately. The combination of direction and gain can also be used in a more sophisticated algorithm that anticipates third-order intermodulation interference from strong UHF taboo channels, or from the n ± 1, two-channel pairs where tuner RF selectivity may be minimal.

A standardized smart antenna interface

While a smart antenna can be an option to the consumer, it will only function if the appropriate interface is available at the receiver. Such an interface has been developed and standardized by the Consumer Electronics Association, and is known as CEA-909A, “Antenna Control Interface.”

This standard describes how a compliant receiver can operate with any compliant antenna, regardless of manufacturer. The standard also specifies the data format used, the connection standards and other requirements.

The antenna configuration is neither specified nor implied, leaving specific design considerations to the manufacturer. As such, an elaborate system can even be designed using a full-blown antenna farm. The more practical design allows for the realization of an affordable, attractive antenna with a small form factor, as seen in Figure 2.

The CEA R4-WG4 working group has also defined a control protocol that works over the antenna coax, resulting in two options for the CEA-909A standard: one that uses a separate connector for the control signal, and one that shares the RF signal connector/coax.

Proven utility

DTV smart antennas have been tested in a variety of field locations with promising results. The new antenna has demonstrated a considerable advantage over various indoor antennas, including the popular mini yagi, with no amplification, and the set-top UHF loop/VHF rod antenna combination with built-in amplification. The smart antenna performs at least as well as a hand-optimized yagi — but doesn't require user intervention. In a few locations, smart antennas may be unable to automatically optimize the signal. However, in those few cases, a yagi would require meticulous adjustment as well, especially when subjected to a high level of multipath.

Preliminary tests by the CEA R-5 Antenna Standards committee have shown impressive results, finding that a smart antenna can be most effective in ghosty areas and can increase DTV system performance by as much as 12dB. In addition, both the MSTV and the NAB have endorsed the technology, and the NTIA is allowing the interface on converter boxes certified in its DTV coupon program.

Various DTV converter box manufacturers now include the smart antenna interface on their products. Nonetheless, and despite the potential of the technology, the public knows little about it, and retail stores are of little help. With a bottom line mentality, the major store chains have been hurt in the past by poor antenna sales, so they don't want to carry a slow mover that won't come down in price unless millions are sold. Just imagine the confused customer who has reception difficulties and blames it on the DTV receiver or converter. As with the DTV transition itself, education is needed in order to fully appreciate the benefits of digital television.

Aldo Cugnini is a consultant in the digital television industry.

Disclosure: The author is a consultant to a company that manufactures smart antennas.

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