Mobile TV transmission polarization boosts reliability

It's said that a picture is worth a thousand words. That's why John Schadler, Dielectric's director of advanced antenna systems development and SPX master inventor, used two short videos to introduce his NAB presentation, “Understanding Reliability in Mobile TV.”

The clips illustrate how people's vastly different expectations about mobile phone call reliability and TV broadcast reliability are central to mobile TV's success. The first is about a dropped cell phone call; the second is a dropped live mobile TV broadcast of a football game. Guess in which clip the customer tosses the phone in the trash.

If you though the live TV broadcast, you're right. "On a cell phone, we expect bad service; we've been trained to accept poor reliability in voice service," Schadler said.

Compare that to our expectations about TV broadcast reliability; namely, that when you turn on the World Series you'll see it from start to finish without service interruptions. "With TV, you probably won't accept poor reliability — you'll discontinue the service," Schadler said. "That's why reliability is key to the [commercial] success of mobile TV."

Mobile TV reliability (high probability of signal) depends on the spatial immunity of the service, he said. A high probability of signal can only be achieved by a large signal-to-noise ratio (SNR) margin. Reducing variability is equivalent to increasing the SNR margin.

So, how can operators do that? Not on the mobile device, according to Schadler. "We found the small antenna in the phone, all it does is excite the phone. In effect, the phone is the antenna, and it's linearly polarized. You can't do a thing about that. So we looked at what can be done at the transmission end: Can the choice of transmit polarization increase reliability of service?"

Schadler's experiments showed that it could, and that circular polarization delivered the best results for mobile TV. "Our tests proved in the lab and in the field — the real world — that circular polarization provides the best signal strength: an extra 5dB of margin over horizontal polarization and extra 7.5dB of margin over vertical."

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