05.04.2007 12:00 AM
BBC Study Says MIMO Techniques Could Increase DTV Data Capacity
A BBC study on achieving higher data rates in DTV signals is being touted as a possible solution for U.K. broadcasters who want to broadcast in hi-def.

Using the ATSC standard, the maximum data rate U.S. TV broadcasters can transmit is just under 19.39 Mbps. For broadcasters looking to offer HDTV programming as well as a multicast channel or two and perhaps a mobile service, more bandwidth would be nice!
Last year, the BBC conducted an experiment that showed it might be possible to achieve higher data rates in the same channel bandwidth while maintaining or improving signal robustness.

While the study is dated December 2006, it received a lot of attention last week in articles promoting it as a way for terrestrial DTV broadcasts in the United Kingdom to offer HDTV. See "Hi-def over Freeview? The BBC has a cunning plan" on CNET. What is this technology? If you bought a WiFi router recently, you may be familiar with it--MIMO. MIMO, the acronym for "multiple input, multiple output" refers to the use of multiple antennas to modulate signals not only in time but in space as well. I described the technology several years ago in my RF Technology column and it is now commonly used to increase data capacity and range in 2.4 GHz wireless networks.

If you fear TV stations that want to use this technology will have to erect three towers with separate antenna to mimic your MIMO or 802.11n wireless router, relax. The BBC experiment achieved the space diversity by using horizontally and vertically polarized signals. This 2x2 MIMO system was able to deliver up to twice the throughput of a conventional DVB-T system. In the experiment, the system transmitted two separate DVB-T transport streams. This required two transmitters and, on the receive side, two tuners. A special synchronizer and channel equalizer was required to process the signals from the two tuners.

The experiment showed the dual polarization MIMO technique worked well over line-of-sight paths but showed that a more sophisticated decoder may be needed to improve performance when the orthogonality of the signal is lost.

For a description of the experiment and the results see BBC Research--White Paper WHP 144--A Dual Polarization MIMO Broadcast TV System by J.D. Mitchell, P.N. Moss, and M.J. Thorp.

While this technology is exciting, it may be difficult if not impossible to make it compatible with existing receivers. Also, the experiment used COFDM multiple carrier modulation instead of the single carrier 8-VSB modulation used in the U.S.; so don't expect to see this technology used for broadcasting any time soon in this country. It may, however, be useful for new services using TV spectrum auctioned after the DTV transition ends in 2009.


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