[opendtv] ATSC Digital On-channel Repeaters at NAB Show
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 3 Jun 2010 17:08:48 -0500
Interesting and encouraging, as these are the simple gap fillers, rather than
the more laborious true SFN systems.
"Distributing signals indoors is a lot different from transmitting at high
power from a single stick. Fortunately wireless carriers have already had to
deal with this and I suspect there is a lot we can learn from them."
Maybe, except that wireless operators do this indoor distribution via the
equivalent of translators (e.g. microcells), not DOCRs.
Bert
---------------------------------
http://www.tvtechnology.com/article/101436
ATSC Digital On-channel Repeaters at NAB Show
by Doug Lung, 06.02.2010.
The amount of floor space devoted to transmitters at the 2010 NAB Show has
dropped significantly over the last five years as the focus has moved from
established high-power tube and solid-state amplifier technology to control
systems, modulators and exciters. These don't require as much room to show or
demonstrate.
Last year ATSC mobile DTV was the hot new technology. This year, after the
adoption of the ATSC A/153 mobile DTV standard last October, broadcasters
expected transmitter manufacturers to have a mobile DTV solution as part of
their transmitter line. For me, the most interesting TV RF products were
digital on-channel repeaters.
DOCR CONCEPTS
Digital on-channel repeaters (DOCRs) receive an off-air DTV signal to amplify
it; down-convert it to an intermediate frequency (IF) for filtering and
equalization; and up-convert it using the same local oscillator on exactly the
same frequency with exactly the same data as the received signal.
DOCR transmit power is limited by coupling between the receive and transmit
antennas. Use of an equalizer provides some rejection of repeater's output
signal, allowing a DOCR to work with less isolation between antennas than a
simple on-channel booster, simplifying installation and allowing higher
transmitted powers. The requirement to keep the time-delay through the DOCR
short enough to avoid interference with the main transmitter (less than 10
microseconds) in areas complicates equalizer design. This year I saw three
companies showing DOCR products on the show floor.
AXCERA AND LINEAR
Axcera builds its DOCR product on its Innovator CX platform. The Innovator CX
can be ordered as an LPTV transmitter, distributed transmission system (DTS)
transmitter or as a DOCR. The DOCR version offers an echo-cancellation option,
which allows the unit to operate with even a net system isolation (coupling
ratio) of -1 dB, the amplitude of the feedback from the repeater's output at
the input to the repeater can be as little as 1 dB less than the signal to be
repeated.
Linear showed the ATG20P ATSC Gap Filler and announced a deal with Teamcast to
manufacture Teamcast's GFX-0300 repeater module at Linear's new plant in
Brazil. As with Axcera's gap filler, Linear's ATG20P works with a coupling
ratio of -1 dB.
In a presentation to the Ohio Association of Broadcasters, Linear Vice
President of Sales Perry Priestley provided additional specifications. Required
receive level is nominally -57 dBm with a range of -40 dBm down to
approximately -75 dBm. MER degradation is less than 2 dB. The delay through the
unit varies depending on required adjacent channel selectivity. When adjacent
channels are present, the transit delay is 6 microseconds. Linear's gap filler
supports output powers up to 30 watts.
HUTECH21 EDOCR
Hutech21 had some of the most interesting DOCRs in their small booth in the
back of the Korean Pavilion. Their DOCRs operate at power levels from 60 mW up
to 100 watts. According to Hutech21, feedback signal-canceling technology
allows the 20-100 watt DOCRs to cancel a feedback signal that's 20 dB stronger
than the desired signal.
This means that for a given receive signal and amount of receive/transmit
antenna isolation, the output power can be significantly higher than with the
Axcera and Linear gap filters.
The 60 mW unit comes in a weatherproof enclosure designed to be mounted
outdoors. It can repeat up to 5 channels anywhere in the current 228 MHz wide
UHF broadcast band if a broadband antenna is used. Cell phone companies have
found micro-cells and repeaters are necessary to provide coverage in locations
like subway platforms and large shopping malls. Broadcasters will need to do
the same to provide ubiquitous mobile DTV service. I can see using a unit like
this mounted on the roof of a building feeding antennas inside or mounted to a
light pole above a subway station feeding antennas on the platforms.
The website, www.hutech21.com, did not have any information on ATSC products
when I checked in May. The company has experience in building repeaters for
wireless broadband and shows Sprint and Clearwire as customers. That experience
could be useful in helping broadcast engineers design systems for indoor
coverage.
Hutech21 didn't provide detailed information on the feedback signal-canceling
circuitry, but I suspect it is similar to the method described in the paper
"Feedback Cancellation with a Pilot-Free Reference for ATSC Terrestrial DTV
Repeaters" co-authored by Ki-Hwan Suh, Young-Jun Lee, and Hyoung-Nam Kim at
Pusan National University and Sung Ik Park, Ho-Min Eum, and Heung Mook Kim at
the Electronics and Telecommunications Research Institute (ETRI) in Korea. The
paper was presented at the IEEE Broadcast Symposium last October and is
available in the proceedings.
DOCR SYSTEM DESIGN
DOCRs provide a way to fill in coverage in locations shadowed by terrain or
buildings. In my November 2, 2009 RF Technology column I showed how a DOCR
could improve ATSC reception in downtown Los Angeles. That hypothetical system
used a transmitter with 20 watts output and an effective radiated power of 473
watts. Let's calculate how much isolation will be required between the receive
and transmit antennas.
Using SPLAT to calculate the path loss and received power, I found that KNBC
should place a -30 dBm signal into a dipole at the top of the Arco Tower, the
location of the repeater. We will want to use a highly directional antenna to
minimize pick up of the transmitted signal and of reflections from surrounding
buildings.
For this example, we'll assume a gain over dipole of 13 dB, resulting in a -17
dBm signal at the antenna. Even after subtracting 3 dB for line loss, at -20 dB
the signal at the input of the repeater will be much higher than the -57 dB
level recommended. Add a 30 dB attenuator to drop it to -50 dBm.
In this example, the output of the DOCR is 20 watts, 43 dBm. Antenna gain,
after subtracting line loss, is 13.74 dB, resulting in a maximum effective
radiated power of 56.74 dBm (473 watts).
Linear-ATG30P ATSC Gap Filler
First, calculate the gain through the DOCR. With -50 dBm in and +43 dBm out,
DOCR gain is 93 dB. Adding the 30 dB attenuator on the input drops it to 63 dB.
Next add in the antenna gain minus line loss. Receive antenna gain, after
subtracting 3 dB cable loss, is 10 dB. Adding the 13.74 dB of transmit antenna
gain results in a total gain of 23.74 dB, increasing the required isolation to
88 dB for a feedback level equal to the received signal.
Because the received signal can fade, the repeater system must be designed with
sufficient margin. The HT-DOCR5U manual recommends 15 dB. For the Axcera or
Linear DOCRs, the feedback signal must be at least 1 dB below the received
signal. Adding 15 dB for margin, these units will require 104 dB of isolation.
Due to its feedback-cancellation circuitry, the Hutech21 HT-DOCR5U repeater
will work with feedback signals 20 dB above the received signal. After
subtracting the 15 dB of margin, 83 dB of isolation will be needed in our
hypothetical configuration.
Fortunately, it is possible to achieve this much isolation. Reviewing papers
presented at IEEE on DOCRs I found the Communications Research Centre Canada
(CRC) low-power DOCR tests were done with antenna isolation around 85 dB. In
Perry Priestley's paper mentioned earlier, the DOCR test configuration in
Baltimore had 110 dB isolation between antennas. Directional antennas have deep
nulls. Careful positioning of the transmit and receive antennas will
significantly increase isolation.
As mobile DTV grows in popularity, viewers will want to watch it in locations
TV transmitters don't reach today. The potential market for digital on-channel
repeaters is huge. For example, consider five stations in the top 50 markets
that roll out mobile DTV. It is likely at least 10 locations in each of those
markets will need a either a DOCR or translator (assuming frequencies are
available). This works out to 2,500 repeaters. In reality, large markets are
likely to need more than 10 repeaters and more than five stations (assuming the
FCC hasn't taken away all the spectrum) are likely to be transmitting MDTV. In
this scenario, the number of repeaters required at all power levels could
increase to 10,000 or more, assuming one repeater per channel per location!
Distributing signals indoors is a lot different from transmitting at high power
from a single stick. Fortunately wireless carriers have already had to deal
with this and I suspect there is a lot we can learn from them. I'll have more
on indoor repeaters in future columns.
Comments are welcome! E-mail me at dlung@xxxxxxxxxxxxxxxx
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:
- Using the UNSUBSCRIBE command in your user configuration settings at
FreeLists.org
- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
unsubscribe in the subject line.
Other related posts:
- » [opendtv] ATSC Digital On-channel Repeaters at NAB Show - Manfredi, Albert E
