[opendtv] Doug Lung: DTV Coverage Problems
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 11 Aug 2009 12:08:28 -0400
In this piece, Doug Lung makes the case for low-powered, on-channel gap
fillers, used in conjunction with a big stick. As far as I can tell, at the
power levels he's assuming here, most of these never need to be synchronized
with the main transmitter. They're just passive repeaters. (I suppose that
could be called passive synchronzation, then.) He says:
"If the transmitter power can be kept around 30 watts and the site is in a
strong signal area, an echo-canceling digital on-channel repeater may be able
to be used, eliminating the need to install a microwave or fiber to the site."
(Or GPS synchronization.)
Two cool tricks he mentions. One is to locate the gap filler downstream of the
obstruction from the main transmitter, then aim the signal back toward the main
transmitter. This is good because it fills in the signal where it's most
needed, yet reduces the signal strength of the gap filler when you get further
away from the obstruction. Where the main signal is strong again.
The second is to use vertical polarization for the gap filler. This could be
useful to reduce the gap filler's impact on outdoor antennas, in areas where
the signals from both transmitters are weak.
On this quote:
"When the two signals are within a few microseconds of each other, interference
is predicted when their amplitude differ by only 1 or 2 dB. Fortunately, it
isn't easy to match levels that closely."
I think what he's saying is that interference is predicted when the two signals
are within more than a few microseconds from each other, and close to the same
strength. For example, three 0 dB echoes 1 usec apart might be okay with modern
receivers(Brazil E), but three 0 dB echoes 15 usec apart may not be okay.
That's where synchronization is needed. Depending on receiver goodness.
Bert
-------------------------
http://www.tvtechnology.com/article/85186
DTV Coverage Problems
by Doug Lung, 08.11.2009.
I've written about distributed transmission systems (DTS, sometimes referred to
as a single frequency network, SFN) before, but thanks to better software
modeling and experience with real-world systems in Las Vegas and New York, I
now have a better understanding of how these systems work. This month I'll
focus on a DTS application I haven't covered before-relatively low-power
synchronized transmitters-and discuss how they can improve reception in the
middle of a station's primary coverage area.
WHERE WILL MORE TRANSMITTERS HELP?
While it may seem strange to add a transmitter in the middle of an area where
coverage already exists, there are situations where it can help improve
coverage.
Indoor reception of DTV, especially at VHF frequencies, can be difficult. A
prime example of this is in communities 30 miles or more from the transmitter.
Outdoor reception should be fine in these areas. Indoor reception may work if
the antenna is near a window, high enough, and facing the right direction, but
it won't be easy. A second transmitter with an ERP of only 1 or 2 kW should
provide plenty of signal over a radius of 1 to 2 miles for indoor reception.
More power will cover a larger area.
Many stations planning to initiate mobile DTV service may be using a
horizontally polarized antenna. Adding a vertically polarized signal from
additional transmitters provides a way to improve reception on mobile antenna,
likely vertical whip antennas, and on handheld devices where the receiver's
antenna polarization will vary.
FCC low-power rules, Section 74.750(f), allow horizontal, vertical or circular
polarization. This would apply to low-power on-channel repeaters licensed under
Part 74. For a DTS licensed under Part 73, a waiver may be needed for vertical
only polarization. Vertically polarized transmit antennas offer another
advantage-reduced interference to viewers' outdoor antennas in weak signal
areas.
Finally, it isn't unusual to find some small shadowed areas in the middle of
otherwise strong signal areas. Look at the Google maps coverage map I created
for WNBC (zoom out if the overlay colors disappear).
You will notice there are many pockets of predicted weak signal due to
obstruction by buildings and hills. Even in Los Angeles (see
http://dtvcoverage.xmtr.com/knbc-dt.html) there are areas obstructed by terrain
(parts of Hollywood and Culver City) and by buildings (Century City). Low-power
transmitters could fill in coverage in many of these areas.
FILLING IN THE HOLES
I'd like to see broadcasters be given the flexibility to use low-power (maximum
30 to 50 watts transmitter output) on-channel repeaters inside their coverage
area with streamlined FCC processing, perhaps registration-only with
notification to adjacent-channel stations, to fill in these coverage holes when
and where needed.
Firing up a fill-in transmitter inside the coverage area of the primary station
takes careful planning, particularly if high power is required. The first step
in the system design is to determine the desired signal strength in the area to
be covered. If the goal is indoor reception, I'd recommend aiming for at least
88 dBµV/m.
On the other hand, if indoor or mobile reception is not critical and viewers
are used to receiving weaker signals on outdoor antennas, a level of 68 dBµV/m
or even less may be sufficient. The next step is finding the optimum
transmitter location. If a strong signal is needed over a small area, putting
an antenna on a tall building or hill in the middle of the area may work. This
approach works well in congested urban areas to boost signal in the "canyons"
between buildings.
When determining the best transmitter location to boost signal in an urban
area, consider the "wrong side of the street, wrong side of the building"
problem. Locating the transmitter at the edge of the area with the antenna
aimed back towards the primary transmitter site may help in this situation.
If the weak signal is due to terrain shielding, the ideal location is likely to
be near the top of the terrain creating the shadow. This allows the use of a
narrower azimuth pattern, reduced transmitter power and perhaps an on-channel
digital repeater.
The antenna's azimuth pattern should concentrate the signal over the area of
interest. In congested areas, the elevation pattern is particularly important.
If the signal close to the transmitter is too strong, it could interfere with
adjacent channel stations. If it is located on a high-rise building, the
antenna will need to be located near the edge of the building or on a tower on
the roof and use enough beam tilt to provide a good signal to the streets below.
Once the location and transmit antenna pattern are known, the effective
radiated power needed to provide the desired signal strength can be determined.
If the transmitter power can be kept around 30 watts and the site is in a
strong signal area, an echo-canceling digital on-channel repeater may be able
to be used, eliminating the need to install a microwave or fiber to the site.
Last but not least, interference from the fill-in transmitter to the primary
signal and to adjacent channel stations has to be calculated. For interference
to other stations, the FCC requires using the root-sum-squared method to
combine the signal level from all transmitters in a DTS licensed under Part 73.
I didn't see any provision, however, for combining signal strength from an
on-channel digital translator licensed under Part 74 and a primary station
licensed under Part 73. Self-interference is more difficult to calculate
because it varies depending on the receiver equalizer range and type of receive
antenna.
I've found it handy to map interference outside the equalizer range by looking
at the difference in signal strength between the stations needed to allow
reception. If a DTS is used, the timing can be adjusted to minimize
interference but if the transmitter is located inside the main coverage area it
won't eliminate it everywhere.
The antenna or power levels may require adjustment to reduce interference. If a
digital on-channel repeater is used, the only handles for controlling
interference are location, antenna and power level.
When the two signals are within a few microseconds of each other, interference
is predicted when their amplitude differ by only 1 or 2 dB. Fortunately, it
isn't easy to match levels that closely. In testing reception in an area where
such interference was predicted from the WNJU transmitter at 4 Times Square and
the high-power transmitter at West Orange, none was observed. Indeed, turning
off the 4 Times Square transmitter caused reception to fail at low antenna
heights. Based on the low probability that signals will be close enough in
amplitude to cause interference and the ease of fixing it if they are-move the
antenna a few inches; interference in this region can usually be ignored.
Interference from fill-in transmitters is more likely in more distant areas
where the primary transmitter is shadowed and the fill-in signal is outside the
receiver's equalizer range. Before getting too concerned about this
interference, remember the primary signal was already too weak for indoor
reception in the area and a directional antenna should provide the gain and
rejection needed to eliminate any calculated interference.
REAL WORLD EXAMPLES
How does it work? The DTS demonstration at the 2009 NAB Show I mentioned in
June's RF Technology column showed multiple transmitters can improve coverage
in urban areas without causing interference.
The main fill-in transmitter was located on top of the Stratosphere Las Vegas
Hotel and operated at an ERP of 1,000 watts. It used a 200 watt Rohde and
Schwarz transmitter and a simple antenna array composed of two vertically
polarized Scala log-periodic antennas with a maximum at 189 degrees.
Offset mechanical beam tilt was used to maximize the signal over the Las Vegas
Convention Center area while providing a strong signal down the Strip. The
primary transmitter operated at 230 kW ERP from Black Mountain using horizontal
polarization only.
From previous articles on DTS, you'll realize this system violated one of my
rules for DTS-minimize signal overlap in the area between two transmitters.
This system worked because the fill-in transmitter ERP was low and it was
vertically polarized.
Timing difference increased in locations further south towards the main
transmitter, but its much higher power allowed receivers to treat the weaker
Stratosphere signal as noise. Interference was predicted in a weak signal
shadowed location west of Black Mountain, but the signals were so weak anyone
would require an outdoor antenna to receive KBLR. Due to the use of vertical
polarization on the Stratosphere and the angle between the sites, interference
was unlikely.
I created the "KBLR-Stratosphere Signal Difference and Delay" map using SPLAT
GIMP and some PERL scripts. The difference between the two signals is shown in
colors ranging from green (16 dB) to cyan or yellow when the difference is only
2 dB. A gray-scale overlay shows the timing difference between the two
transmitters, with the bright areas and narrow bands in the middle showing
delay under 10 microseconds and the broader bands representing 20-microsecond
steps out to 70 microseconds. This figure is based on a 30-microsecond delay in
the signal from the Stratosphere. The PERL scripts can be downloaded from
xmtr.com/splat/perl/.
How did it work? No interference was reported. LG supplied a handheld mobile
DTV receiver that was used to check the signal throughout the Hilton and
Convention Center. In multiple locations in the South Hall meeting area,
reception was poor or non-existent until the Stratosphere transmitter was
turned on. Without out the Stratosphere transmitter, conventional ATSC
reception was impossible in the meeting rooms; the signal couldn't even be
detected. With the Stratosphere transmitter, KBLR could be received on a USB
tuner with a whip antenna, although it wasn't perfect.
The mobile DTV signal was rock solid on a Pixtree USB receiver. Along the
strip, I heard the mobile DTV signal was being received reliably deep in
hotels, even at ground level.
E-mail me at dlung@xxxxxxxxxxxxxxxx
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