Repacking will increase interference because there will more transmitters per channel

CHARLES W.
RHODES 
The FCC has recently expressed concerns
over how they will repack the
UHF band after the spectrum auction
now set for 2015.
Their mandate from
Congress is to protect
broadcasters from cochannel
interference
(CCI) and from firstadjacent channel interference
(ACI).
DTV broadcasters
have never been protected
from interference on what were
called “UHF taboo” channels (channels
N8 to N2 and channels N+2 to N+8 and
also channels N+14, N+15. This is because
although the FCC considered taboo channel
relationships for interference into DTV,
“the D/U [Desired to Undesired] ratios (approximately
–60 dB) were such that they
rarely if ever had an effect on the results,
and the FCC rules adopted in the Sixth Report
and Order do not require attention to
UHF taboo interference to DTV stations,”
according to FCC/OET Bulletin #69.
TABOO CHANNEL INTERFERENCE
I’ve researched the record that suggests
that taboo channel interference to
DTV reception would entail a D/U ratio
for a receiver of –60 dB. At the Advanced
Television Test Center (where I served as
chief engineer), we measured the maximum
undesired (U) signal power in channel
N+2 when the desired (D) signal on
channel N was –68 dBm. We found that
the maximum U power for this prototype
receiver (1995) for N+2 was –8.77 dBm,
so the D/U for the prototype receiver was
–59.13 dB. This is the only UHF taboo test
that gave a specific D/U. The few other
tests conducted support the notion that
–60 dB D/U for this particular receiver
was an appropriate choice if one assumes
that receivers would be built to
perform as well as did this prototype.
The FCC rounded this to –60 dB.

Fig. 1: One U signal two channels above the D signal and –26 dBm power to the receivers 

Fig. 2: Two U signals, two and four channels above the D signal, –26 dBm power each to the receivers 
Consumer receivers do not live up to
the expectation created in the mind of the
FCC by that prototype. Nevertheless, the
commission may still believe that DTV receivers
live up to its 1995 standard.
The FCC ran tests on receivers which
were available in 2005–2006 (Table A4 in
the OET Report 07 TR1003), and found
that the worst receiver tested –28.1 dB for
one U signal on channel N2, and –42.3 dB
for one U signal on channel N+2. The median
receiver was –40.9 dB and the best receiver
had a D/U of –49.7 dB. This should
have sent a message, but it didn’t.
In December 2010, “IEEE Transactions
on Broadcasting” published a paper reporting
on tests conducted by the FCC on the
RF performance of NTIAapproved ATSC
converters. Some of these converters had
a double conversion tuner (DCT) in the
form of an IC built on a silicon chip. Most
used conventional single conversion tuners
(SCT), whose IF is the usual 44 MHz.
These were built of discrete components.
The median SCT converter had a D/U of
–49.5 dB and the median for DCT tuners
was –48.4 dB. These D/U numbers are for
one U signal on channel N2. The best way
to grasp what D/U means is to understand
that a D/U of –49 dB means that if the U
signal received power is 50 dB above the
D signal power, reception fails. If the U signal
is less than 48 dB stronger than the D
signal, the receiver will work.
The above data is for a single U signal.
That will often be the exception, not the
rule, in many communities after repacking.
INTERFERENCE FROM U SIGNALS
ON CERTAIN CHANNEL PAIRS
The FCC tested and reported the effect
of two U signals on channels of the N+K,
N+2K form (the kind which may generate
third order distortion products centered
on channel N). The two most important of
these toxic pairs of U channels are (N+2)
and (N+2, N+4), and (N2) and (N2, N4).
These dump noise into channel N and into
channel N+6 while the other pair dumps
noise into channels N and N6. I consider
these to be the most toxic U channel pairs.
The channel pairs (N+3) and (N+3, N+6)
and (N3) and (N3, N6) are slightly less
toxic.
The FCC test results in 2010 for NTIAapproved
converter boxes had a median
D/U for the (N2), (N2, N4) pair of –43.2
dB. For the N+2, N+4 pair, the median U
was –47.6 dB. These are both a long way
from –60 dB.
As these are medians, it means that 50
percent of these receivers worked at the
stated D/U and 50 percent didn’t.

Fig. 3: Two U signals two and five channels above the D signal and –26 dBm, power each to the receiver
(N+K, N+2K + 1). This test was run on receivers 48–54 only. 

Fig. 4: Two U signals two and six channels above the D signal, each is –26 dBm power to each receiver
(N+K, N+2K + 2) 
The most amazing finding from the
FCC tests of converter boxes is the number
of such toxic channel pairs there are.
The FCC tested with K +/– 2, 3, 5, and 10,
so the toxic pairs included (N+2, N+4),
(N+3, N+6), (N+4, N+8), (N+5, N+10), and
(N+10, N+20). I was amazed that a U signal
20 channels from the D signal would have
any effect at all, but it did. However we
won’t have 40 channels in the UHF band
after repacking, as we only have 37 now.
Our problems lie with N+/–2 and 4, N+/–3
and 6, N+/–4 and 8 and N+/–5, and 10 in
descending order of toxicity.
TEST RESULTS WITH 24 CURRENT
ATSC RECEIVERS
My colleague, Linley Gumm and I have
designed, built, and operated an RF test
bed that can measure the receiver noiselimited
sensitivity of NTIAapproved ATSC
converters and receivers and their robustness
to interference. We have 26 ATSC
converters and 21 DTV modern receivers
(2012 and 2013 models) in this laboratory.
Linley and I tested 24 of these NTIAapproved
converters models and found
that the median D/U for one U on channel
N+2 was –59 dB while for two U on
channels N+2 and N+4, the median D/U
was –47 dB.
These tests involved no double conversion
tuners as most tuners were implemented
as a tuneronachip. They were
single conversion tuners, but with a much
lower IF frequency than we had in the
single conversion converter boxes, so they
could be expected to act differently.
For one U on channel N+2, the median
D/U was –53 dB (see Fig. 1). For two U signals
on N+2 and N+4, the median D/U was
–44 dB as shown in Fig. 2.
We also ran tests on seven modern
receivers for two U signals on channels
N+2 and N+5 (see Fig. 3). For these, the
median D/U was –53 dB, somewhat better
than the really toxic pair N+2, N+4, but
worse than for a single U (N+2) in Fig. 1.
We then tested with a pair of U signals on
channels N+2, N+6 shown (Fig. 4). In this
test, the median D/U was –55 dB, so we
now know that while channel pairs of the
N+K, N+2K kind are the most toxic, other
combinations are also more toxic than is
the case for a single U signal. We did not
test N+2, N+3, or N+2, N+7, but I expect
them to be similar to N+2, N+5. I expect
N+7 to be similar to N+6, but that remains
to be proven. We did not test with the U
channels below the D channel for lack of
costly additional filters, but I believe they
would behave about the same as the channel
pairs we did test.
CONCLUSION
Repacking is going to increase interference
because there will more transmitters
per channel. In my opinion, displaced stations
will be allocated a channel in the lower
portion of the UHF band. Most of these
stations will then be on a channel having a
taboo relationship to other stations in the
UHF band.
If the FCC auction comes up with very
few channels being volunteered, the increase
in interference will be mild, but if
lots of channels are volunteered for repacking
they may create so much interference
that broadcasters face a major problem.
But there is a way out. Interference only
results where there is a large difference between
received signal powers (high negative
D/U), higher than the median D/U of
receivers. Many communities already have
their UHF transmitters colocated on one
antenna farm; these communities may see
no increase in interference. Other communities
have UHF transmitters distributed all
over town and broadcasters serving such
communities are more likely to experience
interference.
If broadcasters centralize their transmitters
they could eliminate this interference.
The strategy to minimize interference is
to reduce the variance of field strengths
between stations to lower the D/U numbers
for field strength towards 0 dB. But
there is nothing wrong with smaller differences
between field strengths of say, 25 dB.
In fact, the FCC rule for adjacent channel
pairs is a D/U of –27 dB (average of Repacking
is going to increase interference
because there will more transmitters per
channel. (–26 and – 28 dB). True, some stations
on adjacent pairs of channel are not
cosited and they are certainly getting by.
There could be two antenna farms serving
the same community if they are within a
few miles of each other.
So, there is light at the end of the tunnel,
and it is not a locomotive. However, this
“light” will come with a price tag attached.
Stay tuned.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email
at cwr@bootit.com.