|Charles W. Rhodes
is proposing 11 scenarios
for a 600 MHz Band Plan
following the spectrum
auctions. Which of these
will be adopted will be
determined by the outcome
of the auction in 2016.
Each scenario for the 600 MHz band
starts with Channel 21 (512–518 MHz).
The highest channel number ranges from
26 to 44, depending on how much spectrum
is offered for sale by broadcasters and
then resold to broadband operators. The re-allocated spectrum
is divided into blocks
of 5 MHz each. There
could be from two to 12 pairs
of blocks. Pairs consist of one block
for uplink transmissions
to base stations; and a
second for downlink
transmission by base
stations to cellphones.
I have restructured
the data in Fig. 1 into
Figs. 2 and 3. Fig. 2 shows the number of UHF TV channels
after repacking. It varies from 6 to 23
depending on how many pairs of 5 MHz
blocks are re-allocated after the auction.
|Fig. 1: The graph is Fig. 23 of the FCC's June 2 R&O, page 453. Light Blue: 5 MHz blocks of spectrum. Orange: Channel 37 reserved for radio astronomy and
medical telemetry (hospitals). Diagonally shaded gray: guard bands. Blocks with numbers from 21–36 and 38–44 should be tinted light green. These
are the remaining DTV channels. The numbers in a column on the left side are the amounts of spectrum broadcasters might offer to sell. The amount
of spectrum that can be re-sold to broadband is less due to the need for guard bands in the 600 MHz band.
In Fig. 1, the striped cells
with numbers of MHz; 11, 9, 7 or
3 represent the guard bands. There is an 11 MHz-wide
guard band between the cellphone uplink blocks and the base station downlink blocks. There
are also some smaller guard bands, notably, a
3 MHz-wide one adjacent to Channel 37; and a 7
MHz-wide guard band between some base
station transmit blocks and DTV channels.
These guard bands are vital to protecting against what the FCC calls “Inter-Service
Interference” or ISIX. Broadcasters are
familiar with the fact that a DTV transmitter
radiates power in both channels adjacent
to the channel it is licensed to use.
This is sometimes called sideband splatter, but it is actually third-order inter-modulation
products generated in the high-power
amplifier of a DTV transmitter; which gets
past the transmitter RF mask filter and is
The 3 MHz guard bands on either side of Channel
37 (608-614 MHz), which is reserved for radio astronomy and medical telemetry, protect that channel from ISIX. The total bandwidth of all guard
bands varies among the 11 FCC scenarios
from 14 MHz to 28 MHz. I call such spectrum
“lost” because, by definition, it cannot
be used either by broadcasters or sold to
broadband. Such spectrum is also lost as a
source of revenue to the U.S. Treasury Department.
BEST LAID PLANS
|Fig. 2: Number of DTV channels after repacking as a function of the
number of pairs of 5 MHz blocks available for auction.
I expect there will be pressure by
white space advocates to let these guard
bands also be used for white space services.
That scares me because I recall the
“good ol’ days” of Citizen’s Band mobile
radios. Chaos soon reigned in the CB Band.
If the FCC chooses either eight or
11 pairs of 5 MHz blocks, it will have to
purchase 28 MHz more spectrum from
broadcasters than it will be able to resell
to broadband operators. Since the commission
is mandated not to lose money in
these auctions, it will have to resell this
spectrum at a price well above what it
paid for the spectrum it purchased from
The 2012 law that authorized this auction
requires the FCC to recover all costs
of conducting it and requires
the commission to turn over to the U.S.
Treasury the net profits. I believe some of the billions of dollars
expected from this spectrum auction
will never be realized. I suspect that the
estimated $44 billion of auction proceeds
do not take into account the fact that some
spectrum the FCC will buy cannot be resold
because it must be used as guard intervals in the 600 MHz band plan.
The best case scenario from this perspective
would have the FCC buy 84 MHz
of spectrum and to resell 70 MHz of this
spectrum. The minimum markup would
be 84/70 or 20 percent. Administrative
costs, the $1.75 billion
to reimburse displaced broadcasters,
and the profit for the
U.S. Treasury will erode those
billions of dollars promised to
|Fig. 3: “Lost UHF Band Spectrum” in MHz. as a function of the number of
pairs of 5 MHz blocks available for auction. Note the steep increase in “Lost
Spectrum” between 7 pairs and 8 pairs of 5 MHz auctionable blocks.
Fig. 3 plots the number of
DTV channels in the 600 MHz
band after repacking as a function
of the number of pairs of
5 MHz blocks auctioned by the
FCC. The outstanding feature of
this plot is the steep decline in
the number of DTV channels between
the scenario with seven
pairs of 5 MHz blocks (16 DTV
channels) and the scenario with
eight pairs of 5 MHz blocks. It
would cost broadcasters four channels to allow one additional pair of 5
MHz blocks instead of seven pairs.
When all is said, it appears the best scenario
identified by the FCC would provide
seven pairs of 5 MHz blocks. This view has
also been expressed by a number of cellphone
operators according to the FCC.
The FCC report provides a very complete
analysis of each of the 11 scenarios
depicted in Fig. 1 (FCC 14–50, p. 453). As
Fig. 1 shows, there are guard bands of 3, 7,
9 and 11 MHz between different kinds or
signals. The 11 MHz guard bands between uplink and downlink signals are obviously
needed to keep the transmitter output from
getting into the receiver input. The others
are also required to avoid third-order distortion
products from causing interference.
For example, the 3 MHz band stop filters
keep received signals in Channels 36 or 38
out of Channel 37.
Every kind of filter attenuates every signal
within its pass band (insertion loss).
With each 1 dB of insertion loss, the receiver’s
noise figure increases by 1 dB, and sensitivity
decreases by 1 dB. Worse yet, where
there is more than one filter in the signal
path, the insertion loss of each filter is additive.
In Fig. 1, you will see that each scenario
requires at least one 11 MHz filter and some require two or three filters. Filters not only
cost in receiver performance, they cannot
be manufactured as an integrated circuit
so they take up space and add slightly to
the weight of handheld cellphones. The
fewer the number of filters in
a handheld cellphone, the less
it will cost and weigh, all things
cellphone users can appreciate.
As Fig. 1 shows, the number of
filters varies significantly for the
There is yet another variable
of importance not shown in Fig.
1, but was covered in the FCC report.
The bandwidth over which
the antenna of a handheld cellphone
is efficient varies between
the scenarios in the FCC
Plan for the 600 MHz Band. Antenna
efficiency directly affects
battery life (time between recharges),
as well as the sensitivity
of the receiver. Scenarios that
provide more than eight pairs of
5 MHz blocks will involve these
antenna bandwidth problems. Where the efficiency of a simple passive
antenna is poor (lots of signal bandwidth),
the antenna can be automatically tuned to
improve its efficiency. However this automatic
antenna tuner feature requires added
circuitry and therefore adds to the manufacturing
cost. The insertion loss of this
reduces the power to the antenna, which
translates to increased power drain on the
battery when transmitting. When receiving,
it adds desensitization of the received signal.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email
at [email protected].