Releases Interim FM IBOC Digital Booster Report
report containing the interim results of an FM IBOC digital booster
test program was released by NAB FASTROAD on November 10. This work
has been undertaken by iBiquity Digital Corporation, developers
of the HD Radio IBOC system used by U.S. broadcasters to transition
to digital radio, and is being co-funded by iBiquity and FASTROAD.
Release of this report follows a recent discussion of this topic
by iBiquity at the IEEE Broadcast Symposium, held in October in
Alexandria, Va. (see the October
25, 2010 issue of Radio TechCheck for additional information).
results have been obtained using experimental station WD2XAB (Columbia,
Md., 93.5 MHz, 1.5 kW ERP analog, 150 W ERP digital, 456 ft HAAT)
and booster WD2XAB-FM1 (Kingsville, Md., 50W ERP digital, 446 ft
HAAT) which is located approximately 20 miles away from the main
transmitter. For these tests, iBiquity is using transmission equipment
modified so that, among other things, both transmission signals
have identical frequency and state synchronization so
that the transmitted signals are identical.
Prior to conducting
measurements in the field, iBiquity ran computer propagation prediction
software to characterize the expected reception conditions for the
main and booster transmitters when operated individually, and for
when both signals were present. The first two figures shown here,
taken from Figures 4-1 and 4-2 in the report, show the propagation
study results for each transmitter operating individually as well
as actual measured results obtained on test runs using an IBOC receiver.
The white and
gray squares in the figures are the computer prediction of where
an IBOC receiver would successfully capture the digital signal (white
squares) or where the receiver would have blended to analog (gray
squares). Also shown on each map is the 60 dBu predicted contour
of the booster transmitter. Note that the booster is using a directional
antenna which is not uncommon for booster applications, since boosters
are often placed at the edge of a stations coverage area and
then, using directional antennas, pointed back towards
the main transmitter to fill-in coverage areas that
are lost due to, for example, terrain shielding.
The green and
yellow dots in the figure are actual IBOC receiver performance as
measured during test runs, for the case where the main transmitter
only is on (left map) and where the booster transmitter only is
on (right map). Note that in the main transmitter only case, as
the test receiver moves away from the main transmitter, the receiver
blends to analog (yellow dots) and for the most part stays that
way for each path tested. When only the booster is on, however (right
map), on those same paths there is good digital reception (green
dots) where the receiver had previously blended to analog (for the
main transmitter only case).
In the final
figure, test results obtained when both transmitters were operating
are shown. These results were obtained after the main and booster
signals were time-aligned for best overall performance, as described
in the report. Note that the signal loss near the booster site indicated
on this map was due to an RF overload condition in the test receiver
and was a consequence of amplifiers used in the test setup. An IBOC
receiver being operated normally (not part of an experimental setup)
would not experience a digital outage at this point.
testing at WD2XAB involved both digital coverage testing as well
as analog compatibility testing, to examine the potential interference
from the digital sidebands on the host analog signal near the booster
site. Digital coverage performance results have been very encouraging
and iBiquity reported that no destructive interference of the digital
signals was encountered. Tests have shown that some receivers may
experience interference to the analog host (due to the digital-only
booster) at digital-to-analog power ratios below 0 dBc.
testing is currently underway at Greater Media station WKLB (Waltham,
Mass., 102.5 MHz, channel 273B). As part of these additional tests,
iBiquity intends to experiment with adding a small analog signal
component to the digital booster output to see if this mitigates
any host interference in the vicinity of the booster, while at the
same time not adversely affecting analog reception at greater distances.
(Flexible Advanced Services for Television & Radio On All Devices)
is the technology advocacy program of the National Association of
Broadcasters. The overall mission of the FASTROAD program is to
seek and facilitate development and commercialization of new technologies
that can be exploited by broadcasters using radio and television
broadcast spectrum. A copy of the digital booster interim results
test report is available for download free-of-charge from the FASTROAD
web site at www.nabfastroad.org.