The world of IBOC encompasses AM and FM operation. At the present
time, AM and FM IBOC operation is in use by a number of stations. The
use of AM IBOC at night is still under discussion due to a possible
problem with skywave signals.
FM IBOC recently received a boost with the introduction of new
antennas capable of dual transmitter operation and not requiring
expensive and elaborate combiners. The FCC is expected to issue new FM
IBOC antenna rules soon.
Table 1. The FCC-required channel occupancy.
Figure 1. Hybrid operation with analog main channel.
Figure 2. Extended hybrid operation with analog.
The FM IBOC system offers four basic services in a single FM
assignment. The main program provides audio, the personal data service
offers miscellaneous data service as required by the user, the station
identification service is provided by a third channel and the fourth
auxiliary application service provides a broad choice of specialized
The IBOC system for FM as developed by Ibiquity is a fascinating
engineering feat. A tremendous amount of engineering design was
involved in complying with the FCC's strict channel width requirements
within which Ibiquity had to stay and still produce the desired
results. The FCC's rules require that all IBOC signals stay within the
FM emission mask in accordance with the commission's rules.
The sum of all parts
IBOC FM is a unique combination of many systems. In fact, multum
in parva is the best way of describing its many parts. The complete
operation is organized into five layers. Each layer performs a
specified job, or jobs and the results in a data transmission system
(including music and speech) that can be considered free from
interference and possible errors within the limits of the transmitter's
The FM IBOC system has been developed as a series of layers, each of
which performs a specific operation. From a standard broadcaster's
point of view, Layer Five is probably the most important. This is the
speech input layer where audio is received, processed if necessary and
passed on to Layer Four where it is encoded for digital transmission.
The channel encoding counteracts the normal RF transmission
imperfections, which can include fading, interference and atmospheric
noise. Forward error correction (FEC) adds correction bits to the
pretransmitted signal with bit redundancy being used to improve system
resistance to errors.
Layers Two and Three perform the multiplexing, which is critical to
the success of the IBOC system, and Layer One determines the format to
be used for the actual transmission of the signal.
A logical channel is used to describe the optimum circuit path laid
out for a signal to follow on its way through Layer One. Because the
IBOC system accepts many different types of signal 10 logical channels
are provided. Not all of these are used all the time depending on the
transmission needs of the audio input. Four of these are classed as
major logical channels, and the remaining six are used only with
digital waveforms. A channel listing protocol has been established that
provides a guide to the type of service required.
The waveform of the transmitted signal is different from anything
heretofore observed in the FM band. IBOC offers different systems or
protocols: hybrid, expanded hybrid and total digital. The three
waveform characteristics, or spectra, are basically similar but have
some clearly defined differences. However, each system's spectrum is
divided into a diverse number of sidebands, which represent different
orthogonal frequency division multiplexing (OFDM) subcarrier
All three of the FM IBOC transmission systems use OFDM for the
digital portion of the signal. It uses the properties of OFDM, which is
a parallel modulation system in which a number of narrowband
subcarriers called partitions are simultaneously modulated. These
narrowband sub carriers produce resistance to multipath, fading and
interference because they are transmitted at a comparatively low symbol
speed and provide a comparatively long information holding time.
One of the major differences between IBOC and other single carrier
digital systems is the use of frequency partitions. Each partition
consists of 18 data subcarriers and one reference subcarrier. The
manner in which the subcarriers are handled depends on whether the IBOC
system in use is the hybrid, extended hybrid or all digital.
The first two waveforms, hybrid and extended hybrid use an analog FM
signal and differ in the sideband usage. As its name implies the
all-digital system does not use any analog signal at all. The bandwidth
of the sidebands from the main digital signal is expanded and lower
power secondary sidebands are inserted in the space formerly used by
the analog signal.
The analog audio channel can include stereo and SCA operation.
Located on each side of the analog audio signal are the primary main
sidebands. The sidebands consist of 10 partitions, which may be
allocated among the various subcarriers. Two additional reference
subcarriers are included in these sidebands.
Extended hybrid operation
To operate in the expanded hybrid condition OFDM subcarriers are
added to the primary main sidebands in the normal hybrid configuration.
As many as four frequency partitions can be added between the edge of
each primary main sidebands and the analog signal. This is called the
primary extended PX sideband. Figure 2 shows the spectrum in the
extended condition and shows four additional subcarriers. The channel
width is greater but still within the FCC's requirements.
All-digital FM operation
Figure 3. All-digital spectrum with full channel occupancy.
The IBOC all digital mode produces a complex waveform. By disabling
or removing the analog signal ample space is left to insert the primary
digital sidebands and lower power secondary sidebands. Extended
frequency partitions are present in this operation. Each secondary
sideband also has 10 secondary main and four secondary extended
frequency partitions and the secondary main frequency partitions are
closer to the channel center.
In addition, each secondary sideband has a small, protected area
located where there is least likelihood of interference from adjacent
analog or digital signals. In this area are 12 OFMD subcarriers and one
reference subcarrier. The center of the channel carries one more
reference subcarrier. The resulting channel spectrum is symmetrical
with several reference subcarriers available to ensure correct decoding
in the receiver.
The frequencies span of the complete spectrum is 3.96803MHz, which
falls completely within the commission's general FM requirements. The
total average power of a primary digital subcarrier is at least 10dB
above the total power of the hybrid primary digital subcarrier.
Thanks to Jeff Detwiler of Ibiquity for providing information for
E-mail Battison at email@example.com.