Spread Spectrum (S/S) radio modem usage in all aspects of
the traffic industry is increasing on a daily basis. The major benefit of S/S
technology is its ability to be deployed rapidly, without the challenge of
regulatory issues or licensing.
An obvious downside to unlicensed technology is the fact
that there may be operating frequency conflicts between other users within the
same geographical area. These radio frequency conflicts, when they affect the
integrity of a system, are referred to as “interfer-ence” and
depending on the degree of conflict may cause the affected systems to perform
intermittently, consistently poor or in extreme cases, not at all.
Due to the unlicensed nature of the available bands, the
concept of unavoidable interference has been synonymous with the concept of
S/S. The first offerings in the S/S radio modem field left a lot to be desired
in terms of quality, performance and reliability. Unfortunately, many end-users
who tried to apply this early technology in their projects came away frustrated
and disenchanted, citing outside interference as a primary point of system
instability once the equipment was installed in the field. This may have been
the case in some systems, but likely not all.
At the frequencies and power levels where typical S/S
equipment operates, initial RF system engineering, if not performed properly,
can result in RF links that are marginal at the best of times. If RF signal
levels reaching remote sites are borderline, the sites may operate poorly. Poor
construction practices during installation of the antenna system (antenna,
lightning suppression, coax cabling, connectors) also can greatly affect S/S
system performance. In both these cases the resulting symptoms may be similar
to interference symptoms, and outside interference may be unfairly blamed for
poor system performance.
Other performance inhibitors
The actual application also may affect overall system
performance. Most of the S/S radio modems in use today in the traffic control
industry are designed for the transfer of asynchronous serial data. A typical
application would be the interconnecting of a local controller to a system
master via the equipment’s respective RS232 serial ports. This is not
news, of course, but it should be realized that if the local and master
application software are not properly configured to compensate for the
requirements of the radio modem the system communications quality may suffer,
showing symptoms similar to an interfered link.
Having said all this, what typically happens when a new S/S
system is turned up in the field, say in a major city’s downtown core?
If there is no data flowing after system power-up, obvious
checks are made for AC power, data cable connections, antenna connections, etc.
If the system is communicating, but with poor results, a typical first response
is to reconfigure the radio modems, flipping DIP switches or flipping bits to
change center frequencies or hop patterns.
Why? The assumption here is that someone else in the
immediate area is interfering with the system. In some cases this could be
true. In others, time and energy could be invested with frustrating results as
a setting that improves communications quality is not found. Because of a lack
of tools, there may be no idea if the spectrum is shared with others, and if
so, who they may be or what channels they occupy.
With copper or fiber circuits, users cannot see the data
carried with the naked eye, but they can at least see and feel the medium
itself. For most people this is comforting. With RF, however, the medium is
invisible and therefore somewhat intimidating. This medium is an extremely
low-level, high-frequency electrical signal received off-air that may be
competing with many others. What is required to efficiently work with this RF
is a tool that will provide a
visual image of all the in-band energy that is being picked up by an
antenna. With knowledge of what else is out there, the equipment can be
configured to avoid obvious congested frequencies, maximizing the chances for
first-round commissioning success. On the other hand, should system
communications quality be poor with no apparent localized RF interference
noted, other potential causes of poor data integrity should be investigated.
Spectrum analysis
To display the minute levels of RF being received by the
antenna, a device called a spectrum analyzer is used. A sensitive receiver in
the analyzer is constantly swept across a selected portion of the RF spectrum,
with its output displayed on a calibrated screen. The resultant visual image is
an almost real-time snapshot of all the in-band energy being picked up by the
antenna and made available to the radio equipment.
As should be expected in any RF system deployment today, the
desired signal may share prime spectrum space with other users or may just have
to compete in a high ambient noise environment. With the proper RF tools and
techniques users can identify potential problems early on, allowing to design
around, or compensate for, them rather than being taken by surprise during
system commissioning. For any RF system being considered in an urban location,
doing the homework up front can significantly increase the odds for success.
During system commissioning, the analyzer provides a
convenient means of antenna alignment: pan for maximum RF signal from the
master station.
Some sources of interference are of an intermittent nature
or occur sporadically. Most analyzers provide a “peak hold”
function, which is functionally a long exposure snapshot. The equipment would
be left operational at the desired site and would log and display all RF
activity.
Spectrum analyzers are an immensely helpful tool. Until
recently they have been prohibitively costly, particularly for end-users of
small systems. Today, however, this functionality is being designed directly
into some radio modem hardware. Coupled with supplied software running on an
end-user’s laptop, spectrum analyzer functionality is then available at
every site, all the time. TME
RSS: Roads & Bridges Articles
