AIM Wireless Solutions
Find how much RF interference is in
n your coverage area, ……
before and after rrebanding!
The experts say that
it’s not possible, it takes too long, it costs too much, it just can’t be
done! We say, “That’s not true!
“In the field, we know knew where deadspots
exist, where our mobile radios don’t work.
When we have had to pull someone over, we’ll we’d follow him
until we we are
in an area where we know knew our radios will would work!” ” says a former Illinois State Police officer.
“I would throw away my mobile phone, there are so many
times when needed to make a call, and I was not able to use it” says an Elk
Grove, IL police officer.
I can tell you exactly where the deadspots are between
Route 83 and Route 53 along Thorndale Road.
I drive it every day, and I know when and where I can’t use my phone!” says the President of a Bensenville, IL
company.
How many deadspots exist
in a major metropolitan area?
Dear Reader,
PPublic safety
officers know they exist. Commuters who
travel the same route daily know they exist.
Drivers Business people using their cell phones have
lost calls their connection in the middle of an important calldiscussions.
How
many times have you not been able to use your cell phone? How many times have you received complaints
about your mobile radio system? Dropped calls, garbled voices, and poor connections are common problems. When a mobile radio or handset becomes suddenly unusable, the person’s probably in a deadspot! These problems are all symptoms of RF
interference, but it happens so often that many people have begun to accept them as a part of wireless communications instead of a problem that should be fixed!
Somebody should do
something about this!
The plan is in place! To quickly recap what the
Federal Communications Commission (FCC) has put together:
July 8, 2004, Washington D.C.
The Federal Communications
Commission adopts the Report and Order “Improving Public Safety Communications
in the 800 MHz band”, a highly anticipated action that took four years of
deliberation, research and debate from the public safety, wireless providers,
and the engineering, economic, legal and policy communities.
“In this Report and Order, we adopt a two-prong
solution to the public safety interference problem in the 800 MHz band, with
each prong having several components.
First, to more adequately respond to individual interference events
immediately, we establish an objective standard for defining “unacceptable
interference” to 800 MHz non-cellular systems, establish rules and procedures
for the expeditious implementation and enforcement of this standard, and
endorse a variety of technical solutions and mechanisms, defined as “Enhanced
Best Practices” to address interference abatement in the short term. Second, to provide a better spectrum
environment for public safety in the long term, we adopt a plan for
reconfiguration of the 800 MHz band and provide a thirty-six month transition
by incumbent licensees from their current frequency assignments to new
frequency assignments in the band.”[1]
The Rule & Order is a great
achievement by the FCC. It is a major step towards
providing a better spectral environment. A standard for unacceptable
interference is
now established.
Three steps make up the
first part of the Rule & Order, which took effect on January 21, 2005.
“We adopt a new, objective definition of
“unacceptable interference” to determine when public safety and other
non-cellular 800 MHz band licensees are entitled to interference protection”
“We assign strict responsibility for eliminating
unacceptable interference to the ESMR or cellular telephone operator(s)
implicated in the interference occurrence, and assign joint responsibility to
all involved commercial operators if unacceptable interference results from a
combination of signals from multiple systems.”
“We require ESMR and cellular telephone licensees,
on request, to notify public safety and CII licensees prior to activating new
or modified cells, and require public safety and CII licensees receiving such
information to notify ESMT and cellular telephone licenses of changes in system
parameters.”[2]
The second part of the
ruling involves the reconfiguration of the 800 MHz band to separate public
safety, critical
infrastructure industries (CII), and other non-cellular systems on one hand, and
ESMR systems, such as Nextel’s, on the other. This too, will move forward, with Nextel’s acceptance
of the terms of
the plan on
Monday, February 7, 2005.
“The method of interference abatement we adopt
herein leaves to the involved parties…the choice of how to best ensure that
their systems do not cause unacceptable interference……to the extent that
interference results from the combination of signals from multiple
transmitters, and potentially multiple licensees, we place joint responsibility
on such CMRS licensees to eliminate unacceptable interference using the
remedies of their choice.”[3]
Our past experience in
interference analysis shows that the problem is not always caused by Nextel. Now that interference reporting will require a response, proper analysis will tell
us who needs to take action. Wireless
providers are on notice to keep from interfering with the networks used by
Public Safety and the Critical Infrastructure Industries.
The Transition Administrator is moving ahead with their plans. The first wave of
reconfigurations will start June 27, 2005.
Over the
next three years, the entire process of rebanding should finish by 2008, if everything goes smoothly!
Just one question left…….
Just how much interference is out there?
What is a drive test?
That’s a big question!
Finding and resolving
interference is
now a high priority, a mandatory action for
certain parties. Interference abatement has
moved to the forefront of regulatory attention. Effective methodologies and tools to
deal with the problem need to be put into use now!
A
Traditionally,
a “fileeld trials” or a “drive test” can help locate deadspotsis done.
For some engineers, this means going
out with a handset and making a series of calls while driving down the highway. The engineer and logginglogs when a call can or can’t be made. The report is simplevery basic. , aA list is compiled with of locationsstreet or highway
intersections, the
time of day,
and whether the
call cois complete, call or incomplete. It is systematic in its process, and used by Department of Transportation engineers
in different states.
At anotherOn a more advanced level,
a scanner is used on the drive test. The
scanner detects what signals exist in the environment. This captures each signal’s “measured” field strength, which is the basis for determining unacceptable interference as specified in the 800 MHz Rule &
Order. The scanner creates a, and the
scanner creates a text file that contains more information
about the environmentthose signals and that file can be transferred to a PC
or laptop. The necessary
informationdata needed for an interference analysis is
transferred to a using a
manual “cut and paste” into an Microsoft
Excel spreadsheet using
a manual “copy and paste” operation. That spreadsheet contains the formulas of calculating
interference,
and uses the data
from the scanner
to solve for interferencewith formulas. It’s a proven method for mathematically calculating interference.
If a few channels are analyzed, the calculation time to do the study can be quick. When hundreds of channels are involved, Ssolving the
equations can could take a while, ; perhaps a dayhours, perhaps or even all
weekend. Sometimes it never finishes
because the computer ran out of memory to finish the problem. This is usually seen on
Monday morning, along withSeeing the infamous “blue screen of death” on the computer monitor on Monday morning is not an uncommon
occurrence!
The limitation of this method is the computing
performance of Microsoft Excel or of the computer, not the methodology!.
Is there
a betterCan
the performance barrier be broken? way?
We did a drive test of 26
miles of highway in the Chicago. area, aAt 5:00pm on a busy
Friday afternoon. W, we loaded up our
equipment, an Andrew Scannerscanner, with 800 MHz receivers, a GPS
antenna and AIM software on a laptop. We started in Hoffman Estates,
Illinois by Barrington Road and I-90. We drove east towards downtown Chicago, through busy Schaumburg
rush hour traffic, through Arlington Heights, past O’Hare airport near Rosemont
and Des Plaines, eastbound south on I-94 through the heart of the
downtown Chicago and back west on I-290.
We got back at 8:00 pm..
At By the end of the
drive, we had collected xx 40 MBGB of
data and more
than 120,000
GPS records..
Every 300 feetAs we drove, the
scanner recorded what frequencies and their corrosponding signal strength in the
800 MHz band were being usedin the environment. It captured the corresponding
signal strength and in the area the latitude and longitude for that spot. We captured every frequency in the 800MHz
range, along with the RSSI strength. (etc, etc.). Sampling
data every 300 feetThe 800 MHz Rule & Order specifies a minimum measurement area of 300 ft. x
300 ft. To meet that requirement,
we’ll do an interference study every
300 feet along that 26 mile route.
gave us 461 separate
sets of data for the 26 mile routeHow many studies is that? A quick bit of math tells us that (26 miles x 5,280 feet) /
300 feet/mile = 461 studies!
,
uniquely identified by the latitude
and longitude.
What did the drive test tell us?
What did we find?
We picked out seven
frequencies belonging to an Illinois Public Safety Agencyagency as an example. We wanted to see how the mobile radios for
this Public Safety agency would be affected along this 26 mile stretch of
highway. We did an interference study
for the seven frequencies against all of the other 800 MHz frequencies that the scanner had
detected, every
300 feet., for every 300 ft along the drive, a total of 461 studies.
For those seven frequencies belonging to an Illinois
Public Safety agency, , we found 477861 interference
points in 13
unique locations along a that 26 mile route. 78Sixty one47
incidents where the interference level (C/I) was less than 20db!
We know that aAt these locations, their
peopleofficers
in the field would be unable to use their mobile radios, to receive a call
or make one. ThThat is an
average of one location
every 2 miles! Not an acceptable
situation for a public safety official!. Look at the cFigurehart
below
for1
for a detailed
look at the results of the analysis.
|
Interference
type |
Intermodulation |
Wideband / TX noise |
RX
Spurious noise |
Interference points |
||||
|
RX (MHz) |
C/I < 20 dB |
C/I % |
C/I < 20 dB |
C/I
% |
C/I < 20 dB |
C/I % |
Total |
Total
(%) |
|
866.xxxx |
4 |
0.9% |
2 |
0.4% |
2 |
0.4% |
8 |
1.74% |
|
866.xxxx |
4 |
0.9% |
3 |
0.7% |
2 |
0.4% |
9 |
1.95% |
|
866.xxxx |
4 |
0.9% |
2 |
0.4% |
2 |
0.4% |
8 |
1.74% |
|
866.xxxx |
5 |
1.1% |
3 |
0.7% |
2 |
0.4% |
10 |
2.17% |
|
867.xxxx |
4 |
0.9% |
2 |
0.4% |
2 |
0.4% |
8 |
1.74% |
|
867.xxxx |
4 |
0.9% |
4 |
0.9% |
1 |
0.2% |
9 |
1.95% |
|
868.xxxx |
4 |
0.9% |
3 |
0.7% |
2 |
0.4% |
9 |
1.95% |
|
Total |
29 |
6.3% |
19 |
4.1% |
13 |
2.8% |
61 |
13.2% |
ChartFigure 1 1. Summary of interference analysis on I-90 chicago
area
(chart)
ChartsNumbers can can be hard to
interpretvisualize,
especially if you’re not an RF engineer.
A graph of
these results is shown in Figure 2. It shows