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