Subject: [NEXTEL-1] Methodologies Of OET71 Compliance Testing

From: [email-masked]
Date: Fri, 01 Aug 2003 16:30:35 -0400

Methodologies Of OET71 Compliance Testing
BY JOHN LOVE
AUGUST 1, 2003

Beginning in 1996, the FCC began publishing a series of orders to improve both the
quality and reliability of 911 service for subscribers of cellular service. The
rules seek to provide emergency service personnel as much information as possible
about the caller's location and state the maximum error in location accuracy for
Phase II compliance.
To ensure accuracy, the FCC requires a representative sample of test points be
randomly selected for a market and tested biannually to ensure that a 90 percent
confidence level of success is met. As a guideline, this requires that sites
selected for testing represent the entire market and are chosen in a random
fashion to ensure statistical accuracy.

Balancing the right number of test points to save costs while ensuring that the
entire market and/or network has been sufficiently tested to minimize risk is the
challenge. For clarity, the FCC has published Table 1 in Appendix A of Pub.  OET71
to provide a guideline for wireless carriers in selecting an appropriate number of
test points, and the allowable failures possible in either the 67 percent or the
95 percent range, or both, depending on the number of test points.

Additionally, locations selected within buildings can provide challenges due to a
lack of GPS signal availability or other reference points. Finally, to be truly
representative of the market, sites selected should have a high degree of
probability of generating a 911 call in a real emergency.

Because RF morphology differs between dense urban, urban, suburban and rural
environments, one approach to compliance testing involves segmenting a market
based on morphology. Once a given morphology has been tested and found to be
compliant, incremental test points provide no additional detail.  However,
extrapolating market-wide conclusions about accuracy from data collected within a
single morphology might prove to be inaccurate.

Based on order statistics, to reach a 90 percent confidence level as prescribed by
the FCC, a minimum of 30 test points must be taken at each morphology, assuming
zero errors. For markets with more densely distributed sites per morphology, a
good testing procedure should include a minimum of 45 test locations to allow for
error within buildings. So for a market with three morphologies represented and
500 sites, an operator should plan on testing a minimum of 30 locations in the
less dense morphologies and at least 45 in the more dense morphologies.

Another approach is to weight, by morphology, the number of test points to be
taken as a percentage of the area covered. This number also can be checked and
verified for independence of morphology, as a check on the practical number of
test points. While this approach will generally exceed the minimum number of test
points required by the FCC, many operators feel this approach minimizes risk.

Without enough data to know the standard error based on historical testing, the
exact number of sites to be tested for compliance cannot be known. In the
meantime, as carriers are implementing Phase II systems and reaching their first
milestones of biannual compliance testing, the approach for site selection should
not only ensure compliance is met, but that risk to the operator is minimized.
These goals can only be balanced by choosing a methodology that is representative
of the entire network within practical guidelines and cost constraints. Any
existing data collected from functional testing and refining the methodology over
time should be leveraged to continue to reduce the cost of compliance testing.

John Love is director of product development at LCC International.

http://www.wirelessweek.com/index.asp?layout=article&articleid=CA314166&spacedesc=Departments&stt=000

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