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Cellular Phone Conformance Testing

An overview or tutorial about the basics of cellular phone conformance test and the way that GSM and UMTS testing is handled by organisations such as GCF, PTCRB, TIA, etc.


Today, vast numbers of mobile phones are in use around the globe. 2006 saw in well excess of 2 billion subscribers connected and over 1 billion phones manufactured. When users buy phones they expect that the system will work. They are not interested in the reasons why there may be problems, often blaming the phone itself. Poor service, for whatever reason will result in users choosing a different network, and adding to the rate of churn.

Ensuring that cell phones operate correctly when deployed is no easy task. Testing of the design is required at all stages of the development. It is necessary to check the hardware, and software. Once the cell phone is assembled, full testing of the complete "system" is required, testing it against its requirements and specification. Then prior to deployment it must undergo formal testing and be "approved" before it can be used.

Dependent upon the type of cellular network, i.e. GSM / UMTS, or CDMA (cdmaOne / cdma2000) this formal "approval" may take one of two forms:

  • Conformance test
  • Interoperability test

Once the formal approval has been gained, the cell phone can then be manufactured in quantity, sold and deployed on the cellular network.

In many respects cell phone conformance test and interoperability test perform the same function but they have some significant differences between them. A conformance test tests for conformance to a particular specification, whereas an interoperability test checks that the phone will work on a given network. Both conformance test and interoperability tests have the advantages and disadvantages. However a conformance test is required for GSM, UMTS cell phones and generally interoperability tests are required for phones for many of the CDMA networks.


Conformance and interoperability tests

To ensure that a mobile phone meets its required standards it has to undergo a variety of types of test. These are often categorised into different areas. In order to undertake these tests different test house may be required.

  1. Basic safety testing     This is a form of test that every piece of equipment, whether mobile phone or otherwise has to undergo to ensure that it is intrinsically safe to use and no injury will be inflicted for example from sharp edges, etc..

  2. SAR, Specific Absorption Rate     This test involvesmeasuring the amount of radio frequency power that a human head will absorb when the cell phone is transmitting. The test uses an anatomically correct model of the human head. Inside the model temperature sensors are set up to measure the temperature rises to ensure that the heating effects caused by the cell phone fall below acceptable limits.

  3. Protocol testing     One of the major areas of cellular conformance testing is the protocol testing of the cell phone. With the complicated protocols used in mobile phones this is a critical area. If the phone protocol software operates incorrectly then it could result not only in problems experienced by the phone, but also on the network. In view of the complexity of the protocols that are used this testing can be very involved. Specialised network simulators are used. These testers emulate a variety of network entities, i.e. base stations or Node B's (in the case of UMTS), RNCs (Radio Network Controller and the like. In this way a host of scenarios from registration to handover, and in fact any situation that can be encountered can be simulated.

  4. RF testing     Conformance testing also includes testing of the RF signal. Many measurements of the transmitter and receiver performance are undertaken in a variety of areas such as the out of band emissions. Measurements of the Radio Resource Management (RRM) are undertaken to ensure that the control capability of the phone is operating correctly. There are for instance very tight limits on the control of the transmitter output power to ensure that the cell phone radiates only as much as is needed under any given conditions and noise in the phone bands is reduced to the minimum level. To achieve this testing a protocol tester is often used to control the phone and set up the relevant scenarios. In addition to this an RF measurement and generation equipment is required. This is often in the form of additional signal generators, power meters, analysers, noise generators, etc. To check operation of the phone with multi-path and fading, special fading simulators are required.

  5. SIM card testing     Another very important area of cellular conformance or interoperability testing is the operation of the SIM card, or in the case of UMTS the USIM. As SIMs are interchangeable between phones it is necessary to rigorously check the interface. It is also vital to check the security aspects of the operation of the SIM, as lapses in security could compromise elements of the network security. To undertake this testing a SIM simulator (or USIM simulator) is required. This simulator emulates the operation of the SIM, and tests on the phone can then be run using a protocol tester to set up the variety of scenarios that are needed.

  6. Audio tests     Finally audio checks of the cell phone are undertaken. These check the correct operation of the audio aspects of the cellular phone, both in terms of the microphone and the earphone. Checks of audio levels, quality and much more are measured using a variety of audio equipment to ensure they conform to the requirements laid down.

Test cases

It is obviously necessary to ensure that each stage in the testing is repeatable regardless of the test equipment is used and the organisation performing the testing.

To achieve uniformity a large number of what are termed "test cases" are defined. These may be expressed in a number of ways, but typically they may be in prose, or they may be in a form of computer notation called TTCN (Tree and Tabular Combined Notation). In the latter format they can often be compiled directly into code that can be run on a given test system.

For any given standard, i.e. GSM UMTS, cdma2000, etc there are many hundreds of test cases that need to be run. They are prepared under the auspices of the governing body, e.g. 3GPP for GSM and UMTS. Manufacturers of conformance test equipment are then able to take these test cases and convert them to run on their test equipment. Often the test cases must be ratified on a given test system before they can be used towards approval of a phone.


Summary

The process of conformance testing and interoperability testing can be time consuming and expensive. However it is essential because the cost of releasing a phone that does not operate correctly is very much higher. As a result network operators and phone manufacturers alike see the importance of conformance testing and interoperability testing.

By Ian Poole

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