Femtocell Testing

- an overview of the essentials of femtocell test requirements and methods used for different areas of femtocell testing.

With femtocell acess points and femtocell technology in general being widely deployed and used in many areas associated particularly with cellular telecommunications, it is necessary that the femtocell technology and equipment is tested to ensure they meet the required standards.

Femtocell testing can take a variety of forms. In some instances it can be performed on an ad-hoc basis, but in other areas the femtocell testing needs to be undertaken on a more rigorous basis, with some elements of the femtocell test regime being against an agreed standard or specification.

Areas of femtocell testing

There are two main areas where femtocell testing is required:

  • Femtocell access point, FAP
  • Femtocell gateway

Of these the form of testing that is gaining the most publicity is that of the femtocell access point, FAP. The reason for this is that FAPs are available from a variety of different suppliers and they are widely deployed in a host of user's homes and offices, etc. While the performance of the femto gateways is equally important, they are not deployed in the same way.

Types of Femtocell, FAP testing

With many manufacturers now developing and supplying femtocell equipment, it is essential that they operate to their individual specifications and also that they also interoperate within a deployment scenario and not just within the laboratory. To ensure that this can happen, manufacturers perform a variety of different types of femtocell test:

  • Development testing
  • Verification and Validation testing
  • Conformance testing
  • Regulatory testing
  • Interoperability testing
  • Production testing

Each of these types of test has its place in the development of any product, and in this case in the development of femtocells.

Femtocell test for development stages

During the development of any product, test equipment is required to ensure that all the circuitry operates correctly. The femtocell testing during these stages is wide ranging and will involve tests of the digital and RF hardware as well as the functionality provided by the software.

During the development testing stage it is always wise to undertake testing of the product against the formal verification and validation tests as well as the conformance test specification and any interoperability testing (see later) as these will have to be undertaken later. By using the verification, validation, conformance and interoperability tests, the product can be made to work correctly and any bugs removed at this stage. On a similar basis, the regulatory tests should also be undertaken.

Another area of testing that is important is security testing. This is a key element of any femto access point as unauthorised entry into a femtocell could compromise the whole femto network.

Femtocell verification and validation testing

Once the design of the product has been completed, in this case a femtocell unit, it is necessary to ensure that it meets the requirements set down in the development specification, and that it also operates satisfactorily. These tests need to be performed formally to ensure that the development has been satisfactorily completed. Test equipment here will include any equipment needed to test the interfaces to the overall unit.

When undertaking the femtocell verification and validation testing, a matrix linking the tests to the requirements / specification to the test procedures is normally compiled. This ensures that all the requirements are tested, and if any fail, which areas of the specification are affected.

Femtocell conformance testing

When deploying any device into a network, it is necessary to ensure that it operates to the standards. Any problems could cause problems within the overall network apart from the inconvenience to the user of poor performance or completely not working. To ensure that problems of this type are not encountered, conformance testing - testing to ensure that a product conforms to a given standard. Testing of this type is normally undertaken by independent test houses working on behalf of the organisation that owns the standard or on behalf of the manufacturer. When the product passes the relevant test it is allowed to be sold and used. All mobile phones used on systems controlled by 3GPP standards undergo conformance testing.

Currently formal tests are being developed under the auspices of 3GPP for the 3G femto conformance testing. Other bodies are developing test regimes for other technologies including WiMAX, etc.

Femtocell interoperability testing

Interoperability testing differs from conformance testing in that, rather than testing conformance to a given specification, it tests the interoperability with other devices. Two devices may be conformance compliant but may have interoperability issues because of an ambiguity or omission in the standard. In this way, interoperability tends to check the real-life scenarios rather than those generated within a laboratory environment.

With a large number of devices from many manufacturers and owing to the immaturity of the technology, interoperability testing is seen as a key element within the test regime for femto access points.

Ultimately there are plans to set down a formal regime for femtocell interoperability testing, but in view of the fact that femtocell technology is relatively new, this has not been undertaken yet. It is anticipated that reference equipments will be defined along with the methods used to check for full femtocell interoperability including full end-to-end interoperability.

Instead femtocell interoperability testing is being achieved through the organisation of "plugfests" where interoperability can be tested when many manufacturers of femtocell equipment come together and check interoperability in various scenarios.

Key areas for testing include the radio interface along with the Fa interface between the femtocell and the femto gateway (this is the Iu-h interface on 3G femtocells).

Femtocell regulatory testing

It is necessary for femtocell access points, FAPs to undergo regulatory testing and to be passed by the relevant regulatory bodies within the country or territory for which it is intended. There are several regulatory tests that are required:

  • EMC:   It is essential that the FAP undergoes EMC testing. This will ensure that the design is able to operate within the anticipated areas without causing interference to other items of electronics equipment within the vicinity. Both conducted and radiated emissions are checked and for susceptibility and unwanted generation. In particular it is necessary to ensure that the radiated emissions do not fall outside the required mask of what is acceptable as the femto access point but its very nature will transmit a signal.

    The FAP will need to conform to the requirements of the relevant bodies for the territory where the equipment is sold. For Europe the R&TTE directive needs to be met. For North America the unit will need to conform to FCC part 15. Japan is one of the first countries to explicitly address the regulatory issues for Femtocells, with it operating within a licenced band, and they require the FAP to be approved but no licence required for the end user.
  • Radio:   The FAP will need to meet the regulations for the radio transmissions it makes. Again these are R&TTE directive for Europe and FCC part 15 for North America.
  • Safety:   Any electronics equipment that is sold must meet the relevant safety requirements. Although the standards are very similar around the globe, different organisations administer their own versions of the standard and there may be requirements associated with a conformity mark for marketing purposes for example.. Within Europe equipment is required to meet EN 60950-1, North America it is UL 60950-1.
  • Telecommunications:   Many femto access points are designed to connect to a router via an Ethernet connection and these do not require any telecommunications regulatory input. However a growing trend will be to have combined femto access points with a router in a single unit. These complete units will connect into the telecommunications networks and will therefore require telecommunications approval. Again, the final approval is dependent upon the location within the globe. In North America ACTA approval is required against specification TIA-968-A/B. In Europe, the approval is market driven - approval being required from the telecommunications provider.

Production femtocell testing

Once an item is in production, testing needs to be performed on it to ensure that each item leaving the production line performs correctly. The testing undertaken at this point in the process is aimed at checking that the unit is built correctly. As this testing adds no value, but only ensure the product is correct, the testing needs to be as fast as possible while still ensuring that no faulty product leaves the factory. Accordingly this form of femtocell testing should not include any development, verification, validation or conformance testing. Instead it should only concentrate on the tests needed to ensure the product has been correctly assembled.

The type of test equipment used for production testing is quite different to that needed for, say, conformance testing. Tests need to be performed very quickly, and therefore the test equipment is honed to ensure that it can undertake a limited number of tests very quickly.

In order to develop a femtocell or femto access point a considerable amount of testing is required. While this is costly, it saves considerable sums over the long term as the femtocell testing provides a high degree of certainty that the femto access point will operate correctly under the scenarios it encounters.

By Ian Poole

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