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Perform virtual field tests on DVB-T receivers

Posted: 18 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:RF testing? GPS? DTV? DVB-T receiver?

One of the most important features of a DVB-T receiver is the performance of the RF front-end, principally consisting of a tuner and the coded OFDM demodulator. The ability to successfully deliver video and audio elementary streams to the user is the acid test that fuels return rate and churn. Good receiver sensitivity is important as DVB-T signal coverage is still marginal, compounded by inefficient analog antennas serving most consumer households. The low antenna gain of increasingly popular mobile devices is also a challenge to receiver designers. With TV spectrum consisting of both analog and digital channels, along with multipath effects from single frequency networks, the successful reception of a channel is hampered by multiple impairments. Front-end receiver performance has typically been validated through limited lab testing to specifications and patchy field test measurements.

Typically, receivers are validated in lab conditions to RF test specifications such the UK D-Book 5.0, the Scandinavian NorDig 1.03 and sometimes E-Book or the Italian DGTVi requirements. However, these specifications test only one type of signal impairment, either an echo or interferer at any one time. Real-field conditions are often a combination of impairments like fading, impulse noise, echoes and multiple interferers. The wide variation of complex waveforms that are actually encountered in the field cannot possibly be modeled with normal lab tests.

Specifications set minimum performance criteria and in practice, there is a wide variation of receiver performance among compliant receivers as the bar is set quite low. Also, test specifications only call for "spot" tests on performance.

Exposing receivers
Field-testing is a method to expose receivers to real RF environments and validate their performance in a consumer setting. The standard approach to field-testing is to send an engineer on a tour of hotels across Europe with a receiver jammed in a suitcase, with larger iDTVs presenting a major challenge. However, the engineer cannot carry all the test equipment required to analyze the RF environments and will struggle to bring any reference receivers for comparison.

Local knowledge of transmission areas is required and getting a reasonable signal in a hotel room can be difficult. The biggest issue is how to reproduce and debug any problem in the field. As new versions of the receiver are developed, it is normally impractical to return to the test sites and the spectrum may have changed. For companies trying to benchmark different front-end configurations or firmware versions, repeatability is very difficult.

In the Advanced TV System Committee world, the use of RF field captures is well known through the A/74 Receiver Performance Guidelines. For the U.S. market, a preselected set of RF recordings have been made that are used to validate RF performance.

However, unlike the homogenous U.S. network, Europe has a wide range of modulation schemes and network topologies. Collecting recordings across 20 European countries from RF trouble spots requires significant investment, local knowledge and time. Presently, there has been no commercially available library of European captures and no reliable services to validate RF front-ends against real-field conditions.

Capturing a library
Digital TV Labs has field tested over 300 sites and developed expert local knowledge of difficult RF conditions. It is constantly collecting recordings throughout Europe, building a library of several hundred RF captures. It also captures each site's RF footprint by recording key parameters such as SNR, BER, modulation error rate, frequency, modulation modes, constellations and echo characteristics of the desired channel, and present any analog and digital interferers. It records the GPS, topography (such as urban, rural, industrial) and weather conditions. Its equipment are capable of 2hr recordings, mobile recordings and measurements for Doppler and fading tests.

Analysis of an RF player output using a professional test receiver is shown.

After collection, RF recordings are analyzed to ensure that they match field conditions by comparing measurements taken in the field to readings taken from the output of the RF capture player. The recordings are benchmarked and the difficulty rated based on the average performance of all receivers tested. The measurements and field collection data are then added to Digital TV Labs' database.

The recordings are graded based on performance of receivers in the field when the recordings were made, and subsequent lab tests. The challenging recordings are incorporated into Digital TV Labs' Virtual RF Test Suite, a collection of the most demanding recordings from across Europe. The test suite enables rapid testing against Europe's RF trouble-spots in a repeatable environment, and evaluation of receiver performance against competitive solutions. The suites available are DVB-T, Analog TV and Mobile DVB-T. DVB-H is in development.

Suite advantages
To validate and benchmark an RF front-end, the RF captures are played using a suitable RF player. The recordings are played at the same frequency and power as the original to fully replicate the field condition.

With each of the recordings, the receiver performance in the normal signal conditions is measured and its minimum signal is recorded. The minimum signal is measured by progressively reducing the signal with a precision in-line attenuator until the quasi error free (QEF) limit is reached. If BER is available, this will also be measured. A test is performed on the DUT, a standard reference receiver and on any comparison receivers. This method allows measuring "margin" of sensitivity as well as assessing normal performance.

The Digital TV Labs Virtual RF Test Suite subjects the receiver to challenging recordings that can differentiate receiver performance in real-field conditions. It can validate silicon tuners, demodulators, set-top boxes, integrated DTVs and even FPGA prototypes.

The captures can be replayed on demand so designs can be debugged and optimized. The Digital TV Labs Virtual RF Test Suite capture library can also be made available to clients who have suitable play-out equipment, allowing European field conditions to be accurately replicated anywhere.

The output of the Virtual RF test is a detailed report containing comprehensive information on the receiver performance; reference receiver performance; and the key parameters of the RF recording. Three key parameters for each capture are recorded such as full power performance, normal power performance and minimum power for QEF point.

Information is provided on the modulation parameters of the channel under test, adjacent interferers, power levels and echo characteristics, as well as measurements. A detailed report is delivered describing how the tests are performed, including graphs of correlated data, comments on performance and recommendations.

Virtual field testing solves the huge problem in validating the RF-performance of an analog, DVB-T or DVB-H receiver in any organization. Within a competitive market, both chip and receiver manufacturers need to augment conformance tests to local specifications with proof of performance in real-field conditions. A variety of DVB-T transmissions across Europe's 45 countries makes in-depth field testing a major undertaking. Because of this, the Digital TV Labs Virtual Field Test Suite enables comprehensive and speedy validation of RF front-end performance.

- Keith Potter
CEO, Digital TV Labs

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