Bit Error Rate Testing: BER Test

- a summary or tutorial of bit error rate, BER testing and how a bit error rate test, BERT is used in telecommunications, network and radio systems.

Bit error rate, BER testing and bit error rate testers are used for testing systems that transmit digital data from one location to another. When data is transmitted there is a possibility of errors being introduced into the system, especially if the medium over which the data is transmitted is noisy. If errors are introduced into the data, then the integrity of the system may be compromised. As a result a bit error rate test can indicate much about the link quality and the ability of the system to accommodate the link characteristics.

Bit error rate, BER may traditionally be associated with radio communications links, however bit error rate and bit error rate testing is also applicable to other systems such as fibre optic links, Ethernet, or any link over which a digital signal is transmitted.

Unlike many other forms of testing, bit error rate, BER measures the full end to end performance of a system including the transmitter, receiver and the medium between the two. In this way, bit error rate, BER enables the actual performance of a system in operation to be tested, rather than testing the component parts and hoping that they will operate satisfactorily when in place.

In order that bit error rate can be measured easily and quickly, a variety of bit error rate testers are available from a variety of manufacturers. Each tester has its own advantages and disadvantages.

Bit error rate testing

The basic concept behind bit error rate testing is quite straightforward. A data stream is sent through the communications channel, whether a radio link, a fibre optic link or whatever, and the resulting data stream is compared with the original. Any changes are noted as data errors and logged. Using this information a bit error rate can be determined.

The basic concept of a bit error rate test is straightforward, but the actual implementation requires a little more thought, and is not as simple. There are a number of issues that need to be addressed.

As data errors occur in a random fashion it can take some while before an accurate reading can be gained using normal data. In order to shorten the time required for measurements, a pseudorandom data sequence can be used.

To expand the reason for using a pseudo random sequence take the example of a typical data link. To make a simple measurement of the number of errors that take place it is possible to use an error detector that compares the transmitted and received data and then counts the number of errors. If one error were detected while sending 10^12 bits, then a first approximation may be that the error rate is 1 in 10^12, but this is not the case in view of the random nature of any errors that may occur. In theory an infinite number of bits should be sent to prove the actual error rate, but this is obviously not feasible.

As the error rates fall so it takes longer for measurements to be made if any degree of accuracy is to be achieved. For Gigabit Ethernet that specifies an error rate of less than 1 in 10^12, the time taken to transmit the 10^12 bits of data is 13.33 minutes. To gain a reasonable level of confidence of the bit error rate it would be wise to send around 100 times this amount of data. This would take 1333 minutes or about 22.2 hours!

It is clearly not convenient to have measurements taking this long. Accordingly to assist making measurements faster, mathematical techniques are applied and the data that is transmitted in the test is made as random as possible - a pseudorandom code is used that is generated within the bit error rate tester. This helps reduce the time required while still enabling reasonably accurate measurements to be made.

System simulation for BER testing

In addition using a pseudo-random data source, it is often necessary to simulate the transmission path. In this way the BER testing can be undertaken in the laboratory with the transmitter and receiver close to each other. To simulate the transmission path it is necessary to set up a "medium" that is representative of the actual data transmission path to be used. In terms of a radio transmission, this includes noise and propagation fading.

  • Noise:   Noise in the radio path comes from a number of sources. It can be generated either externally to the electronics system itself and comes as received noise, or it may be generated internally, chiefly as noise in the front end of the receiver. The receiver noise will be present regardless of whether the system is in a simulated or real environment.

    The remaining noise can be simulated and introduced to the receiver using a noise diode generator.
  • Fading characteristics for radio communications systems:   It is very important to simulate the real life characteristics of the transmission path in as realistic a way as possible. With signals constantly varying as a result of many factors it is necessary to simulate a this. To achieve this for a radio link it is necessary to use a fading simulator that adds Rayleigh fading characteristics to the signal. A sophisticated fading simulator may also use multiple channels with variable time delays to simulate changing path conditions. Although fading simulators are complicated items of test equipment they are able to give a realistic medium for testing bit error rate, BER within the laboratory.

One of the main precautions when testing BER in the laboratory is to ensure that none of the transmitted signal leaks directly into the receiver and avoids passing through the fading simulator. If the transmitter power is relatively high, then it is difficult to give adequate levels of screening and some of the testing may not be valid. Great care must be taken to ensure that all the signal travels via the fading simulator. Considerable levels of screening may be required. In some occasions screened rooms have been used.

Summary

Bit error rate testing, BER testing is a powerful methodology for end to end testing of digital transmission systems. A BER test provides a measurable and useful indication of the performance of the performance of the system that can be directly related to its operational performance. If the BER rises too high then the system performance will noticeably degrade. If it is within limits then the system will operate satisfactorily.

By Ian Poole

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