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Coexistence measurements promise excellent car connectivity

Posted: 20 Oct 2015     Print Version  Bookmark and Share

Keywords:Rohde & Schwarz  WLAN  Bluetooth  connected car  coexistence measurement 

To prevent impairment of GNSS signals by LTE, it makes sense to employ early detection of data transmissions in the time domain. For example, if an LTE chip is aware that a GNSS packet with a length of 20ms will be transmitted, the chip can interrupt its own activity for e.g. 15ms so as not to disrupt the entire transmission, while keeping the interruption short enough that it is not perceived by the subscriber.

The receive quality of audio and video systems in cars can be improved by using flexible diversity reception. The signal is received and assessed by up to three RF tuners, with only the best signal being processed. An additional antenna (3+1 principle) is used to monitor the frequency spectrum. This antenna gathers information about possible interference as well as improved reception on other frequencies, which it then passes on to the other receivers so that they can switch frequencies.

Uninterrupted detection of unwanted signals

A spectrum analyser can be used to capture and display the parameters of unwanted signals in the frequency domain. This information can be used to determine the origin and type of unwanted signal. In practice, these signals can also be very brief in duration while having the same effect.

This is why a real-time spectrum analyser is often used. Instruments like the R&S FSW from Rohde & Schwarz, when equipped with the R&S FSW-K160RE real-time option, measure continuously in real-time operation, thus capturing every event for analysis, no matter how brief. Spectrogram mode is especially suited for verifying the frequency hopping of Bluetooth signals, as it depicts how the signal's spectrum fluctuates versus time.

Bluetooth signal

In spectrogram mode, the individual frequency hops of a Bluetooth signal can be easily viewed and followed.

This provides a good overview of the signal behaviour and allows assessment even for brief signal impairments. Only additional testing can determine the extent to which the individual radio systems are impaired.

Receiver sensitivity assessments

Coexistence measurements are used to determine the degree of desensitisation, i.e. the decrease in receiver sensitivity as a result of strong RF leakage in an adjacent signal. An important assessment criterion in determining the sensitivity of a receiver is the bit error rate (BER).

The device to be tested receives a certain number of bits within a defined time frame, which are then compared against a reference signal. For WLAN and Bluetooth, this is known as the packet error rate (PER) and for LTE it is the block error rate (BLER). With this measurement, the error rate can be seen to increase below a certain receive level, see the blue curve in image three. If an additional unwanted signal is received at the receiver input, the curve slowly shifts to the left, see the red curve in image three. The sensitivity of the receiver decreases dramatically.

WLAN PER progression

The blue curve shows a typical WLAN PER progression without interference. As the receive level decreases, the error rate will increase. This effect occurs sooner when interference is present (red curve), and the sensitivity of the receiver decreases significantly, known as desensitisation.

Multistandard-capable radio communication testers are especially suited to coexistence measurements. With the flexibly configurable R&S CMW500, Rohde & Schwarz offers a test platform that can measure all major cellular and non-cellular wireless communications standards for multiple radio systems simultaneously.

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