Page 28

EETE NOV 2015

Automotive Electronics sions 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. 15 ms 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 analyzer 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 analyzer 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 – see figure 2, as it depicts how the signal’s spectrum fluctuates versus time. This provides a good overview of the signal behavior 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 desensitization, 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 figure 3. If an additional unwanted signal is received at the receiver input, the curve slowly shifts to the left - see the red curve in figure 3. The sensitivity of the receiver decreases dramatically. Multistandard-capable radio communication testers are Fig. 4: The R&S BTC can be used for complex diversity reception scenarios for broadcast signals. The frequency and level of the individual wanted and unwanted signals can be adjusted in real time. 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 noncellular wireless communications standards for multiple radio systems simultaneously. As a result, both the wanted and the unwanted signal can be generated using a single instrument. To ensure a realistic simulation of the signal propagation within the passenger compartment along with any reciprocal interference, the test setup must always include a connection via the air interface instead of the more easily implemented cable connection. For small setups, a compact test chamber can be used for this purpose. The R&S CMW500 frontend includes multiple RF connectors for the transmit and receive signal paths, eliminating the need for a switch matrix in the simplest scenarios. A further multistandard platform is recommended for simulating diversity reception of broadcast signals. The R&S BTC broadcast test center uses two independent real-time signal paths and up to eight arbitrary waveform generators to generate all of the required RF signals for international TV and broadcast standards, including the relevant interferer signals – see figure 4. As a result, developers have access to signals from local public networks as well as those from other countries required for a globally valid scenario. Outlook The described scenarios are not only about permitting users to make phone calls from the car or to connect their portable devices to the vehicle infotainment system. Future cars will have a fixed connection to their environment. This connection can provide quick access from the car to the company server or the home A/C system or from the automotive service center back to the car to permit remote fault diagnostics. WLAN standard 802.11p permits the exchange of information between vehicles, for example to provide a warning about an accident, roadwork or icy road surfaces. Europe, the USA, South Korea and Japan have already allocated frequency bands for this purpose. To further optimize reception in the car, future automobiles are planned to be equipped with their own mobile stations (LTE hotspots). These hotspots, which are already being used in buildings, not only ensure a good connection between the car antenna and the wireless devices, but also permit individual adjustment of the transmit power to reduce interference within the vehicle. Fig. 3: 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). 28 Electronic Engineering Times Europe November 2015 www.electronics-eetimes.com


EETE NOV 2015
To see the actual publication please follow the link above