Coexistence measurements ensure good reception in cars
01 October 2015
Good reception in cars is ensured by an increasing number of transmit and receive antennas from a variety of radio systems located in close proximity to one another.
However, mutual interference is an inherent risk with this type of "in-car coexistence" that must be prevented during development.
Simultaneous collocation of various radio systems is a longstanding issue that is regulated by means of international frequency plans and technical specifications. What is new, however, is that these systems must now both transmit and receive in extremely close proximity.
The passenger compartment in a car poses a particular challenge for developers because of the increasing number of transmit and receive antennas that are collocated in very close proximity in a mostly shielded space. A transmit signal will always affect other systems more here than it would in an open area or a larger space. This is particularly problematic because the noncellular standards are operated in frequency bands that lie very close to one another. As a result, transmit signals can show up as interference in adjacent receivers and cause overdrive, making it impossible to receive wanted signals. The utilisation of the various LTE bands varies by country and network operator, and because there is no way to predict when or where a car might pass into an area serviced by a different band, developers must take all possible scenarios into account in order to ensure interference-free operation.
To address the need for additional frequencies for mobile Internet access, this year the International Telecommunication Union (ITU) released the 700MHz band (digital dividend 2) which previously had been reserved exclusively for broadcasting. This means that car radios and entertainment systems that receive terrestrial TV signals can be impaired by mobile radio signals. As a result, it is essential that developers include all broadcasting signals in any comprehensive test scenario.
There are several possible approaches for improving the reception quality in automobiles. Physically distancing the transmitter from the receiver is not practical because the lack of space is at the root of the problem in the first place. The usual RF shielding methods are likewise not sufficient to resolve the problem. On the other hand, it is possible to use additional bandpass filters on the transmit antennas for WLAN and Bluetooth in order to reduce RF leakage into the at-risk LTE bands. The transmit powers of the individual applications can be reduced as a result of the limited space, thus also reducing any possible unwanted emissions. In the case of noncellular standards, this can be implemented on an individual basis. This is not an option for cellular standards, however, because the transmit power is controlled by the base station.
The essential difference between WLAN and Bluetooth is that WLAN uses fixed frequencies while Bluetooth employs frequency hopping. The Bluetooth signal switches randomly between 40 possible channels in a 2MHz grid (BT4.0) up to 1600 times per second. This prevents the signal from permanently affecting a fixed WLAN signal in the 2.4GHz band. The reception quality within the vehicle can be further improved both in the frequency and in the time domain. The highly integrated computer chips found in today's infotainment systems incorporate the various standards. The frequencies to be used for the various applications are already known in the baseband of this chip. This means that a "black list" can be defined for the Bluetooth signal's frequency hops, listing the channels that could be impaired by an LTE signal and therefore should be avoided. This process is known as adaptive frequency hopping.
To prevent impairment of GNSS signals by LTE, it makes sense to employ early detection of data transmissions in the time domain.
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. Spectrogram mode is especially suited for verifying the frequency hopping of Bluetooth signals, as it depicts how the signal's spectrum fluctuates versus time. 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 (Fig. 3, blue curve). If an additional unwanted signal is received at the receiver input, the curve slowly shifts to the left (Fig. 3, red curve). The sensitivity of the receiver decreases dramatically.
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 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. 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 centre 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 (Fig. 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.
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. To further optimise 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.
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