A ViL-test equipment for ADAS functions has different requirements for a RTS than a test system for verifying the radar sensor itself. The latter has to prove that the sensor is working correctly and validates its performance or electrical parameters. In general this can be accomplished with a single target. Only for testing the radar’s resolution – in range, angle and velocity – a second target is necessary. In the radar’s so-called target mode – no tracking algorithms are used to generate the output – it is not even necessary to change the target’s distance even if the target shows a Doppler-shift. The radar’s range and velocity accuracy can be tested independently.
In contrast, a RTS-system for ADAS V&V has to generate the radar echos of complex and realistic traffic scenarios which are often generated by environment simulation systems (e.g. ). Those virtual targets have to move independently. To ensure that the radar’s tracking algorithm works as desired the stimulated velocity must be equal to the change rate in distance. Otherwise, the radar processor will ignore the target due to its physically impossible attributes.
As mentioned in Sect. 2.2 radar target stimulation on a ViL testbed applies Over-the-air (OTA) injection of the echo signals via the radar’s antennas. Unfortunately, OTA stimulation will introduce another problem: reflections from the surrounding environment which is called clutter. Unlike in reality, where the static environment has a Doppler-shift in regard to the moving radar, on the testbed both, the radar and the environment, are static resulting in zero Doppler-shift. It is difficult for the radar-processor to cope with such non-standard circumstances. For a proper stimulation on a testbed any clutter must be eliminated as much as possible.
Since a ViL testbed should be usable for different vehicles and, therefore, also different radar sensors the design of the RTS shall not impose any constraints on the radar, its design and its waveform. At best, the RTS should successfully stimulate the sensor without any a-priori knowledge about the radar.
Additionally, the intended application for the RTS described in this article imposes another requirement which is difficult to achieve: the focus of the RTS should be on the stimulation of targets at short distances (down to about 2 metres). Nevertheless, it has also to cover the radar’s instrumented range which is typically about 250 m. Furthermore, todays automotive radar have a bandwidth up to 1 GHz resulting in a range resolution of 15 cm. The resolution of the RTS has to be better than that.
The implications of these requirements on the design of our RTS are described in Sect. 3.2.
Concept and general design
In the framework of this article only the hardware of the RTS is described. The parameters necessary to stimulate the targets are prepared by the RASIG software which are provided as digital data .
The concept and design of our RTS was strongly influenced by the requirements given in Sect. 3.1. Especially, the low minimum distance of 2 m (corresponding to a time delay of 13 ns) together with the fact that we have to support a wide range of radar sensors (no a-priori knowledge about the radar’s waveform) forces an analogue implementation. Without a recording and playback mechanism – which is not possible if the radar transmit signals change – such low latencies cannot be achieved in the digital domain.
Unfortunately, the complexity of analogue based RTS increases more or less linearly with the maximum target range and the number of targets to be stimulated. The above given maximum range and delay line resolution would result in nearly 1700 delay sections which is clearly not feasible. To lower the complexity, a hybrid solution consisting of both an analogue and a digital system is used to cover the whole range of an automotive radar (2–250 m). The hand-over distance between the two subsystems is set to 30 m. This is the shortest delay realisable by state-of-the-art digital processing modules consisting of an analogue-digital-converter (ADC), a digital-analogue-converter (ADC) and a field programmable gate array (FPGA) in between (see Sect. 3.7).
Although 30 m can be achieved with acceptable effort, we decided on a concept for the analogue RTS which offers a good scalability, both in the distance and the number of targets. In this way, the RTS can be optimised for the intended test cases. This scalable concept is shown in Fig. 2.
To accommodate the different radar frequencies we decided on processing the radar signal at an intermediate frequency (IF). In this case the RTS can be adapted for different radar centre frequencies by changing the frontend only. Furthermore, the full implementation of the RTS at 77 GHz is of much higher complexity compared to frequencies below 10 GHz. In the end, an IF of 2 GHz was chosen due to the availability of suitable components which support a bandwidth of 1 GHz (most state-of-the-art long-range-radars use a signal bandwidth of up to 1 GHz). Furthermore, the selected IF frequency works well with the ADC and the DAC of the digital RTS module.
The components of the analogue RTS responsible for modifying the signal parameters (as described in Sect. 2.2) are presented in the following sections.
Delay line module
Variable delays are normally implemented by binary switched delay lines because this method scales good in the range dimension (doubling the range needs only one additional delay segment). But it scales badly in regard to the number of targets. Since such a delay line cannot generate delays for multiple independent targets each target would need its own delay line. An alternative approach is the so-called tapped delay-line. It consists of delay segments of identical length corresponding to the minimum desired delay. The number of these segments increases linearly with the maximum range but each possible delay can be achieved simultaneously by using couplers between the segments. This concept is presented in Fig. 3 and was chosen for our RTS. In order to reduce the number of necessary delay segments we chose a length of 60 cm for those (corresponding to 30 cm range resolution). Since the range accuracy was required to be better than this value an additional adjustment of the delay was implemented in the target emulation modules (TEM) as described in Sect. 3.5. This allocation reduced the number of switches and couplers significantly (e.g. the break even point regarding the number of switches between the chosen solution and a high-resolution delay line would be at 50 targets). A photo of the current delay line module with 6 segments and a delay of 360 cm – corresponding to a target range of 180 cm – is given in Fig. 4.
For the first demonstrator the delays themselves were implemented by coaxial cables. In future designs this realisation will be changed to achieve better integration on printed circuit boards (PCB). For example, defective-ground-structure transmission lines could be used.
Switch matrix & modules
The switch modules form the interconnection between the delay modules (Sect. 3.3) and the TEMs (Sect. 3.5) as shown in Fig. 2. A target is moved by switching the input of its TEM from one tap of the delay line to the next. Since hot-switching from one delay to another will cause undesired phase jumps of the stimulator’s output – which does not happen in the real world – measures were taken to prohibit this: instead of a single connection to the TEM there are two, one can be connected to even-numbered delays the other one to odd-numbered ones (see Fig. 5). Using a blend-over mechanism in the TEM cold-switching between those two inputs is ensured.
The chosen design of the switch matrix does not only allow one-to-one routing of a delay line tap to a TEM but also feeding the same delay to multiple TEMs (one-to-many) and even different delays to the same TEM (many-to-one) enabling the generation of targets with a range extension covering multiple delay segments.
Target emulation module
The TEM is the component of the analogue RTS which adjusts the received radar signal in order to generate a virtual target with the desired velocity and RCS properties. Generally, one TEM can only provide a single target. Therefore, the numbers of targets which can be stimulated depends on the number of TEMs in the system. The block diagram of a TEM is provided in Fig. 6.
As mentioned in Sect. 3.3 the TEM improves the range resolution of the stimulator by a factor of 4 by providing additional delay segments. The length of those sub-delays are a quarter of the original delay segments. Variable attenuators allow a blend over from one delay to another. This blend over allows to switch a delay segment on or off while the signal of this channel is not active. This principle is depicted in Fig. 7.
Since the target range is not changed at the radar frequency but by switching the delay at the much lower IF the phase change produced by this is also much lower than that of a target moving in reality (Doppler shift, see Eq. (2)). It is one of the tasks of the TEM to generate the correct Doppler frequency shift which enables the radar to detect the correct target velocity. This is accomplished by a vector-multiplier which is fed with the I/Q-components of the Doppler signal. These Dopper signal components can be simple sine-waves in order to shift the echo-signal in frequency, or, a more complex signal providing also a spectral spread of the target echo. The Dopper signal components are generated by a digital synthesiser implemented on the control board of the TEM.
Furthermore, the TEM has to correctly set the amplitude. As shown in Eq. (3) this depends both on the target distance and its RCS. Especially the dynamic range due to distance variations can be quite large. E.g. the same object positioned at 3 or 300 m has a difference in the echo power of 80 dB. To accomplish this task the TEM applies a cascade of several programmable attenuators. Figure 8 shows a photo of a TEM.
Antenna switch module
For angular target movement the echo signal must be switched between different transmit antennas. The chosen antenna determines the perceived target’s azimuth.
A switch matrix with a similar concept as the delay switch matrix (Sect. 3.4) is used for routing the signal from a TEM to an antenna frontend. It has to support a one-to-many signal routing for lateral extended targets and many-to-one connections for multiple targets in the same direction.
Digital RTS implementation
As indicted in Sect. 3.2 a digital RTS is necessary to make the stimulation of long-range targets feasible. To set the hand-over range as low as possible we selected the cutting edge hardware platform AV104 from ApisSys  (Fig. 9). This module combines an ADC and a DAC with extremely low latencies with a high capacity FPGA. The resulting overall minimum latency of 180 ns – one of the lowest latencies available for civil application – correlates to a target distance of 27 m.
The digital RTS is capable to process signals with a bandwidth of up to 1 GHz (more or less the same as the analogue RTS).
The firmware is based on a classical Digital Radio Frequency Memory (DRFM) structure. Massive parallel processing in the FPGA is necessary to ensure the required throughput. The frequency shift for each target is performed by a complex mixer fed by a numerically controlled oscillator. Afterwards, the delay is implemented independently for each target by a block RAM controlled by a programmable shift register. The resolution of the delay is about 5 cm. Thereafter, the signal’s power level is set by a multiplier. The FPGA should be able to stimulate up to 30 targets. In the last step the signals of all targets are combined and sent to the DAC which has to provide the necessary dynamic range.
The module is equipped with a single DAC which restricts the digital RTS to a single azimuth angle. In order to stimulate multiple directions either multiple boards or a new board with multiple DACs are necessary whose outputs are connected to the antenna switch matrix in the same way as the TEMs.