FR3136134A1 - RadCom low-cost communication device, combining on-board radars and inter-vehicle communication. - Google Patents

Abstract

RadCom low-cost communication device, combining on-board radars and inter-vehicle communication. The present invention consists of a low-cost RadCom communication device combining on-board radars and means of inter-vehicle communication more commonly called V2V. The device uses standards using OFDM type wave modulation on a frequency band of 5 GHz (76 to 81 GHz), transformed into Stepped OFDM by the subdivision of said frequency band into 39 channels of 128 MHz, of so that much less expensive CAD/CDA converters can be used. Instead of using Shiffman phase shifters, this device uses ten phase shifters based on a four-port directional coupler and a capacitance between the Schottky diode supply voltage and ground and uses only one FPGA output (circuit integrated) thus reducing complexity and production costs. Figure for abstract: FIG 4

Description

RadCom low-cost communication device, combining on-board radars and inter-vehicle communication.

We know that the improvement of road safety and the orientation of technologies towards the autonomous driving of transport vehicles, lead to the use most often of a plurality of on-board radars to detect the immediate environment of the vehicles and provide said vehicles circulating on the roads, preventive or primary safety, that is to say a set of elements linked to the vehicle, users and the environment that can prevent the occurrence of an accident.

The concept of the autonomous vehicle sought today corresponds to a motor vehicle capable of traveling on public roads without human intervention in all situations. This is a typical application of mobile robotics, however, with some nuances depending on whether we are talking about a fully autonomous, semi-autonomous or automated driving vehicle, for example in built-up areas or on open roads or even with automated parking. etc.

An autonomous vehicle is most often equipped with digital sensors such as radars, lidars, cameras, etc. whose data collected is processed by specifically adapted processors and software. Said processors and software reconstruct the 3D road situation by pattern recognition, for example roadway boundaries, traffic lanes, surrounding vehicles, signs or various obstacles and use artificial intelligence algorithms to act accordingly on the vehicle controls.

Cooperative communication systems are more generally referred to as V2V for vehicle-to-vehicle communications, V2I for vehicle-to-infrastructure communications, and I2V for infrastructure-to-vehicle communications.

The author of this patent actively participating in this work has already filed several patents with this objective and in particular a patent concerning a cooperative communications device for a transport system, combining on-board radars and V2V inter-vehicle communications known as RadCom, patent filed under the number FR 1902963 of 03/21/2019 and a patent concerning a process for optimizing the performance of an antenna array filed on 04/28/2021 under number FR 2104435.

V2V inter-vehicle communications allow the exchange of data between vehicles and thus to understand dangers. They generally operate according to three common standards such as: 802.11p, Device-to-Device-PC5 of the 3GPP Rel.13 and 14 standard commonly called 4G and 4G+ and Device-to-Device-PC5 of the 3GPP Rel.15 and 16 standard commonly called 5G. These standards use OFDM, FBMC etc. waveforms. OFDM (orthogonal frequency-division multiplexing) is a process for coding digital signals by orthogonal frequency division in the form of multiple subcarriers. This technique helps combat frequency-selective channels by allowing low-complexity equalization. These channels appear in particular in the presence of multiple paths and are all the more detrimental as the transmission rate is high. To ensure that the frequencies of the subcarriers are as close as possible and thus transmit the maximum amount of information over a given frequency portion, OFDM uses orthogonal subcarriers. Thus, the signals from the different subcarriers overlap but thanks to orthogonality do not interfere with each other.

By adopting OFDM modulation of the transmitted wave, simultaneous radar operation (determination of distances and speeds of different targets) and inter-vehicle communications is ensured. It is also ensured that Radcoms located in the same area are not affected by their own mutual interference and are not affected by interference produced by other types of radars, for example current FMCW radars.

The interference emitted by the power of other radars does not affect the RadCom network of this patent given its small beam opening (0.4°), the waves emitted by the other radars having a zero probability of being detected by the scanning antenna.

Since OFDM modulation is produced digitally, to transmit it it is necessary to convert it into an analog signal and convert the analog signals received into digital signals. This involves the use of CDA and CAD converters (Digital/Analogue Converter on transmission and Analogue/Digital Converter on reception).

By using the entire 5GHz frequency band (76 to 81 GHz), we obtain good distance resolution but we impose a high CDA/CAD sampling frequency greater than 2x5=10 giga-samples/s, which implies a very high CAD cost making the cost of RadCom incompatible with the automobile. It is however possible to maintain the same distance precision by using low sampling frequency CADs at reduced cost by transforming OFDM into Stepped OFDM. See Figures 2 and 3.

According to the preferred embodiment of the invention, the low-cost communication device RadCom uses Stepped OFDM modulation by subdividing the 5 GHz band (76 to 81 GHz) into 39 channels of 128 MHz, such that the phase shift of each channel is significantly reduced (considered constant) and allows the use of conventional and therefore low-cost converters, in order to achieve the creation of a RadCom assembly at a cost compatible with the automobile. See it .

The analog phase shifters generally used consist of a Shiffman cell associated with two varicap diodes generating variable capacitances C1 and C2 under the action of a single control voltage. The two coupled lines are usually characterized by a length L of λ/4. See it .

The phase at port 2 “Output port Z0” depending on the phase shift introduced by the Shiffman cell is:

1. - La largeur des pistes doit être supérieure ou égale à 100µm.
2. - La distance séparant deux pistes adjacentes doit être supérieure ou égale à 100µm.

For the production of PCBs to be low cost, certain requirements must be applied, in particular:

1. - The width of the tracks must be greater than or equal to 100µm.
2. - The distance separating two adjacent tracks must be greater than or equal to 100µm.

According to the preferred embodiment of the invention, the above requirements cannot be met by the Shiffman cell-based phase shifter, the low-cost RadCom communication device of the present patent uses ten phase shifters, each based on a four-port directional coupler, so as to cover the entire required phase shift range (0° to 360°) and thus meet the requirements of low-cost PCBs. See it .

The directive antenna of the device being composed of 112 slots distributed 8 over 14 circles, each slot is associated with a phase shifter whose phase varies according to variations in the capacities of the associated Schottky diodes and subsequently the voltage applied to these diodes. To scan the entire horizontal area (-45° to +45°) and the entire vertical area (-10° to +10°) and cover the 11250 pixels, the voltages are applied independently of each other and their values are different. It is therefore necessary to provide 112 independent voltages which must also be varied numerically. For this, the most common command uses 112 CDAs (Digital Analogue Converter). Assuming that these 112 CDAs are at least 8 bits, you would need at least 896 (112x8) dedicated FPGA outputs. This could therefore not be achieved with a single low-cost FPGA and would lead to a complex and expensive implementation.

According to the preferred embodiment of the invention, a capacitor is placed between the supply voltage of the Schottky diodes and ground. This makes it possible to charge this capacitance to the desired voltage and maintain it for the duration of a pixel. The objective of this action is to provide images (Distance/speed) at the rate of 25 images/second, considering 11250 pixels per image, the duration of a pixel is 3.56µs = (1/25: 11250). See it .

According to the preferred embodiment of the invention, the circles are designated from C1 to C14 starting from the smallest diameter to the largest and the tensions to be supplied associated with the slots of a circle C1 from D1_C1 to C1 from D8_C1. See it .

We then consider 14 analog multiplexers with 8 outputs connected to capacitors C and whose inputs are analog voltages supplied by 8-bit CDAs. Each multiplexer is dedicated to a slot circle. In 8 CLK clock strokes, it provides the 8 voltages supplying the Schottky diodes associated with the 8 phase shifters/slots.

The frequency of the CLK clock is adjusted so that the capacitors C have time to charge to the correct values and maintain the charges for the duration of the pixel 3.56µs. The voltages of the 112 capacities are updated in a time equivalent to 1 microsecond, or approximately a third of the time allocated to a pixel (3.55us = 1/11250pixels x 25), which implies a unit charging time for each band of 14 capacities of 125ns (1us/8) since the capacities will be loaded in groups of 14 in 8 cycles. These capacities must then maintain the voltage for the rest of the duration of the pixel, i.e. 2.55us. However, the duration of a pixel may be longer if, for example, the radar image analysis strategy focuses on a few pixels for longer times. See it .

According to the preferred embodiment of the invention, the number of FPGA outputs being limited (112 = 14 x 8), the low-cost communication device RadCom uses a single FPGA (integrated circuit) in a simpler and less expensive. The complexity is therefore reduced from 112 to 14 as if we only had 14 slots instead of 112.

According to the preferred embodiment of the invention, the circulator of the low-cost communication device RadCom is designed based on a ring coupler of circumference equal to 6λ/4 (λ=wavelength) comprising four ports of Characteristic impedance equal to 50 ohms. See it .

Port C is connected to the transmitter, port A to the phase shifter, port B to the receiver and port D is loaded by a 50 ohm resistor.

Ports A and B are separated from each other by a length of λ/4. Port D is separated from port B by a length of 3λ/4 and from port C by a length of λ/4.

When the first port C is excited by a wave (transmission), it divides into two waves. One propagates in the trigonometric direction and the other in the opposite direction. These waves are in phase at port A (Phase shifter / Antenna) and in opposition to phase at port D (linked to the resistance) and B (Receiver). Consequently, on transmission, port C is coupled to port A and isolated from ports B and D.

If port A is excited (Reception of the radar echo), the same reasoning as above shows that this port is coupled to port B and isolated from ports C and D.

The resistor connected to port D consumes very little power. It is necessary to ensure impedance matching for non-continuous frequencies. One of its ends is therefore connected to ground. See it .

Conventionally and given that multilayer PCBs are used to connect the resistance to ground, we have to resort to metallized holes passing through the multilayers, which can only disrupt the useful signals in the 76 to 81 GHz band and lead to design complex solutions to counteract disruptions.

According to the preferred embodiment of the invention, the low-cost communication device RadCom has a planar capacitance connected to the resistance of port D of the circulator and not connected to ground. This resistance acts as an alternating short circuit without causing disturbances. See it .

The use of a 4-port ring coupler provides 5 GHz wideband response (76 to 81 GHz), negligible return loss at all ports, high isolation between ports and a layout (track width) allowing low cost PCBs. Using planar capacitance avoids disruptions.

The supply of the Schottky diodes of a phase shifter propagates to the other phase shifters through the different circulators, which distorts all the supply voltages and induces an antenna pointing result different from that planned. By placing a capacitor between the circulator and the phase shifter this seems to solve the problem. This so-called three-finger capacity transmits useful signals without attenuating them in the 76 to 81 GHz band and it does not transmit DC (supply voltage). However, this does not meet the implementation requirements. See it .

According to the preferred embodiment of the invention, the low-cost communication device RadCom is produced using multilayer PCBs, which induce capacitances located between the different layers. We thus design the insulation capacitance using 2 adjacent PCB layers, called multilayer or sandwich capacitance. See it .

The accompanying drawings illustrate the invention:

shows a Shiffman phase shifter.

is a graph showing the phase shift using OFDM modulation.

is a graph showing the phase shift using OFDM modulation transformed into Stepped OFDM.

shows the mounting diagram of the four-port directional coupler based phase shifter.

shows the assembly diagram of the phase shifter based on four-port directional coupler with a capacitance placed between the supply voltage of the Schottky diodes and the ground.

shows the distribution of the analog multiplexers supplying the Schottky diodes.

is a diagram showing the use of a single FPGA.

shows a circulator based on a ring coupler.

shows a circulator based on a ring coupler with planar capacitance connected to the resistance but not connected to ground.

is a three-finger capacitance view.

shows multilayer capability.

Claims (8)

Radcom low-cost communication device combining on-board radars and V2V communication, characterized in that it uses Stepped OFDM modulation, a single FPGA (integrated circuit), ten analog phase shifters each based on a four-port directional coupler and to a circulator designed based on a ring coupler with four ports. Radcom low-cost communication device combining on-board radars and V2V communication according to claim 1, characterized in that it uses Stepped OFDM modulation by subdividing the 5 GHz band (76 to 81 GHz) into 39 channels of 128 MHz, such way that the phase shift of each channel is significantly reduced (considered constant) thus eliminating pointing errors and allowing the use of very low speed and low cost converters, in order to thus achieve the creation of a RadCom assembly at a cost compatible with the automobile. Radcom low-cost communication device combining on-board radars and V2V communication according to claim 1, characterized in that it uses ten phase shifters, each based on a four-port directional coupler, so as to cover the entire required phase shift range (0 ° to 360°) and thus meet the requirements of low-cost PCBs. Radcom low-cost communication device combining on-board radars and V2V communication according to claim 1, characterized in that a capacitor is placed between the supply voltage of the Schottky diodes and the ground, this making it possible to charge this capacitor to the desired voltage and to maintain this for the duration of a pixel, so as to provide images (Distance/speed) at the rate of 25 images/second considering 11250 pixels per image, the duration of a pixel being 3, 56µs = (1/25: 11250). Radcom low-cost communication device combining on-board radars and V2V communication according to claim 1, characterized in that it uses a single FPGA (integrated circuit) in a simpler and less expensive embodiment, the complexity thus being reduced from 112 to 14 as if we only had 14 slots instead of 112. Radcom low-cost communication device combining on-board radars and V2V communication according to claim 1, characterized in that it uses a circulator designed based on a ring coupler of circumference equal to 6λ/4 (λ=wavelength ) comprising four ports with characteristic impedance equal to 50 ohms. Radcom low-cost communication device combining on-board radars and V2V communication according to claims 1 and 6, characterized in that it has a planar capacitance connected to the resistance of port D of the circulator and not connected to ground. This resistance acts as an alternating short circuit without causing disturbances. Radcom low-cost communication device combining on-board radars and V2V communication according to claims 1 and 7, characterized in that it is produced using multilayer PCBs, which induce capacitances located between the different layers, the insulation capacitance using 2 adjacent PCB layers, being called multilayer or sandwich capacitance.