Physical channels have limits that affect the practicalities of automotive network architecture and communication protocols. Here’s the lowdown on current work aimed at breaking bandwidth bottlenecks.
CHRISTIAN RUSCH, BERT BERGNER | TE Connectivity
It has become a truism that we have an insatiable appetite for data. Not so obvious is the effect our need for data has upon in-vehicle communication systems. Advanced safety, security, and convenience functions put demands on the already crowded data network in the car. For these functions, even one missed byte of data can have a profound impact on vehicle operation.
The evolution toward advanced driver assist systems (ADAS) and automated driving functions has made high-speed data transmission lanes increasingly relevant to vehicle safety. OEMs must now consider the limitations of physical channels when defining the architecture and selecting the communication protocol. Safety considerations make the trade-off between the data channel (the wires and connectors) and the communications protocol performance (the ICs and software) more important for finding a cost-optimized combination of both. This higher complexity and the increasing number of data-links in vehicles lead to a new generation of automotive architectures.
Fragmented vs. converged architectural approach. A modern luxury vehicle can contain up to 100 electronic control units (ECUs) based on multiple proprietary operating systems. These ECUs handle tasks ranging from simple control programs to running complex, real-time, multifunctional embedded platforms that support, for example, increasingly sophisticated infotainment and driver assistance systems.
The ADAS functions that will ultimately lead to fully automated driving are growing more complex. As a result, traditional ECU-based architectures are reaching their limits. Thus OEMs must develop new concepts to manage the high levels of complexity and data through-put.
By clustering functions into domains and converging ECUs, OEMs can optimize the weight of the harness and reduce the complexity of connections. Such measures could reduce the number of components and the overall cost.
Service-oriented architectures. The integration of ADAS applications is one of the most significant challenges OEMs face when designing vehicle architectures. High-resolution cameras and high-performance sensors for radar and lidar generate and require an immense amount of data. Within the vehicle, that data must traverse several meters of cable and be processed by powerful computing systems. For safety reasons, ADAS clusters feature a redundant computing platform. High-priority ADAS data also goes to a secondary processor physically separated from the primary ADAS. This lets the secondary ADAS run in emergency-mode to bring the vehicle safely to a stop.
High-speed vehicular computing domains require a symmetrical (i.e. in which all devices can send and receive data at the same rates), robust, easy-to-implement and standardized networking technology with high-performance backbone connectivity such as Ethernet. Cameras and displays usually need asymmetrical links with higher bandwidth in one direction than in the other. For these connections, less complex physical layers have become established in recent years through use of a serializer/de-serializer (SerDes) chipset that converts parallel data to serial data and vice versa.
Generally, other sensors and actuators operate at much lower data rates which enables the use of less expensive and established bus technologies like CAN(-FD) or LIN. Gateways, that enable the data transfer between the different network technologies and protocols, will play an important role in these new architecture concepts.
Next-generation vehicle communications
Heterogenic high-speed chip landscape and standardization trends. For several years, Ethernet has handled vehicle diagnostics and supported infotainment systems. The addition of deterministic timing functions is now expanding the role of Ethernet. For example, to reduce costs, Ethernet can now serve as a network backbone for inter-domain controller networks and replace serial networks such as MOST and FlexRay.
Ethernet supports line, star, and hybrid ECU connections. As such, it was considered a promising candidate for many topology configurations in automotive applications. However, the original Ethernet standards were not created for time or safety-critical applications. Their adaptation to automotive applications has been the subject of several working groups within the Institute of Electrical and Electronics Engineers (IEEE).
Initially, Ethernet cables for use in buildings were thick, double-shielded, and rather inflexible. Subsequently, Ethernet has become attractive to the automotive market thanks to the development of more lightweight and less expensive unshielded twisted-pair cables.
100BASE-T1 Ethernet technology with a maximum data rate of 100 Mbps became practical for vehicles with the development of the BroadR-Reach physical layer spec (by Broadcom Corp.). This spec received further support from the OPEN Alliance Special Interest Group, different OEMs, and from ECU, chip, and connector suppliers. Possible applications for 100/1000BASE-T1 are connections to rear-view cameras with a 360° panoramic view, radar and lidar systems, as well as driver-cockpit and infotainment solutions.
Soon, 1 Gbps (1000BASE-T1) will be implemented, enabling higher performance. TE Connectivity’s MATEnet portfolio of connectors offers optimized channel parameters for these Ethernet links.
In 2017 another IEEE working group was created for boosting the automotive Ethernet data rate to the multi-gigabit range. The NGAUTO working group is developing the multi-gigabit standard (IEEE P802.3ch) for data rates of 2.5, 5 and 10 Gbps on full-duplex shielded differential cables. The 10 Gbps Ethernet standard includes a preliminary channel specification. Based on a channel analysis by TE (a consortium participant), this specification limits the used channel bandwidth to 4 GHz for return loss and insertion loss and 5.5 GHz for the coupling attenuation.
For high-resolution camera and display connections, OEMs have in recent years deployed asymmetrical point-to-point links instead of Ethernet with SerDes ICs. The current generation with APIX II, GMSL, FPD III-Link allows data rates of up to 3 Gbps on a single coaxial or differential cable. Soon, OEMs will implement the next generation of this technology in vehicle architectures for the first time.
OEMs can increase data rates to 6 Gbps on one channel or 12 Gbps if two channels are combined. Unlike Ethernet, the SerDes protocols are not yet standardized. As a result, chip suppliers are releasing multiple proprietary solutions which are often incompatible with each other. Several OEMs, as well as device and chip manufacturers, have begun working on standardization for automotive display and camera links to reduce the number of non-compatible SerDes variants.
The SerDes ICs usually support both coaxial and differential cables for cameras and displays. In contrast to Ethernet, a SerDes system provides an asymmetrical link in that the data rate for the downstream channel is much higher than for the upstream channel. Asymmetrical connections are sufficient because cameras produce high-speed data but receive control signals at much lower data-rates. Display units, on the other hand, receive high-speed data but need only send control signals to the ECU as, for example, for touchscreen inputs.
This asymmetric approach reduces physical complexity and the channel requirements. Thus OEMs can create systems that are less expensive and more tailored to the application than full-duplex Ethernet systems with the same data rates. Consequently, it is likely that next-generation architectures will feature both Ethernet and SerDes.
TE Connectivity is working closely with chip suppliers of the established SerDes system and tracks the progress of standardization efforts. This enables rapid adaption of products to upcoming data communication protocols.
A full data communication system consists of the channel and the transceiver chipsets within the physical layer (PHY). The channel contains two headers (PCB connectors) and various cable segments that, depending on the link topology, connect via inline connectors. The maximum available data rate of the system depends on a combination of chip and channel complexity.
If the goal is to reduce chipset costs, size, and power consumption, a simple modulation (e.g. pulse-amplitude-modulation with two amplitude levels, PAM-2) scheme could reduce complexity of equalization, filtering, or digital signal processing. However, this approach requires broadband channels with low attenuation and smooth frequency response over a large bandwidth to realize high data rates.
System suppliers often encounter situations where channels provide only limited bandwidth, a non-linear frequency response, or strong echoes caused by channel components. Such sub-optimal scenarios can be addressed by making the chips involved more capable and thus, more complicated.
Thus all parties involved in system development must analyze the trade-off between chip and channel complexity. As an example, TE Connectivity and the Fraunhofer Institute IIS have analyzed channel capacity based on automotive requirements such as topologies featuring link lengths of 10-15 m, EMI performance, signal integrity and IC implementation limitations. This study evaluated maximum data rates of available automotive channels.
As ADAS functions become more sophisticated, the performance and reliability of data links to cameras and sensors becomes increasingly significant. As components get pushed close to their physical limits, the margin narrows between performance ceilings and typical operating parameters. It becomes increasingly important for component developers to consider all critical tolerances. And lower link budgets, driven by the need for more bandwidth, limit link lengths and IC choices for designers working on architectures.
Christian Rusch, Bert Bergner, “Robust Connectivity Solutions for Next-Generation Automotive Data Networks,” TE Connectivity White Paper, https://www.te.com/usa-en/industries/automotive/insights/the-next-generation-of-mobility/robust.html