Rolling Connectivity: Automotive RF communications
Today's passenger cars, especially those in the United States, have become rolling communication centers.
A typical General Motors (GM) vehicle contains AM, FM, and Satellite Radios, two GPS receivers, and a cellular radio. There may also be Bluetooth in the passenger compartment, and in the future, DSRC (dedicated short-range communication, 5.9GHz) will be added for vehicle-to-vehicle and vehicle-to-infrastructure communications. (There may also be anti-collision radar, but that is typically a separate system.)
Most of the electronics for all these devices are located in the "center stack" – that dense area of screens and knobs between the driver and the front-seat passenger. The signals for satellite radio, GPS, AM/FM radio, and cellular all arrive at the vehicle from outside, and must be captured by an antenna, then coupled to the electronics by cabling. To keep cars from bristling with antennas (in contrast to police cars) the OEMs (Original Equipment Manufacturers) have developed multi-function modules containing multiple antennas and boards with over 100 components. A recent example is shown in Figure 1.
(Source: General Motors)
The module in Figure 1 includes the GPS antenna and low-noise amplifier (gain >25dB, noise figure < 1dB – black cable), the XM Satellite Radio antenna and low-noise amplifier (tan cable), and the cellular antenna (blue cable). The three coaxial cables terminate in special SMB connectors that are held in one shell to streamline installation. The 3-plug connector is mated to a cable assembly inside the vehicle, and has locking features to prevent vibration and temperature cycling from causing the connectors to come apart. The total coaxial cable length can be 20 feet or more. Typically, the cable is similar to RG-174. For the 2.4GHz and 1.575GHz Satellite Radio and GPS antennas, respectively, this can introduce a lot of loss.
In the center, you can see a bolt-head and a red-colored clip; that's what fixes the module to the roof. The cables pass through the same opening as the mounting hardware. The hole is around 20mm in diameter. Because so many vehicles are now designed for roof-mount modules, that one hole in the roof has become prime real-estate for any communications entering or leaving the vehicle.
There are also modules integrating "short FM" antennas. These include special matching networks to allow the shorter antenna to be impedance matched to the radio feed, and are very specific to a given vehicle platform. Figure 2 shows one of these modules on a 2013 Chevy Cruze.
Figure 2 2013 Chevy Cruze RS-010 with short FM antenna module mounted just over the rear window in the center.
(Source: General Motors)
Depending on the vehicle and features, the overall architecture for these signals and the user interfaces might look like Figure 3.
In this arrangement, as much as 60ft. of coaxial cable might be run from the roof to the center stack, with Fakra connectors near the antenna and more on the back of the receivers. As a side note, Fakra connectors are special versions of SMB connectors and have evolved to be the standard for RF connections in vehicles. The connector shells come in a variety of colors which have been standardized according to usage. For instance, blue is GPS, and curry yellow is Satellite Radio. Each color has a unique keying, so they cannot be improperly connected. For example Fakra connectors, see TE Connectivity's.
Most of the components in these automotive communication systems are designed and manufactured by so-called Tier 1 suppliers who provide them to the OEMs. I spoke with Dr. Ayman Duzdar, global director of engineering for Laird Technologies' Telematics business unit, a leading Tier 1 communication solutions provider for the US and European markets. Dr. Duzdar agreed to share with me some developments we might see a few years out. Because the Tier 1s are already developing designs for the 2015 to 2019 model years, new approaches now in R&D might not appear in production cars for a few years. Dr. Duzdar explained they are exploring ways to reduce the amount of coaxial cable while adding connectivity features and making the roof-mount unit more modular. Figure 4 shows one possible architecture.
Figure 4 A possible future architecture for a connected car. The receivers for WiFi, GPS, and 4G are moved into the roof-mounted module, which then connects to the head unit using an Ethernet cable.
Jim Ciccarelli, segment head for M2M in Laird's Telematics unit, told me they were also looking ahead to DSRC, which will use RF communications between vehicles (Vehicle to Vehicle, or V2V) and between vehicles and fixed infrastructure (V2I) nodes to add safety features such as collision avoidance, routing, real-time traffic information, etc.
Since the roof is an ideal location for V2V, another antenna and radio could be added to the module for that application. DSRC will use radio spectrum around 5.9GHz, and the losses if the coax were run all the way to the head unit could be as high as 18dB, compared to around 10dB today, making the new architecture more advantageous. What can be seen by comparing Figure 3 and Figure 4 is that the electronics in the center stack can become simpler, and the amount of coaxial cable in the vehicle, and number of RF connectors, is significantly reduced.
Laird has developed working prototypes of a module containing 3G, WiFi (2.4GHz and 5.8GHz), Bluetooth, and GPS receivers/transceivers. Future iterations will move to 4G/LTE, and possibly DSRC. Figure 5 shows a sample of the new module.
Figure 5 Laird Technologies prototype future connectivity module. At the upper right is a GPS patch antenna integrated with a combination cellular/WiFi/Bluetooth antenna assembly. On the left side of the PCB (which is flipped over to show the bottom) is the 3G module; other board sections are visible for Bluetooth and WiFi. Note the chassis is die-cast and is used as a shield can for the electronics, and has compartments to improve isolation.
Although this is an early example, Dr. Duzdar talked about the ability to download entertainment using WiFi, the possibility of supporting WiFi offload for the cellular connection, and providing wireless data capabilities to the rear seat area while driving. Imagine in a few years your kids in the back seat with their tablets connected to your private hot spot using 4G as the data pipe to the Internet.
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