Achieving Wireless Coexistence

Because coexistence is an essential feature for all future automobile electronics, learn the basics of why problems arise and how to mitigate EMI.

By Lou Frenzel, Contributing Editor
[Sponsored by Rohde & Schwarz]

Automobiles have always been major electrical noise generators. Ignition noise blanked AM radio reception in early models until the auto makers figured out how to eliminate it.  Then, as more electrical features were added, more noise was generated.  Motors and relays raised the noise level.  An ever-widening and thickening cloud of electromagnetic interference (EMI) arose from adding electronics like multiple networks (LIN, CAN, Ethernet, etc.), multiple processors in antilock brakes, stability controls, fuel injection, infotainment systems, and others.  And do not forget the entertainment radios, AM, FM, and satellite.

Things have gotten worse with multiple communications systems coming into the mix. Bluetooth, Wi-Fi, GPS, and LTE cellular radios are now commonplace in most modern vehicles, and even more are expected in the future.  An example of a potential forthcoming addition is the Dedicated Short Range Communications (DSRC) vehicle-to-vehicle (V2V) or cellular vehicle-to-everything (V2X) radios that will implement the Intelligent Transportation System (ITS) defined by the Department of Transportation (DoT).  Advanced Driver Assistance Systems (ADAS) add video cameras, radar, and ultrasonics, which all produce another layer of interference.  Self-driving cars will add LiDAR and even more powerful artificial-intelligence (AI) processors to the mix.

This trend brings up the subject of coexistence–the ability of one communications technology to function normally in the presence of others on or near the same frequency.  When designing and adding radios for vehicular activities, be aware that coexistence testing must now be factored into the process.

Coexistence Measurements Ensure Good Reception in Cars
coexistence-measurements-reception-cars-wagner

More and more of today’s cars include infotainment systems that allow occupants to communicate with the outside world. 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.

Download free white paper

The Real Meaning of Coexistence

Coexistence is part of the overall concept of electromagnetic compatibility (EMC).  EMC is defined as the ability of a system or device to function as designed within a surrounding electromagnetic (EM) environment while not causing detrimental interference to other devices or systems operating nearby.  Coexistence implies that one radio will not interfere with another in performing their functions.  That has become a tough design requirement, especially in vehicles.

The coexistence problem arises for two basic reasons.  First, more radios are being added to vehicles, and many of these radios operate in the same unlicensed spectrum.  A good example is Bluetooth operating in the same 2.4-GHz spectrum as Wi-Fi.  Other radios can interfere when their spectrum assignments are close to the operating frequencies of surrounding radios.  LTE bands close to the GPS frequencies are one example.

Second, all of these new vehicular radios are located very close together. Close proximity is the primary cause of coexistence issues.  All of the most widely used wireless technologies are packed within the front driver/passenger areas.  Example content includes Bluetooth hands-free cellular calling, Bluetooth connections to the car’s infotainment audio system, cellular calls to LTE networks, in-vehicle Wi-Fi hot spots, and GPS navigation systems.

The outcome of interference is either complete failure of the radio and its function or reduced operational performance.  A typical occurrence is the reduced receiver sensitivity caused by a nearby radiator.  Receiver sensitivities commonly run from about −80 to over −100 dBm.  A close transmitter can drop that sensitivity down by 10 dB or more. That, in turn, shortens the overall potential range of operation and decreases the reliability of any connection.  Interference will also increase the bit error rate (BER) of the device. Reduced data-throughput rates occur when transmissions are subject to retransmissions or extensive error-correction operations.

Mitigating EMI

The fastest and easiest way to minimize interference is to separate the radios as much as possible.  Improved coexistence comes when the physical distance increases between radios.  That has proven difficult in modern designs, but is still a goal in new designs.

Using different frequencies also improves coexistence, but that too is difficult. How can you not have Bluetooth AND Wi-Fi? Luckily, both of those standards have dealt with the coexistence issue from the beginning of their development. It really helps that Wi-Fi operates on fixed channels and Bluetooth uses a fast frequency-hopping scheme. Interference is not totally eliminated, but the two usually function fine in the presence of one another.

While some radios use different frequencies, the frequencies are close enough to cause adjacent-channel interference from sidebands or harmonics. Or simple receiver de-sensitization will occur. One solution that should be obvious is to minimize the transmitter power where possible.

Timing is also a solution. Keep the different technologies from functioning at the same time. In other words, time-share any common bandwidth. Most technologies have access methods that can and do minimize interference. Examples include CSMA/CA (Wi-Fi) vs. DSSS (older Wi-Fi and Zigbee) vs. TDD LTE.

If those basic methods cannot be implemented, the fallback is to use the well-proven methods for reducing EMI:  grounding, shielding, and filtering. Good grounding is a given in most modern vehicle designs. And shielding may not be sufficient. Filtering almost always works if you can identify the offending devices and frequencies.

Finally, it is comforting to know that coexistence has always been an issue in designing any new wireless standard. It is usually a part of the design—mostly in the physical layer, but more increasingly in the upper communications layers (MAC, data link, network, etc.).  On top of that, separate coexistence standards are and have been established.  These include the ANSI Standard for Evaluation of Wireless Coexistence (C63.27-2017) and the IEEE 802.19 standard for unlicensed spectrum applications that is being developed.

Coexistence Testing

The hard part of ensuring coexistence is finding and diagnosing the nature of the interference. Such testing measures have been developed but are beyond the scope of this article. They involve a good real-time spectrum analyzer or EMI receiver. The great need is to test each device in the presence of the others that could interfere. Most of the major wireless test equipment companies have coexistence solutions.

One good reference on the subject is the National Institute of Standards and Technology (NIST) Technical Note 1885 Complexities of Testing Interference and Coexistence of Wireless Systems in Critical Infrastructure.  The paper titled  A Review of Wireless Coexistence Test Methodologies by William F. Young and Jason B. Coder of the NIST is another good source of information. Also, don’t forget to refresh your knowledge of EMI compliance as stated in the U.S. Code of Federal Regulations (CFR) 47, Parts 15 and 18.

Coexistence Measurements Ensure Good Reception in Cars
coexistence-measurements-reception-cars-wagner

More and more of today’s cars include infotainment systems that allow occupants to communicate with the outside world. 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.

Download free white paper

Start typing and press Enter to search