Coexistence measurements ensure good reception in cars

October 19, 2015 // By Christoph 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.

According to a study by Accenture, more than 48 percent of car buyers today are more interested in electronic features such as driving assistance and infotainment systems than they are in driving performance.

Traditional car manufacturers have long had a presence in the California Silicon Valley as they worked toward early adoption of intelligent, networked mobility, with a view toward enticing especially young buyers into an automobile purchase. The melding of automobile and modern information technology into a smart car is no longer a vision for the future, but rather is now a reality on our roads.

An important function of the smart car is the ability to wirelessly connect smartphones to the vehicle infotainment system using noncellular wireless technologies such as WLAN or Bluetooth. This connection is used to sync data from a mobile phone (e.g. contacts and music) with the on-board unit so that it is available to the car occupants while driving.

Car manufacturers offer integrated WLAN hotspots for connecting smartphones and tablets to the Internet. Cellular standards such as WCDMA and LTE can be used to establish a connection to the mobile radio network. The Bluetooth standard is operated in the license-free ISM band from 2.402 GHz to 2.480 GHz.

For the WLAN standard, country-specific frequencies are available within the 2.4 GHz and 5 GHz bands. The use of multiple wireless communications standards such as LTE, WLAN and Bluetooth in parallel is known as coexistence. Leakage into adjacent channels can lead to quality problems, a reduction in data rates or even to a complete failure.

Radio systems in very tight spaces

Simultaneous collocation of various radio systems is a longstanding issue that is regulated by means of international frequency plans and technical specifications. What is new, however, is that these systems must now both transmit and receive in extremely close proximity.

The primary independent standardization bodies working on this topic are the 3rd Generation Partnership Project (3GPP) for cellular standards and the Wi-Fi Alliance for the WLAN standard. These groups specify, among others, limits for RF leakage into other frequency ranges, which are defined by the adjacent channel leakage power ratio (ACLR), for example.

This parameter specifies the ratio between the transmit power of the wanted signal and the lowest possible power that is leaked into the adjacent channel. Another important parameter when verifying radio standards is the spectrum emission mask (SEM).

This parameter uses tolerance characteristics to describe the permissible signal level versus time both inside and outside the transmission band allocated to a standard in order to limit the interference in the adjacent channels and in other frequency bands.

The passenger compartment in a car poses a particular challenge for developers because of the increasing number of transmit and receive antennas that are collocated in very close proximity in a mostly shielded space. An additional consideration are the associated reflections.

A transmit signal will always affect other systems more here than it would in an open area or a larger space. This is particularly problematic because the described noncellular standards are operated in frequency bands that lie very close to one another.

The LTE standard will also fall within this range depending on which frequency bands are used. For example, LTE time division duplex (TDD) band 40 lies only 1 MHz below and the LTE frequency division duplex (FDD) band 7 uplink only 17 MHz above the 2.4 GHz WLAN band – see figure 1.

Fig. 1: The license-free ISM band is used by WLAN and Bluetooth, in addition to other radio services. The lower band lies very close to LTE band 40 and the upper is adjacent to LTE band 7.

As a result, transmit signals can show up as interference in adjacent receivers and cause overdrive, making it impossible to receive wanted signals.

The utilization of the various LTE bands varies by country and network operator, and because there is no way to predict when or where a car might pass into an area serviced by a different band, developers must take all possible scenarios into account in order to ensure interference-free operation. And the list of potential interferers is expanding, as the satellite-based navigation systems (GNSS) such as GPS, Glonass and Galileo found in vehicles can also be impaired by LTE bands 7, 13 and 14.