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Coexistence measurements promise excellent car connectivity

Posted: 20 Oct 2015     Print Version  Bookmark and Share

Keywords:Rohde & Schwarz  WLAN  Bluetooth  connected car  coexistence measurement 

Automobile manufacturers these days are churning out cars that boast infotainment systems that allow occupants to communicate with the outside world. Good reception in cars is guaranteed by an increasing number of transmit and receive antennas from a number 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 per cent 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 non-cellular 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 2.402GHz to 2.480GHz.

For the WLAN standard, country-specific frequencies are available within the 2.4GHz and 5GHz 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.

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