WLAN IEEE 802.11 met many of the needs of vehicle-to-vehicle (V2V) or car-to-car (C2C) communications required for intelligent transport systems (ITS). For example, the OFDM scheme is well suited to mobile environments, and the capability for ad-hoc communications fits well to the behavior of C2C communications. The IEEE 802.11p amendment for wireless access in vehicular environments ratified in 2010 was becoming an integral part of the related ITS protocol stacks. For example, the amendment was defined for the US market in the IEEE 1609 WAVE standard, for Europe in the ETSI EN 302 665 ITS standard and for Japan in the ARIB STD-109 standard. After comprehensive tests of this technology in trials around the world, an increasing number of commercial cars and roadside equipment support V2V communication based on IEEE 802.11p.
WLAN IEEE 802.11p testing
Rohde & Schwarz test solutions support engineers in bringing V2X communications solutions to market with the required performance, quality and reliability.
Wi-Fi in a very challenging environment
The main requirements for this dedicated short range communications (DSRC) technology were low latency, ad-hoc networking, support for distances of up to one kilometer and the ability to handle the high speed of the vehicles (relative velocities of up to around 500 km/h) in an extreme multipath environment. IEEE 802.11p is based on the first OFDM-based Wi-Fi standard IEEE 802.11a, but uses for example a half-clock mode for the 10 MHz bandwidth channel for more robustness. It operates in dedicated frequency bands reserved for ITS services, typically at 5.9 GHz. In contrast to standard WLAN configurations, there are no access points (AP) in 802.11p. Instead, stations (STA) communicate directly in a peer-to-peer network with each other.
Key IEEE 802.11p parameters based on IEEE 802.11a
| Parameter |
IEEE 802.11a 20 MHz |
IEEE 802.11p 20 MHz |
IEEE 802.11p 10 MHz |
IEEE 802.11p 5 MHz |
|---|---|---|---|---|
| Number of subcarriers | 52 | 52 | 52 | 52 |
| Subcarrier spacing | 312.5 kHz | 312.5 kHz | 156.25 kHz | 78.125 kHz |
| Symbol duration | 4 µs | 4 µs | 8 µs | 16 µs |
| Guard time | 0.8 µs | 0.8 µs | 1.6 µs | 3.2 µs |
| FTT period | 3.2 µs | 3.2 µs | 6.4 µs | 12.8 µs |
| Preamble duration | 16 µs | 16 µs | 32 µs | 64 µs |
| Modulation modes | BPSK, QPSK, 16QAM, 64QAM | BPSK, QPSK, 16QAM, 64QAM | BPSK, QPSK, 16QAM, 64QAM | BPSK, QPSK, 16QAM, 64QAM |
Fading profile: example of highway line of sight scenario as defined by ETSI.
Your 802.11p test challenges
The standard defines several transmitter and receiver tests such as EVM, TX power, spectrum emissions and sensitivity. There are two important adaptations in 802.11p for ITS: a much stricter spectrum mask as well as stricter adjacent and nonadjacent channel rejection requirements. The adjacent channel rejection measures the ability of a receiver to demodulate and decode a desired signal in the presence of an interfering signal in an adjacent or nonadjacent channel. Moreover, for vehicular environments, fading has an enormous impact on the received signal quality. Not only does the channel itself change very quickly versus time, but Doppler shift, which is determined by the relative velocity between the transmitter and receiver, also occurs. This makes repeatable real-time fading simulation essential.
Benefits of Rohde & Schwarz 802.11p test solutions
- Complete set of solutions covering tests for chipsets, modules, on-board units and roadside units
- First 802.11p conformance test system on the market
- Test solutions providing the required accuracy and real-time fading capabilities
