Problem
The 802.11 standard was designed against a set of Physical Layer (PHY) requirements that differ in important ways from the requirements for mobile outdoor communication. In 802.11a/g/j/n/p information is modulated using a technique known as OFDM which, when appropriately configured, is well suited to many wireless communications scenarios.
In wireless communication systems multipath propagation due to reflections is a problem for radio design. The 802.11 standard was designed for low delay spread reflections, compared with outdoor propagation where the delay spreads are an order of magnitude higher. Furthermore when the 802.11 standard was ratified, mobility was not contemplated. In Summary, there are two failure modes of 802.11 radios designed for indoor use when taken outdoors: Mobility and multi-path propagation.
The Figure below shows the regions where, at 6 Mbps, connectivity between a vehicle and AP fitted with a COTS radio is mobility limited. The red region is where neither RMS delay spread nor received power is the constraint, but the mobility induced variation in the radio channel over the packet duration causes failures. We see that the coverage region is severely degraded relative to that expected based on available received power.
When direct communication is attempted between moving vehicles the result is even worse, as the mobility artefacts are amplified.
By designing and building their 802.11 radio from the ground up Cohda Wireless has solved these problems.