Once a TDMA system is implemented another system problem can be readily solved. Today, for safety, some believe the accuracy specification for vehicle position must be one foot or greater to support safety use cases. Unassisted GPS systems are not capable of one-foot position accuracy. However, once the TDMA protocol is implemented, a time of flight algorithm can be used to significantly improve position accuracy. Each car can send its GPS-calculated location to all other cars during its assigned slot time. Each car looks like a GPS satellite to other cars and stochastic noise in the GPS position calculation can be filtered out. When each car transmits it also sends a time stamp. Since the receiving vehicles know the time the message was sent and the time it was received, a time of flight measurement of position can be made. Time of flight information can resolve 1 nanosecond in time difference between the time a car sends out a transmission and the time another car receives the transmission. At the speed of light (radio propagation) 1 nanosecond corresponds to 1 foot. With the GPS information from other vehicles and the time of flight information fed into a Kalmann filter, simulations predict accuracies of one foot are attainable.

As an example, in Figure 3 car A and car B's GPS calculated positions are shown as well as their actual position. GPS error is usually estimated at 15-20 feet and GPS error bands would be 15-20 foot diameter circles around each car's reported position. Additional information from the time of flight calculation shows car A and car B are actually only 1 foot apart. This makes the GPS error bands significantly tighter as portions of the error band would yield an impossible solution. In Figure 3, A and B are estimates based on Kalmann filtering, which show higher accuracy than by GPS alone. As more vehicles are involved, the actual and estimated positions converge.

A third problem that faces the automotive communications environment is the multipath. This is a signal reflection problem where the transmitted signals bounce off other cars, trees, buildings, etc. and distort the actual signal. Most people have readily experienced this problem when listening to a weak FM station in an urban environment as the signal fades in and out. The two effects of multipath are signal cancellation and intersymbol interference, shown in Figure 4.

The problem of multipath has already been seen in VII vehicle test fleets. It is quantifiable based on packet loss in the communications channel. The 802.11a standard utilizes a modulation scheme called orthogonal frequency division multiplexing or OFDM. OFDM has been recognized as being prone to multipath. Newer generations of home and commercial routers (802.11n), WiMAX, and the European broadcast television standards (DVB-T and DVB-H) all use a new modulation scheme called coded OFDM (COFDM), which is much more resistant to multipath.

Putting together the ideas of using an autonomous self-assigning TDMA, along with car-to-car time of flight ranging, and COFDM offers the following significant advantages for V2V and V2I safety systems:

Statistically determinate, low latency messaging, which is required for safety applications.

Time of flight vehicle ranging that significantly improves vehicle-positioning accuracy, as compared to unassisted GPS-based methods. This reduces the component cost of implementing assisted GPS systems.

Advanced coding methods, developed since 802.11a and now commonly in use significantly reduce multipath in systems with rapid motion.

The solution has the promise to be commercially viable at a lower cost than the current protocol being pursued for VII.

Automotive Communications Systems, Inc. has applied for a patent for the above approach. With our development partner, STMicroelectronics, we are working jointly on bringing this new safety protocol to the market as an alternative to the current protocol being used for VII.

Milt Baker is a co-founder of Automotive Communications Systems in Ann Arbor, MI. He has more than 30 years experience in automotive electronics, and began his career working on engine controls and body electronics at General Motors, and subsequently held significant positions at Motorola Automotive.

Larry Hill is also a co-founder of Automotive Communications Systems in Ann Arbor, MI. He has more than 30 years in commercial and military electronics with a specialization in radio communications. He has worked in large companies and led a number of start-ups.

Click here for the enhanced PDF version of this article