# Can RF work while it's in motion?

I have a project idea. It uses a pair of RF transmitter and receiver. The application is a little complicated since it involves motion, typically the speed of a vehicle going on local or even better on highway. I'm wondering if the transmitter and receiver like these can still work under that condition. Supposed both car A and car B have a pair of transmitter and receiver each. Can car A send messages to car B and vice versa?

If not, what different kind of products can achieve this?

The effect of motion is to cause Doppler shift in the received signal. This changes both the carrier frequency and the symbol rate. However, because the speed of propagation of the wave — an electromagnetic wave — is the speed of light, this effect is very small for terrestrial conditions.

A link between a fixed station and a vehicle moving at, let's say, 60 miles per hour, will observe a Doppler shift of order of magnitude $10^{-9}$. Multiply that by 433 MHz and you get 433 millihertz. The effect on the symbol rate is also tiny. These values are likely smaller than the temperature-dependent frequency error in the modules' local oscillators!

So, no, you don't need to worry about the motion of the vehicles. Your biggest problem is likely to be range. (Be prepared to, for example, mount the module and antenna outside the car, ideally in the center of the roof.)

Multipath propagation will probably be the biggest impairment to the link.

You did not mention how far apart the transmitter and receiver will be nor whether you expect to be in dense or sparse traffic.

In dense traffic with some separation between you and the other vehicle, you can assume that the signal you receive will consist of one direct path plus some number of multipath reflections:

$$s(t) = a_0s_0(t-\tau_0) + \sum_{i=1}^{N}a_is_0(t-\tau_i)$$

So you will be dealing with something called delay spread. In an urban environment, you should expect a mean delay spread of around 1 $\mu$s and a total spread of up to 3 $\mu$s.1 In a suburban environment, you can expect lower values: around 0.5 $\mu$s and 2 $\mu$s, respectively.2

Delay spread can cause inter-symbol interference (ISI). I would guess at a 4800 baud rate you would be safe, but I think in the end it will depend on your channel bandwidth, modulation, and error control (if any).

1. W.C.Y. Lee, Mobile Communications Engineering (2nd ed.; McGraw-Hill, 1982), p.49
2. Ibid.