The physical layer in 802.11p is the same as the physical layer in 802.11a except for the different sampling rate. The 802.11-2007 standards define three different PHY layer modes: 20 MHz, 10 MHz, and 5 MHz. The different modes can be achieved by using reduced clock/sampling rates. 802.11a usually uses the full-clocked mode with a 20-MHz bandwidth. In comparison, 802.11p usually uses the half-clocked mode with a 10-MHz bandwidth. The different modes affect the following parameters:
- Bandwidth – In 802.11p, 10 MHz is usually used, making the signal more robust against fading. The 20 MHz mode is optional.
- Carrier spacing – The 802.11p signal uses a carrier spacing reduced by half when compared to 802.11a.
- Symbol length – The symbol length is doubled, making the signal more robust against fading.
Besides the clock rate, the adjacent channel rejection (ACR) and the spectrum emission mask (SEM) are also changed [1, p. 9-10].
Aside from the above differences, there are many similarities between 802.11a and 802.11p. They both use orthogonal frequency-division multiplexing (OFDM) for transmission. It’s is a logical choice with many inherent advantages (see Nutaq’s OFDM reference design). If we watch the video presented in , we see that OFDM is the perfect choice. In V2V communications, the relative speed between a transmitter and a receiver is constantly changing, creating selective transmission channels. With its robustness to fading effects, OFDM is highly suited for this situation. However, let’s not forget about the Doppler effect, which generates carrier frequency offset. OFDM transmission is known to be sensitive to frequency synchronization.
Overall, 802.11p is definitely the future in the automotive market. Nutaq’s OFDM reference design, implemented on the PicoSDR or ZeptoSDR platforms, is a great way to start developing with this technology.