OFDM with CSMA/CA

Question

Most Wifi standards (e.g. 802.11a) use OFDM for orthogonal frequency multiplexing, i.e. assigning different subcarriers/frequencies to different users. On the other hand, it says CSMA/CA is used to manage the access in the time domain and avoid collisions of two users transmitting on the same frequency at the same time.

Why do I need both? If I have different frequencies for different users, that should suffice to avoid collisions. Or are these frequencies not exclusively assigned to a single user but to a group of users? Or is in fact CSMA/CA only used in Wifi standards without OFDM?

Answer 1

I do not see why you think that Wifi assigns subchannels to different users.

Wifi is OFDM but not OFDMA. In fact Wifi does almost nothing to allow simultaneous usage of bandwidth. Other standards that do use OFDMA do both frequency and time sharing to allow for more simultaneous users (Multiple Access).

In Wifi OFDM is used only to increase the datarate at the cost of needing a higher signal/noise ratio. This explains why you get a lower datarate in an environment with a lot of Wifi activity, the system switches back to a lower datarate so that it can cope with a worse signal to noise ratio.