Why we need mixer in UWB or any wideband system?

May I ask:
-Why we need mixer in UWB or any wideband system?
-NB system, requires Mixer due to reduce filter Q factor but for UWB, Q factor will be 3-5, which is low enough for relax design.

Thanks

The use of mixer is for frequency conversion (down convert or upconvert)

you are right, but we use mixer to relax LPF with Lower Q in NB system.
Without mixer it will be impossibl. But in UWB we don't have this problem...
LPF Q, will be low which is doesn't require any Mixer.(I think)

See the image attached

There are a lot of different UWB technologies and not all of them require frequency conversion. For example, I am doing a lot of UWB designs without mixer.

Mixers should be avoided wherever is possible because it is probably the most vulnerable to any interference and non-linear part of the RF system. Conversion usually use to simplify amplification and filtration of the receiving signal as well as have these parameters equal across the whole band. Reducing required Q is not the primary direct reason of mixer usage. There are other ways to change the Q.