DAC & filter spec for TX (e.g. LTE)

[exp]

Hi all,

I am just wondering what are practical specifications of the components for a wideband transmitter (e.g. LTE 20MHz) in terms of:
1) DAC sampling rate
2) DAC reconstruction filter: Type, order, cutoff, image rejection

What puzzles me: Suppose we want to transmit LTE 20MHz (I know it's effectively 18, but let's go with 20). Then the effective bandwidth per I or Q branch is 10 MHz.
The sampling rate according to standard is only 30.72 MHz which suggests that the DAC will also run at 30.72 MHz. True? This is just 1/3 oversampling.

How an ideal zero-order-hold would give us images at 30.72MHz, 61.44MHz, 92.16 MHz etc. Having all 10 Mhz allocated, this means that the first image starts at 30.72-10 Mhz = 20.72Mhz.
This means the reconstruction filter would need to attenuate from 10MHz to 20.72Mhz.
Isn't this infeasible?

Suppose the cutoff frequency would directly be placed at the bandwidth (10MHz). First of all, this would already result in huge phase distortion, right?
Second, suppose we take a fifth order Butterworth filter (which I think is already impractical). Then the image would be attenuated not even by 30dB! (of course, it is a little bit more because the image is already a bit attenuated due to the sinc response of the ZOH).

How does that work in practice?

Thanks!

 

[FvM]

Did you look at LTE chipset specifications? I/Q-modulators are often using DACs with with built-in interpolation filters and oversampling, I won't be surprized if this technique is used for LTE as well.

General rule of thumb in recent DSP design, if you can avoid complex analog filters by digital means without increasing power demand too much, do it.

 

[exp]

Yeah at least I googled but I was not so successful. I found some transceivers, they some of them mention the sampling rate of the DAC (which is not very useful however, because they are usually generic transceivers) but none of them shows specs for the filter.
I know about the interpolation filter but this is effectively just increasing the sampling rate of the DAC. Increasing the sampling rate by a factor of 4-8 does not sound cheap to me too ...

Suppose for example a base station which operates at 100 MHz (aggregated) - or just 20 Mhz with predistortion (that also needs 5x the BW). A factor of 4 would already make up a 2x400 MHz DAC - sounds really much to me...