How to determine the FFT size of LTE downlink receiver?

Question

I'm puzzled about the LTE FFT size.

Assuming an LTE base station works at 20M mode, the FFT size is 2048. Then, many users can be served by this base station.

Do all these users work at 20M mode (i.e., the FFT size of all these users are also 2048)?

OR

This 20M base station can serve users working at various modes (e.g., 1.4M, 5M, 10M, 15M, 20M modes)? For example, one user works at 1.4M mode with 128-point FFT and one user works at 10M mode with 512-point FFT can be served simultaneously by this 20M mode base station?

Answer 1

You don't need to sample at cell FFT rate. Just shifted the received signal to the desired subcarrier and take a lowpass filtering, then sample at user-specific bandwidth (see attached image). The shifting operation can be implemented by a multiplication with phase ramp in the time domain.

However samping at cell specific FFT rate is optimal because the lowpass filter needs some kind of roll-off.

Answer 2

As noted in the following links:

http://www.eetimes.com/document.asp?doc_id=1278096&page_number=2

and

http://www.informit.com/articles/article.aspx?p=1627062&seqNum=3

The uplink is a single frequency where the bandwidth depends on the needs of the user (see first link, third paragraph: "The bandwidth of the single carrier is determined based on the required data rate by the user"). So in answer to your first question - No, not all users use the same bandwidth. Different bandwidths for multiple users are part of the specification (see first link: "In the frequency domain, the number of sub-carriers N ranges from 128 to 2048, depending on channel bandwidth with 512 and 1024 for 5 and 10 MHz, respectively, being most commonly used in practice.").

As for your second question - yes, the base station can serve users of various modes (ie: different FFT sizes).

EDIT regarding comment: I'm not sure how resource blocks are assigned, but I'm sure the user is told which ones belong to them - and yes, I believe the sample rates are different depending on the users bandwidth. As for the clock (from first link): "This results in a sampling rate that's multiple or sub-multiple of the WCDMA chip rate of 3.84 Mcps: LTE parameters have been chosen such that FFT lengths and sampling rates are easily obtained for all operation modes while at the same time ensuring the easy implementation of dual-mode devices with a common clock reference."

Answer 3

LTE is very much "everything described in this humongous collection of standard documents is possible, you will have to accept what the base station tells you your downstream signal will look like".

It's highly adaptive and will give you as many ressource blocks as it deems you deserve, and will add as much coding as necessary to retain a certain BER etc.

If you're looking for something that can at least partially understand LTE downlink, have a look at Johannes Demel's gr-lte:

gr-lte provides blocks to synchronize to a LTE downlink signal. It performs OFDM operation to operate in the frequency domain and provides a channel estimator. At this point physical LTE downlink channels can be extracted from the symbols and be processed. PBCH is fully implemented and decodes MIB. PCFICH is available too. Almost all code for PHICH is implemented and there is lots of code to bring PDCCH support. It is not yet possible to just plug it in, though. Eventually all channels shall be supported.