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LTE, FFT, OFDM

Started by Sharan123 November 15, 2015
On Tue, 01 Dec 2015 03:46:08 -0500, Randy Yates
<yates@digitalsignallabs.com> wrote:

>eric.jacobsen@ieee.org (Eric Jacobsen) writes: > >> On Mon, 30 Nov 2015 03:27:36 +0000 (UTC), spope33@speedymail.org >> (Steve Pope) wrote: >> >>>Eric Jacobsen <eric.jacobsen@ieee.org> wrote: >>> >>>>The CP length or presence or absence or the processing thereof has no >>>>affect on effective subcarrier width in an OFDM system, so I would >>>>disagree with that statement in the context of answering the OPs >>>>questions in that area so far. There are lots of parts and pieces to >>>>OFDM systems that have not been touched on here, appropriately so >>>>because they're not germaine to the question. >>> >>>Well, the question was whether the subcarrier width is the same >>>as the subcarrier spacing; so the possible differences between these >>>two was thereby "touched upon". >> >> And if the idea is to understand the workings of an OFDM system, which >> I think was the case, an answer in that context is, IMHO, better than >> one outside of it. > >Eric / Steve, > >I don't know what you mean by "inside/outside" but this question >has perked my interest in the topic. Why IS the carrier bandwidth >different than the carrier spacing?
I don't think it is different. Steve seems to me to just be pointing out a particular point of view whereby if you hold things up to the light just right one can claim that the subcarrier bandwidths have shrunk a little bit. It's certainly not different to the receiver, though, since the subcarriers *have* to maintain a high degree of orthogonality to get reasonable performance. That won't happen if their bandwidth changes.
>I am woefully ignorant on the subject, but isn't the answer independent >of OFDM? Isn't it that the FFT can viewed as a set of bandpass filters >(a filter bank), and that each of these filters are not Fs / N wide but >something less due to the mechanics of the FFT?
This kind of begs for nitpicking on the definition of bandwidth, sadly. Since window functions change the 3dB width of the sinx/x response of each bin, some will say that that alone will change their bandwidth. So what's the normal BW of an FFT bin? I've always liked the rectangular-window response where the 3dB width equals the bin width as the starting reference, and from that perspective the binwidth = the bin BW, but that's just me. OFDM generally uses rectangular windows partly for this reason, so that there is minimum interference between subcarriers in the presence of synchronization errors.
>By the way, I've watched you guys argue for days (weeks?) now. I wish >you would both acknowledge each other's maturity and capabilities, and >spend all this energy for positive things. I, for one, respect you both >immensely.
I don't think anybody's done any real mud-slinging or claimed anybody else isn't capable or competent (that I recall, anyway). I think it's a fun dialogue. Maybe I'm weird that way. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Dear Eric, Steve, et al.

I have been trying to work out OFDM in a little more detail.

I am taking 20 & 15 MHz channels, as example, to explain the question.

The bandwidth (or sample rate) = 30.72 MHz, 15.36 MHz for 20 and 10 MHz
respectively.

Sample time Ts (1/bandwidth) = 33 ns, 65 ns for 20 and 10 MHz
respectively.

Symbols time (excluding CP) = 1/15000 = 66 us

Cyclic prefix period = for 20 MHz channel, 144 bits x Ts for 20 MHz 
                 144 x 33 ns = 4.6 us

Total symbol time including cyclic prefix for 20 MHz channel = 71 us

In case of 10 MHz channel, cyclic prefix period = 144 x Ts for 10 MHz
                                                  144 x 65 = 9.3 us

Total symbol time including cyclic prefix for 10 MHz channel = 76 us

However, I do understand that the total symbol time including the CP
remains same for all the BW. So, where I am going wrong in my
calculations?
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Dear Eric, Steve, et al.

I have few questions related to LTE/OFDM. I have summarized them below.
I did not kept these questions separate from my previous above to avoid
confusion ...

1) In the base station downlink, does iFFT step handle 
a) only frequency domain to time domain conversion
b) it handles OFDM 
c) just a part of OFDM. 

I believe, it is a) and b) but want to just make sure ...

2) During the OFDM symbol period, Tu, the number of periods for each
sub-carrier is an integer number. Is it 1/2/3 etc. for first/second/third
etc. sub-carriers?

3) I have been thoroughly confused with TTI parameter. What role does this
play on the PHY layer?

Given a bandwidth, I believe that rate of data, symbols in a time slot,
symbol period are all fixed. Hence I am trying to understand how TTI
influences PHY parameters.

4) why the transmit power of eNodeB is higher than power from terminals?
is this to achieve higher downlink speed?

I thank you all in advance ...
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On Fri, 25 Dec 2015 11:59:01 -0600, "Sharan123" <99077@DSPRelated>
wrote:

>Dear Eric, Steve, et al. > >I have been trying to work out OFDM in a little more detail. > >I am taking 20 & 15 MHz channels, as example, to explain the question. > >The bandwidth (or sample rate) = 30.72 MHz, 15.36 MHz for 20 and 10 MHz >respectively. > >Sample time Ts (1/bandwidth) = 33 ns, 65 ns for 20 and 10 MHz >respectively. > >Symbols time (excluding CP) = 1/15000 = 66 us > >Cyclic prefix period = for 20 MHz channel, 144 bits x Ts for 20 MHz > 144 x 33 ns = 4.6 us > >Total symbol time including cyclic prefix for 20 MHz channel = 71 us > >In case of 10 MHz channel, cyclic prefix period = 144 x Ts for 10 MHz > 144 x 65 = 9.3 us > >Total symbol time including cyclic prefix for 10 MHz channel = 76 us > >However, I do understand that the total symbol time including the CP >remains same for all the BW. So, where I am going wrong in my >calculations? >--------------------------------------- >Posted through http://www.DSPRelated.com
If you haven't seen it, this is a good description of the nuts and bolts of the basics of the physical layer for LTE: https://home.zhaw.ch/kunr/NTM1/literatur/LTE%20in%20a%20Nutshell%20-%20Physical%20Layer.pdf The section labelled Physical Layer Pararmeters has some relevant stuff. I had to look this up again as I couldn't recall how the CP lengths were managed. I think the only thing you missed is that the time length of the CP is not bandwidth dependent. Remember that the length of the CP is chosen based on the expected lengths of the channel impulse response, which won't change with the signal bandwidth. So the CP for the 10MHz case will have fewer samples. It looks like fs = 23.04MHz for the 10MHz BW case, so the CP would have around 23.04e6*4.7us = ~108 samples, for the 4.69us CP case. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
On Fri, 25 Dec 2015 12:23:52 -0600, "Sharan123" <99077@DSPRelated>
wrote:

>Dear Eric, Steve, et al. > >I have few questions related to LTE/OFDM. I have summarized them below. >I did not kept these questions separate from my previous above to avoid >confusion ... > >1) In the base station downlink, does iFFT step handle >a) only frequency domain to time domain conversion >b) it handles OFDM >c) just a part of OFDM. > >I believe, it is a) and b) but want to just make sure ...
It's probably a point-of-view thing, but I think it's c). That's a fundamental part of what OFDM is, but a) and b) are essentially also true. Especially a). ;)
>2) During the OFDM symbol period, Tu, the number of periods for each >sub-carrier is an integer number. Is it 1/2/3 etc. for first/second/third >etc. sub-carriers?
Just like any FFT, the bin number describes how many integer cycles there are for that bin over the duration of the transform window. It's the same in OFDM. That being said, numbers or indices can be arbitrarily assigned to bins/subcarriers for bookkeeping purposes that may differ from the usualy signal processing indices. I don't know whether that is done much in LTE, but that is done in some systems.
>3) I have been thoroughly confused with TTI parameter. What role does this >play on the PHY layer?
Not much. It is a constant 1ms slice of physical resource that the MAC can use to keep latency low. The doc I referenced earlier, https://home.zhaw.ch/kunr/NTM1/literatur/LTE%20in%20a%20Nutshell%20-%20Physical%20Layer.pdf shows how one TTI is split into resource block quanta, which are the minimum chunks of assignable phy layer, i.e., 12 subcarrier for 14 symbols. These quanta get filled by the MAC based on traffic demand for each user, as controlled by the scheduler.
>Given a bandwidth, I believe that rate of data, symbols in a time slot, >symbol period are all fixed. Hence I am trying to understand how TTI >influences PHY parameters.
My understanding is that TTI is just a fixed 1ms interval, and is sort of the basic time clock of the MAC. Since the LTE frames and symbol timing are locked with that, it makes translation from the MAC to PHY resources simpler.
>4) why the transmit power of eNodeB is higher than power from terminals? >is this to achieve higher downlink speed?
I usually think of it the other way, that the handset Tx power is limited to manage battery life, which is a managed resource on the handset. It is not as much of a problem at the NodeB (Base Station), so the power can be cranked up (or just not cranked down) to maintain link reliability. There is a fair amount of asymmetry in the LTE system, not the least of which is that the BS (NodeB) can make use of antenna spatial diversity (or other diversity or even MIMO techniques), to enhance the link, while the handset cannot as much due to its size. This makes it possible for the NodeB to achieve diversity or SIMO/MIMO gains at the receiver in the Uplink, which allows the handset to transmit at lower power (and save battery). In the downlink, the BS (NodeB) can potentially do beamsteering or transmit diversity encoding (e.g., Alamouti), but the handset is too small to get much antenna diversity in the receiver. LTE does use MIMO, but the limitations in diversity due to the handset size limit things on that end of the link. Basically, all of those add up to good reasons to use more power in the downlink than uplink, but another good reason is just Becasue You Can. The BS isn't power limited like the handset, so it allows the additional transmit power capability to increase DL reliability and also increase the DL rate, as you mentioned. Since the DL has to be able to spray out a lot more data than the UL from a terminal typically needs, this is just all part of the overall system engineering. It's a very asymmetric system.
>I thank you all in advance ... >--------------------------------------- >Posted through http://www.DSPRelated.com
Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Dear Eric,

Thank you and wishing you season's greetings!

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