PDA

View Full Version : Question Why don't we use SCFDMA for both UL and DL in LTE ?



fahmi
2011-11-15, 08:26 AM
Hi LTE experts, When reading some documents about LTE, I am wondering why don't we use SCFDMA for both UL and DL ? It was clear that the SCFDMA show less PAPR than OFDMA, that's why it was deployed for UL. But why we did not use it also for DL ??? many thkss.

Darko
2011-11-15, 08:52 AM
First, PAPR is of importance for UL, not so for DL. In SC-FDMA you have only one carrier, but you can have many of these carriers in band and they are (usually) NOT orthogonal as in case of OFDMA.

fahmi
2011-11-15, 09:46 AM
ok, I found these explanations : - The reason you wouldn't use SC-FDMA for downlink is that it would significantly increase PAPR because you are adding multiple users' SC-FDMA signals. The whole point of using SC-FDMA over OFDMA is the low PAPR and in the downlink the merit of low PAPR for SC-FDMA is not too obvious. - In SCFDMA there is two extra added blocks ( DFT and freqeuncy mapping) (comared to OFDMA), so this will introduse more complexcity in the processing block of Tx unit. -The main reason using OFDMA is for its fading performance over SCFDMA (selecting only sub-carriers with good channel quality). Thanks for your time and look forward to hear your feedback.

fahmi
2011-11-15, 10:01 AM
Understanding SC-FDMA as used in LTE. Let's assume that there are 4 mobiles talking to one base. If we use a OFDM signal for each one, then the max PAR is equal to IFFT size, assume 64. Now tell each mobile to confine its signal to the first 16 carriers and the second one to the second 16 carriers, so on. How can a mobile do that? If it does a 64 point IFFT, the signal will be spread over all 64 carriers. The way is for it to do a 16 point DFT. Now the data is spread over just 16 carriers. (time to frequency) Each mobile does the same thing, has a 16 carrier signal. Now by pre-arrangement, each mobile is told just exactly where it will array its carriers in the 64 carrier OFDMA space (note the A at the end, which makes OFDM a Multiple Access system.) The first mobile now does a 64 point IFFT by first mapping its data to just the first 16 positions, and filling the rest with zeros, also called zero-padding. The second mobile does the same, except it puts its data in the second block, pads the rest with zeros and does a 64 point IFFT. Each mobile sees a max PAR of only 16, instead of 64. This is a big reduction. The outgoing signal for each mobile after having done a 16 point DFT and then a zero-padded 64 point IFFT is a single-carrier signal. What is the impact of the larger size IFFT? The larger IFFT only adds some time-domain interpolation but other than that this is the same data at the input. Now it is transmitted at a single assigned frequency. When four such signals are received by the base even if at the same frequency, the base can extract out the individual signals. It first does one 64 point FFT (time to frequency) and sees carriers from all four mobiles. It then picks out each of the blocks, first 16 for mobile 1..., and does a IDFT to convert the signal back to time domain. The mapping can be distributed instead of block-based. Some are called LFDMA, MFDMA etc. Charan Langton

T_ADD
2011-11-15, 11:32 AM
Hi, it is true that OFDMA has a higher PAPR (peak to average power ratio) or spikes. But that does not mean it can not be used in the uplink. The practical reason for not using OFDM in the UL is to save battery power. If it was used then the mobile battery power would run down too quick because of the bursty nature of data signals which in turn causes high PAPR.

ahmedtaha
2011-11-15, 04:00 PM
hi all.. plz look to this example...may can help you....BRs

punto
2011-11-15, 04:42 PM
please shares thise documents/trainings where you finds this nice picture
thanks P:)

ahmedtaha
2011-11-15, 04:50 PM
please shares thise documents/trainings where you finds this nice picturethanks P:)you can download it from this post ....(http://www.finetopix.com/showthread.php?23454-LTE-Radio-Planning_RL10&highlight=) ....

..BRs