Hello everyone,
as promised last friday, I can now provide you with more details
regarding Multiple Access in LTE and LTE Advanced. Short context for
everyone that was not present last friday: I got a couple of questions
regarding the Multiple Access Scheme used in LTE. There was some
conflicting information in lecture and exercise, but the problem is now
resolved. Let me address the two major questions I received:
1) The exercise mentions on slide slide 20, that LTE Advanced (4G) uses
a "particular combination of OFDM with CDMA & TDMA". Therefore I got
some questions asking me if cell breathing is still a problem.
*Answer: *No, this is an error. LTE and LTE Advanced both do NOT use
CDMA, therefore cell breathing is not an issue anymore. This is an error
in the exercise slides, which I just removed. Therefore please download
the new and updated version of the exercise slides present on the website.
2) The lecture names the Multiple Access schemes for LTE (3.9G) on slide
4.121, naming OFDMA & SC-FDMA for Downlink and Uplink respectively. On
slide 4.129 however a figure is presented that shows, that frequency
bands are shared over time between users (i.e. TDMA). This resulted in
some confusion, since TDMA is not mentioned on slide 4.121.
*Answer: *If you look at slide 3.8 you can see, that OFDM is considered
a combination of FDM & TDM, since the orthogonal carriers are only
assigned to a user for a certain time. Since OFDM is the basis for
OFDMA, OFDMA implicitly includes TDMA, explaining why the figure on
slide 4.129 shows the usage of time slots.
I hope this resolved all questions regarding LTE / LTE Advanced and the
Multiple Access scheme used. If there was a question regarding this
topic that I forgot to address, please contact me.
Kind regards,
Torben
--
Torben Petersen
Technische Universität Braunschweig
Institut für Betriebssysteme und Rechnerverbund
Mühlenpfordtstr. 23
38106 Braunschweig
E-Mail:petersen@ibr.cs.tu-bs.de
Website:https://www.ibr.cs.tu-bs.de/users/petersen/
