Different Access Schemes for Uplink and Downlink

[a***@gmail.com]

Hi Every One,

I am confused regarding these different access schemes for up link
and
downlink or more precisely different access schemes in forward and
reverse channel of UMTS / LTE(you can generalise my question).


As far as I am getting, say we use OFDMA for downlink (Base Station
to
Mobile Terminal) and SC-FDMA (Mobile Terminal to Base Station), the
main reason is that if a user (MT) is sending any request say he send
www.google.com then he what actually he is doing is sending just few
kbs of data but in return he will get data more then he send that is
why the download data rate should be high, thats why we use different
access schemes.


But still many things are unclear like, okay we are using OFDMA for
higher data rate but what about a video call the user will make?
there
also he will need high data rate because of real time constraints
then
why Sc - FDMA and not OFDMA for that????


Thanks and Regards
Ashwini

[Andreas Wenzel]

Post by a***@gmail.com
[...]
But still many things are unclear like, okay we are using OFDMA for
higher data rate but what about a video call the user will make?
there
also he will need high data rate because of real time constraints
then
why Sc - FDMA and not OFDMA for that????

According to this paper:

http://www.comsysmobile.com/pdf/LTE_PHY_SC_FDMA.pdf

SC-FDMA is beneficial due to its lower PAPR (peak to average power
ratio) which helps power amplifier efficiency. In a mobile device, you
want the most efficient PA possible because you have to deal with tiny
batteries. On the eNB-side, you can afford to trade in a little power
for better speed as you usually run on mains power.

According to the Ericsson website at

http://www.ericsson.com/ericsson/corpinfo/publications/review/2007_01/02.shtml

An SC-FDMA uplink still pumps 50mbit/s, that should easily be enough for
any type of video call.

Andreas