Why different access schemes for uplink and downlink?

[ashwinis...@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

[wirelessman]

Hi Ashwini,

Firstly, WCDMA and LTE are different.
* WCDMA is has same access scheme i.e both we have DS-CDMA based
multiple access

LTE has derived the HSPDA technology from Release 7 and from Release
8 of 3gpp, the PHYSICAL layer mapping has been changed to avoid
multiuser-interference at high data rate transmission.

In downlink, OFDMA based access is possbile because all the
subcarriers are in sync at basestation. while one cant expect that all
users in uplink can sync in frequency. Downlink its a single IFFT
operation.. while in uplink users do an IFFT over a portion of the
band..


rgds,
ssk

On Jan 22, 12:32 pm, "ashwinispatan...@gmail.com"

<ashwinispatan...@gmail.com> wrote:
> 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 sendwww.google.comthen he what actually he is doing is sending just few

[ashwinis...@gmail.com]

Dear SSK,

Thanks for the answer. Can you tell me the name of some good books as
there are for GSM (Wilkies and Garg) and CDMA (Garg) for UMTS and LTE.

Thanks and Regards
Ashwini

> > main reason is that if a user (MT) is sending any request say he sendwww.google.comthenhe what actually he is doing is sending just few

[wirelessman]

Hi Ashwini,

See a previous post.. Book on LTE and WCDMA by Holma is a good one to
begin..

Also recently book on HSDPA and LTE was published by Erik.

regards,
ssk

On Jan 23, 4:59 pm, "ashwinispatan...@gmail.com"

> > > main reason is that if a user (MT) is sending any request say he sendwww.google.comthenhewhat actually he is doing is sending just few