FPGA Central

Hi, i was reading a tutorial on cellular cdma last night (namely Prof.
Andrew's http://www.ece.utexas.edu/wncg/cdmarefs/cdma_talk.pdf) and
have a few questions.

Please be gentle as i am very new with CDMA! Any help is very much
appreciated.

The tutorial talks about uplink and downlink channels. Are they on
different frequencies... if so, why?

It also mentions spreading codes. It says CDMA uses Walsh codes for
downlink, and pseudorandom sequence spreading code for the uplink. I
am ok with the operation of pseudorandom spreading, but havent heard
of Walsh codes before.

What is the difference between the two?

I am only having a guess but would i be right in thinking it is to do
with synchronisation issues? For example, using pseudorandom spreading
you would listen for a number of users to start transmitting. When you
detect a user you start to decode. Meanwhile another user could start
to transmit halfway through the decode of the other user. To sumarise
i guess i am saying that even though the codes arent syncrhonised (the
code sequences might start roling at different poitns in time), it is
still easy to differentiate between different users?

I hope this makes sense, again not 100% sure im even going down the
right road, but any pointers and corrections would be great.

Thanks in advance,

Chris

In the 3GPP standard specifictions there is two modes of operation
FDD: where Uplink and downlink are using different frequencies..
TDD: Uplink and downlink are using the same frequency (but multilpexed
in time)..

In the downlink the codes needed to be orthogonal and synchronized,
the Node_B send to all mobiles at the same time ,so the Hadamard codes
are used in the downlink..

In the uplink, every user sends to the Node-B at anytime.. no synchro,
between the users in the ulpink.


hope i was helpful.

Regards
Hany K. Mahmoud


[email protected] (Chris) wrote in message news:<[email protected] com>...
> Hi, i was reading a tutorial on cellular cdma last night (namely Prof.
> Andrew's http://www.ece.utexas.edu/wncg/cdmarefs/cdma_talk.pdf) and
> have a few questions.
>
> Please be gentle as i am very new with CDMA! Any help is very much
> appreciated.
>
> The tutorial talks about uplink and downlink channels. Are they on
> different frequencies... if so, why?
>
> It also mentions spreading codes. It says CDMA uses Walsh codes for
> downlink, and pseudorandom sequence spreading code for the uplink. I
> am ok with the operation of pseudorandom spreading, but havent heard
> of Walsh codes before.
>
> What is the difference between the two?
>
> I am only having a guess but would i be right in thinking it is to do
> with synchronisation issues? For example, using pseudorandom spreading
> you would listen for a number of users to start transmitting. When you
> detect a user you start to decode. Meanwhile another user could start
> to transmit halfway through the decode of the other user. To sumarise
> i guess i am saying that even though the codes arent syncrhonised (the
> code sequences might start roling at different poitns in time), it is
> still easy to differentiate between different users?
>
> I hope this makes sense, again not 100% sure im even going down the
> right road, but any pointers and corrections would be great.
>
> Thanks in advance,
>
> Chris