Spreading factor difference in UL and DL

[electron]

Hi all,

Can anybody explain why for same air rate for UL we have lesser spreading factor as compare to DL in WCDMA ?


Cheers

[wolverine]

In the UL, BPSK symbols are used which give you a 1:1 ratio between symbols and bits.

In the DL, QPSK symbols are used which give you a 1:2 ratio between symbols and bits. As such you can get double the data rate for the same spreading factor.

[electron]

In the UL, BPSK symbols are used which give you a 1:1 ratio between symbols and bits.

In the DL, QPSK symbols are used which give you a 1:2 ratio between symbols and bits. As such you can get double the data rate for the same spreading factor.

Hi Wolverine,


Would you please share a reference document.Thanks


Cheers

[fdanaei]

Hi dear Friend,
For WCDMA (after Rel.4) in HSDPA and HSUPA you can refer to page 19&20 of attached file.
As we know, in DL (HSDPA) SF of every code is fix (equal 16).
For WCDMA Rel.99, wolverine's description is enough, I think.
I hope it can help you,


Hi all,

Can anybody explain why for same air rate for UL we have lesser spreading factor as compare to DL in WCDMA ?


Cheers

[firstmaxim]

In WCDMA R99:
In DL we have SF = 4 to 512.
For UL, the SF = 4 to 256.

The essence is to bring parity between uplink and downlink speeds as far as the standards go.
The user symbol rate = chip rate/SF. Since, the DL employs QPSK (where each symbol represents 2 bits), and UL employs BPSK (where each symbol represents 1 bit). If you check the data rates for DL with SF 512 and UL with SF 256, you would arrive at the same user throughput. On a sidebar, BPSK is used on UL, rather than QPSK, due to biomedical compatible reasons.