OVSF (Channelization Code) Allocation

Users data is modulated by a channelization code. The orthogonality properties of OVSF enable the UE to recover its bits without being interfered by other users.  This is true only if the system is synchronous, which is the case in downlink, but not in uplink. Thus, the OVSF codes are not used to separate users in uplink and therefore different users can use the same code. But they can be used to distinguish the different physical channels of one user.
Channelization code is defined as Cch SF, k,, where, SF is the spreading factor of the code, and k is the sequence of code, 0≤k≤SF-1. Each level definition length of code tree is SF channelization code, and the left most value of each spreading code character is corresponding to the chip which is transmitted earliest.      SF = chip rate / symbol rate     so, high data rates = low SF code, and low data rates = high SF code.
 


The channelization codes are Orthogonal Variable Spreading Factor (OVSF) codes. They are used to preserve orthogonality between different physical channels. They also increase the clock rate to 3.84 Mcps.  The OVSF codes are defined using a code tree.
A channelization sequence modulates one user’s bit. Because the chip rate is constant, the different lengths of codes enable to have different user data rates.  Low SFs are reserved for high rate services while high SFs are for low rate services.  
      SF in uplink is from 4 to 256.      SF in downlink is from 4 to 512
Purpose of Channelization CodeChannelization code is used to distinguish different physical channels of one transmitter

• For downlink, channelization code ( OVSF code ) is used to separate different physical channels of one cell

• For uplink, channelization code ( OVSF code ) is used to separate different physical channels of one UE  

For voice service (AMR), downlink SF is 128, it means there are 128 voice services maximum can be supported in one WCDMA carrier and for Video Phone (64k packet data) service, downlink SF is 32, it means there are 32 video phone services maximum can be supported in one WCDMA carrier. WCDMA      • Max traffic ch code = 512 (total) – 40 (CCH) = 472
      • Fixed SF=16
      • Hybrid ARQ12
      • Hybrid ARQ
      • No fast power control, no soft HO
      • Adaptive modulation/coding (QPSK or 16QAM)
      • PS scheduler depending on Ec/Nt, QoS
HSPA+
      • DC-HSDPA
      • MIMO support for QPSK/16QAM
      • 64QAM for some MS categories
In general, OVSF Code is occupied by common channel and for HSDPA (HS-SCCH and HS-PDSCH) as well as HSUPA. The rest of the code will be able to use by traffic channel. Upon the reservation of HS-PDSCH code parameter setting , it may occupy between 5-10 codes. 
Therefore, the total code left for traffic channel is about
(normalize at SF256) : 256 – (19+SF256 of HS-PDSCH(5,10)) = 157 – 77 codes at SF256 

OVSF and CE Consumption for DL DCH service 

Even HS-PDSCH will not utilize DL CE but A HSDPA User will  consume 1*SF256 (1 CE) in DL for A-DCH. 
OVSF and CE Consumption for UL DCH service

OVSF and CE Consumption for HSUPA 

By method of reservation by MANUAL then total 10*SF16 = 160 SF256 Code will be reserved for HS-PDSCH Code only.
Total 160 + 19 common channel = 179 codes are occupied and forbidden for traffic channel.  
Free code left for traffic channel = 256-179 = 77 Codes  
However, 1 SF32 is reserved for handover during CAC process . The actual free left code should be about 77- 8 = 69 Codes or about 34 AMR Voice. 

HSPA

Code Number for HS-PDSCH = 10  and it can be configure from 1 until 15 code

Source:http://www.ajusady.com/