1. OVSF Channelization Code Assignment/Arrangement for IMT-2000 Shih - Shien Fang Oct. 18, 2001

2. Outline Introduction to OVSF Channelization Code. Previously Proposed Approaches. My Work Issues

3. Introduction to OVSF Codes Managed by the radio network controller (RNC) –OVSF codes preserve the orthogonality between uplink DPDCH and DPCCH from same terminal and between downlink channels of different rates and spreading factors. –For the OVSF codes, a code can be assigned to a UE if and only if no other code on the path from the specific code to the root of the tree or in the sub- tree below the specific code is assigned.

4. The OVSF Codetree Structure

5. Previously Proposed Approaches Multi-Code Approaches –“ OVSF Code Channel Assignment for IMT-2000 ” by Ray-Guang Cheng and Phone Lin, VTC 2000. –“ A Fair, Efficient, and Exchangeable Channelization Code Assignment Scheme for IMT-2000 ” by Fenfen Shueh, Zu-En Purple Liu and Wen-Shyen Eric Chen, ICPWC 2000.

6. Notations

7. VTC 2000 Approach Multi-code assignment scheme. Criteria to follow –Preserve more small-SF codes to provide a higher utilization. –Use as small amount of codes as possible to reduce the assignment complexity.

8. VTC 2000 Approach

10. ICPWC 2000 Approach Besides the multi-code assignment scheme, an arrangement scheme is proposed to overcome the “ code blocking ” phenomenon. Work flow –If admitted, transform the rate requirement into an appropriate codeword to fit the multi-code capability of the UE. –Assign codes from the right side of the tree. –When codes are released, system performs resource aggregation according to some threshold value, keeping the assigned codes aggregated on the right side and leave available codes on the left side.

11. My Work Integrated multi-code assignment/arrangement scheme. Considerations: –Applying code arrangement scheme globally upon code release is a great time-consuming job. Applying code arrangement scheme when the code resource that the RNC can provide exceed the multi-code capability of the UE. Spare appropriate codes to satisfy the multi-code capability of the UE only, rather than arranging the codetree globally. During code arrangement, the RNC tries its best to maximize the amount of small-SF codes. –The probability to find an available code is low as the SF goes small. Fully utilize the MC capability of the UE, minimizing the amount of small-SF codes required.

12. My Work Metric: the overall reassignment complexity of the codes resident in the candidate ’ s sibling codetree. –The higher rate the resident codes within the codetree occupy, the higher the overall reassignment complexity will be. –The higher the amount of small-SF codes within the codetree is, the higher the overall reassignment complexity will be. Metric calculation –The number of codes resident in the candidate ’ s sibling codetree is computed, denoted as X. –The total data rate provided by these codes is computed, denoted as Y. – (Y-X) is computed, denoted as. –The metric is computed as (Y+ ) = (2Y-X).

13. My Work Code assignment –If the number of available codes is larger than required, the BS chooses the codes whose sibling codetrees are of the highest reassignment complexity. That is, choose the available codes that are of the highest metric values. –Assignment order: from lowest SF to highest SF.

14. Number of codes resident in the sibling codetree = 1 Rate occupied in the sibling codetree = 1 Metric = 2 x 1 – 1 = 1 Number of codes resident in the sibling codetree = 1 Rate occupied in the sibling codetree = 2 Metric = 2 x 2 – 1 = 3 The BS chooses these codes for assignment. Number of codes resident in the sibling codetree = 1 Rate occupied in the sibling codetree = 1 Metric = 2 x 1 – 1 = 1

15. My Work Code arrangement –Occurs when the overall code resource is sufficient but the number of available codes is lower than required. –First the RNC ensures that there ’ re at least available codes for combination, where current indicates the level of the codes used for combination and target indicates the level of the codes we want to generate. Then the BS selects half the number of codes ( ) among these candidates as anchors.

16. My Work –The BS chooses among candidates as anchors the codes whose sibling codetrees are of the lowest reassignment complexity. That is, choose the candidate codes having the lowest metric value. –For codes that are to be reassigned, The BS first seeks outside the candidate code set the available codes that fit them most. If there ’ s no existence of such codes, then the BS picks up available codes from the candidate code set to proceed.

17. My Work Anchor i. Combine two 2R codes into a 4R code ii. Combine two 4R codes into an 8R code Anchor One more 4R code is available! iii. Operation is complete