Transcript OVSF Channelization Code Assignment/Arrangement for IMT …

OVSF Channelization Code
Assignment/Arrangement for
IMT-2000
Shih - Shien Fang
Oct. 18, 2001
Outline
Introduction to OVSF Channelization Code.
Previously Proposed Approaches.
My Work
Issues
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 subtree below the specific code is assigned.
The OVSF Codetree Structure
Cch,4,0 =(1,1,1,1)
Cch,2,0 = (1,1)
Cch,4,1 = (1,1,-1,-1)
Cch,1,0 = (1)
Cch,4,2 = (1,-1,1,-1)
Cch,2,1 = (1,-1)
Cch,4,3 = (1,-1,-1,1)
SF = 1
SF = 2
SF = 4
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.
Notations
Let C   a1 , a2 , a3 , a4 , a5 , a6 , a7  denote the available
codes for SF   4,8,16,32, 64,128, 256  respectively.
Total available rate:
W  C   a1  26  a2  25  a3  24  a4  23  a5  22  a6  21  a7 .
S  n  is a set of codewords than can support a total data rate
up to n and it can be optained by S  n   C | W  C   n, C.
The number of codes N  C  required for transmitting a codeword C
N  C   a1  a2  a3  a4  a5  a6  a7 .
Ct and C 't are the codewords of the system before and after
code assignment, respectively.
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.
VTC 2000 Approach
Explanation by example : Ct   0,0,0,0,2,1,3 , n  6.

 0, 0, 0, 0, 0, 0, 6  ,  0, 0, 0, 0, 0,1, 4  ,  0, 0, 0, 0, 0, 2, 2  

S  6  


,  0, 0, 0, 0,1, 0, 2  ,  0, 0, 0, 0, 0,3, 0  ,  0, 0, 0, 0,1,1, 0  

 C1 , C2 , C3 , C4 , C5 , C6 .
T  7   Ct  C1 , Ct  C2 , Ct  C3 , Ct  C4 , Ct  C5 , Ct  C6 

 0, 0, 0, 0,1,1,1 ,  0, 0, 0, 0,1,1,1 ,  0, 0, 0, 0,1,1,1 , 



0,
0,
0,
0,1,1,1
,
0,
0,
0,
0,1,
0,3
,
0,
0,
0,
0,1,
0,
3





C4 is selected to satisfy the two criteria.
VTC 2000 Approach
Fast code channel assignment algorithm to find the
preferred codeword Copt
Copt  Ct  Copt1  Ct   Ct   0, 0, 0, 0, 0, 0, n  
In the above example,
Copt1  Ct   0, 0, 0, 0, 0, 0, 6    0, 0, 0, 0,1,1,1 .
Copt  Ct  Copt1   0, 0, 0, 0, 2,1,3   0, 0, 0, 0,1,1,1
  0, 0, 0, 0,1, 0, 2   C4 .
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.
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.
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).
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.
The BS chooses these codes
for assignment.
Number of codes resident in Number of codes resident in
the sibling codetree = 1
the sibling codetree = 1
Number of codes resident in
the sibling codetree = 1
Rate occupied in the sibling Rate occupied in the sibling
codetree = 1
codetree = 1
Rate occupied in the sibling
codetree = 2
Metric = 2 x 1 – 1
=1
Metric = 2 x 2 – 1
=3
Metric = 2 x 1 – 1
=1
Example : available codeword   0, 0,1,3, 2  ,
call request codeword   0, 0,1,1, 0  .
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 2current-target 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 ( 2current-target-1) among these
candidates as anchors.
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.
My Work
Anchor
Anchor
i. Combine two 2R codes into a 4R code
ii. Combine two 4R codes into an 8R code
Example : to prepare one 8R code.
One more 4R
code is available!
iii. Operation is complete