Code Placement/Replacement Strategies on the W-CDMA OVSF ...

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Code Placement/Replacement Strategies on the W-CDMA OVSF ...

  1. 1. Chapter 11 Code Placement and Replacement Strategies for Wideband CDMA OVSF /ROVSF Code Tree Management <ul><ul><li>Associate Prof. Yuh-Shyan Chen </li></ul></ul><ul><ul><li>Dept. of Computer Science and Information Engineering </li></ul></ul><ul><ul><li>National Chung-Cheng University </li></ul></ul>
  2. 2. 1. Introduction
  3. 3. 1. Introduction
  4. 4. 1. Introduction 2G GSM 2.5G GPRS 3G UMTS
  5. 5. Structural network architecture 3G UMTS system architecture 1. Introduction Uu Cu Iub Iur Iu USIM ME UE BS BS BS BS RNC RNC MSC/ VLR UTRAN HLR GGSN GMSC SGSN CN Iur-CS Iur-PS UE
  6. 6. OVSF code tree 1. Introduction
  7. 7. 1. Introduction
  8. 8. 2. Problem statement <ul><li>Code placement problem </li></ul><ul><ul><li>Code blocking probability </li></ul></ul><ul><ul><li>Internal fragmentation </li></ul></ul><ul><li>Code replacement problem </li></ul><ul><ul><li>Code reassignment cost </li></ul></ul>
  9. 9. Example of OVSF Code Tree Code blocking : used code : new request :4R
  10. 10. Example of OVSF Code Tree Code blocking : used code : new request :2R
  11. 11. Example of OVSF Code Tree Internal fragmentation : used code : new request : 3R
  12. 12. Example of OVSF Code Tree Internal fragmentation : used code : new request : 3R
  13. 13. 3. Code placement and replacement strategies <ul><li>Y.-C. Tseng and C.-M. Chao, &quot; Code Placement and Replacement Strategies for Wideband CDMA OVSF Code Tree Management &quot;, IEEE Trans. on Mobile Computing , Vol. 1, No. 4, Oct.-Dec. 2002, pp. 293-302. </li></ul><ul><li>Tseng’s Code p lacement schemes </li></ul><ul><ul><li>Random p lacement scheme </li></ul></ul><ul><ul><li>Leftmost p lacement scheme </li></ul></ul><ul><ul><li>Crowded-first p lacement scheme </li></ul></ul>
  14. 14. A new call of rate 2R
  15. 15. 3. Code replacement strateg y <ul><li>Tseng’s Code rep lacement schemes </li></ul><ul><ul><li>Find the minimum-cost branch </li></ul></ul><ul><ul><ul><li>Based on DCA </li></ul></ul></ul><ul><ul><li>Relocate until done </li></ul></ul><ul><ul><ul><li>Based on c ode p lacement schemes </li></ul></ul></ul>
  16. 16. A Code Replacement Example A new call of rate 8R
  17. 17. Multi-Code Approach <ul><li>C.-M. Chao, Y.-C. Tseng, and L.-C. Wang, &quot; Reducing Internal and External Fragmentations of OVSF Codes in WCDMA Systems with Multiple Codes &quot;, IEEE Wireless Communications and Networking Conf. (WCNC) , 2003. </li></ul>
  18. 18. Tseng’s multi-code assignment <ul><li>Order of Assignment: </li></ul><ul><ul><li>increasing </li></ul></ul><ul><ul><li>decreasing </li></ul></ul><ul><li>Co-location of Codes: </li></ul><ul><ul><li>united strategy </li></ul></ul><ul><ul><li>separated strategy </li></ul></ul><ul><li>Assignment of Individual Codes: </li></ul><ul><ul><li>Random </li></ul></ul><ul><ul><li>Leftmost </li></ul></ul><ul><ul><li>Crowded-first-space </li></ul></ul><ul><ul><li>Crowded-first-code </li></ul></ul>
  19. 19. 1 2 3 … n : number of multicode N(i) : ideal (optimal) N ( i ) … 4 n n n n single code multi-code N(i)=Number of 1 s in (i) 2 For any given i, we can find a N(i) Number of code 3. Code Placement and Replacement Strategies <ul><li>Tseng’s i nternal f ragmentation solution </li></ul>
  20. 20. Internal Fragmentations 3. Code Placement and Replacement Strategies
  21. 21. Tseng’s multi-code assignment Possible candidates for 6 R ( n =2: 4 R+2R ) ( decreasing ): Leftmost: {C 8 ,1 , C 16,3 } Crowded-first-space : { C 8 ,8 , C 16,14 } Crowded-first-code : { C 8 ,3 , C 16,7 }
  22. 22. Tseng’s multi-code re-assignment <ul><li>Dynamic code assignment (DCA) scheme was proposed to solve the single-code reassignment problem </li></ul><ul><li>Authors utilize the DCA scheme as a basic construction block. When moving codes around. Authors also consider where to place those codes that are migrated so as to reduce the potential future reassignment cost (this issue is ignored in DCA). </li></ul>
  23. 23. Tseng’s multi-code re-assignment New requested call: 6 R ( n =2: 4 R+2R ) (decreasing): Free capacity : 9 R Leftmost
  24. 24. Our Single-Code Placement and Replacement Strategies <ul><li>Yuh-Shyan Chen and Ting-Lung Lin, &quot;Code Placement and Replacement Schemes for W-CDMA Rotated-OVSF Code Tree Management,&quot; is submitted to The International Conference on Information Networking, ICOIN 2004 , Feb. 18 - Feb. 20, 2004, Korea. </li></ul>
  25. 25. Outline <ul><li>Introduction </li></ul><ul><li>Background Knowledge </li></ul><ul><li>Code Placement and Replacement Strategies </li></ul><ul><li>Performance Analysis </li></ul><ul><li>Simulation Results </li></ul><ul><li>Conclusion </li></ul>
  26. 26. I. Introduction <ul><li>This paper proposes a code replacement scheme based on ROVSF code tree </li></ul><ul><li>This scheme aims to develop </li></ul><ul><ul><li>Code placement strategy </li></ul></ul><ul><ul><ul><li>Reduce blocking probability </li></ul></ul></ul><ul><ul><li>Code replacement strategy </li></ul></ul><ul><ul><ul><li>Reduce reassignment cost </li></ul></ul></ul>
  27. 27. Motivation <ul><li>Existing OVSF-based scheme has a lower spectral efficiency and a higher system overhead </li></ul><ul><li>This study aims to develop a more efficient channelization code scheme </li></ul>
  28. 28. Contributions <ul><li>An alternative solution for code placement and replacement schemes is proposed </li></ul><ul><li>Advantage of the ROVSF-based scheme </li></ul><ul><ul><li>Lower blocking probability </li></ul></ul><ul><ul><ul><li>Better spectral efficiency </li></ul></ul></ul><ul><ul><li>Lower reassignment cost </li></ul></ul><ul><ul><ul><li>Keep the system overhead low </li></ul></ul></ul>
  29. 29. II. Background Knowledge <ul><li>Related Works </li></ul><ul><li>OVSF Code Tree </li></ul><ul><li>Rotated-OVSF Code Tree </li></ul><ul><ul><li>Linear-Code Chain </li></ul></ul>
  30. 30. Related Works <ul><li>OVSF-based Scheme </li></ul><ul><ul><li>Dynamic Code Assignment </li></ul></ul><ul><ul><ul><li>IEEE Journal on Selected Areas in Comm., Aug. 2000 </li></ul></ul></ul><ul><ul><li>Single-code Placement & Replacement </li></ul></ul><ul><ul><ul><li>Proc. of IEEE Trans. on Mobile Computing, 2002. (Y.C. Tseng) </li></ul></ul></ul><ul><ul><li>Multi-code Assignment </li></ul></ul><ul><ul><ul><li>IEEE Wireless Comm. and Networking Conf., 2003. (Y.C. Tseng) </li></ul></ul></ul><ul><li>OVSF-like Scheme </li></ul><ul><ul><li>FOSSIL </li></ul></ul><ul><ul><ul><li>Proc. of IEEE ICC, 2001. </li></ul></ul></ul>
  31. 31. Review of OVSF Property
  32. 32. Our of ROVSF Property
  33. 33. Important Properties of ROVSF Code Tree <ul><li>A ROVSF code is cyclic orthogonal to its two children codes </li></ul>: used code : orthogonal codes
  34. 34. Important Properties of ROVSF Code Tree (cont.) <ul><li>A ROVSF code is cyclic orthogonal to any descendent codes </li></ul>: used code : orthogonal codes
  35. 35. Important Properties of ROVSF Code Tree (cont.) <ul><li>A ROVSF code is not cyclic orthogonal to any descendent of its brother code </li></ul>X X X X : used code
  36. 36. Linear-Code Chain <ul><li>A collection of mutually orthogonal codes </li></ul><ul><ul><li>Every node of a OVSF code tree is mapping to the corresponding node of a ROVSF code tree to form the linear-code chain </li></ul></ul><ul><li>Prior to designate where to allocate each supported request </li></ul><ul><ul><li>Rate restriction of transmission requests </li></ul></ul><ul><ul><li>Reduce blocking of high-rate request </li></ul></ul>
  37. 37. Code Placement in OVSF Code Tree : used code
  38. 38. Example of Linear-Code Chain : used code
  39. 39. Two Types of Linear-Code Chain
  40. 40. III. Code Placement and Replacement Strategies <ul><li>Placement Scheme </li></ul><ul><ul><li>L inear- C ode C hain ( LCC ) Placement Phase </li></ul></ul><ul><ul><li>N on-linear- C ode C hain ( NCC ) Placement Phase </li></ul></ul><ul><li>Replacement Scheme </li></ul><ul><ul><li>Dynamic Adjustment Operation of Linear-Code Chain </li></ul></ul>
  41. 41. LCC Placement Phase <ul><li>If exists ( b k , b k- 1 , b k- 2 , 0,…, 0) and β< j , then the assignment is failed even if b β = 0 </li></ul>1 R (1, (1, 1) , 0) (1, 0, 0) (1, 0, 1 ) X X X X : used code : new request
  42. 42. Example of LCC Placement Phase <ul><li>If b β = 1 and there is b γ = 1 and γ<β , then the assignment is failed </li></ul>(1, 1, 1 ) (1, 0, 1) 2 R (1, 1 , 1) X : used code : new request
  43. 43. NCC Placement Phase <ul><li>If YR is failed in LCC placement phase, then enters NCC placement phase </li></ul><ul><li>If there exists linear-code chain ( b k =1, 0,…,0), where γ =log 2 Y and γ = k , we may assign YR to neighboring node of node N of linear-code chain on the same level of ROVSF code tree, where transmission rate of node N is 2 k </li></ul>
  44. 44. Example of NCC Placement Phase (0, (1, 1) , 0) (1, 1, 1 ) 2 R X X X X X X X
  45. 45. Summary of Code Placement <ul><li>More codes are assigned in linear-code chain will result in a lower blocking probability </li></ul><ul><li>Dynamic adjustment operation of linear-code chain is introduced in code replacement scheme </li></ul>
  46. 46. Replacement Scheme <ul><li>The purpose of this procedure </li></ul><ul><ul><li>Force the code blocking probability to zero </li></ul></ul><ul><li>We adopt the same concept of DCA algorithm </li></ul><ul><ul><li>ROVSF-version DCA algorithm </li></ul></ul><ul><li>Our proposed placement strategy is adopted while relocating each code </li></ul>
  47. 47. Example of Replacement Scheme 4 R cost = 1 cost = 2 cost = 4 cost = 3 : used code : minimum-cost branch : occupied code
  48. 48. Dynamic Adjustment Operation <ul><li>Aims to overcome drawbacks of fixed length of LCC </li></ul><ul><ul><li>Maximum transmission rate is limited </li></ul></ul><ul><ul><li>Not applicable to variable traffic patterns </li></ul></ul><ul><li>If exists BW= ( b k , b k- 1 , b k- 2 ,…, b 1 , b 0 ), where b i = 0 </li></ul><ul><ul><li>If an incoming transmission rate is 2 k+t , where 1 ≤ t ≤ n-k , we can adjust the length of linear-code chain to be k+t+ 1 </li></ul></ul>
  49. 49. Example of Dynamic Adjustment Operation 4 R cost = 1 cost = 2 1 R 2 R : used code : minimum-cost branch : occupied code
  50. 50. IV. Performance Analysis <ul><li>We define the set of allowable states to be </li></ul><ul><li>The steady-state probability π v can be determined using the following equation: </li></ul><ul><li>where π 0 is the steady-state probability being in state 0: </li></ul>
  51. 51. Call Blocking Probability <ul><li>Then we have call blocking probability P B (i)for iR as: where is the call blocking states for iR </li></ul><ul><li>Therefore, the overall call blocking probability P B is simply given by: </li></ul>
  52. 52. Call Blocking Probability at Different Traffic Load when max SF = 16
  53. 53. V. Experimental Results <ul><li>Simulation environment </li></ul><ul><ul><li>Capacity test : code-limited </li></ul></ul><ul><ul><li>Maximum spreading factors are 64 and 256 </li></ul></ul><ul><ul><li>Call arrival process is Poisson distributed with mean arrival rate λ =1 -16 calls/unit time (SF=64), λ =4 -64 calls/unit time (SF=256) </li></ul></ul><ul><ul><li>Call duration is exponentially distributed with a mean value of 4 unit of time </li></ul></ul><ul><ul><li>Possible transmission rates are 1R, 2R, 4R, and 8R </li></ul></ul>
  54. 54. The Compared Targets <ul><li>OVSF-based scheme </li></ul><ul><ul><li>Random </li></ul></ul><ul><ul><li>Leftmost </li></ul></ul><ul><ul><li>Crowded-first </li></ul></ul><ul><ul><li>Mostuser-first </li></ul></ul><ul><li>ROVSF-based scheme </li></ul><ul><ul><li>Leftmost </li></ul></ul><ul><ul><li>Crowded-first </li></ul></ul><ul><ul><ul><li>ROVSF code tree + Crowded-first strategy </li></ul></ul></ul><ul><ul><li>Mostuser-first </li></ul></ul><ul><ul><ul><li>ROVSF code tree + Mostuser-first strategy </li></ul></ul></ul>
  55. 55. Performance Metrics <ul><li>Blocking Probability </li></ul><ul><ul><li>The probability of a new request cannot be accepted because the orthogonality cannot be maintained for this rate, although the system still has enough excess capacity </li></ul></ul><ul><li>Utilization of LCC </li></ul><ul><ul><li>The number of incoming requests assigned on LCC divided by the total number of accepted requests </li></ul></ul>
  56. 56. Performance Metrics (cont.) <ul><li>Number of Reassigned Codes </li></ul><ul><ul><li>The total number of necessary reassignments of all occupied codes to support the new request when occurring code blocking </li></ul></ul>
  57. 57. Impact of Code Placement ( SF =256)
  58. 58. Impact of Code Placement ( SF =64)
  59. 59. Impact of Code Replacement
  60. 60. Impact of the Length of LCC on Blocking Probability
  61. 61. Impact of the Length of LCC on Reassignment Cost
  62. 62. Impact of Call Patterns on Blocking Probability
  63. 63. Impact of Call Patterns on Reassignment Cost
  64. 64. VI. Conclusions <ul><li>This paper proposes a novel approach for channelization code in WCDMA </li></ul><ul><ul><li>Based on the Rotated-OVSF code tree </li></ul></ul><ul><li>The simulation results illustrate that our scheme offers a lower blocking probability and lower reassignment cost, compared to OVSF-based scheme </li></ul>

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