1. Cell Selection in 4G Cellular Networks David Amzallag, BT Design Reuven Bar-Yehuda, Technion Danny Raz, Technion Gabriel Scalosub, Tel Aviv University

2. May 2008 Israeli Networking Seminar 2008 2 Cell Selection and Current 3G Cellular Networks Cell Selection: Which BS covers an MS MSs demands << BSs capacities Mostly voice Data < 15Mb/s Local SNR-based protocols are pretty good Generally, one station servicing every client South Harrow area, NW London (image courtesy of Schema) Cover-by-One (CBO)

3. May 2008 Israeli Networking Seminar 2008 3 Future 4G Cellular Networks High MS demand Video, data, … x10-x100 higher (100Mb/s-1Gb/s) Capacities will be an issue < x20 higher reduced costs missing good planning solutions Technology enables having several stations cover a client 802.16e MIMO South Harrow area, NW London (image courtesy of Schema) Research Goal: Explore the potential of Cover-by-Many (CBM)

4. May 2008 Israeli Networking Seminar 2008 4 Model Bipartite graph (Base) Stations For every, capacity. (Mobile) Clients For every, demand and profit. Coverage Area For every, Notation extended to sets, e.g.,

5. May 2008 Israeli Networking Seminar 2008 5 Model (cont.) Goal: Find a set, and a cover plan (CP) is maximized All-or-Nothing (AoN) Constraint Capacity Constraint All-or-Nothing Demand Maximization (AoNDM) Deceptively “simple” resource allocation problem The same as previously well studied problems?

6. May 2008 Israeli Networking Seminar 2008 6 Previous Work Cell Selection Minimize MSs transmission power [Hanly 95] Maximize throughput (via load balancing) [Sang et al. 08] General Assign. (GAP) 1/2-approx. vs. APX hard [Shmoys-Tardos 93, Chekuri-Khanna 00] Multiple Knapsack PTAS [Chekuri-Khanna 00] Budgeted Cell-planning NP-hard to approximate Sufficient capacities: -approx. [Amzallag et al. 05]

7. May 2008 Israeli Networking Seminar 2008 7 Our Results AoNDM: Hard to approximate to within -AoNDM: Bad News: Still NP-hard Good News: A -approx. CBM algorithm Based on a simpler and faster -approx. CBO algorithm Simulation: CBM is up to 20% better than SNR-based -AoNDM:

8. May 2008 Israeli Networking Seminar 2008 8 A (1-r)/(2-r)-Approx. - Intuition A local-ratio algorithm Based on decomposing the profit function Greedy approach A CP x for S is maximal if it cannot be extended: WLOG, is saturated

9. May 2008 Israeli Networking Seminar 2008 9 If p(j)=d(j) Maximality Suffices! No edge to. -saturated Maximal Solution Algorithm sketch: Decompose profit function: Demand-proportional chunks Recurse! Greedily maximize How?

10. May 2008 Israeli Networking Seminar 2008 10 A (1-r)-Approx. – The Extra Mile Previous algorithm might be wasteful: Solution: Maximize usage of A flow-based algorithm. Slightly increased complexity  Cover-by-Many

11. May 2008 Israeli Networking Seminar 2008 11 Experimental Study - Settings -grid A client in every node

12. May 2008 Israeli Networking Seminar 2008 12 Experimental Study - Settings -grid A client in every node Data Clients: Large demand Few

13. May 2008 Israeli Networking Seminar 2008 13 Experimental Study - Settings -grid A client in every node Picocells: Small capacity Small radius many Microcells: Large capacity Large radius few Data Clients: Large demand Few Voice Clients: Small demand Many High-load: Profit:

14. May 2008 Israeli Networking Seminar 2008 14 Experimental Study - Results

15. May 2008 Israeli Networking Seminar 2008 15 Summary 4G technology will support cover-by-many. Good approximation algorithms for realistic scenarios. CBM is 10%-20% better than SNR-based methods. Future Work: Practical: Online & local CBM policies Theoretical: Approximation independent of r ?

16. Thank You!

17. May 2008 Israeli Networking Seminar 2008 17 Hardness of Approximation Reduction from Maximum Independent Set Theorem: AoNDM Cannot be approximated better than unless 1 2 3 4 5 6 (1,2) (2,3) (3,4) (4,5) (5,6) (3,6) (5,1) 1 2 3 4 5 6

18. May 2008 Israeli Networking Seminar 2008 18 A (1-r)/(2-r)-Approximation Algorithm If return If return Set For every j s.t. try adding j to the cover Return x Cleanup… How?

19. May 2008 Israeli Networking Seminar 2008 19 A (1-r)-Approximation

20. May 2008 Israeli Networking Seminar 2008 20 A (1-r)-Approximation Ignore!!

21. May 2008 Israeli Networking Seminar 2008 21 A (1-r)-Approximation Ignore!!

22. May 2008 Israeli Networking Seminar 2008 22 A (1-r)-Approximation Ignore!!