Dynamic Algorithms for Multiuser OFDM Wireless Cells

Transcription

1 Dynamic Algoritms for Multiuser OFDM Wireless Cells Kolloquium der Fakultät EIM Universität Paderborn James Gross Facgebiet Telekommunikationsnetze Tecnisce Universität Berlin

2 Basic Scenario Access Point Terminals Backbone Down-link Data Queues at Access Point OFDM as transmission sceme! 2

3 Ortogonal Frequency Division Multiplexing traditional FDM OFDM Ortogonal sub-carriers Hig spectral efficiency! [Cang_66], [Wein_71] 3

4 Wy OFDM? -> Wireless Cannel Caracteristics! Multi-pat propagation frequency selective fading! Snap sot of a wireless cannel Freq. sel. fading limits single-carrier systems OFDM overcomes tis problem! Implementations: DVB, a/g, etc. Candidate for future cellular systems (LTE) 4

5 Diversity in Multiuser OFDM Systems Signal-to-noise ratio (SNR) measures cannel quality Adaptive modulation: Bit rate depends on te SNR Multiple Terminals: Frequency and Multi-user Diversity 1,1 N M,1 K O L Terminals 1, J M N, J Sub-carrier Hig Bit Rate Low Bit Rate Term. 1 Term. 2 Term. 3 5

6 System Model & Dynamic OFDM Time slotted into frames Cannel states constant during eac frame Access point knows current cannel matrix Organization of te down-link transmissions? Dynamic Standard metod OFDM: (for Assign example disjoint in IEEE sets of sub-carriers ): to All terminals subcarrierson assigned sub-carrier to depending one state terminal matrix! ( 1,1 1,1 MM NN,1 Sub-carrier t( ) t) ( t( ) t),1 K O L Terminals ( t( ) t) 1, 1, JJ MM ( t( ) t) NN, J, J 6

7 Dynamic OFDM Systems - Issues Data flows Access Point OFDM Sub-carriers 1,1 M N,1 K O L 1, J M N, J Excange of control information Terminals Resource assignment Performance limits How muc dynamics? Practical scemes Heuristics, packet nature? Additional overead Signaling Reduction of overead? Cannel acquisition at te access point Application to example systems (IEEE ) Protocol modifications 7

8 Performance Limits Assumption: 1 large file queued for eac terminal Goal: Optimize te down-link trougput! Integer constraint on te assignments Intuitive approac: Maximize Sum Rate [Jang_03] Eac sub-carrier assigned to terminal wit best rate 1,1 N M,1 K O L Terminals 1, J M N, J Terminal queues Sub-carrier 8

9 Performance Limits II Pure rate maximization leads to fairness problems Instead: Optimize fairness-constrained trougput Maximize te minimal trougput [Ree_00] 1,1 N M,1 K O L Terminals 1, J M N, J Terminal queues Sub-carrier Minimal rate NP-ard optimization problem [Gross_06a] Practical instances are difficult [SNDlib] 9

10 Practical Assignment Algoritm - Relaxation Generate assignments witin milliseconds! Common approac in integer opt.: Relaxation Solve te corresponding linear program Problem: Obtain a feasible and good assignment from te LP (linear program) solution! Static power distribution: Assign eac sub-carrier to terminal wit largest sare Dynamic power distribution: Initially assign eac sub-carrier to terminal/modulation wit largest sare, afterwards ceck and modify regarding transmit power constraint 10

11 Practical Assignment Algoritm Matcing Acieved rate per terminal depends on: Amount of sub-carriers received Specific sub-carriers assigned Simplify te max-min problem by fixing te amount of sub-carriers assigned to eac terminal Resulting problem is equivalent to bipartite weigted matcing, optimal algoritms can be applied [Yin_00] Sub-carrier 1,1 M N,1 K O L Terminals 1, J M N, J 11

12 Example Results: Assignment Algoritms Parameters: 8 terminals 1 m/s object mobility MHz bandwidt 48 sub-carriers 5.2 GHz center freq. 10 s simulated time 2 ms frame lengt 10 mw transmit power static power distr. Relaxation approac provides very good performance, also if power and sub-carriers are assigned dynamically! 12

13 Furter Aspects of Assignment Algoritms Usually queue states differ! Compute ratio between data queued for eac terminal: Relaxation wit weigts Matcing wit adjusted subcarrier amounts per terminal Packets from backbone More sopisticated models matc packets to OFDM resources according to various sceduling policies for different flows (best-effort appl., real-time appl.) Currently a ot topic [Zang_06], [Boge_06], Relaxation approac seems well suited! 13

14 Signaling Cost Access point generates dynamic assignments How do terminals know about teir assignments? Signaling Data Payload Data Problem: Impact due to signaling cost? 14

15 Caracterization of Signaling Cost Initially cosen signaling model: Binary assignment representation Signaling information broadcasted on all sub-carriers Transmitted by robust modulation/coding combination All assignments signaled per down-link frame Evaluation questions: 1. Wat is te loss due to signaling? 2. Is te signaling cost parameter dependent? 15

16 Example Results: Impact of Signaling Cost How to acieve iger trougput? Parameters: 8 terminals 1 m/s object mobility 100 m cell radius MHz bandwidt 0.8 µs guard period 5.2 GHz center freq. 10 s simulated time 2 ms frame lengt 10 mw transmit power static power distr. 0.2 µs delay spread matcing algoritm Quantitative and qualitative impact [Gross_04]! Imp. Param.: Sub-carrier number, bandwidt & frame lengt 16

17 Advanced Signaling Approaces Entropy results indicate correlation of sub-carrier assignments (in time and frequency) Exploit correlation - two approaces investigated: More complex assignment algoritms [Gross_06b] Quadratic optimization model Evaluation: Iterative sequence of integer programs Compression of te signaling information Evaluation: Coice of suitable compression algoritms Bot scemes require additional computational resources 17

18 Example Results: Advanced Signaling Approaces Parameters: 8 terminals 1 m/s object mobility 100 m cell radius MHz bandwidt 0.8 µs guard period 5.2 GHz center freq. 10 s simulated time 2 ms frame lengt 10 mw transmit power static power distr. 0.2 µs delay spread matcing algoritm 18

19 Protocol Modifications for IEEE (a/g) How could IEEE (a/g) networks benefit from dynamic OFDM assignments? Aspects of protocol modifications: Acquisition of cannel knowledge Transmission of signaling information Possible transmission delay due to assignment comp. Compatibility to legacy systems In tis talk: Only point-to-point dynamic scemes (adaptive modulation) 19

20 IEEE (a/g) Mod. Single-User DCF Mode Parameters: 16 terminal positions 1 m/s object mobility 80 m cell radius MHz bandwidt 48 sub-carriers 0.8 µs guard period 5.2 GHz center freq. 10 s simulated time 10 mw transmit power 0.2 µs delay spread Pure transmission time comparison no packet errors, no random cannel access, no retransmissions 20

21 Conclusions Dynamic OFDM is a promising tecnology Meaningfull assignment problems can be difficult! Relaxation approac best suboptimal tecnique Control overead is required Signaling cost as an quantitative and qualitative performance impact Reduction of signaling cost possible Example results of dynamic OFDM for IEEE (a/g) performs well for te point-to-point case 21

22 Some Currently Open Issues Table look-up for dynamic OFDM assignments Diploma tesis at TKN Performance of multiuser dynamic OFDM in Diploma tesis at TKN Assignment scemes for WiMax systems Dynamic OFDM scemes for multi-cell scenario Prototyp implementation of dynamic sub-carrier assignment scemes in OFDM transmission systems Done ere in Paderborn! 22

23 References Boge_06: M. Boge, J. Gross, A. Wolisz: A New Optimization Model for Dynamic Power and Sub-carrier Allocations in Packet-Centric OFDMA Cells, in Proc. International OFDM Worksop, August Cang_66: Ortogonal Frequency Division Multiplexing, US Patent No. 3, 488,4555. Gross_04: J. Gross, I. Paoluzzi, H. Karl, A. Wolisz: Trougput Study for a Dynamic OFDM- FDMA System wit Inband Signaling in Proc. Veicular Tecnology Conference, May Gross_06a: J. Gross and M. Boge: Dynamic Mecanisms in OFDM Wireless Systems: A Survey on Matematical and System Engineering Contributions in Tecnical Report Series Facgebiet Telekommunikationsnetze der TU Berlin, TKN , Gross_06b: J. Gross, H. Geerdes, H. Karl, A. Wolisz: Performance Analysis of Dynamic OFDMA Systems wit Inband Signaling in IEEE Journal on Selected Areas in Communications, Vol. 24, No. 3, , Jang_03: J. Jang, K. Lee: Transmit Power Adaption for Multiuser OFDM Systems in IEEE Journal on Selected Areas in Communications, Vol. 21, No. 2, , Ree_00: W. Ree, J. Cioffi: Increase in Capacity of Multiuser OFDM Systems Using Dynamic Subcannel Allocation in Proc. IEEE Veicular Tecnology Conference, May Wein_71: S. Weinstein, P. Ebert: Data Transmission by Frequency-Division in IEEE Transactions on Communication Tecnology, Vol. 19, No. 5, , Yin_00: H. Yin, H. Liu: An efficient Multiuser Loading Algoritm for OFDM-based Broadband Wireless Systems in Proc. IEEE Globecom, November2000. Zang_06: Y. Zang, K. Letaief: Cross-Layer Adaptive Resource Management for Wireless Packet Networks wit OFDM Signaling in IEEE Transactions on Wireless Communications, Vol. 5, No. 11, ,