Proportional Fair Frequency-Domain Packet Scheduling for 3GPP LTE Uplink

1. Proportional Fair Frequency-Domain Packet Scheduling for 3GPP LTE Uplink Suk-Bok Lee, Ioannis Pefkianakis, Adam Meyerson, Shugong Xu, Songwu Lu IEEE INFOCOM 2009 proceedings. Speaker：Tsung-Yin Lee

2. Outline • Introduction • The Model • Problem Formulation • Heuristic Algorithm • Simulation • Conclusion

3. OFDMA for LTE • Orthogonal Frequency Division Multiple Access (OFDMA) has been considered as a strong candidate for the broadband air interface • robustness to multipath fading • higher spectral efficiency • bandwidth scalability

4. Disadvantage of OFDMA • one major disadvantage of OFDMA is that the instantaneous transmitted RF power can vary dramatically within a single OFDM symbol • high peak-to-average power ratio (PAPR)(decrease battery life)

5. Single-Carrier FDMA • selected for LTE uplink multiple access scheme • keeping most of the advantages of OFDMA • SC-FDMA has significantly lower PAPR • benefits the mobile terminal in terms of transmit power efficiency

6. LTE Uplink Scheduler • a scheduler needs to know the instantaneous radio channel conditions across all users and all resource blocks (RBs) • LTE UL each user transmits a Sounding Reference Signal (SRS) to the BS • channel quality indicator(CQI)

7. Proportional Fair (PF) algorithm • PF algorithm as a basic scheduling principle and apply the PF algorithm directly over each RB one-by-one independently • SC-FDMA requires that all the RBs allocated to a single user must be contiguous in frequency within each time slot [5][6] [5] Moray Rumney. 3GPP LTE: Introducing SIngle-Carrier FDMA Agilent MeasurementJournal, 2008. [6] 3GPP TSG-RAN WG2 Meeting #57, R2-070585, “Resource fragmentation in LTEuplink”, St. Louis, USA, Feb, 2007.

8. The Model • The base station can allocate m RBs to a set of n users • At each time slot multiple RBs (with the contiguity constraint) can be assigned to a single user • indicate whether or not RB c is assigned to user i at time slot t • denote the instantaneous channel rate for user i on RB c at time t

9. Problem Formulation (1/3) • the well known PF algorithm aims to maximize the logarithmic utility function • In order to maximize , one should maximize where di(t) is total data transmitted to user i at time t ( this paper change di(t) to )[7][10][14] [7] M. Andrews. A survey of scheduling theory in wireless data networks. IMA, 2005. [10] H. Kushner and P. Whiting. Asymptotic properties of proportional-fair sharing algorithms. Allerton, 2002. [14] D. Tse. Multiuser diversity in wireless networks. http://www.eecs.berkeley.edu/ dtse/stanford416.ps , 2002.

10. Problem Formulation (2/3) • Let be the PF metric valuethat user i has on RB c at time slot t • We can establish PF objective function when scheduling time slot t as follows: (1)

11. Problem Formulation (3/3) • for LTE UL we need to incorporate the contiguous RB constraint into this objective (1) due to the physical layer requirement of SC-FDMA • serve users with suboptimal PF metric value for some RBs so as to optimize the PF objective (1)

12. Hardness Result • Theorem 1 • LTE UL PF-FDPS problem (i.e. maximizing objective (1) with the contiguous RB constraint) is NP-hard [11] [11] S.-B. Lee, I. Pefkianakis, A. Meyerson, S. Xu, and S. Lu. Proportional Fair Frequency-Domain Packet Scheduling for 3GPP LTE Uplink. UCLA TR-090001, 2009.

13. Heuristic Algorithm • Paper’s heuristics do not give guaranteed error bound, and moreover we believe that no practical greedy algorithms can give an approximation to this particular problem [11] [11] S.-B. Lee, I. Pefkianakis, A. Meyerson, S. Xu, and S. Lu. Proportional Fair Frequency-Domain Packet Scheduling for 3GPP LTE Uplink. UCLA TR-090001, 2009.

14. carrier-by-carrier in turn (1/2) • As a starter, our first greedy heuristic Alg1 schedules data from RB1 to RBm in sequence, and for each RB c it assigns the best user i who • 1) has the maximum PF metric value on c • 2) satisfies the contiguity constraint.

15. carrier-by-carrier in turn (2/2)

16. largest-metric-value-RB-first (1/2) • Assign all the “in-between” RBs to a candidate user • it assigns RB5 to i, which as a result comes with assignment of RB4 to i, since i is already assigned RB3 Assigned RB Between RB3 and RB5 RB5 Assigned Now RB3 Already Assigned RB3 RB4 RB5

17. largest-metric-value-RB-first (2/2)

18. riding peaks (1/3) • Seeing the drawback of Alg2, we would like to utilize each user’s high valued RBs as much as possible • Fundamental physical layer characteristic is that in multi-carrier systems the channel SNR values (i.e. CQI) are correlated in both time and frequency

19. riding peaks (2/3) • if for each user i RB c has good channel rate, then the neighboring RBs (c−1, c+1) have high channel rate as well with high probability

20. riding peaks (3/3)

21. RB grouping (1/2) • Alg3 relies on the strong frequency-domain correlation, it is easily cheated by the small-scale variation

22. RB grouping (2/2) • This RB grouping might be helpful to catch a bit large-scale fluctuation • divide m RBs into n groups • apply the “peak riding” over those RB groups

23. Simulation Parameter (1/2) • use traces generated as specified in 3GPP deployment evaluation [2] [2] Technical specification group radio access networks - Deployment aspects. 3GPP TR 25.943

24. Simulation Parameter (2/2) • paper use an algorithm that optimizes objective (1) without the constraint as our reference, and we refer to this algorithm as OPT∗(upper bound of the optimum) • Jain’s fairness index [9], measured by the data-rate fairness criterion [9] R. Jain, D. M. Chiu, and W. Hawe. A Quantitative Measure of Fairness and Discrimination for Resource Allocation in Shared Systems. DEC Research Report TR-301.

25. System throughput and fairness with varying number of users

26. Average number of users scheduled per 1 TTI

27. Conclusion • Due to its single carrier property of SC-FDMA, LTE UL requires the RBs allocated to a single user to be contiguous in frequency • NP-hard nature of this problem • Four Heuristic Algorithm to solve this problem

28. Comment • In LTE scheduling problem, we will handle uplink and downlink from different scheme • We should combine uplink and downlink to increase network performance