Packet Scheduling for Real-time Communication over LTE Systems
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With various packet-switched networks coming to the fore, real-time services like voice and video, transmitted traditionally using circuit-switched bearers, can have limited capacity due to the limited availability of resource-granting control channels. Such packets are frequent and require more grants compared to other services like FTP. To compound the issue, often these packets are large in size compared to available resources for allocation. To improve the capacity of real-time communication over LTE (-A), various scheduling methods are being studied. However, often the packet sizes are unaccounted for by these studies. This work deals with the development of semi-persistent scheduling (SPS) algorithms based on wide-band time-average SINR information for resource allocation to voice traffic users, with a focus on large packets. A comparative study between dynamic scheduling (DS) and developed SPS algorithms is done to determine the suitable scheduling mechanism for voice packets transmission over LTE (-A) systems in the downlink.
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Packet Scheduling for Real-time Communication over LTE Systems
- 1. PACKET SCHEDULING FOR REAL-TIME COMMUNICATION OVER LTE SYSTEMS A COMPARATIVE STUDY OF DYNAMIC AND SEMI-PERSISTENT SCHEDULING SCHEMES AVISHEK PATRA VOLKER PAULI LANG YU
- 2. 2 © Nomor Research GmbH MOTIVATION LTE – Packet Switched Network Non-Voice Packets: Large, infrequent and non-periodic Dynamic Scheduling (DS) Voice Packets: Small, frequent and periodic Transmitted traditionally over circuit switched bearers VoIP based solution DS limit capacity due to limited control channels Our Contribution – Semi-Persistent Scheduling (SPS) for Real- Time Communication over LTE (VoLTE)
- 3. 3 © Nomor Research GmbH VOICE-OVER LTE SYSTEMS Voice sensitive to latency and packet loss Periodic packets within talk burst (Repeat/ SPS Interval) Scheduling Physical Downlink Control Channels (PDCCHs) Physical Resource Block (PRB) grants limited to free PDCCHs SILENCE PERIOD TALK BURST Fig. 1. Sample Speech Signal Repeat Interval
- 4. DYNAMIC SCHEDULING 4 © Nomor Research GmbH Problem with DS: Multiple users need PRBs in same Transmission Time Interval (TTI) Why? DS grants PRBs to #users = available PDCCH per TTI Result: (1) Call drop rate (2) QoS Repeat Interval DYNAMIC SCHEDULING PDCCH Required Fig. 2. Dynamic Scheduling of VoIP Packets
- 5. 5 © Nomor Research GmbH OTHER SCHEDULING SCHEMES 1. Persistent Scheduling (PS): Fixed PRB allocation PDCCH only required for initial allocation Capacity limited to available PRBs No link adaptation (same MCS) Repeat Interval PERSISTENT SCHEDULING PDCCH Required Fig. 3. Persistent Scheduling of VoIP Packets
- 6. 6 © Nomor Research GmbH OTHER SCHEDULING SCHEMES [CONT.] 2. Semi-Persistent Scheduling (SPS): PRBs reserved for extended period (say, talk burst) PDCCH required for initial allocation and… Capacity less dependent on PRBs or PDCCHs MCS /PRB change possible within talk burst(req. PDCCH) Coming Up! Repeat Interval Talk Burst SEMI - PERSISTENT SCHEDULING Check PDCCH Req. PDCCH Required Fig. 4. Semi - Persistent Scheduling of VoIP Packets
- 7. 7 © Nomor Research GmbH LARGE VOIP PACKETS If PRB req. > PRB available ? Fig. 5. Ratio of Leftover to VoIP Packets in Downlink Queuing increases latency!
- 8. 8 © Nomor Research GmbH SEMI-PERSISTENT SCHEDULING Initial Allocation, Periodic Allocation and Retransmission Effects of large packets taken into account Two strategies of allocating PRBs Based on partitioning method and transmission of leftovers 1. Non-Segmentation based SPS (NS-SPS) 2. Segmentation based SPS (S-SPS)
- 9. 9 © Nomor Research GmbH SEMI-PERSISTENT SCHEDULING [CONT.] Fig. 6. Difference in allocation process of Large Packets in NS-SPS and S-SPS
- 10. 10 © Nomor Research GmbH SEMI-PERSISTENT SCHEDULING [CONT.] Common processes for both variations: Initial Allocation – New MCS and PRBs allocated (PDCCH required) Allocated PRBs and MCS reserved for future use. Periodic Allocation – Check after SPS Interval Packet? New PRBs / MCS required? Allocate reserved / new PRBs / MCS (PDCCH may be required) No packet? Free reserved PRBs! Retransmission – Dynamic Scheduling! (PDCCH required) NOTE: If PDCCH not available, for Periodic Allocation, previously reserved MCS and PRBs used. For other processes, no allocation is possible for the given TTI
- 11. 11 © Nomor Research GmbH 1. NON-SEGMENTATION BASED SPS Fig. 7. NS – SPS Flowchart PERIODIC ALLOCATION FOR NS-SPS USERS LEFTOVER ALLOCATION FOR NS-SPS USERS RETRANSMISSION FOR NS-SPS USERS INITIAL ALLOCATION FOR NEW NS-SPS USERS MCS SELECTION AND PRB RESERVATION FROM PREVIOUS PERIODIC ALLOCATION STORE MCS SELECTION AND PRB RESERVATION FROM INITIAL ALLOCATION DYNAMICALLY SELECT PRBs & MCS SEMI-PERSISTENT SCHEDULING DYNAMICALLY SELECT PRBs & MCS DYNAMICALLY SELECT PRBs & MCS DYNAMIC SCHEDULING DYNAMIC SCHEDULING SEMI-PERSISTENT SCHEDULING
- 12. 12 © Nomor Research GmbH 2. SEGMENTATION BASED SPS Fig. 8. S-SPS Flowchart PERIODIC ALLOCATION OF SEGMENTS FOR S-SPS USERS PERIODIC ALLOCATION OF PACKETS FOR S-SPS USERS RETRANSMISSION FOR S-SPS USERS INITIAL ALLOCATION FOR NEW S-SPS USERS MCS & PRBs FROM PREVIOUS PERIODIC PACKET ALLOCATION STORE MCS SELECTION AND PRB RESERVATION FROM INITIAL PACKET/ SEGMENT ALLOCATION SEMI-PERSISTENT SCHEDULING DYNAMICALLY SELECT PRBs & MCS DYNAMICALLY SELECT PRBs & MCS SEMI-PERSISTENT SCHEDULING DYNAMIC SCHEDULING SEMI-PERSISTENT SCHEDULING MCS & PRBs FROM PREVIOUS PERIODIC SEGMENT ALLOCATION
- 13. 13 © Nomor Research GmbH SIMULATION RESULTS Fig. 9. Comparison of the Downlink Performance Dynamic Scheduling
- 14. Non-Segmentation based Semi-Persistent Scheduling 14 © Nomor Research GmbH SIMULATION RESULTS Fig. 9. Comparison of the Downlink Performance
- 15. Segmentation based Semi-Persistent Scheduling 15 © Nomor Research GmbH SIMULATION RESULTS Fig. 9. Comparison of the Downlink Performance
- 16. 16 © Nomor Research GmbH SIMULATION RESULTS Fig. 9. Comparison of the Downlink Performance
- 17. 1 2 3 0 20 40 60 80 100 PercentageofPRBsused Comparision of Resource Utilization 90 Users 140 Users 190 Users NS-SPS S-SPS DS 17 © Nomor Research GmbH SIMULATION RESULTS [CONT.] Fig. 10. Comparison of Resource Utilization
- 18. 18 © Nomor Research GmbH Comparing NS-SPS AND S-SPS: Low Delay Budget – NS – SPS > S – SPS >> DS High Delay Budget – S – SPS > NS – SPS > DS CONCLUSION PDCCH Usage PRB Usage Capacity System User increase Delay Budget increase DS NS-SPS S-SPS
- 19. 19 © Nomor Research GmbH THANK YOU! QUESTIONS
- 20. 20 © Nomor Research GmbH SIMULATION SCENARIOS & PARAMETERS System Bandwidth 5 MHz Carrier Frequency 2GHz Sub-frame duration 1 ms No. of PDCCH 4 for Uplink; 4 for Downlink Link Adaptation Fast OLLA HARQ Scheme Incremental Redundancy CQI Delay/ Report Rate/ Resolution (for DS) 4 TTI/ 20 ms/ 2 PRBs CQI Report Rate/ Resolution (for SPS) Start of talk-spurt/ Wideband CQI Average talk-spurt duration/ Voice activity 3 sec/ 50% AMR Voice Codec Rate (burst rate) 12.2 kbps Voice packet inter-arrival time (SPS Interval)/ size 20 ms/ 40 bytes SID inter-arrival time/ size 160 ms/ 15 bytes Simulation Scenarios: No. of users from 90 to 200 (with an step of 10 users) Delay budgets from 40 ms to 100 ms (with a step of 10 ms) Simulation for 50 drops of 30 secs for each user-delay budget set Simulation Parameters:
- 21. 21 © Nomor Research GmbH SIMULATION RESULTS Fig. 0. Comparison of User Satisfaction in Downlink at 70 ms 100 120 140 160 180 200 60 70 80 90 100 Number of Users PercentageofSatisfiedUsers Percentage of Satisfied Users in Downlink DS S-SPS NS-SPS (D) Target NS-SPS (D) DS S-SPS NS-SPS (S)
Editor's Notes
- Dynamic Scheduling – 1. No. of users increase decreases capacity due to limited PDCCH. 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency as packets have to wait for turn due to limited PDCCH. Non Segmentation based Semi-Persistent Scheduling (Static TFC)/ Non Segmentation based Semi-Persistent Scheduling (Dynamic TFC)/ Segmentation based Semi-Persistent Scheduling – 1. No. of users increase decreases capacity due to fixed reservation limit (up to a threshold limit). 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency. Compare NS-SPS (S) and NS-SPS (D) – S consumes less PDCCH – also, S-SPS is similar to NS-SPS (D), which has dynamic TFC and consumes more PDCCH. Compare NS-SPS (D) and S-SPS – For lesser delay budget, little less capacity in S-SPS as loss of last segment can increase loss of packets (high latency). Overall, S-SPS performs bit better due to lesser dynamic allocation (of leftovers, etc).
- Dynamic Scheduling – 1. No. of users increase decreases capacity due to limited PDCCH. 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency as packets have to wait for turn due to limited PDCCH. Non Segmentation based Semi-Persistent Scheduling (Static TFC)/ Non Segmentation based Semi-Persistent Scheduling (Dynamic TFC)/ Segmentation based Semi-Persistent Scheduling – 1. No. of users increase decreases capacity due to fixed reservation limit (up to a threshold limit). 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency. Compare NS-SPS (S) and NS-SPS (D) – S consumes less PDCCH – also, S-SPS is similar to NS-SPS (D), which has dynamic TFC and consumes more PDCCH. Compare NS-SPS (D) and S-SPS – For lesser delay budget, little less capacity in S-SPS as loss of last segment can increase loss of packets (high latency). Overall, S-SPS performs bit better due to lesser dynamic allocation (of leftovers, etc).
- Dynamic Scheduling – 1. No. of users increase decreases capacity due to limited PDCCH. 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency as packets have to wait for turn due to limited PDCCH. Non Segmentation based Semi-Persistent Scheduling (Static TFC)/ Non Segmentation based Semi-Persistent Scheduling (Dynamic TFC)/ Segmentation based Semi-Persistent Scheduling – 1. No. of users increase decreases capacity due to fixed reservation limit (up to a threshold limit). 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency. Compare NS-SPS (S) and NS-SPS (D) – S consumes less PDCCH – also, S-SPS is similar to NS-SPS (D), which has dynamic TFC and consumes more PDCCH. Compare NS-SPS (D) and S-SPS – For lesser delay budget, little less capacity in S-SPS as loss of last segment can increase loss of packets (high latency). Overall, S-SPS performs bit better due to lesser dynamic allocation (of leftovers, etc).
- Dynamic Scheduling – 1. No. of users increase decreases capacity due to limited PDCCH. 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency as packets have to wait for turn due to limited PDCCH. Non Segmentation based Semi-Persistent Scheduling (Static TFC)/ Non Segmentation based Semi-Persistent Scheduling (Dynamic TFC)/ Segmentation based Semi-Persistent Scheduling – 1. No. of users increase decreases capacity due to fixed reservation limit (up to a threshold limit). 2. Delay budget decrease decreases capacity due to discarding of packets from queue or huge latency. Compare NS-SPS (S) and NS-SPS (D) – S consumes less PDCCH – also, S-SPS is similar to NS-SPS (D), which has dynamic TFC and consumes more PDCCH. Compare NS-SPS (D) and S-SPS – For lesser delay budget, little less capacity in S-SPS as loss of last segment can increase loss of packets (high latency). Overall, S-SPS performs bit better due to lesser dynamic allocation (of leftovers, etc).
- Resource utilization better for NS-SPS and S-SPS compared to DS. DS limited by PDCCH and hence, less resource usage. But SPS limited by PDSCH/PUSCH for huge packets.