Packet Scheduling for Real-time Communication over LTE Systems

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© 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)


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

DYNAMIC SCHEDULING


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


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


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

PDCCHsMCS /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


LARGE VOIP PACKETS

If PRB req. > PRB available ?

Fig. 5. Ratio of Leftover to VoIP Packets in Downlink

Queuing increases latency!


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)


SEMI-PERSISTENT SCHEDULING [CONT.]

Fig. 6. Difference in allocation process of Large Packets in NS-SPS and S-SPS


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


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




DYNAMIC SCHEDULING

DYNAMIC SCHEDULING



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





DYNAMIC SCHEDULING


MCS & PRBs FROM PREVIOUS PERIODIC

SEGMENT ALLOCATION


SIMULATION RESULTS

Fig. 9. Comparison of the Downlink Performance

Dynamic Scheduling

Non-Segmentation based Semi-Persistent Scheduling


SIMULATION RESULTS


Segmentation based Semi-Persistent Scheduling


SIMULATION RESULTS



SIMULATION RESULTS


1 2 30

20

40

60

80

100

Perc

enta

ge o

f PR

Bs u

sed

Comparision of Resource Utilization

90 Users140 Users190 Users

NS-SPS S-SPS DS


SIMULATION RESULTS [CONT.]

Fig. 10. Comparison of Resource Utilization


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 CapacitySystem

User increaseDelay Budget

increase

DS

NS-SPS

S-SPS


THANK YOU!

QUESTIONS


SIMULATION SCENARIOS & PARAMETERS

System Bandwidth 5 MHzCarrier Frequency 2GHzSub-frame duration 1 msNo. of PDCCH 4 for Uplink; 4 for DownlinkLink Adaptation Fast OLLAHARQ Scheme Incremental RedundancyCQI Delay/ Report Rate/ Resolution (for DS) 4 TTI/ 20 ms/ 2 PRBsCQI Report Rate/ Resolution (for SPS) Start of talk-spurt/ Wideband CQIAverage talk-spurt duration/ Voice activity 3 sec/ 50%AMR Voice Codec Rate (burst rate) 12.2 kbpsVoice packet inter-arrival time (SPS Interval)/ size 20 ms/ 40 bytesSID 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:


SIMULATION RESULTS

Fig. 0. Comparison of User Satisfaction in Downlink at 70 ms

100 120 140 160 180 20060

70

80

90

100

Number of Users

Perc

enta

ge o

f Sat

isfie

d U

sers Percentage of Satisfied Users in Downlink

DS S-SPS NS-SPS (D) Target NS-SPS (D) DS S-SPS

NS-SPS (S)

Engineering

Packet Scheduling for Real-time Communication over LTE Systems