Persistent Packet Scheduling Performance for Voice-over-IP in Evolved UTRAN Downlink Jani Puttonen∗ , Niko Kolehmainen† , Tero Henttonen‡ and Martti Moisio‡ ∗ Magister
Solutions Ltd, c/o Mattilanniemi 6-8, 40101 Jyväskylä, Finland. Email:
[email protected] † University
of Jyväskylä, Department of Mathematical Information Technology, P.O. Box 35, 40014 University of Jyväskylä, Finland. Email:
[email protected] ‡ Nokia,
P.O. Box 407, 00045 Nokia Group, Finland. Email:
[email protected] Abstract—In this paper we present and analyze different packet scheduling mechanisms for Voice-over-IP (VoIP) in UTRAN Long Term Evolution Downlink. Dynamic packet scheduling provides multi-user and frequency domain scheduling gain, but at the expence of high control channel utilization. VoIP service with high number of simultaneous users and small packets can cause the control channel consumption to be the bottleneck for VoIP performance. Thus several persistent packet scheduling mechanisms have been proposed in 3GPP, such as fully persistent, talk-spurt based persistent and semi-persistent packet scheduling. We analyze the VoIP performance of different packet scheduling mechanisms by using fully dynamic system level simulations. We observe that with dynamic packet scheduling the VoIP capacity is restricted by the control channel capacity, whereas with persistent scheduling the capacity is restricted by the delay budget due to worse PRB SINR and resulting increased number of retransmissions.
I. I NTRODUCTION The Evolved UTRAN (E-UTRAN) or the UTRAN Long Term Evolution (LTE) specifications are being finalized in 3GPP. LTE aims at ambitious goals of e.g. peak data rate of 100 Mbps in downlink and 50 Mbps in uplink, increased cell edge user throughput, improved spectral efficiency, scalable bandwidth from 1.25 MHz to 20 MHz, etc. [1]. LTE supports both time (TDD) and frequency division duplex (FDD) modes, but in this article we concentrate on FDD. Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for the downlink multiple access technology and Single Carrier Frequency Division Multiple Access (SCFDMA) for uplink [1]. To achieve the objectives set for LTE, advanced Radio Resource Management (RRM) functions have been defined. The algorithms include e.g. Hybrid ARQ (HARQ), Link Adaptation (LA), Channel Quality Indication (CQI) and Packet Scheduling (PS). More on these can be found e.g. from [2]. Since E-UTRAN is optimized for packet data transfer and the core network is purely packet switched, also speech is transmitted purely with Voice-over-IP (VoIP) protocols. VoIP traffic consists of active (talk-spurts) and silent periods, with relatively small packets transmitted quite rarely. The Adaptive
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Multi-Rate (AMR) codec provides quite bursty traffic; one VoIP packet at 20 ms intervals during active period and one Silence Indicator (SID) packet at 160 ms intervals during silence period. E-UTRAN is expected to support very high number of VoIP users and the Quality-of-Service (QoS) of VoIP is determined by maximum End-to-End delay and tolerable packet loss. These facts set challenges to the resource allocation of VoIP users, for both PS and LA algorithms. Also, the capacity of Physical Downlink Control Channel (PDCCH) induces some restrictions. These restrictions become most relevant with dynamic packet scheduling, since each allocation consumes signaling resources from PDCCH. Thus, several persistent resource allocation schemes, such as fully persistent scheduling, talk-spurt based persistent scheduling and semipersistent scheduling have been proposed in 3GPP. However, these scheduling types limit or even lack entirely the gain from multi-user and frequency domain scheduling. VoIP service in E-UTRAN has been studied e.g. in [3], [4] and [5]. In [3] the VoIP performance in LTE downlink has been studied with dynamic PS. It is concluded that packet bundling together with link adaptation can provide up to 80% gain and partly also compensate the PDCCH capacity constraints. In [4] the realization of VoIP has been outlined for LTE, looking into service requirements, QoS mapping and radio realizations. It is concluded that fully persistent scheduling cannot match the dynamic scheduling approach for VoIP, in spite of the lower protocol overhead. In [5] the talk-spurt based persistent PS has been studied in the context of LTE uplink. Due to worse statistical multiplexing gains with talk-spurt based persistent PS, it has been proposed that dynamic scheduling should be used as the baseline scheduling approach also for the VoIP. The objective of this article is to analyze the VoIP performance with dynamic, talk-spurt based persistent and semi-persistent packet scheduling. The analysis is performed with fully dynamic system level simulator, which models the RRM functionalities, their interactions as well as the UE mobility accurately. The paper is organised as follows: Chapter II discusses the
general aspects of VoIP in LTE and Chapter III presents the scheduling mechanisms proposed for VoIP. Chapter IV lists the simulation assumptions including a short description of the simulator. Chapter V presents the simulation results and analysis. Finally, Chapter VI reviews the main conclusions. II. VOICE - OVER -IP IN LTE VoIP has at least three characteristics that need consideration in LTE: support for lots of simultaneous users, strict packet delay-based QoS and low bitrate bursty traffic. A. High capacity demand The requirements of E-UTRA and E-UTRAN are described in TR 25.813 [6]. E-UTRAN should support various types of services, VoIP being just one of them. VoIP should be offered with at least as good radio backhaul efficiency and latency as voice over UMTS Circuit Switched (CS) networks. Also, voice and other real-time services supported in the CS domain in Release 6 shall be supported by E-UTRAN via the packet switched domain with at least equal quality as supported by UTRAN – over the whole speed range. The effect of UE velocity and Robust Header Compression on VoIP performance has been studied in [7]. B. Strict packet delay-based QoS System loading is limited by the outage limit for each traffic type. These outage limits are defined in TR 25.814 [1] and updated in 3GPP contribution R1-070674 [8]: • The system capacity is defined as the number of users in a cell when more than 95% of the users are satisfied. • A single VoIP user is in outage if less than 98% of its speech frames are delivered successfully within 50 ms air interface delay. These outage limits should ensure that the end-to-end delay with two E-UTRAN users is less than 250 ms, which has been defined as the maximum acceptable mouth-to-ear delay for voice in [9]. C. Low bitrate bursty traffic We consider VoIP traffic as provided by AMR codec. The AMR VoIP traffic is quite bursty: there’s one VoIP frame at 20 ms intervals during active period (talk spurt) and one SID packet at 160 ms intervals during silent period. This means that at any given TTI there may be a lot of users of which only a few need to be scheduled. And at the same time, each user that is not scheduled (but would need to be) contributes to a backlog of requests for later TTIs. This backlog can start accumulating easily, leading to stalling for several users. Thus the PS should take care that the buffering delay of each VoIP user is taken into account. Since VoIP packets are relatively small, there are some challenges in allocating the resources. 1-3 symbols of each carrier in each Physical Resource Block (PRB) are reserved for control data (reference symbols, allocation information, HARQ ACK/NACK channels), depending on need for allocation signaling. With the demand for a several users to be
scheduled simultaneously, the control channel capacity might become a bottleneck for the VoIP capacity due to allocation signaling bits running out. III. VOICE - OVER -IP PACKET S CHEDULING IN LTE Several PS mechanisms have been proposed for VoIP in LTE, which balance between dynamic scheduling gain and control channel consumption. 1) Dynamic packet scheduling: LTE is a packet based system where normally each packet is scheduled by using PDCCH control channels, thus the allocation may change from TTI to another. A fully dynamic scheduling of VoIP packets require a lot of PDCCH resources, since each UE consumes a control channel each time it is scheduled. This may become a problem due to high VoIP capacity demand and small packet sizes of AMR codecs. However, dynamic scheduling has also several benefits, such as time and frequency selective scheduling, LA and packet bundling. According to [3] packet bundling can provide up to 80% gain to VoIP capacity together with LA. Packet bundling and LA may improve the spectral efficiency, which is traditionally a problem with delay critical VoIP with small packet sizes, by being able to utilize the bandwidth more efficiently. However, the gain in spectral efficiency is achieved through with high PDCCH consumption in downlink. Also, accurate and frequency-dependent CQI reporting is needed in uplink shared channel. 2) Persistent packet scheduling: With fully persistent PS, Radio Resource Control (RRC) signaling is used to allocate a fixed time and frequency resources as well as a fixed MCS for a VoIP user for the whole duration of the call. The allocation would include resources for both transmissions and retransmissions, as well as silent and active speech periods. Whenever a VoIP packet is sent to a given VoIP user, the pre-allocated resource and transport format is used. Thus a VoIP user knows exactly when and where to expect a VoIP (re)transmission and no PDCCH is needed. CQI information is not needed for PS and LA, at least when considering the VoIP packets. However, persistent scheduling lacks entirely the gains achievable with dynamic scheduling. Also, persistent allocations waste some resouces, since HARQ retransmission resources as well as resources in silent periods cannot be allocated to other UEs. 3) Talk-spurt based persistent packet scheduling: VoIP users are on average half of the time silent, thus a significant amount of VoIP capacity is wasted, if the silence periods are not reallocated to other (VoIP) users. With talk-spurt based persistent scheduling the allocation is done separately for each talk spurt. The transport block size is the same for the duration of the talk-spurt and resources during silent period can be allocated to some other (VoIP) user. SID frames are transmitted with dynamic PS consuming PDCCH resources. Talk-spurt based PS supports two ways of pre-allocated resources: only for the first transmissions or for both transmissions and retransmissions. We consider the former option, since it supports also the asynchronous HARQ. The first transmissions utilize the pre-allocated resources and HARQ
retransmissions are scheduled with dynamic PS. Resources are not wasted for either silent periods or HARQ retransmissions. Slow talk-spurt based LA is possible achieving a limited dynamic scheduling gain, in which case a wideband CQI information is sufficient. However, compared to fully persistent PS, the control channel consumption is a little higher. 4) Semi-persistent packet scheduling: With semi-persistent PS [5] UE is preconfigured (using e.g., RRC signaling) a limited set of time and frequency resources where initial transmissions can be sent to the UE without utilizing PDCCH resources. Thus if compared to talk-spurt based PS, semipersistent scheduling offers limited multi-user and frequency scheduling gain. The PS is responsible to select which UE is scheduled in each preconfigured resource. The same set of resources can be utilized also for SID frames during silent periods providing a little less PDCCH consumption than talkspurt based persistent PS. The retransmissions are scheduled dynamically using the PDCCH resources. As a drawback, the blind detection becomes more complex, because several allocation possiblities need to be checked. Also, LA is more complex to organize. Channel dependent CQI information is beneficial for the use of PS when selecting pre-configured resource for each UE. IV. S IMULATION ASSUMPTIONS AND MODELING A. System simulator description We have used an own fully dynamic time driven system simulator for studying the VoIP performance. The simulator consists of a fully dynamic system level modeling utilizing exponential SINR link to system level mapping [9]. Both EUTRAN downlink and uplink are simulated with TTI (1 ms) resolution. Simulator has detailed modeling of RRM, mobility and handovers as well as traffic models. B. Packet scheduling modeling Persistent PS (either talk-spurt based or semi-persistent) is performed before the dynamic PS, thus the dynamic PS handles the scheduling of UEs that are left by the persistent scheduler, as shown in Fig. 1. In talk-spurt based PS the VoIP packets are scheduled only to its pre-assigned resources. The transport block assignment and release is performed in the beginning and in the end of the talk-spurt, respectively. The pre-assigned transport block consists of a number of PRBs and a MCS assigned every 20 ms. The pre-assigned TTI is selected by trying to keep the delay in minimum. The PRBs can be chosen either randomly or based on instantaneous PRBspecific CQI reports. After persistent scheduling operations, SID frames and control data are scheduled dynamically to the remaining PRBs. Also, VoIP packets with buffering delay close to the delay budget (HOLdelay > 0.8 * delayBudget) may be scheduled dynamically. With semi-persistent PS each UE is assigned a limited set of resources. One bin (a single time-frequency resource) consists of a parameter defined number of (localized) PRBs and each UE is given a parameter defined number of bins each 20 ms. Persistent bin assignment follows the rules:
L2 buffer data in eNb
Packet Scheduler (PS)
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Fig. 1.
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Number of UEs in each bin is uniformly distributed Number of UEs sharing the same bins is minimized Frequency diversity of a user is maximized
PS prioritizes users based on buffering time and schedules each user to the pre-assigned bin that provides the best average CQI. After semi-persistent scheduling, the HARQ retransmissions and control data are scheduled dynamically. Dynamic PS is handled with de-coupled time domain (TD) and frequency domain (FD) scheduler presented in [10]. TD scheduler chooses a subset of users as candidates for scheduling based on the Medium Access Control (MAC) buffering time and FD allocates one or more PRBs for one or more candidates. FD scheduler first sorts the candidate users based on the buffering time and then allocates enough PRBs to be able to transmit one VoIP packet (or more in case of packet bundling) for each user in turn. The PRBs are sorted according to user experienced CQI. Best possible MCS may be also chosen for dynamically scheduled UEs by LA (including both inner loop (ILLA) and outer loop LA (OLLA)). PDCCH is modelled by allowing a parameter defined maximum number of users to be dynamically scheduled per TTI. C. Scenario setup The VoIP capacity evaluation is based on the UTRAN LTE downlink parameters and assumptions described in [1]. All the evaluated simulations are performed in a three tier diamondpattern macro scenario with 19 3-sector sites, i.e. a total amount of 57 cells. Users are created and move within the 21 cells in the middle. The 26 cells at the edge of the scenario are just generating interference at the same magnitude as the average load in the center cells. The 3GPP simulation case 1 has been utilized with 5 MHz system bandwidth, 500 m intersite distance and 3 kmph UE velocity. Rest of the simulation parameters have been presented in Table I.
TABLE I S TATIC SIMULATION PARAMETERS
3GPP case Receiver type Channel model Simulation length Subframe lenght (TTI) Symbols per TTI Subcarriers per RB Duplexing Power control HARQ
ARQ Initial MCS (LA off) Possible MCSs Link Adaptation Fullband CQI
Segmentation Handovers
Dynamic PS Talk-spurt PS Semi-persistent PS
VoIP AMR 12.20
Value 57 macro cells, synchronous, reuse 1, downlink Modified case 1: 5 MHz, 500m ISD, 3kmph MRC 1x2 Typical Urban 20 1M steps (about 71 seconds) 1 ms 14 (4 reserved for control) 12 FDD Off Asychronous with Chase Combining 8 SAW channels Max 3 retransmissions Off QPSK 2/3 QPSK, 16QAM and 64 QAM, each with 3 coding rates Both inner and outer loop BLER target 0.2 Measurement period: 5 ms Error variance: 1 dB Quantization step: 1 dB Reporting delay: 2 ms CQI resolution: 2 RBs per CQI Off Hard handovers Sliding window size: 200 ms Handover margin: 3 dB Time to trigger: 200 ms TD: Round Robin FD: Even Resources PRBs are selected based on CQI QPSK 2/3 with 2 PRB allocation per 20 ms 3 FD bins per FD bin 2 TD bin positions per 20 ms QPSK 2/3 with 2 PRB allocation Localized PRBs per bin VoIP packet: 38 bytes SID packet: 14 bytes Call length: neg. exp. distr. 20s mean Active period: neg. exp. distr. 2s mean Silent period: neg. exp. distr. 2s mean
V. R ESULTS A. Effect of amount of PDCCHs VoIP capacities as a function of PDCCHs with different packet schedulers are presented in Fig. 2. It can be seen that with dynamic PS the VoIP capacity is quite much linearly dependent on the PDCCH capacity. Without LA the VoIP capacity is approximately doubled when the amount of PDCCHs has been doubled. With LA the gain of increased number of PDCCHs is even larger (even tripled), but with 8-10 control channels the VoIP capacity is closening the maximum. Thus, with dynamic PS the PDCCH clearly restricts the VoIP capacity if the amount of PDCCHs is lower than 8. With persistent PS, the VoIP capacity is not that dependent on the PDCCH capacity. VoIP capacity is at maximum already with 4 control channels with talk-spurt based persistent PS (200 UEs per cell) and even with 2 control channels with
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semi-persistent PS (185 UEs per cell). This is because the 1st transmissions of VoIP packets are transmitted with a persistent allocation, thus not requiring any PDCCHs. Only the 1st transmission of either talk-spurt (talk-spurt based persistent PS) or the entire call (semi-persistent PS) consumes a PDCCH. HARQ retransmissions are dynamically scheduled. Thus assuming 20% BLER for the first transmission, 80% of the VoIP packets do not need PDCCH resources. Interesting in Fig. 2 are the points where the dynamic PS and persistent PS VoIP capacity curves cross. This means that if we have less than 9-10 control channels, then there is no sense in using dynamic PS without LA. The dynamic PS with LA start providing better VoIP capacity than persistent PS already with 5 control channels. However, if we consider that we have also best effort type traffic to be scheduled, the 5 control channels may not be enough. Thus in mixed traffic case the need for persistent PS is emphasized. B. Scheduled PRB SINR and delay distributions The scheduled PRB SINR and L3 delay distributions have been presented in Fig. 3 and Fig. 4 with 200 UEs per cell, respectively. It can be seen that dynamic PS provides about 78 dB PRB SINR gain compared to persistent PS. This is clear, since dynamic PS as a full freedom of selecting the best time (TTI) and frequency (PRBs) for each UE every scheduling instant, based on periodical, and in this case quite, accurate CQI reports. Whereas the with persistent PS there are limited set of PRBs possible for allocation. For talk-spurt based persistent PS the allocation remains constant for the duration of a talk-spurt while with semi-persistent there are three sets of allocations possible for the duration of the entire call. Semipersistent provides about 1-2 dB SINR gain compared to talkspurt based persistent PS due to limited scheduling freedom in selecting the bin to schedule. Thus there is a tradeoff between control channel usage and scheduled PRB SINR. Worse SINR
L3 delay CDF with 200 UEs per cell 1
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C. Scheduled users per TTI The scheduled users per TTI with dynamical and persistent PS have been presented in Fig. 5 and Fig. 6, respectively. With dynamic PS the PDCCH limitations control the amount of scheduled UEs per TTI, while with persistent PS most of the UEs are scheduled persistently leaving only a small proportion of UEs in need for dynamic PDCCH resources. Semi-persistent PS consumes less PDCCH resources than talkspurt based persistent PS, since also the SID frames are scheduled persistently.
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results in higher number of retransmissions and worse residual BLER. However, this could be improved by using a slow LA, where the transport block size could be changed from talkspurt to another or even more frequently. The L3 delays show that 200 UEs per cell is clearly over the capacity limit with dynamic PS without LA. It is worth noting from Fig. 4, that with semi-persistent PS there is a small proportion of UEs which experience too long delay. This causes the fact the semi-persistent PS offers a little less VoIP capacity than talk-spurt based persistent PS, even though the PRB SINRs are a 1-2 dB better. For semi-persistent PS the assignment of UEs into the TD and FD bins is extremely challenging. There should be an even number of UEs in each bin and UEs sharing the same bin should be minimized. More importantly the number of active and silent UEs per bin should be tried to be uniform. This is challenging, since VoIP calls, active periods and silent periods have variable lengths (negative exponentially distributed). But, with some improvements, the VoIP capacity with semi-persistent PS is expected to increase over the talk-spurt based persistent PS: e.g. blocks of different transport block sizes could be assigned for UEs, thus UE could utilize the best one for a certain radio conditions.
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VI. C ONCLUSIONS AND FUTURE WORK In this article we have studied different packet scheduling mechanisms for VoIP service, such as talk-spurt based persistent, semi-persistent and dynamic PS. The results show persistent PS provides a tradeoff between VoIP capacity and PDCCH usage. With higher amount of control channel demand, persistent PS becomes unavoidable. Based on the results, talk-spurt based persistent PS seems to provide a little better VoIP capacity than semi-persistent PS. Future work includes studying the VoIP capacity with persistent PS and more realistic PDCCH modelling. ACKNOWLEDGEMENTS The authors acknowledge the valuable feedback from Mr. Jussi Ojala from Nokia.
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R EFERENCES [1] “Physical Layer Aspects for Evolved UTRA,” 3GPP Technical Report 25.814, version 7.1.0, October 2006. [2] H. Ekström, A. Furuskär, J. Karlsson, M. Meyer, S. Parkvall, J. Torsner, and M. Wahlqvist, “Technical Solutions for the 3G Long Term Evolution,” in IEEE Communications Magazine, vol. 44, March 2006, pp. 38–45. [3] J. Puttonen, T. Henttonen, N. Kolehmainen, K. Aschan, M. Moisio, and P. Kela, “Voice-over-IP Performance in UTRA Long Term Evolution Downlink,” in Proceedings of the IEEE Vehicular Technology Conference (VTC’S08), May 2008. [4] F. Persson, “Voice over IP Realized for the 3GPP Long Term Evolution,” in Proceedings of the 66th IEEE Vehicular Technology Conference (VTC’F07), September 2007. [5] D. Jiang, H. Wang, E. Malkamäki, and E. Tuomaala, “Principle and Performance of Semi-Persistent Scheduling for VoIP in LTE System,” in Proceedings of the International Conference on Wireless Communications, Networking and Mobile Computing (WiCom’07), September 2007, pp. 2861–2864. [6] “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (EUTRAN),” 3GPP Technical Report 25.813, version 7.3.0, March 2006. [7] T. Henttonen, K. Aschan, J. Puttonen, N. Kolehmainen, P. Kela, M. Moisio, and J. Ojala, “Performance of VoIP with Mobility in UTRA Long Term Evolution,” in Proceedings of the IEEE Vehicular Technology Conference (VTC’S08), May 2008. [8] “LTE Physical Layer Framework for Performance Verification,” 3GPP R1-070674, February 2007. [9] “One Way Transmission Time,” ITU-T reconmmendation G.114, May 2003. [10] P. Kela, J. Puttonen, N. Kolehmainen, T. Ristaniemi, T. Henttonen, and M. Moisio, “Dynamic Packet Scheduling Performance in UTRA Long Term Evolution Downlink,” in Proceedings of the International Symposium on Wireless Pervasive Computing (ISWPC’08), May 2008.
