An Efficient Scheduling Scheme to Enhance the Capacity of VoIP Services in Evolved UTRA Uplink
© Yong-Seok Kim. 2008
Received: 14 July 2007
Accepted: 27 March 2008
Published: 20 April 2008
An efficient scheduling scheme is proposed to increase the available capacity of VoIP services over evolved UTRA uplink. On top of the advantages of persistent scheduling, the proposed scheme adaptively share the resources of two VoIP users to get early-termination gain of dynamic scheduler. Through system-level simulations, the performance of the proposed algorithm is evaluated in terms of the capacity enhancement of VoIP services. Comparisons with the original persistent scheduling and the HSUPA scheduler reveal that the proposed scheme increases the capacity of VoIP services up to 20%.
Evolved universal terrestrial radio access (E-UTRA), which is known as long-term evolution (LTE) of third-generation cellular system, is being specified by the third generation partnership project (3GPP). In September 2006, the study item of the LTE has been completed and the corresponding work item was scheduled to be finalized within about one and a half years, that is the second half of 2007, so that the subsequent initial deployment can be possible in the year of 2009 or 2010. The E-UTRA is regarded as the preliminary version of next generation wireless communication system because of its capability to satisfy demand for higher user bit rates [1, 2]. In order to obtain such higher user bit rates, the E-UTRA is being designed by only packet-switched (PS) network without circuit mode, requiring that all the available LTE services should be implemented on top of internet protocol (IP). At this point, the transmission of real-time data such as voice traffic through PS IP network becomes arguably the hottest issue today because voice over IP (VoIP) has high visibility in consumer space.
Managing such real-time data transmission, scheduling algorithm at medium access control layer can be a core function because the algorithm directly controls the level of quality-of-service (QoS). As a basic scheduling scheme for packet-based services, proportional fairness (PF) scheduler was designed to support the high data rate of 3GPP2 wireless system in . The PF scheduler provides effectiveness from the view point of throughput and fairness by judiciously selecting frames based on the average and the instantaneous data rate of each user. Because of its simple yet effective scheduling capability, the scheme is still being used in the LTE as dynamic scheduler. However, such dynamic scheduler could provide limited performance for some delay sensitive real-time services in that the scheduling algorithm passed over delay constraint in its frame prioritization.
To cope with such delay problem and to satisfy the specific QoS parameter of maximum allowable service delay, the author proposed a frame bundling scheme in . The scheme modified PF scheduler such that the estimated delay of each user controls the priority of frame schedule with frame bundling according to the user's channel condition, resulting in the significant enhancement of the capacity of VoIP over high-speed downlink packet access network. Nevertheless, this work did not reflect on the overhead of control signaling, although it grows fast as the number of VoIP users increases. Note that the control signaling is required to assign and distribute resources for each user at every transmission time interval (TTI), and the TTI is usually very short, that is, 1 millisecond in most of the emerging systems. This overhead may degrade the spectral efficiency of radio systems seriously, making its minimization essential to the enhancement of systems performance.
As a method to reduce such control signaling overhead, persistent scheduling scheme has been investigated in [5, 6]. By use of the inherent characteristics of voice traffic such as frame size and period, the scheme efficiently reduced control channel signaling overhead. With the aid of such overhead reduction, the persistent scheduling scheme has been discussed as an option for VoIP services in E-UTRA uplink. However, such persistent resource allocation makes lack of early-termination gain, bringing about the waste of frequency resources with the reduced fairness of users. This is because the allocated resources through this persistent scheduling scheme (i.e., the TTI and frequency resource block (RB)-index) are assigned to each VoIP user for a relatively long period of time without changes. Accordingly, the persistent scheduling scheme could limit the capacity of VoIP service using LTE system, and thus more efficient scheduling should be required.
In this paper, an efficient LTE scheduler is proposed to increase the capacity of VoIP in E-UTRA uplink. The proposed scheme modified the persistent scheduling algorithm such that the resources of two VoIP users can be coupled. By letting these coupled resources adaptively shared by the two VoIP users, the proposed scheme achieves a significant amount of early-termination gain without the need of additional control signals.
The remainder of this paper is organized as follows. In the next section, the conventional and the proposed scheduling schemes of E-UTRA uplink system will be described. Then, Section 3 presents the details of simulation configurations and the criteria to measure the capacity of VoIP services. Section 4 follows to discuss the performance of the proposed scheme by system-level simulations. In this section, we also present the comparison results of the proposed algorithm with the original persistent scheduler of LTE as well as that of high-speed uplink packet access (HSUPA) in 3GPP Release'6 , which is the latest version of already deployed wireless network, for more thorough comparison. Finally, the conclusion of this paper is drawn in Section 5.
2. Scheduling for VoIP Services in E-UTRA Uplink
This section deals with the question of what kind of resource allocation scheme is suitable for satisfying the required QoS of various services having different transmission characteristics. Those services can be voice, streaming, web-browsing, file transfer, and so on.
2.1. Conventional Dynamic Scheduling
Basically, dynamic resource allocation has been chosen for integrated scheduler of E-UTRA uplink transmission scheme. The integrated scheduler includes packet scheduler, adaptive modulation and coding (AMC) unit, hybrid automatic repeat request (HARQ) manager, power management unit, and buffers . They are all located at eNB to support fast channel-dependent scheduling.
The packet dynamic scheduler is the main and the representative part of such integrated scheduler to select users for the assignment of time/frequency resources at every TTI. Resource block (RB), which consists of 12 subcarriers, is the minimum scheduling granule of such packet scheduler in frequency-domain. With the unit of RB, in E-UTRA uplink, control parameters such as payload sizes and modulation coding sets (MCS) are determined by the eNB's packet dynamic scheduler, depending on queue states in the user's data buffer. Therefore, the reporting of the buffer status to eNB is essential. This allows for all the tight QoS control taken by the eNB and no QoS handling done at user-site.
The algorithm of packet scheduling can be divided into two parts, saying the selection of users and the assignment of RBs. In the users selection procedure, dynamic scheduler chooses a particular user according to his/her priority that is calculated based on the PF algorithm. As for the RBs assignment, the scheduler exploits the benefit of multiuser frequency diversity. To do so, the RBs assignment for selected retransmission is done by picking up relatively good quality of channels for the previous TTI. However, once RBs were allocated to a user, the retransmission packets of HARQ uplink should be maintained until the packets are correctly received or the maximum allowable time interval. This is because the E-UTRA system employed synchronous HARQ and nonadaptive AMC scheme for uplink to reduce the amount of control signals. Due to this limitation, it becomes less flexible to assign good RBs for the retransmission of uplink data, but HARQ combining gain still can benefit the retransmission quality.
However, downlink control channel signaling is classified into two groups to support uplink services. The first group is used to indicate the assigned RB's information. It contains some user specific signals such as user identification (UEID), cyclic redundancy check (CRC), RB assigned frequency-domain location, assignment time-domain duration, modulation, coding, and payload size . Also, this control channel signaling is required at the beginning of each first transmission TTI because of the synchronous HARQ in E-UTRA uplink. The second is used to assign RBs in order to apply ACK/NACK information for uplink data.
2.2. Original Persistent Scheduling
Since the eNB handles a large portion of VoIP users, the amount of the required control channel signals for voice services increases significantly. It is critical that the control channel signaling overhead is minimized as much as possible. Therefore, for voice services, this paper focuses on the persistent resource assignment with the static control signaling. The persistent scheduling is mainly aimed for the voice services. It has been discussed for E-UTRA as a solution to overcome the limitation of layer1/layer2 (L1/L2) control channels.
By definition, the persistent assignment means that the resources are assigned by the eNB's persistent grant for a relatively long period of time (i.e., talk-spurt period). Once an allocation it is not changed regardless of channel quality and queue status except when entering silent period. It is not required to inform the scheduling L1/L2 control channel signals except for the first establishing time [5, 6]. So it is very efficient for the control signaling overhead reduction. The eNB's scheduler can simply use the predefined persistent RBs in the allocated every TTI. The assigned resources are released only when the VoIP user at the talk-spurt state enters the silent state. During the silent period, the silent payloads are transmitted by using the conventional dynamic scheduling method. This is because the L1/L2 control channel signaling overheads are not burden since the period of the silent frame is more than talk-spurt frame (i.e., 160 milliseconds).
Although the persistent scheduling is very efficient to reduce the overheads of L1/L2 control channel signals, it is very inefficient to achieve some dynamic scheduling benefits such as early-termination gain and channel-dependent scheduling. The early-termination gain means that the allocated resources, which are assigned to a scheduled user, can be potentially available to other user when they are not used by the allocated user. It may be efficient in resource utilization. But, in the original persistent scheduling scheme, the allocated resources are not changed during the talk-spurt period. In other words, since the original persistent scheduling does not provide the early-termination gain, the available capacity of VoIP is limited. Therefore, there is a need for a solution to improve the available VoIP capacity.
2.3. A Proposed Scheduling Scheme
2.3.1. Concept of the Proposed Scheduling
In this section, an adaptive resource sharing scheme using user pairing, which can maintain the properties of the original persistent scheduling method, is proposed. Design objective of the proposed scheme is to achieve the early-termination gain by using user pairing of two VoIP users.
Event status for indicating the authority of a shared RB.
1st pairing user's
2nd pairing user's
In order to perform the above process, one paired VoIP user in each pairing group only monitors the other paired user's ACK/NACK channel information. So, there is no burden for control channel signaling to change the transmitting authority adaptively. However, the rest resources, except for the shared resources for pairing groups, are used according to the original persistent scheduling rule. Moreover, if the only one user in a pairing group is staying alone because the other paired user's assigned resources are released, the remaining user can be operated according to the original persistent configurations.
2.3.2. User Pairing Method
The resource sharing algorithm that we propose for VoIP services in E-UTRA uplink is designed to obtain the early-termination gain. It can be achieved by employing the user pairing. The user pairing can be classified as two methods basically. One is a random pairing that two supported VoIP users can be paired as a pairing group without any consideration. It does not take into account the individual user radio channel condition, the data buffer status, and the packet transmission delay in making a paired group. The other is a best pairing scheme which pairs two VoIP users to a paired group considering the individual status of user conditions. In this scheme, the best pairing chooses one user under good conditions and the rest under bad status of conditions in making a paired group. In other words, the delay sensitive user, who requires more retransmission opportunities due to a bad channel conditions, is favored by pairing a group with the delay nonsensitive users under a good channel condition. This pairing configuration can increase the number of event 2 or 3 occurrence. So, the discarded packets, which occur because of a timeout of the delay sensitive user, can be reduced by using the shared resources of the delay nonsensitive user. Therefore, best pairing can give a large efficiency to the proposed resource sharing scheduling scheme. However, in this paper the above two pairing methods are compared to each other.
2.3.3. Description of Overall Operation for a Proposed Scheduling Scheme
In this section, we describe the overall procedure of the proposed resource sharing scheduling scheme. The authority adaptation is executed every repeated pattern period. Also, the user pairing can be also adjusted several times of repeated pattern periods. The overall procedure can be described on the eNB-site and user-site, which is summarized as follow.
The Flow on the eNB-Site
pair two users according to the user pairing method;
assign RBs to each pairing group persistently using the measured priority that can be calculated by the modified PF in ;
scheduler monitors the event status of the paired users in each pairing group;
change the transmission authority of shared RBs when they meet the boundary of repeated pattern period and occur event 2 or 3.
The Flow on the User-Site
monitor his own ACK/NACK channel as well as the ACK/NACK channel allocated to the rest user in a pairing group;
change the available transmission RBs when they meet the boundary of repeated pattern period and occur event 2 or 3;
transmit VoIP packet using the allocated RBs at his assigned time.
2.3.4. Discussion on the Control Channel Signaling Overhead
In this section, we investigate a quantitative comparison of two VoIP scheduling schemes from the control channel signaling overhead point of view. However, in the proposed resource sharing scheme each paired user must know the authority of the shared resources to perform according to the proposed scenario. Therefore, one concern for the proposed scheme is the additional control channel signals. The paired user's control channel information for ACK/NACK channel needs to be once transmitted in the first time when the voice radio bearer is established. Therefore, the achievable capacity gains using the proposed scheduling are enough even if the additional control channel signaling overhead is taken into account, since the additional control channel signals are a very small quantity.
2.3.5. Power Control and Interference over Thermal in Uplink
where represents the serving sector of user , is the total received power from user at sector , and is the thermal noise variance. The IoT plays an important role in determining the system throughput since the transmission rate depends on the signal-to-interference plus noise ratio (SINR) and not just on the signal-to-noise ratio (SNR). The IoT bounds the additional interference to the cell, and thereby limits the power required for new users to access the network. Typical IoT values in commercial networks range from 3 to 10 dB and we will employ the worst constraints the same as or near the average 3 dB in this paper .
However, similar to HSUPA, the average IoT level received at the eNB is effectively reduced by using uplink power control approach. To achieve good tradeoff between the cell-edge performance and the overall spectral efficiency, power control scheme should be considered carefully. The only inter-cell interference exists in the uplink for E-UTRA due to an orthogonal access. Therefore, the slow power control is sufficient to compensate the influence of IoT. In this paper, the slow power control may be implemented in each eNB by sending power control command slowly. It should be noted that the other approach is possible. For example, each user can derive its own transmit power based on the path loss measurement from downlink pilot.
3. VoIP Services over E-UTRA Uplink
3.1. The Properties of VoIP Services
3.1.1. Traffic Model and Protocol
In this paper, the conversation traffic can be approximated to the two state Markov model with a suitable voice activity factor (VAF) . The adaptive multirate (AMR) voice codec is mandatory for voice services in E-UTRA systems. The AMR codec generates a 32-bytes voice payload every 20 milliseconds during talk-spurt period. During silent period, a 7-bytes payload carries a silence descriptor (SID) frame every 160 milliseconds . Also, we can set by 50% VAF.
However, a typical VoIP protocol stack, which employs the real-time transport protocol (RTP), is encapsulated to the user datagram protocol (UDP). This, in turn, is carried by IP. These combined protocols demand a 40-bytes IPv4 header or a 60-bytes IPv6 header. Therefore, the overhead, caused from these headers, seriously degrades the spectral efficiency in supporting VoIP service. Therefore, efficient and robust header compression (ROHC) technique must be used to reduce the amount of the large headers in the IP/UDP/RTP layers. This technique can reduce the size of the IP/UDP/RTP headers as little as 2 or 4 bytes using IETF RFC 3059 [12, 13]. Therefore, in this paper we assume that the IP/UDP/RTP headers are reduced as 4 bytes using ROHC.
3.1.2. Definition of VoIP Capacity
In PS network, packets will be dropped due to packet error and packet delay exceeding the target latency. Although some packet loss occurs, the voice quality is not affected if the amount of packet loss is less than outage threshold. At this point, the VoIP capacity is defined by the maximum number of VoIP users that can be supported without exceeding a given outage threshold. The outage criterion means that packet error rate (PER) of VoIP user is kept within 2%. Moreover, at least 95% of total VoIP users should meet the above outage criteria .
3.1.3. End-to-End Delay Latency for QoS Support
3.2. System-Level Simulation Setup
3.2.1. Physical Layer in E-UTRA Uplink
For uplink transmission, the important property is to allow user equipment (UE) for power efficient transmission to maximize coverage. The choice of single-carrier frequency-domain multiple access (SC-FDMA) is therefore preferable in E-UTRA uplink. This is because the resulting peak-to-average power ratio (PAPR) is lower than orthogonal frequency division multiple access (OFDMA) in downlink. Also, the fast fourier transform (FFT) and inverse fast fourier transform (IFFT) are used in transmitter to produce the FDMA signal. So, multipath propagations are handled by frequency domain equalization at the eNB, aided by the insertion of a cyclic prefix (CP) in the transmitted signal. A subcarrier spacing of 15 kHz is adopted, which allows for simple implementation of dual mode between wideband code division multiple access (WCDMA) and E-UTRA terminal. Also, the number of individual subcarrier as 12 consists of 1 RB. The RB is the basic time-frequency transmission resource unit in E-UTRA system. To minimize delays, the subframe duration, that is TTI, is selected as short as 1 millisecond, corresponding to 14 OFDM symbols .
However, in case of scheduled access the eNB-driven scheduling scheme has been chosen in uplink. It means that unlike HSUPA, the uplink data transmission format of a UE such as payload size and MCS level is controlled by eNB.
3.2.2. Simulation Environments
To investigate the capacity performance of VoIP traffic, the system-level Monte-Carlo computer simulation is accomplished in this paper. In all simulations, 5 MHz system bandwidth is considered among the flexible bandwidths. The simulations are carried out with a regular hexagonal 19 cellular model, where the intersite distance (ISD) between eNB is 500 m. Mobile terminals should be uniformly distributed on the 19-cell layout for each simulation run and assigned by typical urban (TU) channel model according to channel assignment probability specified in . Note that a realistic model of the wave propagation plays an important role for the significance of the simulation results. Mobile speed is 3 km/h since 3GPP E-UTRA system should be optimized for low mobile speed. Shadowing is modeled by log-normal fading of the total received power. An attenuation is determined by Hata model.
System-level simulation parameters.
Source traffic packet overhead
AMR 12.2 kbps,
VAF = 0.5, 2-state Markov,
ROHC 4 bytes [IETF RFC 3059]
Hexagonal grid, 19 sites, 3 sectors
(eNB-to-eNB 0.5 km)
Carrier frequency 2 GHz
Path loss = −128.1–37.6*log(R)
Log normal Std. dev. 8 dB,
Node B 14 dB/UE 0 dB
User Tx. Max. power
Typical urban (TU)
Thermal noise density
Number Rx ant.
No RLC retransmission (No ARQ),
sync. HARQ (max. retrial = 4),
HARQ process number = 8,
50 Hz (sounding RS-based closed-loop method)
Persistent scheduling with a resource sharing using user pairing,
(repeated pattern period = 20 milliseconds)
QPSK, 0.55 coding rate,
talk-spurt period (2 RBs assign);
silent period (1 RB assign)
Link curve mapping
Effective SIR method (ESM)
Link curve TTI
29%(pilot and control overheads)
2 long blocks for DM RS
4 RBs for control signals
4. VoIP Capacity Evaluation
In this section, we evaluate the available capacity of VoIP traffic with the proposed scheduling scheme in the typical urban fading channel environments. The proposed scheme is also compared with the original persistent method.
Summary of VoIP capacity using proposed scheduling scheme.
Original persistent scheme
Proposed (random pairing)
Proposed (best pairing)
In this paper, we propose the efficient scheduling method employing a resource sharing approach. This proposed scheme employs the random user pairing and the best user pairing method to improve the capacity of VoIP services over E-UTRA uplink. Results are investigated by the system-level simulation. Our simulation results show that the employment of proposed scheduling scheme makes a larger available capacity than that resulted by the original persistent scheduling. In addition, we also conclude that E-UTRA is attractive for supporting of VoIP services if compared to HSUPA (Release'6).
The consideration of the combination of other traffic types such as best-effort, web, and streaming may be an interesting issue for future study. Moreover, the influence of ACK/NACK decoding errors may be considered.
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