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  • Research Article
  • Open Access

An Efficient Scheduling Scheme to Enhance the Capacity of VoIP Services in Evolved UTRA Uplink

EURASIP Journal on Wireless Communications and Networking20082008:732418

  • Received: 14 July 2007
  • Accepted: 27 March 2008
  • Published:


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%.


  • Schedule Scheme
  • Resource Block
  • Transmission Time Interval
  • User Pairing
  • Voice Packet

1. Introduction

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 [3]. 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 [4]. 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 [7], 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

3. VoIP Services over E-UTRA Uplink

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.

The percentage of VoIP users satisfying outage limitation (e.g., 2% PER) is presented in Figure 4. It is shown as a function of the available delay latency in scheduler. From the figure, we observe that the percentage of users satisfying outage criterion increases according to the increase of the available delay latency in scheduler. This is because the probability of packet loss because of a timeout of target delay latency becomes more decreased with higher delay latency in scheduler. In addition, a proposed scheme results in the smaller required delay latency in scheduler to achieve the same outage performance than the corresponding original persistent scheduling scheme. For example, If we aim for an identical percentage value of 0.8 and the same number of users as 190, the required delay latency using a proposed scheme is 43 milliseconds when employing a random user pairing and 39 milliseconds when employing a best user pairing, respectively. On the other hand, when using an original persistent scheme the required latency may be 52 milliseconds. This implies that the VoIP capacity can be increased if the proposed scheduling scheme is employed. Specifically, more capacity benefits can be obtained by employing a best user pairing approach.
Figure 4
Figure 4

Outage probability versus delay latency in scheduler.

Figure 5 characterizes the achievable VoIP capacity, when the delay latency in scheduler is 50 milliseconds statically. According to the figure, we note that the available VoIP capacity with a proposed scheme employing a best user pairing is 200 against that the 168 VoIP users can be serviced by using original persistent scheduling.
Figure 5
Figure 5

Outage probability versus number of VoIP users.

However, Figure 6 shows the IoT distributions for the different number of VoIP users. The IoT is computed per sector per subframe. Unlike downlink environments, in uplink case IoT level will be an important factor determining the VoIP capacity. We will employ the worst constraints the same as or below the average 3 dB in the paper.
Figure 6
Figure 6

VoIP user IoT cumulative distributions.

Finally, Table 3 summarizes the capacity of VoIP using proposed scheduling scheme in E-UTRA uplink. The results confirm that the VoIP capacity with a proposed scheme can be improved of 10% when using the random user pairing and 20% when using the best user pairing approach against the original persistent scheduling. Moreover, the available VoIP capacity employing a best user pairing scheme over E-UTRA uplink provides significantly highly capacity (e.g., 160%) if compared to the VoIP capacity over HSUPA (Release'6).
Table 3

Summary of VoIP capacity using proposed scheduling scheme.


Capacity for

IoT [dB]


VoIP services


Original persistent scheme



Proposed (random pairing)



Proposed (best pairing)



HSUPA (Release'6)



5. Conclusion

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.

Authors’ Affiliations

Telecommunication R&D Center, Telecommunication Network Business, Samsung Electronics, 416, Maetan-3dong, Yeongtong-gu, Suwon-si, Gyeonggi-do, 443-742, South Korea


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© Yong-Seok Kim. 2008

This article is published under license to BioMed Central Ltd. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.