Benefits of Group Scheduling for VoIP on WiMax

Real-time services are envisioned to be an essential component of next generation mobile broadband networks (4G), and like 2G and 3G, voice is still expected to be the most desirable service over these networks. However, mobile-broadband networks, based on IP technologies, are well-known for high packet-data efficiency, but not for voice (VoIP) efficiency. A key requirement for IEEE 802.16m, the next-generation WiMAX standard, currently under definition, is support of a large number of VoIP users. Hence, efficient support of VoIP over next-generation WiMAX is needed.

VoIP is characterized by small-size, periodic packets and supporting a large number of simultaneous VoIP users in a WiMAX network is a challenge. To understand why, let’s review the IEEE 802.16e WiMAX frame structure, shown in Fig 1.

Voip_fig1.jpg

WiMAX frames comprises of the downlink (DL) and uplink (UL) subframe. The DL subframe contains the MAP region and the DL traffic region. The MAP region describes the data allocations and signals the position, size and modulation and coding scheme (MCS) for each user scheduled in the DL and UL traffic regions. The size of the MAP is proportional to the number of users in the DL and UL region. The small size of VoIP packets allows scheduling a large number of VoIP users in each frame and proportionally the size of the MAP region increases. Analysis shows that for VoIP traffic up to 40 to 45% of the DL subframe is occupied by the MAP. As shown in Fig 2a, the scheduling mechanism in WiMAX requires that MAP overhead for the user is repeated in every frame in which the user is scheduled. Hence, reducing the MAP overhead is necessary to enable scheduling additional VoIP users and to increase the network capacity to handle VoIP traffic.

voip_fig2.jpg

Persistent scheduling is a mechanism adopted in IEEE 802.16e Rev2, that reduces the MAP signaling overhead for VoIP traffic by allocating resources persistently for VoIP connections, i.e. provide periodic fixed allocation to a connection for multiple frames, eliminating the need to send the signaling information in every frame. Omitting the signaling information over multiple frames saves considerable MAP overhead. Persistent scheduling is shown in Fig 2b where the first frame includes MAP information for the VoIP burst and the information is skipped in subsequent frames. However, persistent scheduling does not allow the position and size of the VoIP allocation to change in subsequent frames, which results in reduced flexibility for the base station for scheduling. The reduced flexibility in resource allocation may cause “resource holes” in the frame when some users are reallocated. Due to this inefficient resource utilization, persistent scheduling may not achieve optimum VoIP capacity. Resource holes can be avoided by repacking users within the frame, however this causes additional overhead. The reduced flexibility with persistent scheduling also makes it harder to achieve statistical multiplexing between users. Group scheduling overcomes the above mentioned limitations of persistent scheduling.

Group scheduling clusters several users into groups based on their channel conditions and the VoIP codec used. The grouping of users makes it redundant to specify common parameters for each user, thereby saving overhead. In addition, the relative positions of users within the group are fixed with respect to each other, eliminating the need to signal the resource location of each user. Further overhead reduction is achieved by using smaller codes to specify the MCS and allocation size. A bitmap is used to specify which users of the group have allocations in a given frame, thereby allowing efficient packing of resources and avoiding resource holes. Group Scheduling thus reduces signaling overhead as well as provides flexibility in resource allocation, thereby providing efficient resource utilization.

To demonstrate the benefits of group scheduling we have built a prototype modifying an IEEE 802.16e BS and MS to support group scheduling. The video below describes the prototype.

As seen in the video, with group scheduling the size of the MAP reduces from 54 bytes (9 slots) to 42 bytes (7 slots) for 3 VoIP calls. Given that the gains in the number of slots increases linearly with number of users, we have extrapolated the result to fill an entire WiMAX frame with VoIP data bursts and the potential reduction in the MAP over head is 48% and the number of VoIP users in each frame increases from 20 to 29. Based on these initial encouraging results we are extending the prototype to support multiple simultaneous VoIP calls under real channel conditions.

This prototype provides an industry-first hardware demonstration of group scheduling an important feature for improving VoIP capacity for next-generation WiMAX systems.

The prototype was developed by Wey-Yi Guy, Minnie Ho, Vijay Kesavan, Kyle McCanta and Somya Rathi. The team acknowledges the contributions from David Bormann, Shweta Shrivastava and Jerry Sydir.

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