Capacity of VoIP Services in TDD OFDMA System with Different Delay Requirements Hoyoung Choi * , Hyunwoo Choi * , Daehyoung Hong * , Jinwoo Choe * , and Soonyoung Yoon † * Department of Electronic Engineering, Sogang University, † Samsung Electronics Inc. {hychoi, hwchoi, dhong, xinu}@sogang.ac.kr, soon.young.yoon@samsung.com Abstract In this paper, we evaluate the voice service capacity of the TDD OFDMA system based on IEEE 802.16d/e. Voice traffic must be transmitted in real time with very small delay when we use packet transmission networks. Voice over IP (VoIP) services can be provided in this wireless packet network as Unsolicited Granted Service (UGS) or Real Time Polling Service (rtPS) class. As UGS, a fixed amount of bandwidth can be allocated to transmit the voice traffic for the duration of a call. On the other hand, as rtPS, bandwidth is allocated variably as needed to transmit the talk spurts using polling process. In this analysis, we also take into account the delay limit as one of the main Quality of Service (QoS) requirements of the voice services. The capacities of VoIP services for the classes with different delay limits are derived and compared in this paper. Simulation results show that the voice capacity is larger when we treat the VoIP service as rtPS than that as UGS. 1. Introduction The demand of using mobile voice telephony services as well as multi-media data services through the mobile or wireless packet network (Internet) is increasing gradually. The IEEE 802.16d/e WirelessMAN TM is one of the networks which can support these needs. The conversational voice traffic typically presents a symmetric or quasi-symmetric nature and requires small end-to-end transmission delays[7]. This voice traffic has a pattern of voice activity with talk spurts and silence periods. These periods correspond to talking, pausing and listening patterns of a conversation. The voice traffic which has these sorts of natures can be transmitted as UGS or the rtPS service class in the IEEE 802.16d/e system. As UGS, we allocate a fixed amount of bandwidth to transmit voice traffic for the duration of a call. The necessary bandwidth and allocation period are negotiated only at the initialization process of the voice session, and it doesn’t need any additional periodic bandwidth request or polling process during the session. Therefore, MAC overhead and uplink access delay can be minimized. However, a fixed size of the bandwidth is allocated continuously even during the silent period of voice traffic; it can cause a waste of the bandwidth resources. On the other hand, as rtPS, bandwidth resources are assigned as much as needed only for the talk spurts. Because this service class always uses a bandwidth request or polling process for the suitable size of talk spurs, it can utilize the bandwidth resource efficiently only for the amount of voice traffic generated. However, the bandwidth needs to be requested multiple times in a call duration for each talk spurt, the bandwidth resource can be wasted as the MAC overhead. The access delay can also become larger in this case. Voice is a delay-sensitive service. To support the voice service on the packet network as VoIP services, the end-to-end delay of the voice packet needs to be smaller than the maximum delay required. It is an important quality factor and needs to be set as one of the QoS requirements. Because both of these service classes are not suitable perfectly for voice services, it is important to evaluate the system performance of VoIP services using both service classes. In addition, it is very crucial to estimate how many voice users can be served at the same time ahead of deploying base stations for the IEEE 802.16d/e WirelessMAN TM system. This is quite helpful for system operators to configure the system economically. In this study, the system capacity for VoIP services is evaluated with the delay limits. In previous studies, the voice capacity was evaluated in the IEEE 802.11b WLAN as well as in the This work is supported in part by Samsung Electronics Inc. 1-4244-0667-6/07/$25.00 © 2007 IEEE 44