1398 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 6, NO. 12, DECEMBER 2007 Modeling UMTS Power Saving with Bursty Packet Data Traffic Shun-Ren Yang, Sheng-Ying Yan, and Hui-Nien Hung Abstract—The universal mobile telecommunications system (UMTS) utilizes the discontinuous reception (DRX) mechanism to reduce the power consumption of mobile stations (MSs). DRX permits an idle MS to power off the radio receiver for a predefined sleep period and then wake up to receive the next paging message. The sleep/wake-up scheduling of each MS is determined by two DRX parameters: the inactivity timer threshold and the DRX cycle. In the literature, analytic and simulation models have been developed to study the DRX performance mainly for Poisson traffic. In this paper, we propose a novel semi-Markov process to model the UMTS DRX with bursty packet data traffic. The analytic results are validated against simulation experiments. We investigate the effects of the two DRX parameters on output measures including the power saving factor and the mean packet waiting time. Our study provides inactivity timer and DRX cycle value selection guidelines for various packet traffic patterns. Index Terms—Bursty packet data traffic, discontinuous reception, power saving, universal mobile telecommunications system (UMTS). Ç 1INTRODUCTION HE third-generation mobile cellular system universal . In Cellular Digital Packet Data (CDPD) [5], [12] and Tmobile telecommunications system (UMTS) offers high IEEE 802.11 [8], a sleeping MS is not forced to wake data transmission rates to support a variety of mobile up at every announcement instant. Instead, the MS applications including voice, data, and multimedia. In may choose to skip some announcements for further order to fulfill the high-bandwidth requirement of these reducing its power consumption. A wake-up MS has different services, the mobile station (MS) power saving is a to send a receiver-ready (RR) frame to notify the crucial issue for the UMTS network operation. Since the network of its capability to receive the pending data bandwidth is significantly restricted by the battery frames. However, such RR transmissions may capacity, most existing wireless mobile networks (includ- collide with each other if the MSs tend to wake up ing UMTS) employ discontinuous reception (DRX) to con- at the same time. Thus, RR retransmissions may serve the power of MSs. DRX allows an idle MS to power occur and extra power is unnecessarily consumed. off the radio receiver for a predefined period (called the . UMTS DRX [2], [4] improves the aforementioned DRX cycle) instead of continuously listening to the radio mechanisms by allowing an MS to negotiate its DRX channel. Some typical DRX mechanisms are briefly cycle length with the network. Therefore, the net- described as follows: work is aware of the sleep/wake-up scheduling of each MS and only delivers the paging message when . In Mobitex [15], all sleeping MSs are required to the MS wakes up. hSVP6i synchronize with a specific frame and wake In the literature, DRX mechanisms have been studied. up immediately before the hSVP6i transmission Lin and Chuang [12] proposed simulation models to starts. When some MSs experience high traffic loads, investigate the CDPD DRX mechanism. In [10], an analytic the network may decide to shorten the hSVP6i model was developed to investigate the CDPD DRX announcement interval to reduce the frame delay. mechanism. This model does not provide a closed-form As a result, the low-traffic MSs will consume extra solution. Furthermore, the model is not validated against unnecessary power budget. simulation experiments. In our previous work [21], we proposed a variant of the M=G=1 vacation model to explore . S.-R. Yang is with the Department of Computer Science and Institute of the performance of the UMTS DRX. We derived the closed- Communications Engineering, National Tsing Hua University, No. 101, form equations for the output measures based on the Sec. 2, Kuang Fu Rd., Hsinchu, Taiwan 300. E-mail: [email protected]. Poisson assumption. However, the Poisson distribution has . S.-Y. Yan is with the Telecommunication Laboratories, Chunghwa Telecom been proven to be impractical when modeling bursty packet Company, Ltd., 12, Lane 551, Min-Tsu Road Sec. 5 Yang-Mei, Taoyuan, data traffic [20]. In [11] and [23], simulation and analysis Taiwan 32617. E-mail: [email protected]. H.-N. Hung is with the Institute of Statistics, National Chiao Tung were utilized to examine the UMTS DRX mechanism. The University, 1001 Ta Hsueh Rd., Hsinchu, Taiwan 30050. authors studied the impact of an inactivity timer on energy E-mail: [email protected]. consumption for both real-time and non-real-time traffic. Manuscript received 6 Oct. 2006; revised 9 Mar. 2007; accepted 3 Apr. 2007; However, they do not consider the mean packet waiting published online 25 Apr. 2007. time under DRX. This paper proposes a novel semi-Markov For information on obtaining reprints of this article, please send e-mail to: [email protected], and reference IEEECS Log Number TMC-0267-1006. process to model the UMTS DRX for bursty packet data Digital Object Identifier no. 10.1109/TMC.2007.1072. applications. The analytic results are validated against 1536-1233/07/$25.00 ß 2007 IEEE Published by the IEEE CS, CASS, ComSoc, IES, & SPS YANG ET AL.: MODELING UMTS POWER SAVING WITH BURSTY PACKET DATA TRAFFIC 1399 Fig. 1. A simplified UMTS network architecture. Fig. 3. The timing diagram for UMTS MS receiver activities. simulation experiments. Based on the proposed analytic and simulation models, the DRX performance is investi- . In the Cell_DCH state, the MS occupies a dedicated gated by numerical examples. Specifically, we consider two traffic channel. performance measures: . In the Cell_FACH state, the MS is allocated a common or shared traffic channel. Power saving factor. This is the probability that the MS . In the Cell_PCH state, no uplink access is possible receiver is turned off when exercising the UMTS and the MS monitors paging messages from the DRX mechanism. This factor indicates the percen- RNC. tage of power saving in the DRX (compared with the . In the URA_PCH state, the MS eliminates the case where DRX is not exercised). location registration overhead by performing URA . Mean packet waiting time. This is the expected waiting updates instead of cell updates. time of a packet in the UMTS network buffer before In the Cell_DCH and Cell_FACH states, the MS receiver it is transmitted to the MS. is always turned on to receive packets. These states 2 UMTS DRX MECHANISM correspond to the power active mode. In the RRC Idle mode and the Cell_PCH and URA_PCH states, the DRX is As illustrated in Fig. 1, a simplified UMTS architecture consists of the core network and the UMTS terrestrial radio exercised to conserve the MS power budget. These states/ access network (UTRAN). The core network is responsible for mode correspond to the power saving mode. Under DRX, the switching/routing calls and data connections to the external MS receiver activities could be described in terms of three networks, whereas the UTRAN handles all radio-related periods (see Fig. 3): functionalities. The UTRAN consists of radio network . controllers (RNCs) and Node Bs (that is, base stations) that In the busy period (see Fig. 3a), the MS is in the power active mode and the UMTS core network are connected by an asynchronous transfer mode (ATM) delivers packets to the MS through the RNC and network. An MS communicates with Node Bs through the Node B in the first-in, first-out (FIFO) order. radio interface based on the wideband CDMA (WCDMA) Compared with WCDMA radio transmission, ATM technology [7]. is much faster and more reliable. Therefore, the The UMTS DRX mechanism is realized through the radio ATM transmission delay is ignored in this paper resource control (RRC) finite-state machine exercised be- and the RNC and Node B are regarded as a FIFO tween the RNC and the MS [1]. There are two modes in this server. Furthermore, due to the high error rate and finite-state machine (see Fig. 2). In the RRC Idle mode, the low bit rate nature of radio transmission, the Stop- MS is tracked by the core network without the help of the and-Wait Hybrid Automatic Repeat request (SAW- UTRAN. When an RRC connection is established between Hybrid ARQ) flow-control algorithm [3] is executed the MS and its serving RNC, the MS enters the RRC to guarantee successful radio packet delivery: When Connected mode. In this mode, the MS could stay in one of Node B sends a packet to the MS, it waits for a the four states: positive acknowledgment (ack) from the MS before it can transmit the next packet. Hybrid ARQ was originally proposed for the High-Speed Downlink Packet Access (HSDPA) system and has also been adopted by next-generation wireless networks in- cluding the IEEE 802.16 WiMAX system. SAW- Hybrid ARQ is one of the simplest forms of ARQ, requiring very little overhead. Hybrid ARQ using this stop-and-wait mechanism offers significant improvements by reducing the overall bandwidth demanded for signaling and the MS memory. Due to its simplicity, SAW-Hybrid ARQ could also be implemented in the earlier UMTS releases without Fig. 2. The RRC state diagram. HSDPA support. 1400 IEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. 6, NO. 12, DECEMBER 2007 downloading of a WWW page). If the request is permitted, then a burst of packets (for example, as a whole constituting a video clip in the WWW page) will be transmitted to the MS through the RNC and Node B. When the RNC receives the positive ack for the last packet from the MS, the current packet call transmission has completed.