MAC Sleep Mode Control Considering Downlink Traffic Pattern and Mobility

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1 1 MAC Sleep Mode Control Considering Downlink Traffic Pattern and Mobility Neung-Hyung Lee and Saewoong Bahk School of Electrical Engineering & Computer Science, Seoul National University, Seoul Korea Tel: Fax: {nhlee, Abstract Energy of a mobile terminal (MT) is an important resource, so recent wireless network protocols like IEEE and IEEE include energy saving techniques. These protocols, however, have a problem in minimizing energy loss under varying traffic pattern and mobile environment. In this paper, we propose a sleep mode interval control algorithm that considers downlink traffic pattern and terminal mobility to maximize energy-efficiency. Through simulations, we show that our proposed algorithm improves energy saving and delay performance. There is no energy loss because it does not incur unnecessary cell searching in multi-cell environments. Index Terms energy saving, sleep mode, doze state. I. INTRODUCTION In wireless communications, the battery power is an important resource because it is limited. There are many ways of reducing energy consumption throughout various layers. In MAC layer, the sleep mode control is a key technique. A terminal turns its transmitter and/or receiver off when there is nothing to send and/or receive. In active mode, the terminal is on and communicates with an access point (AP). Proper switching between active mode and sleep mode is necessary to save energy. When a terminal makes a transition from active to sleep mode, it checks whether there exists any packet in the AP buffer. If there is no packet, the terminal enters the sleep mode. If the terminal has some, it stays with the active mode. IEEE and IEEE 82.16e protocols have energy saving algorithms in MAC layer. In the IEEE protocol, the sleep mode uses a fixed interval that is determined and announced by the AP, so the terminal wakes up periodically. This kind of periodic scheme is easy to implement, but it works inefficiently in terms of energy saving because the terminal wakes up periodically even though it has nothing to receive. In contrast, IEEE 82.16e uses a sleep window update algorithm. Its procedures are similar to those of IEEE except that it doubles the sleep interval. So the terminal can have a long sleep interval at light load. The drawback of this algorithm is its insensitivity to traffic pattern. In this paper, we propose a simple sleep mode control algorithm. It is adaptive to traffic pattern variation and terminal mobility. Our proposed algorithm shows good performance when the terminal is moving. A lot of previous researches in [1], [2], and [3] have dealt with the MAC protocol for sensor or ad-hoc networks, because This work was supported by NRL Program of KISTEP, KOREA Fig. 1. Doze Idle MAC state diagram. Active CID asssigned state Power Save sometimes the remaining energy represent network lifetime and connectivity in those networks. Some researches in [4] and [5] use the battery dynamics for energy saving. The standard working groups produced several drafts about the sleep mode [6]-[8] that are modifications of the standard. Our work is different from these works, because our work deals with the MAC layer energy saving in multi-cell environments. In Section II, we introduce our considered system model. Section III describes our proposed sleep mode interval algorithms. We evaluate these algorithms through simulations in Section IV, followed by the conclusion in Section V. II. SYSTEM MODEL A. MAC State and The sleep mode means that a terminal does not communicate with AP though it can do. Basically the purpose of using sleep mode is to improve energy efficiency. So a terminal controls the state of its transmitter and/or receiver according to whether it has something to send and/or receive. The sleep mode can be used to let the terminal do jobs independent of the associated AP (e.g., cell searching and signaling with other APs). There are two states in the sleep mode as shown in the simplified MAC state diagram of Fig. 1. If a terminal is associated with the AP but there is no connection ID (CID) for data transmission, the MAC state is idle. If the terminal needs to communicate with the AP, it should remain in idle state. After the AP assigns a CID to it, it enters the active state. In idle state, the terminal can enter the doze state for power saving if it has no data to send or receive. In doze state, the terminal goes to sleep. Another sleep mode is the power save state. Although the terminal is active, which means that it has been assigned a CID for data transmission, there can be a

2 2 Frame Frame MT A AP broadcast Up/Down transmission period Awake mode - Estimating s pilot power - Synchronizing with in downlink HO Request (HO Indicattion 1) ( information) B Fig. 2. transition in the unit of frame. relatively long no-traffic interval because of traffic burstiness. In this case the terminal enters the power save state which period is shorter than that of idle state. We classify four MAC states in Fig. 1 into two modes. One is the sleep mode for doze and power save states, and the other is the awake mode for idle and active states. transition is done by the unit of frame, but its name can be anything. For instance, it is a beacon interval in IEEE and a frame in IEEE 82.16e. Fig. 2 shows an example of brief frame structure and mode transition. At the beginning of a frame, the AP broadcasts which terminal s downlink data is buffered. Each terminal can sleep during multiple frames. When it wakes up, it receives the AP broadcast and determines whether it remains in sleep mode or transits to awake mode. In Fig. 2, terminal A knows that the AP buffers data, so it comes back to the awake mode. Terminal B enters the sleep mode for two frame time at least, so it is not awake in the second frame. A long sleep mode interval is good for energy efficiency, but makes long buffering delay. On the other hand, a short interval can decrease delay, but makes unnecessary awakes for checking a packet arrival. Therefore there is a tradeoff between energy efficiency and delay. A sleep mode interval control should reflect a traffic pattern, but we do not concern simple packet arrival rate. When a terminal s MAC operates in active state, packet arrives the form of burst, which means train of packets. The algorithm in active state needs to consider the intermediate time between bursts. In idle state, there is no connection, so the algorithm reflects the intermediate time between connections. B. Mobility and Handoff Terminal mobility is an important factor in wireless system. It is related to channel condition, so it makes difficulties in physical layer design. In upper layer mobility is also important, because it incurs handoff. If a handoff occurs when the terminal is in active state, it follows normal handoff procedures without any disruption. Fig. 3 shows the procedure of active handoff. In scanning period, the terminal, MT, detects target. After signaling message exchange, the terminal does reentry to. Sometimes when the terminal wakes up from the sleep mode, it realizes that it has moved into another AP area while sleeping. According to the scanning result before a sleep mode, there are two kinds of handoff s. Fig. 4 shows an idle handoff Fig. 3. Fig. 4. HO Response HO Indication 2 Active handoff procedure. MS - Estimating s pilot power - Synchronizing with in downlink Data transmission Idle handoff procedure. XHO Request (XHO Indication) XHO-Response procedure. An idle handoff occurs when the terminal already has detected a target AP in scanning period before a sleep mode. Although the terminal moves to a target AP during a sleep mode, it can do as almost same handoff procedure as an active handoff. The difference from an active handoff is that the terminal starts a handoff procedure to a target AP. A doze handoff occurs when a terminal could not detect a target AP in scanning period, and Fig. 5 shows the handoff procedure. In this case, the terminal can not communicate a target AP, because it moves too far during sleep mode, so it should do cell searching first. After cell searching, remaining handoff procedure is same to idle handoff s. To avoid having a doze handoff as much as possible, the terminal executes cell searching before entering the sleep mode. However cell searching needs lots of energy, so it is inefficient. Even though cell searching is perfect, a doze handoff still occurs when the terminal moves around while sleeping as shown in Fig. 1. This example shows the worst case of a doze handoff. We need an assumption that the terminal can detect 3 different APs by analyzing AP s pilot power and broadcasting signal. When the terminal is in position 1, it saves a AP list as {3, 2, 1} according to the pilot power. In position 1, it enters sleep mode, and it moves to position 2. When it wakes up in position 2, it wants to find AP3,, and, but

3 3 Fig Not detecting Data transmission Doze handoff procedure. MS Cell searching - Synchoronizing with downlink XHO Request (XHO Indication) XHO-Response A. Sleep interval control in active state We suggest an algorithm for a sleep interval control in an active state. In active state, the intermediate time of burst is as short as we do not need to consider a doze handoff. Our proposed algorithm can be written as follows. Initial value I is set to 1, which means one frame. Then for i 1, { 2I i 1 if 2I i 1 N th1 I i = (1) I i if 2I i 1 > N th1. A sleep mode interval increases twice until it is less than a threshold N th1. After it becomes greater than the threshold, it increases linearly. We call this algorithm (Multiple Increase and Linear Increase). Actually it has the form of the slightly modified conventional TCP congestion window update algorithm. N th1 is a threshold value that divides exponential increase period and linear increase period. N th1 is adaptive to burst intermediate time and is given by N th1 = T A /2, where T A is the estimated intermediate time of downlink traffic burst. We consider the estimation formula like following, Fig. 6. AP 1 AP 2 AP 3 Doze handoff example. (1) they are not the target AP transmitting highest pilot power. It should do cell searching to find AP4. This example shows that a doze handoff can occur to the terminal which moves half radius of a cell during a sleep mode. A doze handoff occurs more frequently when cell size is smaller and when a terminal moved more quickly. When a doze handoff occurs, the terminal has to do cell searching, which consumes much power. So a sleep interval control algorithm should reflect not only a traffic pattern but also mobility to avoid a doze handoff. (2) III. PROPOSED ALGORITHM Important factors in a sleep mode interval control are a traffic pattern and mobility, so our proposed algorithm has the form reflecting them. According to the two states in awake mode, we need to apply a different sleep interval control mechanism. This is because the periods of a doze state and a power save state are very different. The period of idle and doze state is the intermediate time of connection, so it can be very long. On the other hand, the period of active and power save state is relatively short because it is the intermediate time of burst. AP 4 T A new = (1 a)t A old + a I B, (2) where < a < 1. This estimation is updated when a sleep interval ends. When it ends a terminal knows the number of buffered packets, B, by AP s broadcast. I is the sum of previous sleep intervals, so I/B is the intermediate time of burst. To average the intermediate time, the formula has a moving average form. The estimation of the intermediate time can be done by using other better algorithms. The reason we suggest the simple formula is that the estimation is done in a terminal. If the calculation can be done in AP, more complex and exact estimation would be possible. B. Sleep interval control in idle state We suggest a sleep interval control algorithm in an idle state. Because of long intermediate time, the algorithm should consider the possibility of doze handoff, so it has form as follows with I = 1. For i 1, { min (2I i 1, M est /4) if 2I i 1 N th2 I i = (3) min (I i 1 + 1, M est /4) if 2I i 1 > N th2. N th2 is a threshold value and very greater than N th1, because in an idle state, the sleep mode interval is related to intermediate time of connection. N th2 is also adaptive to the connection intermediate time and is given by N th2 = T I /2, where T I is the estimated intermediate time of a downlink traffic connection. The estimation of T I is similar to the one of active state. M est is an estimated time for the terminal to pass through a cell and it needs to be estimated at physical or MAC layer. M est /4 comes from the worst case of Section II-B. We assume that the physical layer reports mobility information to the upper layer. For instance, M est may be simply estimated by using the receiver power of pilot signal.

4 4 Awake time(sec) (Proposed) Periodic(2) Periodic(1) Average delay(sec) (Proposed) Periodic(2) Periodic(1) Fig. 7. Awake time according to packet inter arrival time. Fig. 8. IV. SIMULATION RESULTS We performed simulations to evaluate our proposed algorithms performance. We assume that there are 5 cells (not sectored) and the cell radius is 1m. A frame, the unit of sleep mode interval, is 1ms and the length of AP broadcast is 1ms. And The mobile terminal moves across cells with the speed of 2m/s. We compare our proposed algorithm with other competitive ones, a periodic method of and a doubling method of The terminal using the periodic method wakes up with a fixed sleep mode interval, and the terminal using the doubling method doubles the sleep mode interval whenever it knows there is no buffered packets in an AP. In simulation, we assume that a terminal can estimate intermediate times of burst and connection, and mobility very precisely. Fig. 9. Waking time(sec) Average delay(sec) Waking time-delay tradeoff curve. A. Sleep interval control in active state Simulation results in Fig. 7 and 8 represent the effectiveness of sleep interval control in active state. Fig. 7 shows the total awake time of terminals. Awake time means the remaining time at an active state of a terminal, it is the main factor of energy consumption. The results show that Period(1) having a interval of 1 frames shows the best performance, and Period(2) shows the worst performance, because it wakes up too frequently. Proposed has similar awake time with the doubling method. But in Fig. 8 the result of delay comparison shows significant difference. Periodic methods show fixed average delay as the packet inter arrival time changes. Period(2) shows best performance. The average delay of and doubling methods increase according to packet inter arrival time, but their slopes are very different. shows lower average delay than the doubling method. Fig. 9 shows the tradeoff curve of and the doubling method. We know that is better than the doubling method because has lower waking time than the doubling method in the condition of a same average delay. B. Sleep interval control in idle state We simulate a sleep interval control in an idle state. Fig. 1 and 11 show the simulation results. In an idle state, the most important energy waste is an avoidable cell searching caused by doze handoff. So we compare the number of doze handoffs rather than awake time. Fig. 1 shows the number of doze handoffs according to an idle interval. The idle interval means the intermediate time of connection. A doubling method shows the worst performance. Its number of doze handoffs is highest. shows better performance when an idle interval is short, but s performance is as bad as doubling method s in a long idle interval. +MOES is our proposed algorithm expressed equation (3) where MOES means mobility estimation. It shows the best performance. Periodic(25) is a periodic method using fixed sleep mode interval of 25 frames, which is the time for the mobile terminal to move half of cell radius. So it never causes a doze handoff. Fig. 1 itself can not show the difference between +MOES and Periodic(25). When we compare their the average delay performance as Fig. 11 shows, the difference is revealed. When an idle interval is short, +MOES shows the shortest average delay as. Compared to simple, +MOES has a bound given by mobility estimation. Periodic(25) shows fixed average delay independently on an idle interval. If the period is greater than 25, average delay becomes longer, and if the period is less than 25, doze handoff occurs. The doubling method shows the worst performance.

5 5 Number of doze handoffs Periodic(25) +MOES Idle interval(s) Fig. 12. Waking time(sec) % over-est. 1% under-est Awake time according to packet inter arrival time. Fig. 1. Number of doze handoffs..8 Delay(s) Periodic(25) +MOES Delay(sec) % over-est. 1% under-est Idle interval(s) Fig Fig. 11. C. Effect of estimation error In previous simulation works, we assume that traffic patterns, burst intermediate time and connection intermediate time, are perfectly estimated. If we can know the exact values, we can determine the exact sleep interval by using the simulation results of periodic methods, because they also show good performances. So here we simulate when there is an estimation error in traffic pattern. We compare with no estimation error to s with 1% estimation error. Fig. 12 shows their awake times according inter arrival time. Simulation results show that there is only negligible difference among them. Fig. 13 shows average delays. If the inter arrival time is overly estimated, shows a little increased delay, but in almost cases it shows same performance with of no estimation error. of under-estimated case shows similar simulation results. V. CONCLUSION We proposed sleep mode interval control algorithms according to MAC states. To formulate the problem precisely, we described the definition of the sleep mode in each MAC state. Comparing with other sleep mode algorithms of and 82.16e, we found that traffic pattern is an important factor for the sleep mode interval control. In addition to we could find an energy waste problem caused by doze handoff under the condition of high mobility and small cell size. We showed that our proposed algorithms can solve these problems by simulation works. Our proposed algorithm for the sleep interval control in an active state shows better energy and delay tradeoff performance than doubling method. It also shows good performance through there is an estimation error in traffic pattern compared to periodic method. Our proposed algorithm in an idle state does not make a doze handoff and shows the lowest average delay. REFERENCES [1] W. Ye, J. Heidemann, and D. Estrin, An Energy-efficient MAC Protocol for Wireless Sensor Networks, IEEE INFOCOM Proceedings, vol. 3, pp , 22. [2] L. M. Feeney and M. Nilsson, Investigating the Energy Consumption of a Wireless Network Interface in an Ad Hoc Networking Environment, IEEE INFOCOM Proceedings, vol. 3, pp , 21. [3] Y.-C. Tseng, C.S. Hsu, and T.-Y. Hsieh, Power-Saving Protocols for IEEE Based Multi-Hop Ad Hoc Networks, IEEE INFOCOM Proceedings, vol. 1, pp. 2-29, 22. [4] C. F. Chiasserini and R. R. Rao, Combining Paging with Dynamic Power Management, IEEE INFOCOM Proceedings, vol. 2, pp , 21. [5] C. F. Chiasserini and R. R. Rao, Improving Energy Saving in Wireless Systems by Using Dynamic Power Management, IEEE Transactions on Wireless Communications, vol. 2, no. 5, pp , Sep. 23. [6] IEEE C82.16e-3/15r1 IEEE 82.16e Sleep Mode [7] IEEE C82.16e-3/7r3 IEEE TGe Working Document [8] IEEE C82.16e-3/45 Enhancement on Sleep Mode Operation in the 82.16e [9] IEEE Std 82.11b, [1] IEEE P82.16-REVd/D4-24, Mar. 24. [11] IEEE P82.16e/D2, 2, Apr. 24.