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(H2H) communications, Machine to Machine (M2M) communications have to confront much more challenges. In this paper, we focus on solutions to provide ...
International Workshop on Machine-to-Machine Communications

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Power Saving for Machine to Machine Communications in Cellular Networks Hua Chao, Yu Chen, Jinsong Wu Bell Laboratories, Shanghai 201206, China

{hua.chao, yu.b.chen}@alcatel-sbell.com.cn, [email protected]

Abstract— Compared with conventional Human to Human (H2H) communications, Machine to Machine (M2M) communications have to confront much more challenges. In this paper, we focus on solutions to provide longer battery lifetime. To obtain better power saving for M2M devices, this paper proposes improved mechanisms for M2M devices, radio access networks and core networks, in which the M2M devices can work well with simplified activities under optimized signaling flow. Meanwhile, the proposed approaches do not introduce negative impacts on legacy H2H terminals. This paper provides both qualitative and quantitative analysis to show advantages of the proposed comprehensive solutions. Index Terms—Cellular Network, Group-based Paging, Instant Report, LTE/LTE-A, M2M, Mobility Management, Power Saving

I. INTRODUCTION

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he applications of Machine to Machine (M2M) services in industry, civilian and national defense are facilitating and will further change our daily life a lot. In recent years cellular operators are expecting system solutions to deploy M2M service on their systems due to advantages of ubiquitous coverage and low deployment cost [1]. However, present cellular networks are designed for human to human (H2H) communication and may be suboptimal for M2M services. Therefore, since 2005, 3GPP (the 3rd Generation Partnership Project) have initiated the study item of M2M communications in GSM and UMTS to investigate structure optimizations [3]. Study for LTE (Long Term Evolution) systems started in September 2008, where network improvement of machine-type communications (MTC) has been discussed, including requirements analysis [4], improvements for core network [5] and improvements for radio access network [12].

Power management is one of the most important issues to be dealt with for M2M in cellular networks. The power source of M2M devices, i.e. batteries, cannot be replaced or recharged as easily as those of human mobile phones [4]. Therefore, cellular networks should provide M2M services with the least possible energy required in order to extend the lifetime of M2M devices. There have been considerable efforts in power control issues in wireless sensor networks. Pantazis and Vergados provided a thorough survey in [2], and they classified and presented different power control approaches for different contexts. However, those approaches in [2] for wireless sensor networks cannot be directly adapted into cellular networks, since the

978-1-4673-0040-7/11/$26.00 ©2011 IEEE

system characteristics of two networks are different. Unlike cellular networks with a hierarchical architecture, wireless sensor networks have a dynamic network topology, which can vary from a simple star network to an advanced multi-hop wireless mesh network. Moreover, although each scheme in [2] is well suited for some certain scenarios, it is not guaranteed that any of them is the best for all situations. Therefore, dedicated solutions are required for cellular networks. High-level functionalities of cellular networks are grouped into Access Stratum (AS) and Non-Access Stratum (NAS). 3GPP has provided mechanisms in AS and NAS respectively to reduce power consumption for MTC devices in terms of reducing the frequency for them to perform certain activities. In AS, longer paging cycle is proposed to reduce the frequency for MTC devices to monitor paging channel. In NAS, similarly, longer timer is introduced to reduce the frequency of periodic location area update (LAU) procedure for circuit switched (CS) systems and routing area update (RAU) procedure (in UMTS) or tracking area update (TAU) procedure (in LTE/LTE-A (Advanced)) for packet switched (PS) systems [5]. Downlink data transfer for MTC can be limited in an allowed time after MTC device performed TAU/RAU procedure. Then MTC devices may power off after the allowed time period. In this paper, we consider improving power savings for both M2M devices and network operations. On the one hand, removal of unnecessary M2M activities will be beneficial to conserve the power of M2M devices directly. On the other hand, improvements in operations and optimized signaling flow are proposed for both AS and NAS, which may help to reduce the power consumption of M2M devices indirectly. This paper is organized as follows. In section II, we first propose modified idle mode for M2M devices. Then, we discuss our proposed power saving approaches in radio access network (RAN) for M2M in section III. The core network (CN) side optimization for M2M is presented in section IV. In section V, performance results are provided for proposed approaches, followed by conclusions in section VI. II. EXTENDED IDLE MODE FOR M2M DEVICE Unlike human to human terminals, a lot of machines related to specific M2M applications are fixed and not expected to move randomly. As a consequence, the Mobility Management (MM) for M2M in cellular systems can be optimized to capture the new feature called as low mobility in [5]. In this section optimizations to MM are proposed in both AS and NAS. The basic idea is to introduce a new sleep state in idle mode, in

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2 which unnecessary activities of low mobility M2M devices are removed. It should be emphasized that the proposed “sleep state” is different from the standalone “sleep mode” of H2H communications in IEEE 802.16 [6]. In fact, the H2H mobile stations in “sleep mode” of IEEE 802.16 are switched between unavailability intervals and availability intervals. This power saving manner is similar to the Discontinuous Reception (DRX) defined for legacy H2H terminals in “idle mode” of 3GPP. MS in “idle mode” of IEEE 802.16 always works in availability, while 3GPP does not define a corresponding mode. A. Sleep State in AS and NAS Proposed MM model for M2M devices is shown in Figure 1. States with a shape of rectangle indicate the available states defined in current cellular networks, i.e. 3GPP. The proposed new states with a shape of capsule show separate sub-states of AS and NAS because 3GPP uses different specifications to define AS and NAS functions. As listed in Table 1, these new states extend the legacy idle mode into more economical mode with simplified activities. Specifically, the state 4 is used to avoid unnecessary AS measurement and filtering activities for serving cell and cell re-selection. The state 5 is used to avoid unnecessary periodic NAS activities, i.e. LAU/RAU procedure for UMTS system and TAU procedure for LTE system.

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• 5+3: Device could detach from the network based on network’s configuration, e.g. through controlled timer. It should be noted that measurement and filtering for cell-reselection is performed to select the best cell each time when M2M devices transit from state 4 or state 5 to state 1. This rule is defined to guarantee successful UL data transmissions.

A. Limitations of existing H2H Paging Mechanism To receive paging messages, H2H terminals should monitor paging indication periodically in physical channel at certain paging occasion (PO). In order to increase paging capacity multiple terminals are allocated with the same PO. The H2H paging mechanism cannot be reused directly for M2M. At first, existing paging mechanism for H2H cannot perform paging for a certain type or group of M2M devices separating from H2H terminals. Secondly, if H2H terminals and M2M device are paged together, larger number of H2H terminals and M2M devices need to wake up at the same PO. This results in both terminal power wasting and a higher false alarm probability. Although 3GPP discussed possible usage of M2M group ID to identify a group of M2M devices, how to define the M2M group ID and how to properly utilize it in existing radio access network is still an open issue.

    

MM model for M2M device

TASKS DEFINED FOR NEW STATES Tasks defined Detect paging (may apply a longer DRX cycle than that of H2H terminals) No serving cell measurement and filtering No measurement and filtering for cell re-selection Perform periodic LAU/RAU/TAU procedure (may apply a longer duration than that of H2H terminals) Detect paging (may apply a longer DRX cycle than that of H2H terminals) No serving cell measurement and filtering No measurement and filtering for cell re-selection Stop periodic LAU/RAU/TAU procedure

TABLE I State

• 4+2: The state transfer from state 4 to state 2 could be triggered by urgent UL data transmission; then transition to state 1 as prerequisite is avoided to save time for radio access network connection establishment.

In this section, we propose to improve AS operations of the radio access network other than decrease the activities of M2M devices, which contribute to power saving for M2M devices indirectly.

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• 5+4: In the case that M2M devices in state 5 are deactivated low mobility feature or are informed that the network is recovered from overload/congestion, they transfer back to state 4 and restart the periodic LAU/RAU/TAU procedure.

III. M2M PAGING MECHANISM

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• {4, 5}+1: Both time-controlled UL data transmission or paging response could trigger M2M devices in state 4 or 5 back to state 1. When back to state 1 from state 5, M2M devices shall restart periodic LAU/RAU/TAU procedure.

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their AS enters sleep state. Alternatively, the network could configure them to stop periodic LAU/RAU/TAU procedure in order to reduce signaling load and save power once overload or congestion happens.

B. Mechanism for State Transition As shown in Figure 1, the power of M2M devices is conserved gradually when transferred from state 1 to state 4 and further to state 5. Key state transitions are explained below. • 1+4: M2M devices in state 1 select and monitor paging channel, if no paging is found it enters state 4. • 4+5: M2M devices with activated low mobility feature could be self-controlled to stop periodic LAU/RAU/TAU procedure. It mean that they enter state 5 once they detect

B. Proposed Three-Layer Paging Mechanism for M2M Motivated by above observation, this section proposes an efficient three-layer paging mechanism for M2M communications. Architecture of proposed mechanism is shown in Figure 2. It solves above-mentioned problems without introducing negative impact to legacy H2H paging. 1) Layer one: paging occasion In layer one, dedicated paging occasion for M2M is defined to narrow down the paging range from all camped terminals to M2M devices alone. Unlike H2H communication, whose PO is calculated by terminal ID, PO of M2M is calculated by M2M

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3 group ID. The network determines the radio frames (the paging frame (PF)) and the subframes within that PF (i.e. the PO) used for M2M paging. M2M devices calculate PF and PO using: ǂSFN mod T = (T/Nf) × (group ID mod Nf) i_s = (group ID/Nf) mod Ns

(1) (2)

where, SFN is the System Frame Number identifying the PF, T is paging cycle, Nf is the number of paging frames within one paging cycle, i_s is the index of subframe pointing to a pre-defined table [8], Ns is the number of paging subframes within one radio frame. M2M group ID may indicate different M2M applications. Certainly, each M2M device can has its own device ID as terminal ID for H2H. To page a certain group of M2M devices, paging indication is set in corresponding PO in PF, in which idle mode M2M devices wake up and detect the paging indication. If indication is found, the paging procedure moves to the 2nd layer. Otherwise the procedure is finished.

Figure 2

example, metering devices report their measured values periodically and vendor machines send report, once they find some goods are sold out. In this paper, we focus on another typical scenario, which is especially meaningful for surveillance, tracing and tracking systems. In the case that urgent or sudden events occur, the system administrator always needs instant reports from certain monitoring machines to analyze what is happening or what may probably happen. In this section, we firstly analyze the limitations of the existing method and then present our improved solution. A. Available Application-Based Method A MT (Mobile-Terminating) message can be applied to trigger uplink data transmissions. Target M2M device shall respond an instant report in terms of SMS (Short Message Service) or MMS (Multimedia Messaging Service) if received the MT message. The whole signaling flow is shown in Figure 3. At first M2M server/user sends one SMS to the SMS Service Center (one of the SMS entities involved in the signaling flow, not shown separately for simplicity). Secondly SMS entities forward the SMS to the right core network element. Then the target M2M devices are informed that there is an SMS message to receive. Each informed device needs to setup connection with the network to receive the SMS message. At last, until correctly decoding the received message, the M2M device cannot know it is required to send M2M data via SMS/MMS to the M2M server/user.

Three-layer paging mechanism for M2M

2) Layer two: paging target In the 2nd layer, the paging range is further narrowed down to expected M2M devices. Paging message is sent in the air interface, in which M2M device group ID or M2M device ID is carried as indicated paging target. If the group ID is carried, all the M2M devices belonging to the group need to respond to received paging message. If the device ID is carried, only the indicated device needs to respond. If an M2M device does not find its own group ID or device ID in the received paging message, the paging procedure is finished. Otherwise, the paging procedure enters the third layer. 3) Layer three: paging reason The last layer is to tell M2M devices why they are paged. In a paging message, candidate paging reasons are call setup, M2M report or M2M system information acquisition, each of which is carried for a specific M2M group ID or a specific M2M device ID. If the reason is call setup, it requests the paged device(s) to set up call connection with the network. If the reason is M2M report the network wants the paged device(s) to respond a report. Regarding the reason of M2M system information acquisition, it means that the network requires the paged device(s) to update latest M2M system information.

Figure 3

In our understanding, this method is not good enough as it is an application level solution and therefore it is unavoidable to waste time and power. It also wastes system resources if more than one M2M devices are involved, since separate messages have to be sent. In this paper the whole procedure is optimized to improve the 4th step and avoid application level steps (i.e. the 1st, 3rd, 6th and 7th steps). B. Instant UL Transmission as Response

Figure 4

IV. UPLINK DATA TRANSMISSION NAS operations and signaling flows may also be optimized for M2M in the core network scope. M2M devices can send data to the network in a periodic way or an event-triggered way. For

Available method to trigger instant UL transmission

Proposed method to trigger instant UL transmission

The basic idea of the proposed solution is to allow M2M server or M2M user require instant M2M report from M2M devices by triggering paging in AS. The power of M2M Devices

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4 as well as system resource is thus saved for uplink data transmission required by the network. The proposed method is applicable to various cellular networks. As shown in Figure 4, taking 3GPP systems as an example, the whole optimized signaling flow consists of five steps. 1. M2M server/user is allowed to initiate the existing routing information exchange procedure. It is proposed to enclose one or more M2M device IDs or one or more group IDs in the request message from the M2M server/user to the HSS/HLR. With this step M2M server/user knows to which core network elements shall send the message of step 2. 2. M2M server/user initiates the new Instant report requisition message to right core network elements (CNE) indicated by HSS/HLR in step 1. Logical area information is enclosed to show the paging range. The correct CNE is MSC (Mobile Switching Center) for LTE systems or CS domain UMTS systems while it is SGSN (Serving GPRS (General Packet Radio Service) Support Node) for PS domain UMTS systems.

B. Quantitative Analysis 1) Idle mode activities In fact, idle mode H2H terminal are quite “busy”. In this paper we do not consider measurement of inter-RAT (Radio Access Technology) cell. Thus activities for idle mode H2H terminals include following seven types: system information reading, paging detecting, serving cell measurement and evaluation (including measurement and evaluation selection criterion S for serving cell), intra-frequency cell measurement and evaluation (including intra-frequency cell detection, measurement and evaluation), inter-frequency cell measurement and evaluation (including inter-frequency cell detection, measurement and evaluation), periodic LAU/RAU/TAU procedure and UL data transmission. Except for UL data transmission, the frequencies of above activities are defined in [10] for LTE systems. Power saving for idle mode M2M devices is evaluated in terms of sum amount activities. In Table III other applied evaluation parameters are given. TABLE III PARAMETERS FOR IDLE MODE ACTIVITIES Parameters Value Evaluation time 60 min Number of inter-frequency carriers 1 Number of intra-frequency cells 2 Periodic tracking area update timer 54 min (defined in [11]) DRX cycle for H2H (Quoted from [10]) 0.32, 0.64, 1.28, 2.56 DRX cycle for M2M is N times of H2H 2, 3 Frequency of M2M device transport UL data 1 times per hour

3. The core network element interprets the received message into paging message to relevant base station, which then constructs the AS paging message and delivers it in the radio interface. 4. M2M device decodes the received paging message. If the reason “M2M Report” is detected, the M2M device establishes a connection with the network for uplink data transmission. 5. Paged M2M devices encapsulate M2M report into pre-defined type, i.e. SMS or MMS. The terminating point of the instant report is pre-configured M2M server/user, which is stored in the devices via operation & maintenance. V. ANALYSIS AND EVALUATION

It is shown in Figure 5 that 74.96% tasks are saved for idle mode low mobility M2M devices compared to idle mode H2H terminals. It can be observed that longer H2H DRX cycle brings more tasks saving. Up to 91.75% tasks can be avoided in the case that the H2H DRX cycle is 2.56s. In addition, it is no doubt that longer M2M DRX cycle brings more task saving.

A. Qualitative Analysis In this paper, the power of M2M devices is reduced using following optimizations, which is mapped to different network elements in TABLE II.

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Amount of activities

40000

1) Unnecessary activities removal Periodic AS measurement and filtering for cell reselection and periodic NAS LAU/RAU/TAU procedure are removed for low mobility M2M devices in order to reduce power consumption.

30000 25000 20000 15000 10000 5000 0

2) Network operation optimization Group-based paging reduces the power of M2M devices for detecting paging indication. The network could configure the M2M devices to enter into NAS sleep state for power saving. 3) Signaling flow optimization Paging procedure is optimized to adapt to new features of M2M services. A solution to trigger instant report with optimized signaling flow is also provided. TABLE II MAPPING OPTIMIZATIONS TO NETWORK ELELEMENT Optimizations M2M Device RAN CN Unnecessary activities removal • • Network operation optimization • • • Signaling flow optimization • •

Idle H2H terminal Idle M2M device N=2 Idle M2M device N=3

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0.32

0.64

1.28

2.56

DRX cycle (s)

Figure 5

Tasks comparason of idle mode

2) False alarm probability We also do simulations of power saving in terms of false alarm probability for paging, since the larger the probability is the more power is consumed for all the M2M devices. Applied parameters are tabulated in Table IV [8][12]. Smaller mod coefficient is applied for M2M compared to H2H as the number of paging targets and the amount paging resources are reduced with the proposed group-based paging mechanism. TABLE IV PARAMETERS FOR FALSE ALARM PROBABILITY Expression Semantics Description Value N Number of H2H terminals 10000 m Number of M2M devices / 3, 5, 10, 15, 20, 25, 30

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5 number of H2H terminals Number of M2M devices mod coefficient of H2H Number of M2M groups mod coefficient of M2M

Nm Nmod Ngroup Mmod

m×N 4096 128, 256, 512, 1024 256

For each paged H2H terminal, those terminals having the same result of “IMSI mod Nmod” wake up in the same PO. IMSI is the International Mobile Subscriber Identity used to identify a H2H terminal. Thus the false alarm probability is defined by equation (3) for H2H. In the case that shared PO is used for M2M devices and H2H terminals, the false alarm probability for all terminals camped in the system is given by equation (4). P = (floor (N/Nmod)-1) / floor (N/Nmod)

(3)

Pm = (floor ((N+Nm)/Nmod)-1) / floor ((N+Nm)/Nmod)

(5)

1 0.9

False alarm probability

0.8 0.7 0.6 0.5 0.4

VI. CONCLUSION

(4)

When applying dedicated PO for M2M, legacy terminals do not need to wake up at M2M PO. The false alarm probability for legacy H2H terminals is a constant while the false alarm for M2M happens when different M2M groups are configured at the same M2M PO. It is calculated by equation (5), which is determined by the number of groups and mod coefficient of M2M. Pm_d = (floor (Nm/Ngroup/Mmod -1) / floor (Nm/Ngroup/Mmod)

avoids step 1, 6 and 7 of available solution. Therefore, the power of devices is saved in return for processing those application activities. The triggering for CN operation is changed from SMS (step 3 in Figure 3) to a new signaling (step 2 in Figure 4), which avoid larger payload. What’s more, comparing step 4 in Figure 3 with step 3 in Figure 4 having the same functionality, the former one contains one more signaling from RAN to CN to request SMS data (not shown in Figure 3 for simplicity). In summary, both device power and system resources are saved. However, we remark that compared with application-based method, the proposed instant report method needs direct connections (wired or wireless) between the M2M user and the MSC/SGSN.

In this paper we focus on the power saving issue for M2M communications in cellular networks. In the device side, we introduce sleep state with removal of unnecessary activities in both AS and NAS, through which the power is saved directly for low mobility M2M devices. For RAN, three-layer M2M paging is proposed to reduce the power of M2M devices. Finally, the CN solution to support instant report, with which the power of M2M devices is saved, as well as both radio and core network resources are used more efficiently. Although we have addressed a class of general solutions for M2M cellular communications, we have not specifically studied the power impact on other M2M data sources coming from capillary deployments, such as routing, element discovering, data aggregation, and device management, which may also have a significant impact in M2M architectures. For future work, we plan to expand our research to other potential factors contributing to power saving.

H2H terminals

REFERENCES

M2M, Shared PO

0.3

M2M, Dedicated PO, Ngroup = 128 M2M, Dedicated PO, Ngroup = 256

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M2M, Dedicated PO, Ngroup = 512

[1]

M2M, Dedicated PO, Ngroup = 1024

0.1 0

0

5 10 15 20 25 Number of M2M devices / Number of H2H terminals

Figure 6

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False alarm probability for paging

In Figure 6, false alarm probability for paging cases discussed above is illustrated. It is obviously that larger Mmod introduces smaller Pm_d. However larger Mmod in return brings more scattered POs in the time scale for different M2M groups, which makes the network consume more resources to deliver paging indications for all M2M groups. No matter what number of Mmod is the trend of the curves is the same. Thus we do not show simulation results for different Mmod. When using shared PO for both H2H terminals and M2M devices, the probability is significantly increased. It is indicated that the larger number of M2M devices results in the higher probability. With the idea of group-based paging and dedicated paging occasion, the false alarm probability of different M2M groups is obviously smaller than that of the shared paging occasion. 3) Signaling flow optimization It is difficult to calculate how much power is saved due to signaling optimization, and thus only qualitative analysis is provided here. Comparing Figure 3 with Figure 4, our solution

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