A Cluster-based Random-access Scheme for LTE

A Cluster-based Random-access Scheme for LTE/LTE-A Networks Supporting Massive Machine-Type Communications Tiago P. C. de Andrade, Luiz R. Sekijima and Nelson L. S. da Fonseca Institute of Computing - State University of Campinas, Brazil [email protected], [email protected], [email protected] Abstract—In the Internet-of-Things (IoT), it is desirable that every IoT device will be capable to communicate with the network at any time. Among various technologies to enable network connectivity, the Long Term Evolution (LTE) is by far the most ubiquitous technology to provide large coverage of IoT devices. Machine-Type Communications (MTC) is seen as a major service in the next generation cellular mobile networks for IoT. However, the predicted large number of MTC devices will overload the Radio Access Network (RAN), with impact on non-MTC devices. In this work, we propose a design of a RAN overload control mechanism based in clustering that is exclusively engineered for delay-tolerant MTC devices. The proposed solution is intended for delay-tolerant MTC devices with a Radio Resource Control (RRC) context (have a Cell Radio Network Temporary Identity (C-RNTI)) but without being synchronized with the network. By simulation, we show that the proposed scheme leads to acceptable collision levels, besides ameliorating the energy efficiency. Keywords—LTE networks, machine-to-machine communications, random-access procedure and RAN overload control.

I.

I NTRODUCTION

The Internet-of-Things (IoT) refers to a world of physical and virtual objects with sensing, computing and communication capabilities. Its realization heavily relies on technologies such as Machine-to-Machine (M2M) communications, also known as Machine-Type Communications (MTC) in the Third Generation Partnership Project (3GPP) terminology, which enables communication between machines that autonomously generates traffic with no human intervention [1]. The MTC requires that a massive number of geographically distributed devices to be able to communicate in a reliable and efficient manner. Given the ubiquity coverage, the Long Term Evolution (LTE) cellular technology is a key enabling technology for the IoT connectivity landscape. To cellular operators, the growth of new M2M applications has opened up a new market opportunity in the cellular industry. However, there are two critical problems that need to the addressed for efficient cellular M2M communications. One is the high signalling overhead [2] and the other is congestion [3], [4]. The traditional cellular network, which is originally engineered for Human-to-Human (H2H) communications, has not been considered suitable to handle the unique traffic of M2M applications. The M2M applications, such as smart metering, e-health and intelligent transportation are characterized by small-sized data intermittently transmitted by a massive number of MTC devices. The connection-oriented communication in the conventional cellular system can induce

excessive signalling overhead in the case of transmitting smallsized data [2]. Moreover, the situation in which a huge number of MTC devices attempt to access cellular networks at the same time can lead to severe congestion especially in the Physical Random Access Channel (PRACH) [3], [4], which can impact on both MTC and non-MTC devices. This congestion may cause intolerable delays, packet loss and service unavailability to current H2H communications. Tackling this problem necessarily includes the design of Mobile Network Operators (MNOs)-friencly MTC applications. However, MNOs can not leave the control of signaling to MTC applications’ developers. One of the most compelling requirement of MTC devices is to have seamless channel access all the time. The energy constraint nature of these devices makes the task of channel access more challenging. Moreover, the slotted ALOHA of Random Access Channel (RACH) brings additional difficulties in terms of performance and success rate of the randomaccess (RA) requests. A requirement of the RA procedure in MTC devices is the energy efficiency, and one of the main problem to the energy efficiency is the collision of the preamble sequences. In the past years, the 3GPP has proposed several improvements to the LTE standards, focusing mainly on the Radio Access Network (RAN) overload control problem and the reduction of user equipment (UE) cost [5]. In LTE networks, a user or group of users can receive prioritized access during the initial phase of the RA procedure. For instance, in the contention-free RA procedure, the RAN explicitly informs the UEs the preamble sequence to be used in the RA procedure in order to avoid collisions. This preamble sequence is chosen from a poll of unique preamble sequences reserved for procedures requiring low latency such as handover. Another example is the RAN overload control scheme called RACH Resource Separation (RRS) which allows preamble sequences separation between Human-Type Communication (HTC) and MTC in order to alleviate the effect of the MTC on HTC [6] [7]. By reserving an exclusive set of preamble sequences to a small number of UEs, the collision probability in the preamble transmission phase of the RA procedure is significantly reduced in the presence of massive access attempts [8]. One of the solutions that solve the collision problem is the paging method, in which the MTC devices will start the RA procedure just after receiving a paging message. However, this method is not practical in the presence of a huge number of MTC devices since it will take a long period to page all the devices.

This paper proposes a mechanism to avoid the collisions on the RACH for delay-tolerant MTC devices using a clusterbased approach. In this mechanism, the delay-tolerant MTC devices within a cell are divided into several clusters, being each cluster assigned to one RA slot and each device of the clusters assigned to an exclusive preamble sequence. In this way, there is no contention between the delay-tolerant MTC devices, and therefore, no collision occurs between them.

[20], the authors analyzed signal-to-interference-plus-noise ratio (SINR) distributions and derive efficient resource allocation schemes for spatial multi-group random-access in multi-cell systems.

The rest of the paper is organized as follows. Section II provides a summary of some works related with RAN overload problem. Section III briefly introduces the background on UE state machine and on RA procedure of the LTE technology. Section IV presents our proposed solution to the RAN overload problem in details. Section V assesses the performance of the proposed solution via extensive simulations and discusses the simulation results. Finally, Section VI concludes the paper.

A UE in LTE network can be in one of the two states: RRC CONNECTED and RRC IDLE. In the former, a Radio Resource Control (RRC) context is established between the UE and the eNB, and in the latter, no context is established.

II.

R ELATED W ORK

In this section, we review previous works on M2M communication in cellular networks. The foremost problem in accommodating M2M traffic into cellular networks is identified as a RAN overload problem [3]. The 3GPP has suggested a variety of solutions to the RAN overload problem [7]. RAN overload control approaches can be classified into push-based and pull-based approaches [2]. In the former approach, the MTC devices push their traffic to the network until a RAN overload is detected, and these approaches are considered as decentralized control models. In the latter approach, the network pulls the MTC devices traffic to control the load. The RAN overload control for M2M communications bears resemblance to the traffic load control in traditional slotted ALOHA system [9], [10], [11]. Inspired by these researches, work on the RAN overload control for M2M communication has been published [12], [13], [14], [15]. In [12], the transient behavior of finite-user multi-channel slotted ALOHA systems is analyzed for M2M communication. The authors of [13] proposed a prioritized random-access scheme to solve the RAN overload problem while providing Quality of Service (QoS) to different M2M classes. The cooperative access class barring scheme among different evolved NodeBs (eNBs) is proposed in [14]. In [15], the authors propose a fast adaptive slotted ALOHA scheme that accelerates the tracking process of network status. The RAN overload problem can also be tackled by more efficiently using the radio resources for the random-access procedure as proposed in [16], [17], [18]. In [16], the authors propose a novel random-access scheme based on fixed-location MTC devices to reduce the collision probability. In [17], the amount of available contention resources is increased by expanding the contention space to the code domain, enabling the support of an increased number of M2M users. In the scheme proposed in [18], M2M users form coalitions and perform relay transmission with the objective of reducing network congestion. Group-based schemes are proposed to efficiently control a massive number of M2M users [19], [20]. The group-based feature proposed in [19] supports a large number of M2M users with small data transmission and diverse QoS requirements. In

III.

LTE BACKGROUND

A. UE States in LTE

In the RRC CONNECTED state, the UE can transmit/receive data to/from the eNB. In this state, the UE has, among other things, a temporal identity, named as Cell Radio Network Temporary Identity (C-RNTI), assigned by the eNB when the UE is attached. Depending on whether there is uplink synchronization, the RRC CONNECTED state can be divided into two substates: IN SYNC and OUT OF SYNC. As long as the uplink is synchronized, the uplink transmission is possible. Otherwise, the UE must perform the RA procedure in order to restore the uplink synchronization. B. Random-access procedure The random-access procedure in LTE networks can be executed in two operational modes: contention-free and contention-based. The former is used to perform handover and to re-establish synchronization prior to downlink data transmission. On the other hand, the latter is commonly used in the following cases: (i) initial access to the network, i.e., when the radio interface is turned on; (ii) to request uplink resources upon arrival of uplink data at the UE buffer if data and control resources are not assigned to the UE; (iii) to reestablish a connection after a radio failure; and (iv) loss of uplink synchronization. In both contention-free and contention-based RA procedures, the UE transmits a preamble sequence (msg1) on the RACH. In the contention-based mode, the preamble sequence is randomly selected by the UE from a set of available preambles sequences, which is periodically updated by the eNB. Conversely, in the contention-free mode, the preamble sequence is explicitly allocated either by the source eNB (for handover) or by the serving eNB (for downlink transmission). After transmitting a preamble sequence, the UE monitors the Packet Downlink Control Channel (PDCCH) during a certain time, and, if no response is received, it enters a backoff period and then tries to transmit a new preamble sequence. This is repeated until the UE receives an Random Access Response (RAR) message or until the maximum number of preamble sequence transmissions has been achieved. Upon detection of a preamble sequence, the eNB transmits an RAR message (msg2) on the Physical Downlink Shared Channel (PDSCH) addressed to the random-access Temporary Identifier (RA-RNTI), which is generated by using the preamble sequence index and the subframe in which the sequence was received. This message contains a timing advance command as well as an uplink grant for the transmission of a message in the following step. In the contention-free mode, upon receiving this message, the procedure is finished. To

inform the UE about this transmission, a Downlink Control Information (DCI) message is allocated on the common search space (CSS) region of the PDCCH to indicate the PDSCH resources in which the RAR message is transmitted. The number of RAR messages that an eNB can send is limited, e.g., three RAR messages per millisecond. Once the RAR message is received by the UE, it transmits an L2/L3 message (msg3) on the Uplink Share Channel following the uplink grant contained in the received RAR message. This message will indicate the reason of the contention-based RA procedure was triggered. This message is addressed to the RA-RNTI and contains either the identity of the UE or a temporary UE identity. If two or more UEs chose the same preamble sequence in a certain random-access opportunity (RAO), they will receive the same grant in the RAR message, and, thus, all their L2/L3 message transmissions will collide. In such a case, after the maximum number of msg3 retransmissions is achieved, the UE re-initiates the randomaccess procedure. When the number of RA attempts reaches a threshold, the network is considered unavailable by the UE, and an access problem exception is reported to the upper layers. Finally, upon successful reception of an msg3, the eNB sends a Contention Resolution (CR) message (msg4) to the UE. Once the msg4 is successfully received by the UE, the contention-based RA procedure is finished. IV.

C OLLISION - AVOIDANCE C LUSTER - BASED OVERLOAD C ONTROL R ANDOM - ACCESS M ECHANISM

In the proposed solution, it is assumed that all delay-tolerant MTC devices are in the RRC CONNECTED OUT OF SYNC state and, therefore, all the delay-tolerant MTC devices have C-RNTI. In other words, the delay-tolerant MTC devices have RRC context, but there is no synchronization between the devices and the network (i.e., there is no uplink transmission). In our approach, instead of the eNB signalling when and where the delay-tolerant MTC devices can transmit the preamble sequence, we implemented a mechanism to allow the devices to send the preamble sequences at the correct time without extra information from the eNB. The idea behind this mechanism is that instead of leaving each delay-tolerant MTC device to choose randomly a RAO, we divide the delay-tolerant MTC devices in clusters and assign exclusive RAO to each delay-tolerant MTC device within the clusters. Thereby, the clusters will transmit the preamble sequences in different RA slots, with each delaytolerant MTC device within the cluster using an exclusive preamble sequence. For that, it was defined the utilization of virtual random-access slots inside rounds, which each round has between 1 and a maximum number of RA slots and the rounds are re-initiated at the end. The number of required virtual RA slots to allow all clusters to send the preamble sequences is given by:

where, R is the number of reserved preamble sequences to the CC-RA mechanism and M is the number of delaytolerant MTC devices assigned to the eNB. When the value of R increases, the number of required virtual RA slots decreases, and this causes a decrease in the number of preamble sequences for the other devices (e.g. HTC and delay-constraint MTC devices). As the number of HTC and delay-constraint MTC devices is very small in comparison to the number of delay-tolerant MTC devices, the small number of preamble sequences does not affect the performance of the randomaccess procedure. Because of the limitation of the number of responses grants in each RAR message, the number of preamble sequences reserved to be used by the clusters in the CC-RA mechanism should be less or equal to the maximal number of UEs that can be responded to with RAR per RAR Occasion (RO). This is equal to the number of grants (R) that can be included in one RAR multiplied by RAR window (WRAR ) and is defined as: R = GRAR × WRAR

where GRAR is the number of responses grants that can be carried in a RAR message. It is important to recall that the proposed solution is dedicated to the case in which the delay-tolerant MTC devices are in the connected state but without being synchronized with the network and, thus, the devices have, among other things, the C-RNTI. A pair of values S and P to each delaytolerant MTC device is used, where S is the virtual RA slot and P is the identifier of the preamble sequence to be used. In order to calculate this pair of values, we avoided directly assigning them once the delay-tolerant MTC device joins the network, because this requires additional signaling overhead to be exchanged with each device. This mechanism depends on the C-RNTI, which can take values comprised in the interval 0x003D to 0xFFF3. We propose allocating a sub-interval of these values (32, 768 values) to the delay-tolerate MTC devices and whenever a device requires an C-RNTI, the eNB assigns a value of this sub-interval sequentially and re-assign the values that are not in use. The pair of values S and P can be obtained directly from each delay-tolerant MTC device using the exclusive C-RNTI for the following steps: •

Using a mask value (M SK) to normalize the C-RNTI values of the delay-tolerant MTC devices to a range from 0 to 32, 767. This value can be sent using the System Information (SI) message, jointly with the R value.

•

Each delay-tolerant MTC device perform the logical operation XOR between the M SK and its C-RNTI as follows: P ID = M SK ⊕ C − RN T I

Vslots

M = R



(1)

(3)

where P ID is the normalized devices’ ID. •



(2)

The pair of values S and P , which represent the virtual RA slot and the identity of the preamble sequence to be used respectively, is given by:

TABLE I.

S =1+



P ID R

P = 1 + (P ID



mod R)

(4)

Parameter

Value Main

(5)

where, S takes values in [1, Nslots ] and P takes values in [1, R]. A disadvantage of this mechanism is the increase in access delay as a result of waiting for the correct RA slot. Thereby, the higher the number of devices assigned in the network, the larger is the delay increase. However, as each device chooses a different preamble sequence and RA slot in relation to all other devices, there is no preamble collision. Furthermore, there is no need for messages msg3 and msg4 (used for the contention RA procedure) as the eNB knows exactly which device transmits which preamble and on which RA slot. Therefore, the RA scheme in the proposed solution behaves as if it were a contention-free RA procedure, decreasing the final delay of the procedure. V.

S IMULATION PARAMETERS

P ERFORMANCE E VALUATION

In this section, we evaluate the performance of the proposed algorithm via extensive simulations by using the LTE-Sim simulator [5]. LTE-Sim is a discrete-event packetlevel simulator developed in C++, widely used for simulating Medium Access Control functions of LTE/LTE-Advanced (LTE-A) networks. A. Simulation Model and Setup All delay-tolerant MTC devices are assumed to be in the connected state but without synchronization with the network, and they try to perform the RA procedure at the same time. We assumed that the RA configuration parameters jointly with the CC-RA configuration parameters (e.g. the M SK and R values) have already been received by the UEs in the beginning of the simulation. In this simulation, it is assumed that each device has data to transmit in the beginning of the simulation. However, to measure the performance of RACH process we have assumed that during the simulation time the devices stop contending for the channel after the first successful RA completion. A preamble sequence is successfully received with probability 1 − e−i , where i is the number of preamble sequence transmissions [7]. This assumption captures the effect of the power ramping technique and physical impairments of the PRACH. A UE device considers that a preamble sequence transmission has failed after pre-defined interval with no reception of the corresponding RAR message. Unlike the unrealistic assumption about preamble collision in the 3GPP model [7], our simulation model considers that the eNB is not able to detect collision at the reception of the preamble sequence. Thus, two or more UEs can receive the same Packet Uplink Shared Channel (PUSCH) resources in the RAR message to transmit the msg3 messages. In this way, collisions are only detected when a UE does not receive the msg4 message. The simulation scenarios comprise a single cell with a 0.5 km radius. An eNB with 5 MHz cell bandwidth in the Frequency Division Duplexing mode is located at the center of

System type Single cell System bandwidth 5 MHz Cell radius 0.5 km PRACH configuration index 6 RA preamble format 0 Contention-based preambles 52 # of grants per RAR message (GRAR ) 3 # of CCEs allocated for the PDCCH 16 # of CCEs per used per UEs 4 Backoff period 20 ms preambleTransMax 10 Response Window Size (WRAR ) 5 ms Contention Resolution Timer 48 ms maxHARQ-Msg3Tx 5 CC-RA R 15 Access Class Barring (ACB) BarringFactor 0.9 BarringTime 4s Resource Separation Scheme (RSS) # of preambles to HTC devices 22 # of preambles to delay-tolerant MTC devices 30

the cell with several UE uniformly distributed around it. We have simulated the RA attempt by varying the total number of delay-tolerant MTC devices from 50 to 800, with step size of 50. The number of HTC and delay-sensitive MTC devices are assumed as 50 in the total. We compare the proposed mechanism with other two RAN overload control mechanism proposed by 3GPP in [7] besides without the overload control: Access Class Barring (ACB), Resource Separation Scheme (RSS) and traditional (without overload control). The configurations parameters used in the simulations are summarized in Table I. B. Simulation Results and Discussion The metrics analyzed are the access success probability, which is defined as the probability of devices that successful perform the RA procedure; the average random-access delay, which is the average time of execution of successful RA procedures; the average msg2 message delay, which is the time between the transmission of the first preamble sequence and the reception of the RAR messages; and the energy consumption, which is the average energy consumed by each successful RA procedure. Fig. 1 demonstrates the percentage of access probability of different RAN overload control mechanisms as function of the number of delay-tolerant MTC devices in each cell. As it can be observed, the proposed CC-RA mechanism reaches an access probability of 100% for both HTC/delaysensitive MTC and delay-tolerant MTC devices, independently of the total number of devices in the system. This happens due to separation of the preamble sequences allocated to the HTC/delay-sensitive MTC and delay-tolerant MTC devices, besides separating the delay-tolerant MTC devices in different clusters with an unique preamble sequence to each one. As the delay-tolerant MTC devices never chooses the same RAO, there is no preamble sequence collision in this mechanism.

ACB

CC−RA

RSS

Traditional

ACB

80 60 40 20

80 60 40

800

200 400 600 Number of delay−tolerant MTC devices

(a) Access probability to HTC and delay-sensitive MTC devices.

(b) Access probability to delay-tolerant MTC devices.

Traditional

CC−RA

120

Average msg2 delay [ms]

Average msg2 delay [ms]

RRS

100 80 60 40 20 200 400 600 Number of delay−tolerant MTC devices

100 80 60 40 20 200 400 600 Number of delay−tolerant MTC devices

800

(b) Average msg2 message delay to delay-tolerant MTC devices.

CC−RA

RSS

Traditional

ACB

Average access delay [s]

Average access delay [s]

Traditional

Average msg2 message delay as function of the number of delay-tolerant MTC devices. ACB

2.5 2.0 1.5 1.0 0.5 200 400 600 Number of delay−tolerant MTC devices

CC−RA

RSS

2.0 1.5 1.0 0.5

800

200 400 600 Number of delay−tolerant MTC devices

Energy Consumption [J]

CC−RA

800

(b) Average access delay to delay-tolerant MTC devices.

Average access delay as function of the number of delay-tolerant MTC devices. ACB

Traditional

2.5

(a) Average access delay to HTC and delay-sensitive MTC devices.

RRS

Traditional

1.2 1.0 0.8 0.6 0.4 0.2 200 400 600 Number of delay−tolerant MTC devices

Figure 4.

RRS

120

800

(a) Average msg2 message delay to HTC and delay-sensitive MTC devices.

Figure 3.

800

Access probability as function of the number of delay-tolerant MTC devices. CC−RA

Figure 2.

Traditional

20 200 400 600 Number of delay−tolerant MTC devices

Figure 1.

RSS

100 Access probability [%]

Access probability [%]

100

CC−RA

800

Energy consumption per successful RA procedure as function of the number of delay-tolerant MTC devices.

For a number of delay-tolerant MTC devices above 400, when the traditional mechanism is used, the access probability for HTC/delay-sensitive MTC and delay-tolerant MTC devices begin to decrease. This means that a portion of the users are not able to get access to the network under massive number of access attempts. In addition, as there is no need to re-transmit preamble sequence many times, the power consumption is reduced in the CC-RA mechanism, as shown in Fig. 4. Regarding the average msg2 message delay, Fig. 2 compares the performance of the four mechanisms and shows their behaviours. As it can see, although not expected, our proposed CC-RA mechanism has the largest average msg2 message delay between the mechanisms. To better understand this effect, we need to remember that the CC-RA mechanism is cluster-based, and therefore, when the msg2 message timeout, the delay-tolerant MTC devices need to wait until the next slot allocated to the preamble sequence transmission. Thus, the delay between a first preamble sequence transmission and the first msg2 message received can increase due to the increase of the devices in the network. However, as can be seen in Fig. 3, the average access delay of the proposed CC-RA mechanism is the lowest between the mechanisms. This result can be explained by the absence of the messages msg3 and msg4 in the RA procedure (i.e. the RA procedure in the proposed CC-RA mechanism became like contention-free). Therefore, the access delay in the proposed solution depends only on the time required to transmit the preamble sequence and to receive the msg2 message. As the average access delay is calculated only for UEs which RA procedure are successfully finished, it cannot reflect the main gain for the CC-RA mechanism since traditional and RRS mechanisms have quite similar average access delay. For users utilizing contention-based schemes, the transmission of the msg3 message may collide when two or more users transmit the same preamble sequence, thus increasing the overall access delay. This happens this collision is usually detected only when the msg4 message is not received by the UE. In summary, more important than the reduction of the access delay for UEs, the use of the proposed CC-RA mechanism in coexisting HTC/MTC scenarios leads to no blocking of access request of HTC, delay-sensitive MTC and delaytolerant MTC devices. In addition, it can also improves the energy efficiency. VI.

C ONCLUSION

In this paper we presented the CC-RA mechanism to improve the performance of the random-access procedure in LTE networks. The proposed CC-RA mechanism is dedicated to the case where the delay-tolerant MTC devices are with a RRC context but without being synchronized with the network, and ensures that the random-access procedure is as if it were contention-free to the delay-tolerant MTC devices. Numerical results obtained by simulations have confirmed the superiority of the CC-RA mechanism compared to the others 3GPP RAN overload control mechanisms. Besides, reducing collisions and increasing access probabilities, the CC-RA mechanism allows reduction of the energy consumption, which is an important issue to MTC devices with limited power resources.

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