This article has been accepted for publication in the Proceedings of the 2016 IEEE International Conference on Communications (ICC). PLEASE, CITE THE PAPER AS FOLLOWS: Plain text: M. Polese, M. Centenaro, A. Zanella, and M. Zorzi, M2M Massive Access in LTE: RACH Performance Evaluation in a Smart City Scenario, 2016 IEEE International Conference on Communications (ICC), Kuala Lumpur, 2016, pp. 1-6. BibTex: @INPROCEEDINGS{7511430, author={M. Polese and M. Centenaro and A. Zanella and M. Zorzi}, booktitle={2016 IEEE International Conference on Communications (ICC)}, title={M2M massive access in LTE: RACH performance evaluation in a Smart City scenario}, 6 year={2016}, 1 pages={1-6}, 0 keywords={Long Term Evolution;multi-access systems;public domain software; 2 smart cities;Internet of Things;LTE Random Access Channel; t c LTE cellular standard;LTE module;LTE random access procedure; O Long Term Evolution cellular standard;M2M massive access;RACH; Smart City scenario;mobile terminals;open-source network simulators; 5 system-level simulators;Delays;Indexes;Internet of things; ] Long Term Evolution;Signal to noise ratio;Smart cities;Uplink}, I doi={10.1109/ICC.2016.7511430}, N month={May},} . s c [ 4 v 8 9 0 5 0 . 1 0 6 1 : v i X r a ThispaperwasacceptedforpresentationattheIEEEICC2016 conference,May23-27,2016,KualaLumpur,Malaysia. M2M Massive Access in LTE: RACH Performance Evaluation in a Smart City Scenario Michele Polese, Marco Centenaro, Andrea Zanella, and Michele Zorzi Department of Information Engineering, University of Padova – Via Gradenigo, 6/b, 35131 Padova, Italy Email: {polesemi,marco.centenaro,zanella,zorzi}@dei.unipd.it Abstract—Several studies assert that the random access proce- to study the impact of M2M traffic in LTE networks in dure of the Long Term Evolution (LTE) cellular standard may urban scenarios. Simulation results show that if a few hundred not be effective whenever a massive number of simultaneous smart sensors simultaneously require network access, e.g., to connectionattemptsareperformedbyterminals,asmayhappenin report some kind of failure event, an MTD would experience atypicalInternetofThingsorSmartCityscenario.Nevertheless, simulation studies in real deployment scenarios are missing extremely long delays to complete the access procedure, thus because many system-level simulators do not implement the LTE not respecting the delay constraints of important Smart City random access procedure in detail. In this paper, we propose a applications, such as alarms. patch for the LTE module of ns–3, one of the most prominent Theremainderofthepaperisorganizedasfollows.InSec.II, open-source network simulators, to improve the accuracy of the random access procedure in LTE is described and issues the routine that simulates the LTE Random Access Channel (RACH). The patched version of the random access procedure related to massive access are briefly explained. In Sec. III the is compared with the default one and the issues arising from implementation of LTE Random Access Channel (RACH) in massive simultaneous access from mobile terminals in LTE are ns–3 is discussed and its weaknesses are highlighted; then, a assessed via a simulation campaign. patchtothedefaultroutineisproposedtoenhancetheaccuracy of the simulator. In Sec. IV the patched routine is compared with the default one and is used to evaluate the impact of a I. INTRODUCTION massive number of simultaneous access attempts in a realistic Ahugeincreaseinfullyautomatedcommunicationsbetween SmartCitiesscenario.Finally,conclusionsaredrawninSec.V. devices is forecasted in the next few years as new use cases, II. THERANDOMACCESSPROCEDUREINLTE e.g., connected cars, e-health, environmental monitoring, are being identified. This new paradigm is called Machine-to- The RA procedure in LTE is initiated when (i) a User Machine(M2M)communication,sinceitistypicallyperformed Equipment (UE) is in the RRC_CONNECTED state1 and has withoutanyhumanintervention,andisconsideredafundamen- new data to transmit or receive but no uplink synchronization; tal enabler of the Internet of Things (IoT) vision. (ii) it recovers after radio link failure; (iii) it switches from AdesirablerequirementforthedeploymentofMachine-Type RRC_IDLE to RRC_CONNECTED, or, finally, (iv) it performs Devices (MTDs) is the place & play concept [1], i.e., MTDs a handover. The procedure takes place in a dedicated physical should just need to be deployed in a certain area to be ready channelcalledPhysicalRandomAccessChannel(PRACH)[6], to operate. Indeed, the expected number of devices (40 MTDs which is multiplexed in time and frequency with the Physical per household according to [2]) makes a manual configuration Uplink Shared Channel (PUSCH). The PRACH consists of 6 infeasible. For this reason, the cellular network infrastructure Resource Blocks (RBs) for an overall bandwidth of 1.08 MHz is suitable to provide connectivity for M2M communications, and has a duration between 1 and 4 subframes. Its periodicity sinceitcanprovision(ideally)worldwide,ubiquitouscoverage, is variable and is defined by the PRACH Configuration Index, in contrast to many ad hoc proprietary technologies. However, which is broadcast by the base station (eNodeB, eNB) on deploying such a huge number of devices in current cellular the System Information Broadcast 2 (SIB2) along with the networks, e.g., the Long Term Evolution (LTE), poses new following signaling information: issues that need to be addressed. In particular, an important • numContentionPreambles, i.e., the number of problem is the overload of the LTE Random Access (RA) preamblesreservedforcontention-basedRA(atmost64); procedureunderamassivenumberofsimultaneousconnection • preambleInitialReceivedTargetPower, i.e., attempts, since the LTE standard has been designed to provide the target power (in dBm) to be reached at the eNB for high-rate access to a fairly limited number of terminals. transmissions on PRACH; In this paper, we aim at evaluating the delay that a device • powerRampingStep,i.e.,thepowerrampingstepused may undergo while accessing an LTE network in the case of to increase the transmission power after every failed a massive number of access requests in a real deployment. attempt; In particular, we address a Smart City scenario using one of • preambleTransMax, i.e., the maximum number of themostaccurateopen-sourcesystem-levelnetworksimulators, preamble transmission attempts. i.e., network simulator 3 (ns–3, [3]). We found that the current 1A UE is in the RRC_CONNECTED state when a Radio Resource Control implementation of the RA procedure in ns–3 is idealized; (RRC)connectionhasbeenestablished;ifthisisnotthecase,theUEisinthe therefore, we developed a patch to make the routine suitable RRC_IDLEstate[4,Sec.4.2.1]. 532 F.J.López-Martínezetal./DigitalSignalProcessing22(2012)526–534 Table3 PRACHmisseddetectionrequirementsfornormalmode. Numberof Propagation Burst0 Burst1 Burst2 Burst3 RXantennas conditions(FO) 2 AETWUG7N0((207H0zH)z) −148.2dBdB −147..82ddBB −1160..04ddBB −1106..15ddBB − − − − 4 AETWUG7N0((207H0zH)z) −1162..91ddBB −1161..77ddBB −1194..01ddBB −1183..89ddBB − − − − Werecallthatthepreambleisasignaturecomposedofacyclic prefix and a Zadoff-Chu (ZC) sequence that is obtained by shifting a root sequence, which is common to all the UEs connected to a certain eNB. Preambles containing different sequencesareorthogonaltooneanother.2 Thereare4different preambleformats,withdurationfrom1to4subframes,inorder to guarantee coverage of different cell sizes. The RA procedure consists of 4 messages as follows. a) Preamble Transmission: The UE selects a random ZC sequence and transmits a preamble on one of the resources specified by the PRACH Configuration Index. The eNB will Figure 1: PFmig.i8s.sMDvPsvsSSNRN.2RanteΓnn,as,tAaWkGeN.n from [14]. detectthesequencebyapplyingacorrelatorandapeakdetector to the received signal [6]. However, since the number of ZC sequences is finite, it may happen that more than one UE repeats the RA procedure from the beginning after a random select the same sequence, thus incurring in a collision. If the backofftime.Again,whenthenumberofunsuccessfulattempts colliding UE preambles are received with high enough Signal- reachessomespecifiedmaximumvalue,thenetworkisdeclared to-NoiseRatio(SNR),andaresufficientlyspacedapartintime, unavailable by the UE and an access problem exception is two energy peaks separated by a time that is longer than the raised to the upper layers. Maximum Delay Spread (MDS) are detected and the eNB will interpret this event as due to a collision. On the other hand, if A. Massive Random Access onlyoneofthecollidingpreamblesisreceivedwithhighSNR, The LTE RA process is efficient when a small number of orthedelayofthedifferentpreamblesissimilar,3 theeNBwill devices require access to the network, which is the typical not be able to recognize the collision. case for Human-to-HFigu.9m.MaDPnvs(SNHR.24Hant)enntars,aAWffiGNc.. However, the num- We remark that MDS and the preamble detection algorithm ber of terminals is envisioned to grow exponentially in IoT are not standardized but left to the eNB vendor. However, the scenarios, especially for what concerns Smart Cities where 3GPP report [11] requires a missed detection probability lower smart meters will be deployed to monitor a large variety of than 10−2 for an SNR value of −14.2 dB and a 2-antenna parameters, from air pollution to supply levels. In the case of receiver in AWGN channel. extraordinary events, e.g., power outages, many MTDs may b) RandomAccessResponse(RAR): TheeNBanswersto be activated simultaneously, thus causing a PRACH overload. correctly decoded preambles (including those with undetected The consequences are that the constraints of delay-sensitive collision) by sending a RAR message on the Downlink Shared M2M applications can be violated, the power consumption of Channel (DLSCH). RAR carries the detected preamble index, the sensors is increased, and the Quality of Service (QoS) of which corresponds to the sequence sent by the UE, a timing H2H applications can be degraded. alignment to synchronize the UE to the eNB, a temporary In this paper we aim at evaluating how this massive event- identifier (RNTI), and an uplink scheduling grant that specifies triggered reporting mFiga.1y0.MiDmPvpsaSNcRt.2tahnteennaLs,TETEU70.network performance the resources assigned to the UE to transmit in the next phase in a Smart City scenario using a well-known open-source oftheRAprocedure.IfaUEreceivesaRAR,thenitproceeds network simulation tool, i.e., ns–3, written in C++, which is with the third step; otherwise, it restarts the RA procedure particularly suitable to simulate an urban propagation environ- anew (unless it has reached the maximum number of preamble ment. Other open-source simulation platforms are available, transmission attempts) after a backoff time that is uniformly e.g., the LTE Vienna Simulator [9], which is based on Matlab, distributed in the interval [0,BI], where BI is the Backoff and Omnet++ [8] and LTE-sim [7], both written in C++. Indicator carried by the RAR. If the counter of consecutive However,theycannotbedirectlyusedforourpurposes.Infact, unsuccessful preamble transmissions exceeds the maximum the Vienna Simulator is a link level simulator for the uplink, number of attempts, a RA problem is indicated to the upper andthereforelackssomeofthenecessaryfeaturestoadequately layers. model a network of MTDs, whereas Omnet++ and LTE-sim c) Connection Request: The UE transmits a Radio Re- focus on the higher networking layers through an idealized source Control (RRC) message containing its core-network abstraction of the lower layers, and therefore do not capture terminal identifier in the uplink grant resources and starts a the level of detail we need to model the RACH performance. ContentionResolutiontimer.NotethattheUEsthattransmitted the same preamble but whose collision remained undetected III. LTERACHINNS–3 will transmit on the same resources, colliding again. In this section the LTE RACH implementation in ns–3 d) Contention Resolution: If the eNB correctly receives is addressed and an enhancement to evaluate IoT traffic is theRRCmessage,itreplieswithanRRCConnectionSetupthat proposed. signals to the UE that the RA phase is successfully completed. We refer to version 3.23 of the ns–3 simulator, which Instead, if the Contention Resolution timer expires, the UE uses the LTE-EPC Network simulAtor (LENA) [10] module to simulate the LTE protocol stack and the Evolved Packet 2For eNBs with a large coverage area there may be more than one root Core (EPC) network. In the current implementation of LENA, sequence.Howeverthesequencesobtainedhavelowcross-correlation[5]. 3ThisistypicalinSmallCellsscenarios[6]. however, the RACH preamble is an ideal message, i.e., not Parameter Value DUowplninliknkcacrarrireirerfrferqeuqeunecnycy 994050MMHHzz 250 AvaRilBabbleanbdawniddwthidth 15800RkHBz 200 eNBesNHbBeexasmafgowornideatalhcsh(emcsteaocirntsorlobe) 3(co6-l51o◦cated) Y [m] 150 TXpowerusedbyeNBs 43dBm 100 MaxTMXeNTpBDownneooriissueesfiefidgguubrryeeMTDs 2335dddBBBm 50 Shadowing log-normalwithσ=8 0 Numberofbuildings 96 0 50 100 150 200 250 300 350 400 Apartmentsforeachfloor 6 Floorsforeachbuilding 3 X [m] MTDspeed 0Km/h Figure 2: Smart City network deployment example. The rect- NumberofMTDsN {50,100,150,200,300,400,500,600} Simulationtime∀N {60,60,120,120,300,300,400,400}s angles are the buildings, the small triangles are the MTDs, and the black square is the position of the three co-located eNBs. Table I: Simulation parameters [2]. Parameters Value and transmits it in the next PRACH opportunity. Note that PRACHConfigurationIndex 1 we are assuming that, in a PRACH-dedicated subframe, no BackoffIndicatorBI 0ms PUSCHtrafficisallocatedbythescheduler,whilethePRACH preambleInitialReceivedTargetPower −110dBm powerRampingStep 2dB is allocated in the first 6 RBs available. The power (in dBm) numContentionPreambles 54 for the transmission is computed according to the standard as preambleTransMax ∞ [12] Contentionresolutiontimer 32ms P =min{P , Table II: Simulation parameters of LTE RACH. prach UE,max (1) PREAMBLE_RECEIVED_TARGET_POWER+P } lc where P is the maximum transmit power UE,max subject to radio propagation; moreover, Connection Request for a UE, P is the estimated pathloss and lc and Connection Resolution messages are not modeled and, PREAMBLE_RECEIVED_TARGET_POWER is given by therefore, all collisions are detected and solved at the first the MAC layer, as [13] step of the RA procedure. Furthermore, we found that it is not possible to simulate the connection and disconnection of PREAMBLE_RECEIVED_TARGET_POWER= UEs during runtime, since LENA allows every UE to switch preambleInitialReceivedTargetPower only once from RRC_IDLE to RRC_CONNECTED states at (2) +∆ +(PREAMBLE_TX_COUNTER−1) preamble the beginning of the simulation. Therefore, we implemented ×powerRampingStep a more realistic RA procedure, along with the possibility to disconnectUEsfromtheeNB(i.e.,switchingfromRRC_IDLE where PREAMBLE_TX_COUNTER is the number of consec- toRRC_CONNECTEDandviceversa).Theenhancedmoduleis utive preamble transmissions and ∆ = 0 for format preamble called LENA+. 4 However, to maintain the backward compat- 0. The other parameters are given by the eNB with SIB2, as ibility with the current release, an option has been introduced explainedinSec.II.AttheeNBside,theSNRiscomputedfor to use the idealized LENA RA procedure if desired. In the each preamble and a decision on correct or missed detection is following, we describe in detail the features of LENA+ that made.In[14]differenteNBdetectionalgorithmsareintroduced were not present in LENA. and evaluated in terms of missed detection probability vs SNR atthereceiver.Themisseddetectionprobabilityperformanceof A. PRACH Characterization these algorithms is reported in Fig. 1. Note that our improved PRACH is implemented as a real physical channel, re- PRACH model for ns–3 assumes a time domain detector with lying on the already developed and tested channel model. decimation (denoted with LT in the legend), which is the Nonetheless, PRACH preambles are now subject to noise and simplest algorithm which satisfies the 3GPP requirements in radio propagation, since they are sent on specific time and [14], as can be seen from Fig. 1. The ns–3 LTE module has frequency physical resources, and therefore the eNB can fail alsobeenmodified,inordertohandlethereceptionofmultiple their detection. We remark that only format 0 of PRACH signals in the same time and frequency resources. Indeed, the preambles is implemented. Whenever a UE starts the RA default implementation raises an exception whenever two or procedure,itcheckswhetherithasreceivedSIB2,whichcarries more signals are received in the same time and frequency the RACH configuration. Then, it chooses a random index resources by the eNB, since the MAC scheduler forbids mul- drawn uniformly in [0, numContentionPreambles - 1] tiple transmissions in physical channels other than PRACH. In PRACH, indeed, transmissions cannot be scheduled and the 4The source code is available at https://github.com/ signetlabdei/lena-plus. ZCsequencesallowmultipleaccessbyCodeDivisionMultiple 1 1 1 0.8 0.8 0.8 0.6 0.6 0.6 F F F D D D C C C E 0.4 E 0.4 E 0.4 0.2 LENA 0.2 LENA 0.2 LENA LENA+ LENA+ LENA+ 0 0 0 10−2 100 102 10−2 100 102 10−2 100 102 Delay[s] Delay[s] Delay[s] (a)N =100 (b)N =200 (c)N =300 1 1 1 LENA LENA LENA LENA+ LENA+ LENA+ 0.8 0.8 0.8 0.6 0.6 0.6 F F F D D D C C C E 0.4 E 0.4 E 0.4 0.2 0.2 0.2 0 0 0 10−2 100 102 10−2 100 102 10−2 100 102 Delay[s] Delay[s] Delay[s] (d)N =400 (e)N =500 (f)N =600 Figure 3: ECDFs of access delay for N ∈{100, 200, 300, 400, 500, 600}. The x-axis is expressed in logarithmic scale. Access (CDMA). If a preamble is correctly received but there LENA, has been added as well. aretwoormorepreambleswiththesameZCsequence,thenthe collisionisdetectedornotaccordingtothefollowingheuristic. IV. PERFORMANCEEVALUATION Since the PDP of different users is not simulated in a system- In this section the proposed enhanced version of the LENA level simulator such as ns–3, as a rule of thumb a collision is module is evaluated and compared with the current release. detected if Moreover, we will use our module to evaluate the impact of d −d max min >T , (3) massive simultaneous accesses to an LTE network in a Smart c chip City scenario. where d and d are the distances from the eNB of the The scenario we simulated is compliant with the specifica- max min farthest and closest colliding UE, respectively, c is the speed tions in [2]; the main simulation parameters are in Table I, oflight,T =1/(2B),andB =1.08MHzisthebandwidth while the RACH-related parameters can be found in Table II. chip of PRACH. We refer to an urban environment with a high density of tall RAR message transmission was already implemented as buildings; the deployment of buildings and MTDs is depicted a message on the DLSCH. For each not-collided preamble in Fig. 2. For what concerns the radio propagation model, or undetected collision, an uplink grant is allocated by the we employed the ns–3 Hybrid Buildings Propagation Loss schedulerandaddedtotheRARresponse;theBackoffindicator Model, which exploits different propagation models to account has also been added. for several factors, such as the positions of the UE and the Connection Request transmission on granted resources was eNB (both indoor, both outdoor, one indoor and the other already implemented, as well. However, since in the default outdoor), the external wall penetration loss of different types implementation of RACH all the collisions are resolved at the of buildings (i.e., concrete with windows, concrete without first step, collisions of messages 3 were not handled and the windows,stoneblocks,wood),andtheinternalwallpenetration simulatorraisedanexception.Thisexceptionhasbeenhandled loss. We remark that all the MTDs have been placed inside as follows. Firstly, no capture effect has been considered. the buildings and their positions are not changed during the Then, if two or more Connection Request messages collide, simulation according to the specifications found in [2]. they are considered as received with errors, triggering an Let us denote with N the number of MTDs that are trying HARQ (layer 2) retransmission until the maximum number of to simultaneously access the LTE network. For every value of attempts is reached; after that, the RA procedure starts again. N ∈ {50,100,150,200,300,400,500,600}, 10 Monte Carlo The Contention Resolution timer, that was also not present in simulations have been run and the Empirical Cumulative Dis- 1 50 pts 40 100 0.8 51000 Attem 125000 H ECDF 00..46 123450000000 cessfulRAC 20 345600000000 c 0.2 500 Su 600 0 0 0 2 4 6 8 10 10−2 10−1 100 101 102 Time[s] Delay[s] Figure 5: Successful RACH attempts vs time. Figure 4: ECDF for various values of N. The x-axis is expressed in logarithmic scale. of the higher number of collision events. For N > 300, we N Meanµ[s] Std.Dev.σ [s] µ/σ cannot observe any meaningful peak, denoting that the RACH 50 0.235 1.855 0.127 is congested and the success probability is very low. 100 0.498 2.608 0.191 150 0.780 2.605 0.300 V. CONCLUSION 200 1.481 4.453 0.333 300 5.268 5.359 0.983 TheimplementationofamorerealisticmodeloftheLTERA 400 21.400 15.126 1.415 procedureinthenetworksimulatorns–3hasbeenproposedand 500 64.234 52.852 1.215 600 77.423 59.256 1.307 evaluated in a Smart City scenario. An enhanced ns–3 LENA module, called LENA+, has been developed to overcome the Table III: Statistics of the access delay experienced by the limitationsofthedefaultroutine,whichisratheridealized.The MTDs that succeeded in completing the access procedure. simulation results show that the default release of the LENA module underestimates the impact of M2M traffic in cellular networksandconfirmstheconcernsaboutLTERACHoverload tribution Functions (ECDFs) of the access delays have been incaseofmassivesimultaneousaccessattempts.Aspartofour produced.Fig.3showstheECDFsofthedelay(inlogarithmic futurework,wewillenrichtheLENA+implementation,totest scale) for the various values of N, obtained using both the the effectiveness of some of the strategies proposed in [1] to defaultLENAmoduleandtheLENA+module.Weremarkthat, relieve the RACH overload problem. forwhatconcernstheLENA+performancecurves,theaverage ECDF is represented by the solid line and we plotted the REFERENCES ECDFs of the individual Monte Carlo simulations with dashed lines to show the dispersion around the average value. 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