Radio Resource Management In LTE Networks

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1Radio Resource Management in LTE NetworksRicardo Sousa, Instituto Superior Técnico, Universidade de LisboaAbstract—This paper proposes a newly developed prioritybased call admission control algorithm, that performs the all thedecision making regarding which strategies to adopt in orderto successfully serve a new user, even if the load level in thecorresponding cell is above a defined threshold. The combinationof methods such as forced handover, queue list and servicedegradation allows for a customizable approach to admissioncontrol, between rigid Quality of Service (QoS) limits and highcapacity in number of simultaneously active users. To assess andobtain valid results about its performance, one adapted and usedthe LTE System Level Downlink Simulator in real world basedscenarios.Index Terms—Radio Resource Management, LTE, AdmissionControl, LTE System Level Downlink Simulator, Load Balancing.I. I NTRODUCTIONTHE main objective of this work rests on studying anddeveloping strategies in the area of Long Term Evolution’s (LTE) admission control algorithms. Its scope includesthe design, implementation and testing of a new algorithmbased on several existing techniques, as well as the evaluationof its impact by assessing the performance of the analysednetwork. To further increase the value of this work and in anattempt to show the validity of a real world application of thedeveloped algorithm, the network architecture data used for thedistribution of sites and cells, along with path loss data, wasprovided from a company present in the Portuguese mobiletelecommunications operator’s market.The simulation tool used as the main basis for new implementations and necessary adaptations is the LTE-A DownlinkSystem Level Simulator [1], made available for academicresearch by the Vienna University of Technology. The currentwork also included the addition of several new methods androutines crucial to the implementation of a working admissioncontrol module.The final goal is to devise an algorithm that applies newstrategies regarding the management of user entries, handoversand exits in the network, allowing for a more finely tunedperformance and QoS approach. The new algorithm is to betested and evaluated by application on the referred LTE-ADownlink System Level Simulator to assess its value andvalidity.II. F UNDAMENTAL A SPECTSLTE is a standard developed by the Third Generation Partnership Project (3GPP), based on Global System for Mobilecommunications(GSM)/Enhanced Data Rates for GSM Evolution (EDGE) and Universal Mobile Telecommunication System(UMTS)/High Speed Packet Access (HSPA) technologies,specified in its Release 8 and enhanced in Release 9. Althoughthis standard only met some of the requirements defined by theInternational Telecommunication Union RadiocommunicationSector (ITU-R) for Internatonal Mobile Telecommunications Advanced (IMT-A), it was still marketed as 4G. However, inRelease 10, the next step is taken, and those requirements arefully satisfied, creating a new standard, LTE-Advanced (LTEA). To distinguish between the two, LTE-A has been definedby ITU-R as ”True 4G”.This section describes the basic aspects of LTE, fromthe architecture used in its network to its radio interfacespecifications. Also, a summary of new characteristics andaspects of LTE-A is given. All content is based on [2]-[5].As all other Radio Access Technologies (RAT), LTE has itsnetwork divided in two main components, the Core Networkand the Access Network. Each of them will be addressed individually and also how they, together with the User Equipment(UE), form the Evolved Packet System (EPS) to accomplishthe goal of IP based radio connectivity. In Figure II.1 aschematic representation of that system is displayed.Fig. II.1. EPS architecture, extracted from [2].A. Services and ApplicationsUltimately, mobile technologies such as LTE are means toprovide a service, which is communication between peoplethrough a variety of different applications. So, it is evidentthat these RATs must be developed bearing in mind the typeof service they will be offering. A number of different typesmust be treated according to the purpose they serve and to therequirements they impose. For example, voice is a service that

2requires very low and constant bit rates, but at the same time avery small delay. Other services, however, like buffered videostreaming, can be much more demanding in terms of bit ratebut with no delay requirements.In order to adapt the QoS standards to a data-oriented RATlike LTE, 3GPP defined new and more optimised aspects tocompose the QoS profile of an EPS bearer: Allocation Retention Priority (ARP) - an integer in the1-16 range, is used mainly as a priority indicator in ACdecisions; uplink and downlink guaranteed bit rate (GBR) - minimum guaranteed bit rate for the bearer in question.Specified independently for downlink and uplink, onlyfor EPS GBR bearers. In the case of non-GBR bearer, anaggregate maximum bit rate (AMBR) is defined; QoS class identifier (QCI) - identifies the QoS class,that includes several QoS parameters, such as schedulingpriority, delay budget and loss rate. 3GPP specified amapping for nine values of QCI, associating differentparameters to typical services, represented on Table II.1.TABLE II.1QCI MAPPING TO Q O S PARAMETERS FOR THE EPS BEARER , ADAPTEDFROM [6]QCI#Delay budgetLoss rateExample services1 (GBR)Priority2100 ms10 2Conversational voice2 (GBR)4150 ms10 3Conversational video3 (GBR)350 ms10 3Real-time gaming4 (GBR)5300 ms10 6Buffered Streaming5 (non-GBR)1100 ms10 6IMS signaling6 (non-GBR)6300 ms10 3Video, TCP-based services7 (non-GBR)7100 ms10 6Voice, Video, Interactive gaming8 (non-GBR)8300 ms10 6Video, TCP-based services9 (non-GBR)9300 ms10 6Video, TCP-based servicesB. Radio Resource Management in LTETo ensure an efficient use of radio resources, severaltechniques are used as part of Radio Resource Management(RRM), in order to provide the users with a service inaccordance with the configured Quality of Service (QoS)parameters. The main RRM algorithms in LTE are packetscheduling, admission control, power control and interferencecontrol. Since 3GPP only defines signalling regarding theseprocedures, they can be vendor and operator dependant. Admission control is the tackled subject in this case, as it is themain topic of this work.1) Admission Control: One of the eNodeBs’ (eNB) responsibilities is the Admission Control (AC) algorithm. Ithandles the requests for new EPS bearers in the correspondingcell. The decision to admit a new user is made taking intoaccount several aspects, such as resource availability, QoSrequirements of the new bearer, priority levels, and providedQoS to the current bearers served. So, the new request isonly accepted if the bearer’s QoS is estimated to be fulfilledwhile still serving the remaining bearers, always within theirdefined QoS, with the same or higher priority. Simply, theAC algorithm will mostly only accept new bearers until thepacket scheduler is no longer able to reach a solution thatfulfils the QoS requirements of high priority bearers. As theRRM protocols are not fully defined by 3GPP, the admissionrules and the actual algorithm are eNodeB vendor specific.Some parameters, namely the packet loss rate and priority,are used to configure the Automatic Repeat-reQuest (ARQ)operation point, the error control mechanism for data transmission in the eNodeB, necessary for the Radio Link Control(RLC) protocol.The reference admission control algorithm applies the simple method of accepting the new bearer if the number ofResource Blocks (RB) requested added to the number ofRBs used by the active connections in the network doesnot surpass the total number of RBs available. However, thisapproach neglects the QoS requirements of the several traffictypes such as VoIP, which require tight constraints in order tohave good performance. The Call Admission Control (CAC)algorithm proposed in [7] Delay-aware CAC (DACAC) takesinto account these requirements, and is based on windowstatistics for measured packet delay and RB utilization. Thisenables the ability to guarantee a certain level of packet delay.Also on the same subject, [8] proposes a Radio AccessControl (RAC) algorithm (LTE-FIAC) based on the combinedidea of complete sharing and virtual partitioning. In order tofavour high priority traffic when resources are limited, a stepwise degradation scheme is used, reducing resources availableto lower priority bearers. Results show lower blocking probability and fair sharing of bandwidth, in comparison with thereference scheme. Currently, the algorithm only considers theincoming user’s QoS requirements and the channel conditionsat the connection’s time of arrival. Considering a situation inwhich channel conditions deteriorate during the connection,its QoS may be affected in a negative way. So, a possibleimprovement may be the ability to deal with channel fluctuations.III. A LGORITHM D EVELOPMENT AND S IMULATIONE NVIRONMENTA. Algorithm DescriptionFrom now on, the term call will refer to any type ofconnection attempt from an UE to an eNB. Therefore, thedeveloped module will be labelled CAC.The main objective of implementing CAC capabilities restson the greater control over distribution of resources amongusers, lower blocking probabilities and overall increase inservice capacity while respecting minimum QoS thresholds.To accomplish these intentions, one must tackle the problemby enforcing a sequence of procedures to perform whentriggered, i.e. when a new UE tries to connect or an handoverto the eNB in question is attempted.The overall functioning of the algorithm as well as how themain components interact with each other is represented inFigure III.1. This flowchart gives an overview of the process,from a high level standpoint, before going into detail abouteach component.Since the simulator handles all the data processing andmeasurement such as RB assignment, traffic generation and

3transmission, link quality and performance estimation, amongother vital processes, the CAC algorithm needs informationabout the position, type of traffic and a list of accessible cellsof the new UE as inputs.It is worth noticing that the CAC algorithm runs on an eNBlevel, which means each eNB will have its own algorithm operating solely on that cell, but with access to information aboutthe load situation of the surrounding cells, as well as simpleinformation about their attached UEs, such as traffic type andmeasured Signal-to-interference-plus-noise ratio (SINR).and throughput, the UE can be allocated, although it causesthe degradation of service to others of same or lower priority.On the other hand, if the check for lower priority UEsreturns positively, the possibility of performing an handover ofa such an UE is taken into account, in order to free resourcesfor the allocation of the new UE. To this effect, the algorithmgets an ordered list of possible UEs to handover, with respectto their priorities (lower to higher), always below the one ofthe new UE, and the SINR values stored in the simulator forpossible connections to neighbour cells.If the returned list is empty, which can occur due to thelower limit for SINR value set, then the CAC handover is notpossible and the new UE is immediately inserted in the queuelist, for which the whole operation is detailed in SubsectionIII-D. Otherwise, the list is iterated through, evaluating thepossibility of performing each handover and recursively alsochecking if the possibilities themselves have lower priorityUEs in the neighbour cell that can be handed over and so on,until a solution is found for everyone and the initial UE canbe allocated to the chosen eNB.These steps sum up the overall functioning of the CACalgorithm, and Subsections III-B to III-D will then detail thespecifics about the smaller processes of which the CAC iscomposed.B. Load Estimation1) Load Definition: The estimation of the load situation isdone similarly to as described in [9], but with a slight changeon what is used as a numerator in the ratio, due to the way theinformation is available from the simulator, corresponding tothe number of assigned RBs at each TTI. Therefore, the loadsituation of an eNB is defined as:NRBused 100(1)NRBtotalWhere NRBused corresponds to the average number ofRBs allocated per TTI, according to an implemented slidingwindow average and NRBtotal to the number of RBs availablefor scheduling each TTI, according to the defined bandwidth.Also, the size of the averaging window was implementedas parametrizable, and the value used for simulations was of100 TTIs, as it allows to encompass all the transmission typesof used traffic models.A very important additional aspect is that the traffic typesbased on best effort, such as File Transfer Protocol (FTP) andHypertext Transfer Protocol (HTTP), are not accounted for inthis load calculation process, since the allocation of resourceson their case is done according to available RBs after all otherQoS constrained types do not have bits to transmit in a givenTTI, as they have no minimum QoS requirements.This definition allows for an estimation of the load situationin the eNB, on which the whole CAC algorithm is based on,a vital point for decision making in this process.2) Process Description: This important element of the CACalgorithm performs the calculation of the load situation of theeNB in the event of a new UE’s attempt to connect.The calculation is done according to the definition of loadin Section 1, only accounting for QoS constrained trafficρ(%) Fig. III.1. Overview of CAC module’s operation.The first step consists of choosing a cell from the accessiblelist, on which to run the algorithm. Once chosen, the loadsituation of the eNB on which the UE attempts its connectionis evaluated. The details of this small process are well definedon Subsection III-B, including the definition of load consideredin this thesis, and result on a simple positive or negativeresponse. In case the response is affirmative, the operationends here and the new UE is accepted and attached to theeNB in question. Otherwise, the process continues and furtherevaluation is necessary.Next, a quick check for the existence of lower priorityUEs, concerning their type of traffic, in the chosen cell, isdone. If none are present, the analysis proceeds on the nexteNB. If there are no more remaining cells in the list, anothergreat part of this algorithm is applied, the QoS verification,described in detail on Subsection III-E. In short, it evaluatesthe current performance of UEs in the best cell, and if theyare above previously set QoS minimum thresholds for delay

4types, and if it falls below the defined threshold, this processimmediately responds positively. On the other hand, if theresult is above the threshold, the response is negative.In this thesis’ work, the threshold value used as the frontierbetween accepted and rejected connection attempts was 70%.This number was chosen due to being an usual value usedby mobile operators, being amply utilised throughout theliterature.C. CAC HandoverCAC handover is a crucial part of the algorithm’s functioning. It allows for the offer of service even if the first choiceeNB is currently above the load threshold. Unfortunately thatmay imply a certain degradation of service, since the neweNB, as the second (or more) choice, will serve the UE witha lower SINR value, which mean worse rates, and thereforepossibly worse QoS. To the user however, this may not evenbe an issue, depending on his type of traffic, that can have nominimum QoS requirements or even have them and still donot notice any kind of degradation.This process is used extensively, as the situation demands,and can be applied to either the new UE itself, or even lowerpriority ones in the cell (or surrounding ones), as shortlydepicted in Section III-A.The flowchart on Figure III.2, represents the sequenceo

fully satisfied, creating a new standard, LTE-Advanced (LTE-A). To distinguish between the two, LTE-A has been defined by ITU-R as ”True 4G”. This section describes the basic aspects of LTE, from the architecture used in its network to its radio interface specifications. Also, a summary of new characteristics and aspects of LTE-A is given.

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