WIXLIB

Enhancements for Enabling Point-to-MultipointCommunication Using Unlicensed SpectrumAthul Prasad† , Petteri Lunden‡ , Zexian Li† , and Mikko A. Uusitalo††NOKIA Bell Labs, Finland. ‡ Nokia Networks, Finland.Email: † {firstname.lastname}@Nokia-Bell-Labs.com ‡ al increase in data rate demand has leadto the periodic upgrade of mobile network infrastructure, witha new generation of wireless access technology being developedevery decade. Currently, the fifth generation (5G) of mobile networks – supporting higher data rates and reliability, with lowerlatency are being developed and planned to be deployed. Pointto-Multipoint or multicast / broadcast communication, which hasreceived limited attention so far in 5G, enables the deliveryof common content to a multitude of users, while consumingminimal amount of radio resources. In this work, we considerthe usage of unlicensed spectrum for enabling such transmissionsusing enhancements of the currently specified LTE-AdvancedeMBMS network. The proposed enhancements are fully compatible with distributed radio access network deployments, andrequire limited coordination between base stations. Using realistic5G network assumptions, we also evaluate performance of suchenhancements in delivering media content to a large number ofusers.I. I NTRODUCTIONMulticast and broadcast (Point-to-Multipoint / PTM) communication enabled using the evolved Multimedia Broadcast/ Multicast Service (eMBMS) feature [1], has been a keycomponent in Third Generation (3G) and Fourth Generation(4G) LTE-Advanced (LTE-A) wireless networks, in enablingresource efficient content distribution [2]. The content hasmainly been TV broadcast and public safety (public warningsystems and mission critical communication systems) in legacybroadband networks. Due to the improvement in the contentquality requirements and time criticality, the amount of radioresources consumed for delivering the content has constantlybeen increasing with the passage of time. The content qualityrequirements have been constantly increasing with advancedvideo and audio codecs enhancing the quality of experienceof the end users, and the network operators need to allocatehigher amount of radio resources to efficiently and effectivelydeliver this content to the end user. The scarce amount ofavailable spectral resources makes such content delivery overthe air, increasingly challenging, especially when the media isbroadcasted over a wide area.Current research focus on the 5G system and architecturedesign has been mainly related to traditional wide-area macroand ultra-dense small cell network deployments for mediadelivery using unicast [3], [4]. The implications on the radioaccess and core network for delivering PTM traffic using5G has received limited attention in the past, both fromacademic and industry perspective. Recently, there has beenan increasing focus on technologies that enable such communications with study items for the development of standardsbeing discussed in 3GPP [5] and active research being donefor developing technology enablers [6]–[8]. The use casesconsidered in [6] indicate the need for PTM communicationsto enable the efficient transport of data for a multitude ofverticals apart from media and entertainment, such as - automotive, internet of things and public safety. The detailedperformance benchmarking of 4G LTE-A Pro networks isdone in [9], evaluating the need for enhancements in 5G. Theoverall content delivery framework required for such types oftraffic is evaluated in [10]. For the media and entertainmentvertical, the challenges for the mass delivery of virtual realitydata which consumes significant amount of network bandwidthhas been investigated in [11]. The use of device-to-device(D2D) augmented broadcast as a possible technology enableraddressing such challenges have been presented in [12].Vertical micro-operator (µO) networks that can be deployedwith minimal inter-working with the wide-area macro networkoperators is considered to be a key disruptor in 5G [13].The µO networks enable the wider technology adoption of5G networks, with its native support of services that requireultra-reliability and low-latency. The main advantage of suchnetworks is the ability to tailor the network to specific usecases, thereby enhancing the quality of service and experiencefor the end users. A key enabler for such deployments wouldbe its operational capability in unlicensed bands, in order tofurther reduce the deployment costs for µOs. The enhancedarchitecture and mechanisms considered in this work couldbe applied to any type of operator network deployed forprovisioning PTM services.One of the most important problem related to µO networks and PTM transmissions, is related to the setting up ofspectrally-efficient single frequency network (SFN) [2] usingunlicensed band deployments, while complying to the regulatory requirements of unlicensed band transmissions. Thiswould be an important enabler for µO deployments using 5GXcast transmissions, and a key driver for the support of newverticals and use cases in 5G. Here Xcast implies an efficientmix of uni-, multi-, and broad- cast transmissions. There area significant number of use cases for which such solutionswould be relevant and important. The key issue here is thesetting up of SFN in an unlicensed band, solutions for whichcurrently does not exist. The main reason behind this hasbeen the consideration of unicast service provisioning over

SFN AreaEnhance SYNC protocol forunlicensed band operationXnM1*Enable transmission coordinationusing MCE enhancements withsub-optimal network operationMBMS-GWM2*MCEFig. 1. Channel access for listen-before-talk.M3*MMEBMSCSm *ContentProviderunlicensed bands.Unicast transmissions using unlicensed band has receivedsignificant attention in the context of 4G networks. LTEoperation using unlicensed spectrum with license-assisted access (LAA) feature which augments the capacity of existingdeployments using licensed spectrum has been studied in [14].The co-existence of LTE and WiFi, especially with the usageof Multefire standalone unlicensed network deployments havebeen considered in [15]. In order to obtain the significantamount of bandwidth required for 5G networks, they needto natively support unlicensed band operation [16]. Hence,the future releases of the standards are expected to supportstandalone operation using unlicensed frequency bands.The challenge in operating SFN on unlicensed spectrumis that due to the listen-before-talk (LBT) mode of operationrequired by the regulations [17]: 1) the access to the mediumcannot be guaranteed at any specific moment, and 2) even theother transmitters of the same network (SFN) block the accessto the channel. Another key challenge is that the transmitpower is limited on unlicensed spectrum leading to smaller cellsize. Thus, there are potentially a large number of neighborcells competing for the same channel, which is an issue whenthey all try to transmit at the same time (as in SFN) but stillneed to apply LBT. These challenges make it non-trivial toenable SFNs on unlicensed spectrum. While some countriesthe regulation does not enforce listen before talk, it would bechallenging to enable large scale deployments that would notfollow such an operational paradigm.The rest of the paper is structured as follows: Section IIprovides an overview of the system model used, Section IIIdiscusses the key challenges for indoor VR broadcast. Section IV presents the simulation assumptions and system levelparameters used for simulations, together with performanceresults of the proposed scheme. Section V gives summary ofthe paper.II. S YSTEM M ODELIn this work, we consider a system operating using unlicensed frequency bands, constrained by listen-before-talk with each base station listening for channel occupancy beforetransmitting data and occupying the channel, similar to theone considered in [18]. The channel access mechanism usedby two 5G base stations (gNBs) is as shown in Fig. 1. Dueto the limited transmit power of such a system, we considersmall cell network deployments, with the user equipment (UE)connected to the network for its communication needs. It isFig. 2. System considered in this work.Frame Type-0(a) Use Frame Type-0 for channel sensing timeinstances/subframes, other types for transmissionsSpareTime Stamp(b) Indicate Transmission Type– Transmit / Channel SensingPacket CounterElapsed Octet CounterControl PartTotal No. Of PacketFrame Type-2Total No. Of OctetTime StampCRCPaddingFrame Type-1SpareChecksum PartSparePacket CounterElapsed Octet CounterControl PartTotal No. Of PacketTime StampTotal No. Of OctetPacket CounterElapsed Octet CounterControl PartCRCPaddingChecksum PartPayload CRCTotal No. Of PacketPayloadPayload PartTotal No. Of OctetCRCPaddingPayloadCheck sum PartPayload Part(c) Use null-payload to configuresensing time instancesFig. 3. Possible SYNC protocol enhancement options.assumed that all the UEs request common data which is thendelivered by the network using over-the-air broadcast.For the sake of simplicity, we assume the LTE-A eMBMS network architecture shown in Fig. 2 for delivering thebroadcast traffic, with enhancements to enable SFNs usingunlicensed bands. The enhancements are mainly proposed onthe synchronization protocol (SYNC) [19], which limits theimpact on the radio access and core network entities. Herethe multi-cell coordination entity (MCE) which configuresand coordinates the radio transmission parameters within theRAN would also require enhancements, especially for the casewhen the channel is occupied by other transmitters. While themechanisms described in this work are mainly from LTE-AeMBMS perspective, the solution is applicable to any multicast/ broadcast capable network, which supports a central networkentity for providing the timing for synchronized over-the-airtransmissions.III. U NLICENSED M ULTICAST / B ROADCASTIn this work, we consider the setup SFN transmissions withthe help of SYNC protocol [19] enhancements in the Broadcast Multicast Service Center (BMSC), which translates theincoming data packets into SYNC protocol data unit (PDUs).The timestamp information in the SYNC PDUs enable allthe BSs which are part of the SFN area to schedule the