Coverage Enhancement for Vehicles
11 Coverage Enhancement for Vehicles
Stefan Runeson , Ali Zaidi , and Ana Cantarero Ericsson Ericsson BMW Group
Abstract —The Third Generation Partnership Project (3GPP)has standardized Coverage Enhancement (CE) for Internet-of-Things (IoT) to connect devices in challenging radio conditionswith cellular networks. CE is based on the principle of prolongedtransmission time that exploits the fact that many IoT applica-tions have relaxed requirements on data rate and latency, and thecoverage can be significantly boosted by repeating transmissionsfor such applications. However, CE consumes a lot of radioresources and should be implemented carefully for differentapplications. This paper presents an end-to-end concept forrealizing dynamic use of CE for connected vehicles in a resourceefficient way. The proposed framework has the potential ofimproving coverage by around 10dB for low data rate connectedvehicle applications, based on the 3GPP Long Term Evolution(LTE) standard.
I. I
NTRODUCTION T HE automotive industry is going through a fast-pacedevolution, with the connected vehicle as one of itsfocus. A connected vehicle refers to one which can connectover wireless networks to other devices and its surroundings.This may go from vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications, which are designed toincrease situation awareness and road safety, to vehicle-to-network (V2N) communication, which supports a wider areaover a cellular network. This paper is focused on V2N serviceswhich can be benefited from CE and how could this featurebe enabled on a vehicular modem.Cellular V2N communication enables a wide range ofservices in the vehicle, including Cooperative IntelligentTransport Systems (C-ITS), Advanced Driver Assistance Sys-tems (ADAS), dynamic high-definition maps, vehicle-centricOriginal Equipment Manufacturer (OEM) telematics, fleetmanagement, logistics services and infotainment. Many ofthese services require high data rates and low latency fortheir reliability to be guaranteed. In this case, CE is not anoption for optimization, due to the high amount of requiredradio resources. There are others however, whose data raterequirements are low and on which coverage and networkconnectivity should always be guaranteed. Here, CE could playa major role for optimization.These low data rate services include, among others, basicvehicle status updates to the OEM backend servers, remotevehicle lock or unlock and sharing of vehicle diagnostics,which are of major importance for fleet management and OEMlogistics. Specially with the evolution of mobility concepts,such as car sharing, the fleet manager must have knowledge on the location and status of each vehicle on its fleet. Thevehicles on the other side, should be able to connect to thebackend servers when needed, even in areas of poor coverage.Areas such as underground garages or rural zones are ofspecial interest, since their network coverage may be worsein comparison with urban zones or garages above ground.II. C
ONCEPT DESCRIPTION
A. 3GPP standardization of Coverage Enhancement
Third Generation Partnership Project (3GPP) has standard-ized Coverage Enhancement (CE) techniques for Internet-of-Things (IoT) devices in Release 13 and further evolved it inRelease 14 [1]. Coverage Enhancement is primarily achievedby repetition or retransmission techniques for applications thathave relaxed requirements on data rate and latency [1].3GPP has standardized two CE modes: i) CE mode A, sup-porting up to 32 subframe repetitions with estimated coveragegain of around 10 dB, ii) CE mode B, supporting up to 2048subframe repetitions. The repetitions are costly in terms ofspectrum use, which is a scarce resource, and therefore thispaper focuses on CE mode A and low data rate connectedvehicle applications.There is a wide range of LTE UE categories supportingdifferent capabilities and peak data rates [2]. CE mode A ismandatory for LTE Category-M1 UE, which is a very lowcomplexity UE suitable for very low cost modems targetinglow data rate connections (below 1Mbps peak rates). Theconnected vehicles typically employ higher category LTE UEs(capable of supporting tens or hundreds of Mbps) due to theneed for delivering high data rates. CE mode A is optionalfor high category LTE UEs and is beneficial only when thedevice is out of normal LTE coverage and is supporting oneor multiple low data rate applications.
B. Usage of Coverage Enhancement
One way to take advantage of the benefits of CE in avehicle would be to install an additional modem of LTECat-M1. This modem would then only be used for serviceswith low requirements on data rates and latency. However,the cost would be increased with dual modems and theadministrative effort would rise with dual Subscriber IdentityModules (SIMs).The preferred concept is rather to use a single modem ofhigh LTE UE category and allow the vehicle to dynamicallyenable the use of CE mode A in this modem. a r X i v : . [ c s . N I] J a n When the vehicle is outside normal coverage and only useservices with low requirements on data rates and latency, itcan enable the use of CE mode A in the modem of high LTEUE category.One example where CE may be used is when the vehicleis parked outside normal coverage and only uses “remoteservices” and “real time monitoring services” , see Fig. 1. Fig. 1. Example of usage of Coverage Enhancement
Another example where CE may be used is an emergencysituation outside normal coverage, where the vehicle may shutdown other services and only send emergency messages.
C. Enable and disable the use of CE mode A in the vehicle’smodem
Different UEs have different capabilities. The capabilitiescould be what frequency bands the UE supports, what radiofeatures it supports, to name a few. The UE informs themobile network of its capabilities when the UE attaches tothe mobile network. The mobile network takes the individualUE’s capabilities into account when it interacts with the UE.During the attach procedure, the base station (eNB) in themobile network uses the Radio Resource Control (RRC) mes-sage “UE Capability Enquiry” to retrieve the UE capabilitiesfrom the UE. The UE capabilities are stored in the servingbase station (eNB) but also in the Mobility Management Entity(MME), in order to keep the UE capabilities in the mobilenetwork when the UE moves between different base stations.Support for Coverage Enhancement is one such UE capa-bility. The UE may indicate its support for CE mode A inits UE capabilities by setting the parameter “ce-ModeA-r13”to “supported” [3]. In order to update the mobile networkwith new UE capabilities, the UE must start a new attachprocedure, see Fig. 2. Radio Resource Control (RRC) is thecontrol plane protocol between UE and eNB. Non-AccessStratum (NAS) is the control plane protocol between UE andMME. S1 Application Protocol (S1-AP) is the control planeprotocol between eNB and MME. Transmission of a small information packet to the vehicle in order totrigger some action, such as starting the heating or flashing the headlights. Periodic transmissions from electrical vehicles to a backend server in orderto track the current status of the battery. Fig. 2. 3GPP procedures to enable support of CE mode A
D. Restriction of use of Enhanced Coverage
The usage of Enhanced Coverage may require use ofextensive radio resources from the mobile network. 3GPPhas standardized a restriction that enables the mobile networkoperator to prevent specific subscribers from using EnhancedCoverage [4].The UE indicates its capability of support for restrictionof use of Enhanced Coverage in the attach procedure tothe MME. MME receives “Enhanced Coverage Restricted”parameter from the Home Subscriber Server (HSS). Thisparameter is retained as part of subscription data in the HSSand indicates whether the enhanced coverage functionality isrestricted (i.e. disallowed) for the UE or not.For a UE that indicates support for restriction of useof Enhanced Coverage, MME sends a “Enhanced CoverageRestricted” parameter to the UE in the “Attach Accept”message. The UE assumes Enhanced Coverage is allowedunless explicitly restricted by the mobile network.If the MME has sent the “Enhanced Coverage Restricted”parameter to the UE, the MME provides the “EnhancedCoverage Restricted” parameter to the eNB via S1 Applica-tion Protocol (S1-AP), see Fig. 3. Diameter is the name ofan authentication, authorization, and accounting protocol forcomputer networks. It is used between MME and HSS.
Fig. 3. 3GPP procedures to restrict usage of Enhanced Coverage
Since the “Enhanced Coverage Restricted” parameter is keptas part of subscription data in the HSS, the mobile networkoperator has a possibility to control and also charge for theCoverage Enhancement service for individual subscriptions.
E. Coverage Enhancement for roaming vehicles
Roaming is an important aspect of all automotive services.Vehicles may be manufactured in one country, sold in anothercountry, and driven in a third country.3GPP has standardized the roaming interface between HomePublic Land Mobile Network (H-PLMN) and Visited PublicLand Mobile Network (V-PLMN) for Coverage Enhancement.S6a is the roaming interface between MME in the visitednetwork and HSS in the home network.There are a few prerequisites for usage of Coverage En-hancement in the visited network:1) eNB and MME in the visited network must support CEmode A for UEs of high LTE UE category.2) MME in the visited network must get the subscriptiondata “Enhanced Coverage [Not] Allowed” from HSS inthe home network for roaming UEs.There might also be a need for an update of the commercialagreement between the home network operator and the visitednetwork operator regarding the roaming UE’s use of CoverageEnhancement in the visited network.
F. End-to-end concept for usage of Coverage Enhancement
Coverage Enhancement may be used when the vehicle isoutside normal coverage and only uses services with lowrequirements on data rates and latency. On the other hand,Coverage Enhancement shall not be used when the vehicleuses services with high requirements on data rates and latency.In order to use Coverage Enhancement in a proper way,an end-to-end concept for the system is needed. The systemconsists of three entities: vehicle, mobile network, and cloud.It is described in Fig. 4.
Fig. 4. End-to-end system using Coverage Enhancement
The services in the vehicle are connected via the mobilenetwork to the OEM backend servers in the cloud. Proprietaryapplication layer protocols are often used for these connec-tions.The vehicle has a Telematics Control Unit (TCU) with amodem and a connection manager.The modem is connected to the mobile network via the3GPP wireless interface (Uu). The modem establishes andmonitors the connection to the mobile network. It exposesits services via an Application Programming Interface (API).The API could be based on AT-commands [5], or other typesof interfaces.The connection manager in the TCU establishes and mon-itors the connection from the vehicle to the OEM backend servers. In addition, the connection manager keeps track ofwhether the UE support CE mode A and whether any servicein the vehicle needs data traffic with high requirements on datarates and latency.
G. End-to-end procedure to enable the use of CE mode A
The end-to-end procedure to enable the use of CE mode Aconsists of four steps, see Fig. 5.
Fig. 5. End-to-end procedure to enable the use of CE mode A
1) The connection manager detects that the vehicle isoutside normal coverage. This could be based on in-formation through the modem API.2) If one or more services in the vehicle need data trafficwith high requirements on data rates and latency, it isnot possible to use CE mode A and the procedure endshere.3) Otherwise, the connection manager updates the UEcapabilities in the modem through the modem API toindicate that CE mode A is supported.4) The connection manager starts an attach procedure inthe modem through modem API . H. End-to-end procedure to disable the use of CE mode A
The end-to-end procedure to disable the use of CE mode Aconsists of five steps, see Fig. 6.
Fig. 6. End-to-end procedure to disable the use of CE mode A If AT-commands are used as modem API, the “AT+CGATT=1” may beused [5].
1) A service notifies the connection manager that the ser-vice needs data traffic with high requirements on datarates and latency.2) If the UE capabilities in the modem has no support forCE mode A, data traffic with high requirements on datarates and latency is permitted and the procedure endshere.3) Otherwise, the connection manager starts a detach pro-cedure in the modem through modem API .4) The connection manager updates the UE capabilities inthe modem through the modem API to indicate that CEmode A is not supported.5) The connection manager starts an attach procedure inthe modem through modem API.III. C ONCLUSION
The paper presented an end-to-end concept for enabling adynamic use of CE for vehicles connected to cellular networksin a resource efficient manner. The proposed framework canbe realized with LTE based on 3GPP Release 14 specification,with a potential of around 10 dB coverage improvement forlow data rate applications. The framework presented in thepaper can be generalized to other types of moving IoT devices,for example, drones and aerial vehicles. The concept can alsobe extended to 5G IoT devices, when the CE functionality isfully standardized for 5G in the future 3GPP releases.R
EFERENCES[1] O. Liberg, M. Sundberg, E. Wang, J. Bergman, and J. Sachs,
CellularInternet of Things, Technologies, Standards and Performance , AcademicPress, 2018.[2] E. Dahlman, S. Parkvall, J. Skold,
4G LTE-Advanced Pro and The Roadto 5G , Academic Press, 2016.[3] 3GPP TS 36.306, Rel. 14, ”Evolved Universal Terrestrial Radio Access(E-UTRA); User Equipment (UE) radio access capabilities,” 2020.[4] 3GPP TS 23.401, Rel. 14, ”General Packet Radio Service (GPRS)enhancements for Evolved Universal Terrestrial Radio Access Network(E-UTRAN) access,” 2019.[5] 3GPP TS 27.007, Rel. 14, ”AT command set for User Equipment (UE),”2018.
Stefan Runeson ([email protected]) is a Senior Specialist Con-nected Vehicles at Ericsson Business Area Networks. He received his Master’sdegree in Engineering Physics from Lund University, Sweden. He joinedEricsson in 1997 to work with data communications in mobile phones.Since 2016, he has been working with the impact of connected vehicles onmobile networks. Since 2017, he is Ericsson’s delegate in the 5G AutomotiveAssociation (5GAA).
Ali Zaidi ([email protected]) is a strategic product manager for CellularIoT at Ericsson and also serves as the company’s head of IoT Competence.Since joining Ericsson in 2014, he has been working with technology andbusiness development of 4G and 5G radio access at Ericsson. He is currentlyresponsible for LTE-M, URLLC, Industrial IoT, vehicle-to-everything andlocal industrial networks. He holds a Ph.D. in telecommunications from KTHRoyal Institute of Technology, Stockholm. If AT-commands are used as modem API, the “AT+CGATT=0” may beused [5]