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Iran Journal of Computer Science

, Volume 1, Issue 3, pp 187–197 | Cite as

LTE-D2D for connected cars: a survey on radio resource management schemes

  • Mehdi Harounabadi
  • Andreas Mitschele-Thiel
  • Abbas Akkasi
Original Article
  • 315 Downloads

Abstract

Long-Term Evolution (LTE) is a promising technology to be applied for different applications due to its high penetration, high data rate, reliability, and QoS support. One of the emerging applications for LTE is vehicular networks. Vehicular networks can raise traffic safety, improve traffic management, or provide infotainment services for road users. LTE device-to-device (LTE-D2D) communication which was proposed by 3GPP working group is also a good candidate to be applied for vehicle-to-vehicle communications (V2V). The V2V communication for traffic safety has strict QoS requirements in terms of latency and reliability. LTE-D2D may provide high reliability and low latency for such a communication. However, an efficient radio resource management for V2V communications in an LTE network is required to support such requirements. In the current article, we introduce vehicular networks, their requirements, and existing technologies to be employed in them and specially look at LTE-D2D to connect cars. Most recent works in radio resource management for V2V communications in LTE are reviewed in this work.

Keywords

LTE 3gpp Vehicular networks Radio resource management V2V 

1 Introduction

Connecting transportation elements is an industrial trend since the last decade. It has been mentioned in the literature that cars will be one of the major connected devices in the near future [1]. The idea of “connected cars” has been proposed to connect all vehicles in roads together. Equipping cars with wireless interfaces makes data communication among them possible. They may exchange messages periodically or on demand to avoid accidents or improve the flow of traffic. Moreover, cars can connect to the mobile devices of pedestrians such as mobile phones, tablets, etc., to avoid probable accidents.

Intelligent transportation systems (ITS) are systems where utilize synergy of different technologies to improve different types of transportation systems. Vehicle-to-X (V2X) communications in the ITS are employed for safety of road users by avoiding accidents or for non-safety use cases such as infotainment. An ITS consists of four elements which are the On-Board Unit (OBU), i.e., the installed equipment in vehicles, vulnerable users such as pedestrians, bicycles, etc., Road Side Units (RSUs) which are the communication infrastructures in roads, and an ITS server as a centralized entity in the vehicular networks for management [1]. An OBU in a car generates message to broadcast in wireless medium. The messages may be sent to other vehicles or an RSU. The RSU in an ITS collects information about state of the road where it is installed or acts as a relay or router to forward messages between ITS entities. The ITS server may centrally collect information from RSUs, which make decisions to improve the safety in roads or traffic flows. Therefore, cars may need to communicate directly together or other elements in an ITS system.

This paper starts with an introduction of vehicular networks and use cases where the communication among vehicles can provide safety or improve the traffic flow in roads. Then, the existing communication technologies to connect cars and other road users are presented and drawbacks of different technologies are discussed. Vehicle-to-vehicle (V2V) communication in safety application has rigorous quality-of-service (QoS) requirements in terms of latency and reliability. LTE-D2D is explained as a promising technology to provide QoS requirements of the V2V communication. The radio resource management is an influencing factor in the performance of LTE-D2D communication which defines the radio resources for each pair of devices to communicate. Radio resource allocation for V2V communication may be done in a centralized or distributed way. Moreover, spectrum sharing between legacy users (traditional cellular users) and d2d users is an important challenge which should be addressed in radio resource management schemes. Conventional radio resource allocation schemes for d2d communication in LTE networks may not fit for vehicular networks due to the high mobility of vehicles and strict QoS requirements of safety applications. Existing radio resource management schemes are surveyed in this paper which allocate resources in highly dynamic vehicular networks for direct V2V communications. We differentiate the existing work based on where the resources are allocated, how the spectrum is shared between legacy users and V2V communication, and how the challenge of high mobility of vehicles is overcome.

The paper is organized as follows: Sect. 2 introduces QoS requirements for safety applications in vehicular networks. Use cases for vehicular networks are described in Sect. 3. The existing communication technologies to connect cars together is addressed in Sect. 4. Section 5 reviews the existing radio resource management schemes for V2V communication in an LTE network. Sections 6 and 7 are open problems and summary of the paper.

2 Communication for traffic safety and QoS requirements

The European Telecommunications Standards Institute (ETSI) defined two types of messages in vehicular networks to increase safety of road users [2]. The messages are as follows:
  • Cooperative Awareness Messages (CAM): It is a periodic message that is generated by a vehicle to inform all other road users in the proximity about its status. The status of a vehicle is set of information such as its geographical position, speed, device certificate, etc. The CAM message may have a variable size but is generated with a static arrival rate. The rate of CAM messages has been defined 10 Hz and may tolerate 100 ms end-to-end latency.

  • Decentralized Environmental Notification Message (DENM): It is a warning and mostly event triggered message that informs road users about the state of roads such as accidents or hazards. Either the DENM message can be generated periodically if it is needed.

The V2V communication which is part of Advanced Driver Assistance Services (ADAS) has very strict QoS requirements such as maximum tolerable delay and reliability for safety applications.

The European project METIS [3] has mentioned that traffic safety applications have a 5 ms latency requirement for end-to-end delay of messages that are generated periodically with a rate of 10 Hz and the maximum payload size of 1600 bytes. Moreover, the network should provide a transmission reliability of more than 99.999% for such applications.

3 Use cases for vehicular networks

Several use cases for ITS were mentioned by ETSI [4] which can be classified into two main categories:
  • Cooperative road safety use cases;

  • Traffic efficiency use cases.

Some example use cases from each category are mentioned in the next sections and their requirements are studied.

3.1 Cooperative road safety

Vehicles increase road safety by message exchange among each other. The example use cases are as follows.

Emergency electronic brakelights A hard braking vehicle informs its followers by a signal. This causes an emergency electronic brake light in the following vehicles be switched on. By informing other vehicles about sudden slowdown in a vehicle, the risk of collision is reduced. In this case, an event-driven time-limited periodic broadcast message is required. Minimum frequency of periodic message is 10 Hz for this use case and maximum tolerable latency for messages is 100 ms.

Out of normal condition warning A vehicle which detects abnormal functionality in its features such as steering, braking, etc. informs other vehicles about this abnormal condition. It needs a limited periodic message broadcasting triggered by an out of normal condition. Minimum frequency of the periodic message is 1 Hz and its maximum tolerable delay is 100 ms.

Emergency vehicle warning In this case, an emergency vehicle signals its presence. Therefore, other vehicles in the road may give the way faster. It also reduces the risk of collision between vehicles in the road and the emergency vehicle. The message is broadcasted by the emergency vehicle periodically depending on its state. The frequency of message broadcast by the emergency vehicle is 10 Hz and latency of message reception should be less than 100 ms.

Slow vehicle warning Slow vehicles such as trucks broadcast messages to signal other road users about their presence. This use case increases road safety by reducing the probability of collision between slow vehicles and other vehicles, and improves the flow of traffic by encouraging cars to take other lanes if it is possible. The messages should be sent by a slow vehicle with the frequency of 2 Hz and the message latency must be less than 100 ms.

Motorcycle warning It is to warn drivers about arriving motorcycle to avoid collision. It is beneficial when there is a limitation on visibility of drivers. A motorcycle broadcasts messages to indicate its presence. Maximum latency of the broadcast message in this case should be less than 100 ms and the motorcycle must broadcast the message with minimum 2 Hz frequency.

Lane change assistance This use case provides assistance for drivers to change their lane in a road by informing them about cars in the neighboring lanes. A vehicle which is changing its lane must broadcast cooperative awareness messages indicating its action. For this use case, the accuracy of vehicles positioning should be at least 2 m and the frequency of V2V messages is 10 Hz. The tolerable latency is also 100 ms for this use case.

There are more use cases for road safety. We list some of them here, but readers are referred to [4] to find the definition and requirements of each use case. The use cases are as follows:
  • vulnerable road user warning;

  • wrong way driving warning;

  • stationary vehicle warning;

  • traffic condition warning;

  • overtaking vehicle warning;

  • pre-crash sensing warning;

  • cooperative merging assistance.

3.2 Traffic efficiency

Another category of use cases which was mentioned by ETSI was to improve the traffic efficiency of roads. In this class of use cases, vehicles and the network infrastructure cooperate by message exchange to improve the flow of traffic. Different types of communication such as V2V, vehicle-to-infrastructure (V2I), and infrastructure-to-vehicle (I2V) are required for this class of use cases. For clarity, we introduce an example use case from this category and list other use cases.

Intersection management is an example use case for traffic efficiency. In this use case, traffic lights act as RSU and send vehicles periodically traffic management advises. The frequency of such periodic messages from a traffic light to vehicles is 1 Hz. The maximum message latency must be less than 500 ms and the positioning accuracy of vehicles should be better than 5 m.

The list for some of possible use cases in this category is as follows and can be found in [4] with more details:
  • regulatory/contextual speed limits;

  • traffic light optimal speed advisory;

  • traffic information and recommended itinerary;

  • enhanced route guidance and navigation;

  • limited access warning, detour notification.

4 Communication technologies for vehicular networks

In this section, we introduce the existing communication technologies for vehicular networks. Some of the most important features of each technology are mentioned, and finally, they are compared. The communication technologies are Dedicated Short-Range Communication (DSRC) and Long-Term Evolution (LTE). We describe the motivation to apply LTE for vehicular networks and its benefits.

4.1 IEEE 802.11p standard

Dedicated Short-Range Communication (DSRC) is the proposed solution for vehicular networks, which works based on the definitions of physical and data link layers in the IEEE 802.11p standard. DSRC works on 5.9 GHz licensed frequency band [5, 6]. The frequency band is divided to one Control CHannel (CCH) and six Service Channels (SCH). The MAC layer of 802.11p provides a contention-based medium access, i.e., Enhanced Distributed Channel Access (EDCA). EDCA is based on the IEEE 802.11e definition and considers packets priorities by shorter Arbitration Inter Frame Space (AIFS) for high priority packets. Four different channels’ access class was defined in EDCA which are AC0, AC1, AC2, and AC3 from lowest to highest channel access priorities, respectively. Figure 1 illustrates channel access classes in EDCA, the length of AIFS, and the size of Contention Window (CW) for each class. The higher priority of a classes leads to the shorter waiting time before accessing the channel.
Fig. 1

Different channel access classes in EDCA

The physical layer in 802.11p is based on Orthogonal Frequency Decision Multiple access (OFDM) same as the IEEE 802.11a [7]. Vehicles communicate in ad hoc mode employing 802.11p when they are in the proximity of each other.

Due to the back-off mechanism in nodes using EDCA, 802.11p cannot guarantee QoS in terms of latency. The contention-based medium access is not scalable and increasing number of vehicles may impose unbounded delays to the safety messages. The random access schemes also suffer from high number of collisions in dense networks which is far from the reliability requirements of safety applications. Furthermore, the short radio transmission range in 802.11p standard causes frequent disconnections in highly mobile networks such as vehicular networks. Messages may experience long buffering delay caused by the lack of connectivity in the network. Based on the 5G definitions, a maximum 0.125 ms buffering delay is tolerable for a 1 ms end-to-end delay requirement of messages [8].

Some example applications of 802.11p short-range communication in vehicular networks for traffic safety are mentioned in [9, 10, 11] and are as follows:
  • forward collision warning;

  • emergency electronic brake lights;

  • blind spot warning;

  • intersection collision warning.

4.2 Long-term evolution

The Long-Term Evolution (LTE) is an alternative for 802.11p based DSRC. LTE is the recent technology for cellular networks that provides high data rate, long communication range, low latency, and high reliability for mobile user equipment (UE). High penetration of this technology makes it a good candidate to be utilized it in vehicular networks. It can support mobility of UEs up to 300 kmph which fits to connect cars. Support of one-to-many communications by LTE downlink is another beneficiary feature that can be employed for vehicular networks [12]. It is supported in LTE using Single-Cell Point To Multi-point (SC-PTM) or Multimedia Broadcast Multicast Services (MBMS) from multiple cells.

In ETSI [13], a back-end server was considered to support geocasting. Geocasting was proposed to distribute information among concerned vehicles. To identify concerned vehicles, the back-end server needs to know the coordinates of geographic regions, the position of cars, and their IP address. Each time when a vehicle enters a region, the server informs it about its region. Through multicast, the server distributes data safety messages to concerned vehicles. Considering the requirements that has been mentioned by ETSI, LTE is able to support such a geocast service.

Third-Generation Partnership Project (3GPP) specified three types of communication in a vehicular network that employs LTE. The communication types are as follows [14]:
  • Vehicle-to-Vehicle (V2V): It is LTE-D2D-based communication among vehicles.

  • Vehicle-to-Pedestrian (V2P): It is also LTE-D2D-based communication between vehicles and vulnerable road users such as pedestrians, motorcyclists, bikers, and so on.

  • Vehicle-to-Network/Infrastructure (V2N/I): It is the LTE-based communication between UEs (vehicles or other road users) and RSU.

In an LTE-based vehicular network, the RSU is implemented in a eNodeB or any stationary UE in the cellular network which collects topology and road information from vehicles, vulnerable road users and installed sensors, radars, cameras, and etc. The ITS server could be outside of LTE network or in EPC to reduce latency [15].
Fig. 2

Different types of communication in an LTE-based vehicular network

Figure 2 illustrates different communication types in vehicular networks. V2V communication between cars is based on LTE-D2D communication that will be discussed in next sections. V2P is the communication between vehicles and devices (UEs) that are carried by pedestrians (vulnerable road users). V2N is also a logical communication between a vehicle and a cloud service in the Internet that takes place through a V2I communication.

4.2.1 Challenges with conventional LTE in vehicular networks

LTE is a cellular network technology where every decision is made in a central entity. There is no contention in channel access and resources are allocated by a base station (eNodeB) in each cell. However, replacing LTE instead of 802.11p which works in ad hoc mode for V2V communications imposes high traffic loads to the core of LTE network. In an LTE network, all messages are transmitted through Evolved Packet Core (EPC) as the core of network. Having periodic CAM messages with high frequency of generation, the core of the LTE network will be highly loaded even if the vehicles are located in vicinity of each other. Moreover, the round-trip time of messages may increase due to the multi-hop delivery of messages via EPC [16]. Doppler shift, frequency errors, and inter-carrier interference due to the very high relative speed of vehicles in highways are the existing challenges with the current PHY design of the LTE [1]. There are some efforts in the literature to apply LTE for vehicular networks [17, 18, 19].

4.2.2 LTE-D2D for V2V communication

3GPP proposed in its release 12 and 13 a direct Device-to-Device (D2D) communication for mobile devices that are located geographically in proximity of each other. The D2D communication in 3GPP was considered as Proximity Services (ProSe) [20], and its feasibility and architecture were studied in [21, 22], respectively.

In release 14, it was mentioned that D2D communication can be applied for V2V communication in vehicular networks. Localized nature of vehicular networks, which is similar to D2D communication in LTE (ProSe), provides some gains such as proximity gain, reuse gain, and hop gain [23].

The resources for a D2D communication can be orthogonal or non-orthogonal to the resources of Cellular Users (CUs). Allocation of orthogonal resources to D2D and CU communications is called overlay D2D which dedicated resources are allocated to D2D users by eNodeB. In this case, there will be no resource reuse between CUs and D2D users. D2D overlay provides higher reliability for a D2D communication but decreases the spectrum utilization [24].

The other case is underlay D2D which D2D communications take place in the same resources that are utilized by CUs keeping the interference level to the cellular network below a certain threshold. Figure 3 shows overlay and underlay D2D communication. In underlay D2D, a radio resource is utilized by a D2D and Cellular Communication (CC) at the same time. However, D2D and CC take place on different times in a radio resource. D2D communication may occur either in uplink or downlink of LTE. In downlink, a D2D communication may cause interference to CUs if the distance between them is not kept enough. In the uplink case, the D2D communication may cause interference only in the eNodeB. Figure 4 demonstrates D2D communication on uplink and downlink and interference of each case on cellular network.
Fig. 3

Overlay and underlay D2D communication

Fig. 4

D2D communication on uplink and downlink and its imposed interference

D2D communications in uplink can provide a higher radio transmission range, while the eNodeB tolerates a higher level of interference. Therefore, devices may transmit with a higher transmission power. With the higher radio transmission ranges of V2V communication, vehicles can transmit to further distances which gives the drivers (or cars in case of driver-less cars) more time for reaction in case of accidents. An experiment in [25] showed that a D2D communication may have the radio transmission range of 3–9 m in downlink and 3–38 m in uplink.

4.2.3 Benefits of LTE compared with DSRC

Considering all features in LTE including D2D communication, we mention some benefits of LTE for V2X communication over DSRC to conclude this section. The advantages of LTE over DSCR are as follows [1]:
  • Better coverage Due to the higher sensitivity of LTE receiver comparing with DSRC, the performance of LTE is much better when the received signal is weak. This cause better coverage of LTE network than short-range communication in the IEEE 802.11p physical layer. The coding scheme in physical layer of LTE is turbo coding which provides better gain than the convolutional coding applied in DSRC. Moreover, the single-cell and multi-cell multicast in LTE provides lager V2X coverage.

  • Cost effectiveness High penetration of LTE network and its established infrastructure such as eNodeBs and core network makes it an efficient solution to be applied in vehicular networks to provide connectivity for cars and other road users without any extra cost. As mentioned, the RSU can be implemented in eNodeBs and the ITS server can be placed in the EPC of LTE network.

  • Higher multiplexing LTE supports the transmission of multiple UEs at the same time by frequency domain multiplexing. This is not the case for DSRC, while only one vehicle may access to a channel at a specific time in the IEEE 802.11p. By frequency domain multiplexing in LTE, communication in high density of vehicles is supported.

  • Scalability By orthogonal resource allocation to UEs, the collision between transmissions of vehicles is avoided in LTE. For this reason, LTE can support existence of a large number of vehicles in the network. The scalability of the communication network for connectivity of vehicles is essential, while congested scenarios are frequent. The random access in DSRC does not scale well and causes message collisions in the network. While a high reliability is the requirement for safety applications in vehicular networks, DSRC fails to provide such a requirement in highly congested networks.

5 Radio resource management for V2V communication in LTE

Four physical (PHY) layer channels are defined in [14, 26] for a D2D link which is called Side Link (SL). The channels are as follows:
  • Physical Side link Broadcast Channel (PSBCH): It is a broadcast channel for side link and is utilized for system information and synchronization.

  • Physical Side link Control Channel (PSCCH): It is the side link control channel which carries control information about the resources for the side link and transmission parameters such as transmission power.

  • Physical Side link Discovery Channel (PSDCH): It is the side link channel for UEs direct discovery. The discovery procedure takes place to find the devices in proximity for D2D communication.

  • Physical Side link Shared Channel (PSSCH): It is the side link shared channel which is used by UEs for user plane data transmissions.

Two modes have been defined by 3GPP standard release 14 for resource allocation or selection in vehicular networks where LTE side link is employed for V2X communication. They are mode 3 and mode 4 which were defined by the standard as follows.
  • Mode 3 In this mode, the radio resources are allocated by eNodeB and vehicles must be in coverage of the cellular network. eNodeB may receive requests from vehicle UEs and allocate resources. This type of resource allocation is called dynamic scheduling. An eNodeB may also reserve a set of resources for a vehicle for its periodic transmissions. In this case, the eNodeB defines how long resources will be reserved for the vehicle.

  • Mode 4 This mode is the autonomous resource selection by vehicular UEs. A vehicle autonomously decides for resources within the control (resources) pool for the transmission of the control messages and resources within the transmission (resources) pool for data messages. In this mode, a vehicle may be in coverage or out of coverage of the cellular network. If a vehicle is in coverage, the eNodeB can define the resource pools and communication parameters for side link communication of vehicles. If vehicles are out of coverage, pre-configured parameters and resource pools can be used by vehicles based on their geographical location. A sensing base resource selection was defined in the standard which a vehicle senses the medium for a period of time before selection of resource blocks. A distributed congestion control mechanism is also applied which calculates channel busy ratio and channel occupancy ratio. Then, a vehicle reserves resource for a random interval and sends a reservation messages using Side link Control Information (SCI). The reservation message is also called Scheduling Assignment (SA). Using SA, other vehicles which sense and listen to medium find out the list of busy resources and avoid selection of those resources. To increase the reliability, a vehicle may send a data message more than once in this mode.

Radio Resource Management (RRM) for D2D communication is the decision procedure about radio resources [resource blocks (RBs)] that can be used by any D2D pair and their radio transmission parameters such as transmission power. An RB is the smallest radio resource unit that can be allocated to users. Figure 5 presents RBs within a radio frame.
Fig. 5

Illustration of a radio frame and resource blocks

Most of the existing works in the literature are centralized approaches which an eNodeB decides about RBs for D2D pairs in the cellular network. There are a few works that RRM is done in a decentralized way or only by assistance of eNodeB.

Authors in [27] did an extensive survey on LTE-D2D communication in the literature and divided the existing work based on underlay and overlay D2D approaches. Most of the existing works in LTE-D2D context belong to underlay D2D communication. By resource reuse and interference management for underlaying D2D communications, spectrum efficiency is increased. Therefore, underlay D2D communication increases system capacity. However, the interference to cellular communication and among D2D communications must be limited. Authors in [28, 29, 30, 31, 32] increased spectrum efficiency in an underlay D2D network. Others enhanced power efficiency for underlay D2D communications in a cellular network [33, 34, 35].

For the overlay D2D where the radio resource of cellular communication and D2D communications is orthogonal, efforts was done also to improve spectrum and energy efficiency [36, 37, 38].

Applying the existing D2D RRM schemes without any modification is not efficient for some reasons. We introduce the reasons as follows:
  1. 1.

    Majority of the D2D RRM schemes rely on the full Channel State Information (CSI) in the network that is collected and sent by UEs. For V2V communication where cars are highly mobile, collecting CSI information is infeasible and costly.

     
  2. 2.

    In most of the works, the objective of RRM is to maximize the sum rate for CUs and D2D communications which have a lower priority.

     
  3. 3.

    Signal Interference–Noise Ratio (SINR) is measured as a reference for QoS which, for V2V communication with strict QoS requirements, is not a feasible metric.

     

5.1 Centralized resource allocation schemes

In this paper, we survey the existing work in LTE-V2V RRM schemes that have been proposed for highly mobile vehicular networks with strict QoS requirements. In this section, we look first at centralized schemes where the allocation of radio resources is done in an eNodeB.

In [39], a Location-Dependent Resource Allocation Scheme (LDRAS) was proposed which aims to reduce signaling and guarantees level of interference to CUs and V2V communications with spatial reuse in uplink. They divided each cell to z disjoint zones and in each zone set of RBs which can be used by V2V communications in a dedicated way. The same set is reused by CUs that are far enough from the zone to limit the interference to V2V communication. The zones and set of RBs for each zone are predefined and the eNodeB does not need full CSI information to assign an RB to a V2V pair. By entering to a new zone, the vehicle sends request to the eNodeB and the latter assigns an RB from the set of RBs in that zone.

The number of zone is not optimized and the placement of zones is by intuition. Figure 6 shows zones and set of RBs that can be utilized by CUs and D2D communication which is V2V in case of vehicular networks.
Fig. 6

Location-dependent resource allocation

In [40], authors improved the work in [39] by optimizing transmission power of V2V communications and maximizing number of zones. They applied sequential quadratic programming to minimize the transmission power of V2V communications. For maximizing number of zone, they considered the outage probability in both cellular and V2V communication to fulfill their QoS requirements. Zone formation was done by assessing the similarity of different location and a hierarchical clustering. They employed sequential graph coloring to determine set of RBs that can be utilized by V2V communications in each zone. By definition of zones and RBs in each, they decreased number of necessary channel measurements. Moreover, they increased number of successful transmission by maximizing number of reuses (by maximizing number of zones).

Authors in [41] proposed a centralized resource allocation scheme and two distributed power control. The resources are reused based on the geographical information of CUs and V2V communications. Three zones are defined for each resource based on the amount of interference that the CU communication can impose to V2V communications. The CUs are selected from zones to minimize their interference to V2V communications. Besides, they proposed two decentralized power control mechanisms which each vehicle decides itself about its transmission power. eNodeB only selects the power control mechanism for each pair and informs them about the mechanism which they must apply. The V2V pair makes the detailed decision about the transmission power independently. The power control mechanisms are geo-based and uniform power control schemes. They mentioned that performance of each power control mechanism depends on the requirements in the network, while the geo-based scheme provides better sum rate, whereas the uniform scheme leads to more balanced data rate even if the V2V communications take place in the vicinity of eNodeB.

An optimization problem has been solved in [23] which transforms QoS requirements of V2V communications to the constraints of the problem. They solve the problem utilizing slowly changing CSI information instead instantaneous CSI information. Their objective for optimization was to maximize CUs sum rate with proportional bandwidth fairness under the QoS constraints of V2V communications. Separate resOurce bLock and powEr allocatioN (SOLEN) algorithm in this work allocates RBs to CUs and V2V communications applying the maximum weight matching and then transforms power control problem to a convex optimization problem and solves it by dual decomposition method.

The work in [42] also considers slowly changing long-term fading information instead of individual CSI information. The novelty of their work is to consider queuing delay and reliability of V2V communications in resource allocation. A joint resource allocation and power control has been proposed that exploits geographical information and queuing dynamics. An optimization problem was formulated that aims to maximize sum rate and minimum ergodic capacity of V2I communications ensuring the reliability in V2V communications. They showed that their approach is robust to the fast channel variations in a vehicular network.
Table 1

Comparison of the existing RRM schemes in vehicular networks

References

Underlay

Overlay

Hybrid (DSRC-LTE)

Zone-based

CSI-based

[39]

\(\checkmark \)

\(\checkmark \)

[40]

\(\checkmark \)

\(\checkmark \)

[41]

\(\checkmark \)

\(\checkmark \)

[23]

\(\checkmark \)

Long-term CSI

[42]

\(\checkmark \)

Long-term CSI

[43]

\(\checkmark \)

\(\checkmark \)

[44]

\(\checkmark \)

\(\checkmark \)

[45]

\(\checkmark \)

\(\checkmark \)

[46]

\(\checkmark \)

\(\checkmark \)

\(\checkmark \)

In [43], an RSU-assisted approach was proposed which each RSU groups vehicles into virtual zones. The zone formation in a RSU is based on topology of the vehicles, and their traffic and latency requirements. Set of RBs are allocated to each zone by the RSU. Moreover, a power control mechanism was proposed for V2V communications in each zone which V2V pairs minimize their transmission power based on the constraints on queuing delay and reliability applying Lyapunov stochastic optimization method. Their results illustrated a significant reduction in queuing delay and improvements in the reliability of V2V communications.

The work in [44] maximizes number of concurrent reuses for an RB by multiple V2V communications instead of maximizing the sum rate. The RB reuse decision is done in a centralized way in an eNodeB. They employed spectral radius estimation theory to increase spectral efficiency by an RB sharing algorithm. They showed that their approach can take care of high-density scenarios in vehicular networks.

An V2V approach was presented in [45] for a hybrid system. The hybrid system consists of both LTE-D2D and IEEE 802.11p contention-based V2V communication. The resource allocation for LTE-D2D-based communication is done in eNodeB optimally. The proposed approach is different from the existing overlay resource allocation schemes. In the traditional schemes, the D2D pair is given to the eNodB and it only decides about the RBs. However, by a request from a vehicle in the proposed approach, the eNodeB jointly pairs the vehicle to another vehicle and allocates the RBs to the pair considering a guaranteed SINR for all links. In this way, the total delay is minimized by decreasing number of vehicles that competing for medium access in 802.11p-based V2V communication. Figure 7 shows a hybrid network where eNodeB pairs vehicles for D2D communication and allocates resources to them. In the figure, node A sends message to node D. Messages are forwarded through nodes B and C towards node D employing ad hoc mode of IEEE 802.11p. eNodeB pairs A and D and allocates resources for their communication. In this case, the number of hops is decreases. Moreover, nodes B and C do not need to access the channel to forward messages.
Fig. 7

Hybrid IEEE 802.11p and LTE network

Table 1 compares the existing work on centralized RRM schemes in vehicular networks.

5.2 Distributed resource selection schemes

In this section, distributed resource selection schemes, where each vehicle decides autonomously for the radio resources to utilize for V2X communication, are reviewed.

Unlike [39, 40] which resources were allocated statically to zones, a distributed self-organizing mechanism was proposed in [46]. They exploited geographical and traffic pattern information of vehicles and came up with a dynamic zone formation algorithm. In this work, vehicles optimize their resource allocation considering CSI information, network topology, and reliability requirements. They modeled the problem as a matching game problem which players are RBs and V2V pairs. They showed that the distributed approach can increase SINR for V2V communications and number of satisfied users in terms of QoS. Figure 8 illustrates the dynamic zone formation of vehicles and intra-zone interference of V2V communications that utilize the same RBs.

In [47], a two-stage distributed resource selection for V2V communication in urban scenarios was proposed. In the first stage, resources are partitioned based on vehicles heading direction, i.e., the cars moving in the same direction have same set of resources (a resource pool) to select and there will be no collision in junctions due to the different mobility direction of cars. On the second stage, a sensing-based approach is applied to avoid the collision of V2V communication between cars which travel in parallel in a street and interference of resource pools. Each vehicle prioritizes resource blocks based on their level of interference and selects resources based on their priorities.

Another approach in distributed resource selection is Semi-Persistent Scheduling (SPS) with Collision Avoidance (CA). In the SPS with CA, vehicles broadcast SA messages to reserve resource blocks for the next data transmission phase. However, collision of SAs will lead in collision in multiple data frames. To increase the reliability, the [48] proposed piggybacking of SA information in some data frames in addition to reservation phase to inform neighboring vehicles about reserved data frames.
Fig. 8

Dynamic zone-based approach

6 Open problems

Many of the proposed solutions are centralized which an eNodeB allocates resources for both V2V pairs and CUs. Radio resource allocation schemes which can make decisions in a decentralized and self-organized way to support applications with strict QoS requirements and adapt themselves to the dynamics of vehicular networks are missing. In terms of spectrum sharing, the existing radio resource management schemes share radio resources between legacy cellular users and V2V pair. The allocated resources for V2V or V2I communications are orthogonal. To increase the spectral efficiency, radio resources can be shared among V2V pairs or V2V and V2I communication. This type of spectrum sharing needs more complex radio management schemes, while, due to the emerged interference between V2V communications or between V2V and V2I communication, achieving QoS requirements of safety application will be more challenging.

7 Summary

Random medium access scheme in the IEEE 802.11p standard does not scale to the dense vehicular networks. A number of packet collisions are increased and the medium access scheme fails to support delay and reliability requirements of safety applications. LTE is a technology for cellular networks and an alternative for the IEEE 802.11p. However, still, the traditional LTE may face some challenges in dense networks and provide long-round-trip latency due to its core network. Direct communication between mobile device, i.e., LTE-D2D, is an add-on feature which have many applications. LTE-D2D is an appropriate candidate for V2X communications in traffic safety applications. LTE-D2D may provide high reliability and low latency in vehicular networks. However, the radio resource management scheme for allocation of resource blocks for both CUs and V2V pairs highly impacts on the performance of communication in a cellular network. In this paper, recent radio management schemes in the scope of vehicular networks were reviewed.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Integrated Communication Systems GroupIlmenau University of TechnologyIlmenauGermany
  2. 2.Text Analysis and Knowledge Engineering LabUniversity of ZagrebZagrebCroatia

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