D2D-communication is a novel concept for data transmission between mobile users in vicinity. Besides the potential of providing very high data rate and low ...
On D2D-Communication for Resource Efficient Data Transmission of Delay-Critical Services Jens Mueckenheim, Hochschule Merseburg André Puschmann, Dariush M. Soleymani, Elke Roth-Mandutz, Abubaker M. Waswa, Andreas Mitschele-Thiel, Technische Universität Ilmenau
Abstract D2D-communication is a novel concept for data transmission between mobile users in vicinity. Besides the potential of providing very high data rate and low power communication, this technique can also be used to increase the capacity of a mobile network by means of, e. g., offloading some of the traffic from the network. The present paper discusses the application of D2D-communication to delay critical services, which can hardly be supported over infrastructure based networks. Within this context, we options and requirements for resource control are indicated and some ways for improving especially the sharing of the resources between cellular and D2D-users are shown. Moreover, first insights into the simulation of D2D-communication using open-source software and some early results are provided.
1
Introduction
Today, modern cellular communication systems, such as Long Term Evolution (LTE), already support a wide variety of services including human generated traffic as well as machine type traffic (M2M). While for human based communication (e.g. web browsing or video streaming) the provided throughput is the most important quality metric, for M2M-communication other parameters have more relevance. In particular, for delay critical M2Mapplications (e.g., safety critical communication between cars or machines) there is the requirement to provide a low latency of the overall communication. In consequence, for next generation mobile communication systems an overall delay down to 1 ms has been defined as a requirement for such services [1]. So far, cellular networks are mostly based on mobile devices exclusively communicating with a base-station. This principle allows the network operator best control over the assigned resources to provide an optimal trade-off between full coverage for mobile communication and maximum capacity of the network. Because the traffic is always routed via the network infrastructure this might impose some latency for the data transmission. While for many services the delay of existing LTE networks is fully acceptable, it might be too large for the delay critical services. One promising way for future mobile networks is the application of direct communication between devices when in vicinity (D2D) [2]. Due to the (usually) short distances between such devices, very high data rates with low transmit power could be achieved by using this approach. Even more, D2D-communication provides the possibility for offloading some of the traffic from the cellular network, hence, increasing the overall spectrum efficiency of the system. On the other side, D2D-communication imposes new challenges especially for controlling the network operation. With D2D the communication principle
is now moving towards a de-centralized structure, and, hence, the current structure of resource control needs to be modified. Moreover, new interference situations may arise due to the sharing of the same radio resources between cellular and D2D users. In 3GPP Release-12 the support of D2D-communication has been added to LTE under the proximity based service feature [3],[4]. The initial specification supports a basic service mainly for public safety communication with the target of less impact onto the existing LTE standard. Following Release-12, 3GPP is planning to support additional D2D-services, e.g. M2M-data, to benefit from the advantages of direct communication. Because the communication is directly performed between the peer D2D-users, additional delays imposed by traveling through other network elements (like in conventional cellular communication) are avoided. In consequence, D2D-communication can be also seen as a good way to provide delay critical services. One exposed candidate is vehicular communication (V2X), where the use case of adjacent vehicular users communicating with each other using small messages with low latency is perfectly matching with the D2D environment [5]. In order to exploit the short latency, special resource allocation methods are required. For example, permanent resource allocation might be used to avoid additional delays imposed by dynamic scheduling methods. However, resources maybe wasted in case, the service is not consuming it due to inactivity or small message size. In consequence, a new approach is needed to efficiently support delay critical services in this environment. In the present paper we will examine the application of D2D-communication to support delay critical services and present first results of the performance impacts on other cellular communication services. The paper is organized as follows: In Section 2 we discuss the required functionality to support D2D-communication for delay-critical services within the existing cellular communication networks. Then, the problem of efficient resource manage-
ment between cellular and D2D-users is described and performance results of two promising resource sharing methods are provided in Section 3. Section 4 illustrates the adaptation of open source network simulations to D2D communication and provides some first simulation results. Finally, conclusions of the current status are drawn and an outlook on future work is given in Section 5.
2
D2D-Communication for DelayCritical Services
Until now, the principle of mobile cellular networks is based on mobile stations (UE in LTE) only communicating with base-stations (eNodeB or eNB in LTE). With this principle, the resource control and management is mainly done by the network, where in LTE most functionality regarding radio resource management resides in the eNB. D2D-communication changes this paradigm. This imposes new challenges on the design of radio resource management (RRM) algorithms. Figure 1 shows the system model that is used for D2D communication. Besides the uplink communication from a legacy cellular user (C-UE), a communication can now be also established directly between D2D users (D2DUE). For this sidelink communication the uplink transmission format is used. The management of the consumed radio resources is done in the eNB, which allocates also eNB grants for the resources of the sidelink communication.
Uplink data
To make the most efficient usage of D2D-communication the transmission mode needs to be thoroughly selected. For example, the best effects of sidelink communication are given at low distance between candidate D2D-UEs. However, with increasing distance the advantages of a sidelink communication might be outweighed by the impact to other C-UE, such as increased interference. In this case it would be more beneficial to perform a cellular communication for the affected UEs. On the other hand, the expected delay for D2D is significantly lower compared to the infrastructure based mode. In order to handle the various service requirements, different criteria can be used for selecting the communication mode, such as channel quality, interference or load of the eNB [6]. Another important aspect for controlling the D2Dcommunication is the coverage of the underlying cellular network. From the basic transmission model as shown in Figure 1 the following scenarios have been defined by 3GPP [4]: 1A) Out-of-Coverage: There is no cellular connection to either of the D2D-UEs. In this case, the UEs are using pre-defined grants for D2D-communication. 1B) Partial Coverage: Only one of the D2D-UEs has a connection to the cellular network. There is a mix between pre-defined and eNB provided resource grants. 1C) In Coverage, Single Cell: Both D2D-UEs are within the coverage of the same cell. Here, one eNB can fully control the resources for D2D communication. 1D) In Coverage, Multiple Cells: Each D2D-UE is within the coverage of a different cell. For efficient resource allocation rules need to be defined for handling multiple eNB grants. In principle, D2D-communication can also be used to enhance the coverage or capacity of mobile communication by means of D2D-relaying or clustering of D2D-UEs with same communication channel. In these cases even more scenarios can be defined [6].
eNB grant
Sidelink data
C-UE
phase based on the transmission grants that have been provided for sidelink communication.
D2D-UERx
D2D-UETx
For the support of delay-critical services there are two components to be considered when employing D2Dcommunication: ·
Figure 1: System Model for D2D-communication
From a D2D-UE perspective, the communication can be divided into two phases [4]: ·
·
During the Discovery phase the D2D-UE needs to find out, whether any other candidate UE for sidelink communication is in its vicinity. If this is not the case, communication can only be done via the cellular network. If a suitable candidate UE has been found the D2DUE can transmit the data during the Communication
·
In cellular communication, there is a significant impact to the delay as every message has to travel through multiple hops, where each hop represents a network element. The usage of direct communication provides significant reduction to only wireless transmission delay plus some latency due to signal processing on the transmitting and receiving side. Modern cellular communication systems such as LTE are purely based on dynamic scheduling to allocate the resources. Because of the request-grant scheme imposed by this method, there could be also an additional contribution to the latency. The fallback to permanent resource allocation, as applied in earlier cellular systems, can avoid this latency impact. However, resources are wasted when not used for D2Dcommunication.
In order to fulfill the given use case for public safety communication at the beginning, a simple resource allocation scheme has been specified in 3GPP Release 12. The D2D-sidelink resources are allocated by the eNB on the broadcast control channel (BCCH) for a pool of D2Dusers. They cannot be used by C-UEs. For UL diversity, frequency hopping is supported. It is obvious that this simple implementation is able to fulfill the basic scenario for public safety communication, where for a limited number of D2D-users, a simple sending of broadcast data without cellular coordination and feedback from the receiver is assumed. However, for future use cases with increasing number of D2D-users and higher traffic-variations improvements are required in order to fully utilize the potential of D2D-communication.
3
Efficient Resource Management for D2D-Communication
From an RRM point of view, one of the most important challenges is the sharing of the resources, which in LTE are given by resource blocks of 12 subcarriers in frequency and one slot of 0.5 ms in time, between the new D2D and existing cellular users.
other side, the occurrence of intracell-interference needs to be considered, which could be significant in case of transmissions using the same resources in vicinity. An interesting way provides the combination of both modes by using a hybrid method [8]. With this approach, depending on the location, D2D-UEs are divided into two groups, which are then allocated according to the overlay and underlay mode, respectively. By dynamic mode selection, the advantages of each mode are efficiently exploited. The results of a mathematical analysis for the different modes in a macrocell environment are shown in Figure. 2. The curves clearly indicate that the advantage of reusing the same resources in the underlay mode is only given with a low number of D2D-UE within a cell. With increasing traffic, the overlay mode provides better performance. Using hybrid allocation, the performance of the two other schemes can be combined. In case of low D2Dtraffic, resource allocation combines both modes, while at higher D2D-traffic the allocation is mainly according to the overlay mode. In consequence, the performance can be significantly improved for low D2D-traffic, while avoiding degradation for high D2D-traffic.
The current basic implementation of D2D-communication within LTE is based on an exclusive reservation of the radio resources to the D2D users [3]. As the reserved resource blocks cannot be used by other cellular users, the overall cell throughput will be affected. While this reduced throughput could be acceptable for low D2D traffic scenarios at the beginning, the throughput maybe significantly degraded with increasing number of D2D users in the future. In the following subsections we will discuss two promising approaches for advancing the resource sharing beyond these limits. 3.1
Sharing of Resources between D2D-Communication and Cellular Users – Hybrid Allocation
One important question for the resource management of the D2D-communication is on how to distribute the resource blocks between C-UE and D-UE. The efficient resource allocation depends on the location and amount of D2D-traffic. In principle, there are two different approaches to efficiently share the resources between both categories [6]: ·
·
In the Overlay Mode, sometimes also called orthogonal mode, the resource blocks are divided between D2D-UE and C-UE. Because of the full separation between the UE classes, a transmission without intracell-interference is provided. However, the separate allocation of the resources may lead to underutilization especially in areas with low D2D-traffic. Using the Underlay Mode, the same resource blocks are used for C-UE as well as for D2D-UE, simultaneously. This allows a reuse of the same resources for communication of C-UE and several D2D-UE pairs with higher distance and, hence, even higher utilization than in a pure cellular network. On the
Figure 2: Impact of D2D-communication to the spectral efficiency [8]
3.2
Reuse of Scheduling Resources – Sub-granting
As mentioned at the beginning, M2M-communication can be seen as one typical application for D2Dcommunication. Typically, the amount of data consumed by many M2M applications is relatively small, i.e. the payload length of a single transmission can be assumed to range between 10 to 50 Bytes. In this case, the current LTE resource allocation fails to efficiently utilize the available communication resources as the granularity of the allocation is too coarse.
On the other hand, in order to guarantee low latency for data transmission, the resources maybe permanently assigned for D2D-communication even if not used. In consequence, a typical M2M-transmission may not fully occupy the allocated resources and valuable radio resources would be wasted in current radio networks, in case of multiple deployed M2M applications. In order to use the available radio resources in an efficient manner and to overcome the challenges mentioned above, one idea is to allow C-UEs to use the remaining portion of a scheduling grant that is not used by the D2D-UE itself [7]. This scheme is based on the assumption that in each transmission interval, a D2D transmitter is aware that it may not fully occupy the allocated resources, reserved for D2D application. In order to enable users in its vicinity to exploit those resources, the D2D transmitter takes a small portion of its resources to indicate to C-UEs about the remaining portion of the scheduling grant. Referring to Figure 1 the scheme works as follows: In a first step the eNB allocates the resources for D2Dcommunication by means of eNB scheduling grants. Afterwards, the D2D-UE is using the grant for a sidelink data transmission. Then, if the D2D-UE detects that it is not fully using the allocated resources it can delegate the unused resources to a nearby C-UE during a second step. The forwarded resources are referred as sub-grants. The C-UE can use the sub-grant to transmit data on the uplink to the eNB. Figure 3 shows the impact of D2D traffic to the normalized throughput of one C-UE. For this analysis one RB was allocated to one D2D User and a certain mix of different modulation and coding schemes (MCS) has been assumed [7].
The results in the figure indicate that an increasing amount of D2D traffic may significantly degrade the remaining throughput for the C-UE. In particular, when sub-granting is not used, the throughput may go down by more than 40% in the worst case. With the suggested subgranting method, depending on the number of reused OFDM symbols, the degradation is smaller. For example, with 10 symbols, the C-UE throughput increases by about 30% compared to the no reuse case. Even a reuse of only 4 symbols may improve the throughput between 10% and 20%.
4
Simulation of D2D RRM Algorithms
As most of the involved RRM algorithms are highly interrelated, investigations by using dynamic simulation tools are needed to obtain the performance and to study the impact of the algorithms for D2D communication. In the following subsections we will discuss the usage of available simulation tools and present first results achieved from one selected tool for D2D-communication 4.1
Dynamic Network Simulation
In the past, mostly proprietary simulation software has been used to study the effects of different RRMalgorithms onto the network performance, cf. [9]. Those software tools included a detailed implementation of specific algorithms. Using these simulators, the performance of the algorithms and the impact of important parameter settings have been demonstrated, successfully. Because of their proprietary character, the usage was limited to vendor specific scenarios. Moreover, results between different simulators were hard to compare. Fortunately, in the meantime there are already various open software tools available for simulating LTE based systems. In [10], the simulator LTE-Sim is described, which is based on a standalone C++ LTE software. It contains parts of a LTE protocol stack implementation as well as different RRM algorithms, such as handover and dynamic scheduling. Both duplexing modes, FDD as well as TDD are supported. The simulation tool LENA is based on the popular network simulator ns-3 [11]. In addition to a detailed implementation of the LTE protocol stack in the user as well as in the control plane, many other aspects of LTE networks are implemented. There are models for the radio access network (RAN) as well as the evolved packet core (EPC) Extensive testing of the models has been done, so that LENA is now part of the official ns-3 distribution.
Figure 3: Impact of D2D traffic to the C-UE throughput for different resource reuse [7]
SimLTE is a LTE system simulator that is based on the public OMNeT++ platform. By means of its graphical user interface (GUI) the researcher is able to directly observe the behavior of the network. The U-plane side of the LTE protocol stack has been implemented in detail, such as HARQ and RLC. With its connection to the INET framework, upper layer protocols such as TCP and UDP are supported.
4.2
Implementation of D2D-Communication
To investigate D2D-communication in detail, the simulation models need to be adapted, accordingly. Next, we will present first results of the basic implementation of D2D communication on a system simulation tool. Although shown for the SimuLTE package, the modifications can be also applied for the other simulation tools mentioned above. The main difference between D2D and cellular communication results from the different transmission concept used by the two schemes. In cellular communication, there is a fixed eNB, where all traffic from the moving CUEs is directed towards the core network or other C-UEs, respectively. With D2D-communication the traffic is now directed between adjacent D2D-UEs, which are both moving. In consequence, for simulation of D2D communication the following needs to be modelled: ·
·
In an cellular system one communication partner, the eNB is fixed, while in D2D-communication, both the sender and the receiver are moving. This has an impact on mobility as well as on the association of D2D-UEs to a specific cell e.g. in case of handover. In cellular communication, there is a strict relation of the interference, which in the uplink is caused by other C-UEs and in the downlink by other eNBs. With D2D-communication, in theory, any D2D-UE can interfere with any other D2D-UE.
In a first approach, D2D communication has been implemented by a master-slave architecture. In particular, the transmitting D2D-UE was modelled as a master that also controls the relation of the receiving D2D-UE to a specific communication direction. Figure 4 shows a basic D2Dscenario. In this figure, there are two D2D communication pairs, where each master UE has a constant data rate transmission towards its associated slave UE. The impact of the cellular system was not modelled in this simple case.
Figure 5: SINR for a single D2D-pair
Within a second simulation setup, two D2D-UE-pairs have been simulated as shown in Figure 4. The D2D pair 1 was moving with constant speed and crossing the fixed D2D-pair 2 at a certain time. Both pairs are using the same time-frequency resources, cf. section 3. Figure 6 shows the resulting SINR for the described scenario. The curves clearly indicate the impact of the interference between the D2D-transmissions. When further apart the SINR of each communication is relatively high, while at lower distance the SINR is significantly reduced. Because of different configuration between the D2D-pairs (distance, tx-power) the minimum occurs at a different point.
Figure 4: Basic D2D-simulation scenario
In the first experiment, there was only one pair active with the master remaining at a fixed location and the slave moving away at constant speed. The resulting SINR is depicted in Figure 5, which shows the usual behavior of a wireless communication, i.e. the SINR going down with increasing distance.
Figure 6: SINR for two D2D-pairs
In a next step, D2D-communication for U-plane data transmission will be implemented. This will include the
modifications required to get the D2D-protocol stack as well as the control of the resources between cellular and D2D users.
5
Conclusions
In the present paper we discussed the application of D2Dcommunication to efficiently support delay critical services. Because of the new paradigm to allow a direct communication between D2D candidate users, a change of the existing method for controlling the transmission is needed. In the paper we highlighted some of the required new functionality and the issues that may arise, when supporting delay critical services via D2Dcommunication. One important aspect of the modified RRM mechanisms is the sharing of the available radio resources between the cellular and D2D-users. Within this context we discussed the basic underlay and overlay sharing principle and presented the performance of hybrid operation between both modes. Moreover, we showed how an improved resource reuse method can overcome the problem of wasting unused resources by delay critical services. Dynamic network simulations are needed to evaluate the performance impact of the D2D-communication on the overall network performance. Here, the usage of simulation tools based on open-source software seems to provide a good solution. We discussed the basic items to be implemented for simulating D2D-communication and provided first results of a basic D2D-implementation. The presented work will be continued by studying the performance of RRM algorithms in more detail, e. g., using the dynamic simulation tools with enhanced D2Dmodelling. Besides RRM management on lower level, self-organized procedures could be used to achieve the ambitious low latency goal of 1 ms in 5G. This may include the introduction of latency classes to prioritize the services according to their latency requirements. By using this approach, the share of reserved resources per latency class is autonomously adapted to guarantee fair resource sharing between all types of services. In addition, network operators are then enabled to control the allocation of reserved resources per latency class to meet their policy. Acknowledgement This work has been partly supported by the German Federal Ministry of Education and Research (BMBF) under the project “fast-wireless”.
6
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