Virtual mesh networking for achieving multi-hop D2D communications in 5G networks

Abstract

In 5G networks, when D2D communications or the quality of a cellular link fails to support data transmission of a terminal, it is indispensable to exploit data transmission opportunities via multi-hop D2D communications. However, to achieve multi-hop D2D communications, two problems need to be solved: 1) A routing and packet forwarding protocol is required to enable terminals to forward data packets across multi-hop D2D or cellular links; 2) This protocol must conform to the existing specifications of D2D communications. To this end, a framework of virtual mesh networking is developed in this paper to support multi-hop D2D communications in 5G networks. More specifically, based on the 5G network architecture, routing and packet forwarding are split into two separate mechanisms in the control plane and the user plane, respectively. In the control plane, base stations collect link and topology information of terminals and then form a virtual mesh network among terminals accordingly. The routing path for end-to-end communications is then determined by a virtual mesh routing algorithm, which also takes into account terminal mobility and link failure. In the user plane, a packet forwarding mechanism is designed to deliver data packets following the routing path as determined by the routing algorithm in the control plane. This mechanism extends but also conforms to the existing specifications of D2D communications. The virtual mesh networking protocol and its underlying algorithms are evaluated via simulations. Performance results demonstrate the effectiveness of multi-hop D2D communications via virtual mesh networking.

Introduction

D2D communications are promising for high speed direct communications when two terminals are in the proximity of each other [1]. It has been specified in 3GPP standard in Release 12 and further strengthened in Release 13–15. Two types of communication scenarios are specified in Release 13 [2]: 1) single-hop direct data transmission between two terminals, as shown in scenario 1 in Fig. 1; 2) two-hop data transmission between a terminal and the core network using another terminal as a relay, as shown in scenario 2 in Fig. 1. When two terminals are not in the proximity of each other, they have to communicate through the core network via cellular links. However, such links may not be available in two typical scenarios: 1) small cell base stations lack wireless resources to support massive access in large gatherings; 2) communication infrastructure crashes at the occurrence of natural disasters. In these situations, multi-hop D2D communications are indispensable to exploit data transmission opportunities. Such opportunities are shown in scenarios 3–5 in Fig. 1, where a two-hop link can be further extended to a multi-hop link.

Multi-hop D2D communications play an important role in improving end-to-end connectivity between two terminals. This is particularly critical for providing communication opportunities in public safety applications, as studied in [3] and analyzed in [4]. Moreover, it is crucial when mmWave is used for multimedia applications [5] in 5G access networks, since mmWave suffers from high attenuation during transmission and can be blocked by the surrounding objects. Moreover, multi-hop D2D communications also expand the network coverage significantly, which is essential to support massive devices in Internet of Things (IoT) applications [6]. Although the UE-to-network relay technology for IoT and wearables has been enhanced in [7], multi-hop D2D communications have not been specified in the latest 3GPP standards so far. Therefore, it is critical to study multi-hop D2D communications in 5G networks.

Two issues need to be addressed in multi-hop D2D communications. First, if a terminal generates a packet or receives a packet not for itself, it needs to send the packet to an appropriate next-hop terminal. Second, when receiving a packet, a terminal needs to determine whether the packet is for itself, and forwards the packet once it is not. Consequently, a routing and forwarding mechanism needs to be designed for multi-hop D2D communications.

Although there exists a routing and packet forwarding mechanism in current D2D standards for uplink data transmission (i.e., scenario 2 in Fig. 1), it cannot be extended to support multi-hop D2D communications for the following reason. In this mechanism, evolved packet switched system (EPS) bearers [8] to the core network are first established at a relay, and a traffic flow template (TFT) [2] is constructed if the relay sets up a D2D link with a distant terminal. When the relay receives a packet from the distant terminal, it sends the packet to the core network through the EPS bearer based on the TFT. Since the EPS bearer is a tunnel from the relay to the core network, the packet is not visible at the base station until it reaches the core network. As a result, this mechanism cannot be leveraged to support routing and packet forwarding in multi-hop D2D communications.

Multi-hop communications have been extensively studied in ad hoc and mesh networks. So far, many routing protocols are available to support multi-hop communications, and most of them can be classified into distributed routing protocols or hybrid routing protocols. In a distributed routing protocol, control messages are distributively disseminated in a network to find the route from a source to a destination. There are two widely used distributed routing protocols: 1) optimized link state routing (OLSR) [9], where the aforementioned procedure is performed proactively; 2) ad hoc on-demand distance vector (AODV) [10], where the routing procedure is performed reactively. In a hybrid routing protocol, each node proactively maintains a routing path to a root node. If a source node cannot find an existing route for a packet, it sends the packet to the root node and triggers a route setup procedure reactively at the same time. As a result, a route is set up and packets can be transmitted via the newly set up route. A typical example of hybrid protocols is hybrid wireless mesh protocol (HWMP), which is specified in IEEE 802.11s [11].

The above two types of routing protocols are not suited for multi-hop D2D communications for two reasons. First, their route setup procedures do not conform to the standardized D2D specifications. In both types of routing protocols, an intermediate terminal broadcasts route setup request messages after it receives one from its neighbors. However, in the current D2D standards, a terminal needs to be triggered by an application and authorized by the ProSe Function Application Server in the core network before conducting broadcasting. As a result, lots of modifications to the current standards are necessary to adopt the existing mesh or ad hoc routing protocols in multi-hop D2D communications. Second, the route setup procedures are not efficient for multi-hop D2D communications. Signaling messages from a source need to go through multiple hops before reaching a destination, resulting in large latency in route setup and route failure recovery. However, such hop-by-hop signaling procedures are not necessary in 5G networks, since signaling can be conducted via a single-hop cellular link, following a path different from that of multi-hop data communications.

Besides the above routing protocols, multipath routing can be another option for routing mechanisms of mesh networks. Typical examples include disjoint multipath routing [12] and braided multipath routing [13]. Multipath routing does not only facilitate load balancing but also improves transmission reliability. However, since transmitting packets via a large number of hops leads to large delay, the number of hops of a routing path in multi-hop D2D communications is actually limited. Consequently, there exist few totally disjoint or partially disjoint routing paths between a given source-destination pair, which makes the benefits of multipath routing marginal. Moreover, to make a multipath routing mechanism effective, associated mechanisms need to be designed [14], e.g., dynamically allocating traffic to different routing paths, reordering packet received from multiple routing paths at the destination, or integrating multipath routing with packet aggregation [15] to reduce overhead. These mechanisms increase the complexity of multi-hop D2D communications and cause high terminal energy consumption when they are executed in terminals. Thus, multi-path routing is not taken into consideration in this paper. However, with the densification of small-cell base stations and the growing interest in integrated access and backhaul (IAB) networks, it is worth future study to consider multipath routing in IAB networks to improve network reliability and perform load balancing dynamically.

Related research work is carried out in [5], [16], [17], [18], [19] to study multi-hop D2D communications. Such work is focused on designing routing algorithms that optimize routes under various constraints such as QoS [5], interference [16], and scheduling [17], [18], [19]. However, how to design routing mechanisms that conform to the standard mechanisms of D2D communications is not considered. Several routing mechanisms surveyed in [20] only consider multi-hop D2D communications as a type of ad hoc networks, and thus they are inapplicable to multi-hop D2D communications. Consequently, a new routing and packet forwarding mechanism that comply with the existing D2D standards needs to be developed to support multi-hop D2D communications.

In this paper, a routing and packet forwarding mechanism is developed to achieve multi-hop D2D communications via standardized one-hop cellular or D2D communication procedures, and it is critical for both low-frequency and mmWave 5G networks. The underlying routing algorithm selects a routing path that may contain a mixture of D2D links and cellular links, but it neglects the interference between cellular and D2D communications. Such a design is valid for two reasons. First, D2D communications can be carried out in the spectrum orthogonal to that of cellular communications, which is called the overlay mode in [21]. Second, resource allocation can be considered separately from routing in a different algorithm, as shown in [22]. To improve the efficiency of the routing mechanism, a routing algorithm that jointly considers routing, resource allocation, and interference needs to be developed, which is an interesting topic subject to future research.

To develop the routing and packet forwarding mechanism, key features of the hierarchical network architecture of 5G networks are leveraged. First, communications between a small-cell base station and terminals or between a macro-cell base station and the small-cell base station can be accomplished within one hop. Thus, neighbor and link information of terminals can be collected and managed by the small-cell base station, and such information is then reported to the macro-cell base station. Based on such information, a virtual mesh network of terminals can be formed in small-cell base stations to accomplish multi-hop D2D communications within a cell, and virtual mesh networks of multiple cells are integrated in the macro cell base station to achieve inter-cell multi-hop communications. Second, signaling messages between terminals can be exchanged via base stations, so they do not have to follow the same path as that of data communications between terminals. In this way, the control plane and the data plane of multi-hop D2D communications are separated to improve protocol efficiency and reliability.

Based on the above two features, the new routing and packet forwarding protocol is designed as follows:

• A virtual mesh network involving multi-hop D2D communications is formed and managed by macro-cell or small-cell base stations.

• A virtual mesh routing algorithm is developed based on the virtual mesh network to determine an routing path that may consist of a mixture of cellular links and D2D links.

• A packet forwarding mechanism conforming to the standard single-hop cellular and D2D communication procedures is designed for a terminal to forward packets according to the routing path determined by the virtual mesh routing algorithm.

Moreover, the challenging issues related to terminal mobility and link failure are also addressed in the new protocol.

The virtual mesh routing and forwarding mechanism developed in this paper is evaluated via extensive simulations. Performance results demonstrate its effectiveness in supporting multi-hop D2D communications. Moreover, it is adaptive to terminal mobility and is also resilient to link failure. Besides, the framework of virtual mesh networking conforms to the existing 3GPP D2D standards.

The rest of the paper is organized as follows. The system model is described in Section 2. The virtual mesh network formation and maintenance procedure is presented in Section 3. A routing algorithm is designed in Section 4, while the corresponding routing mechanism and route failure recovery procedures are illustrated in Section 5. The virtual mesh packet forwarding mechanism is also studied in Section 5. The performance of virtual mesh networking for multi-hop D2D communications is evaluated and analyzed in Section 6. The paper is concluded in Section 7.