Traditional communication systems were built using a layered structure to provide well-defined but limited interfaces among protocols in adjacent layers. The modular design of the layered structure, where the details of each protocol are hidden, promotes the interoperatability of the communication protocols. Although the protocol details (e.g., states and internal functions) are encapsulated in each layer, this structure prevents the protocols sharing, accessing, and controlling the operations of other protocols, which might be required for efficient data transmission. Therefore, the concept of cross-layer design has been introduced to allow tighter integration of different protocols. It is not necessary for these protocols to be in adjacent layers. The benefits of cross-layer design are that it allows one to improve both the flexibility of protocol implementations and network performance. Cross-layer design and optimization have been adopted in various wireless systems. A few surveys on cross-layer design exist in the literature, e.g., [190, 191, 192, 193].

The cross-layer design has been adopted for optimizing D2D communication. In this chapter, we give an overview of the cross-layer design by introducing its definition and different approaches. Then, we present one of the most commonly used cross-layer design models, i.e., the coordination model, which incorporates different functionalities. These functionalities are security, QoS, mobility, and wireless link adaptation. We next present the cross-layer implementation and challenges.

We introduce cross-layer optimization, which is part of the cross-layer design. We provide examples of the cross-layer optimization problems, including opportunistic scheduling, OFDM resource allocation, and congestion control. We then review the cross-layer design framework proposed for D2D communications. They are information correlation routing, routing in sensor networks, and traffic scheduling for a video application. Finally, we outline some research directions of the cross-layer design and D2D communications.

An overview of cross-layer design

In this section, we first introduce the basic concepts including the definition of, and approaches to, cross-layer design. Then, we discuss the coordination model, which incorporates other functionalities (i.e., security, QoS, mobility, and wireless link adaptation) into the protocols.