1. Field
Device-to-device. (D2D) communication is one alternative to communication between devices through a radio access network. One interesting D2D communication method is reuse-mode D2D, where D2D nodes communicate with each other using the same uplink (UL) radio resource that is being used by some other cellular user equipment(s) (UEs). In other words, the UL cellular resources occupied by cellular UEs are reused by D2D nodes in their short-range communications.
2. Description of the Related Art
One way to decide which cellular resource can be reused in a reuse D2D link is by each D2D pair themselves. In that approach, each D2D candidate first gets the UL scheduling information from the eNB and measures the corresponding UL channels. Then, the D2D candidate itself finds the cellular UEs whose resource allocation can be reused in its future D2D communication. After that, related information is exchange between a D2D pair, and a final list of reused cellular UEs (UEs whose resource allocation can be reused in their D2D communication) is decided. That D2D pair then starts monitoring the eNB's scheduling again. Once a cellular UE in the decided list is allocated a radio resource, that D2D pair will reuse that part of radio resource to perform their short-range D2D communication.
As each D2D pair decides which resource to reuse independently, eNB has no information on the existence of D2D pair at all. Therefore, the eNB cannot consider the D2D communications in its radio resource allocation. As a result, D2D UEs can, in this approach, only communicate with each other when a resource suitable to be reused comes along. Therefore, that distributed reuse-mode D2D can only support a service opportunistically without any guarantee of availability.
Additionally, if two reuse-D2D pairs start their communication simultaneously, resource contention may occur if both D2D pairs decide to reuse the resources of the same cellular UE. To overcome this problem, a 2-way handshake mechanism can be employed similar to RTS/CTS/Data/ACK mechanism in WLAN systems. In an RTS/CTS handshake, a reusable list of cellular UE resources can be exchanged between the D2D pair and a final reuse list can be decided for each D2D communication direction. If a nearby D2D pair hears the transmitted RTS signal, it will postpone its RTS/CTS exchange and perform interference measurement a second time. However, it should be noted that, in this approach, the RTS/CTS message cannot be exchanged by reusing the cellular resource. This is because resource reusable in a cell for a specified D2D pair is not fixed for all time. It changes with the scheduling of cellular UEs, positions of cellular UEs, positions of D2D UEs, etc. Hence, it may be that there will be an absence of information as to which resources can be reused for the delivery of such messages. Even if there were a dedicated orthogonal channel for RTS/CTS exchanges, RTS/CTS collision between two D2D pairs may significantly postpone the link setup of reuse-mode D2D links.
In the approaches above, the eNB has no awareness of the reuse-D2D links. Thus, the eNB is unable to assist operators in imposing a service charge for reuse D2D communications.