## 1 Introduction  
The demand for services that provide current sensing data (data) at a specified location is expected to grow in the future [1]. Examples of such services are (1) a driver obtains information on the traffic ahead of him or her during a traffic jam or (2) a sightseer asks a data service about the crowding at his or her destination. The cost of installing, operating, and maintaining sensors to obtain such sensing data is a problem when providing such services. Mobile crowdsensing (MCS) [2, 3] is a method that can collect sensing data without the pre-installation of sensors in the field. An MCS system consists of mobile hosts with sensors, such as smartphones and cars, that are willing and able to help a service obtain data and a server operated by a service provider, which collects data with the help of the mobile hosts. The cost of cellular traffic can be a problem in MCS. Let us assume an MCS system that provides a user with current data about the point of interest (POI) specified by the user. To make the MCS system applicable to various services providing current sensing data, it is desirable to impose no restriction on the location of a POI. Therefore, we assume that a server communicates with mobile hosts through a cellular network because a cellular network allows mobile hosts to communicate with the server almost everywhere. It is also desirable to have many mobile hosts participate in MCS so that the system can provide sensing data for many locations. However, the cellular traffic between the server and mobile hosts increases as the number of mobile hosts increases. Thus, an increase in cellular traffic can increase the operation cost of the MCS system. In this paper, we propose a new communication method for MCS that reduces cellular traffic between the server and the mobile hosts by exploiting device-to-device (D2D) [4] communication, which enablesneighboring mobile hosts to communicate with each other through a free wireless link, such as an IEEE 802.11p wireless network. In the proposed method, only a limited number of mobile hosts directly communicate and exchange messages with the server via a cellular network and forward the messages to other mobile hosts through D2D communication. Thus, cellular traffic can be reduced. We identify design challenges arising from the use of D2D communication and propose solutions for them. To successfully obtain data with MCS, it is necessary to motivate mobile hosts to help. Incentive-based MCS [5; 6] is a method that increases data availability by motivating mobile MCS hosts to obtain the data. The design goal of the incentive-based MCS is that the server can obtain higher-quality data by offering fewer incentives. The reverse auction (RevAuc) [7], in which the bidder requesting the smallest reward wins the bid, is a solution to achieve the goal (Figure 1). An example of a reward is money paid to a successful bidder. In MCS using RevAuc, the server acts as an auctioneer that sends bid invitations and determines a successful bidder. Each mobile host sends a bid to the server. The bid includes information on the quality of data that the mobile host plans to obtain and transmit and the reward that it requests. In this paper, we assume that the quality of data depends on factors such as the distance between the POI and the place where the data is obtained, the time when the data is obtained, and the resolution of the data. The evaluation score of each bid is calculated based on the reward requested by the bidder and the quality of data proposed by the bidding mobile host. The server asks the mobile host that has tendered the bid with the highest evaluation score among bids sent by mobile hosts to obtain and transmit the data. There are two methods for a server to send a bid invitation in MCS using RevAuc. One is pull-type, in which the server sends a bid invitation to a mobile host when it receives a query for bid invitations from the mobile host. Another is push-type, in which the server sends a bid invitation spontaneously (Figure 2) [2].