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the channel capacity, it is still feasible for the communica-tion devices to quickly detect each other using the method we proposed which employs several dierent frequency channels [11]. We then congured a transmission frame within which transmission time slots were assigned to each communication device installed in a test terminal. e time slot assignment took into account the role each terminal plays. is process ensured that signals transmitted from communication devices properly diuse. Within the trans-mission frame, we determined time slot duration and transmission frequency for each communication device while taking into account the role of each test terminal which houses the communication device. e communica-tion devices are capable of transmitting data only during their assigned time slots (a frame structure example is shown in Fig. 7). In our system, the transmission frame duration was set to be 60 seconds and this cycle was re-peated. We installed GPS receivers in all test terminals distributed across the experimental area in order to enable them to receive time pulses transmitted from GPS satellites and thereby achieve transmission frame synchronization. Some test terminals installed inside buildings were unable to receive GPS signals. Accordingly, we synchronized these terminals by arranging other terminals—which have been synchronized using GPS time pulses—to send synchroniza-tion signals to them.(2) TDMA- and CSMA/CA-based channel access schemesAll communication devices—which use the same fre-quency channel as described in the previous section—ac-cess the channel primarily using a TDMA (time-division multiple access) scheme appropriate for these types of devices. However, when multiple test terminals of the same type are within communication range, data packets trans-mitted from them might collide. To avoid this, we also adopted a CSMA/CA (carrier sense multiple access with collision avoidance) scheme in addition to TDMA. e scheme allows a communication device to transmit data only when no other device is doing so. e combined use of TDMA and CSMA/CA enables communication devices in dierent types of test terminals to autonomously estab-lish a communication network between them.(3) Information buering functionMutual communications between terminals are possible only when they are within communication range. To in-crease the number of terminals that can share information, we added an information buering function to the termi-nals. When a terminal receives data packets, the data packets are stored in a buer for a certain time period. If a new communication terminal enters communication range during this time period, the stored data packets are transmitted to the entered terminal. In our system, com-munication devices shi their locations, causing informa-tion propagation routes to change continuously. Under this condition, a communication device might receive the same data packets multiple times. To prevent this, we assigned a unique ID to each data packet to make it distinguishable. In addition, a communication device might receive old data packets in correlation with the network routes that the data packets followed. To avoid this, we attached time informa-tion to each data packet so as to update them accordingly and continuously.3Structure of test terminals in the testbedWhile various types of test terminals are used in the testbed, they all operate at 920 MHz, which is a specied frequency band dened in the ARIB STD T108 standard [12]. We are using either 20 mW (which can be used without a license) or 250 mW (which can be used upon being registered) communication devices depending on their usage. Both types can be used free of communication fee. e power dierence in decibels between these two types of communication devices can be calculated using the following equation.10log(250) − 10log (20) ≈ 10 (1)is dierence can be converted into the dierence in propagation distance using the following equations which take into account the propagation loss in free space.L(dB) = 20log 20() (2)L() −L() = 20log() − 20log () (3)rough the calculation, it was found that the dier-ence in propagation distance between the 20 mW and 250 mW communication devices was D2 ≈ 3.5 × D1, given that the same type antenna was used.e signal bandwidth of both communication devices was 400 kHz. ey both employed the GFSK modulation and the CRC-16 error-detecting code. eir transmission rate was 115,200 bps. We describe below the role of each type of test terminal we developed.452-7 Development and Field Trial of D2D Networks for Local Area Information Collection and Distribution