Koji Kamakura

Alamouti-type coding for visible light communication based on direct detection using image sensor

This paper proposes Alamouti-type space time (ST) coding to visible light communication (VLC) using light emitting diodes (LEDs) and image sensor-based direct detection with a high-speed camera. Although multiple LEDs of an array are used as transmitters to communicate to individual pixel elements of an image sensor which act as multiple receive elements, the received signal is essentially the image of the transmitting element, whose size in pixel reduces with distance. When the receiver captures the LED array at a distance where images of two neighboring LEDs in the array are not separated at the image plane, such distortion depreciates the symbol decision quality. Our proposed scheme aims at expanding the communication range limited by the pixel resolution of image sensor. Our implemented prototype demonstrates that ST coding is feasible to achieve 48-meter error-free transmission range while the repetition transmission achieves 26-m error-free range.

Motion modeling of mobile transmitter for image sensor based I2V-VLC, V2I-VLC, and V2V-VLC

Visible light communication (VLC) using light-emitting diodes (LEDs) is drawing much attention. There are two types of VLC reception devices, a photodiode and an image sensor. VLC using image sensor is called image sensor based VLC. As the receiver only uses the image sensor pixels that sense LED transmission sources and discards other pixels, including those sensing noise sources, image sensor based VLC is an attractive solution for outdoor mobile applications. However, little evidence is available for channel modeling of image sensor based VLC in mobile outdoor environments, especially for motion modeling of VLC transmitter. In this paper, we propose a motion model of VLC transmitter. In particular, we consider three cases; infrastructure-to-vehicle VLC (I2V-VLC), vehicle-to-infrastructure VLC (V2I-VLC), and vehicle-to-vehicle VLC (V2V-VLC). As a result, we show that the motion model of the VLC transmitter on the captured image for the three aforementioned situations (i.e., I2V-VLC, V2I-VLC, and V2V-VLC) can be expressed in the identical pinhole camera model.

Layered space-time coding using LED array for image-sensor-based visible light communications

This paper demonstrates the feasibility of layered space-time coding (STC) in an outdoor image-sensor-based (IS-based) visible light communication (VLC) system. We examined that for low-resolution IS-based VLC channel where intensity modulated signals from two different light emitting diodes (LEDs) are detected at one pixel on the image plane, STC allows to decouple them, thus receiving data with no errors. Consequently, STC offers extending the transmission distance. In the layered STC presented here, additional bit streams are aligned in the 2n×2n LED array for increasing the transmission rate per symbol duration for the case where the spatial resolution is improved. A prototype of three layered STC is built with an 8×8LED array, where each of the LEDs is modulated at 1 kbps and a high-speed camera with IS operating at 1000 fps. Our experimental results validate that additional bit streams (layer-2 and -3), aligned in the layer-1 STC matrix pair, are extracted with no errors when the receiver comes within 155m and 55 m, respectively, from the LED array, without decreasing 210m of the transmission distance of layer-1 bit stream.

Dimensioning an scheduler buffer in OBS networks using forward resource reservation

This paper indicates a desired scheduler buffer size which minimizes the overall packet blocking probability of optical burst switching (OBS) networks based on forward resource reservation (FRR). A packet is blocked either if it arrives at the edge buffer that is full or if the control packet (CP) corresponding to the burst including the packet arrives to find the scheduler buffer full. In conventional one-way reservation schemes, simply increasing the size of a scheduler buffer in which CPs are enqueued may avoid its overflow, but it involves increasing per-hop delay because buffering a CP prolongs to process it, thus increasing the burst delay. In FRR-based OBS networks, since the burst assembly procedure at edge nodes and the resource reservation by CPs at core nodes are processed in parallel, the packet blocking probability can be reduced with no increased delay. To analyze the FRR-based scheme, we develop an edge node model comprising a set of independent edge buffers that feed bursts to core networks and their CPs to a scheduler. We demonstrate that for a set of parameters, an optimal scheduler buffer size exists and depends on the number of packets comprising a burst and the packet arrival rate, and that the minimum packet blocking probability is much lower than that of non-FRR-based OBS systems.

Modified TDMA-based MAC protocol for vehicular ad hoc networks

This paper proposes a modified version of vehicular ad hoc network media access control (VeMAC) protocol for further increased throughput. In the modified VeMAC, an additional bit is appended to the field for each time slot in the header used in VeMAC as the collision flag. Upon detecting a collision for a time slot, a node sets the collision flag for that time slot without changing the ID field and sends its packet in its acquired time slot. By receiving the packet with the collision flag, nodes notice a collision happen in the time slot. One of the nodes involved in the collision knows from its ID field that it is the node itself that is supposed to use the time slot, and therefore it continues to use the time slot. The other nodes recognize themselves as nodes that should not use the time slot because their IDs are not included in its ID field of the packet. Consequently, they release the time slot and begin by acquiring new time slots. Since not all nodes involved in the collision release the time slot in our modified VeMAC, the possibility that access collisions following the merging collision happen is reduced, compared to the original one. Simulation results show that our modified one outperforms the original one in terms of the average number of collisions and throughput.

Experimental validation of cooperative approach near road side units

This paper reports an experimental validation of the feasibility of cooperative approach for extending the coverage of vehicular-to-infrastructure (V2I) communication using our software platform named Airplug. Although the access time between a vehicle and a roadside unit (RSU) providing Internet access is limited in the real-world condition, our field trials demonstrate that it is extended by just passing by an RSU with forming a convoy of five vehicles installed Airplug. An overview of our software design and experimental setup is described, and discussed together with results using a WiFi access point (AP) as the RSU. Our field results show that the minimum of increased access time of our cooperative approach is always longer than the maximum of the access time of the single-vehicle case, even though the coverage varies.