Batteryless Wireless Sensors
To emancipate sensors from the limit of batteries, we proposed new MAC protocol named energy-neutral receiver-initiated MAC (ENRI-MAC). Our proposed protocol enables every sensor to autonomously decide its own intermittent interval based on their available energy and the number of neighboring sensors. Furthermore, we implemented our protocol using Lazurite 920J which is an off-the-shelf Arduino-compatible board and showed that our sensor network could operate very well only with unstable energy-harvesting power supplies.
 R. Tanabe, T. Kawaguchi, and K. Ishibashi, “Receiver-Initiated MAC Protocol with Distributed Intermittent-Interval Control based on Energy Inflow-Outflow,” to be appeared in IEEE CCNC, Las Vegas, NV (2018.1)
 H. Kawabata, K. Ishibashi, S. Vuppala, and G. Abreu, “Robust Relay Selection for Large-Scale Energy Harvesting IoT Networks ,” IEEE Internet of Things Journal, vol. 4, no. 2, pp. 384-392, Apr. 2017. DOI: 10.1109/JIOT.2016.2586581[available here]
Relays without Batteries
Typical wireless channels suffer from multipath fading and shadowing which significantly reduce communication capacity for a given average transmission power and hinder reliable transmission. Although an effective option is using multiple antennas to obtain spatial diversity gain, it is practically difficult to equip multiple antennas in some applications such as sensor networks because of the size, complexity, and cost. In order to overcome this issue, another concept has been proposed in the literature; when the source cannot reliably communicate with the destination, other available nodes can temporarily work as relays in order to support the communication by expending their own energies regularly supplied by pre-charged batteries, which is called cooperative diversity and allows nodes to enjoy spatial diversity gain without equipping additional antennas. Cooperative diversity inherently consumes the battery of users to support some users having with small channel capacities, which may results in shorter network life since more nodes drain their batteries at the same time.
A remedy for this crucial battery issue is the use of energy harvesting. Energy harvesting technologies enable devices to harness energy from ambient sources such as solar, vibration, themoelectric effects, and so on. Since this might be an ultimate solution of the crucial energy constraint, it has gained much attention from researchers. Especially, radio frequency (RF) energy harvesting does not depend on availability of ambient energy sources where ambient RF radiation is captured by the receiver antennas and converted into a direct current (DC) voltage through appropriate circuits such as rectennas. Therefore, this RF energy transfer is considered as one of the most attractive candidate technologies to realize self-sustaining networks.
In our laboratory, we investigate the cooperative transmissions with RF energy harvesting and published lots of papers. It is noteworthy that we proposed a novel cooperative diversity technique based on dynamic decode-and-forward cooperation with RF energy harvesting named dynamic harvest-and-forward (DHF) cooperation. This cooperation allows us to obtain diversity gain with consuming neither extra energy nor extra bandwidth by exploiting the relay’s proximity advantage over the destination.
 K. Ishibashi, “Relays without Batteries: A New Paradigm of Cooperative Diversity”, in Proc. of SmartCom2014, The Institute for Infocomm Research (I2R), Singapore (2014.10) [Invited Talk]
 K. Ishibashi, “Dynamic Harvest-and-Forward: New Cooperative Diversity with RF Energy Harvesting,” in Proc. of WCSP’14, Hefei, China (2014.10) [Invited Talk]
 K. Ishibashi and G. Abreu, “Analysis of RF Energy Harvesting in Large-Scale Networks Using Absorption Function” in Proc. of ICASSP 2014, Florence, Italy, (2014.5)
 H. Kawabata and K. Ishibashi, “RF Energy Powered Feedback-Aided Cooperation,” in Proc. of IEEE PIMRC’14, Washington D.C., USA, (2014.9)
 K. Ishibashi, H. Ochiai, and V. Tarokh, “Energy Harvesting Cooperative Communications,” IEEE PIMRC 2012, Sydney, Australia, (2012. 9)
Coded cooperation which can be considered as one of DF relaying protocols has been proposed to efficiently obtain both diversity and coding gain. For a large-scale network, adaptive network coded cooperation (ANCC) has been proposed in order to exploit the structure of the large-scale network to encoding graph-based error correcting codes.
However, this coded cooperation requires two orthogonal wireless resources due to the half-duplex constraint, which reduces the bandwidth efficiency. To overcome this issue, we proposed superimposed ANCC (S-ANCC) which simultaneously conveys the current information and redundant bits corresponding to the previous information bits by using higher-order modulation. This approach can be considered as the spatial coupling in complex domain and thus the efficient window decoding can be used to achieve the superior performance to the conventional ANCC.
Recently, we further proposed new coded cooperation scheme called spatially coupled repeat-accumulate coded cooperation (SC-RA-CC). SC-RA-CC can be considered as a distributed version of spatially coupled repeat-accumulate codes (SC-RA) which combines repeat-accumulate (RA) codes and spatially coupled codes and can achieve threshold saturation with lower coding complexity. This new coded cooperation achieves the performance close to the capacity even when the nodes do not have enough long information bits.
 N. Takeishi and K. Ishibashi, “Spatially Coupled Repeat-Accumulate Coded Cooperation,” in Proc. of IEEE WCNC2015, New Orleans, LA (2015,3) [Accepted]
 N. Takeishi, K. Ishibashi, and Y. Yamao, “Superimposed adaptive network coded cooperation for wireless sensor networks,” Personal Indoor and Mobile Radio Communications (PIMRC), 2013 IEEE 24th International Symposium on, pp.1180-1184, 8-11 (2013.9)
Semi-blind Interference Alignment
Multiuser channels inherently have higher channel capacity than single-user channels although the receivers have to cope with strong interference. Interference alignment (IA) is an effective approach which can achieve K/2 degrees of freedom (DoF) gains over K user interference channels. However, IA requires global channel state information at transmitters (CSIT) and accurate time synchronization among transmitters. Wang et al. have proposed blind interference alignment (BIA) which can align interference signals with a given signal space without CSIT and can achieve same DoF gains as original IA. The receivers, however, have to equip reconfigurable staggered antennas to create short term channel fluctuation patterns for alignment.
To alleviate this practical difficulty, we have recently proposed semi-blind IA (S-BIA) in combination with orthogonal frequency division multiplexing (OFDM) for 2 user X channels. In this scheme, each transmitter only requires CSIT related to itself (i.e., local CSIT), to creat the desired channel fluctuation and thus receivers do not need staggered antennas. Furthermore, accurate time synchronization among users is not required with the aid of OFDM.
 M. Takai, K. Ishibashi, W.-Y. Shin, H. S. Yi, and T. Wada, “Semi-Blind Interference Alignment Based on OFDM over Frequency Selective X Channels”, IEEE International Conference on Communications (ICC) 2013, pp.5236-5241, Budapest, Hungary, (2013.6)
 M. Takai, K. Ishibashi, and T. Wada, “Semi-Blind Interference Alignment over Correlated Wireless Channels,” Proc. of IEEE Radio and Wireless Symposium, (2014.1)