In this paper, we propose a near-ultrasound chirp signal-based communication for the TV's 2nd screen services. While near-ultrasound (with under 20 kHz frequency) communication has been developed for various applications recently, none of the previous work provides a perfect solution for 2nd screen services between TVs and smart devices. This is due mainly to the following real world challenges. The embedded signal in TV contents should be successfully received in a typical TV-watching environment by (i) delivering information at least at 15 bps with significantly low volume to avoid human perception, (ii) despite the presence of ambient noise, e.g., a tick, a snap, or a knock. To fulfill (i), we design chirp quaternary orthogonal keying (QOK) symbols. Especially, we aim to minimize inter-symbol interference (ISI) effects by symbol design in order to completely eliminate guard intervals. To resolve (ii), we propose the novel J-shape detection algorithms for both frame synchronization and carrier sensing. The proposed modem achieves almost zero frame error rate on a smartphone 2.7 m away from the TV even with minimal receive sound pressure level of 35 dBSPL, i.e., the noise level in a very quiet room. Moreover, throughout experiments and log analysis of 2nd screen service deployed in a nation-wide TV broadcasting system, J-shape detection algorithms are proven to achieve highly resilient performance for both frame synchronization and carrier sensing compared to previous schemes.

Smart devices such as smartphones and tablet/wearable PCs are equipped with voice user interface, i.e., speaker and microphone. Accordingly, various aerial acoustic communication techniques have been introduced to utilize the voice user interface as a communication interface. In this paper, we propose an aerial acoustic communication system using inaudible audio signal for low-rate communication in indoor environments. By adopting chirp signal, which is widely used for radar applications due to its capability of resolving multi-path propagation, the proposed acoustic modem supports long-range communication independent of device characteristics over severely frequency-selective acoustic channel. We also design a backend server architecture to compensate for the low data rate of chirp signal-based acoustic modem. Via extensive experiments, we evaluate various characteristics of the proposed modem including multi-path resolution and multiple chirp signal detection. We also verify that the proposed chirp signal can deliver data at 16 bps in typical indoor environments, where its maximum transmission range is drastically extended up to 25 m compared to the few meters of the previous research.

Recently, it has been shown that a simple, distributed backlog-based carrier-sense multiple access (CSMA) algorithm is throughput-optimal. However, throughput optimality is established under the perfect or ideal carrier-sensing assumption, i.e., each link can precisely sense the presence of other active links in its neighborhood. In this paper, we investigate the achievable throughput of the CSMA algorithm under imperfect carrier sensing. Through the analysis on both false positive and negative carrier sensing failures, we show that CSMA can achieve an arbitrary fraction of the capacity region if certain access probabilities are set appropriately. To establish this result, we use the perturbation theory of Markov chains.

Recently, it has been shown that a simple, distributed CSMA algorithm can achieve throughput-optimality. However, the optimality is established under the ideal carrier sensing assumption, i.e., each link can precisely sense the presence of other active links in its neighborhood. This paper, in contrast, investigates the achievable throughput of the CSMA algorithm under imperfect carrier sensing. The main result is that CSMA can achieve an arbitrary fraction of the capacity region if certain access probabilities are set appropriately. To establish this result, we use the perturbation theory of Markov chains.

The multiple-input multiple-output (MIMO) broadcast channel uses multiple antennas at the transmitter to concurrently deliver information to multiple receivers. Despite the theoretical capacity gains over single user MIMO, the actual throughput of the system with limited feedback on channel state information suffers due to interspatial stream interference. On top of the orthogonal beamforming with limited feedback , this paper introduces two novel stream schedulers to mitigate the interference. By the proposed adaptive spatial stream schedulers, an adaptive number of spatial streams are scheduled and thus, reduces the interference, which also achieves power gains by concentrating power into fewer streams. The two proposed algorithms are evaluated and shown to effectively mitigate the interference.

While a network with stationary nodes can provide large capacity in many current wireless systems, a network with mobile users suffers from severe performance degradation. This happens due to quick channel variation and its resulting protocol overheads that are especially large for multiuser multiple-input multiple-output (MIMO) systems. In this paper, we propose a relay-assisted multiuser MIMO downlink system to support highly mobile wireless access. Through a theoretical throughput analysis that explicitly considers the protocol overheads, we show that the MIMO relay significantly enhances the sum throughput of the system.

The number of frequency channels specified for IEEE 802.15.4 low-rate wireless personal area networks (LR-WPANs) does not suffice to operate a variety of collocated WPAN applications that the standard is targeting. To overcome this limit, we introduce virtual channel, a novel concept to increase the number of available channels by efficiently managing given spectral and temporal resources. A virtual channel is created by scheduling a superframe and selecting a logical channel. This extends the notion of a channel from spectral domain to spectral and temporal domain. Specifically, we propose a superframe scheduler using throughput estimation (SUTE) of the IEEE 802.15.4 carrier sense multiple access with collision avoidance (CSMA/CA). In addition, nearest vacancy search (NEVS) is proposed, both of which are for temporal efficiency of the collocation. For both spectral and temporal efficiency, virtual channel selector (VCS) is proposed. The simulation results show that a remarkable improvement on the collocation efficiency of IEEE 802.15.4 can be achieved by our proposals. Moreover, this study also reveals the fundamental drawback of the current standard in terms of the collocation efficiency that the beacon interval and superframe duration are adjustable only by exponent parameters.

The multiuser multiple-input multiple-output (MIMO) broadcast channel uses multiple antennas at the transmitter to simultaneously deliver information to multiple users. Despite the theoretical capacity gains, the actual throughput of the system with limited feedback of channel state information suffers due to interspatial stream interference caused by quantization errors. This paper introduces a new method to mitigate the interference: transmit-beam matching. Using multiple receive antennas, transmit-beam matching minimizes the inter-spatial stream interference by each user's effective channel to the transmit-beam. It is evaluated based on a recent MIMO broadcast channel technique with limited feedback, per-user unitary rate control (PU2RC) and shown to effectively mitigate the interference.

The number of channels specified for IEEE 802.15.4 low-rate wireless personal area networks (LR-WPANs) is too few to operate many applications of WPANs in the same area. To overcome this limit, we introduce virtual channel, a novel concept to increase the number of available channels when various WPAN applications coexist. A virtual channel is basically created via superframe scheduling within the inactive periods in a logical channel preoccupied by other WPANs. To maximize the coexistence capability of WPANs using virtual channels, we propose (1) least collision superframe scheduler (LC-scheduler), (2) less complex heuristics, and (3) virtual channel selector (VCS) to efficiently manage multiple available logical channels. In addition, a simple but practical synchronization method is developed to compensate different time drifts among coexisting WPANs. The simulation results demonstrate that a remarkable improvement on the coexistence capability of the 802.15.4 can be achieved through the proposed schemes.

We investigate the problem of the lifetime maximization in wireless sensor networks under the constraint of end-to-end transmission success probability, by adopting a cross-layer strategy that considers physical layer (i.e., power control), MAC layer (i.e., retransmission control) and network layer (i.e., routing protocol) jointly. We decouple the problem into separate sub-problems of each layer and propose the optimal algorithm as well as an alternative heuristic algorithm with lower complexity for each sub-problem. We demonstrate through computer simulations that a trade-off relation exists between the network lifetime maximization and the reliability constraint, and the strategy that is designed by combining the proposed algorithm at each layer can significantly increase the network lifetime. We also investigate the effect of the retransmission control on the energy efficiency for different energy consumption models. Simulation results reveal that multiple retransmissions with low power yield little gain when the link distance is short and the power conversion efficiency of the amplifier increases with the transmission power

We investigate the problem of lifetime maximization in a wireless sensor network under the constraint of target end-to-end transmission success probability, by adopting a cross-layer strategy that considers physical layer (i.e., power control), MAC layer (i.e., ARQ control) and network layer (i.e., routing protocol) jointly. We first present a near-optimal retry limit allocation algorithm for a given routing path. This allocation determines the per-hop success probability for each link along the path in order to minimize the total energy consumption while guaranteeing the reliability constraint. Then, we develop an optimal routing and power control algorithm that maximizes the network lifetime while keeping the network stable, and an alternative heuristic algorithm that has lower and tractable complexity. Simulation results reveal that a trade-off relation exists between the network lifetime maximization and the reliability constraints and that the network lifetime can be increased significantly by employing the proposed algorithms.

In this paper, we investigate the problem of the lifetime maximization in a wireless sensor network under the constraint of the target end-to-end transmission success probability, by adopting a cross-layer strategy that considers both physical layer (i.e., power control) and network layer (i.e., routing protocol) jointly. We first present the optimal power allocation algorithm for a given routing path. This allocation determines per-hop success probability for each link along the path in order to minimize the total energy consumption while guaranteeing the reliability constraint. Then, we develop an optimal routing algorithm that maximizes the network lifetime and a heuristic algorithm that has lower and tractable complexity. Simulation results reveal that a trade-off relation exists between the network lifetime maximization and the reliability constraint, and that the proposed routing algorithm combined with the optimal power allocation algorithm increases the network lifetime significantly.