Near Field Communication (NFC) applied with resonant coupling would lead to an innovative improvements such as power transfer efficiency and robustness, which can be most suitable for medical applications. However, there are several technical issues of the conventional NFC system that makes it difficult to apply resonant coupling. One of the critical issues is that the system performance becomes excessively sensitive to impedance matching and bandwidth of its antenna. In this paper, we propose a novel high-Q NFC system that overcomes the limitations of applying resonant coupling to conventional NFC system. As a result, the high-Q NFC system showed remarkable improvement in energy efficiency. Moreover, the high-Q NFC system showed perfect compatibility with conventional NFC system, while it quadrupled the power transmission efficiency.

Near field channel is fundamentally different from conventional far field channel. Instead of using electromagnetic wave propagation, magnetically coupled resonance is utilized in near field. In order to apply this resonance phenomenon to communication system, Resonance Isolation (RI) system was recently proposed. In the RI system, a resonator stored with initial energy is mutually coupled with another resonator in near field, alternately exchanging its energy with the resonator. We establish two corresponding communication channel models using circuit theory and coupled mode theory, considering three classical network scenarios: two resonators (peer-to-peer), three resonators (equilateral), and three resonators (relay). We analyze the channel models for each scenario by both models and compare the performance with numerical results. We found significant relaying gain in the relaying scenario, and we defined this scenario as the Resonate and Forward technique.

In wireless sensor networks, limited battery capacity in each sensor node restricts the reliability of the network. This restriction can be somewhat removed by recharging the node batteries through wireless energy transmission. In a mobile wireless sensor network, if there is only one energy station that can transmit energy, the sensor nodes have to rely on their mobility to receive energy from an energy station. When a node battery is depleted, the node stops activating and stays idle until it recharges its battery. As the number of such idle nodes increases, the reliability of the wireless sensor network will decrease. If energy can be relayed via mobile nodes, a battery-depleted node can recharge its battery with higher rate, which affects the reliability of the network. In this paper, we show that if the energy transmission efficiency exceeds a certain threshold then the reliability of the network can always be improved.

Technology of using mid-range magnetic resonant coupling has enabled efficient power transmission in near field. The advantage of using resonant coupling as a power transmitting method lies in the high efficiency, omnidirectional, and no electromagnetic radiation on human body. When a continuous wave (AC) is used as the power source, the resonant coupling method is required to match the impedances at both transmitting and receiving ends in order to achieve the high efficiency. Instead of using an AC, the Resonator Isolation (RI) system uses an instantaneous periodic delivery (IPD) method to effectively transfer power and data at the same time. Using IPD method, the RI system does not require impedance matching at both ends. However, the operation between the transmitter and the receiver in the RI system needs to be synchronized. Since the transmission efficiency highly depends on the synchronization performance, the RI system requires an accurate synchronizing method. In this paper, we describe how to synchronize the transmitter and the receiver in the RI system, and present an accurate synchronization can be achieved with simple circuitry and operation.

In this paper, we propose a power autonomous sensor platform with ultra-low power radio frequency (RF) transceiver and wireless charging. The ultra-low power RF transceiver consists of super-regenerative receiver and direct-modulation transmitter with multi-cell power amplifier. Although the proposed transmitter and receiver only consume 1.79 and 0.683 mW, respectively, the wearable sensor platform requires more power efficient transmitter, due to the limited capacity of rechargeable thin film battery, e.g, 50 ~ 250 mWhr [1]. Therefore, we apply wireless charging in order to prolong the lifetime and investigate the impact of wireless power transfer on the power autonomy of sensor platform.

Wireless power transmission using mid-range magnetic resonance has become an important technology in body area sensor systems. Recent researches have used fine-tuned impedance matching for wireless power transmission at fixed distance. However, this method is significantly sensitive to the distance change between two devices since the impedances at both ends must be perfectly matched. Hence, it is a major challenge to achieve efficient wireless power transmission in mobile environment. We propose a new circuit architecture for wireless power transmission in the near field; the Resonator Isolation (RI) structure. Using the RI structure, the wireless power transmission becomes highly robust under mobile environment. In this paper, we present the circuit design of the RI structure and evaluate its performance with the actual systems we constructed.

This paper proposes a spectral efficient cooperative diversity technique with multi-layered modulation for high data rate transmissions. The proposed system transmits with different cyclic delay and phase rotation patterns, in order to achieve cooperative diversity gain for all transmit signals from the source. Since these patterns increase the interference between the layers, we derive optimal cyclic delay and phase rotation patterns to minimize mean square error. With the optimal patterns, the proposed system still suffers from inter-layer interference, especially in high-order modulations. To overcome this problem, multiple layers are separately interleaved by layer-specific interleavers and are mapped by hierarchical symbol mapping for the high data rate transmission with cooperative diversity. A destination structure with iterative equalization is also presented to separate each layer and extract diversity gain for high data rate transmissions. The proposed system can use reliable feedback from the layers and effectively remove the inter-layer interference. To estimate the performance of the proposed system, we develop a density evolution technique. It is shown that the predicted performances agree with simulated performances. Simulation results indicate that the proposed system achieves cooperative diversity and has better performance than conventional cooperative systems.

This paper proposes spectral efficient relay-assisted transmit diversity techniques, i.e., space-time and space-frequency block codes (STBC/SFBC), for single carrier frequency-domain equalization (SC-FDE). The proposed systems achieve spatial diversity over frequency selective channels in a distributed fashion without cyclic prefix (CP), which increases spectral efficiency of conventional relay-assisted systems. In practical mobile applications, the CP-less distributed STBC (D-STBC) SC-FDE suffers from the time selectivity of wireless fading channels, resulting in a deviation from the basic assumption of Alamouti codeword. In order to obtain a reliable performance over doubly selective channels, the CP-less D-SFBC SC-FDE and its efficient implementation are presented. Further, the performance of these two distributed SC transmissions is analytically compared over fast fading (i.e., time selective) environments. For evaluation of mean square error over fast fading channels, we present a channel model that captures the time-varying nature of wireless channels. It is shown through analysis that the CP-less DSFBC SC-FDE outperforms the CP-less D-STBC SC-FDE when there exists a severe Doppler spread. Timing synchronization and highly accurate channel estimation of the proposed systems are also addressed for practical implementations. Simulation results show that the proposed systems approach the lower bound of full-CP systems, thus increasing the spectral efficiency.

Peak-to-average power ratio (PAPR) regrowth after clipping is one disadvantage of space-frequency block coded orthogonal frequency-division multiplexing (SFBC-OFDM). In this letter, we propose an effective PAPR reduction technique of SFBC-OFDM for multinode cooperative transmission. To reduce PAPR at the source (mobile equipment), the relay applies SFBC encoding, which enables the source to transmit clipped single-input single-output (SISO)-OFDM signals without any increase of PAPR. Simulation results show that the clipped signals of proposed scheme are effectively recovered, and the proposed scheme achieves the diversity of SFBC without the complexity of multiple antennas at the source.

This letter proposes a multiple-input multiple-output (MIMO) technique for single carrier frequency-domain equalization (SC-FDE). The proposed system achieves spatial multiplexing and transmit diversity gains altogether, thus improving error performance and increasing spectral efficiency compared to conventional spatial multiplexing and transmit diversity techniques, respectively. In order to achieve both of the gains, independent data streams are transmitted simultaneously through all transmit antennas with different cyclic delay patterns for each data stream. Corresponding receiver structure is also presented, where a successive interference cancellation (SIC) algorithm without ordering process is introduced. Simulation results show that the proposed system with iterative receiver structure provides considerable performance gains over conventional MIMO techniques.

Space-time block coded (STBC) single carrier (SC) transmission was extended in a distributed fashion for practical implementation of user cooperation. However, STBC was designed under the assumption that the channel is static over the duration of a space-time codeword. Thus, the distributed STBC SC system suffers from the time selectivity of wireless fading channels. To achieve a reliable performance over fast fading channels, we propose a distributed space-frequency block coded (D-SFBC) SC transmission. This paper analytically compares the performance of these two distributed SC transmissions over fast fading environment. For evaluation of mean square error (MSE) over fast fading channels, we present a channel model that captures the time-varying nature of wireless channels. It gives an insight into the characteristics of inter-carrier interference and simplifies the evaluation of MSE. Using this model, it is proven that the D-SFBC SC transmission outperforms the D-STBC SC transmission when there exists a severe Doppler spread. Simulation results are also provided to validate our analysis and to compare two distributed single carrier transmission schemes.

This paper proposes a full-rate cooperative communication technique with spatial diversity for single-carrier transmissions. In order to achieve both the spatial diversity and the full-rate, data streams are simultaneously transmitted through all direct and relay channels with different phase rotation and cyclic delay patterns. The phase rotation and cyclic delay patterns are derived in the sense of minimizing interlayer interference and a corresponding destination structure is presented for decoupling each layer at the destination. Simulation results show that the proposed technique achieves spatial diversity without sacrificing spectral efficiency and outperforms conventional full-rate cooperative communication techniques.

Multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) has become a promising candidate for next generation broadband wireless communications. However, like a single-input single-output (SISO)-OFDM, one main disadvantage of the MIMO-OFDM is the high peak-to-average power ratio (PAPR), which can be reduced by using an amplitude clipping. In this paper, we propose clipped signal reconstruction methods for the MIMO-OFDMs with spatial diversity, such as space-time and space-frequency block codes (STBC/SFBC). The proposed methods are based on the technique called iterative amplitude reconstruction (IAR) for SISO-OFDM. It is shown that the IAR can be easily employed for the STBC-OFDM, but it cannot be directly applied to the SFBC-OFDM, because the transmitted sequences over different antennas are dependent due to the use of space-frequency code. We propose a new SFBC transmitter for clipped OFDM, which has approximately half the computational complexity of conventional SFBC-OFDM. The proposed clipping preserves the orthogonality of transmitted signals, and the clipped signals are iteratively recovered at the receiver. Further, we theoretically analyze the performance of IAR with optimum equalization, and also provide highly accurate channel estimation of the OFDM with amplitude clipping. Simulation results show that the proposed receivers effectively recover contaminated OFDM signals with a moderate computational complexity.

Relay-assisted space-time block code (STBC) and space-frequency block code (SFBC) for single carrier frequency-domain equalization (SC-FDE) were presented. They achieve spatial diversity over fading relay channels under the assumption of perfect channel state information (CSI). In this paper, we propose a pilot position selection/detection technique for channel estimation of those systems. Unlike the conventional block-type channel estimation techniques, the proposed scheme superimposes pilots on data-carrying tones whose positions are selected to minimize the distortion of original signals. Without additional pilot overhead, the proposed technique can track the CSI even when the mobile equipment speed is high. The corresponding destination structure and frequency domain equalization are also presented, where the pilot positions are blindly detected and the distorted data symbols are iteratively reconstructed. Simulation results show that the proposed method gives better BER performance than the block-type channel estimation for the distributed SFBC (D-SFBC) SC-FDE over fast fading relay channels, without the loss of spectral efficiency.

This paper proposes a relay-based transmit diversity technique for single carrier frequency-domain equalization (SC-FDE). The proposed system achieves spatial diversity over fading channels in a distribution fashion without cyclic prefix (CP), which increases spectral efficiency of conventional relay- based systems. The destruction of channel cyclicity due to the lack of CP is recovered at the input of relay and destination. In order to obtain spatial diversity, the transmit sequence of relay is efficiently generated in time domain, realizing space-frequency block code (SFBC). Corresponding destination structure using FDE, timing synchronization, and highly accurate channel estimation for the proposed system are also presented. Simulation results show that the proposed system approaches the lower bound of full-CP system.

This paper proposes a relay-assisted space- frequency block code (SFBC) for single carrier frequency-domain equalization (SC-FDE) in a distributed fashion. The proposed scheme achieves spatial diversity over uplink fast fading channels without the complexity of multiple antennas. The source (mobile equipment) of the proposed system has a very simple transmitter structure with constant amplitude transmit sequences. In order to obtain spatial diversity, the transmit sequence of relay is efficiently generated in the time domain, which is equivalent to the SFBC. The corresponding destination structure and frequency domain equalization are also presented. Simulation results show that the proposed system considerably outperforms the distributed space-time block code (D-STBC) SC-FDE over fast fading channels.

Two of the major drawbacks of orthogonal frequency-division multiplexing (OFDM) are its large peak-to-average power ratio (PAPR) and long cyclic prefix (CP), which decrease the system power and spectral efficiencies, respectively. Clipping is an efficient and simple method to reduce the PAPR of OFDM. However, it causes distortion that degrades the system performance. In this paper, an iteration technique, called iterative CP and amplitude reconstruction, is first proposed, which is to recover the insufficient CP and clipped OFDM signals simultaneously with one iteration loop at the receiver. Simulation results show that the proposed receiver effectively recovers the contaminated OFDM signals with a moderate computational complexity

As with orthogonal frequency-division multiplexing (OFDM), one main disadvantage of multi-input multi-output (MIMO) OFDM is the prohibitively large peak-to-average power ratio (PAPR) of transmitted signals on different antennas, which can be reduced by a deliberate amplitude clipping. However, clipping causes distortion that degrades the system performance. In this letter, we propose clipping noise mitigation techniques for space-time and space-frequency block coded (STBC/SFBC)-OFDMs. A new SFBC transmitter for clipped OFDM is proposed, and the optimum equalizer is derived for reconstruction of the clipped signals. Simulation results show that the proposed receivers effectively recover contaminated STBC/SFBC-OFDM signals with a moderate computational complexity.