This paper proposes an interference avoidance technique that allows wireless device with similar frequency bands to be operated adjacent to each other for compact mobile wireless routers (MWRs). This MWR implements two devices of Wireless LAN (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX). The MWR connects WLAN terminals to the backbone network by using WiMAX-WLAN relay. Generally, different frequency channels are assigned for the wireless systems assign in order not to interfere among multiple systems. However, mutual system interference is generated if the space between each device is very close and if the frequency using each system is adjacent. To suppress this interference, this paper proposes a novel interference avoidance technique that leverages IEEE802.11n Power Save Multi-Poll (PSMP). First, we clarify the conditions that raise the issues of mutual interference by experiment. Simulations are conducted to show that the proposed scheme outperforms the conventional schemes. Finally, the effectiveness of the proposed scheme is shown by the computer simulation.

This paper proposes a channel capacity maximization method for Multiple-Input Multiple-Output (MIMO) antennas with parasitic elements. Reactive terminations are connected to the parasitic elements, and the reactance values are determined to achieve stochastically high channel capacity for the environment targeted. This method treats the S-parameter and propagation channel of the antenna, including the parasitic elements, as a combined circuit. The idea of the 'parasitic channel,' which is observed at the parasitic antenna, is introduced to simplify the optimization procedure. This method can significantly reduce the number of necessary measurements of the channel for designing the antenna. As a design example, a bidirectional Yagi-Uda array, which has two driven antennas at both ends of the linear array, is measured in an indoor environment. The resulting design offers enhanced channel capacity mainly due to its improved signal-to-noise ratio compared to the antenna without the parasitic antennas.

A novel analog decoding method using only 90-degree phase shifters is proposed to simplify the decoding method for short-range multiple-input multiple-output (MIMO) transmission. In a short-range MIMO transmission, an optimal element spacing that maximizes the channel capacity exists for a given transmit distance between the transmitter and receiver. We focus on the fact that the weight matrix by zero forcing (ZF) at the optimal element spacing can be obtained by using dividers and 90-degree phase shifters because it can be expressed by a unitary matrix. The channel capacity by the proposed method is next derived for the evaluation of the exact limitation of the channel capacity. Moreover, it is shown that an optimal weight when using directional antennas can be expressed by using only dividers, 90-degree phase shifters, and attenuators, regardless of the beam width of the directional antenna. Finally, bit error rate and channel capacity evaluations by both simulation and measurement confirm the effectiveness of the proposed method.

This paper presents a novel decoupling network consisting of transmission lines and a bridge resistance for a two-element array antenna and evaluates its performance through simulations and measurements. To decouple the antennas, the phase of the mutual admittance between the antenna ports is rotated by using the transmission lines, and a pure resistance working as a bridge resistance is inserted between the two antenna ports to cancel the mutual coupling. The simulation results indicate that the proposed decoupling network can provide a wider bandwidth than the conventional approach. The proposed decoupling network is implemented and tested as a demonstration to confirm its performance. The measurement results indicate that the mutual coupling between the two antenna ports is lowered by about 47dB at the resonant frequency.

Controlling interference from the secondary system (SS) to the receiver of the primary system (PS) is an important issue when the SS uses the same frequency band as the television broadcast system. The reason includes that the SS is unaware of the interference imposed on the primary receiver (PS-Rx), which does not have a transmitter. In this paper, we propose an interference control method between PS-Rx and SS, where a load modulation scheme is introduced to the PS-Rx. In this method, the signal from the PS transmitting station is scattered by switching its load impedance. The SS observes the scattered channel and calculates the interference suppression weights for transmitting, and controls interference by transmit beamforming. A simulation shows that the Signal-to-Interference Ratio (SIR) with interference control is improved by up to 41.5dB compared to that without interference control at short distances; the results confirm that the proposed method is effective in controlling interference between PS-Rx and SS. Furthermore, we evaluate the Signal-to-Noise Ratio (SNR) and channel capacity at SS.

This paper proposes a new method that combines signal modulation and FDTD (Finite-Difference Time-Domain) simulations to reduce the computation time in multiple-antenna analysis. In this method, signals are modulated so as to maintain orthogonality among the excited signals; multiple antennas are excited at the same time. This means just one FDTD simulation is needed whereas the conventional method demands as many simulations as there are transmitting antennas. The simulation of a 2×2 multi-antenna system shows that the proposed method matches the performance of the conventional method even though its computation time is much shorter.

A series-fed beam-scanning array employing a MUlti-Stage Configured microstrip Antenna with Tunable reactance devices (MUSCAT) is proposed. The proposed antenna significantly expands the beam scanning range and achieves high efficiency. This antenna comprises unit element groups, whose elements are placed close to each other and employ tunable reactance devices. Analyses and experiments on the unit element groups show that their multi-stage configuration extends the phase shift range and increases the radiation efficiency, e.g., a 120phase shift and the radiation efficiency of more than 50% are achieved, when three stages are employed. The radiation pattern of the fabricated MUSCAT array antenna comprising eight unit element groups is measured. A beam scanning range of 27, which is greater than twice the beam scanning range of a non-multi-stage configuration, is achieved.

As wireless LAN systems become more widespread, the number of access points (APs) is increasing. A large number of APs cause overlapping cells where nearby cells utilize the same frequency channel. In the overlapping cells, inter-cell interference (ICI) degrades the throughput. This paper proposes an interference-aware multi-cell beamforming (IMB) technique to reduce the throughput degradation in the overlapping cells. The IMB technique improves transmission performance better than conventional multi-cell beamforming based on a decentralized control scheme. The conventional technique mitigates ICI by nullifying all the interference signal space (ISS) by beamforming, but the signal spaces to the user terminal (UT) is also limited because the degree of freedom (DoF) at the AP is limited. On the other hand, the IMB technique increases the signal space to the UT because the DoF at the AP is increased by selecting the ISS by allowing a small amount of ICI. In addition, we introduce a method of selecting the ISS in a decentralized control scheme. In our work, we analyze the interference channel state information (CSI) and evaluate the transmission performance of the IMB technique by using a measured CSI in an actual indoor environment. As a result, we find that the IMB technique becomes more effective as the number of UT antennas in nearby cells increases.

Intruder detection method by utilizing a time variation of Multiple Input Multiple Output (MIMO) channel (MIMO Sensor) has been proposed. Although the channel capacity on the MIMO transmission is severely degraded in time variant channels, we can take advantage of this feature in MIMO Sensor applications. We have already demonstrated the effectiveness of 2×2 MIMO sensor using 2.4GHz band at a small room (Size is 50m2). In this paper, we compare the detection probability of SIMO/MIMO sensors when the number of channel responses are same between SIMO/MIMO sensors: The numbers of transmit and receive antennas are 1 and 4 (SIMO), it is clarified that 2 and 2 (MIMO). The measurement was carried out at the room with the size of 140m2. From the measured results, 2×2 MIMO sensor obtains the same or higher detection probability compared to 4×1 SIMO sensor regardless of the measured location.