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 90degree 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.

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In the wireless communications, packet collisions due to the interfering signals are significant issue. Since the wireless local area network (WLAN) may not detect packets collision while transmitting the signals, right/wrong of the reception will be judged by the reply of the acknowledgment (ACK) frame. Therefore, the WLAN of transmission efficiency is very low and approximately 60%. In this paper, a novel access control scheme is proposed for the collision detection which utilizes multiple-input multiple-output (MIMO) technology. The proposed scheme improves transmitting efficiency compared to the conventional scheme. Via a computer simulation, it is confirmed that the proposed scheme achieves the transmitting efficiency of over 90%.

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This paper proposes an interference detection method in MIMO transmission, which utilizes periodical preamble signals in a frequency domain. In the propose method, second antenna receives the signals while first antenna transmits the preamble signals. Hence, the interference can be detected by observing the subcarriers which are not mapped short preamble signal using the received signal after the FFT processing. Moreover, we utilize the orthogonal polarized antennas to reduce the mutual coupling between the transmitter and receiver. By a computer simulation and measurement with the use of orthogonal polarized antenna, it is shown that the proposed method can successfully detect interference from the other system when the interfering power is greater than the noise power.