## 1 Introduction  
Since many kinds of wireless communication systems such as wireless fidelity (Wi-Fi), worldwide interoperability for microwave access (WiMAX) or long-term evolution (LTE) (-Advanced) have emerged, frequency resources are becoming exhausted, especially in the microwave band. In order to accommodate the explosive increase in data traffic, many approaches are now being actively discussed, e.g., the use of higher frequencies, small cell deployment, and Multiple-Input Multiple-Output (MIMO) enhancement [1, 2]. All of them are essential in improving the spectral efficiency. To further enhance frequency resource utilization, the spectrum sharing approach, well known as cognitive radio [3], is being investigated. The basic principle is opportunistic spectrum access (OSA) or dynamic spectrum access (DSA) [4, 5]: the secondary system detects which time/frequency resources are active or which are idle, and instantly accesses vacant channels so as not to interfere with the primary systems. It can maximize the use of available frequency resources, but spectral utilization efficiency cannot be increased beyond 100%. This approach also necessitates spectrum usage of the primary system. The secondary system should perform request-to-send (RTS) and clear-to-send (CTS) handshake, rendezvous protocols [6; 7] or requires signaling knowledge of the primary transmitter [5]. These preprocessing might cause non-negligible overhead. A concept for utilizing unlicensed spectrum such as LTE in unlicensed spectrum (LTE-U) [8] or licensed assisted access (LAA) [9] has emerged as one of the most promising solutions to the spectrum exhaustion. Although unlicensed spectrum can be effectively utilized by the LTE function, there is apprehension that it will cause inter-system interference for Wi-Fi systems. With the LAA, listen before talk (LBT) [10] is essential to ensure co-existence with Wi-Fi, but it still permits resource sharing in time domain. Therefore, spectrum sharing in a spatial domain, i.e., spectrum superposing, is a significant approach to further enhance the spectral efficiency. Opportunistic analysis for the spectrum superposing among different wireless communication systems was conducted [11]. It derived a possibility of spectrum sharing in spatial domain for the secondary system, but its communication range is limited to be short. To expand the spectrum superposing region, some additional processing could be required. The null-steering approach [12; 13] disclosed the spectral efficiency improvement by suppressing inter-system interference in the superposed spectrum. The most important issue in order to realize fully spectrum superposing is how to estimate channel state information (CSI) of both primary and secondary systems since obtaining such information about different systems (inter-system CSI) is impossible. Given the above constraint, the following problems must be resolved:  
* suppressing inter-system interference from the primary transmitter at the secondary receiver;
* reducing inter-system interference to the primary receiver by the secondary transmitter;
* both interference suppression/reduction must be realized in a blind manner. Our original approach is to exploit the blind adaptive array (BAA) signal processing. It is quite effective in suppressing interference without inter-system CSI estimation. Constant modulus algorithm (CMA) [14; 15] and power inversion (PI) [16; 17] are well known BAA algorithms that do not require any a priori information such as a reference signal or direction of arrival. However, these algorithms are limited to some operational signal-to-interference power ratio (SIR) range at the input of receiver antenna elements. CMA cannot suppress the interference when SIR is smaller than 0 dB and PI cannot suppress it when SIR is greater than 0 dB. In other words, these algorithms face strong limits on their applicable region when the desired signal and the interference signal have almost the same power (SIR \(\simeq\) 0 dB).