Simultaneous Transmission and Null Space Tracking Using Interference Feedback for Spatial Division Multiple Access

In recent years, the radio frequency (RF) spectrum used by cellular operators has become an expensive resource. Therefore there is a need to develop creative methods for optimizing the use of the RF spectrum. To address this, we propose ST(P)-ENSL, a method consisting of two downlink cognitive radio transmission algorithms (ST-ENSL and STP-ENSL) that allow the secondary user (SU) on a frequency band to both transmit data symbols simultaneously, along with the primary user (PU), and perform MIMO-OFDM channel estimation and tracking, using the PU's interference level feedback. ST(P)-ENSL can be integrated into existing communication systems, improving the SU's throughput while inflicting minimal interference on the PU. We examine a scenario consisting of a PU and SU that share the same MIMO-OFDM frequency band. When using ST(P)-ENSL, the SU estimates the interference channel using the interference level feedback received from the PU and projects its transmissions onto the null space of the PU's interference channel to minimize interference. The PU is unaware of the SU, which only requires the noise level samples sensed by the PU in order to estimate the interference channel. Our method offers a distinct advantage over existing methods by optimizing the utilization of frequency band in next-generation cellular systems. This is particularly beneficial for user equipment (UE) situated at the edge of a cell, ensuring efficient frequency band use. We evaluated our method by simulating an SU that employs our transmission method on a narrowband fading channel model. The results demonstrate ST(P)-ENSL's ability to accurately estimate and track an interference channel over time and thus reduce the interference inflicted by an SU on a PU when transmitting data symbols to the UE registered on the SU's base station.