Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> ■ Extensive research has been done on the use of antenna arrays for direction finding and beamforming; this research focuses on the detailed behavior of specific techniques rather than on actual signal processing applications. In most applications, there is a fundamental signal feature that provides essential leverage for an effective processing approach. This article, which is structured around such features, presents a comprehensive framework for selecting an appropriate adaptive approach for processing cochannel narrowband signals. We address the roles of antenna calibration and prior waveform knowledge, and give examples of effective, practical direction-finding and beamforming procedures that cover a wide range of potential applications. <s> BIB001 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> In this paper, we consider spectrum sharing between multiple-input multiple-output (MIMO) radar system and a communication system modeled as MIMO interference channel. We derive a zero-forcing precoder for radar transmitter which completely eliminates the radar interference to communication users. Obtaining the precoder requires the knowledge of an effective interference channel matrix composed of the channel matrices to all of the communication receivers and the postprocessing matrices employed by them. We propose a channel estimation phase in which all of the communication receivers coordinate in their choice of training symbols and power transmission and the radar transmitter can estimate the effective interference channel. We investigate the effect of radar precoder and channel estimation error on the performance of radar and interference to communication receivers. Our results indicate that while the precoder null steers the radar interference to communication users it degrades the radar performance by introducing correlation to the probing signals. We show that this performance loss can be compensated for by increasing the number of radar antennas. <s> BIB002 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> The theoretical feasibility is explored of spectrum-sharing between radar and wireless communications systems via an interference mitigation processing approach. The new approach allows radar and wireless systems to operate at the same carrier frequency if the radar possesses a multiple-input multiple-output (MIMO) structure. A novel signal processing approach is developed for coherent MIMO radar that effectively minimizes the arbitrary interferences generated by wireless systems from any direction, while operating at the same frequency using cognitive radio technology. Various theoretical aspects of the new approach are investigated, and its effectiveness is further validated through simulation. <s> BIB003 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> 1. History 2. Notational and mathematical preliminaries 3. Probability and statistics 4. Wireless communications fundamentals 5. Simple channels 6. Antenna arrays 7. Angle-of-arrival estimation 8. MIMO channel 9. Spatially adaptive receivers 10. Dispersive and doubly dispersive channels 11. Space-time coding 12. 2x2 network 13. Cellular networks 14. Ad-hoc networks 15. Medium access control protocols 16. Cognitive radios 17. Multiple antenna acquisition and synchronisation 18. Practical issues. <s> BIB004 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> In this paper, we propose an information theoretic waveform design algorithm for multiple-input multiple-output radars to support co-existence with a communication system under additional constraints on interference reduction from the clutter, minimizing the peak-to-average-power ratio and minimizing correlation levels of the radar. Numerical results are presented which show the performance of the proposed waveform design algorithm in terms of mutual information, interference reduction and sidelobe levels of the radar waveform. <s> BIB005 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> Spectrum sharing is a new way forward to solve spectrum scarcity problem. In this paper, we propose a spatial approach for spectrum sharing between a MIMO radar and an LTE cellular system with N BS base stations (BS). The MIMO radar and LTE share N BS interference channels i.e. H i , i = 1,2, …, N BS . We propose projecting the radar signal onto the null space of interference channel between the MIMO radar and LTE using our proposed interference-channel-selection algorithm, in order to have zero-interference from the MIMO radar. We select interference channel with the maximum null space i.e. arg max 1≤i≤NBS dim [N(H i )] and project the radar signal onto the null space of this channel. Our proposed spatial spectrum-sharing algorithm is radar-centric such that it causes minimum loss in radar performance by carefully selecting the interference channel and at the same time protects the ith LTE BS from the radar interference. Through our analytical and simulation results we show that the loss in the radar performance is less when the proposed interference-channel-selection algorithm is used to select the channel onto which radar signals are projected. <s> BIB006 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> This paper presents a system concept of a MIMO OFDM Radar with sensing and communication abilities. With the proposed spectrally-interleaved OFDM signal model, multi-user access and object position estimation based on spatial diversity can be accomplished within the same system. Since the OFDM signals are sensitive and susceptible to inter-carrier misalignment, an interference handling technique must be incorporated into the system. This paper presents the signal model used to accomplish the multi-user access capability and analyzes the inter-carrier interference due to 1) the non-synchronized local oscillators between two MIMO OFDM Radar systems, and 2) the phase rotations caused by radar channel, and proposes an interference cancellation algorithm to overcome the problem. <s> BIB007 </s> Survey of RF Communications and Sensing Convergence Research <s> E. ADAPTIVE SPATIAL MITIGATION & MIMO SYSTEMS <s> In this paper, we present our spectrum sharing algorithm between a multi-input multi-output (MIMO) radar and Long Term Evolution (LTE) cellular system with multiple base stations (BS)s. We analyze the performance of MIMO radars in detecting the angle of arrival, propagation delay and Doppler angular frequency by projecting orthogonal waveforms onto the null-space of interference channel matrix. We compare and analyze the radar's detectable target parameters in the case of the original radar waveform and the case of null-projected radar waveform. Our proposed spectrum-sharing algorithm causes minimum loss in radar performance by selecting the best interference channel that does not cause interference to the $i^{th}$ LTE base station due to the radar signal. We show through our analytical and simulation results that the loss in the radar performance in detecting the target parameters is minimal when our proposed spectrum sharing algorithm is used to select the best channel onto which radar signals are projected. <s> BIB008
Others looked at spatial mitigation as a method for enabling joint radar-communications coexistence. Ultimately, the systems adaptively cancel specific users by exploiting system degrees of freedom and performing array processing. For example, some researchers have looked at sharing spectrum between an S-band radar with LTE cellular systems by projecting the radar signal onto the null space of the interference matrix BIB006 - BIB008 . Some researchers have arrived at such solutions by equalizing MIMO radar systems in the presence of MIMO communications in-band interference BIB002 , while others have developed similar results by solving for spatial filters to mitigate communications interference by exploiting MIMO degrees of freedom BIB003 . However, all of these spatial methods are merely a form of spatial isolation managed by radiation patterns of steered elements, and so once again rely on the classical assumption or driving to requirement of some form of isolation. Nulling interference using spatial degrees of freedom comes at a cost as well, as discussed in Reference BIB001 . For example, as shown in Figure 14 , the degradation to radar SNR as the radar attempts to spatially mitigate an interfering communications signal is dependent on the signal strength of the interferer, as well as the beamwidth separation BIB004 . This unit of separation for two steering vectors v 1 and v 2 is defined as follows BIB004 , BIB001 : where (·) † denotes the Hermitian conjugate. This definition is normalized such that b = 1 corresponds to orthogonal array responses. Note also that a residual signal-to-interferenceplus-noise ratio (SINR) term also remains at high interference SNR levels, as adaptive techniques match the pace of the increasing interferer strength BIB001 . True MIMO radar techniques have also been proposed, given the independent transmit elements could enable FIGURE 14. Effect of interference on radar SNR as a function of beamwidth separation and interferer strength. As is seen, the further the angular separation, the less the radar SNR is affected by the presence of the interference. Non-trivial beamwidth separations degrade radar performance, especially at low interference SNR where the estimation and cancellation of the interference is impacted by the degradation of the signal. multiple communications receivers naturally within its framework. Some have looked at an information-based waveform design for MIMO systems that trades detection performance with favorable correlation properties while attempting to minimize communications interference BIB005 . Others have extended the null space projection methods previously mentioned to MIMO architectures to allow more fine control of the degrees of freedom  . Similarly, researchers have extended the interleaved subcarrier OFDM approaches of other work to function as a MIMO radar over traditional phased arrays BIB007 . Finally, matrix completion MIMO radar techniques that are inherently less susceptible to interference from an in band MIMO communications user have been investigated in both non-cooperative and cooperative configurations  . The matrix completion methodology also has the advantage of requiring less bandwidth to send the radar image data to another site than traditional radar systems.