Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> This paper presents a derivation of the probability distribution function (pdf) of the aggregate interference in a secondary access network where multiple secondary users cause interference to a single primary user. The derivation considers a practical interference protection mechanism that the transmission of each secondary user is regulated by an interference threshold. Analytic pdf of the interference from a secondary user is obtained. Then, the distribution of the aggregate interference is approximated based on its cumulants. The derived pdf shows a good agreement with Monte Carlo simulation. <s> BIB001 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> In this paper, we investigate the impact of aggregate interference in a secondary spectrum access system. Particularly, meteorological radar operating in 5.6 GHz band is considered to be a primary user. Secondary users are WLAN devices spreading in a large area which induce aggregate interference to the radar. We develop a mathematical model to derive the probability distribution function (PDF) of the aggregate interference. The derivation considers dynamic frequency selection (DFS) mechanism for the protection of the radar such that the transmission of each WLAN is regulated by an interference threshold. Numerical experiments are performed with various propagation environments and densities of WLAN devices. It is observed that the effect of aggregate interference is severe in a rural environment. The interference threshold for individual WLAN should be much lower than the maximum tolerable interference at the radar. Thus, only a limited number of WLANs can transmit at the same time. On the other hand, adverse effect of the aggregate interference is not shown in an urban environment, where up to 10 WLANs per square kilometer can use the radar spectrum without considering the aggregate interference. The framework discussed in this paper can readily be adapted to assess the aggregate interference for other types of radars. <s> BIB002 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> In this paper, we have studied the potential for secondary usage of radar bands by 3GPP LTE eNB devices in different scenarios, such as HeNB transmitters located at street level, HeNB transmitters located at high-rise buildings, macro LTE transmitter, etc. Different pathloss models are used to best suite the scenarios. By using different types of radar characteristics (e.g. radio navigations radars, meteorological radars, etc) and a protection requirement of −10dB Interference-to-Noise Ratio (INR), we have shown that in some scenarios, the required distances for adjacent channel radar usage in 2.7–2.9GHz band are quite reasonable. This means, in those scenarios, it could be possible to utilize the radar bands for secondary LTE systems. A protection margin of 18 to 20dB can be added for capturing the aggregate interference effects from multiple secondary interferers for downlink direction. More detailed system level investigations are required in this direction for further understanding the secondary usage in this band. <s> BIB003 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> An overview of the challenges related to spectrum sharing between radar and communication systems is provided and some potential ways to address them are indicated. <s> BIB004 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> National Telecommunications and Information Administration (NTIA) has proposed vast exclusions zones between radar and Worldwide Interoperability for Microwave Access (WiMAX) (WiMAX) systems which are also being considered as geographic separations between radars and 3.5 GHz Long Term Evolution (LTE) systems without investigating any changes induced by the distinct nature of LTE as opposed to WiMAX. This paper performs a detailed system-level analysis of the interference effects from shipborne radar systems into LTE systems. Even though the results reveal impacts of radar interference on LTE systems performance, they provide clear indications of conspicuously narrower exclusion zones for LTE vis-\`a-vis those for WiMAX and pave the way toward deploying LTE at 3.5 GHz within the coastline populous areas. <s> BIB005 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> In this paper, we investigate a friendly spectrally shaped radar waveform design that can be used in a spectral band being utilized by one or more communication systems. We specifically consider legacy communication systems as opposed to cooperative communication systems to address the ever present problem of legacy technologies. This radar waveform is able to share the spectrum with the existing communication systems such that its detection performance is not compromised while trying to help the legacy systems maintain their own symbol error rates (SERs). We show with various scenarios that the spectrally shaped radar waveform outperforms the traditional wideband pulse waveform in terms of detection performance with the communication signals acting as interference to the radar. Moreover, SERs of the legacy systems employing quaternary phase-shift keying modulation in the presence of the shaped radar waveform (acting as interference) outperform SERs of systems under traditional radar pulse interference. These SERs are very close to theoretical noise-only SERs. <s> BIB006 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> Spectrum sharing between radar and communications systems is currently under investigation because of the high demand for new wireless services and shortage of available bandwidth. To effectively design their coexistence, it is crucial to understand how current unaltered radar and communication systems would affect one another. This paper investigates the effect of radar interference on an uncoded data communication system where the optimal Maximum-A-Posteriori decoder is used and where the bandwidth of the radar system is much larger than the one of the communication system. Conclusions depend on how the radar interference power, measured by the Interference-to-Noise ratio (INR), compares with the intended signal power, measured by the Signal-to-Noise ratio (SNR). For the case of real-valued modulation schemes, three regimes emerge: (a) Treat interference as Gaussian noise: when INR >SNR the optimal receiver estimates the radar interference and subtracts it from the received signal; in the process of canceling interference, part of the useful signal is also cancelled, which reduces the effective SNR at the receiver. The probability of error exhibits an irreducible error floor, which can be exactly characterized and behaves like a narrow-band fading channel with multiplicative fading that is perfectly known at the receiver. (c) When INR≈SNR, the probability of error attains its maximum value, thus indicating that there is a worst operating INR for any given SNR. <s> BIB007 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> This document derives approximate expressions for the Cramer-Rao lower bounds on the variance of unbiased estimates of the parameters of a narrow-band radar model in the presence of additive white Gaussian noise as well as interference with known structure. We show that the Cramer-Rao lower bounds with interference are comprised of the bound when the interference is not present and a term that is proportional to the squared normalized-correlation between the radar signal and the interfering signal. Numerical simulations demonstrating these bounds are shown and the threshold effect is observed. The bounds are then used to define an objective function to be used for waveform co-design, and a simple example of this is shown. <s> BIB008 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> The ability of radar and communication applications to share the radio spectrum will require the use of innovative agile circuit techniques for radar and communications. Reconfigurable circuits can provide real-time adjustment of operating frequency and spectral output, while maintaining system performance and maximizing power efficiency. This paper discusses recent developments in circuit optimization techniques for power efficiency and spectral performance. Optimization of a single parameter (load reflection coefficient) for multiple criteria is first addressed, followed by multiple-parameter, multiple-criteria optimizations. The use of the recently innovated Smith Tube to optimize additional parameters, such as input power and bias voltage, simultaneously with the load impedance is discussed. Optimization examples and a forward look to fast, emerging multidimensional circuit optimization techniques are provided. <s> BIB009 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> Radar signal design in spectrally dense environments is a very challenging and topical problem. This paper deals with the synthesis of waveforms optimizing radar performance while satisfying multiple spectral compatibility constraints. Unlike some counterparts available in the open literature, a specific control on the interference energy radiated on each shared bandwidth is enforced. To tackle the resulting NP-hard optimization problem, a polynomial computational complexity procedure based on SemiDefinite Relaxation (SDR) and randomization is developed. Hence, some numerical results are shown to highlight the effectiveness of the new technique to devise high performance radar waveforms complying with the spectral compatibility requirements. <s> BIB010 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> In this paper, we investigate the demodulation of a communication signal that is interfered by a high-powered radar signal. We choose quaternary phase-shift keying (QPSK) modulation for illustration. A least squares estimator (LSE) is used to estimate the amplitude and phase of the interfering radar signal to be used for interference cancellation. Then a maximum likelihood detection (MLD) receiver is used. We show that the QPSK SER improvement in the radar interference environment depends on collection time. For large collection times, the SER approaches the theoretical SER for QPSK. Unfortunately, as the collection time decreases the variance of the radar estimate increases, which degrades the SER. Further, we show increases in radar estimate variance as the QPSK SNR increases, which affects the SER. We propose a form of interference cancellation based on estimating the radar signal and then subtracting it from the received signal prior to MLD of the communication signal. <s> BIB011 </s> Survey of RF Communications and Sensing Convergence Research <s> A. COEXISTENCE METHODS <s> We study the interference from a rotating shipborne radar system that spectrally and spatially coexists with a Long Term Evolution (LTE) cellular communications network in the 3.5 GHz band to investigate the feasibility of LTE deployment in the United States coastal metropolitan cities in that band. First, we simulate the radar systems with realistic operational parameters. Furthermore, we leverage a detailed 3GPP-compliant LTE simulation with a sophisticated air interface modeling and investigate sensitivity of LTE to radar interference in macro cell, outdoor small cell, and indoor small cell scenarios. We simulate the propagation conditions between the radar and LTE system by adopting the Free Space Path Loss and Irregular Terrain Model commonly leveraged by National Telecommunications and Information Administration (NTIA), to account for propagation, diffraction, and troposcatter losses that the radar pulses undergo before they reach the LTE system. As a performance metric, we evaluate the throughput of the LTE system in the uplink direction for various distances between the radar and the cellular system. Our simulation results indicate an LTE link will remain operational even in severe interference conditions. In fact, the LTE system as close as 100 km away from the radar undergoes less than 10 % throughput loss from the LTE total throughput, and the throughput loss is less than 30 % when the radar is only 50 km away from the LTE. <s> BIB012
Preliminary research looked at the problems posed by spectrum sharing relative to each user. Immediately it was FIGURE 12. Magnitude of the complex ambiguity function, or CAF, for both the linear frequency-modulated chirp (shown in blue), and an OFDM communications signal (dashed orange). The random data of the OFDM waveform has noise like properties, and so the CAF surface falls off rapidly away from the zero-Doppler, zero-time shift origin. Shown are contours at −3 dB, −6 dB, and −10 dB from the peak at (0,0). LFM ridge. The linear contours are a result of range-Doppler coupling. This can be favorable in some processing conditions, where Doppler shifts do not eliminate range-only matched filters. However, they present a bias in range as a function of Doppler. The near perfect OFDM spike means the cross-ambiguity matched filter may need to be sampled at extremely fine granularity. clear that unaddressed or unacknowledged, the effect on radar for even mild interference is large for in-band operation BIB004 . Therefore, coexistence methods have acknowledged the interference posed by spatially adjacent users to incorporate this interference model into their processing BIB001 . For example, recent works have looked at modifications to the optimal symbol decision regions for communications based on additive radar interference statistics and the resulting average bit error rate (BER) performance in different SNR regimes BIB007 . Using the results in Reference BIB007 , we demonstrate an example of the modified detection region for an 8-PSK system in Figure 13 , with an SNR of 5 dB and a signal-to-interference ratio of -5.5 dB. This partitioning is determined by employing maximum likelihood symbol detection with a random radar interference phase term: where Y rx is the received signal, S is the communications signal power, I is the interference-to-noise ratio, X is the communications symbol from constellation X = {x 1 , ..., x N }, is the radar interference random phase term, and Z is a standard Gaussian noise source. Under the random radar phase term, the maximum likelihood symbol for a given received point in the constellation is given by BIB007 l(y) = arg min where y is the received signal, x l is a hypothesis constellation point, and I 0 (x) is the modified Bessel function of the first kind, order 0. It can be seen in Figure 13 that far from the origin, the decision regions are the standard, interference free regions. However, the decision symbol regions close to the origin are distorted by the spatial Bessel function term. This illustrates that even in this simple interference model, the communications decision logic is significantly altered by the presence of the radar. Conversely, others have looked at the effect of communications interference with significant structure on radar estimation variance bounds BIB008 . Some researchers have investigated optimization at a circuit level, using Smith Tubes to adaptively manage radar and communications regulatory spectral masks through dynamic impedance matching BIB009 . Others have investigated radar waveform design in legacy communications bands where the communications systems are rigid and unable to adapt BIB006 , where the radar waveform employs a form of water-filling. Researchers have also looked at spectrally constrained radar waveform design to determine the feasibility of future systems believed to be faced with growing regulatory requirements BIB010 . Some researchers have looked at radar estimation performance and integration time impacts on a communications user's ability to cancel radar returns, specifically in high power radar scenarios BIB011 . Others have developed computationally feasible models for performance impact on meteorological radar from secondary communications users with dynamic frequency selection capability BIB002 . The link budget of LongTerm Evolution (LTE) systems operating in legacy radar bands at finite stand-off distances in urban environments has been calculated to determine the impact for cellular customers BIB003 . Others have studied regulatory exclusion zones for radar and cellular users, concluding that the stipulated stand-off range is overly conservative relative to the impact both users present to one another BIB005 . Some researchers have fused multiple LTE simulation and radar models to examine the impact of rotating radar users on LTE systems at various distances and cellular configurations BIB012 .