An integrated radar and communications system (IRCS) where a monostatic radar transceiver is employed for target classification while simultaneously used as a communications transmitter is considered. The radar combined propagation-target response (joint response of the radar propagation channel and target) and communications channel response are generally frequency selective but the corresponding frequency response functions are not exactly known. In particular, these frequency response functions are only known to lie in an uncertainty class. To ensure the IRCS simultaneously provides acceptable target classification performance and communications rate, a robust orthogonal frequency division multiplexing (OFDM) integrated radar and communications waveform (IRCW) design method is proposed. The approach finds a waveform that simultaneously provides a sufficiently large weighted sum of the communications data information rate (DIR) and the conditional mutual information (MI) between the observed signal and the radar target over the entire uncertainty class. First, the conditional MI and DIR based on the integrated OFDM radar and communications waveform are derived. Then, a robust OFDM IRCW optimization problem based on the minimax design philosophy is developed such that closed-form solution is derived. Finally, several numerical results are presented to demonstrate the effectiveness of the proposed method.
With the increased acceptance of communications and radar systems for both commercial and defense applications, the study of integrated systems has attracted significant attention in the signal processing community [1]- [3]. These systems have advantages in reducing the hardware cost and improving the spectrum usage. For example, [4] describes how radar, communications, and electronic warfare functionality can be integrated into the same platform with array antennas, signal processing, and display hardware shared. The work in [4] uses the advanced multifunction radio frequency concept in order to decrease the system size, weight, and electromagnetic interference while performing multiple functions. However, different functions are carried out by using different waveforms, independently. One important application involves intelligent transportation systems (ITS) [5], [6]. The research in [5] and [6] investigates the integration of radar and communications in an intelligent vehicle system, where the radar can sense collisions and traffic while the communications device connects a vehicle to other vehicles and information sources or collection points. Some different alternative for the design of such systems are surveyed in [3]. In this paper, we will consider an integrated radar and communications system (IRCS) where a monostatic radar transceiver (transmitter and receiver) is employed for target classification while simultaneously used as a communications transmitter.
The integration of radar and communications hardware is a topic of great interest [7]. It seems crucial to explore the best approaches to design the integrated radar and communications waveform (IRCW) that is a single transmitted waveform by the IRCS to perform both radar and communications functions as suggested in [7]. The promising approaches that have already been suggested can be classified into two major categories: multiplexing-waveform and identical-waveform. The multiplexing-waveform approaches employ multiplexing techniques, such as space division multiplexing (SDM) [1], [2], time division multiplexing (TDM) [5], [6], frequency division multiplexing (FDM) [8], and code division multiplexing (CDM) [9]. Multiplexing allows one to easily separate the communications and radar signals so they will not interfere with each other. The identical-waveform approach picks a single waveform which may be similar to the traditional radar waveforms [10] or the conventional communications waveforms [11]. A popular approach uses an orthogonal frequency division multiplexing (OFDM) waveform [12]- [15], a waveform that has been widely applied in communications [16]- [18] and recently suggested for radar applications [19]- [26]. In fact, OFDM waveforms have been proposed for car-to-car (C2C) and car-to-infrastructure (C2I) communications [27]- [29] which is an important application for IRCSs. In this paper, we consider a single pulsed OFDM waveform, called as OFDM IRCW, is transmitted by the IRCS. Such a waveform can simultaneously perform radar and communications functions.
To effectively allocate the limited total power in an IRCS, many design criteria have been proposed.
The research in [30] splits the total power between the data symbols that perform the information transmission and the training symbols which accomplish the radar function. The study in [31] and [32] splits the total bandwidth into two subbands, one for communications only and the other for both radar and communications. By employing different allocations of the total power between these two subbands, the joint radar and communications system performance with respect to the data information rate (DIR) and estimation rate is explored. To simultaneously improve the radar target parameter estimation performance and the communications channel capacity, optimal assignment of the subcarrier power profile for an integrated OFDM waveform is proposed in [33] by using a multiobjective design criteria. Similarly, to improve the detection performance and channel capacity of the IRCS, an adaptive transmit power allocation for the subcarriers of an integrated OFDM waveform is proposed in [34]. In fact, there is not a uniform criterion to guide IRCW design.
In this paper, our focus is on a radar used for target classification based on the target-impulse response so our ability to estimate the target-impulse response and thus the minimum mean-square error of that estimate become the appropriate criterion [35]. However, based on [36], this can be shown to be directly related to the maximization of the conditional mutual information (MI) between the observed signal and the radar target return under some reasonable assumptions. This information theoretic criteria [31] [32] has since emerged as a popular criterion for radar waveform design [25], [35]- [37]. On the other hand, information theory provides much of the foundation of communications theory [38], and various communications waveform design me
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