Differs from conventional phased-array, frequency diverse array (FDA) employs a frequency increment across the array elements and thus produces a range-angle-dependent beampattern. This additional range dependence finds applications in target detection/localization, high-resolution synthetic aperture radar (SAR) imaging, and SAR deceptive jamming. To capture the essence behind those applications, we analyze the range, angle, and Doppler resolution capabilities of FDA radar signal via ambiguity function. As conventional ambiguity function is defined for a single-input single-output (SISO) signal, we extend it to the generalized ambiguity function for FDA radar signal, parameterized with the range, angle, and Doppler shifts of a target. Both theoretical analysis and numerical results show that the FDA radar with a large frequency increment has potentials in high-resolution ranging but will suffer from additional range ambiguity within the range bin and Doppler ambiguity. Moreover, the use of frequency increment brings FDA radar a virtual transmit aperture but decreases the directional gain.

This paper considers the application of widely linear (WL) receivers in an uplink multi-user system using real-valued modulation schemes, where the cellular base station (BS) with multiple antennas provides connectivity for randomly deployed single-antenna users. The targeted use case is massive machine type communication (mMTC) with grant-free access in the uplink, where the network is required to host a large number of low data rate devices transmitting in an uncoordinated fashion. Four types of WL receivers are investigated, namely the WL zero-forcing (ZF) and the WL minimum mean-squared error (MMSE) receivers, along with their enhanced versions employing successive interference cancellation (SIC) with channel-dependent ordering, i.e., the WL-ZF-SIC and WL-MMSE-SIC receivers. The outage performances of these receivers are analytically characterized in the high signal-to-noise ratio (SNR) regime and compared to those of conventional linear (CL) receivers using complex-valued modulation schemes. For the non-SIC receivers, we show that, when compared to the CL counterparts, the WL receivers yield a higher diversity gain when decoding the same number of users and have the same diversity gain but a decreased coding gain when the number of users is nearly doubled. The outage performance analysis of WL-SIC receivers is facilitated by the marginal distribution of ordered eigenvalues of a real-valued Wishart matrix. It is shown that the SIC operation with channel-dependent ordering brings no additional diversity gain to the WL receivers but instead increases the coding gain. Moreover, the coding gain of WL-SIC receivers grows as the number of users increases and even exceeds that of CL-SIC receivers under suitable conditions. For the mMTC scenario with grant-free transmission, it is demonstrated that the WL receivers outperform their CL counterparts in terms of offering a lower outage (and packet drop) probability and a higher system throughput for a given packet drop probability.

Future wireless cellular communication networks are expected to provide connectivity for massive machine-type communication (mMTC) devices. The main challenge of supporting mMTC traffic for a cellular network lies in the high density of these devices, which individually have relatively little data to transmit. This suggests the use of low overhead, grant-free access scheme for uplink data transmission, which, however, suffers from packet collisions when devices attempt to access the channel. In this article, we suggest the use of real-valued transmission together with widely linear (WL) reception for improving resource access and thus data throughput in the uplink of mMTC traffic scenarios. We show that not surprisingly, the WL scheme can virtually double the number of receive antennas at the base station (BS). We analyze the effect of this on the supported user density and data throughput for grant-free uplink transmission. As a specific example, we consider the narrowband Internet-of-Things (NB-IoT) cellular communication system, which already includes real-valued modulation modes. Our numerical results show that the supported user density and data throughput of grant-free NB-IoT systems can be significantly improved due to the use of WL receivers. For example, considering an NB-IoT system with 1% packet drop probability, we obtain a tenfold (for single-antenna BS) and a sixfold (for dual-antenna BS) increase in the supported user density by using WL receivers instead of their conventional linear counterparts.

In this letter, we propose a scattered-wave deceptive jamming approach using a frequency diverse array (FDA) radar against spaceborne synthetic aperture radars (SARs). As an emerging array radar, FDA transmits multiple carrier frequencies across its elements, and thus, a multiple deceptive jamming is potentially produced in the range dimension of the SAR image. Moreover, we consider the scattered-wave jamming, since it has low complexity when used to create spurious scenes and control the jammer locations. The theoretical analysis and numerical simulations show that the scattered-wave jamming with FDA can generate multiple fake targets with the number of targets being dependent on the number of FDA carrier frequencies.

Narrowband Internet of Things (NB-IoT) is one of the most prominent low power wide area (LPWA) technologies to support massive connections among low-cost devices with enhanced coverage. Addressing the further increase in the number of supported connections in IoT applications, we investigate the use of real-valued modulated signals together with widely linear detection (WLD) to NB-IoT. In this paper, we show that the use of WLD to resolve collisions of two simultaneously transmitted real- valued signals generated from two user equipments (UEs) enhances the capability of NB-IoT systems to facilitate massive connectivity. More specifically, to demonstrate the performance improvements, we analyze the uplink transmission reliability and the device density supported in NB-IoT systems with WLD in a grant-free scenario. The numerical results show that, for a 1% probability of decoding failure, the supported device density can be increased by one order of magnitude compared to that of conventional complex-valued signalling schemes.

Joint direction-of-arrival (DOA), direction-of-departure (DOD), and Doppler estimation for multiple-input multiple-output (MIMO) radar usually are too time-consuming to implement because of high-dimensional searching procedures. This letter proposes a computational efficient algorithm for MIMO radar joint DOA, DOD, and Doppler estimation, which adopts an unstructured maximum likelihood method to sequentially estimate the Doppler, DOD, and DOA by three one-dimensional searching procedures based on the extended invariance principle. Numerical results validate the effectiveness together with its superiorities over conventional maximum likelihood algorithms.