I. Introduction
Ultrawideband (UWB) is a radio technology for future wireless communication, radar, and imaging systems [1], [2]. Although this technology offers several unique advantages such as multipath immunity, high data rate, and strong antijamming ability [3], there are a number of practical challenges, which are topics of current research. One key challenge is the increased multipath dispersion, which results because of the fine temporal resolution [4]. Although such challenges have been investigated to some extent, they have not been fully explored in connection with transmit beamforming techniques in realistic multipath environments [5]– [11]. Beamforming is a transmission technique that can be employed to combat the multipath dispersion and provide temporal and spatial focusing at the target receiver. The temporal compression leads to an improved signal-to-noise ratio (SNR), which reduces the intersymbol interference in high-speed wireless communications [5], [11]. On the other hand, spatial focusing means that the received power decreases away from the target location, leading to lower probabilities of intercept by an eavesdropper or interuser interference in a secure multiuser communication channel [7], [12]– [14]. Due to this spatiotemporal focusing, the receiver complexity is shifted to the transmitter side, and a simpler receiver's structure can be employed to capture the received energies. As an example, time reversal (TR) [15]– [17] is one of the well-known beamformers in which the flipped version of the channel impulse response is used as a prematched filter. When the time-reversed signal is transmitted back through the same channel, multipath components arrive constructively aligned at the target receiver at a particular time.