Passive harmonic transponders are used in applications where weight and size restrictions are prohibitive to the use of active radio transmitters, e.g., for insect tracking. A typical harmonic transponder tag consists of a wire dipole antenna and a Schottky diode with a parallel inductance. Despite the simplicity in tag circuit, designing lightweight, compact, and efficient harmonic tags are challenging. In this letter, we describe a design that combines the reliability of a printed circuit with the simplicity and small mass of a wire dipole antenna. We study the interplay of individual system parameters and show that optimizing the tag antenna together with the tuning inductance provides further opportunities for tag miniaturization.

A BLAST/MIMO detector with 3.84 bit/s/Hz spectral efficiency supporting up to 4×4 QPSK MIMO is presented. The detector shows over a 5 dB improvement over VBLAST for an uncoded data system. The device is 438 k effective logic gates (7.3 mm²) in 0.18 µm CMOS and consumes 192 mW for the 19.2 Mbit/s uncoded rate.

In this paper, we propose an iterative structure for channel estimation and data detection for multiple-input-multiple-output orthogonal frequency-division multiplexing systems (MIMO-OFDM) with insufficient cyclic prefix (CP) and a limited number of pilot subcarriers. Insufficient CP leads to increased intersymbol interference (ISI) and intercarrier interference (ICI). The interference corrupts pilot subcarriers used for channel estimation and affects the detection process. We propose a multistep channel estimation process, which is assisted by a small number of pilot subcarriers. First, the channel covariance matrix and the number of channel paths are acquired from the channel least square estimates at the pilot subcarriers. We then formulate a maximum likelihood process to approximate the time-domain channel. For data detection, a high-performance bidirectional M-algorithm (BDMA) is proposed for trellis-based equalization. Simulation results show that the channel estimate mean square error converges to the Cramer-Rao bound (CRB) after a few iterations. In addition, the achieved bit error rate (BER) can reach that of the sufficient CP case, even when the delay spread is much longer than the CP. ¹ .

Millimeter-wave (mmWave) channel sparsity is widely exploited to reduce the complexity of compressed sensing and beamtracking algorithms for 5G, yet there are still many misconceptions about it: that it arises from pencilbeam antennas admitting just a few paths, that the channel in fact has just a few paths, that the channel is always sparse. What is missing to confirm these conjectures is measurement-based analysis, to our knowledge conducted by only two papers thus far, however both at microwave (5 GHz), in a single environment, and based on just a few ( $\leq$ 49) measurements. For conclusive analysis on mmWave sparsity, we have collected measurements with our high-resolution 3D double-directional 60 GHz channel sounder in three indoor and two outdoor environments, comprising a total of 750 measurements, while investigating both line-of-sight (LoS) and obstructed LoS (OLoS) conditions. We learned that in LoS the channel is always sparse, that sparsity drops notably from LoS to OLoS, and that mmWave sparsity is inherent to the channel itself – not to pencilbeam antennas – since our sounder has omnidirectional field-of-view. We also propose a sparsity metric that accounts for channels with just a few paths, which we found to occur in some environments.

With massive multi-input multi-output, it may be the case that large numbers of antennas are closely packed to fit in some available space. Here, channel correlations become important and it is of interest to investigate the space requirements of different array shapes. We focus on uniform square and linear arrays and consider a range of correlation models. We show that the benefits of two-dimensional arrays are dependent on the type of correlation. When the correlation decays slowly over small antenna separations then square arrays can be far more compact than linear arrays or they can offer substantial sum rate enhancements. When the correlation decays more quickly, then the main benefit is compactness.