Alexandru Tatomirescu

Novel Architecture for LTE World-Phones

The 4th Generation of mobile communications (4G) came with new challenges on the antenna bandwidth and on the front-end architecture of mobile phones. This letter proposes a novel architecture overcoming these challenges. It includes narrowband tunable antennas, co-designed with a tunable front rnd (FE). Simulations and measurements demonstrate the concept for low and high bands of the LTE frequency spectrum.

Concurrent Communication and Sensing in Cognitive Radio Devices: Challenges and an Enabling Solution

Cognitive radios (CRs) need to continuously monitor the availability of unoccupied spectrum. Prior work on spectrum sensing mainly focused on time-slotted schemes where sensing and communication take place on different time periods in the same frequency. This however leads to a) limited CR throughput as data transmissions need to be interrupted for the sensing task, and b) unreliable detection performance since sensing happens in specific confined time durations. The paper describes the basic design challenges and hardware requirements that restrain CRs from simultaneously and continuously sensing the spectrum while transmitting in the same frequency band. The paper then describes a novel approach based on spatial filtering that promises to empower CRs with concurrent transmission and sensing capabilities. The idea is to equip the CR with redundant transmit antennas for forming an adaptive spatial filter that selectively nulls the transmit signal in the sensing direction. By doing so, a wideband isolation level of ~ 60 dB is obtained by the antenna system. Finally, by following the spatial filtering stage with active power cancellation in the radio-frequency stage and in the baseband stage, a total isolation in excess of a 100 dB required for enabling concurrent communication and sensing can be obtained.

Breaking the Transmitter–Receiver Isolation Barrier in Mobile Handsets With Spatial Duplexing

In full-duplex radio communication systems like e-UTRAN, CDMA-2000, the radio transmitter (Tx) is active at the same time as the radio receiver (Rx). The Tx and the Rx will be using separate dedicated frequency bands and the Tx-Rx isolation is ensured by duplex filters. However, agile duplexers required for multiband operation are almost nonexistent while dedicating a bank of narrowband filters is bulky and incurs considerable switching losses. In this paper, we propose an approach that dramatically reduces the complexity of the RF frontend, first by replacing the duplex filter with a spatial filter and second, by codesigning the filtering antennas and the RF frontend. The spatial filter is synthesized by equipping the Tx with redundant antennas. By properly weighting the Tx antennas, the Tx signal is selectively attenuated in the Rx direction. The spatial filter can be tuned to different frequency bands as long as the antennas are made tunable. Moreover, the spatial filter may directly benefit from the balanced architecture of the power amplifiers (PAs) thus reducing the total system complexity and insertion loss. Finally, simulation and initial measurement results are provided in a challenging low-frequency band, serving as a proof-of-concept.

On Small Terminal MIMO Antennas, Harmonizing Characteristic Modes With Ground Plane Geometry

multiple input-multiple output (MIMO) antenna systems are defined by fundamental figure of merits (FoM), such as branch imbalance, total efficiency, mean effective gain (MEG), and the correlation coefficient. Those FoM requirements are challenging, specially when applied to electrically small mobile devices, i.e., smart-phones, due to the form factor-reduced dimensions and multiplicity of adjacent frequency bands of operation. Uncorrelated antennas are especially important for MIMO antenna systems; other than few exceptional cases, the reduction of magnitude of complex correlation coefficient will increase the system capacity and data throughput. This work proposes an MIMO antenna system for mobile terminals, based on a realistic form factor and packaging implementation, with a very low magnitude of the complex correlation coefficient and an impressive isolation. An isolation better than 20 dB has been achieved using a folded monopole and a commonly adopted planar inverted F antenna (PIFA) on a platform with a very small form factor (100 × 50 × 10 mm). The proposed implementation is based on the synergy of two known techniques that consists in the: 1) reference ground plane geometry manipulation and 2) in the application of the characteristic mode theory to obtain orthogonal radiation modes. Since MIMO antenna systems at frequencies higher than 1.7 GHz are naturally proper isolated and decorreleated, this work demonstrates the proposed antenna topology enabling higher isolation and uncorrelated antenna system at 750 MHz, which is more difficult to achieve in form factors smaller than 1λ, while maintaining high total efficiency and adequate gain imbalance. In this paper, a simulation model and prototype (1/4λ long) measurement results are presented, demonstrating the implementation feasibility of such antenna system in realistic mobile device embodiment.

Port Isolation Method for MIMO Antenna in Small Terminals for Next Generation Mobile Networks

This paper presents a practical method to increase the isolation between two ports of a Multiple Input Multiple Output (MIMO) antenna for mobile hand-held devices. The method consists in a connecting strip containing lumped components placed in between the low impedance zones of the two antennas. The lumped components modify the impedance of the connecting strip so that, together with the mutual coupling impedance, it acts as a band-stop filter between the two ports. As a consequence, the efficiency due to scattering parameters almost doubles and the envelope correlation coefficient decreases dramatically. Furthermore, the use of lumped components makes this method less susceptible to the presence of other components in the mobile device or the user. This method has been verified on two antenna configurations, through simulation and measurements.

Compact multiband sensing MIMO antenna array for cognitive radio system

This paper presents a new design for a multiband sensing MIMO antenna system that can be used in cognitive radio communication for a mobile platform. The antenna system consists of two identical small-size multiband antennas with dimensions 29.5 × 17 × 5 mm3 and each of them is comprised of a driven strip monopole and a coupled parasitic shorted strip. These antennas are located diagonally to limit the mutual coupling and antenna correlation at low frequencies. The obtained operating bands are 698-990 MHz and 1710-5530 MHz, which include most of the LTE and Wi-Fi frequency bands. A prototype was fabricated and the measured results indicate that the realized efficiency is acceptable for a mobile application considering the compact size.

Effect of antenna bandwidth and placement on the robustness to user interaction

Modern smart phones require antenna systems that can deal with an ever-growing number of bands. This study compares two different approaches to the coverage of large bandwidths: Wide-band (WB) antennas covering a whole band at once and tunable narrow-band (NB) antennas covering only one channel at a time but tunable to all channels within the band of interest. To investigate the effect of antenna placement the antennas are placed both at the top and the bottom of the phone. All antenna configurations are simulated in talk position with a head and hand included. The hand is constructed with a movable index finger and the index finger is swept at 6 positions on the backside of the phone. The study shows that WB antennas detune a lot more than NB antennas and that top-mounted antennas are experiencing more than 6 dB higher losses than bottom-mounted antennas. It is proposed to expand this study with more antenna types and placements as both of these parameters are known to influence the immunity to the user. It is also proposed to compare the simulation results to measurements to increase the confidence in the results.

On small terminal MIMO antenna correlation optimization adopting characteristic mode theory

This work proposes a MIMO antenna system for mobile terminals, based on a compact form factor and packaging implementation, with a very low magnitude of the complex correlation coefficient and an impressive isolation. An isolation better than 20 dB has been achieved using a folded monopole and a commonly adopted PIFA (planar inverted F antenna) on a platform with a very small form factor (100×50 mm). The proposed implementation is based in the application of the characteristic mode theory to obtain orthogonal radiation modes. The adopted topology enables uncorrelated antenna system at 750 MHz, while maintaining high total efficiency and adequate gain imbalance. In this present paper simulation model as well as prototype measurement results are presented, demonstrating the implementation feasibility of such antenna system in compact mobile device embodiment.

Tx-Rx isolation exploiting tunable balanced - Unbalanced antennas architecture

The duplex filter is probably still the most expensive component in mobile handsets for the significant Transmitter (Tx) - Receiver (Rx) isolation required. This paper suggests to relax or even replace the duplex filter by equipping the Tx and the Rx with two separate antennas. The two antennas are different in the sense that one is balanced and the other is unbalanced. By properly controlling the two arms of the balanced antenna, impressive Tx-Rx isolation is obtained by canceling the coupling trans-impedance between the two antennas.

Multi-Band Compact Antenna for Smartphones Supporting LTE Carrier Aggregation

In this paper, we are proposing a dual feed compact antenna for a mobile phones that is capable of supporting the latest features of LTE, Carrier Aggregation (CA), which has been introduced to satisfy the ever increasing demand for data-rate. The antenna makes use of a dual resonator design to cover the low bands used in Long Term Evolution (LTE) from 699 to 960 MHz. The design of the RF Front End(FE) is simplified by the dual feed design which can have distributed components of the FE to relax its design and packaging requirements. The main antenna switch design can be relaxed and the high isolation between the two antennas removes the need for a diplexer. The antenna supporting the high bands (from 1710 to 2700 MHz) is fed through a matching network to increase the bandwidth while having a small volume. Inductive loading is used for the minimization of both elements. Even though the antenna occupies only an area of 30×10mm2, many combinations of low and high band are available for CA. Despite the small volume it has a good efficiency which is close to -3 dB. The design principle has been evaluated through simulations and measurement with good agreement between them.