Gert Frølund Pedersen

Novel small-size directional antenna for UWB WBAN/WPAN applications

This paper presents a novel small-size directional antenna design for ultrawide-band wireless body area networks/wireless personal area networks applications. The design is based on a typical slot antenna structure with an added reflector in order to achieve directionality. The effects of different antenna parameters and human body proximity on the radiation characteristics are analyzed. Antenna measurements with an optic RF setup were performed in order to characterize the small-size antenna far field radiation pattern. The different structural antenna parameters were optimized via extensive numerical simulations. Results show that for frequencies above 3.5 GHz, where the power front-to-back ratio of the directional antenna is greater than 10 dB, its impedance is nearly the same as in the free space. It is not the case neither for the omnidirectional slot antenna nor the monopole antenna next to the body. Between 3 and 6 GHz performance of the novel directional antenna, in terms of radiation efficiency and SAR values, is significantly improved compared to omnidirectional antenna designs.

The potential use of time reversal techniques in multiple element antenna systems

Based on outdoor measurements we study the feasibility of applying the time reversal techniques (TR) in multiple element antenna (MEA) wireless communication systems. It is demonstrated that the use of TR in wireless communication has a promising potential in mitigating the effect of channel dispersion and especially in reducing the cochannel interference where a margin of 18 dB interference reduction has been obtained.

Spherical outdoor to indoor power spectrum model at the mobile terminal

Mobile terminals are often used indoor with the base station outdoors. At the mobile terminal the major part of the signal energy comes through openings in the building such as windows. Typically, only one of the sides in a room has windows, and seldom does a room have windows on all sides. Hence, the dominating signal can be expected to arrive at the mobile terminal from a narrow range of angles. Mobile terminal antennas used next to the head in the speaking position will be directional due to the fact that part of the radiation pattern facing the head will be attenuated and reflected. Having a directive antenna in a directive environment, the performance will depend on the orientation of the antenna in the radio environment. A new statistical spherical outdoor to indoor power spectrum model has been proposed to be able to calculate the directional performance of mobile terminals with a single or multiple antennas. The model consists of a major scattering area in one direction and more uniformly distributed minor scatterers in the other directions. A verification of the proposed model was performed and 60 data sets of spherical power spectrum measurements were collected in a typical urban environment. Using the new model, the directional performance of mobile terminal antennas including a human operator has been investigated through directional mean effective gain, branch power ratio, and correlation calculations using spherical radiation pattern measurements of a mobile terminal including the effect of 42 different persons. The accuracy of the calculated values was verified by directly measured values using 200 persons walking with the mobile terminal in the same office-like environments as where the spherical power spectrum measurements were performed.

Antenna Proximity Effects for Talk and Data Modes in Mobile Phones

Based on a recent study of the ways a phone is held (a grip study), CAD models of the human hand have been generated, and antenna proximity effects for both talk and data modes in mobile phones have been investigated using an FDTD code. The simulation results showed that the hand, and especially the index finger, exhibited a major contribution in determining the total loss when compared to the upper torso alone. The influence of the position of the fingers on the handset was found to be more important when close to the antenna. The palm-handset gap and the index-finger location were the main factors for both absorption and mismatch loss. Different data-mode hand phantoms and configurations were investigated, showing that both “overlapped” and “interlaced” grip styles similarly influenced the antennas communication performance.

Time reversal in wireless communications: a measurement-based investigation

Interference caused by other users in a multi-user environment degrades the system performance significantly. Conventionally, user separation in a multi-user environment is achieved by signal separation in time, frequency or code. The use of multiple elements antenna (MEA) systems can apply separation of the users in space by targeting the transmitted power to the user of interest. Inevitably in that case, the separation is not ideal and there is interference on the other users in the systems. Moreover in a wideband transmission, frequency selective fading causing inter symbol interference (ISI) is a big challenge for point to point wireless communications. In this paper, we study the feasibility of applying the time reversal technique (TR) in multiple input single output (MISO) systems to alleviate the effect of ISI. The ability of TR to focus the signal on the receiver of interest in a multi-user environment is also considered. The studies are supported by measured complex channel impulse responses with 10 MHz bandwidth centered at 2.14 GHz. For an 8times1 MISO system, using TR the root mean square delay spread is reduced by a factor of 2. To evaluate the capability of the TR in reducing the ISI, simulation of the bit error rate (BER) was made. The irreducible BER of the TR-MISO system was shown to be lower than that of the SISO system by at least an order of magnitude. By using TR and assuming that two users are communicating simultaneously with the same BS the two signals have 17 dB of isolation

Cable-less measurement set-up for wireless handheld terminals

Attaching cables to mobile terminals for accurate measurements is progressively influencing the radiation characteristics with the diminishing size of the terminals. Simulations show the large influence of cables and their exact curvature on the radiation properties of mobile terminals that are small with respect to the wavelength. These results are confirmed by measurements in an anechoic chamber. As corrections for these almost irreproducible influences are hardly feasible, we propose to omit conducting cables completely. Instead, optical fibres are used for transparent transport of the RF signals. Off-the-shelf system components allow for two channels in a box of size comparable to modern small terminals. When driven at an appropriate level this set-up shows ample dynamic range for mobile radio channel sounding.

Characterization of the Indoor Multiantenna Body-to-Body Radio Channel

In this paper, we investigate the wideband body-to-body radio channel with multiple antennas at both ends based on a time-domain radio channel measurement campaign. Four single-element transmitters and eight quad-element receivers were mounted on three test people. Both directional and omnidirectional antennas have been investigated. A comparison between electromagnetic antenna simulations and the measurements shows that the multipath environment reduces the body losses effectively. Channel characterizations in terms of path loss, body shadowing, small-scale fading, and spatial correlation have been derived. Small path-loss exponents (< 2.0) are observed in the investigated environments. Considerable power loss due to body blockage makes the body shadowing a prominent factor in the short-range body-to-body communications. Distributed antenna selection diversity is presented to mitigate the body shadowing. A 5-dB diversity gain in the average received power at the mean and 10% outage levels have been identified based on the measurements with two distributed antennas mounted on the transmitter and receiver person and without channel-state information feedback to the transmitter.

Mutual Coupling Reduction for UWB MIMO Antennas With a Wideband Neutralization Line

A wideband neutralization line is proposed to reduce the mutual coupling of a compact ultrawideband (UWB) multiple-input-multiple-output (MIMO) antenna. With the introduced decoupling method, the designed UWB MIMO antenna covers the band of 3.1-5 GHz with an isolation of higher than 22 dB. The proposed wideband neutralization line is not necessarily placed in the clearance area between two MIMO elements and can be put above the copper ground. A small clearance (antenna area) of 35 ×16 mm2 is achieved. The designed UWB MIMO antenna is fabricated. Sparameters, radiation patterns, total efficiency, and realized gain of the prototype are measured and compared to the simulations.

Beamforming via Large and Dense Antenna Arrays Above a Clutter

The paper sheds light on the beamforming (BF) performance of large (potentially unconstrained in size) as well as dense (but physically constrained in size) antenna arrays when equipped with arbitrarily many elements. Two operational modes are investigated: Single-layer BF and multi-layer BF. In the first mode, a realistic BF criterion namely the average BF gain is revisited and employed to understand the far-field and the near-field effects on the BF performance of large-scale antennas above a clutter. The diminishing throughput returns in a single-layer BF mode versus the number of antennas necessitate multi-layering. In the multi-layer BF mode, the RF coverage is divided into a number of directive non-overlapping sector-beams in a deterministic manner within a multi-user multi-input multi-output (MIMO) system. The optimal number of layers that maximizes the users sum-rate given a constrained antenna array is found as a compromise between the multiplexing gain (associated with the number of sector-beams) and the inter-beam interference, represented by the side lobe level (SLL).

Bodyloss for handheld phones

Measurements in an anechoic room of three-dimensional radiation patterns for 44 test persons using a handheld phone with two antennas are investigated in order to find the bodyloss. The measured bodyloss range from 0.9 dB to 16 dB. From the measurements is was possible to separate the absorption and the loss due to mis-match of the antenna. The antenna mis-tuning accounted for less than 2 dB to the bodyloss. The uncertainties in the measurements is in the order of one dB.