Boyan Radkov Yanakiev

Shout to Secure: Physical-Layer Wireless Security with Known Interference

This paper proposes a physical-layer security scheme for wireless networks, aiming to achieve communication secrecy by making the eavesdropper incapable of decoding the secret wireless message. The considered scenario features one user within the range of two access points, API and AP2. The APs are assumed to be connected through an alternative secure (e.g. wired) connection. The goal is to secure the wireless link between the user and API. While the user transmits to API, AP2 simultaneously transmits an interfering signal, which is a priori provided to API, such that API is likely the only node capable of decoding the users entire transmission. Evaluation is done through simulation by measuring the upper bound of the information-theoretic secrecy and error performance. In the latter case it is shown that the eavesdropper experiences significantly higher error rates than the intended receiver, thus providing evidence of practical security.

User Influence on MIMO Channel Capacity for Handsets in Data Mode Operation

The current paper concerns realistic evaluation of the capacity of the MIMO channel between a BS and handheld device, such as a PDA or smartphone, held in front of the users body (data mode). The work is based on measurements of the MIMO channel between two widely separated BSs in a micro-cellular setup, and six handsets located in an indoor environment. The measurements are done simultaneously in both the 773.5-778.5 MHz and 2250-2350 MHz bands, and from the two BSs. The handsets are realistic types and were measured both in free space and with twelve different users, using both one and two hands. The random capacities of the channels are evaluated in terms of outage capacity. For an SNR of 10 dB, median capacities in free space of about 4.4-4.7 bit/s/Hz for the low band and about 3.3-3.8 bit/s/Hz for the high band were found. The mean decrease in outage capacity due to the user was found to be up to about 2.2 bit/s/Hz, depending on the band and handset. More results are presented in the paper.

On Small Terminal Antenna Correlation and Impact on MIMO Channel Capacity

Analysis of the antenna correlation at the design stage is made, and then compared to real life performance in a typical propagation environment and in typical use cases. A traditional design flow is followed and conclusions are made on the performance of several handsets. These conclusions are then contrasted to measurements and an explanation is sought for the variations. It is concluded that correlation estimation and optimization at the design stage, using conventional methods, brings little benefit in real life situations. Impact on channel capacity has been the figure of merit.

On Antenna Design Objectives and the Channel Capacity of MIMO Handsets

The branch correlation coefficient (BCC), the branch power ratio (BPR), and the total mean power (TMP) are often used to characterize the mobile multiple-input multiple-output (MIMO) channel. This work investigates to which degree these parameters are useful for maximizing the channel capacity of multiple-input multiple-output (MIMO) handheld devices used in data mode. A statistical point of view is applied, using about 2800 outdoor to indoor channel sounder measurements obtained with combinations of ten different handsets, four to eight test users, and a variety of different use cases (UCs). All measurements were made in an urban environment in a setup with two different base stations (BSs) and with the users inside a single building. For each measurement combination, the mean capacity (MC) and associated values of BCC, BPR, and TMP are obtained. From the data it is found that the MC is only weakly correlated with both the BCC and the BPR, while the MC is highly correlated with the TMP.

Long-Range Channel Measurements on Small Terminal Antennas Using Optics

In this paper, details are given on a novel measurement device for radio propagation-channel measurements. To avoid measurement errors due to the conductive cables on small terminal antennas, as well as to improve the handling of the prototypes under investigation, an optical measurement device has been developed. It utilizes thin, light, and flexible glass fibers as opposed to heavy, stiff, and conductive coaxial cables. This paper looks at the various system parameters such as overall gain, noise figure, and dynamic range and compares the solution to other methods. An estimate of the device accuracy is also given. Selected parts of the circuitry are given in more detail. Typical measurement results are also shown.

Small Device for Short-Range Antenna Measurements Using Optics

This paper gives a practical solution for implementing an antenna radiation-pattern measurement device using optical fibers. It is suitable for anechoic chambers, as well as for short-range channel sounding. The device is optimized for small size, and provides a cheap and easy way to make optical antenna measurements using off-the-shelf components. Verification measurements were made to confirm the benefits.

MIMO channel measurements using optical links on small mobile terminals

Most up-to-date and future communication standards (WiMax, LTE, LTE-A 802.11n) include some sort of Multiple Input Multiple Output (MIMO) scheme for the purposes of boosting the overall throughput in the system. Many of these standards also include small and portable (mobile phone size) device categories [1], where the antenna design is in general challenging due to the fundamental limitation of antennas [2], [3]. It is also well known what if the antenna can be identified as electrically small, measurement cable effects become a significant source of errors in nearly all types of antenna measurements, [4]. The additional antennas required for the MIMO schemes, pose even further challenges to the design as well as performance evaluation. The already scarce space for leading out measurement cables, becomes even less and new problems arise due to the cables (one for each antenna), for example increased coupling. Removing the errors that the conductive measurement cables add, brings the antenna design closer to reality, which can be extremely beneficial since there are no devices available to date supporting the new standards. Moreover, channel sounding data is more complete than any chip receiver anyway.

MIMO Reference Antennas Performance in Anisotropic Channel Environments

The new generation of cellular wireless communications, long term evolution (LTE), requires multiple antennas at both the transmit and receive ends of the radio link to improve system performance. This use of the radio link is termed multiple input-multiple output (MIMO), and in this work, the focus is on the downlink (base station transmit and mobile receive). Efforts to define over-the-air (OTA) downlink MIMO measurement methodologies are still being carried out within several standards bodies and are based on the evaluation of so-called good, nominal, and bad MIMO reference antennas. The reference antennas were necessarily defined independently of the channel environment, which is still under discussion. The reference antenna figures of merit, for an isotropic channel environment, were chosen as efficiency, branch power ratio (BPR), and correlation. The objective of this work is to evaluate the radiated performance of the MIMO reference antennas using anisotropic (nonuniform power angular spread) channel environments. The performance of the reference antennas in anisotropic environments is different to that in an isotropic environment, due to the antenna orientation in the three-dimensional space relative to the channel. This leads to a statistical description of the branch power ratio (BPR), correlation, and efficiency, which is expressed in terms of mean effective gain (MEG). The outcome of this new anisotropic evaluation is that the reference antennas are seen to vary significantly for BPR and mean effective gain (MEG) and somewhat less for correlation, relative to the isotropic environment.

Portable MIMO-LTE antennas for different hand-held device sizes

This paper presents a dual-band internal planar antenna suited for different phone form factors. It is shown that a high-Q self-resonating antenna element is adaptable to different platforms. The antenna aims at being frequency reconfigurable and a varactor will be used for that purpose. Simulations and measurements of S parameters, total efficiency and envelope correlation are presented at 796 MHz and 2.3 GHz for two different devices.