Ahmed Hussain

Bio: Ahmed Hussain is an academic researcher from Chalmers University of Technology. The author has contributed to research in topics: Electromagnetic reverberation chamber & Multipath propagation. The author has an hindex of 11, co-authored 33 publications receiving 445 citations. Previous affiliations of Ahmed Hussain include University of Gothenburg.

Threshold Receiver Model for Throughput of Wireless Devices With MIMO and Frequency Diversity Measured in Reverberation Chamber

Abstract: We present a simple theoretical model for the throughput data rate of a wireless LTE device including the improvements of data rate due to diversity in frequency (OFDM) and spatial domains (MIMO) under frequency selective fading. The model is based on defining an ideal threshold receiver for the line-of-sight (LOS) case, corresponding to reception with advanced error-correcting codes. The theoretical throughput model is in agreement with measurements in a reverberation chamber of a commercial LTE device for the 1 × 2 SIMO case, both regarding diversity and MIMO array gains, and it can therefore be used to complement measured results in evaluation of performance of LTE devices.

Wideband Compact 4-Port Dual Polarized Self-Grounded Bowtie Antenna

Abstract: We present a dual-polarized self-grounded bowtie antenna. The antenna is mechanically simple and wideband. One of the characteristics is its flexibility: it can be used either as a 4-port antenna in multiple-input-multiple-output (MIMO) systems or as a directional 2-port dual-polarized antenna in radar or sensor systems. The latter application requires a balun. The 4-petal geometry of the antenna has been optimized for the best matching to 50 coax over 1.5-3 GHz. Both simulated and measured results are presented to verify the design. The antenna radiates over parts of the bandwidth similar to four Huygens source, one from each petal, each having directivity close to 4.8 dBi.

Interpreting the Total Isotropic Sensitivity and Diversity Gain of LTE-Enabled Wireless Devices From Over-the-Air Throughput Measurements in Reverberation Chambers

Abstract: The characterization of the performance of wireless devices is key to developing new RF products conforming to the latest communications protocols. Traditionally, communication performance tests have focused on the RF performance of the tested devices, e.g., smart phones, pads, laptops, and so on. In particular, the focus has shifted from conducted (i.e., cabled) measurements to more realistic over-the-air (OTA) characterization of the RF performance of these devices in transmit or receive mode. For example, the receiver performance of 2G and 3G wireless devices can be measured in terms of the total isotropic sensitivity (TIS) that depends on the antenna and the receiver parts of a wireless device. These measurements can be performed in a reverberation chamber setup. However, standard TIS measurements can be time-consuming and do not reflect the actual performance gains of multiple-input multiple-output antenna systems operating over orthogonal frequency division multiplexing channels, such as those encountered in 4G long-term evolution (LTE) systems. Therefore, in order to meet both time and cost efficiency requirements, we propose here a new method to determine the TIS, as well as the diversity performance, of an LTE device based on throughput measurements. The proposed method shows that the TIS of an LTE device is characterized much faster directly from OTA throughput measurements than from standard TIS measurements and with excellent accuracy.

Ray based multipath simulation tool for studying convergence and estimating ergodic capacity and diversity gain for antennas with given far-field functions

Abstract: We propose that the rich isotropic multipath environment should be a reference environment for OTA (over-the-air) characterization of wireless stations for use in multipath, in the same way as the anechoic environment is a reference environment for antennas mounted on masts and rooftops. The rich isotropic multipath environment is emulated by a well designed reverberation chamber, but also by a spherical array of probes in an anechoic chamber. The present paper describes a ray-based simulation tool, ViRM-Lab, which enables users to study the performance of arbitrary user-defined multiport antenna located in user-specified multipath environment. The antennas are defined by their far-field functions of the embedded element (with the other ports matched-load terminated). The environment can be determined by its Angle-of-Arrival (AoA) distribution, richness, and polarization balance. The code is made in such a way that the convergence of the cumulative distribution function (CDF) received on all ports can be studied, as well as the convergence of the statistical expectations when estimating the ergodic Shannon capacity and diversity gain.

A New Compact Wideband MIMO Antenna—The Double-Sided Tapered Self-Grounded Monopole Array

Abstract: We present a new compact wideband multiple input multiple output (MIMO) antenna—the double-sided 4-port arm-tapered self-grounded monopole array, briefly referred to as the butterfly antenna, in the communication. The antenna is very compact with low correlation between ports and high diversity gain. The genetic algorithm optimization scheme has been employed in the design. Simulation results have been verified against measurements. The measured reflection coefficients at all ports are below $-$ 7 dB over 0.5–9 GHz and below $-$ 4.5 dB over 0.4–0.5 GHz and 9–15 GHz. The measured correlation coefficients are below 0.4 over 0.4–15 GHz and lower than 0.1 in most of the frequency band. This new MIMO antenna is developed as a transmit antenna in reverberation chambers, and we believe that it will find more applications in other systems, such as micro base station antennas in wireless communication systems.

Characterization of Reverberation Chambers for OTA Measurements of Wireless Devices: Physical Formulations of Channel Matrix and New Uncertainty Formula

Abstract: The paper deals with reverberation chambers for over-the-air (OTA) testing of wireless devices for use in multipath. We present a formulation of the S-parameters of a reverberation chamber in terms of the free space S-parameters of the antennas, and the channel matrix in the way this is known from propagation literature. Thereby the physical relations between the chamber and real-life multipath environments are more easily explained. Thereafter we use the formulation to determine the uncertainty by which efficiency-related quantities can be measured in reverberation chamber. The final expression shows that the uncertainty is predominantly determined by the Rician K-factor in the reverberation chamber rather than by the number of excited modes, assumed by previous literature. We introduce an average Rician K-factor that is conveniently expressed in terms of the direct coupling between the transmitting and receiving antennas (corresponding to a line-of-sight contribution) and Hill's transmission formula (corresponding to a multipath or non-line-of-sight contribution). The uncertainty is expressed in terms of this average K-factor and geometrical mode stirring parameters, showing strong reduction by platform and polarization stirring. Finally the formulations are verified by measurements, and the new understanding of uncertainty is used to upgrade an existing reverberation chamber to better uncertainty.

Type of fatty acids in maternal diets during pregnancy and/or lactation and metabolic consequences of the offspring

Abstract: During pregnancy and/or lactation, maternal nutrition is related to the adequate development of the fetus, newborn and future adult, likely by modifications in fetal programming and epigenetic regulation. Fetal programming is characterized by adaptive responses to specific environmental conditions during early life stages, which may alter gene expression and permanently affect the structure and function of several organs and tissues, thus influencing the susceptibility to metabolic disorders. Regarding lipid metabolism during the first two trimesters of pregnancy, the maternal body accumulates fat, whereas in late pregnancy, the lipolytic activity in the maternal adipose tissue is increased. However, an excess or deficiency of certain fatty acids may lead to adverse consequences to the fetuses and newborns. Fetal exposure to trans fatty acids appears to promote early deleterious effects in the offspring's health, thereby increasing the individual risk for developing metabolic diseases throughout life. Similarly, the maternal intake of saturated fatty acids seems to trigger alterations in the liver and adipose tissue function associated with insulin resistance and diabetes. The polyunsaturated fatty acids (PUFAs), particularly long-chain PUFAs (long-chain PUFA-arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid), play an important and beneficial physiologic role in the offspring who receive this fatty acid during critical periods of development. Therefore, the maternal nutritional condition and fatty acid intake during pregnancy and/or lactation are critical factors that are strongly associated with normal fetal and postnatal development, which influence the modifications in fetal programming and in the individual risk for developing metabolic diseases throughout life.

OTA Testing in Multipath of Antennas and Wireless Devices With MIMO and OFDM

Abstract: New over-the-air (OTA) measurement technology is wanted for quantitative testing of modern wireless devices for use in multipath. We show that the reverberation chamber emulates a rich isotropic multipath (RIMP), making it an extreme reference environment for testing of wireless devices. This thereby complements testing in anechoic chambers representing the opposite extreme reference environment: pure line-of-sight (LOS). Antenna diversity gain was defined for RIMP environments based on improved fading performance. This paper finds this RIMP-diversity gain also valid as a metric of the cumulative improvement of the 1% worst users randomly distributed in the RIMP environment. The paper argues that LOS in modern wireless systems is random due to randomness of the orientations of the users and their devices. This leads to the definition of cumulative LOS-diversity gain of the 1% worst users in random LOS. This is generally not equal to the RIMP-diversity gain. The paper overviews the research on reverberation chambers for testing of wireless devices in RIMP environments. Finally, it presents a simple theory that can accurately model measured throughput for a long-term evolution (LTE) system with orthogonal frequency-division multiplexing (OFDM) and multiple-input-multiple-output (MIMO), the effects of which can clearly be seen and depend on the controllable time delay spread in the chamber.

A Compact Kapton-Based Inkjet-Printed Multiband Antenna for Flexible Wireless Devices

Abstract: A low-cost inkjet-printed multiband antenna envisioned for integration into flexible and conformal mobile devices is presented. The antenna structure contains a novel triangular iterative design with coplanar waveguide (CPW) feed, printed on a Kapton polyimide-based flexible substrate with dimensions of $ 70\times 70\times 0.11~\hbox{mm}^{\bf 3}$ . The antenna covers four wide frequency bands with measured impedance bandwidths of 54.4%, 14%, 23.5% and 17.2%, centered at 1.2, 2.0, 2.6 and 3.4 GHz, respectively, thus, enabling it to cover GSM 900, GPS, UMTS, WLAN, ISM, Bluetooth, LTE 2300/2500 and WiMAX standards. The antenna has omnidirectional radiation pattern with a maximum gain of 2.1 dBi. To characterize the flexibility of the antenna, the fabricated prototype is tested in convex and concave bent configurations for radii of 78 mm and 59 mm. The overall performance remains unaffected, except a minor shift of 20 MHz and 60 MHz in S11, for concave bending at both radii. The compact, lightweight and conformal design as well as multiband performance in bent configurations, proves the suitability of the antenna for future electronic devices.

Measurements of total isotropic sensitivity and average fading sensitivity of mobile phones in reverberation chambers

Abstract: The authors describe two ways to measure receive performance of active terminals by using a reverberation chamber, referred to as total isotropic sensitivity (TIS) and average fading sensitivity. The latter is a new method which can be measured very fast in a reverberation chamber. It is also more representative of the actual performance of a terminal in a real fading environment than the former. Based on measured results, they conclude that the average fading sensitivity is a promising alternative to TIS measurements for characterizing receive performance of mobile terminals. The reverberation chamber is well suited for both these measurements. Average fading sensitivity measurements can be done faster than TIS and give similar relative results i.e. the average fading sensitivity shows similar difference between the different phones as the TIS results, when they read the sensitivity levels at 0.5% error level.