Magnus Franzén

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

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 Hills 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.

Investigation of Heavily Loaded Reverberation Chamber for Testing of Wideband Wireless Units

Traditionally, the reverberation chamber has been used as a low-loss cavity to gain high signal levels. For testing of the radiated properties of wireless terminals and their antennas, this is not an issue, and lossy objects like for instance a head phantom can be introduced in the chamber without problem. However, for testing of active units, the bandwidth of the system itself plays an important role. In order to avoid distortion of the signal transmitted in the chamber, the bandwidth of the chamber must be larger than the bandwidth of the signal. This is one reason for loading the reverberation chamber. Another reason is a potential increase in the measurement accuracy, since an increased bandwidth makes more modes excited at a particular frequency. The purpose of this paper is to investigate the effect of loading on the statistics of the chamber and its bandwidth

MIMO performance of enhanced UMTS four-antenna structures for mobile phones in the presence of the user’s head

In this paper, we have presented the reverberation chambers (RC) evaluation of the multiple-input-multiple-output (MIMO) system performance of several UMTS four-antenna systems with low and high planar inverted-F antennas (PIFAs) isolation. It has been measured that the neutralization technique is always improving the performances even with the presence of the users head, due to the high isolation obtained between the antennas and their high total efficiencies.

Evaluation of the performances of several four-antenna systems in a reverberation chamber

This paper results from a short-term mission granted by the COST 284 where the antenna-design competences of the LEAT have been gathered with the measurement skills of Chalmers Institute of Technology. Three multi-antenna prototypes have been characterized in the Bluetest reverberation chamber in terms of total efficiency and diversity gain. Particularly the performances of an initial four-antenna system with poor isolation have been evaluated with and without the presence of a phantom head and then we characterized in the same manner two prototypes with highly decoupled antennas. The paper especially focused on demonstrating that the neutralization technique is still giving improvements in the presence of the users head.

Diversity characterization of optimized two-antenna systems for UMTS handsets

This paper presents the evaluation of the diversity performance of several two-antenna systems for UMTS terminals. All the measurements are done in a reverberation chamber and in a Wheeler cap setup. First, a two-antenna system having poor isolation between its radiators is measured. Then, the performance of this structure is compared with two optimized structures having high isolation and high total efficiency, thanks to the implementation of a neutralization technique between the radiating elements. The key diversity parameters of all these systems are discussed, that is, the total efficiency of the antenna, the envelope correlation coefficient, the diversity gains, the mean effective gain (MEG), and the MEG ratio. The comparison of all these results is especially showing the benefit brought back by the neutralization technique.

Designing reverberation chambers for measurements of small antennas and wireless terminals: Accuracy, frequency resolution, lowest frequency of operation, loading and shielding of chamber

The last five years the reverberation chamber has been developed to an accurate instrument for measuring the performance of small antennas and active mobile terminals in Rayleigh fading. The present paper gives an overview of the research done by the authors in relation to achieving an accuracy of 0.5 dB RMS or better, when measuring efficiency and radiated power. The accuracy has been verified by comparison with measurements in anechoic chambers, and between reverberation chambers of different size, and from participation in benchmarking of measurement ranges done within ACE (Antenna Center of Excellence, a European Network of Excellence). There have also been developed procedures for measuring quantities that are specific for the Rayleigh environment, such as diversity gain, MIMO capacity, and static as well as dynamic receiver sensitivity at certain data error rate (BER or FER). The paper will describe important topics related to the mode stirring and loading of the chamber, such as the accuracy, frequency resolution, average transfer function, and the system bandwidth when measuring active terminals. Numerical simulations play an important role in controlling the chamber performance.

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

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.

Measuring the Impact of Multiple Terminal Antennas on the Bit Rate of Mobile Broadband Systems Using Reverberation Chambers

The bit rate of new mobile broadband services will be directly affected by the configuration and efficiency of the terminal antennas. Each dB not lost through fading, mismatch and ohmic loss can be used by new (HSDPA and WiMAX) and future (LTE) mobile broadband services for higher user bit rate or use of less spectrum by increasing the modulation rate and/or reducing the coding rate. Fading can be combated using diversity antennas or utilized for higher capacity using partially uncorrelated communication channels created through the use of multiple antennas at both the radio base station and the terminal (MIMO). Low mismatch and low ohmic loss in the antenna and its environment are crucial for high bit rates with terminals using either diversity or MIMO. The only measurement instrument to directly measure the SNR increase of diversity antennas or the increase in capacity of using MIMO antennas is the reverberation chamber. The reverberation chamber simulates a fading environment similar to the one encountered in urban or indoor environments. Measurements in the reverberation chamber are fast and repeatable. The alternative is complex and often unreliable drive tests. Measurements of diversity gain and MIMO capacity in a reverberation chamber are explained and results are presented.

Realized diversity gain of active DECT phones in talk position measured in reverberation chamber

The reverberation chamber is a rectangular metal cavity that supports several modes. Each mode can be interpreted as eight plane waves propagating in different directions. To simulate a rich scattering environment, the modes are stirred. The net effect is fading with Rayleigh statistics, being very similar to what handheld units experience in indoor and urban areas. DECT is an acronym for Digital Enhanced Cordless Telecommunications. The cordless units receive and transmit on the same frequency, which in Europe lies in the band 1880-1900 MHz, and this makes it easy to implement antenna diversity on transmit. The maximum output power from a DECT unit is limited to 24 dBm.

Active throughput measurements for complete MIMO WLAN systems using dual reverberation chamber technique

A reliable, repeatable and fast measurement procedure for a complete Multiple-Input-Multiple-Output (MIMO) Wireless Local Area Network (WLAN) system is presented. Application level throughput is presented as a performance metric to reflect the actual throughput delivered to the end user by a WLAN system. The employed technique involves the use of two reverberation chambers to cope with the real world scenario of low Signal to Noise Ratio (SNR) proximity and to effectively control the rank of a communication channel.