Jesper Ødum Nielsen

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.

Emulating Spatial Characteristics of MIMO Channels for OTA Testing

This paper discusses over-the-air (OTA) testing for multiple-input multiple-output (MIMO) capable terminals with emphasis on channel spatial characteristics emulation. A novel technique to obtain optimum power weights for the OTA probes based on convex optimization is proposed. The proposed technique emulates spatial correlation as well as introduces constraints on the maximum deviation between the target power azimuth spectrum (PAS) and the emulated PAS in terms of mean angle of arrival (AoA) and azimuth spread (AS). Simulation results show that the proposed emulation technique presents better performance compared with existing techniques in the literature. This improvement is further demonstrated by measurement results in a practical MIMO OTA setup.

Mobile handset performance evaluation using radiation pattern measurements

The mean effective gain is an attractive performance measure of mobile handsets, since it incorporates both directional and polarization properties of the handset and environment. In this work the mean effective gain is computed from measured spherical radiation patterns of five different mobile handsets, both in free space and including a human head & shoulder phantom. Different models of the environment allow a comparison of the mean effective gain obtained for realistic models based on measurements with the total radiated power and the total isotropic sensitivity. All the comparisons are based on the mean effective gain values obtained for different orientations of the handsets in the environments. For practical measurements it is important to minimize the measurement time. The paper includes a study of the variation in mean effective gain when the number of samples in the spherical radiation pattern is reduced. Furthermore, the frequency dependence of the mean effective gain is investigated, and a method is proposed for reducing the required number of measurements on different frequencies.

Experimental Assessment of Specific Absorption Rate Using Room Electromagnetics

A closed room environment is viewed as a lossy cavity, characterized by possibly a line-of-sight component and diffuse scattering parts from walls and internal obstacles. A theory used in acoustics and reverberation chambers is applied for the electromagnetic case, and main issues related to measurement systems, antennas characteristics, diffuse energy properties, and human exposure are investigated. The goal of this paper aims first toward validation of the assessment of the reverberation time in an environment using a virtual multiple-input-multiple-output channel system. Second, the reverberation time in an adjacent room is investigated, and hence, a measurement-based method is readily developed to assess the absorption cross section and the whole-body specific absorption rate of humans at 2.3 GHz in a realistic closed environment.

Doppler spectrum from moving scatterers in a random environment

A random non-line-of-sight environment with stationary transmitter and receiver is considered. In such an environment movement of a scatterer will lead to perturbations of the otherwise static channel with a resulting Doppler spectrum. This is quite a general situation in outdoor environments with moving traffic or indoor situations with moving people. Here we study the latter situation in detail with experimental results from a large office environment. A general theory of Doppler spectra is developed. The impact of a scatterer depends on the angular distribution of scattered energy, and uniform as well as sharply peaked distributions are considered in the theory. The Doppler spectra are in all cases sharply peaked at zero frequency due to forward scattering, but the actually measured distribution depends on the degree and type of activity in the environment, as well as the spectrum estimation accuracy.

Statistics of measured body loss for mobile phones

The variation in body loss for different users of a cellular handset is investigated. Measurements involving 200 test users of mobile communications (GSM) handsets have been performed and statistics are presented for a handset with three types of antennas. Differences in the body loss of up to 10 dB have been observed between users, thus indicating that body loss measurements for handsets should include several test persons. Depending on the antenna type, 8-13 test persons are required to obtain an estimate of the mean body loss with a /spl plusmn/1 dB confidence interval at a 90% level.

Towards very large aperture massive MIMO: A measurement based study

Massive MIMO is a new technique for wireless communications that claims to offer very high system throughput and energy efficiency in multi-user scenarios. The cost is to add a very large number of antennas at the base station. Theoretical research has probed these benefits, but very few measurements have showed the potential of Massive MIMO in practice. We investigate the properties of measured Massive MIMO channels in a large indoor venue. We describe a measurement campaign using 3 arrays having different shape and aperture, with 64 antennas and 8 users with 2 antennas each. We focus on the impact of the array aperture which is the main limiting factor in the degrees of freedom available in the multiple antenna channel. We find that performance is improved as the aperture increases, with an impact mostly visible in crowded scenarios where the users are closely spaced. We also test MIMO capability within a same user device with user proximity effect. We see a good channel resolvability with confirmation of the strong effect of the user hand grip. At last, we highlight that propagation conditions where line-of-sight is dominant can be favourable.

A Dynamic Model of the Indoor Channel

This paper proposes a new approach to modeling the radio channel experienced by transceivers moving in an indoor environment. For modeling the time-varying impulse response (IR) a randomly time-varyingpower-delay profile (PDP) is used, which enables the use of PDPs specific to local environments while widely different environments still can be included in the model. The PDPs are expanded in terms of so-called cluster functions modeling ray scattering on major physical objects in the environment. By using random weighting of the cluster functions, transceiver movements in some imaginary environment is modeled. In addition to the new model, the paper also presents results obtained from measurements of the indoor channel. A good agreement is found between measurement and model data, and thus the concepts applied in the new model seem to be useful in practice.

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.

Multiuser MIMO Channel Measurements and Performance in a Large Office Environment

We consider a multiuser MIMO-OFDMA scheme which exploits multiuser diversity in all dimensions: time, frequency and space. The main contribution of this paper is the evaluation and explanation of multiuser MIMO in a real world scenario, i.e. a large office room, based on measured channels. We report interesting results of a measurement campaign which suggest that significant MIMO gains are possible in an indoor environment even under strong line-of-sight condition as long as the distance of the users from the base station is larger than a reverberation distance which only depends on room surface and material. We show results on the achievable multiuser MIMO data rates for the given scenario compare to theoretical limits and discuss the results in the light of the insights gained from the measurement campaign. We also introduce restrictions on the rate distribution between users, i.e. QoS constraints. It is shown that the theoretical limits can be approximately achieved provided that the users which compete for the spatial resources are carefully chosen.