Afroza Khatun

Measurement-Based Analysis of Spatial Degrees of Freedom in Multipath Propagation Channels

A method of estimating spatial degrees of freedom (DoF) from measured multipath propagation channels in the multiple-input single-output regime is presented. The DoF of the multipath channels on the transmit (Tx) side is derived by means of the spherical-wave expansion of electromagnetic fields radiated from a Tx antenna array having a certain aperture size. The DoF estimates are independent of particular realization of antenna elements on the Tx aperture. For a given aperture size, the DoF provides the number of the Tx antenna elements for efficient improvement of the system performance by utilizing the spatial diversity. Having confirmed the soundness of our DoF estimation method by channel measurements in an anechoic chamber, the DoF of indoor multipath channels is analyzed. The DoF on the Tx side of the considered multipath channels reveals larger values when the antenna aperture size is increased at a fixed frequency, and when the frequency increases for a fixed antenna aperture size. For a fixed frequency, increasing the antenna aperture size is more effective in observing extra DoF in the obstructed and non-line-of-sight channels than in the line-of-sight channels. Furthermore, for a fixed antenna aperture size, the use of the higher frequency brings larger DoFs in many propagation scenarios. The results also show that electrically smaller antennas are more efficient in observing the DoF. Finally, the solid angle of multipath is derived as a Tx antenna-independent metric of the multipath richness. It is defined as an angular range of dominant multipath clusters subtended on a unit sphere. Our analysis method is extendable to the multiple-input multiple-output regime in a straightforward manner.

Radio Propagation Measurements and WINNER II Parameterization for a Shopping Mall at 60 GHz

In this paper, we derive parameters of the WINNER II channel model for a shopping mall environment at 60 GHz band, covering both line-of-sight and obstructed-line-of-sight scenarios. The model parameters are derived mostly based on channel measurements reported in this paper. Due to a large measurement bandwidth and the highly specular nature of the channel in the shopping mall, only weak clustering effects of propagation paths are found. Still, we demonstrate that the WINNER II model structure is applicable to 60 GHz channel modeling since the model reproduces the measured channels well.

Experimental Verification of a Plane-Wave Field Synthesis Technique for MIMO OTA Antenna Testing

This paper evaluates the feasibility of a plane-wave field synthesis (PWS) technique for multiple-input multiple-output (MIMO) over-the-air (OTA) test facility, where a reference channel model is implemented. The test facility is based on a fading emulator and an anechoic chamber, equipped with multiple field emulating probes. The test facility emulates a radio channel condition using the PWS technique, based on the spherical wave theory. A simulation tool implementing the MIMO OTA field synthesis based on the PWS technique, named WIN-OTA, is established, where the WINNER II is chosen as the reference channel model. The simulation results show that the PWS technique reproduces the reference channels accurately in terms of envelope distribution, spatial and temporal correlation, and channel capacity. The WIN-OTA implementation was verified by comparing the emulated fields and the throughput from the simulations with the measurements for a practical MIMO OTA test facility. The results support the feasibility and accuracy of the field synthesis technique and the WIN-OTA implementation in MIMO OTA antenna testing.

Spatial Degrees-of-Freedom of 60 GHz Multiple-Antenna Channels

This paper studies the potential of spatial multiplexing for measured 60 GHz radio channels by estimating the number of eigenchannels. It is well-known that the number of available eigenchannels is dependent on a propagation channel condition as well as antenna array conguration. We evaluate a spatial degrees- of-freedom (SDoF) metric that depends on the propagation conditions and the antenna aperture size, but is otherwise independent of the realization of antenna elements on the aperture. The SDoF metric is a measure of the effective number of antenna elements for spatial multiplexing. We evaluate the SDoF for 60 GHz multiple-input multiple-output channels measured in a conference room environment under a line-of- sight (LOS) condition. Our analysis reveals that the SDoF is more than one for a transmit- receive antenna separation of more than 2 m, which indicates that spatial multiplexing can work in such LOS scenarios. Our results indicate that using only 10 antenna elements at each of the transmit and receive antenna aperture is sufficient for effective spatial multiplexing performance in the considered propagation scenario. Furthermore, by using just 5 antennas at each aperture, eigenchannels down to -15 dB relative magnitude can still be captured.

Spatial Coexistence of Millimeter-Wave Distributed Indoor Channels

Millimeter-wave radios have great potential of spatial interference management through its physically small but possibly electrically large antenna aperture. It is expected that the interference management becomes a major challenge after massive deployment of millimeter-wave devices. This paper therefore discusses design guidelines and an influential radio link property of two beamforming schemes for spatial coexistence at 60 GHz; conventional gain focusing and the interference alignment. The guideline is derived from an example of distributed channels in a small room. The investigation shows that, under a moderate receive signal-to-noise ratio, 1) having 2λ2 antenna apertures at the Tx and Rx realizes 3 to 7 bit/s/Hz under the presence of mutual interference, 2) the gain focusing is as robust as the interference alignment, and finally, 3) inter-link correlation is a primary bottleneck in the interference alignment.

Azimuthal Mode Decomposition Method for Cubical Surface Scanning With the Application to Spherical-Mode Field Synthesis

In this letter, a method of azimuthal mode decomposition is presented for cubical surface scanning. This method can be applied either in near-field-to-far-field (NF-FF) transformation in near-field antenna measurements or in synthesizing the fields using the virtual array principle. The method is presented for spherical-wave-mode field synthesis application in this letter. The method allows a reduction in the computational burden of the matrix inversion for synthesizing the fields. Requirements for the sampling locations on the cubical scanning surface and for the probe orientations and polarizations at each sampling location allowing the azimuthal-mode decomposition are provided.

Feasibility of multi-probe over-the-air antenna test methods for frequencies above 6 GHz

This paper discusses the feasibility of two multi-probe over-the-air antenna test methods at frequencies higher than 6 GHz. The two methods considered in this paper are prefaded signals synthesis (PFS) and plane wave synthesis (PWS) methods. Both methods suffer from huge challenges of the large number of dual-polarized probes required in emulating realistic standing waves in the antenna test zone, because physically small antennas become electrically large at the higher frequencies. However, the PFS method can take advantage of the nature of radio propagation channels at the higher frequencies, i.e., the sparsity of multipath channels and their angular selectivity, in order to reduce the number of active probes and hence signal feeds from a fading emulator. In contrast, the PWS method always needs signal feeds to all the probes and hence the implementation complexity is not reduced even under sparse multipath channels. The required number of probes is derived for the two methods using heuristic theoretical analyses and through practical antenna examples on a mobile-phone-sized ground plane at 5, 15, 30 and 60 GHz.

Attainable Capacity of Spatial Radio Channels: A Multiple-Frequency Analysis

This paper reports an attainable channel capacity and spatial degrees-of-freedom of multiple-antenna radio channels at 15, 28 and 61 GHz. The analysis is based on channel sounding in a same street canyon. The attainable capacity is evaluated so that it depends only on the antenna aperture size but is independent of implementation of antenna elements on the aperture. The analysis shows that multipath richness decreases as the frequency increases, as indicated by the smaller spatial degrees-of-freedom at higher frequencies for the same electrical aperture size of the receive antenna. The analysis furthermore reveals that channels at the three radio frequencies can attain almost the same level of capacity for a given transmit power and physical size of the receive antenna. The result is explained by a greater electrical size of the antenna aperture at the higher frequencies that can leverage higher gains and finer angular resolution and hence normalize the availability of fewer multipaths.