Ghassan S. Dahman

Spatial separation of closely-spaced users in measured massive multi-user MIMO channels

Fully-synchronous measurements of a massive multi-user multiple-input multiple-output (MU-MIMO) radio propagation channel are presented. We evaluate the ability of a massive MIMO system to spatially separate users located close to each other in line-of-sight (LOS) propagation conditions. The system consists of a base-station (BS) antenna array equipped with 64 dual-polarized antenna elements (128 ports) arranged in a cylindrical configuration, and eight single-antenna users. The users are confined to a five-meter diameter circle and move randomly at pedestrian speeds. The BS antenna array is located on top of a 20 m tall building and has LOS to the users. We examine user separability by studying singular value spread of the MU-MIMO channel matrix for several subsets of BS antenna array ports, along with sum-rate capacity and achievable sum-rates with both zero-forcing and matched-filtering linear precoders. We also analyze the performance of the user with the lowest rate. Finally, a comparison between the performance offered by the massive MIMO system and that of a conventional MU-MIMO system is provided. To the best of our knowledge, this is the first report of fully-synchronous dynamic measurements of a massive MIMO system. Our investigation shows that even users located close to each other in LOS propagation conditions can be spatially separated in a massive MIMO system.

On the directional reciprocity of uplink and downlink channels in Frequency Division Duplex systems

The possibility of using the channel reciprocity between the uplink and downlink channels in Frequency Division Duplex (FDD) systems to improve the efficiency has been deeply investigated. Previous studies have come to different conclusions for the characterization of the dissimilarity in uplink and downlink channel properties. This paper analyzes the mismatch in directional properties of the uplink and downlink channels of FDD systems based on the power of their multipath clusters. At a system level, due to the limited directional resolution, the multipath components arriving at the base station are seen as clusters, rather than individual signal paths. This fact is used to describe the mismatch of directional properties between uplink and downlink. The contribution of this paper is the use of a spectral dissimilarity metric as a measure to characterize this mismatch; a detailed study of this dissimilarity metric is also presented. It is found that under favorable propagation conditions, for both actual channel measurement data and ray-tracing simulations, the directional and power properties of the downlink multipath clusters can be estimated from the uplink channel with high reliability. Therefore, directional-based beamforming transmission techniques for FDD systems will be able to benefit from such similarity in order to improve the system performance.

The Use of Frequency-Orthogonal Pseudonoise (FOPN) Sounding Signals for Identifying Transmissions From Different Transmit Antennas

A method is described for identifying simultaneous signals from different transmit antennas in multiantenna sounders. Multiple-orthogonal signals are produced from one pseudonoise code by using it to modulate RF carriers at precalculated offset frequencies. Although their frequency spectra are interleaved and they occupy the same band, the spectral lines from different transmitters are separable after Fourier transformation at the receiver output. Since there is a one-to-one relationship between the length of the signal required for Fourier analysis and the number of orthogonal sounding signals, any arbitrary number of such signals can be generated, but Doppler spreads limit the number that can be employed with acceptable isolation among them.

Angle-of-Departure-Aided Opportunistic Space-Division Multiple Access

In this paper, a novel transmission technique for the multiple-input multiple-output (MIMO) broadcasting directional channels is introduced. During the training period, the base station sends a set of mutually orthogonal signals which enables each user to estimate the angle-of-departure of the anticipated strongest beam and the corresponding SNR. Then, the base station uses the reported angles-of-departure and SNRs to select a subgroup of users for data transmission in such a way that the sum capacity is maximized. Compared to the opportunistic space-division multiple access with beam selection algorithm [12], the advantage of the proposed algorithm is that it provides throughput gains up to 95% with low to medium number of users, under the same number of feedback bits. In addition to our new proposed algorithm, we introduce a modified version of the opportunistic space-division multiple access with beam selection algorithm, which shows a significant throughput improvement when evaluated using directional channel models such as WINNER.

Experimental evaluation of the effect of BS antenna inter-element spacing on MU-MIMO separation

In this paper, multiple-input multiple-output (MIMO) channel measurements in an outdoor micro-cell environment are used to study the effect of base-station (BS) inter-element spacing on multi-user MIMO signal separation. Users with two dual-polarized patch antennas at the mobile station (MS), and four equally-spaced dual-polarized patch antennas at the BS are considered. At the BS, the inter-element spacing (i.e., the spacing between the four dual-polarized patch antennas), is varied from half a wavelength to 8 m. For each BS inter-element spacing, the 4 × 8 MIMO channels are used to evaluate the systems capability separating closely located users. This evaluation is done by means of the correlation matrix distance metric (between each pair of closely located users) and the sum-rate capacity of the system (when 8 equally-spaced closely located users are served simultaneously). It is found that the system is capable of separating closely located users as long as the distance between them and the BS is less than the Fraunhofer distance associated with the BS antenna array. In the measured environment, for both LOS and NLOS propagation conditions, users located as close as 0.5 m from each other, having the same orientation, are separated successfully in the multi-user MIMO sense.

On the cross-correlation properties of large-scale fading in distributed antenna systems

Spatially distributed transmission points connected to the same source, known as distributed antenna systems, is one of the promising communication techniques that is capable to improve system performance. The distribution of the transmission points over a large geographical area increases the chances for the receive mobile station (MS) to experience favorable channel conditions from one link or more, simultaneously. However, this anticipated gain depends heavily on the cross-correlation properties of the parameters of different links, especially the large scale parameters (LSP). In this contribution, propagation measurements were used in order to study the cross-correlation of the large-scale fading (LSF) of different links in a distributed antenna system. Multi-site fully-coherent wideband (40 MHz bandwidth) measurements for a semi-urban microcell environment at 2.6 GHz with four base stations (BSs) and one MS were performed. The study of the LSF cross-correlation properties was performed on the global scale (i.e., using the LSF values of the full measurement route at once), and on the local scale (i.e., over local areas of hundreds of wavelengths long). It was found that, for the investigated environment, analyzing the LSF cross-correlation properties on the local scale gives better insight compared to performing the analysis on the global scale; the variations of the LSF of the different links are caused mainly by independent interacting objects (IOs), which reduce the probability of having all the links degrade simultaneously; and a Gaussian distribution can be used to model the cross-correlation coefficients for each link pairs.

Exploiting antenna correlation in measured massive MIMO channels

We investigate antenna correlation of an M-antenna massive multiple-input multiple-output (MIMO) setup with the purpose of obtaining a low-rank representation of the instantaneous massive MIMO channel. Low-rank representation bases using short-term and long-term antenna correlation statistics are defined, and their performance is evaluated with data sets obtained from channel measurements in both indoor and outdoor environments at 2.6 GHz. Our results indicate that the short-term bases can capture a larger amount of the channel energy compared to the long-term ones, but they have a limited timespan, one coherence time or less. On the other hand, the long-term bases are stable over time-spans of a few seconds. Hence, they can be obtained relatively easily. We also investigate a rank-p vector-scalar LMMSE channel estimator that exploits antenna correlation. Our results show that the investigated estimator can achieve a performance similar to that of full-rank LMMSE at a (2p + 1)/M times lower cost. The investigated estimator may be used in conjunction with estimators that exploit correlation in the frequency and time domains or, alternatively, in situations in which these estimators cannot be used, e.g., when pilot separation is larger than the channel coherence bandwidth or time.

Utilizing Multipath Clusters in Cognitive Radio Systems

In this paper, we introduce a new technique that allows the coexistence of secondary users (SUs) with a primary user (PU), in the same frequency and time. The proposed technique (i.e., the cluster-based spatial opportunistic spectrum sharing (CB-SOSS) technique) exploits the spatial degrees of freedom resulting from the use of multiple antennas at both the secondary base station (SBS) and SUs; also it does not have any requirement regarding the number of antennas at the primary base station nor the PU. The CB-SOSS benefits from the fact that in wireless channels, the multipath components travel in clusters that are non-uniformly distributed in the spatio-temporal space. The key idea is to have the SBS become aware of the locations of the strong clusters in the surrounding environment and utilize them opportunistically for data transmission to the SUs, without causing a harmful interference at the PU receiver.

Double directional radio propagation measurements and radio channel modelling pertinent to mobile MIMO communications in microcells

Power spectra for AOAs in microcells are typified by a clustering of AOAs in multiple overlapping groups, which, when analysed as a single conglomeration, have large spreads. Centroids of such conglomerations are in the direction along street canyons towards the base station. Associated AOD spectra are markedly different, and have several distinct modes, with each mode, as well as the total spectra having smaller spreads than those of the AOA spectra. This, as well as evidence reported from scatter plots of AODs vs. AOAs, suggests deterministic behavior, in which AOD centroids change from street section to street section in accordance with street geometry with respect to a base station. It would therefore be very difficult to recommend a generic PAS model for AODs, although it is considered that this might be a possibility in the case of AOAs. The fact that multiple runs along many street sections resulted in very similar angular spectra also indicates that features of the built-environment dominate, and that variations due to different traffic situations are less significant than might be expected. Narrowband fading statistics can be modelled as Rician or Rayleigh, depending on the street section.

A method for identifying simultaneously-transmitted signals from different transmit antennas in multi-antenna channel sounding experiments

A method is described for identifying simultaneous signals from different transmit antennas in multi-antenna sounders. Multiple orthogonal signals are produced from one pseudorandom code by using it to modulate RF carriers at pre-calculated offset frequencies. Although their frequency spectra are interleaved, the spectral lines from different transmitters are separable by Fourier transform at the receiver output. Since there is a one-to-one relationship between the length of the signal required for Fourier analysis and the number of orthogonal sounding signals, any arbitrary number of such signals can be generated, but Doppler spreads limit the number that can be employed with acceptable isolation among them.