Ramya Bhagavatula

Adaptive Limited Feedback for Sum-Rate Maximizing Beamforming in Cooperative Multicell Systems

Base station cooperation improves the sum-rates that can be achieved in cellular systems. Conventional cooperation techniques require sharing large amounts of information over finite-capacity backhaul links and assume that base stations have full channel state information (CSI) of all the active users in the system. In this paper, a new limited feedback strategy is proposed for multicell beamforming where cooperation is restricted to sharing only the CSI of active users among base stations. The system setup considered is a linear array of cells based on the “soft hand-off model,” where each cell contains single-antenna users and multi-antenna base stations. Beamforming vectors are designed using a generalized eigenvector approach to maximize the sum-rate in a single-interferer scenario, at high signal to noise ratio. Users are assumed to feedback quantized CSI of the desired and interfering channels using a finite-bandwidth feedback link. An upper bound on the mean loss in sum rate due to random vector quantization is derived. A new feedback-bit allocation strategy, to partition the available bits between the desired and interfering channels, is developed to reduce the mean loss in sum-rate due to quantization for the soft hand-off model. The proposed feedback-bit partitioning algorithm is shown, using simulations, to yield sum-rates close to the those obtained using full CSI at base station.

Limited feedback with joint CSI quantization for multicell cooperative generalized eigenvector beamforming

Existing work on limited feedback for cooperative multicell beamforming quantizes the desired and interfering channel state information (CSI) using separate codebooks. In this paper, it is shown that comparatively higher sum-rates can be obtained by jointly quantizing the desired and interfering CSI using a single codebook. A selection criterion is developed for random vector quantization (RVQ) to show that joint quantization with RVQ yields higher sum-rates than those obtained using separate codebooks. The generalized Lloyd algorithm is then used to generate codebooks using the codeword design strategy proposed in this paper. Simulations are used to show that the proposed joint quantization approaches perform almost as well as the full CSI case.

A New Double-Directional Channel Model Including Antenna Patterns, Array Orientation, and Depolarization

Multiple-input-multiple-output (MIMO) wireless channel models are often too simplistic to accurately model wireless propagation effects or too complex and/or site specific to be used for analytical purposes. In this paper, we develop a double-directional MIMO channel model that accounts for important propagation effects like scattering, clustering, and channel depolarization and antenna effects like antenna diversity, cross polarization, and random array orientation, while still retaining an intuitive representation. The proposed model can be parameterized using channel measurements obtained from site-specific measurement campaigns or from standard-based channel models. We show, using simulations, that the proposed model captures channel and antenna effects not included in other models, like the third-generation partnership program (3GPP) spatial channel model, the WINNER, and the IEEE 802.11n channel model. We use the model to study the impact of random orientation and channel depolarization on the data rates of a MIMO system.

Optimizing MIMO Antenna Placement and Array Configurations for Multimedia Delivery in Aircraft

In this paper, the feasibility of multiple-input multiple-output (MIMO) systems in aircraft is examined for the specific application of seatback entertainment, using an approach built around a site-specific capacity analysis methodology. The average capacity is evaluated as a function of the seat location, using the proposed methodology that relies on inputs from measurement results or ray-tracing simulations. The extension of the methodology to optimize the access point and client antenna placement locations and array configurations is also described in the paper. While the specific results are for the deployment of MIMO communication in aircraft for seatback entertainment, the same methodology can be applied to other deployment scenarios as well.

Sizing up MIMO arrays

In this article, we illustrate the proposed methodology by applying it to wireless deployment in aircraft, to reduce expenditure associated with the installation and maintenance of wires for seatback entertainment. The large data rates required can be easily supported by MIMO systems.

Sum-rate maximizing beamforming in multicell systems with limited feedback

Multicell cooperative transmit strategies typically require full channel state information (CSI) at the base stations. In this paper, a more practical limited feedback scenario is considered for a recently proposed multicell beamforming approach that approximately maximizes the sum-rates in a two-cell two-user network. In the limited feedback system presented, the desired and interfering CSI is quantized by the users and fed back to the base stations over a finite-bandwidth link. Each receiver is assumed to use two separate codebooks to quantize the desired and interfering channels. It is proposed to vary the codebook sizes adaptively as a function of the relative strength of the channels to minimize the loss in sum-rate. To this end, the mean loss in sum-rate due to random vector quantization is quantified. Closed-form expressions are derived to partition the available feedback bits adaptively between the desired and interfering channels to minimize the mean loss in sum-rate. Finally, simulations are used to verify the results presented in the paper.

Computing the Receive Spatial Correlation for a Multi-Cluster MIMO Channel Using Different Array Configurations

Spatial correlation among received signals has a significant impact on the performance of a multiple- input multiple-output (MIMO) system. In prior research, expressions were derived for the spatial correlation between the signals received at a uniform linear array (ULA) by assuming that the propagation environment has a single cluster, low angular spread or that the received rays arrive only along the broadside of the array. In this paper, analytical expressions are derived for the spatial correlation between the received signals of a ULA and uniform circular array without making any of the above mentioned assumptions. The derivation can be extended to suit most planar antenna array configurations. This makes the derived expressions valid for most practical propagation channels. Finally, we show using simulations that the derived expression evaluates spatial correlation more accurately than the existing models.

Impact of Mutual Coupling and Antenna Efficiencies on Adaptive Switching Between MIMO Transmission Strategies

Previous research has shown that adaptive switching between multiple-input multiple-output (MIMO) transmission strategies like spatial multiplexing and beamforming increases link reliability and capacity gains, as compared to fixed transmission strategies. To get the full benefit of adaptive switching it is necessary to obtain accurate estimates of the SNR values when we switch between the transmission strategies. In this paper, it is shown that (relatively more) accurate switching point estimates can be obtained by taking into account real-life effects like mutual coupling and antenna efficiencies, for switching between statistical beamforming and spatial multiplexing. Using simulations, it is shown that accounting for these effects can make the switching point estimate more accurate by as much as 12 dB, compared to the case when the practical effects are not considered.

A new MIMO channel representation including spatial diversity, array orientation and depolarization effects

This paper proposes an analytical channel model that accounts for spatial, pattern, polarization diversities, channel depolarization, random mobile terminal orientations and mutual coupling effects. by utilizing simulations, this study verifies that the model accounts for more propagation effects than the 3GPP SCM and IEEE 802.11n model.