21 research outputs found
Kronecker Product Correlation Model and Limited Feedback Codebook Design in a 3D Channel Model
A 2D antenna array introduces a new level of control and additional degrees
of freedom in multiple-input-multiple-output (MIMO) systems particularly for
the so-called "massive MIMO" systems. To accurately assess the performance
gains of these large arrays, existing azimuth-only channel models have been
extended to handle 3D channels by modeling both the elevation and azimuth
dimensions. In this paper, we study the channel correlation matrix of a generic
ray-based 3D channel model, and our analysis and simulation results demonstrate
that the 3D correlation matrix can be well approximated by a Kronecker
production of azimuth and elevation correlations. This finding lays the
theoretical support for the usage of a product codebook for reduced complexity
feedback from the receiver to the transmitter. We also present the design of a
product codebook based on Grassmannian line packing.Comment: 6 pages, 5 figures, to appear at IEEE ICC 201
Limited Feedback Design for Interference Alignment on MIMO Interference Networks with Heterogeneous Path Loss and Spatial Correlations
Interference alignment is degree of freedom optimal in K -user MIMO
interference channels and many previous works have studied the transceiver
designs. However, these works predominantly focus on networks with perfect
channel state information at the transmitters and symmetrical interference
topology. In this paper, we consider a limited feedback system with
heterogeneous path loss and spatial correlations, and investigate how the
dynamics of the interference topology can be exploited to improve the feedback
efficiency. We propose a novel spatial codebook design, and perform dynamic
quantization via bit allocations to adapt to the asymmetry of the interference
topology. We bound the system throughput under the proposed dynamic scheme in
terms of the transmit SNR, feedback bits and the interference topology
parameters. It is shown that when the number of feedback bits scales with SNR
as C_{s}\cdot\log\textrm{SNR}, the sum degrees of freedom of the network are
preserved. Moreover, the value of scaling coefficient C_{s} can be
significantly reduced in networks with asymmetric interference topology.Comment: 30 pages, 6 figures, accepted by IEEE transactions on signal
processing in Feb. 201
High-Dimensional Stochastic Gradient Quantization for Communication-Efficient Edge Learning
Edge machine learning involves the deployment of learning algorithms at the
wireless network edge so as to leverage massive mobile data for enabling
intelligent applications. The mainstream edge learning approach, federated
learning, has been developed based on distributed gradient descent. Based on
the approach, stochastic gradients are computed at edge devices and then
transmitted to an edge server for updating a global AI model. Since each
stochastic gradient is typically high-dimensional (with millions to billions of
coefficients), communication overhead becomes a bottleneck for edge learning.
To address this issue, we propose in this work a novel framework of
hierarchical stochastic gradient quantization and study its effect on the
learning performance. First, the framework features a practical hierarchical
architecture for decomposing the stochastic gradient into its norm and
normalized block gradients, and efficiently quantizes them using a uniform
quantizer and a low-dimensional codebook on a Grassmann manifold, respectively.
Subsequently, the quantized normalized block gradients are scaled and cascaded
to yield the quantized normalized stochastic gradient using a so-called hinge
vector designed under the criterion of minimum distortion. The hinge vector is
also efficiently compressed using another low-dimensional Grassmannian
quantizer. The other feature of the framework is a bit-allocation scheme for
reducing the quantization error. The scheme determines the resolutions of the
low-dimensional quantizers in the proposed framework. The framework is proved
to guarantee model convergency by analyzing the convergence rate as a function
of the quantization bits. Furthermore, by simulation, our design is shown to
substantially reduce the communication overhead compared with the
state-of-the-art signSGD scheme, while both achieve similar learning
accuracies