3 research outputs found
Volume of Metric Balls in High-Dimensional Complex Grassmann Manifolds
Volume of metric balls relates to rate-distortion theory and packing bounds
on codes. In this paper, the volume of balls in complex Grassmann manifolds is
evaluated for an arbitrary radius. The ball is defined as a set of hyperplanes
of a fixed dimension with reference to a center of possibly different
dimension, and a generalized chordal distance for unequal dimensional subspaces
is used. First, the volume is reduced to one-dimensional integral
representation. The overall problem boils down to evaluating a determinant of a
matrix of the same size as the subspace dimensionality. Interpreting this
determinant as a characteristic function of the Jacobi ensemble, an asymptotic
analysis is carried out. The obtained asymptotic volume is moreover refined
using moment-matching techniques to provide a tighter approximation in
finite-size regimes. Lastly, the pertinence of the derived results is shown by
rate-distortion analysis of source coding on Grassmann manifolds.Comment: 26 page
Modulation-Specific Multiuser Transmit Precoding and User Selection for BPSK Signalling
Motivated by challenges to existing multiuser transmission methods in a low
signal to noise ratio (SNR) regime, and emergence of massive numbers of low
data rate ehealth and internet of things (IoT) devices, in this paper we show
that it is beneficial to incorporate knowledge of modulation type into
multiuser transmit precoder design. Particularly, we propose a transmit
precoding (beamforming) specific to BPSK modulation, which has maximum power
efficiency and capacity in poor channel conditions. To be more specific, in a
multiuser scenario, an objective function is formulated based on the weighted
sum of error probabilities of BPSK modulated users. Convex optimization is used
to transform and solve this ill-behaved non-convex minimum probability of error
(MPE) precoding problem. Numerical results confirm significant performance
improvement. We then develop a low-complexity user selection algorithm for MPE
precoding. Based on line packing principles in Grassmannian manifolds, the
number of supported users is able to exceed the number of transmit antennas,
and hence the proposed approach is able to support more simultaneous users
compared with existing multiuser transmit precoding methods
Frequency Selective Hybrid Precoding for Limited Feedback Millimeter Wave Systems
Hybrid analog/digital precoding offers a compromise between hardware
complexity and system performance in millimeter wave (mmWave) systems. This
type of precoding allows mmWave systems to leverage large antenna array gains
that are necessary for sufficient link margin, while permitting low cost and
power consumption hardware. Most prior work has focused on hybrid precoding for
narrowband mmWave systems, with perfect or estimated channel knowledge at the
transmitter. MmWave systems, however, will likely operate on wideband channels
with frequency selectivity. Therefore, this paper considers wideband mmWave
systems with a limited feedback channel between the transmitter and receiver.
First, the optimal hybrid precoding design for a given RF codebook is derived.
This provides a benchmark for any other heuristic algorithm and gives useful
insights into codebook designs. Second, efficient hybrid analog/digital
codebooks are developed for spatial multiplexing in wideband mmWave systems.
Finally, a low-complexity yet near-optimal greedy frequency selective hybrid
precoding algorithm is proposed based on Gram-Schmidt orthogonalization.
Simulation results show that the developed hybrid codebooks and precoder
designs achieve very good performance compared with the unconstrained solutions
while requiring much less complexity.Comment: 42 pages, 8 figures, IEEE Transactions on Communications (Invited
Paper) [Some typos are fixed in this version