Linear Precoding in Cooperative MIMO Cellular Networks with Limited Coordination Clusters

In a cooperative multiple-antenna downlink cellular network, maximization of a concave function of user rates is considered. A new linear precoding technique called soft interference nulling (SIN) is proposed, which performs at least as well as zero-forcing (ZF) beamforming. All base stations share channel state information, but each user's message is only routed to those that participate in the user's coordination cluster. SIN precoding is particularly useful when clusters of limited sizes overlap in the network, in which case traditional techniques such as dirty paper coding or ZF do not directly apply. The SIN precoder is computed by solving a sequence of convex optimization problems. SIN under partial network coordination can outperform ZF under full network coordination at moderate SNRs. Under overlapping coordination clusters, SIN precoding achieves considerably higher throughput compared to myopic ZF, especially when the clusters are large.Comment: 13 pages, 5 figure

Polarized gamma rays from dark matter annihilations

In this paper, we explore the possibility of a linearly polarized gamma-ray signal from dark matter annihilations in the Galactic center. Considering neutral weakly interacting massive particles, a polarized gamma-ray signal can be realized by a two-component dark matter model of Majorana fermions with an anapole moment. We discuss the spin alignment of such dark matter fermions in the Galactic center and then estimate the intensity and the polarizability of the final-state electromagnetic radiation in the dark matter annihilations. For low-mass dark matter, the photon flux at sub-GeV energies may be polarized at a level detectable in current X-ray polarimeters. Depending on the mass ratio between the final-state fermion and DM, the degree of polarization at the mass threshold can reach $70\%$ or even higher, providing us with a new tool for probing the nature of dark matter in future gamma-ray polarization experiments.Comment: 19 pages, 7 figures. v2: version accepted by Physics Letters

A DS-CDMA system using despreading sequences weighted by adjustable chip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDS's) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDS's outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for band-limited spreading signals is also provided to reveal the practical implications of the proposed technique. © 1999 IEEE.published_or_final_versio