2,532 research outputs found
Iterative decoding for MIMO channels via modified sphere decoding
In recent years, soft iterative decoding techniques have been shown to greatly improve the bit error rate performance of various communication systems. For multiantenna systems employing space-time codes, however, it is not clear what is the best way to obtain the soft information required of the iterative scheme with low complexity. In this paper, we propose a modification of the Fincke-Pohst (sphere decoding) algorithm to estimate the maximum a posteriori probability of the received symbol sequence. The new algorithm solves a nonlinear integer least squares problem and, over a wide range of rates and signal-to-noise ratios, has polynomial-time complexity. Performance of the algorithm, combined with convolutional, turbo, and low-density parity check codes, is demonstrated on several multiantenna channels. The results for systems that employ space-time modulation schemes seem to indicate that the best performing schemes are those that support the highest mutual information between the transmitted and received signals, rather than the best diversity gain
Degrees of Freedom of Full-Duplex Multiantenna Cellular Networks
We study the degrees of freedom (DoF) of cellular networks in which a full
duplex (FD) base station (BS) equipped with multiple transmit and receive
antennas communicates with multiple mobile users. We consider two different
scenarios. In the first scenario, we study the case when half duplex (HD)
users, partitioned to either the uplink (UL) set or the downlink (DL) set,
simultaneously communicate with the FD BS. In the second scenario, we study the
case when FD users simultaneously communicate UL and DL data with the FD BS.
Unlike conventional HD only systems, inter-user interference (within the cell)
may severely limit the DoF, and must be carefully taken into account. With the
goal of providing theoretical guidelines for designing such FD systems, we
completely characterize the sum DoF of each of the two different FD cellular
networks by developing an achievable scheme and obtaining a matching upper
bound. The key idea of the proposed scheme is to carefully allocate UL and DL
information streams using interference alignment and beamforming techniques. By
comparing the DoFs of the considered FD systems with those of the conventional
HD systems, we establish the DoF gain by enabling FD operation in various
configurations. As a consequence of the result, we show that the DoF can
approach the two-fold gain over the HD systems when the number of users becomes
large enough as compared to the number of antennas at the BS.Comment: 21 pages, 16 figures, a shorter version of this paper has been
submitted to the IEEE International Symposium on Information Theory (ISIT)
201
Sum Throughput Maximization in Multi-Tag Backscattering to Multiantenna Reader
Backscatter communication (BSC) is being realized as the core technology for
pervasive sustainable Internet-of-Things applications. However, owing to the
resource-limitations of passive tags, the efficient usage of multiple antennas
at the reader is essential for both downlink excitation and uplink detection.
This work targets at maximizing the achievable sum-backscattered-throughput by
jointly optimizing the transceiver (TRX) design at the reader and
backscattering coefficients (BC) at the tags. Since, this joint problem is
nonconvex, we first present individually-optimal designs for the TRX and BC. We
show that with precoder and {combiner} designs at the reader respectively
targeting downlink energy beamforming and uplink Wiener filtering operations,
the BC optimization at tags can be reduced to a binary power control problem.
Next, the asymptotically-optimal joint-TRX-BC designs are proposed for both low
and high signal-to-noise-ratio regimes. Based on these developments, an
iterative low-complexity algorithm is proposed to yield an efficient
jointly-suboptimal design. Thereafter, we discuss the practical utility of the
proposed designs to other application settings like wireless powered
communication networks and BSC with imperfect channel state information.
Lastly, selected numerical results, validating the analysis and shedding novel
insights, demonstrate that the proposed designs can yield significant
enhancement in the sum-backscattered throughput over existing benchmarks.Comment: 17 pages, 5 figures, accepted for publication in IEEE Transactions on
Communication
Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks
An uplink-downlink two-cell cellular network is studied in which the first
base station (BS) with antennas receives independent messages from its
serving users, while the second BS with antennas transmits
independent messages to its serving users. That is, the first and second
cells operate as uplink and downlink, respectively. Each user is assumed to
have a single antenna. Under this uplink-downlink setting, the sum degrees of
freedom (DoF) is completely characterized as the minimum of
,
, , and , where denotes
. The result demonstrates that, for a broad class of network
configurations, operating one of the two cells as uplink and the other cell as
downlink can strictly improve the sum DoF compared to the conventional uplink
or downlink operation, in which both cells operate as either uplink or
downlink. The DoF gain from such uplink-downlink operation is further shown to
be achievable for heterogeneous cellular networks having hotspots and with
delayed channel state information.Comment: 22 pages, 11 figures, in revision for IEEE Transactions on
Information Theor
Free Probability based Capacity Calculation of Multiantenna Gaussian Fading Channels with Cochannel Interference
During the last decade, it has been well understood that communication over
multiple antennas can increase linearly the multiplexing capacity gain and
provide large spectral efficiency improvements. However, the majority of
studies in this area were carried out ignoring cochannel interference. Only a
small number of investigations have considered cochannel interference, but even
therein simple channel models were employed, assuming identically distributed
fading coefficients. In this paper, a generic model for a multi-antenna channel
is presented incorporating four impairments, namely additive white Gaussian
noise, flat fading, path loss and cochannel interference. Both point-to-point
and multiple-access MIMO channels are considered, including the case of
cooperating Base Station clusters. The asymptotic capacity limit of this
channel is calculated based on an asymptotic free probability approach which
exploits the additive and multiplicative free convolution in the R- and
S-transform domain respectively, as well as properties of the eta and Stieltjes
transform. Numerical results are utilized to verify the accuracy of the derived
closed-form expressions and evaluate the effect of the cochannel interference.Comment: 16 pages, 4 figures, 1 tabl
Optimum Pilot Overhead in Wireless Communication: A Unified Treatment of Continuous and Block-Fading Channels
The optimization of the pilot overhead in single-user wireless fading
channels is investigated, and the dependence of this overhead on various system
parameters of interest (e.g., fading rate, signal-to-noise ratio) is
quantified. The achievable pilot-based spectral efficiency is expanded with
respect to the fading rate about the no-fading point, which leads to an
accurate order expansion for the pilot overhead. This expansion identifies that
the pilot overhead, as well as the spectral efficiency penalty with respect to
a reference system with genie-aided CSI (channel state information) at the
receiver, depend on the square root of the normalized Doppler frequency.
Furthermore, it is shown that the widely-used block fading model is only a
special case of more accurate continuous fading models in terms of the
achievable pilot-based spectral efficiency, and that the overhead optimization
for multiantenna systems is effectively the same as for single-antenna systems
with the normalized Doppler frequency multiplied by the number of transmit
antennas.Comment: Submitted to IEEE Trans. Wireless Communication
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