[Show abstract][Hide abstract] ABSTRACT: In a multiple-antenna relay channel, the full-duplex cut-set capacity upper bound and decode-and-forward rate are formulated as convex optimization problems. For half-duplex relaying, bandwidth allocation and transmit signals are optimized jointly. Moreover, achievable rates based on the compress-and-forward strategy are presented using rate-distortion and Wyner-Ziv compression schemes.
Preview · Article · Nov 2011 · IEEE Transactions on Communications
[Show abstract][Hide abstract] ABSTRACT: With wavelength-division multiplexing (WDM) rapidly nearing its scalability limits, space-division multiplexing (SDM) seems the only option to further scale the capacity of optical transport networks. In order for SDM systems to continue the WDM trend of reducing energy and cost per bit with system capacity, integration will be key to SDM. Since integration is likely to introduce non-negligible crosstalk between multiple parallel transmission paths, multiple-input multiple output (MIMO) signal processing techniques will have to be used. In this paper, we discuss MIMO capacities in optical SDM systems, including related outage considerations which are an important part in the design of such systems. In order to achieve the low-outage standards required for optical transport networks, SDM transponders should be capable of individually addressing, and preferably MIMO processing all modes supported by the optical SDM waveguide. We then discuss the effect of distributed optical noise in MIMO SDM systems and focus on the impact of mode-dependent loss (MDL) on system capacity and system outage. Through extensive numerical simulations, we extract scaling rules for mode-average and mode-dependent loss and show that MIMO SDM systems composed of up to 128 segments and supporting up to 128 modes can tolerate up to 1 dB of per-segment MDL at 90% of the system's full capacity at an outage probability of 10(-4).
[Show abstract][Hide abstract] ABSTRACT: We consider a flat fading multiantenna downlink system with a large number of users where the objective is to deliver equal rates to nonoutage users with a low complexity. We show that in the limit of a large number of users, a zero-forcing beamforming strategy combined with a low complexity user grouping algorithm based on a semi-orthogonal user selection achieves asymptotically optimal performance, with respect to an upper bound that can be achieved when no interference is present among users.
No preview · Article · Jul 2011 · IEEE Transactions on Information Theory
[Show abstract][Hide abstract] ABSTRACT: We compare the performance of half-duplex relays in downlink cellular system against a baseline system without relays. We simulate the performance of (i) a collaborative power addition scheme, where the relay boosts the received power (P- CPA) at the mobile locations, and (ii) a CPA scheme with power control (PC-CPA) at the base station and relays. Evaluations are done in the context of a 19-cell, 57-sector set-up in which each of the served users must be delivered a message. The user messages are taken to have the same size and 90% of users in the network must be served. Improvements over the baseline due to relay deployments are measured in terms of increase in common rate of users as well as power savings in terms of reduction in peak or average power transmitted by base stations. In the CPA schemes with base stations and relays transmitting at full power, the peak power saving is 1.46 dB, alternately, the throughput improvement over a 1 bit/sec/Hz baseline rate is 21%. In the PC-CPA scheme, the peak power saving is 2.6 dB and the average total power in the system can be reduced by 3 dB.
Full-text · Article · May 2011 · IEEE Transactions on Wireless Communications
[Show abstract][Hide abstract] ABSTRACT: For one isolated wireless link we take a unified look at simple beamforming
(BF) as contrasted with MIMO to see how both emerge and under which conditions
advantage goes to one or the other. Communication is from a high base array to
a user in clutter. The channel propagation model is derived from fundamentals.
The base knows the power angular spectrum, but not the channel instantiation.
Eigenstates of the field spatial autocorrelation are the preferred apodizations
(APODs) which are drivers of the natural modes for exciting lectric fields.
Preference for MIMO or BF depends on APOD spectra which are surveyed pointing
to various asymptotic effects, including the maximum BF gain. Performance is
studied under varying eigenmode power settings at 10% outage. We focus on (1,4)
driving the strongest mode for BF and (4,4) driving the 4 strongest for MIMO.
Results are obtained under representative parameter settings, e.g. an angular
spread of 8 deg, 2 GHz carrier, 0 dB SNR and an array aperture of 1.68m (4
field decorrelation lengths) with antenna elements spaced as close as lambda/2.
We find MIMO excelling for array apertures much larger than the decorrelation
length; BF does almost as well for smaller apertures.
[Show abstract][Hide abstract] ABSTRACT: We calculate multiple-input-multiple-output (MIMO) outage probabilities for optical fiber links employing spatial multiplexing across linearly coupled propagation modes in the presence of mode-dependent loss and distributed optical noise.
[Show abstract][Hide abstract] ABSTRACT: We apply information theory to optical fibers in the context of transmitting information in optical networks. We define a Â¿fiber channelÂ¿ suitable for such networks and present fiber capacity estimates for transmission over 500 km. We compare our capacity estimates to recent record capacity experiments.
[Show abstract][Hide abstract] ABSTRACT: We present a simulation study of two relaying schemes in downlink cellular systems. We simulate the performance of two relaying strategies: (i) a collaborative power addition (CPA) scheme, cooperative scheme where the relay collaborates with the base station, and (ii) a simple orthogonal relaying strategy where base stations and relays transmit in orthogonal time slots. Evaluations are done in the context of a 19-cell, 57-sector set-up in which each of the served users must be delivered a message. The user messages are taken to have the same size and 90% of users in the network must be served. All simultaneous transmissions mutually interfere. Improvements due to relay deployments are measured as throughput improvements in terms of increase in common rate of users as well as power savings in terms of reduction in peak or average power transmitted by base stations. We observe that the simpler orthogonal relaying scheme performs nearly as well as the more complex collaborative relaying in producing throughput gains and power savings.
[Show abstract][Hide abstract] ABSTRACT: We describe a method to estimate the capacity limit of fiber-optic communication systems (or Â¿fiber channelsÂ¿) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.
Full-text · Article · Mar 2010 · Journal of Lightwave Technology
[Show abstract][Hide abstract] ABSTRACT: We evaluate the peak and average power savings due to relay deployments in cellular systems via a simulation study. The peak power savings translate to cost reduction in power amplifiers. The average power savings lead to savings in electricity bills. Half-duplex relays are placed one per sector in a 19-cell, 57-sector cellular network. In the baseline case, the base stations control their transmit powers to achieve a common rate among users. When relays are present in the system, optimal powers are found when the relays get the complete message to be relayed to the user. The codebooks at the relays are chosen such that the users obtain a rate corresponding to the sum of the received powers from the base station and the relay. We observe that when power control is employed, the peak power saving is 2.6 dB and the average total power in the system can be reduced by 3 dB.
[Show abstract][Hide abstract] ABSTRACT: In this letter, we calculate theoretical limits to fiber-optic transport capacity under the constraint of requiring the use of concentric multiring modulation alphabets. Subject to these conditions, we vary ring amplitude ratios and occupation probabilities to determine the capacity-optimized ring constellations. The technique used for fiber transmission calculations accurately captures all fiber nonlinear effects, with the accuracy limited by the noise and signal statistical properties only.
[Show abstract][Hide abstract] ABSTRACT: • Shannon's information theory allows to determine an asymptote of the channel information rate for a signal impaired by additive white Gaussian noise • Determining the limiting information rate in point-topoint fiber transmission in optically-routed network can allow to set limits on optical network capacity • Achieving capacity requires an array of advanced technologies • Many important open issues remain to be addressed to solve the problem of maximizing fiber capacity in optical networks!
[Show abstract][Hide abstract] ABSTRACT: The instantaneous optical Kerr effect in optical fibers is a nonlinear phenomenon that can impose limits on the ability of fiber-optic communication systems to transport information. We present here a conservative estimate of the "fiber channel" capacity in an optically routed network. We show that the fiber capacity per unit bandwidth for a given distance significantly exceeds current record experimental demonstrations.
Full-text · Article · Nov 2008 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: The problem of evaluating the maximum possible rate of information transmission in fibre-optic communication systems is discussed. A spectral efficiency of ∼7 bits/s/Hz over 1000 km in one polarization is shown to be theoretically achievable.
[Show abstract][Hide abstract] ABSTRACT: We discuss the impact of amplification technologies, multi-level constellations and fiber nonlinearity on the spectral efficiency achievable in fiber-optic networks.
[Show abstract][Hide abstract] ABSTRACT: It has been demonstrated that base station cooperation can reduce co-channel interference (CCI) and increase cellular system capacity. In this work we consider another approach by dividing the system into microcells through denser base station deployment. We adopt the criterion to maximize the minimum spectral efficiency of served users with a certain user outage constraint. In a two-dimensional hexagon array with homogeneous microcell structure, under the proposed propagation model denser base station deployment outperforms suboptimal cooperation schemes (zero-forcing) when the density increases beyond 3 - 12 base stations per km<sup>2</sup>, the exact value depending on the rules of outage user selection. However, close- to-optimal cooperation schemes (zero-forcing with dirty-paper- coding) are always superior to denser deployment. Performance of a hierarchial cellular structure mixed with both macrocells and microcells is also evaluated.
[Show abstract][Hide abstract] ABSTRACT: We present a capacity estimate of fiber-optic communication systems limited by fiber nonlinearity. The analysis reveals that a capacity of ~5 bits/s/Hz in a single polarization for transmission over 2000 km is possible using advanced technologies.
[Show abstract][Hide abstract] ABSTRACT: It has been demonstrated that base station cooperation can reduce co-channel interference (CCI) and increase cellular system capacity. In this work, we consider another approach by dividing the system into picocells through denser base station deployment. For a two-dimensional hexagon cellular array and the propagation model under consideration, we observe that the operating regime shifts from interference-limited to noise-limited when the density increases to about 20 base stations per km<sup>2</sup>. To compare the performance of both approaches, we adopt a criterion to maximize the minimum served spectral efficiency with a certain user outage constraint. Simulations show that denser base station deployment outperforms suboptimal cooperation schemes (zero-forcing) when the density increases beyond 3~12 base stations per km<sup>2</sup>, the exact value depending on the rules of outage user selection. However, close-to-optimal cooperation schemes (zero-forcing with dirty-paper-coding) are always superior to denser base station deployment.
[Show abstract][Hide abstract] ABSTRACT: We investigate optimum zero-forcing beamforming in multiple antenna broadcast channels with per-antenna power constraints. We show that standard zero-forcing techniques, such as the Moore-Penrose pseudo-inverse, considered mainly in the context of sum-power constrained systems are suboptimal when there are per-antenna power constraints. We formulate convex optimization problems to find the optimum zero-forcing beamforming vectors. Our results indicate that optimizing the antenna outputs based on the per-antenna constraints may improve the rate considerably when the number of transmit antennas is larger the number of receive antennas. Having more transmit antennas gives rise to additional signal space dimensions that may be exploited effectively to reduce transmit power at particular antennas with limited power budget.