Hamid Khodakarami

Flexible Optical Networks: An Energy Efficiency Perspective

High energy efficiency is expected to become a mandatory design criterion in optical networks of the future. This paper investigates schemes for enhancing the energy efficiency of core optical networks based on multicarrier transmission. Such networks of the future will incorporate flexible techniques, namely: 1) adaptive modulation and coding, 2) flexible spectrum allocation, 3) wavelength conversion, and 4) traffic grooming. We investigate the problem of energy efficient routing and spectrum allocation in core optical networks incorporating these flexible techniques. We propose a heuristic solution that provides an energy minimized design of long-haul optical networks by avoiding under-utilization of network resources such as optical fibers, transponders, and amplifiers. A fixed architecture that does not employ the aforementioned flexible techniques is used as a benchmark for comparison. The numerical results in the west European (24-node US) core optical network show that the energy efficiency of the flexible architecture can outperform the fixed architecture by a factor of 4.2 (6.4) for low and 1.8 (1.9) for high traffic demands, respectively.

Link adaptation for physical layer security over wireless fading channels

A secure link adaptation framework is proposed, which exploits the inherent fluctuations of wireless fading channels for high-performance communications and physical layer security in the presence of an eavesdropper. The authors use very sharp channel codes intended for reliability and demonstrate that they also provide security, when successfully incorporated in the link adaptation design framework and the security constraint is not very stringent. Two scenarios are considered in which the transmitter has access to the eavesdropper channel state information either instantaneously or statistically. The proposed secure link adaptation framework is formulated to maximise the spectral efficiency of the communication, whereas both reliability and security constraints are provisioned. Different designs are considered when the security constraint is quantified by instantaneous bit error rate (BER), average BER or leakage probability. For the problem with instantaneous BER constraint, an efficient analytical solution and a numerical solution are presented. A closed-form analytical solution is also provided for secure link adaptation with average BER constraint, whereas the problem with the leakage constraint is tackled numerically. Extensive results and detailed analysis are provided to draw insights on the effects of different design parameters on the performance.

Link Adaptation with Untrusted Relay Assignment: Design and Performance Analysis

In this paper, a link adaptation and untrusted relay assignment (LAURA) framework for efficient and reliable wireless cooperative communications with physical layer security is proposed. Using sharp channel codes in different transmission modes, reliability for the destination and security in the presence of untrusted relays (low probability of interception) are provided through rate and power allocation. Within this framework, several schemes are designed for highly spectrally efficient link adaptation and relay selection, which involve different levels of complexity and channel state information requirement. Analytical and simulation performance evaluation of the proposed LAURA schemes are provided, which demonstrates the effectiveness of the presented designs. The results indicate that power adaptation at the source plays a critical role in spectral efficiency performance. Also, it is shown that relay selection based on the signal to noise ratio of the source to relays channels provides an interesting balance of performance and complexity within the proposed LAURA framework.

On the Energy Efficiency of Modulation Formats for Optical Communications

An information theoretic framework for performance evaluation of high-dimensional modulation formats in an optical communication link is proposed. The spectral and energy efficiency of different candidates for future coherent optical communications are analyzed to explore the advantages of exploiting different degrees of freedom for symbol coding, construction, and transmission. Our analysis demonstrates the advantage of fully utilizing both polarization states from the spectral and energy efficiency perspective. In particular, at the same spectral efficiencies of 2.5 and 2.8 b/s/Hz, dual polarization quadrature phase shift keying (DP-QPSK) has, respectively, 0.8 and 1.6 dB energy efficiency advantage over polarization switched QPSK (PS-QPSK).

Upper bound for the performance metrics of amplify-and-forward cooperative networks based on harmonic mean approximation

In many studies of amplify-and-forward cooperative networks, a harmonic mean approximation has been used for the instantaneous signal-to-noise ratio of each dual hop relaying branch. This approximation overestimates the signal-to-noise ratio and results in a lower bound for the performance metrics, such as outage probability and average error probability. In this paper, a framework is introduced to obtain a tight upper bound for these performance metrics, as a function of harmonic mean approximation. It is shown that this upper bound is the tightest possible one among all harmonic mean related approximations. This bound is derived by modifying the same analysis used in the previous studies to compute the lower bound, without increasing the order of computational complexity. Numerical results and simulations show that the proposed upper bound for the performance metrics well follows the exact values.

Quality of service provisioning and energy minimized scheduling in software defined flexible optical networks

The over-provisioning of capacities in optical networks is not a sustainable approach in the long run. In this paper, we propose a software defined networking scheme for quality of service provisioning through energy efficient assignment of optical transponders, employing bandwidth variable distance adaptive modulation and coding. Our scheme enables avoiding over-provisioning of transponder capacity as well as short-term major changes in equipment allocation for networks with dynamic traffic. We make use of the seasonal auto-regressive integrated moving average model to forecast the statistics of network traffic for an arbitrary time span based on the requirements and the constraints of the service provider. The quality of service measure is defined as the probability of congestion at the core router ports. A stochastic linear programming approach is used to provide a solution for energy efficient assignment of optical transponders and electronic switching capacity while ensuring a certain level of quality of service to core routers. The scheduling of optical lightpath capacities is performed for the entire duration of time under consideration, whereas the scheduling of electronic switching capacities is performed based on the short-term dynamics of the traffic. Numerical results show up to 48% improvement in the energy efficiency of optical networks and 45% reduction in the number of optical lightpaths through the implementation of the proposed technique, compared to a design based on employing conventional fixed optical transponders and no traffic rerouting, where both schemes satisfy the congestion probability requirements.

Invited Article: Polarization diversity and modulation for high-speed optical communications: architectures and capacity

Polarization is one of the fundamental properties of optical waves. To cope with the exponential growth of the Internet traffic, optical communications has advanced by leaps and bounds within the last decade. For the first time, the polarization domain has been extensively explored for high-speed optical communications. In this paper, we discuss the general principle of polarization modulation in both Jones and Stokes spaces. We show that there is no linear optical device capable of transforming an arbitrary input polarization into one that is orthogonal to itself. This excludes the receiver self-polarization diversity architecture by splitting the signal into two branches, and then transferring one of the branches into orthogonal polarization. We next propose a novel Stokes vector (SV) detection architecture using four single-ended photodiodes (PD) that can recover a full set of SV. We then derive a closed-form expression for the information capacity of different SV detection architectures and compare the capacity of our proposed architectures with that of intensity-modulated directly-detected (IM/DD) method. We next study the 3-PD SV detection architecture where a subset of SV is detected, and devise a novel modulation algorithm that can achieve 2-dimensional modulation with the 3-PD detection. By using cost-effective SV receivers, polarization modulation and multiplexing offers a powerful solution for short-reach optical networks where the wavelength domain is quickly exhausted.

Link adaptation for fixed relaying with untrusted relays: Transmission strategy design and performance analysis

Wireless cooperative communication through link adaptation with untrusted relay assignment is considered. Using sharp channel codes in different transmission modes, reliability for destination and security in the presence of untrusted relays are provided through rate and power allocation. Scenarios with single available relay and opportunistic relaying in the presence of multiple relays are investigated. These scenarios are analyzed separately in terms of performance over Rayleigh fading channel for constant power and adaptive power transmission. In constant power case, closed form expressions for average spectral efficiency is derived through an effective approximation. Retaining the sum transmission power of source and relay unchanged, the performance of system is enhanced through power allocation. Numerical results reflect the effectiveness of the approximations for analytic performance evaluation when comparing to the simulation results.

Subcarrier Reliability Aware Soft-Decision LDPC Code in CO-OFDM Systems

We propose a subcarrier reliability aware low-density parity-check (SRA-LDPC) soft-decision coding scheme for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The SRA-LDPC code takes consideration of varying signal-to-noise ratio performance among different OFDM subcarriers. At the SRA-LDPC encoder, flexible forward error correction is applied to different ranges of subcarriers. At the decoder, the algorithm keeps track of noise variance of each subcarrier and utilizes such variance for decoding. Our proposed algorithm is substantiated in a 120-Gb/s CO-OFDM system where near-dc noise originated from the local oscillator is detrimental to the system performance. The experimental results show that the SRA-LDPC code is effective in combating near-dc noise and enhancing the receiver sensitivity.

Duobinary pulse shaping for frequency chirp enabled complex modulation.

The frequency chirp of optical direct modulation (DM) used to be a performance barrier of optical transmission system, because it broadens the signal optical spectrum, which becomes more susceptible to chromatic dispersion induced inter-symbol interference (ISI). However, by considering the chirp as frequency modulation, the single DM simultaneously generates a 2-D signal containing the intensity and phase (namely, the time integral of frequency). This complex modulation concept significantly increases the optical signal to noise ratio (OSNR) sensitivity of DM systems. This Letter studies the duobinary pulse shaping (DB-PS) for chirp enabled DM and its impact on the optical bandwidth and system OSNR sensitivity. DB-PS relieves the bandwidth requirement, at the sacrifice of system OSNR sensitivity. As DB-PS induces a controlled ISI, the receiver requires one more tap for maximum likelihood sequence estimation (MLSE). We verify this modified MLSE with a 10-Gbaud duobinary PAM-4 transmission experiment.