Physical Layer Constraints Research Articles

Photonic-aware neural networks

Photonics-based neural networks promise to outperform electronic counterparts, accelerating neural network computations while reducing power consumption and footprint. However, these solutions suffer from physical layer constraints arising from the underlying analog photonic hardware, impacting the resolution of computations (in terms of effective number of bits), requiring the use of positive-valued inputs, and imposing limitations in the fan-in and in the size of convolutional kernels. To abstract these constraints, in this paper we introduce the concept of Photonic-Aware Neural Network (PANN) architectures, i.e., deep neural network models aware of the photonic hardware constraints. Then, we devise PANN training schemes resorting to quantization strategies aimed to obtain the required neural network parameters in the fixed-point domain, compliant with the limited resolution of the underlying hardware. We finally carry out extensive simulations exploiting PANNs in image classification tasks on well-known datasets (MNIST, Fashion-MNIST, and Cifar-10) with varying bitwidths (i.e., 2, 4, and 6 bits). We consider two kernel sizes and two pooling schemes for each PANN model, exploiting 2times 2 and 3times 3 convolutional kernels, and max and average pooling, the latter more amenable to an optical implementation. 3times 3 kernels perform better than 2times 2 counterparts, while max and average pooling provide comparable results, with the latter performing better on MNIST and Cifar-10. The accuracy degradation due to the photonic hardware constraints is quite limited, especially on MNIST and Fashion-MNIST, demonstrating the feasibility of PANN approaches on computer vision tasks.

Optical networks management and control: A review and recent challenges

In the last twenty years, optical networks have witnessed recurrent changes in their management and control architecture. In this paper, we present a historical timeline and a future perspective of the evolution of optical network management and control deployed for Wavelength Switched Optical Networks (WSON), Elastic Optical Networks (EON) and (multilayer) Data Center Networks.Early implementations of WSON envisaged a static and centralized provisioning approach supported by the Management Plane only. Gradually, the requirement of accommodating more network dynamicity in WSON, and later in EON, pushed the adoption of a distributed control, mostly supported by vendor-dependent implementations of the Generalized MultiProtocol Label Switching (GMPLS) protocol suite. The drawbacks of the fully distributed GMPLS-based control, such as resource contention, suboptimal resource usage, and complex computations (e.g., to account for physical layer constraints) showed the necessity to bring back some of the routing/provisioning functions to a centralized Path Computation Element (PCE) capable of accounting for e.g. physical impairments and interworking with GMPLS.The centralized control then gained its momentum and brought a radical change in network control, through the separation of data and control plane introduced by the paradigm of Software Defined Networking (SDN). Such an approach has been gradually extended to optical network control.The paper, eventually, presents the most advanced control techniques, namely the intent-based networking, the observe/decide/act state-based approach providing for autonomic optical network and the (closed-loop) zero-touch service management approach. Advanced traffic conditioning techniques are also detailed, namely the in-band telemetry and the exploitation of Programming Protocol-Independent Packet Processors (P4) language capabilities as well as solutions tailored for data center networks: all of them are still in a research stage and to be integrated within future optical network architectures.

Analysis of Available Components and Performance Estimation of Optical Multi-Band Systems

Optical multi-band (OMB) systems exploit the low-loss spectrum of the single mode fiber (SMF) and are key enablers to increase the transportation capacity and node connectivity of already deployed systems. The realization of OMB systems is mainly based on the technological advances on the component and system level, and for this purpose, a broad gamut of various structural elements, such as transceivers, amplifiers, filters, etc. have been commercialized already or are close to commercialization. This wide range of options, which aid in unlocking the concurrent transmission in all amplification bands, is reviewed here for the first time, whilst their pros and cons as well as their limitations are discussed. Furthermore, the needs for additional components in order to fully exploit the ≈390 nm low-loss wavelength range of SMF, which spans from 1260 to 1650 nm, are highlighted. Finally, based on a physical layer formalism, which incorporates the impact of the most important physical layer constraints for an OMB system, the attainable capacity and transparent reach of each amplification band are quantified.

Impairment-aware fixed-alternate BSR routing heuristics applied to elastic optical networks

The introduction of elastic optical networks (EONs) leads to new challenges, such as the problem of routing, modulation level, and spectrum assignment. In the literature, the best among the shortest routes (BSR) algorithm was proposed as fixed and fixed-alternate routing for EONs, but without considering physical impairments. Therefore, call requests were handled not taking into account the different bandwidth requirements as well as the impact on the optical signal quality of transmission when different modulation levels are considered. This is the first study to undertake heuristics based on BSR algorithm considering a fixed-alternate routing approach jointly with physical layer constraints in EONs. In order to assess the effectiveness of the proposed BSR heuristics in this paper, a genetic and a particle swarm optimization algorithms were also proposed. The results obtained by numerical simulations in different network physical topologies available in the literature show the effectiveness of our proposals in reducing the blocking probability of call requests in comparison with the traditional Yen algorithm.

Cross-Layer Design and Analysis of Wireless Geophone Networks Utilizing TV White Space

Traditional seismic data acquisition systems used for surveying during the exploration of oil and gas rely on cables between geophones and the data collection center. Despite the fact that cable-based systems provide reliable seismic data transfer, their deployment and maintenance costs increase substantially as the survey area increases in scale. Therefore, a three layer wireless network architecture is proposed in this work, which consists of wireless geophones (WG) and a data center with an intermediate wireless gateway node (WGN). This paper investigates the aggregate data throughput, transmission time, and energy consumption from WGs to the WGN in a wireless geophone network architecture based on the IEEE 802.11af standard. This standard is considered in order to have the maximum possible range and low power consumption due to operating in TV bands. Analytical expressions of the aforementioned quantities are derived using Markov chain models. Two Markov models are considered for this purpose: one for modeling the access method that allows multiple WGs to connect to a WGN and the other for representing a buffer in a WG. Since seismic data is recorded at regular intervals, arrivals of data packets in the buffer of the WG is deterministic. On the other hand, departure is random due to the multiple access method. Hence, in this work D/M/1/B queue is used for the first time to model the buffer in a wireless geophone. Furthermore, the physical layer constraints are also taken into account together with proper wireless path-loss channel models. The results obtained are useful for designing such wireless seismic networks without extensive simulations. In particular, the proposed joint medium access control, physical layer, and D/M/1/B model enables us to optimize the required number of WGNs. Finally, sectoring is also introduced in order to minimize the total number of WGNs needed to cover the whole surveying area.

SLA Formulation for Squeezed Protection in Elastic Optical Networks Considering the Modulation Format

In spectrum-sliced elastic optical path networks (SLICE), the lightpath bandwidth is variable, and the virtual topology overlay on a physical topology shall be designed to optimize the spectrum utilization. Under static traffic, SLICE networks are typically designed through a mixed integer linear programming (MILP) with the aim of minimizing the spectrum utilization. In this paper, a new MILP formulation for protection in SLICE networks is proposed, which uses the concept of bandwidth squeezing and grooming to guarantee a minimum agreed bandwidth for each source-destination pair in the surviving bandwidth. The route for each demand on the physical topology is determined by balance equations together with physical layer constraints in the formulation, so that no pre-calculated routes are required and the modulation format of each established lightpath may be chosen with enough quality of transmission and to save network spectrum. Therefore, the proposed formulation jointly solves the virtual topology design and physical topology design problems. The first results evaluate the effectiveness of the MILP formulation for two small networks when connections are under different service-level agreement (SLA) requirements and are provisioned by an appropriate protection scheme and different modulation formats. Due to the NP-hard nature of the proposed MILP formulation, a heuristic algorithm for moderately large networks is also proposed. Case studies are carried out to analyze the basic properties of the formulation and the performance of the proposed heuristic. With the proposed formulation, it is possible to identify the configurations that ensure minimum spectrum occupation with different kinds of protection for each lightpath. Different kinds of modulation formats are considered and contrasted to the benchmark case of a single modulation format and using the same kind of protection for all lightpaths.

On the Optimal Convergence Speed of Wireless Scheduling for Fair Resource Allocation

In this paper, we study the design of joint flow-rate control and scheduling policies in multihop wireless networks for achieving maximum network utility with provably optimal convergence speed. Fast convergence is especially important in wireless networks that are dominated by the dynamics of incoming and outgoing flows as well as the time-sensitive applications. Yet, the design of fast converging policies in wireless networks is complicated by: 1) the interference-constrained communication capabilities, and 2) the finite set of transmission rates to select from due to operational and physical-layer constraints. We tackle these challenges by explicitly incorporating such discrete constraints to understand their impact on the convergence speed at which the running average of the received service rates and the network utility over a finite time horizon T converges to their limits. In particular, we establish a fundamental fact that the convergence speed of any feasible policy cannot be faster than Ω( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> /T) under both the rate and utility metrics. Then, we develop an algorithm that achieves this optimal convergence speed in both metrics. We also show that the well-known dual algorithm can achieve the optimal convergence speed in terms of its utility value. These results reveal the interesting fact that the convergence speed of rates and utilities in wireless networks is dominated by the discrete choices of scheduling and transmission rates, which also implies that the use of higher-order flow-rate controllers with fast convergence guarantees cannot overcome the aforementioned fundamental limitation.

Capacity Analysis and Simulation for Mobile WiMAX System in Distributed Subcarrier Permutation Modes in a Proposed Channel

In this paper, capacity analysis for mobile WiMAX working in distributed subcarrier permutation modes is investigated mathematically depending on the physical layer simulation results. These simulation results of physical layerdispose exact values for the number of best burst profiles that candidate to carry Medium Access Control (MAC) payload in a proposed ITU-R mixed user channel. The complete simulation model for physical and MAC layers has been made using OPNET 14.5 to validate the analytical resultsof the obtained capacity. This approach for capacity analysis based on simulation results is proposed to overcome the lack of interests by other researches in the significant influence of the physical layer constraints on the MAC layer capacity calculations. As well as in this paper,a proposed ITU-R mixed user channelis performed to suit the high mobility users involved under distributed subcarrier permutation modes. Moreover, this paper focused on the exact overhead calculations by taking into account all the significant overhead parameters in the MAC layer frame structure that affects the overall system capacity. Keywords IEEE802.16e-2005, OFDMA, PUSC, FUSC, QoS, proposed ITU-R mixed user channel model, PER.

On the Efficiency of CSMA-CA Scheduling in Wireless Multihop Networks

This paper establishes that random access scheduling schemes, and more specifically CSMA-CA, yield exceptionally good performance in the context of wireless multihop networks. While it is believed that CSMA-CA performs significantly worse than optimal, this belief is usually based on experiments that use rate allocation mechanisms that grossly underutilize the available capacity that random access provides. To establish our thesis, we first compare the achievable rate region of CSMA-CA and optimal in a number of carefully constructed multihop topologies and find that CSMA-CA is always within 48% of the optimal. Motivated by this result, we next characterize the worst-case performance of CSMA-CA in neighborhood topologies representing the congested regions of larger multihop topologies by deriving the neighborhood topology that yields the worst-case throughput ratio for CSMA-CA and find that in neighborhood topologies with less than 20 edges: 1) CSMA-CA is never worse than 16% of the optimal when ignoring physical-layer constraints; and 2) in any realistic topology with geometric constraints due to the physical layer, CSMA-CA is never worse than 30% of the optimal. Considering that maximal scheduling achieves much lower bounds than the above, and greedy maximal scheduling, which is one of the best known distributed approximation of an optimal scheduler, achieves similar worst-case bounds, CSMA-CA is surprisingly efficient.

Performance evaluation of multiuser diversity in multiuser two‐hop cooperative multi‐relay wireless networks using maximal ratio combining over Rayleigh fading channels

SUMMARYMultiuser diversity (MUD) cooperative wireless networks combine the features of the MIMO systems without confronting the physical layer constraints by providing multiple copies of the transmitted signal from the source to the destination with the help of the relay node. Cooperative wireless networks have attracted the full attention in the last few years and are implemented widely in many wireless communication systems to adapt for the fading impairments, provide higher data rates, and improve the performance of the wireless communication systems. In this paper, we present an informative study for the reason of evaluating the performance of the MUD in the multiuser two‐hop cooperative multi‐relay networks using maximal ratio combining. Furthermore, we derive tight closed‐form expressions of outage probability and symbol error probability for the amplify‐and‐forward and fixed decode‐and‐forward protocols with the MUD. Additionally, we conduct a simulation study to show to what extent our analytical and simulation results agree with each other. It is worthy to mention that our analytical and simulation results agree fairly with each other under high average signal‐to‐noise ratio, whereas they show that our proposed system with multiple relays provides significant improvements over those previously proposed systems having only one relay. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Effect of Physical Layer Limitations on the MAC Layer Capacity of Mobile WiMAX System, Analysis and Simulation

this paper, the effect of physical layer limitations on the MAC layer capacity of mobile WiMAX system is analyzed mathematically. The downlink physical layer simulation results show the best burst profiles for a mobile WiMAX system working in a multipath fading channel under the Quality of Service (QoS) requirements for video streaming applications. The capacity mathematical results are obtained based on the physical layer constraints after discarding the MAC layer overheads. Then, a complete simulation model for physical and MAC layers are built to validate the capacity analytical results. The simulation results show a good degree of convergence between the analytical and simulation capacity results. Also, the results show that the maximum number of users that can be served instantaneously by the BS are different depends on the used distributed subcarrier permutation mode. KeywordsWiMAX, physical layer, MAC layer, PUSC, FUSC, ITU-R mixed users channel model, QoS.

The Major Physical Layer Constraints of Fiber Optical fiber Packet Switch Architectures

Many optical packet switch (OPS) architectures are proposed and demonstrated. In the design of these architectures, most of the attention is paid on the network layer parameters like high through put (low packet loss probability) and low latency, etc. and to achieve these goals. The structure of the architectures becomes very complex. In real scenarios, these architectures may not work efficiently because of physical layer constraints. Hence, cross layer optimization needs to be considered. This paper addresses the major physical layer constraints, and it has been found that the optical packet switch architecture can work efficiently within the bounded regimes which can be called as an operation window.

Impairment-Aware Optical Networking Using Cross-Layer Communication

We study strategies for relaxing physical layer constraints in flexible transparent optical transport networks through traffic engineering as well as through impairment-aware traffic management supported by cross-layer communication. We show that impairment-aware traffic management can effectively counteract inter-channel impairments on the physical layer and yield significant throughput gains. Using extensive Monte Carlo simulations, we find these throughput gains to be fairly independent of network topology and size. Based on analytical formulations for the performance gain resulting from impairment-aware networking we evaluate the dependence of the cost per bit on the applied traffic management method as well as on parameters such as the inter-channel impairment probability and the average network utilization.

When Channel Bonding is Beneficial for Opportunistic Spectrum Access Networks

Transmission over multiple frequency bands combined into one logical channel speeds up data transfer for wireless networks. On the other hand, the allocation of multiple channels to a single user decreases the probability of finding a free logical channel for new connections, which may result in a network-wide throughput loss. While this relationship has been studied experimentally, especially in the WLAN configuration, little is known on how to analytically model such phenomena. With the advent of Opportunistic Spectrum Access (OSA) networks, it is even more important to understand the circumstances in which it is beneficial to bond channels occupied by primary users with dynamic duty cycle patterns. In this paper we propose an analytical framework which allows the investigation of the average channel throughput at the medium access control layer for OSA networks with channel bonding enabled. We show that channel bonding is generally beneficial, though the extent of the benefits depend on the features of the OSA network, including OSA network size and the total number of channels available for bonding. In addition, we show that performance benefits can be realized by adaptively changing the number of bonded channels depending on network conditions. Finally, we evaluate channel bonding considering physical layer constraints, i.e. throughput reduction compared to the theoretical throughput of a single virtual channel due to a transmission power limit for any bonding size.

Global Optimization of Multi Radio Mesh Access Point Location in Underground Area

This paper shows that with a proper placement of Mesh multi radio devices in a confined area, it is possible to reduce the congestion in the WMN (Wireless Mesh Network) by 50 to 60 percent according to the loaded traffic. The best portal deployment is obtained by minimizing the maximum traffic aggregation based on several constraints such as collision domain; routing and physical layer constraints. The different steps that must be fulfilled to reach this adequate deployment are clearly detailed. The effect of uneven resources utilization due to a non uniform spatial density of the workers in the confined area is also addressed. A well designed traffic distribution and a deployment algorithm for underground confined area are proposed and its effects on the WMN’s performance are evaluated by a Monte Carlo simulation.

Multicast Routing Algorithms Based on $Q$-Factor Physical-Layer Constraints in Metro Networks

We use novel ldquolight-tree balancing techniquesrdquo to investigate the problem of provisioning multicast sessions in metropolitan all-optical networks. The Q-factor for every path of a derived light-tree is calculated taking into account several physical layer constraints in the network and using a Q-budgeting approach. Based on the above performance, tree balancing techniques are applied to maximize the number of multicast connections that can be admitted to the network.

A Linear Programming Approach to Cross-Layer Optimization in Multiclass VSG-CDMA Over Rayleigh Fading

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents a new linear programming approach for throughput maximization on the uplink of a multiclass variable spreading gain code-division multiple-access (CDMA) multicellular system in Rayleigh fading for both binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) modulations. Based on the improved Gaussian approximation technique, we derive exact closed-form expressions of the outage probability, which are used as a physical (PHY) layer constraint of the maximization problem. We show that it is possible to transform the nonlinear constraint into a set of equivalent linear expressions. This facilitates the formulation of a new linear throughput maximization that noticeably requires less computational complexity than the known nonlinear approaches. Furthermore, due to the simplicity of the linear approach, we can include the constraints of such higher layers, i.e., media access control (MAC) and call admission control (CAC) layers, into the proposed formula. Accordingly, we introduce two linear programming optimization formulas. One is for MAC-PHY optimization, and another is for cross-layer optimal CAC policy. In the case of MAC-PHY optimization, the throughput is maximized, at low background noise level, when every user retains the same bit energy, particularly in the case where the difference in data rates is high. Nonetheless, at high levels of background noise, the throughput would be maximized when a larger amount of bit energy is allocated to the high-rate users. For the joint optimal policy, the throughput and the blocking probability are optimized and improved up to 50% in comparison to those of the conventional CAC policy, which is known as the <emphasis emphasistype="boldital">complete sharing</emphasis> policy. </para>

Adaptive opportunistic fair scheduling in power-controlled code division multiple access systems

The authors treat the multiuser scheduling problem for practical power-controlled code division multiple access (CDMA) systems under the opportunistic fair scheduling (OFS) framework. OFS is an important technique in wireless networks to achieve fair and efficient resource allocation. Power control is an effective resource management technique in CDMA systems. Given a certain user subset, the optimal power control scheme can be derived. Then the multiuser scheduling problem refers to the optimal user subset selection at each scheduling interval to maximise certain metric subject to some specific physical-layer constraints. The authors propose discrete stochastic approximation algorithms to adaptively select the user subset to maximise the instantaneous total throughput or a general utility. Both uplink and downlink scenarios are considered. They also consider the time-varying channels where the algorithm can track the time-varying optimal user subset. Simulation results to show the performance of the proposed algorithms in terms of the throughput/ utility maximisation, the fairness, the fast convergence and the tracking capability in time-varying environments are presented.

Adaptive subchannel allocation in multiuser multicarrier systems

The problem of multiuser scheduling in multicarrier (MC) systems under practical physical-layer constraints and implementations is considered. Subchannel allocation is an important resource assignment issue in multiuser MC systems. The multiuser scheduler is decoupled into a multiuser selector and a subchannel allocator, which result in a sub-optimum multiuser scheduler with significantly reduced computational complexity. Given an active user subset and a channel set, the multiuser scheduling problem then refers to the optimal subchannel allocation to maximise the instantaneous system throughput subject to certain fairness constraints. Efficient adaptive algorithms are developed for optimal subchannel allocation. The extension of the algorithms for tracking the time-varying optimum, which occurs in non-stationary environments, is also addressed. Simulation results are presented to demonstrate the performance of the proposed algorithms in terms of the throughput maximisation, the fast convergence, the excellent tracking capability in time-varying environments, the achievable throughput of the proposed multiuser scheduler as well as the long-term fairness.

Capacity analysis and system optimization for the forward link of multi‐beam satellite broadband systems exploiting adaptive coding and modulation

AbstractThis paper proposes a methodology for designing and optimizing the performance of adaptive coding and modulation (ACM) schemes for the forward link of unicast satellite broadband networks. The proposed technique of general applicability is based on the overall system capacity maximization conditioned on meeting a certain service area coverage for a given outage probability. The paper first derives in detail the method for computing the Shannon‐based capacity of a multi‐beam satellite system adopting ACM. Then the link efficiency is optimized under different physical layer constraints allowing a more realistic system performance assessment. Finally, to exemplify the methodology application and to quantify the ACM system performance improvement capabilities a study case corresponding to a Ka‐band state‐of‐the‐art bent pipe satellite network is provided. The example illustrates the very substantial capacity gain offered by state‐of‐the‐art adaptive modulation and coding schemes utilization compared to current broadband satellite system design approach. Copyright © 2004 John Wiley &amp; Sons, Ltd.