We propose and analyze a unipolar generalized frequency division multiplexing (GFDM) modulation scheme with dimming support for visible light communications (VLC). In the proposed method, the dimming level can be controlled at a transmitter using a single parameter that a receiver need not know. In addition, the dimming parameter can be adjusted by the transmitter at any time without affecting the quality of data transmissions. Expressions of the illumination percentage and the bit error rate (BER) are derived taking into account the dimming parameter, and validated through comparisons with simulation results. Using these expressions, it is possible to specify the dimming parameter at the transmitter based on the desired illumination percentage and estimate the corresponding BER for data transmissions. Numerical results indicate that, compared to adjusting the direct current (DC) bias for dimming control, the proposed method provides a wider dimming range subject to a constraint on the BER.

We analyze the performance of a dual-hop relay system with correlated antennas at the source and the destination. Orthogonal space time block coding (OSTBC) is employed at the source. Single-antenna relays with and without direct transmission between the source and the destination are stud- ied. Assuming non-identically distributed Nakagami-m fading channels, closed- form expressions of outage probability, average symbol error rate (SER), and moment generating function (MGF) of the instantaneous end-to-end signal-to-noise ratio (SNR) are derived for the single- relay case. Asymptotic analysis is also performed to obtain the diversity order for the single-relay system. Furthermore, the analysis is extended to obtain expressions for the outage probability and SER for multiple-relay systems. Analytical results are validated with Monte-Carlo simulations using Mathematica and MATLAB programs.

The cooperation among cognitive users (CUs) is considered as an efficient means to improve the primary user (PU) sensing in cognitive radio environment. Almost all previous related works employ the cooperation among CUs for PU activity detection whereas in this paper, the notions of cooperation among PUs and CUs are applied. This is achieved by allocating the available PU power among OFDMA subcarriers with two objectives 1. Guaranteed spectral efficiency to PU 2. Maximize detection of PU presence to a CU. The dynamic nature of channel parameters significantly improves the detection probability of CU with a minor reduction in PU's spectral efficiency. Analysis shows that a reduction of 5% spectral efficiency could be utilized to gain more than 15% in terms of the correct detection probability. The convexity of the formulated problem is analyzed. Finally, we propose deploying our scheme with CU collaborative sensing schemes to improve the detection probability in the low signal-to-noise ratio regions.

In wireless multi-hop networks, the allocation of resources is influenced by mechanisms for medium access control (MAC), routing, congestion control, and flow control. Designing these mechanisms jointly can increase the capacity of wireless networks. We attempt to introduce routing into an existing framework for the joint design of flow control and MAC on random access multi-hop wireless networks. The problem of joint flow control, routing and MAC in random access multi-hop wireless network is formulated as an optimization problem. However, a direct formulation yields a non-convex optimization problem. To overcome the difficulty in solving a non-convex problem, we introduce a harmonic rate function to convexify the formulation. The joint optimization mechanism is presented as an iterative process to compute a solution. The resultant distributed algorithm is proved to yield a global optimal solution when it converges. Numerical results are provided to show the convergence of the proposed algorithm and properties of the harmonic rate function.

We propose an optical line terminal (OLT) centric dynamic bandwidth allocation (DBA) scheme based on individual requests from service queues in optical network units (ONUs) for a quality-of-service (QoS) aware Ethernet passive optical network (EPON). The goal is to provide fairness for allocating bandwidth among different service classes based on their service level agreements (SLAs). The proposed DBA scheme makes use of the excess bandwidth of lightly loaded queues to meet the bandwidth demand of heavily loaded queues. To implement this scheme, an effective polling mechanism is incorporated to solve the idle period problem. In addition, to reduce the scheduling overhead, the DBA scheme utilizes a novel different cycle policy which involves selective bandwidth allocations to different service classes based on their delay bounds. We conduct detailed simulation experiments and compare the results with the well-known interleaved polling with adaptive cycle time (IPACT) algorithm that utilizes strict priority policy.

We compare the effectiveness of uniform versus non-uniform waveband switching under the dual cost metrics of switching requirements and fiber capacity. We consider a star topology and begin by characterizing the optimal performance frontier achievable under no restrictions on waveband sizing, and provide algorithms employing non-uniform waveband sizing that approach or achieve this optimum. We then consider the special case of uniform waveband sizing, and show that the performance compares very favorably. We also extend our results to general topologies.

We extend two existing narrowband beamforming techniques for ultra-wideband applications: minimum mean square error (MMSE) beamforming and MMSE beamforming subject to the constraint on complete nulling of interfering signals. Assuming that the signal at each antenna element is processed by a linear time invariant (LTI) filter, we derive the optimal filters for each beamformer. We then focus on the scenarios in which the number of antenna elements L is much larger than the number of users K in the system, e.g. multi-element antenna arrays on satellites. To improve on the expensive baseline implementation in which one electronic LTI filter is used for each antenna element, for a total of L filters, we propose the use of simple optical signal processing to reduce the cost of electronic hardware. We show that, by performing conventional beamforming in optics using fiber delay lines as a preprocessing step, the number of electronic filters required for both MMSE and constrained MMSE beamforming is reduced to K. The key observation is that, for L>K, the number of filters depends on the number of users K and not on the number of antenna elements L.

We develop an on-line wavelength assignment (WA) algorithm for a wavelength-routed WDM tree network. The algorithm dynamically supports all k-port traffic matrices among N end nodes, where k denotes an integer vector [k/sub 1/...,k/sub N/] and end node i,1/spl les/i/spl les/N, can transmit at most k/sub i/ wavelengths and receive at most k/sub i/ wavelengths. Our algorithm is rearrangeably nonblocking, uses the minimum number of wavelengths, and requires at most d/sup */-1 lightpath rearrangements per new session request, where d/sup */ is the degree of the most heavily used node. We observe that the number of lightpath rearrangements per new session request does not increase as the amount of traffic k scales up by an integer factor. In addition, wavelength converters cannot reduce the number of wavelengths required to support k-port traffic in a tree network. We show how to implement our WA algorithm using a hybrid wavelength-routed/broadcast tree with only one switching node connecting several passive broadcast subtrees. Finally, using roughly twice the minimum number of wavelengths for a rearrangeably nonblocking WA algorithm, we can modify the WA algorithm to be strict-sense nonblocking.

Switching traffic together in bundles of wavelengths called wavebands can greatly reduce the switching costs of the network. We consider the problem of partitioning wavelengths into wavebands for star networks using the minimum number of total wavebands. We provide a greedy algorithm for waveband partitioning and show that it is optimal in that it requires the minimum number of wavebands subject to using the minimum possible number of wavelengths. We also give an algorithm for allocating calls from any admissible traffic set to the wavebands in a non-blocking manner. Finally, we show that the increase in the number of wavebands required is logarithmic in the number of calls and polynomial in the size of the network.

We develop on-line routing and wavelength assignment (RWA) algorithms for WDM bidirectional ring and torus networks with N nodes. The algorithms dynamically support all k-allowable traffic matrices, where k denotes an arbitrary integer vector [k/sub 1/, k /sub 2/, ..., k/sub N/], and node i, 1/spl les/i/spl les/N, can transmit at most k/sub i/ wavelengths and receive at most k/sub i/ wavelengths. Both algorithms support the changing traffic in a rearrangeably nonblocking fashion. Our first algorithm, for a bidirectional ring, uses /spl lceil/(/spl Sigma//sub i=1//sup N/k/sub i/)/3/spl rceil/ wavelengths in each ring direction and requires at most three lightpath rearrangements per new session request regardless of the number of nodes N and the amount of traffic k. When all the k/sub i/s are equal to k, the algorithm uses /spl lceil/kN/3/spl rceil/ wavelengths, which is known to be the minimum for any off-line rearrangeably nonblocking algorithm. Our second algorithm, for a torus topology, is designed for the special case with all the k/sub i/s equal to k. For a square torus network with N nodes, the algorithm uses /spl lceil/k/spl radic/N/2/spl rceil/ wavelengths in each fiber, which is shown to be at most two times a lower bound obtained by assuming full wavelength conversion at all nodes. In addition, the algorithm requires at most /spl radic/N-1 lightpath rearrangements per new session request regardless of the amount of traffic k.

A network consisting of transparent optical nodes (TONs) can provide high speed end-to-end communication paths with very low bit-error rates (BERs). However, owing to component crosstalk and other degradations at TONs, the BER of a particular communication path traversing several TONs can be degraded by a few orders of magnitude even in the absence of component failure. Monitoring the quality-of-service (QoS) of a communication path has typically relied on sporadic BER testing and operation monitoring by the nodes using probe signals. Intermittent BER testing cannot provide continuous monitoring of the network QoS. On the other hand, the use of probe signals is not sensitive enough to detect the BER degradation. This work investigates a novel approach of monitoring service degradation at individual nodes using a wrap-around device which taps and compares signals from the input and the output at each TON along the lightpath. We propose a modification using hard limiters at TON inputs and derive the BER value that this modified method can guarantee in the presence of signal degradation due to coherent crosstalk at TONs.