Wai Pang Ng

Performance analysis of free space optical links over turbulence and misalignment induced fading channels

The performance of free-space optical (FSO) communication system is investigated from the information theory perspective, under the influence of atmospheric loss and channel fading due to the turbulence and pointing error. The outage probability and average channel capacity of the FSO link are evaluated using a combined slow-fading channel model and a Gaussian beam wave model, unlike the limiting Rytov-based scintillation model considered in earlier work. Taking into account various link design criteria, our study reveals that lower outage probability can be achieved with larger receiver aperture due to the aperture averaging effect. Optimization of the capacity metrics is best performed through proper selection of the aperture size and beam width for known laser wavelength, particularly for long-distance links and strong turbulence scenarios. Hence, findings and results from our analysis can be treated as the design benchmark for optimal planning and deployment of optical links.

Experimental full duplex simultaneous transmission of LTE over a DWDM directly modulated RoF system

In this paper, we experimentally demonstrate the seamless integration of full duplex system frequency division duplex (FDD) long-term evolution (LTE) technology with radio over fiber (RoF) for eNodeB (eNB) coverage extension. LTE is composed of quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM) and 64-QAM, modulated onto orthogonal frequency division multiplexing (OFDM) and single-carrier-frequency division multiplexing for downlink (DL) and uplink (UL) transmissions, respectively. The RoF system is composed of dedicated directly modulated lasers for DL and UL with dense wavelength division multiplexing (DWDM) for instantaneous connections and for Rayleigh backscattering and nonlinear interference mitigation. DL and UL signals have varying carrier frequencies and are categorized as broad frequency spacing (BFS), intermediate frequency spacing (IFS), and narrow frequency spacing (NFS). The adjacent channel leakage ratio (ACLR) for DL and UL with 64-QAM are similar for all frequency spacings while cross talk is observed for NFS. For the best case scenario for DL and UL transmissions we achieve error vector magnitude (EVM) values of ~2.30%, ~2.33%, and ~2.39% for QPSK, 16-QAM, and 64-QAM, respectively, while for the worst case scenario with a NFS EVM is increased by 0.40% for all schemes.

Investigation of Optical Modulators in Optimized Nonlinear Compensated LTE RoF System

In this paper, we investigate a nonlinear compensation technique with two different architectures using direct modulation (DM) and external modulation (EM) techniques, termed as DM based frequency dithering (DMFD) and EM based frequency dithering (EMFD). We show that DMFD and EMFD methods operate substantially different in radio-over-fiber (RoF) system by optimizing the dithering technique relative to the LTE technology. The proposed techniques is only applicable if the condition of {fL <; fd <; fRF} is met, where fL represents the dithering boundary limit of 14 MHz, fd is DMFD signal frequency and fRF is the RoF carrier frequency. Analysis of the optical launch power for DMFD and EMFD methods reveal that the stimulated Brillouin scattering threshold is above ~6 dBm for the LTE-RoF system. In addition, we also unveil that DMFD and EMFD methods do not introduce additional distortion for the linear and optimum optical launch power regions, which are frequency chirp driven regions. If the given condition is met, the proposed method improves the LTE-RoF system without any shortcoming. Finally, at 10 dBm launch power, DMFD and EMFD methods exhibits an average signal-to-noise ratio gain of ~5.95 and ~7.71 dB, respectively.

Optimization of Optical Modulator for LTE RoF in Nonlinear Fiber Propagation

This letter proposes an optimized launch power for the direct detection of optical orthogonal frequency-division multiplexing (DD-OOFDM) for radio-over-flber (RoF). The aim is to optimize the physical layer connectivity for the third generation partnership program-long-term evolution employing RoF technologies. We also analytically derive an expression for the distributed-feedback laser, laser-induced positive frequency chirping, incorporating the phenomena that induce phase distortion at the receiver, and explicitly explain the transient chirp of DD-OOFDM. Results show that transmission at the optimized launch power of -4 dBm improves the system power efficiency of 16-quadrature amplitude modulation (QAM) DD-OOFDM by ~20% and ~37%, 64-QAM DD-OOFDM by ~21% and ~35% compared to launch powers in the linear and nonlinear regions, respectively.

Effects of aperture averaging and beam width on a partially coherent Gaussian beam over free-space optical links with turbulence and pointing errors

Joint effects of aperture averaging and beam width on the performance of free-space optical communication links, under the impairments of atmospheric loss, turbulence, and pointing errors (PEs), are investigated from an information theory perspective. The propagation of a spatially partially coherent Gaussian-beam wave through a random turbulent medium is characterized, taking into account the diverging and focusing properties of the optical beam as well as the scintillation and beam wander effects. Results show that a noticeable improvement in the average channel capacity can be achieved with an enlarged receiver aperture in the moderate-to-strong turbulence regime, even without knowledge of the channel state information. In particular, it is observed that the optimum beam width can be reduced to improve the channel capacity, albeit the presence of scintillation and PEs, given that either one or both of these adverse effects are least dominant. We show that, under strong turbulence conditions, the beam width increases linearly with the Rytov variance for a relatively smaller PE loss but changes exponentially with steeper increments for higher PE losses. Our findings conclude that the optimal beam width is dependent on the combined effects of turbulence and PEs, and this parameter should be adjusted according to the varying atmospheric channel conditions. Therefore, we demonstrate that the maximum channel capacity is best achieved through the introduction of a larger receiver aperture and a beam-width optimization technique.

Characterization and performance analysis of a TOAD switch employing a dual control pulse scheme in high-speed OTDM demultiplexer

This letter presents the characterization, analysis and simulation of a novel dual control pulse (CP) scheme for the Terahertz Optical Asymmetric Demultiplexer (TOAD) switch used as a demultiplexer in optical time division multiplexing (OTDM) system. The dual-CP TOAD offers symmetrical switching window (SW) which reduces inter-channel crosstalk (CXT) in switched/demultiplexed channels, thus significantly improving the bit-error-rate (BER) and received power penalty. The improvements in CXT and received power penalty are 5 and 1.5 dB (at BER = 10-9), respectively, for demultiplexing high bit rates of 200 and 300-to-10 Gb/s.

OPN06-3: Multiple-Hop Routing Based on the Pulse-Position Modulation Header Processing Scheme in All-Optical Ultrafast Packet Switching Network

In this paper we present modeling and simulation of multiple-hop all-optical packet routing based on the pulse- position modulation header processing (PPM-HP) scheme in ultrafast optical packet switching network. Optical core node employing PPM-HP requires a dedicated routing table based on PPM scheme that will result in significantly reduced number of routing table entries. In this scheme, the header address correlation, using optical AND gates, is carried out by employing only a single bit-wise for each entry rather than a multiple-bit stream as in the existing systems thus resulting in reduced header processing time and avoiding the slow gain-recovery of AND gates. The reduced routing table also offers embedding multiple transmitting modes (unicast, multicast, broadcast) in optical layer and improves core network scalability where the number of core/edge nodes could be altered without the need for changing the number of routing table entries. The simulation results for the optical signal-to-noise ratio (OSNR) of packets at each hop are presented and compared with the theoretical calculations.

Impact of optical modulators in LTE RoF system with nonlinear compensator for enhanced power budget

Nonlinear propagation compensation for LTE-RoF system is experimentally demonstrated with proposed DMFD and EMFD methods. We discovered that the RoF dithering frequency requirement have to be much smaller with an average improvement of ~5.35 dB.

Joint optimization of a partially coherent Gaussian beam for free-space optical communication over turbulent channels with pointing errors

Joint beam width and spatial coherence length optimization is proposed to maximize the average capacity in partially coherent free-space optical links, under the combined effects of atmospheric turbulence and pointing errors. An optimization metric is introduced to enable feasible translation of the joint optimal transmitter beam parameters into an analogous level of divergence of the received optical beam. Results show that near-ideal average capacity is best achieved through the introduction of a larger receiver aperture and the joint optimization technique.

Ultrafast All-optical Self Clock Extraction Based on Two Inline Symmetric Mach-Zehnder Switches

In this paper, we report a novel ultrafast all-optical self clock extraction module (CEM) based on two inline symmetric Mach-Zehnder switches. In the proposed CEM, incoming data packet is used as the control signal to set/reset the imbalance/balance states of the SMZs for extracting the clock pulse, CEM offers fast clock extraction response and high on/off contrast ratio between extracted clock pulse and residual signals. Simulation results show that contrast ratio of more than 20 dB at low input energy (1 fJ) and small required processing time in 100 Gb/s transmission are achievable