Prevalence, incidence of and risk factors for vertebral fracture in the community: the Vietnam Osteoporosis Study.

The epidemiology of vertebral fractures (VF) in underrepresented populations is not well-documented. This cohort study was part of a longitudinal osteoporosis research project with the aim of determining the prevalence, incidence, and risk factors for VF. 401 individuals (155 men) aged 50 years and older without a clinical diagnosis of VF were took radiographs at baseline and 2 years later. VF were ascertained using the Genant's semi-quantitative method. Bone mineral density (BMD) of femoral neck and lumbar spine were measured by dual-energy X-ray absorptiometry (Hologic Inc). The association between VF and risk factors was analyzed by the multiple logistic regression. The 95% confidence interval for prevalence and incidence was estimated by exact Poisson test. At baseline, the prevalence of VF was 12.2% (n = 49, 95% CI 9.0-16.2%) and increased with advancing age with one-fifth of those aged 70 and older having a VF. During the follow-up period, we observed 6 new VF, making the incidence of 6.6/1000 person-years (n = 6, 95% CI 2.4-14.3). The risk of prevalent VF was associated with male gender (OR: 2.67; 95% CI 1.28-5.87) and T-score at the femoral neck (OR per one SD decrease: 1.1; 1.03-1.17). These data indicate that VF is common among adults, and that lower femoral neck BMD was a risk factor for VF.

Prevalence and Clinical Profile of Undiagnosed Diabetes Mellitus: Data from a Tertiary Hospital.

BACKGROUND: The prevalence of diabetes mellitus in Vietnam is relatively low compared to other Asian countries, but it is accelerating with the economic and cultural transition. This study aimed to estimate the current prevalence and clinical profile of undiagnosed diabetes mellitus in a tertiary hospital in the south of Vietnam. METHODS: A cross-sectional investigation was conducted to recruit 1, 250 participants, who were at least 18 year-old and randomly sampled from Cho Ray Hospital, Ho Chi Minh City, Vietnam. Fasting plasma glucose concentration and HbA1c were measured for each individual. The American Diabetes Association criteria were used to diagnose diabetes. Demographic data and other clinical characteristics of diabetes were also documented, including age, sex, residence, educational status, weight, height, waist and hip circumferences, blood pressure, familial history of diabetes, and lipid profile. RESULTS: The prevalence of undiagnosed diabetes mellitus was 7.5% in the population studied. Age, waist circumference, waist-hip ratio, body mass index, and hypertension, as well as dyslipidaemia were well-correlated with the diabetes rate. CONCLUSION: The prevalence of undiagnosed diabetes mellitus is increasing far more than expected. Newly diagnosed diabetic patients usually presented with multiple comorbidities, including overweight/ obesity, hypertension, and dyslipidaemia.

Flexible bandwidth DGD estimation for coherent optical OFDM system.

A Stokes vector based non-data-aided (NDA) differential group delay (DGD) estimation scheme with flexible bandwidth requirements is investigated. The scheme is demonstrated in both experiments and simulations for 40-Gb/s polarization-multiplexed coherent optical OFDM systems with up to 1000-km long haul transmission with an estimation error of less than 2.5 ps after 1000-km standard single-mode fiber transmissions.

Applying implicit training to polarization-division-multiplexed coherent optical systems.

We report the use of implicit training-aided channel estimation (ITA-CE) for polarization-division-multiplexed (PDM) single-carrier coherent optical systems. With the concept of ITA-CE, where the training sequence (TS) is arithmetically added onto the information symbols rather than appearing in separate time slots, overhead-free transmission can be achieved with minor optical signal to noise ratio (OSNR) penalty. ITA-CE system enables simple digital pilot-tone based frequency-offset compensation, and inherits the benefits from typical training-aided channel acquisition and data recovery. Furthermore, we address the issue of self-interference to channel estimation (CE) of ITA-CE system in this paper. The data-induced interference can be eliminated by removing the cyclic mean of the transmitted signal, which can accelerate the convergence of the CE and allows smaller power to be allocated to the TS, leading to enhanced system performance. Numerical simulations and experiments for both PDM-QPSK and PDM-16-QAM systems have been conducted to verify the proposed scheme. Comparable performance to the conventional blind equalization system can be achieved when employing the transmitter side pre-distortion approach to the ITA-CE system.

Optical performance monitoring for OFDM using low bandwidth coherent receivers.

We propose using low bandwidth coherent receivers for distributed optical performance monitoring. We demonstrate optical signal-to-noise ratio (OSNR) monitoring of both 20-Gb/s single-polarization and 40-Gb/s polarization-multiplexed coherent optical orthogonal frequency-division multiplexing (CO-OFDM) signals with a 0.8-GHz receiver using both data-aided (DA) and non-data-aided (NDA) approaches. The sampling rate of the performance monitor is much lower than the signal baud rate, so provides a cost-effective solution for distributed optical performance monitoring. The proposed method is demonstrated experimentally and through simulation. The results show that after calibration the OSNR monitoring error is less than 1 dB and the two approaches are not affected by fiber dispersion after 800-km transmission and 30-ps differential group delay (DGD).

Improved two-stage equalization for coherent Pol-Mux QPSK and 16-QAM systems.

We report a two-stage blind frequency domain equalization method for long-haul coherent polarization-multiplexed (pol-mux) systems using quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16-QAM). In the first stage, blind CD parameter prediction is conducted prior to a CD equalizer. This supports flexible path switching in optical networks. In the second stage, a frequency-domain multi-modulus algorithm (MMA) equalizer is used to cope with the residual fiber impairments and perform polarization de-multiplexing. Compared with the conventional constant modulus algorithm (CMA), MMA shows advantages including better steady state performance and a faster convergence rate. Furthermore, all the estimation and equalization algorithms are implemented in the frequency domain which potentially provides the least complexity for the pol-mux optical coherent systems. The proposed algorithm is experimentally demonstrated with an 800-km 10 Gbaud coherent optical pol-mux system. For QPSK signal, the proposed method achieves error-free transmission and shows superior convergence speed against CMA, and for 16-QAM signals, the proposed MMA outperforms CMA with more than 1-dB improvement in Q-value.

Mitigation of Rayleigh noise and dispersion in REAM-based WDM-PON using spectrum-shaping codes.

This paper presents a novel technique capable of Rayleigh backscattering (RB) mitigation and chromatic dispersion (CD) compensation for wavelength-division-multiplexed passive optical network (WDM-PON). The reduction of the interference caused by RB and CD in the uplink based on reflective electro-absorption modulator (REAM) is realized by the proposed correlative level (CL) coding. We investigate the RB-induced interferometric crosstalk for different fiber lengths. 10 Gb/s and 20 Gb/s transmissions over 70 km and 35 km fiber are demonstrated using the CL codes of dicode and modified duobinary (MD), respectively. Significant improvement in system resilience to backscattered seed light is verified for both dicode and MD coding. MD-coded signal also exhibits considerable robustness against the effects of CD.

Combined utilization of partial-response coding and equalization for high-speed WDM-PON with centralized lightwaves.

In this paper, we investigate the transmission impairments in a high-speed single-feeder wavelength-division-multiplexed passive optical network (WDM-PON) employing low-bandwidth upstream transmitter. A 1-GHz reflective semiconductor optical amplifier (RSOA) is operated at the rates of 10 Gb/s and 20 Gb/s in the proposed WDM-PON. Since the system performance is seriously limited by its uplink in both capacity and reach owing to inter-symbol interference and reflection noise, we present a novel technique with simultaneous capability of spectral efficiency enhancement and transmission distance extension in the uplink via coding and equalization that exploit the principles of partial-response (PR) signal. It is experimentally demonstrated that the proposed system supports the delivery of 10 Gb/s and 20 Gb/s upstream signals over 75-km and 25-km bidirectional fiber, respectively. The configuration of PR equalizer is optimized for its best performance-complexity trade-off. The reflection tolerance of 10 Gb/s and 20 Gb/s channels is improved by 8 dB and 6 dB, respectively, with PR coding. The proposed cost-effective signal processing scheme has great potential for the next-generation access networks.

Statistical moments-based OSNR monitoring for coherent optical systems.

We present a novel statistical moments-based method for optical signal-to-noise ratio (OSNR) monitoring in polarization-multiplexed (pol-mux) coherent optical systems. This technique only requires the knowledge of the envelope of the equalized signal before phase correction, which can be achieved by using any two arbitrary statistical moments, and it is suitable for both constant and non-constant modulus modulation formats. The proposed estimation method is experimentally demonstrated for 10-Gbaud pol-mux coherent systems using QPSK and 16-QAM. Additionally, numerical simulations are carried out to demonstrate 20-Gbaud systems using 16-QAM and 64-QAM. The results show that the OSNR can be estimated accurately over a wide range of values for QPSK, 16-QAM or 64-QAM systems up to 1920-km long and with up to 50-ps all-order polarization mode dispersion. By setting a proper reference value for calibration, the proposed algorithm also shows good tolerance when the received signal is not well compensated.

Level coding technique for a wavelength-division-multiplexed optical access system using a remodulation scheme.

In the wavelength-division-multiplexed optical access network, the system architecture based on remodulating the downstream (DS) signal for upstream transmission resolves the issues of wavelength management and hardware cost . However, this amplitude remodulation scheme introduces intensity noise in the uplink and leads to a system performance penalty. Therefore, the DS extinction ratio (ER(d)) has to be optimized in order to balance the performance of up- and down- links. In this Letter, a novel technique is presented to overcome the restrictions on ER(d) by introducing correlations into the levels of the DS signal to alter its spectrum. 4 dB improved tolerance to ER(d) is achieved in the proposed system. The level-correlation coding technique is also able to mitigate the in-band crosstalk induced by Rayleigh backscattering. It is demonstrated that the system reach is extended by 40 km with the proposed approach.

Cost optimal allocation of amplifiers and DCMs in WDM ring networks.

Designing metropolitan wavelength division multiplexing (WDM) ring networks with minimum deployment cost is a demanding issue in Telecommunication Network planning . We have already presented amplifier placement methods to minimize the number of amplifiers in WDM rings for the case all amplifiers follow a unique gain model. In this paper, we take into account different types of amplifiers with predefined fixed characteristics and costs. We also formulate fiber dispersion limitations on the ring design, and present two efficient methods for placing amplifiers and Dispersion Compensation Modules (DCMs) in WDM rings to minimize the total deployment cost of the system. The first method deals with both linear and nonlinear equations and uses a mixed integer nonlinear programming (MINLP) solver where the second method applies the linear approximation of nonlinear constraints, and uses a mixed integer linear programming (MILP) solver to minimize the total cost of the system. We carry out Simulation experiments to confirm the applicability of the methods and compare the results for various network configurations.