PAM-N—Fundamental Limits in Chromatic Dispersive-Uncompensated Channels

The fundamental chromatic dispersion limit for an optical communication N-level pulse amplitude modulation (PAM-N) format without any dispersion compensating module is calculated. The main result of this analysis shows that in a non-dispersion-compensated channel, the product β2B2L (where β2, L, and B are the dispersion coefficient, fiber length, and the Baud rate, respectively) is bounded by a number, which depends only on the number of levels N. In particular, β2B2L < 0.318, β2B2L < 0.212, and β2B2L < 0.14 for N values of 2, 4, and 8, respectively. Moreover, an analytical expression for a noisy channel’s power penalty was formulated. This analytic expression shows high agreement with numerical simulations. To the best of our knowledge, this is the first time that such a fundamental limit has been formulated for PAM-N systems.

Performance evaluation of a multihop WDM network with share-per-node L-WIXC architecture

This paper investigates the impact of coherent and incoherent crosstalks on bit error rate of a multihop wavelength division multiplexing (WDM) transmission system with limited-wavelength-interchanging cross connects (L-WIXCs). The relation between the crosstalk and signal to crosstalk plus noise ratio at the receiver output is analyzed and redefined with an improved mathematical modeling. Quantitative analysis of the bit error rate (BER) in terms of system parameters like number of hops, number of fiber and number of wavelength channels per fiber are evaluated. The power penalty due to accumulated crosstalk at a given BER of 10−9 and the optimum system design parameters are also determined numerically. Results show that there is a significant power penalty of the order of 10–12 dB due to accumulated crosstalk depending on the number of hops. Maximum allowable number of hops at a given BER of 10−9 and number of input fibers are also evaluated.

Transmission challenges in metropolitan area optical networks

For implementing wide/metropolitan area network in optical fiber communication system the key technology that can be utilized is wavelength division multiplexing (WDM). We discuss the use of WDM in metropolitan networks along with its special feature of using optical add drop multiplexers and also design issues in implementing these systems. The issue of nonlinear crosstalk induced in metropolitan networks is considered in terms of power penalty. Also, the effect of dispersion induced in optical fibers is considered by using two types of transmission systems. One is considering the non-zero dispersion shifted fiber (NZDSF) in dispersion effected environment and second is, implementing dispersion compensation after normal single mode fiber (SMF + DCF). The effect of nonlinear cross talk which can limit transmission performance is discussed in both cases in context with different parameters like power penalty, frequency difference between signal & crosstalk and transmission distance etc.

Performance Analysis of a Multiple Subcarrier Modulated FSO Communication System using Direct Detection Optical Receiver under the Effect of Weak Atmospheric Turbulence

In this paper, an analytical approach is presented to evaluate the bit error rate (BER) of a free space optical (FSO) link with radio frequency (RF) multiple subcarrier modulation, taking into account the effect of weak atmospheric turbulence considering a direct detection optical receiver followed by RF synchronous demodulator for each sub-channel. Analysis is carried out to find a closed-form expression for conditional BER at the output of the RF demodulators conditioned on a given value of atmospheric turbulence-induced fading and intermodulation distortion (IMD). The average BER for each sub-channel is then found by averaging the conditional BER over the probability density function of the atmospheric turbulence modeled as log-normal distribution. Degradations of BER due to atmospheric turbulence are evaluated for several values of system parameters like number of RF subcarrier, turbulence variance, link distance, data rate and power penalty suffered by the system due to atmospheric turbulence and IMD. For a given system bandwidth, it is found that the maximum power penalty occurs when the subcarrier number is around four and the performance gradually improves with increase in the number of subcarrier. For example, at a system bandwidth of 20 GHz with subcarrier number 4, the power penalty at a BER of 10–9 is found to be 30 dB for a link distance of 3.6 km, whereas the power penalty reduces to 13 dB when the number of subcarrier is increased to 32.

Impact of Pointing Error on the BER Performance of an OFDM Optical Wireless Communication Link over Turbulent Condition

An analytical approach is developed in this paper to evaluate the bit error rate (BER) performance of an optical wireless (OW) communication system with multiplexing of the RF orthogonal frequency division (OFDM) over turbulent condition taking into account the effect of pointing error. The received signal is detected through direct detection receiver followed by RF synchronous demodulation including the effect of OW channel and different form of noises such as receiver thermal noise, background channel noise and photo detector shot noise. Analysis is developed for an OFDM system over the OW channel, taking into account the effect of pointing error between the transmitter and the receiver in turbulent condition and the analysis reveals that the OFDM OW system is less affected by pointing error with deference to the major power penalty at BER performance. For instance, power penalty at BER 10−9 is found to be 3 dB for 256 OFDM subcarriers with 9 millidegree displacement angle at a data rate of 10 Gbps under turbulent condition. It is found that the system is more influenced by the atmospheric turbulence at a higher data rate.