Oversampling to Reduce the Effect of Timing Jitter on High Speed ofdm systems Abstract O



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Oversampling to Reduce the Effect of Timing Jitter on High Speed OFDM Systems
Abstract
ORTHOGONAL frequency division multiplexing (OFDM) is used in many wireless broadband communication systems because it is a simple and scalable solution to inter symbol interference caused by a multipath channel. Very recently the use of OFDM in optical systems has attracted increasing interest. Data rates in optical fiber systems are typically much higher than in RF wireless systems. The impairments caused by timing jitter are a significant limiting factor in the performance of very high data rate OFDM systems. In this letter we show that oversampling can reduce the noise caused by timing jitter. Both fractional oversampling achieved by leaving some band-edge OFDM subcarriers unused and integral oversampling are considered. The theoretical results are compared with simulation results for the case of white timing jitter showing very close agreement. Oversampling results in a 3 dB reduction in jitter noise power for every doubling of the sampling rate.
OFDM is a multicarrier modulation scheme that provides strong robustness against inters -symbol interference (ISI) by dividing the broadband channel into many narrowband sub- channels in such a way that attenuation across each sub channel stays flat. Orthogonalization of sub channels is performed with low complexity by using the fast Fourier transform (FFT). The serial high-rate data stream is converted into multiple parallel low-rate streams, each modulated on a different subcarrier.

INTRODUCTION
ORTHOGONAL frequency division multiplexing (OFDM) is used in many wireless broadband communication systems because it is a simple and scalable solution to intersymbol interference caused by a multipath channel. Very recently the use of OFDM in optical systems has attracted increasing interest.

Data rates in optical fiber systems are typically much higher than in RF wireless systems. At these very high data rates, timing jitter is emerging as an important limitation to the performance of OFDM systems. A major source of jitter is the sampling clock in the very high speed analog-to-digital converters (ADCs) which are required in these systems. Timing jitter is also emerging as a problem in high frequency band pass sampling OFDM radios. The effect of timing jitter has been analyzed in . These project focuses on the colored low pass timing jitter which is typical of systems using phase lock loops (PLL). They consider only integral oversampling. In OFDM, fractional oversampling can be achieved by leaving some band-edge subcarriers unused. In this letter we investigate both fractional and integral oversampling. We extend the timing jitter matrix proposed in to analyze the detail of the inter carrier interference (ICI) in an oversampled system. Very high speed ADCs typically uses parallel pipeline architecture not a PLL and for these the white jitter which is the focus of this project is a more appropriate model.


APPLICATIONS:

  1. Timing jitter is the unwelcome companion of all electrical systems that use voltage transitions to represent timing information.

  2. Especially for telecom it is crucial that the pulses are synchronous to a clock signal, which is supplied by the system

  3. It Characteristics of RZ Pulse Nonlinear Transmission on Dispersion Managed Fiber Link.

Block Diagram:



REFERENCES
[1] J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol.,

vol. 27, no. 1, pp. 189-204, Feb. 2009.


[2] V. Syrjala and M. Valkama, “Jitter mitigation in high-frequency bandpasssampling

OFDM radios,” in Proc. WCNC 2009, pp. 1-6.


[3] K. N. Manoj and G. Thiagarajan, “The effect of sampling jitter in OFDM systems,” in Proc. IEEE Int. Conf. Commun., vol. 3, pp. 2061-2065, May 2003.
[4] U. Onunkwo, Y. Li, and A. Swami, “Effect of timing jitter on OFDM based UWB systems,” EEE J. Sel. Areas Commun., vol. 24, pp. 787-793, 2006.
[5] L. Yang, P. Fitzpatrick, and J. Armstrong, “The Effect of timing jitter on high-speed OFDM systems,” in Proc. AusCTW 2009, pp. 12-16.
[6] L. Sumanen, M. Waltari, and K. A. I. Halonen, “A 10-bit 200-MS/s CMOS parallel pipeline A/D converter,” IEEE J. Solid-State Circuits, vol. 36, pp. 1048-55, 2001.

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