A Chaotic Ultra-Wideband (UWB) over Fiber Link Jianyu Zheng1,2, Mingjiang Zhang1 * , Liu Liu1, Lina Meng1 and Yuncai Wang1,3 1
Institute of Optoelectronic Engineering, Taiyuan University of Technology, Taiyuan 030024, China Phone: +(86)3516018134, Fax: +(86)3516018134, Email:
[email protected] (2State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, CAS, Beijing 100083, China) (3State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, China)
Keywords: chaotic laser;ultra-wideband; radio over fiber
Abstract A chaotic Ultra-wideband (UWB) over fiber link with steerable and flatted power spectrum is demonstrated, which generates the photonic UWB signals using chaotic dynamics of the semiconductor laser with optical feedback.
Fig. 1. Experimental setup of the Chaotic-UWB over fiber link.
1 Introduction Impulse radio based Ultra-wideband (IR-UWB) technology is considered to be a highly hopeful technology for short-range and high-throughput wireless communications and sensor network owing to its advantages such as high immunity to multipath fading, low power consumption, enhanced penetration capability and high range resolution. As for the chaotic-UWB technique, one of the physical layer candi-dates of the IEEE 802.15.4a standard, information is usually transmitted by means of the chaotic pulse signal with an extremely peculiar shape [1]. Although low power spectral density severely limits the coverage of UWB signals, the adoption of microwave photonics technique is expected to solve this problem by means of generating UWB signals in optical domain, and then distributing the signals over a long distance fiber [2]. However, some reported approaches for generation and distribution of the photonic IR-UWB signals, for example describe in Ref. [3], fail to eliminate the harmful effects of discrete spectrum lines of the regular UWB pulses train which include the interference with narrow band wireless systems and limitation of total power that the transmitter is allowed to radiate. Although this problem could be mitigated by means of drawing into the random jitter [4] or optimizing modulation format [5], they are bound to increase the complexity and cost of the UWB over fiber link. We demonstrated a UWB pulse generator based on chaotic dynamics of laser diode [6, 7]. In this paper, we experimentally demonstrate a chaotic-UWB over fiber link in which the generated and distributed photonic UWB signals own steerable and flatted power spectrum.
2 Experimental principle
setup
and
operation
Figure 1 shows the experimental setup of the chaoticUWB over fiber. The chaotic light source is served by a distributed feedback laser diode (DFB-LD) subject to optical feedback via a fiber ring cavity. The strength and polarization state of the feedback light are controlled by an optical variable attenuator (OVA1) and polarization controllers (PC1 and PC2), respectively. The information sequences with variable code rate provided by the nonreturn-to-zero code random data generator (NRZRDG) are modulated via a Mach-Zehnder modulator (MZM) to realize on-off keying (OOK) modulation. The chaotic optical pulse sequence is converted into chaoticUWB radio pulse via a 50-GHz bandwidth photodetector (PD) after propagating through a 10-km single-mode fiber (SMF) and reamplifying by an erbium-doped fiber amplifier (EDFA). A pair of broadband horn antennas with bandwidth ranging from 1.0 to 18.0 GHz is used to transmit and receive the chaotic-UWB signals, whose spacing is about 0.6 m. The received signals are processed by a data processor comprising of a high sample rate digital storage oscilloscope and a custom offline signal processing algorithm.
3 Generation of the Chaotic UWB signals We could optimize the power spectrum of the chaotic source’s output before modulating the data in order to realize regulation for power spectrum of the photonic chaotic-UWB signals through adjusting the bias current and feedback strength whose variations will affect the energy distribution of the chaotic laser in frequency domain as in [6, 7].
Fig. 2. Power spectra (a) and time sequences (b) at different bit rates recorded before transmission.
Figure 2 (a) and (b) describe the spectra and corresponding time sequences of the chaotic-UWB signals with OOK modulation format at three different bit rates of 360 Mb/s, 720 Mb/s, and 1.44 Gb/s, which measured and recorded at test point 1. As shown in Fig.2 (a), compared with the original chaotic laser, the information modulation lead to generation of new frequency components at low frequency band which correspond to the spectrum characteristics of the baseband signals, but the power spectrum of the chaoticUWB pulse sequence still kept approximate profile as ever, which is in accord with theory anticipation. Some burrs randomly occurred at the edge of the spectrum profile because of the random and continues analog signals, i.e. the original chaotic output, were replaced by separate random pulse sequences which have been displayed in Fig. 2(b). Especially, the discrete spectrum lines were not appeared in frequency domain because the discretization of spectrum were restricted owing to the diversity of envelopment between each chaotic pulse, which means that the allowable radiated power of generated UWB signals could be maximized.
4 Terminal signal demodulation When the bit rate was fixed at 1.44 Gb/s, we completed the message demodulation by using a high sample rate oscilloscope and an algorithm of correlated demodulation at the antenna terminal after propagating through a 10-km SMF followed by 0.6-m wireless. Resembling the algorithm presented in Ref. [6], our algorithm possesses noise filtering, bit correlation, and distinguishing between “0” and “1” bits by comparing the average power within the center window of each bit slot to an optimal decision threshold. The original data sequence and demodulated data sequence are shown in Fig. 3 (a) and (b), respectively. The eye diagram with 4000 bit is displayed in Fig. 3(c), which was composed by computer. The bit error rate was subsequently computed using a digital signal processing algorithm in a bit-for-bit comparison between the original and decoded data. Finally, the 105 bits transmitted with error-free were achieved.
Fig. 3. (a) Encoded data, (b) decoded data, (c) corresponding eye diagram with 4000 bit after transmitting over 10-km fiber link and 0.6m wireless link.
Conclusions In conclusion, we achieved a chaotic-UWB over fiber link based on a photonic chaotic-UWB generator. The 1.44Gb/s chaotic-UWB signals were sent over 10-km fiber without dispersion compensating followed by 0.6m wireless. Moreover, there is no discrete spectrum line in their corresponding power spectra, which means that harmful effects of discrete spectrum lines could be avoided with this method.
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