ОПТИКА И СПЕКТРОСКОПИЯ, 2013, том 114, № 5, с. 877-880
ЛАЗЕРЫ ^^^^^^^^^^^^^^
И ИХ ПРИМЕНЕНИЕ
УДК 621.391
PHOTONIC GENERATION MULTIBAND UWB IMPULSE RADIO SIGNAL
© 2013 г. Guodan Sun, Rong Wang, Tao Pu, and Zhihu Wei
Institute of Communication Engineering PLA University of Science & Technology, Nanjing, 210007 China
E-mail: Chinacard4@163.com Received September 23, 2012
Abstract—A novel method to photonic generation multiband ultra-wide band (MB-UWB) impulse radio signal based on the summation of multiple doublet pulses with different weights and proper time delays is proposed. Microwave sinusoidal signal is used to amplitude modulate a tunable laser to get multi-wavelength source. The optical signal is then phase modulation with Gaussian pulse and fed into a section of single-mode fiber (SMF). The SMF services as phase modulation to intensity modulation (PM-IM) conversion to generate Gaussian doublet pulse, and offers proper time delays to Gaussian doublet pulses with different weights. MB-UWB pulses with sidelobe suppression levels over 25 dB, central frequency from 5 to 8 GHz, 10 dB bandwidth from 2.9 to 3.8 GHz are obtained in experiment.
DOI: 10.7868/S0030403413050073
INTRODUCTION
Ultra-wideband (UWB) technology has attracted considerable interests for short-range large capacity wireless communication systems and sensor networks due to many advantages, such as high data rate, low power consumption, low spectral density, and immunity to multipath fading [1, 2]. Due to the low power density —41.3 dBm/MHz in the frequency band of 3.1—10.6 GHz regulated by the Federal Communications Commission (FCC), the communication distances are limited, typically extending less than 10 meters [3]. To increase the area of coverage and offer the availability of undisrupted service across different networks, a technique to distribute UWB over fiber is considered as a promising solution [4—7]. In UWB system, there are two main signaling formats, singleband and multi-band UWB (MB-UWB) waveforms [8, 9]. The frequency spectrum of single-band UWB waveform is uneven, so the frequency efficiency is low. Compared to single-band UWB, MB-UWB waveforms utilize overlapping groups of UWB signals with bandwidth more than 500 MHz. This format can fully utilize rectangle spectra of FCC requirements. MB-UWB guarantees the adherence to FCC minimum bandwidth requirements and allows dynamic adjusting of frequency spectrum to coexistence with local frequency spectrum standard. However, up to now, there have been few attempts in generating MB-UWB pulse in optical domain [8, 9]. In addition, these techniques present complicated optical or electrical structure designs.
In this paper, a method to photonic generation MB-UWB pulses based on the summation of multiple doublet pulses with different weights and proper time delays is proposed. Microwave sinusoidal signal is used to amplitude modulate optical source. The optical sig-
nal is then phase modulation by Gaussian pulse and fed into a section of single-mode fiber (SMF). The SMF services as phase modulation to intensity modulation (PM-IM) conversion to generate Gaussian doublet pulse, and offers proper time delays to sum different weights Gaussian doublet pulses. In experiment, MB-UWB pulses with sidelobe suppression ratio (SLSR) over 25 dB, central frequency from 5 to 8 GHz, 10 dB bandwidth from 2.9 to 3.8 GHz are obtained.
PRINCIPLE
Yao has proposed when an optical carrier is phase modulated by a Gaussian pulse, after SMF transmission, UWB doublet pulses can be generated [7]. The frequency response of PM—IM conversion based on chromatic dispersion can be expressed as:
Hout(ffl) = sin(1 Da№. (1)
Where
I2
Da = - -n D(X) z, (2)
2 n c
where X is optical central wavelength, c is light velocity in vacuum, D is fiber chromatic dispersion, and z is fiber length.
The proposed method is based on the summation of generated multiple doublet pulses with different weights and proper time delays. The main principle is shown in Fig. 1. A single wavelength laser diode (LD) is amplitude modulated by a tunable microwave oscillator with an angular frequency fm using a Mach-Ze-hnder modulator (MZM), which is biased at the maximum transmission bias point (MATP). The normal-
878
GUODAN SUN et al.
RF I
Gaussian,
SMF
LD <X>, MZM m(a) PM \Ub> PD (c)
o u
Fig. 2. (a) waveform and (b) power spectrum of the Gaussian doublet pulse at the end of the fiber link.
ized optical field at the output of the MZM can be expressed as
Eout (t) = Eoexp \J( 2 nf01 + ^c )]j J (ß) +
+<»
+ £ (-1)k +1 J2k(ß)\exp(J2k x 2nfmt) + (3)
k = 1
+ exp(-J2k x 2ft)] L
Where f is the frequency of the optical carrier, p is the modulation index of the MZM (p = nVfFl2VK). It is seen that only optical carrier and the even-order sidebands are present at the output of MZM; the odd-order optical sidebands are suppressed [Fig. 1 inset (a)]. The phase modulator (PM) is driven by a Gaussian pulse train which representing the data sequence and then transmitted to remote site by SMF. The SMF performs two functions: PM-IM conversion to generate Gaussian doublet pulse, proper time delays to sum different weight Gaussian doublet pulses [Fig. 1 inset (b)]. At the receiver, the optical signal is detected by a PD
with 3-dB bandwidth of10 GHz. Fig. 1 inset (c) shows the generation of MB-UWB pulses with different microwave frequency. It can be seen hat the higher microwave frequency the larger time duration of generated signal pulses.
EXPERIMENT AND DISCUSSION
An experiment based on the setup shown in Fig. 1 is performed. The light wave from a tunable laser source (TLS) with wavelength 1550.2 nm and line-width about 150 kHz is modulated at a 40 GHz MZM by a 40 GHz EVNA (Agilent N5230A) which acts as electrical sinusoidal source. The MZM is biased at the MATP with modulation voltage 1.15Vn. The PM is driven by pulse pattern generator (PPG, MP1763C). The PPG is set one "1" every 32 bits and a bit rate of 10 Gbit|s. The input pulse has a shape close to a Gaussian with full-width at half-maximum about 80 ps. A SMF with a length of 20 km is used to distribute the optical signal to the remote site. At the remote site, the optical signal is detected by a 10-GHz PD then measured by a digital sampling oscillator (NR09000) and electrical spectrum analyzer (Agilent E4447AU). Figure 2 shows the waveform and power spectrum of the Gaussian doublet pulse after PM and fiber transmis-
OOTHKA H CnEKTPOCKOnH^ tom 114 № 5 2013
PHOTONIC GENERATION MULTIBAND UWB IMPULSE RADIO SIGNAL
879
(a)
(b)
200 ps/div
m
!JJIjU.
Ï
Fig. 3. Eye of generated UWB pulse with different RF frequency: (a) 20 GHz, (b) 40 GHz.
sion. The spectrum of the generated Gaussian doublet pulse has a central frequency 5.8 GHz, and a 10 dB bandwidth 7.8 GHz. However, it is noted that the spectra have distinguished frequency components in low-frequency band (0.96—1.61 GHz). Those frequency components violate the dip in FCC spectral mask for noninterference operation with other wireless technologies, especially in the global positioning system (GPS) band.
Figure 3 shows the eye diagram of generated UWB pulse with 20 and 40 GHz microwave frequency. It can be seen that the pulse duration of (b) is longer than (a) because the higher microwave frequency is selected, the larger time delay of different weight Gaussian doublet pulses is produced. The generated UWB pulses with 20 GHz frequency suffer more noise impact than that of 40 GHz, this is because of the residual 20 GHz frequency component after PD.
It is known that the waveform apodization profile determines the shape of the generated signal spectrum. The generated MB-UWB pulses are all with apodization, this is because the power of optical carrier is larg-
er than ±2 order sidebands. There are obvious rising and falling edges in pulse duration, so its electrical spectrum has high sidelobes suppression ratio. From Fig. 4, we can see that two bands of MB-UWB are generated in FCC mask by tuning the RF frequency from 20 to 40 GHz. The sidelobes are suppressed over 25 dB for all sub-bands, which is in line with FCC definition.
Figure 5 shows the relationship between microwave frequency and central frequency, 10 dB bandwidth of generated MB-UWB signal. The central frequency and 10 dB bandwidth of generated MB-UWB signal is decreasing with trigger frequency, which is mainly because the pulse duration is longer with higher microwave frequency.
This method is simple in principle. The center frequency and bandwidth of generated pulse are determined by microwave frequency. The SLSR is the contribution of apodization shape, which is determined by MZM modulation voltage. So it's easy and possible to tune the center frequency, bandwidth and SLSR of UWB signal.
Power/dBm -40 L
-50
-60
-70
-80
-90
-100
0
9 12 15 Frequency, GHz
Fig. 4. Spectra of generated multiband UWB pulse in different frequency.
z Hz
O
y, g7
e
u q
e
6
n
e Ce
v -
O-
4.0
z Hz
0 3.5 £
id dwi
n
3.0 ba
B
d
0
2.51
20 25 30 35 RF frequency, GHz
40
Fig. 5. Relationship between RF frequency and central frequency, 10 dB bandwidth of generated MB UWB pulse.
5
6
OOTHKA H CnEKTPOCKOnHH tom 114 № 5 2013
880
GUODAN SUN et al.
CONCLUSION
A simple method to photonic generation MB-UWB pulses based on the summation of multiple doublet pulses with different weights and proper time delays is proposed. In experiment, MB-UWB pulses with side-lobe suppression levels over 25 dB, central frequency from 5 to 8 GHz, 10 dB bandwidth from 2.9 to 3.8 GHz are implemented. The center frequency and bandwidth of generated pulse is determined by microwave frequency. The SLSR is the contribution of apodization shape, which is determined by MZM modulation voltage. So it is easy and possible to tune the center frequency, bandwidth and SLSR of UWB signal. This method can be easily realized and used in future multiband modulation UWB over fiber systems.
ACKNOWL
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.