Chapter 1 Introduction
1.2 Motivation
Thanks to the unlimited bandwidth and low transmission loss of optical fiber, 60-GHz RoF technology is a promising solution to provide wireless broadband communication services, wide converge, and mobility. OFDM modulation format provides a solution with great potential at 60-GHz band [6].
High capacity transmission beyond 10 Gb/s at 60-GHz band within the 7-GHz free licensed band can be achieved due to the high spectrum efficiency of the OFDM modulation format. The uneven channel response can be readily compensated using one-tap equalizer due to the small bandwidth multi-carrier property of the OFDM signal. In summarization, no dispersion induced fading is observed, high spectral efficiency vector signal can be utilized and wavelength reuse is also achieved.
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Chapter 2
The Concept of New Optical Modulation System
2.1 Preface
Optical communication systems include three parts: optical transmitter, communication channel and optical receiver. Optical transmitter plays an role to convert an electrical input signal into the corresponding optical signal and then launches it into the optical fiber providing as a communication channel.
The utility of an optical receiver is to convert the optical signal back into electrical signal and recover the data transmitted through the lightwave system.
In this chapter, we will introduce the external Mach-Zehnder Modulator (MZM), constructing a model of new ROF system.
2.2 Mach-Zehnder Modulator (MZM)
The modulation method of optical signal generation are direct modulation and external modulation. Comparing with the two modulations, if the bit rate of direct modulation signal is above 10-Gb/s, the frequency chirp imposed on signal becomes large enough. Because of this reason, it is difficult to utilize direct modulation to generate microwave/mm-wave. Nevertheless, the band width of signal generated from external modulation can be above 10-Gb/s. In recent year, most RoF systems use external modulation with MZM and Electro-Absorption Modulator (EAM) [7]. Most of the MZMs are based on LiNbO3 (lithium niobate) technology. The types of LiNbO3 device with the applied electric field are x-cut and z-cut. With the number of electrode, there are two types of LiNbO3 device: dual-drive Mach-Zehnder Modulator and single-drive Mach-Zehnder Modulator [8].
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2.3 Single-drive Mach-Zehnder modulator
There are two arms and an electrode in the single-drive Mach-Zehnder Modulator. By changing the voltage applied on the electrode, the optical phase in each arm can be controlled. We call that the modulator is in “on” state if the lightwave are in phase. On the other hand, if the lightwave are in opposite phase, the modulator is in “off” state. The lightwave can not propagate by waveguide for output when the modulator is in “off” state.
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2.4 The architecture of ROF system 2.4.1 Optical transmitter
In the optical transmitter system, there are optical source, modulator, local oscillator, etc. So far, most of the RoF system use laser as the optical source.
There are a lot of advantages of using laser as optical source. For example, the advantages include compact size, high efficiency, good reliability small emissive area compatible with fiber core dimensions, and possibility of direct modulation at relatively high frequency. Electrical signal converts into optical form by using modulator. Due to the external integrated modulator composing of MZMs, we select MZM as modulator to build the system of optical transmitter.
Laser data
MZM 1 MZM 2
(a)
LOLaser data
MZM
(b)
LOmixer
LO:Local Oscillator
Figure 2-1 (a) and (b) are two schemes of transmitter and (c) is duty cycle of subcarrier biased at different points in the transfer function. (LO: local
oscillator)
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Figure 2-1 shows two schemes of optical transmitter and duty cycle of subcarrier biased at different points in the transfer function. In figure 2-1 (a), it is one of the two schemes that we mentioned. In the scheme, it includes one laser, two modulators, one local oscillator, and some data. First MZM is utilized to generate optical carrier which carried the data, and the output optical signal is baseband (BB) signal. The second MZM generates the optical subcarrier which carried the BB signal and then output the RF signal. In figure 2-1 (b), it is another kind of the two schemes of optical transmitter. In the scheme, there are one laser, one modulator,one electrical mixer one local oscillator, and some data. The scheme is used a electrical mixer and a local oscillator (LO) to up-convert elecrical signal and then sent it into a MZM to generate the optical signal. Figure 2-1 (c) depicts the duty cycle of subcarrier biased at different points in the transfer function.
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2.4.2 Optical signal generations based on LiNbO3 MZM
Laser
Double sideband scheme (DSB)
Large bandwidth requirement
compact
Limited OMI (≤1)
RFfading
Single sideband scheme (SSB)
Large bandwidth requirement
No RF fading
complex
Limited OMI (<1)
Double sideband scheme with carrier Suppression (DSBCS)
No RF fading
compact
Full OMI
Can not transmit vector signal
Figure 2-2 Optical microwave/mm-wave modulation scheme by using MZM.
The main techniques in RoF system are mm-wave and microwave generation. Optical mm-wave generation using external MZM based on double-sideband (DSB), single-side band (SSB), double-sideband with carrier suppression (DSBCS) have been demonstrated. Figure 2-2 shows the three schemes that we mentioned. For DSB, the optical signal is generated when the DC bias voltage of single-drive MZM is setted at quadrature point. The DSB modulation scheme suffers performance fading problems due to fiber dispersion, resulting in degradation of the receiver sensitivity. So the signals only can be transmitted over several kilo-meters. The RF fading means that when an optical signal is modulated by an electrical RF signal, fiber chromatic dispersion causes the detected RF signal power to have a periodic fading
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characteristic. Due the reason, the SSB modulation scheme is proposed to overcome the fading issue.
In the SSB modulation scheme, the DC bias voltage of dual-drive MZM is setted at quadrature point and the phase difference of π/2 is applied between the two RF electrodes of the dual-drive MZM. Although the SSB modulation scheme can reduce the fiber dispersion but it suffers worse receiver sensitivity due to the limited optical modulation index (OMI).
In recent year, DSBCS has been demonstrated for optical mm-wave generation. In DSBCS modulation scheme, the DC bias voltage of single-drive MZM is setted at null point. There is no fiber dispersion problem in the scheme It has best receiver sensitivity following transmission over a long distance because the OMI is equal to one. The bandwidth requirment of DSBCS modulation scheme for RF signal, electrical components, electrical amplifier, and optical modulator is lower than DSB and SSB modulation scheme. The sprectral occupancy of DSBCS modulation scheme is the lowest in the three modulation scheme. However, the disadvantage of the DSBCS modulation scheme is that it can not transmit vector signal, such as phase shift keying (PSK), quadrature amplitude modulation (QAM), or orthogonal frequency-division multiplexing (OFDM) signals. It only can support on-off keying (OOK) format, which are of utmost importance for wireless application.
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2.4.3 Communication channel
Figure 2-3 shows the common model of communication channel.
Communication channel includes fiber, optical amplifier, etc. Most RoF systems use single mode fiber (SMF) or dispersion compensated fiber (DCF) as the transmission medium. The dispersion will be happened when the optical signal transmits in the fiber. DCF is ulitized to compensate the dispersion phenomenon. The transmission distance of any fiber-optic communication sysem is limited by fiber losses. For long-haul transmission systems, the loss limitation has traditionally been overcome using regenerator witch the optical signal is first converted into an electric current and then regenerated using a transmitter. It is quite expensive and complex for WDM lightwave systems. A method of loss management is utilizing optical amplifier to amplify the optical signal directly without requiring its conversion to electric domain [9]. Most RoF systems use erbium-doped fiber amplifier (EDFA) to amplify the optical signal. The optical bandpss filter (OBPF) is necessary to filter out the ASE noise.
EDFA OBPF Fiber Modulated
Optical Signal
· EDFA:Erbium-doped Fiber Amplifier
· OBPF:Optical Bandpass Filter
Figure 2-3 The model of communication channel in a RoF system.
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2.4.4 Demodulation of optical millimeter-wave signal
· PD:Photo-detector
· LPF:Low-pass Filter
· BERT:Bit-error-rate Tester LPF BERT LO
PD
mixer After
transmitting optical signal
Figure 2-3 The model of receiver in a RoF system.
Figure 2-4 shows the receiver model of the RoF system. As the figure depicts, there are PD, mixer, LO, LPF and BERT. PD usually includes trans-impedance amplifier (TIA) and photo diode. The PIN diode is usually utilized due to it’s lower transit time in the microwave or mm-wave system.
TIA function converts photo-current to output voltage.
After PD square-law detection, the BB and RF signals are the same. The RF signal is down converted to baseband signal by using an electrical mixer, and then filtered by low-pass filter (LPF).
The down-converted signal will be sent into a signal analyzer to test its performance, just like bit-error-rate (BER) tester or real time scope, as shown in figure 2-4.
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LD PC
data
MZM
LO mixer
EDFA OBPF Fiber
PD mixer
LPF BERT
LO
· LD:Laser Diode
· PC:Polarization Controller
· EDFA:Erbium-doped Fiber Amplifier
· OBPF:Optical Bandpass Filter
· PD:Photodetector
· LPF:Low-pass Filter
· BERT:Bit-error-rate Tester
Figure 2-4 The model of RoF system.
Figure 2-5 shows the model of RoF system. In the model, it concludes transmitter communication channel and receiver. The transmitter combining with communication channel and receiver is called the model of RoF system.
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2.5 The new proposed model of optical modulation system
Figure 2-6 depicts the conceptual diagram of the proposed sysem. A dual-drive MZM is biased at quadrrature point to achieve SSB modulation scheme. The OFDM signal is generated from an AWG and the OFDM optical sideband is shown in figure 2-6 (a). A new optical carrier is generated from a pure sinusoidal signal with frequency of ωrf. In the figure 2-6 (b), the optical sideband of sinusoidal is located at frequency ωrf. In order to achieve tandem SSB modulation scheme, a 900 phase shifter is added to the upper path of OFDM signal and the lower path of the sinusoidal signal. The upper path of OFDM signal and sinusoidal signal are combined and sent into one of the electrode of the dual-drive MZM. On the other hand, the lower path of OFDM signal and sinusoidal signal are also combined and sent into the other electrode of the dual-drive MZM.
At the output of the dual-drive MZM, the optical spectrum is shown in figure 2-6 (C). There are original optical carrier (ωc), new optical carrier (ωc − ωrf), and OFDM optical sideband (ωc + ωrf) in the optical spectrum.
And then, after SSMF transmission, a fiber bragg grating (FBG) and optical circulator are utilized to separate the original optical carrier for the reuse of uplink data. After the FBG, the OFDM optical sideband (ωc+ ωrf) and the new optical carrier (ωc − ωrf) are received for wireless application. The figure 2-6 (d) shows the optical spectrum which includes the OFDM optical sideband (ωc + ωrf) and the new optical carrier (ωc − ωrf), and the original optical carrier is separated by FBG. The original optical carrier using a RSOA [10] for uplink application. The figure 2-6 (e) shows the separated original optical carrier. In this work, generation and transmission of 60-GHz signal using
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modified tandem single sideband (TSSB) modulation scheme [11].
LD
· AWG:Arbitrary Waveform Generator
· SSMF:Standar Single Mode Fiber
· FBG:Fiber Bragg Grating
(a)
(b)
(c) (d)
(e)
Figure 2-5 Concept of the proposed system. (LD: laser diode, MZM:
Mach-Zehnder modulator, SSMF:standar single mode fiber, FBG: fiber bragg grating, RSOA: reflective semiconductor optical amplifier)
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Chapter 3
The Theoretical Simulations of Proposed System
3.1 preface
Some theoretical simulations are shown in this chapter. The VPI software is utilized here. The system is modified TSSB as shown in figure 2-6. In the simulation, the signal is OFDM signal, and the issue that we discuss is the linewidth of the laser source. First part, the bandwidth of the OFDM signals are 3.125G, 5G, 7G, respectively and the signals are all located at 60-GHz millimeter-wave band. Second part, the bandwidth of the OFDM signal is all 7G, but the signals are located at 60-GHz, 40-GHz, 20-GHz millimeter-wave band, respectively. In next several sections, the diagrams of signal to noise ratio (SNR) and BER versus linewidth of the laser.
In the theorical simulation, the input power of photodetector is setted at -8.5 dBm. There are three cases about BTB, 25km transmission length, 50km transmission length in the simulations which are tested. The range of laser linewidth which we discuss is from 1K to 100000K. In the range of laser linewidth, the division which we set is per 101. We will itemize to illustrate the simulation of every case
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3.2 Theoretical simulation of proposed system for OFDM signal with 3.125G bandwidth at 60-GHz millimeter-wave band
2 3 4 5 6 7 8 9
Figure 3-1 the SNR and BER curves of 3.125G bandwidth at 60-GHz millimeter-wave band
In the section, the OFDM signal with 3.125G bandwidth is utilized as the signal source. The system is modified TSSB scheme. In the simulation, the BTB, 25km transmission, 50km transmission cases are tested. The saved files are analyzed using outline MATLAB program. Figure 3-1 (a) shows the diagram of signal to noise ratio (SNR) versus linewidth of laser source. In the figure, when the linewidth of laser increases, the SNR gets worse at BTB, 25km transmission length, 50km transmission length cases. Figure 3-1 (b) shows the diagram of BER versus linewidth of laser source. When the linewidth of laser increases, the BER gets worse at BTB, 25km transmission length, 50km transmission length cases. Compared with the figure3-1 (a) and (b), if the SNR is much better, the BER is also much better at the three cases. In figure 3-1, we find that the SNR and BER at 25km transmission case is better than at BTB case due to the self modulation of the fiber.
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BTB_linewidth-1k
BTB_linewidth-100M
25km_linewidth-1k 50km_linewidth-1k
25km_linewidth-100M 50km_linewidth-100M
(a)
(b)
(c)
(d)
(e)
(f)
Figure 3-2 constellations of BTB, 25km, 50km with 3.125G bandwidth at 60-GHz millimeter-wave band
Figure 3-2 shows the constellstions of OFDM signal with 3.125G bandwidth. Figure 3-2 (a) and (b) are at BTB case and the linewidth of laser are 1K and 100M respectively. Figure 3-2 (c) and (d) are at 25km transmission case and figure 3-2 (e) and (f) are at 50km transmission case. In figure 3-2 (c) and (e), the linewidth of laser source is 1K at 25km transmission and 50km transmission case respectively. As shown in figure 3-2 (d) and (f), the linewidth of laser source is 100M at 25km transmission and 50km transmission case respectively. When the linewidth of laser source is 1K, the constellation is very clear. The constellation gets blured when the linewidth of laser source increases at the three cases (BTB, 25km, 50km).
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3.3 Theoretical simulation of proposed system for OFDM signal with 5G bandwidth at 60-GHz millimeter-wave band
2 3 4 5 6 7 8 9
Figure 3-3 the SNR and BER curves of 5G bandwidth at 60-GHz millimeter-wave band
In the section, the OFDM signal with 5G bandwidth is utilized as the signal source. The system is modified TSSB scheme. In the simulation, the BTB, 25km transmission, 50km transmission cases are tested. The saved files are analyzed using outline MATLAB program. Figure 3-3 (a) shows the better, the BER is also much better at the three cases. In figure 3-3, we find that the SNR and BER at 25km transmission case is better than at BTB case because the self modulation of the fiber.
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BTB_linewidth-1k
BTB_linewidth-100M
25km_linewidth-1k 50km_linewidth-1k
25km_linewidth-100M 50km_linewidth-100M
(a)
(b) (d)
(c) (e)
(f)
Figure 3-4 constellations of BTB, 25km, 50km with 5G bandwidth at 60-GHz millimeter-wave band
Figure 3-4 shows the constellstions of OFDM signal with 5G bandwidth.
Figure 3-4 (a) and (b) are at BTB case and the linewidth of laser are 1K and 100M respectively. Figure 3-4 (c) and (d) are at 25km transmission case and figure 3-4 (e) and (f) are at 50km transmission case. In figure 3-4 (c) and (e), the linewidth of laser source is 1K at 25km transmission and 50km transmission case respectively. As shown in figure 3-4 (d) and (f), the linewidth of laser source is 100M at 25km transmission and 50km transmission case respectively. When the linewidth of laser source is 1K, the constellation is very clear. The constellation gets blured when the linewidth of laser source increases at the three cases (BTB, 25km, 50km).
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3.4 Theoretical simulation of proposed system for OFDM signal with 7G bandwidth at 60-GHz millimeter-wave band
2 3 4 5 6 7 8 9
Figure 3-5 the SNR and BER curves of 7G bandwidth at 60-GHz millimeter-wave band
In the section, the OFDM signal with 7G bandwidth is utilized as the signal source. The system is modified TSSB scheme. In the simulation, the BTB, 25km transmission, 50km transmission cases are tested. The saved files are analyzed using outline MATLAB program. Figure 3-5 (a) shows the better, the BER is also much better at the three cases. In figure 3-5, we find that the SNR and BER at 25km transmission case is better than at BTB case because the self modulation of the fiber.
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BTB_linewidth-1k
BTB_linewidth-100M
25km_linewidth-1k 50km_linewidth-1k
25km_linewidth-100M 50km_linewidth-100M
(a)
(b)
(c)
(d)
(e)
(f)
Figure 3-6 constellations of BTB, 25km, 50km with 7G bandwidth at 60-GHz millimeter-wave band
Figure 3-6 shows the constellstions of OFDM signal with 7G bandwidth.
Figure 3-6 (a) and (b) are at BTB case and the linewidth of laser are 1K and 100M respectively. Figure 3-6 (c) and (d) are at 25km transmission case and figure 3-6 (e) and (f) are at 50km transmission case. In figure 3-6 (c) and (e), the linewidth of laser source is 1K at 25km transmission and 50km transmission case respectively. As shown in figure 3-6 (d) and (f), the linewidth of laser source is 100M at 25km transmission and 50km transmission case respectively. When the linewidth of laser source is 1K, the constellation is very clear. The constellation gets blured when the linewidth of laser source increases at the three cases (BTB, 25km, 50km).
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3.5 Theoretical simulation of proposed system for OFDM signal with 7G bandwidth at 40-GHz millimeter-wave band
2 3 4 5 6 7 8 9
Figure 3-7 the SNR and BER curves of 7G bandwidth at 40-GHz millimeter-wave band
In the section, the OFDM signal with 7G bandwidth is utilized as the signal source. The system is modified TSSB scheme. In the simulation, the BTB, 25km transmission, 50km transmission cases are tested. The saved files are analyzed using outline MATLAB program. Figure 3-7 (a) shows the better, the BER is also much better at the three cases. In figure 3-7, we find that the SNR and BER at 25km transmission case is better than at BTB case because the self modulation of the fiber.
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BTB_linewidth-1k
BTB_linewidth-100M
25km_linewidth-1k 50km_linewidth-1k
25km_linewidth-100M 50km_linewidth-100M
(a)
(b)
(c)
(d)
(e)
(f)
Figure 3-8 constellations of BTB, 25km, 50km with 7G bandwidth at 40-GHz millimeter-wave band
Figure 3-8 shows the constellstions of OFDM signal with 7G bandwidth.
Figure 3-8 (a) and (b) are at BTB case and the linewidth of laser are 1K and 100M respectively. Figure 3-8 (c) and (d) are at 25km transmission case and figure 3-8 (e) and (f) are at 50km transmission case. In figure 3-8 (c) and (e), the linewidth of laser source is 1K at 25km transmission and 50km transmission case respectively. As shown in figure 3-8 (d) and (f), the linewidth of laser source is 100M at 25km transmission and 50km transmission case respectively. When the linewidth of laser source is 1K, the constellation is very clear. The constellation gets blured when the linewidth of laser source increases at the three cases (BTB, 25km, 50km).
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3.6 Theoretical simulation of proposed system for OFDM signal with 7G bandwidth at 20-GHz millimeter-wave band
2 3 4 5 6 7 8 9
Figure 3-9 the SNR and BER curves of 7G bandwidth at 20-GHz millimeter-wave band
In the section, the OFDM signal with 7G bandwidth is utilized as the signal source. The system is modified TSSB scheme. In the simulation, the BTB, 25km transmission, 50km transmission cases are tested. The saved files are analyzed using outline MATLAB program. Figure 3-9 (a) shows the better, the BER is also much better at the three cases. In figure 3-9, we find that the SNR and BER at 25km transmission case is better than at BTB case because the self modulation of the fiber.
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BTB_linewidth-1k
BTB_linewidth-100M
25km_linewidth-1k 50km_linewidth-1k
25km_linewidth-100M 50km_linewidth-100M
(a)
(b)
(c)
(d)
(e)
(f)
Figure 3-10 constellations of BTB, 25km, 50km with 7G bandwidth at 20-GHz millimeter-wave band
Figure 3-10 shows the constellstions of OFDM signal with 7G bandwidth.
Figure 3-10 shows the constellstions of OFDM signal with 7G bandwidth.