Chapter 4 Experimental demonstration of proposed system
4.2 Comparison of different DD-OFDM modulation scheme
(a) Conventional SSB
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4.2 Comparison of different DD-OFDM modulation scheme
Before showing the concept of the proposed system, we see the comparison of different DD-OFDM modulation scheme shown as Fig4-1. Conventionally, optical direct-detection OFDM (DD-OFDM) signal is generated based on single-sideband
(b) Virtual SSB
(c) Proposed System
Figure 4-1 Comparison of different DD-OFDM modulation shceme.
(B: bandwidth of DAC, f0: optical carrier frequency, f0: intermedium frequency)
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(SSB) modulation scheme via an external Mach-Zehnder modulator (MZM) as shown in inset (a) of Fig. 4-1. However, conventional SSB modulation approaches suffer from inferior sensitivities because the optical modulation index (OMI) is limited.
Furthermore, to avoid beat noise after photo detection, an electrical I/Q mixer with a typical noise figure (NF) of more than 8 dB is needed to up-convert OFDM signal to intermedium frequency, which severely hinders implementation of highly spectral efficiency QAM OFDM signal for narrowband applications in the 10-Gb/s and above regimes. Recently, several modified SSB modulation schemes have been proposed to overcome this limitation. However, an electrical mixer is still needed to avoid the beat noise interference. Another DD-OFDM modulation scheme is virtual SSB modulation scheme without electrical I/Q mixer is proposed as shown in inset (b) of Fig. 4-1. The drawback is that half bandwidth of the DAC is wasted to avoid the beat noise
interference.
This work presents an optical I/Q up-conversion for direct-detection OFDM signal generation as shown in inset (c) of Fig. 4-1. Since an electrical mixer is not needed to avoid the beat noise interference, the proposed system can support high level OFDM signal (i.e. 16 QAM or 64 QAM). Moreover, the bandwidth of
digital-to-analog converter can be fully utilized.
4.3Concept of the proposed system
Figure 4-2 schematically depicts the principle of the proposed optical I/Q
up-conversion for direct-detection signal generation. Inset (a) of Figure 4-2 shows how we generate optical subcarrier with carrier suppression.To realize direct-detection signals generation, we insert an optical subcarrier to induce remote beating by using
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SSB modulation with carrier suppression . Except with a phase difference of 90°, the RF signals sent into the MZ-a and MZ-b are exactly the same. We bias MZ-a and MZ-b at the null point, the generated optical spectrum consists of an upper sideband (USB) and a lower sideband (LSB) with optical carrier suppression as shown in inset (a) of Fig. 4-2. When MZ-c is biased at the quadrature point, the polarity of LSB in lower arm opposes that in upper arm. The LSB will be eliminated whereas the USB is obtained.
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And then, see inset (b) of Fig. 4-2. I want to use a single-carrier signal case to tell you how the I/Q modulation works. Data I and Q of single-carrier signal are sent to MZ-a and MZ-b of the optical I/Q modulator, respectively. To achieve high optical modulation depth and to operate in E-field linear region of the MZM, both MZ-a and MZ-b are biased at the null point. Therefore, optical single-carrier signal at the center frequency of optical carrier with carrier suppression is generated as shown in inset (b) of Fig. 4-2. And we utilize the π/2 phase delay modulation to rotate Data-Q to Q-axis (90°-axis) to forma m-QAM signal. Inset (c) of Fig. 4-2 is the case of OFDM signal.
Therefore, as both RF and OFDM signals are simultaneously sent to I/Q modulator, the generated optical DD-OFDM signal consisting of an un-modulated subcarrier and an OFDM-modulated carrier. After square-law photo-diode (PD) detection, electrical RF-OFDM signals are obtained. Note that the proposed OFDM transmitter does not need an electrical mixer with a typical NF of more than 8 dB to
Figure 4-2 The Concept of All Optical IQ Up Conversion System.
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avoid the beating noise interference, which is very important for highly spectral efficiency OFDM signals (i.e. 16-QAM or 64-QAM OFDM signal). Additionally, the relative intensity between the un-modulated and OFDM-modulated subcarriers can be easily tuned by adjusting the individual power of the electrical sinusoidal and OFDM signals to optimize the performance of the optical RF signal.
4.4 Experimental results for all optical IQ up conversion system
4.4.1 Experimental setup
Figure 4-3 depicts the experimental setup for all optical I/Q up conversion system via a I/Q modulator. We use a continue wave tunable laser (CW-TL) to generate light source about 1550nm. The light source is then passed through a polarization controller to achieve max optical power when the I/Q modulator is biased at full point. The signals (Data-I and Data-Q) are generated by a Tektronix® AWG7102 arbitrary waveform generator (AWG) using a Matlab® program. The sample rate and digital-to-analogue converter resolution of the AWG are 10 Gb/s and 10 bits,
respectively. And then we use two the same type low noise amplifier (LNA) to amplify Figure 4-3 The Experimental Setup of All Optical IQ Up Conversion System.
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Date I and Data Q to reach the best modulation index (MI) condition. Following, we use low pass filter to remove unwanted noise which is created by DSP signal
generation method and AWG-machine. To realize optical direct-detection signal, a new optical subcarrier is generated at the LSB of the original carrier by 7.5 GHz.
Generate sinusoidal wave as electrical driving signals that are used to generate an optical subcarrier. We use a 90°phase delay coupler to divide the sinusoidal wave into two parts. And then we bias both MZ-a and MZ-b at null point and set the phase modulator to delay π/2. To be able to transmit the signals, we utilize an EDFA to amplifier the optical power and an optical band pass filter to reduce ASE noise. Finally we use a Real Time Scope which is a Tektronix® DPO 71254 with a 50-Gb/s sample rate and a 3-dB bandwidth of 12.5 GHz. to capture the receiver-signals. We use off-line way to demodulate the signals.
4.4.2 Optimal condition for RF signals
Figure 4-4 Optical Spectrum of single carrier QPSK signal.
OPR= Pd/Ps(dB), Pd: optical power of data-modulated optical carrier; Ps: optical power of 7.5GHz subcarrier.OPR=-4dB (b) OPR=0dB (c) OPR=4dB
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Fig. 4-4 and Fig. 4-6 show the optical spectrums in different OPR (OPR= Ps/Pd, Ps and Pd are the optical power of the 7.5-GHz subcarrier and the 0-GHz data-modulated optical carrier, respectively.) for single carrier QPSK and OFDM 16QAM signals. The BER vs. OPR curves of single carrier QPSK and OFDM 16QAM signal are shown in Fig. 4-5 and Fig. 4-7. Single carrier QPSK signal best OPR condition is 0dB as shown in Fig 4-5. The blue star is the case without imbalance-correction and the red circle is the case with imbalance correction. OFDM 16QAM signal best OPR condition is 3dB as shown in Fig 4-7. The blue star case is captured at PD power equal to -14dBm and the other is at PD receiver power equal to -13dBm.
Figure 4-5 BER vs. OPR curve for single carrier QPSK signal.
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Figure 4-6 Optical Spectrum of OFDM 16QAM signal.
OPR= Pd/Ps(dB), Pd: optical power of data-modulated optical carrier; Ps:
optical power of 7.5GHz subcarrier.OPR=5dB (b) OPR=3dB (c) OPR=1dB
Figure 4-7 BER vs. OPR curve for OFDM 16QAM signal.
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4.4.3 Transmission Results
Figure 4-8 is the electrical spectrum for single carrier QPSK signal and a subcarrier. The single carrier QPSK signals are generated by a Tektronix® AWG7102 arbitrary waveform generator (AWG) using a Matlab® program. An optical 10-Gb/s single carrier QPSK signal that occupies a total bandwidth of 5 GHz can be generated, and its receiver signal electrical spectrum is shown as Figure 4-8 (b). We just need half bandwidth of receiver signal to generate Tx single carrier QPSK signal as shown Figure 4-8 (a), and a optical subcarrier is generated at the LSB of the original carrier by 7.5 GHz.
. The OFDM 16QAM signals are generated by a Tektronix® AWG7102 arbitrary waveform generator (AWG) using a Matlab® program, too. The IFFT length is 64. A 156.25-MSym/s 16-QAM symbol is encoded at 32 channels (i.e. channels 3-18 and 48-63) with the remaining 32 channels set to zero. Therefore, an optical 20-Gb/s 16-QAM OFDM signal that has 32 subcarriers and occupies a total bandwidth of 5 GHz can be generated, and its receiver signal spectrum as shown in Figure 4-9(b). The cyclic prefix is set to 1/256
Figure 4-8 Electrical Spectrums for Single Carrier QPSK.
(a) Tx terminal Electrical Spectrum.
(b) Rx terminal Electrical Spectrum.
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symbol time. To realize optical direct-detection OFDM signal, a new optical subcarrier is generated at the LSB of the original carrier by 7.5 GHz, as shown in Fig. 4-9(a).
Figure 4-9 Electrical Spectrum for OFDM 16QAM.
(a) Tx terminal Electrical Spectrum.
(b) Rx terminal Electrical Spectrum.
Figure 4-10 BER-Receiver Power curve for QPSK.
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Figure 4-10 shows the BER performance of the single carrier QPSK signals, a receiver sensitivity of -13dBm is achieved at a BER of 10-9 in the BTB case. The penalty at a BER of 10-9 is negligible following 100-km SMF transmission. And left hand side constellation is the BTB case, right hand side is the after 100-km transmission case.
The constellation size of each point of the two diagrams is almost the same. Figure 4-11 shows the constellations and eye diagrams of BTB, 25km, 50km, and 100km. And they are captured at PD input power equal to -12.5dBm.
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Figure 4-11 Constellations and eye diagrams for single carrier QPSK signals at PD Input Power=-12.5dBm. (a)BTB (b) 25km (c) 50km (d)100km
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Figure 4-12 shows the BER performance of the OFDM 16QAM signals, a receiver sensitivity of -14dBm is achieved at a BER of 10-3 in the BTB case. The penalty at a BER of 10-3 is about 1dB following 100-km SMF transmission. The constellation size of each point of Figure 4-13 constellations is almost the same. The inset (a), (b), (c), and (d) are BTB, 25km, 50km, and 100km, respectively. And they are captured at PD input power equal to -8dBm.
Figure 4-12 BER-Receiver Power curve for OFDM 16QAM.
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4.5 The influence of phase noise
In this section, we investigate the laser linewidth impact on optical 60-GHz OFDM performance. With laser linewidth of 10MHz, BER of both 20-Gb/s 16-QAM and 14-Gb/s QPSK OFDM signals within 7-GHz license-free band is below the FEC limit following 100-km SMF transmission.
Figure 4-14 shows BER and SNR change with laser line-width of 28Gb/s OFDM 16-QAM for transmission 50-km and 100-km. We can find that when the laser line-width exceeds 100 KHz, the SNR drops quickly.
Figure 4-13 Constellations and eye diagrams for OFDM 16QAM signals at PD Input Power=-8dBm.
(a)BTB (b) 25km (c) 50km (d)100km
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Figure 4-15 shows that the BER curves of 60-GHz 20-Gb/s 16-QAM OFDM signals using DFB laser. Note that the laser linewidth has more influence on 16-QAM signals than QPSK signals. As the transmission distance increases from 0 km to 100 km, the performance of 60-GHz 16-QAM OFDM signals gradually degrades due to de-coherence of optical un-modulated and OFDM-modulated sidebands. As
transmission distance is 100 km, the BER of 60-GHz 20-Gb/s 16-QAM OFDM signals can be below the FEC limit with receiver power of more than -6dBm. If the ECL with linewidth of 10kHz is utilized, no significant receiver power penalty is observed after the transmission of 100-km SSMF as shown in Fig. 4-16.
Figure 4-14 BER and SNR vs. Laser Line-Width for simulation 28Gb/s OFDM 16-QAM.
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Figure 4-15 BER curves of 60-GHz 20-Gb/s 16-QM OFDM signals. The linewidth is 10 MHz.
Figure 4-16 BER curves of 60-GHz 20-Gb/s 16-QM OFDM signals. The linewidth is 10 KHz.
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To provide higher data rate within 7-GHz license-free band at 60 GHz, multi-level OFDM is utilized. As higher spectral efficiency modulation format is used, the laser linewidth will play more and more important due to de-coherence of optical un-modulated and OFDM-modulated sidebands caused by fiber chromatic dispersion.
In this paper, we experimentally demonstrate that BERs of both 20-Gb/s 16-QAM and 14-Gb/s QPSK OFDM signals within 7-GHz license- free band are below the FEC limit following 100-km SMF transmission as DFB laser with the line-width of 10MHz is used.
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Chapter5 60GHz system
5.1 Introduction of frequency quadrupling scheme
In order to carry more and wider bandwidth signals, we have to utilize higher frequency system. And now I want to show you the concept of a frequency quadrupling 60GH system without using high speed components in transmitter terminal. It is used after I/Q modulator to up convert signals to 60GHz.
This part of the transmitter in the proposed system is frequency quadrupling, as shown in Figure 5-1. For simplicity, the figure shows only one optical input. Since MZ-a and MZ-b are biased at full point, the output spectrum are both double-sideband (DSB). The only difference between MZ-a and MZ-b is phase term. Compare to MZ-a, MZ-b has +π phase shift at +2ωRF term and -π phase shift at -2ωRF term. After that, a π phase shift is introduced at lower arm, And then combine the two arms. Finally, frequency quadrupling result is obtained.
Figure 5-1 Frequency quadrupling scheme
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5.2 Concept of the 60GHz system
Figure 5-2 schematically depicts the concept of the proposed 60-GHz OFDM RoF system employing all-optical up-conversion. The proposed 60-GHz OFDM transmitter consists of two dual-parallel modulators for photonic up-conversion and frequency quadrupling, respectively. For photonic up-conversion, OFDM I and Q signals are sent to MZ-a and MZ-b of the first dual-parallel modulator, respectively.
To achieve high optical modulation depth and to operate in E-field linear region of the MZM, both MZ-a and MZ-b are biased at the null point. To realize direct-detection OFDM signals, we insert an optical subcarrier as a remote heterodyne scheme by using single side band (SSB) modulation with carrier suppression. Therefore, the generated optical OFDM signal consisting of an un-modulated subcarrier and an OFDM-modulated carrier, which can be converted into electrical RF OFDM signals by square-law photo-diode (PD) detection, can be produced as shown in inset (a) of Fig.
5-2. Then, the generated OFDM signal is up-converted by using the frequency Figure.5-2 Conceptual diagram of the 60-GHz optical/wireless system using all-optical up-conversion.
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quadrupling technique as shown in inset (b) of Fig. 5-2. After an interleaver to filter out the unwanted sideband, frequency quintupling is achieved as shown in inset (c) of Fig. 5-2. Note that the proposed OFDM transmitter does not need any electrical mixer with a typical noise figure (NF) of more than 8 dB for up-conversion to 60 GHz. This is very important for highly spectral efficiency OFDM signals (i.e. 16-QAM and above) that requires higher signal-to-noise ratio (SNR). Additionally, the relative intensity between the un-modulated and OFDM-modulated subcarriers can be tuned by adjusting the individual power of the electrical sinusoidal and OFDM signals to optimize the performance of the optical RF signals.
5.3 Experimental setup
Figure.5-3 Experimental setup for proposed system.
Figure 5-3 depicts the experimental setup. The OFDM signals are generated by a Tektronix® AWG7102 arbitrary waveform generator (AWG) using a Matlab® program.
The sample rate and digital-to-analog converter resolution of the AWG are 10 GHz and
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8 bits, respectively. For 14Gb/s OFDM QPSK signals, the IFFT length is 64. A 156.25-MSym/s QPSK symbol is encoded at 45 channels (i.e. channels 3-25 and 42-63) with the remaining 9 channels set to zero. Therefore, an optical 14-Gb/s QPSK OFDM signal that has 45 subcarriers and occupies a total bandwidth of 7 GHz can be generated. For 20Gb/s OFDM 16QAM signals, the IFFT length is 64. A 156.25-MSym/s QPSK symbol is encoded at 32 channels (i.e. channels 3-18 and 48-63) with the remaining 32 channels set to zero. Therefore, an optical 20-Gb/s 16-QAM OFDM signal that has 32 subcarriers and occupies a total bandwidth of 5 GHz can be generated. For 28Gb/s OFDM QPSK signals, the IFFT length is 64. A 156.25-MSym/s QPSK symbol is encoded at 45 channels (i.e. channels 3-25 and 42-63) with the remaining 9 channels set to zero. Therefore, an optical 28-Gb/s QPSK OFDM signal that has 45 subcarriers and occupies a total bandwidth of 7 GHz can be generated. To realize optical direct-detection OFDM signal, a new optical subcarrier is generated at the lower sideband of the original carrier by 12 GHz. The generated OFDM signal is up-converted by using optical frequency quadrupling technique.
After a 50/100 GHz optical interleaver, OFDM signal at 60 GHz with frequency quintupling is generated at (c) of Fig. 5-3. A 100-km SMF is used to evaluate the transmission penalty of the system. After square-law PD detection, an electrical OFDM signal at 60 GHz is generated and down-converted to 5 GHz at (d) of Fig. 5-2.
The down-converted OFDM signal is captured by a Tektronix® DPO 71254 with a 50-GHz sample rate and a 3-dB bandwidth of 12.5 GHz. An off-line DSP program is employed to demodulate the OFDM signal. The demodulation process includes synchronization, Fast Fourier Transform (FFT), one-tap equalization, and QAM symbol decoding. The bit error rate (BER) performance is calculated from the measured error vector magnitude (EVM).
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5.3.1 Optimal condition for RF signals
The relative intensity between optical un-modulated and data-modulated subcarriers strongly influences the performance of the optical OFDM signals. One of the advantages of the proposed OFDM transmitter is that the relative intensity between optical un-modulated and data-modulated subcarriers can be easily tuned by adjusting the individual amplitude of the driving sinusoidal and OFDM signals to optimize the performance of the direct-detection OFDM signals, respectively. Figure 5-4 illustrates the BER of the QPSK OFDM signals versus different optical power ratios (OPR) of the un-modulated subcarrier to the OFDM-encoded subcarrier as optical powers of 60-GHz OFDM signals are normalized before detection. The optimal OPR is 0.4 dB.
Figure 5-4.BER vs. OPR for OFDM QPSK.
OPR=Pd/Ps(dB). Pd: the optical power of data-modulated optical carrier.
Ps: the optical power of un-modulated subcarrier.
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Figure 5-5.Optical Spectrums for OFDM QPSK.
(a)OPR=3dB (b) OPR=3dB after 4f-system (c) OPR=0.4dB
(d) OPR=0.4dB after 4f-system(e) OPR=-1.4dB (f) OPR=-1.4dB after 4f-system
Figure 5-5 shows the optical spectrums of different OPRs. We define OPR is
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optical power of un-modulated subcarrier over optical power of signals modulated optical carrier. And inset (a), (c), (e) of Figure 5-5 are OPR=3dB, OPR=0.4dB, OPR=-1.4dB before frequency quadrupling system, respectively. Inset (b), (d) and (f) illustrate OPR=3dB, OPR=0.4dB, OPR=-1.4dB after frequency quadrupling system and interleaver.
Figure 5-6 illustrates the BER of the 20Gb/s 16-QAM OFDM signals versus different optical power ratios (OPR) of the un-modulated subcarrier to the OFDM-encoded subcarrier as optical powers of 60-GHz OFDM signals are normalized before detection. The optimal OPR is 3 dB.
Figure 5-7shows the optical spectrums of different OPRs. And inset (a), (c), (e) of Figure 5-7 are OPR=1dB, OPR=3dB, OPR=5dB before frequency quadrupling system,
Figure 5-6.BER vs. OPR for 20Gb/s OFDM 16QAM.
OPR=Pd/Ps(dB). Pd: the optical power of data-modulated optical carrier.
Ps: the optical power of un-modulated subcarrier.
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respectively. Inset (b), (d) and (f) illustrate OPR=1dB, OPR=3dB, OPR=5dB after frequency quadrupling system and interleaver. We find that when signals bandwidth wider, the optimal OPR seems to be larger.
Figure 5-7.Optical Spectrums for 20Gb/s OFDM 16-QAM.
(a) OPR=1dB (b) OPR=1dB after 4f-system (c) OPR=3dB
(d) OPR=3dB after 4f-system(e) OPR=5dB (f) OPR=5dB after 4f-system
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Figure 5-8.BER vs. OPR for 28Gb/s OFDM 16QAM.
OPR=Pd/Ps(dB). Pd: the optical power of data-modulated optical carrier.
Ps: the optical power of un-modulated subcarrier.
Figure 5-8 shows the BER of the 20Gb/s 16-QAM OFDM signals versus different optical power ratios (OPR) of the un-modulated subcarrier to the OFDM-encoded subcarrier as optical powers of 60-GHz OFDM signals are normalized before detection.
The optimal OPR is 4 dB.
Figure 5-9 illustrates the optical spectrums of different OPRs. And inset (a), (c), (e) of Figure 5-7 are OPR=0dB, OPR=4dB, OPR=8dB before frequency quadrupling system, respectively. Inset (b), (d) and (f) illustrate OPR=0dB, OPR=4dB, OPR=8dB
Figure 5-9 illustrates the optical spectrums of different OPRs. And inset (a), (c), (e) of Figure 5-7 are OPR=0dB, OPR=4dB, OPR=8dB before frequency quadrupling system, respectively. Inset (b), (d) and (f) illustrate OPR=0dB, OPR=4dB, OPR=8dB