Chapter 4 Experimental demonstration of proposed system
4.4 Experimentl results for all optical I/Q up conversion system
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 after frequency quadrupling system and interleaver. We also find that when signals bandwidth wider, the optimal OPR seems to be larger. The reason is as the data-rate becomes larger, it needs higher SNR.
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Figure 5-9.Optical Spectrums for 28Gb/s OFDM 16-QAM.
(a)OPR=0dB (b) OPR=0dB after 4f-system (c) OPR=4dB
(d) OPR=4dB after 4f-system(e) OPR=8dB (f) OPR=8dB after 4f-system
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5.4 Experimental results and discussion
Figure 5-10 illustrates the electrical spectrums for QPSK OFDM signals and Figure 5-11. BER curves of the OFDM QPSK signals
Figure 5-10. Electrical Spectrums of QPSK OFDM signals.
(a) Tx OFDM and subcarrier electrical spectrum.
(b) OFDM electrical spectrum after I/Q modulator.
(c) Rx OFDM electrical spectrum.
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indicates where the spectrums belong to. Inset (a) of Figure 5-10 is the electrical spectrum of transmitter terminal OFDM signals and 12GHz-subcarrier. Inset (b) of Figure 5-10 is the one of OFDM signals after I/Q modulator. We can see the bandwidth is 7GHz. And Inset (c) of Figure 5-10 is the electrical spectrum of receiver QPSK OFDM signals. We down convert the signals to center frequency 5GHz.
Figure 5-12 shows QPSK constellation diagrams before and after the frequency quadrupling system in back-to-back (BTB) and following SMF transmission cases.
The equalizer in OFDM transceiver is used to combat both frequency response of various millimeter-wave components at 60 GHz and fiber dispersion. Since the proposed OFDM transmitter can generate high-purity two-tone lightwave, the generated OFDM signals do not suffer from periodic fading issue due to fiber dispersion. Only in-band distortion of the OFDM-encoded subcarrier induced by fiber dispersion is considered. Since the symbol rate of each subcarrier is only 156.25 MSym/s, the fiber chromatic penalty can be ignored. Figure 5-11 shows the BER curves of the 14-Gb/s QPSK OFDM signals using optimal OPRs after transmission over 100-km SMF. After transmission 100-km, the sensitivity penalties is about 1dB.
Compare purple circular symbols with red diamond symbols, the penalty of frequency quadrupling system is 2dB. Although it still worsens the performance a little, it does better than using electrical mixer with noise figure (NF) 8dB. Inset (a), (b), (c) and (d) of Figure 5-12 are BTB before frequency quadrupling, BTB, 50km and 100km, respectively. They are captured at PD input power equal to -11dBm.
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Figure 5-12. Constellations of QPSK OFDM signals (-11dBm).
(a) BTB before 4f (b) BTB (c) 50km (d) 100km
Figure 5-13. Electrical Spectrums of 20Gb/s 16-QAM OFDM signals.
(a) Tx OFDM and subcarrier electrical spectrum.
(b) Rx OFDM electrical spectrum.
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Figure 5-13 illustrates the electrical spectrums for 20Gb/s 16-QAM OFDM signals and indicates where the spectrums belong to. Inset (a) of Figure 5-13 is the electrical spectrum of transmitter terminal OFDM signals and 12GHz-subcarrier. Inset (b) of Figure 5-13 is the electrical spectrum of receiver 16-QAM OFDM signals. The bandwidth is 5GHz. We down convert the signals to center frequency 5GHz.
Figure 5-15 shows 16-QAM constellation diagrams after the frequency quadrupling system in back-to-back (BTB) and following SMF transmission cases.
The equalizer in OFDM transceiver is used to combat both frequency response of various millimeter-wave components at 60 GHz and fiber dispersion. Since the proposed OFDM transmitter can generate high-purity two-tone lightwave, the generated OFDM signals do not suffer from periodic fading issue due to fiber dispersion. Only in-band distortion of the OFDM-encoded subcarrier induced by fiber dispersion is considered.
Figure 5-14. BER curves of the 20Gb/s OFDM 16-QAM signals using DFB Laser.
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Figure 5-15. Constellations of 20Gb/s 16-QAM OFDM signals using DFB Laser (-5dBm). (a) BTB (b) 25km (c) 50km (d) 75km (e) 100km
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Since the symbol rate of each subcarrier is only 156.25 MSym/s, the fiber chromatic penalty can be ignored. Figure 5-14 shows the BER curves of the 20-Gb/s 16-QAM OFDM signals using optimal OPRs and DFB Laser after transmission over 100-km SMF. After transmission 100-km, the sensitivity penalties is very large. The reason to explain large penalty is laser line-width. Following pages will show you a better result with smaller line-width light source. Inset (a), (b), (c), (d) and (d) of Figure 5-15 are BTB, 25km, 50km, 75km and 100km, respectively. They are captured at PD input power equal to -5dBm.
Figure 5-16. Electrical Spectrums of 20Gb/s 16-QAM OFDM signals with tunable laser (TL).
(a)Tx OFDM and subcarrier electrical spectrum.
(b)Rx OFDM electrical spectrum.
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Figure 5-16 illustrates the electrical spectrums for 20Gb/s 16-QAM OFDM signals with tunable laser and indicates where the spectrums belong to. Inset (a) of Figure 5-16 is the electrical spectrum of transmitter terminal OFDM signals and 12GHz-subcarrier. Inset (b) of Figure 5-16 is the electrical spectrum of receiver 16-QAM OFDM signals. The bandwidth is 5GHz. We down convert the signals to center frequency 5GHz.
Figure 5-18 shows 16-QAM constellation diagrams after the frequency quadrupling system in back-to-back (BTB) and following SMF transmission cases.
The equalizer in OFDM transceiver is used to combat both frequency response of various millimeter-wave components at 60 GHz and fiber dispersion. Since the proposed OFDM transmitter can generate high-purity two-tone lightwave, the generated OFDM signals do not suffer from periodic fading issue due to fiber dispersion. Only in-band distortion of the OFDM-encoded subcarrier induced by fiber Figure 5-17. BER curves of the 20Gb/s OFDM 16-QAM signals using Tunable Laser.
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dispersion is considered. Since the symbol rate of each subcarrier is only 156.25 MSym/s, the fiber chromatic penalty can be ignored. Figure 5-17 shows the BER curves of the 20-Gb/s 16-QAM OFDM signals using optimal OPRs and DFB Laser after transmission over 100-km SMF. After transmission 100-km, the sensitivity penalties is almost negligible. The two cases only have one difference, laser line-width.
We replace DFB Laser with Tunable Laser to reduce light source line-width. Inset (a), (b), (c), (d) and (d) of Figure 5-18 are BTB, 25km, 50km, and 100km, respectively.
They are captured at PD input power equal to -5dBm.
Figure 5-18. Constellations of 20Gb/s 16-QAM OFDM signals using Tunable Laser (-5dBm).
(a) BTB (b) 25km (c) 50km (d) 100km
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Figure 5-19 illustrates the electrical spectrums for 28Gb/s 16-QAM OFDM Figure 5-19. Electrical Spectrums of 28Gb/s 16-QAM OFDM signals with tunable laser (TL).
(a) Tx OFDM and subcarrier electrical spectrum.
(b) Rx OFDM electrical spectrum.
Figure 5-20. BER curves of the 28Gb/s OFDM 16-QAM signals using Tunable Laser.
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signals with tunable laser and indicates where the spectrums belong to. Inset (a) of Figure 5-19 is the electrical spectrum of transmitter terminal OFDM signals and 12GHz-subcarrier. Inset (b) of Figure 5-19 is the electrical spectrum of receiver 16-QAM OFDM signals. The bandwidth is 7GHz. We down convert the signals to center frequency 5GHz.
Figure 5-21 shows 16-QAM constellation diagrams before and after the one-tap equalizer in back-to-back (BTB) and following SMF transmission cases. The equalizer in OFDM transceiver is used to combat both frequency response of various millimeter-wave components at 60 GHz and fiber dispersion. Since the proposed OFDM transmitter can generate high-purity two-tone lightwave, the generated OFDM signals do not suffer from periodic fading issue due to fiber dispersion. Only in-band distortion of the OFDM-encoded subcarrier induced by fiber dispersion is considered.
Since the symbol rate of each subcarrier is only 156.25 MSym/s, the fiber chromatic
Since the symbol rate of each subcarrier is only 156.25 MSym/s, the fiber chromatic