Simulation comparison - The Theoretical Simulations of Proposed System

Chapter 3 The Theoretical Simulations of Proposed System

3.7 Simulation comparison

First, in 3.2, 3.3, 3.4 prat, the OFDM signals are also located at 60-GHz millimeter-wave band but the bandwidth of the OFDM signals are 3.125G, 5G and 7G respectively. Compared with the three parts, the difference between the three parts is bandwidth of OFDM signal. From the three parts, when the bandwidth of OFDM signal increases, the SNR and BER get worse as shown in figure 3-1, 3-3, 3-5 and the constellation gets blured as shown in figure 3-2, 3-4, 3-6.

Second, in 3.4, 3.5, 3.6 part, the bandwidth of OFDM signals are all 7G but the OFDM signals are located at 60-GHz, 40-GHz, 20-GHz millimeter-wave band respectively. Compared with the three parts, the difference between the three parts is millimeter-wave band which is the OFDM signal located. The SNR and BER is almost the same as shown in figure 3-5, 3-7, 3-9 and the constellations are clear as shown in figure 3-6, 3-8, 3-10.

Chapter 4 Experimental Results of Proposed System for Single Carrier

4.1 preface

In chapter 3, we provide the simulation for the concept of the proposed system. Therefore, the result can be tried to apply to the radio-over-fiber system. In the chapter, we will build the experimental setup for the proposed system. In the proposed system, we use tunable laser and distributed feedback (DFB) laser to fit the linewidth of the laser source because the linewidth of tunable laser and DFB laser are located at the range of several KHz and several MHz respectively.

4.2 Experimental results of TSSB 4.2.1 Experiment setup

Figure 4-1 displays the experimental setup of proposed system. Tunable lasers and DFB lasers are used to be the optical source. The OFDM signal with 88 carriers is gnerated from an arbitrary waveform generator (AWG). Every subcarrier is encoded with 78.125MHz QPSK symbol. The inverse fast Fourier transform (IFFT) size is 256.

Because the sampling rate is 20GHz, so the total bandwidth of OFDM signal is (20GHz/256)*88=6.875GHz (about 7GHz). The OFDM signal is up-converted to 30GHz using an electrical mixer. The OFDM signal is generated on BB, the lower sideband and upper sideband of the up-converted electrical signal are not mutually independent. Because every subcarrier is demodulated independently,the limit between the subcarriers’ symbols has negligible effect on the system performance.

· LD:Laser Diode

· AWG:Arbitrary Waveform Generator

· EDFA:Erbium-doped Fiber Amplifier

· SSMF:Standar Single Mode Fiber

· FBG:Fiber Bragg Grating

· PD:Photodetector

· BPF:Bandpass Filter

· RSOA:Reflective Semiconductor Optical Amplifier

· LPF:Low-pass Filter

· BERT:Bit-error-rate Tester

Figure 4-1 shows the experimental setup of the proposed system

The up-converted 30-GHz RF OFDM signal with 88 subcarriers,the total

bandwidth of the signal is about 7-GHz, and the data rate is about 14-Gb/sec. With a 90⁰ hybrid coupler, the 30-GHz RF OFDM signal is split two paths. A 30-GHz

sinusoidal signal is also split two paths using 90⁰ hybrid coupler. As Figure 4-1 shows, the upper 30-GHz RF OFDM signal combines with the upper 30-GHz sinusoidal signal and the lower 30-GHz RF OFDM signal combines with the lower 30-GHz sinusoidal signal. These combined signals are amplified to drive the dual-drive MZM. The MZM is biased at quadrature point to achieve SSB modulation scheme. At the output of the MZM, an EDFA is employed to boost the total optical power. After the EDFA, the optical power is transmitted through the SSMF, At the output of the SSMF, the circulator is employed to separate the original optical carrier and the other signals. At 2 port of the circulator, the FBG is utilized to eliminate the optical carrier. At remote node, the 60-GHz RF OFDM signal is obtained from the beating signal of the OFDM optical sideband and new optical carrier at a 60-GHz photo diode. A bandpass filter (BPF) with center frequency 60-GHz is employed to filter out the 60-GHz electrical OFDM signal. The 60-GHz OFDM signal is down-converted to 5-GHz using an electrical mixer with 55-GHz LO signal.

The down-converted 5-GHz OFDM signal is sent to a real time scope to capture the time domain waveform. The OFDM signal is demodulated with off-line digital signal processing (DSP) program. The demodulation process includes synchronization, Fast Fourier Transform (FFT), one-tap equalization, and QPSK symbol decoding. The BER is obtained from the measured error vector magnitude (EVM). At 3 port of the circulator, the original optical carrier is utilized for uplink application with RSOA and the electrical signal 1.25-Gb/s OOK with generating from a pattern generator. After RSOA and SSMF, the uplink signal is received by a low-speed photo diode. After a LPF, the BER of the uplink signal is obtained directly using bit error rate tester (BERT).

4.3 Experimental results of TSSB using single carrier QPSK 4.3.1 Optimal condition of single carrier QPSK

-8 -6 -4 -2 0 2

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55 1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55 1554.4 1554.6 1554.8 1555.0 1555.2 1555.4

-55

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55 1554.4 1554.6 1554.8 1555.0 1555.2 1555.4

-55

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55

Figure 4-2 OPR curve, optical spectrums and constellations of optimal condition with single carrier QPSK

In this part , the signal is single carrier QPSK with data rate 2.5Gb/s. The optical power ratio (OPR) is power of clock over QPSK signal. The OPR is a very important factor which effects the receiver sensitivity of signal. Figure 4-2 shows the BER versus OPR curve, optical spectrums and constellations. In optical spectrum, the left sideband is single carrier QPSK signal and the right sidband is clock signal. The original optical carrier is removed by FBG. Figure 4-2 (a) and (b) shows the optical spectrums and constellations when the OPR is -8. In optical spectrum, the left sideband is single carrier QPSK signal and the right sidband is clock signal. The original optical carrier is removed by FBG. In figure 4-2, the BER of blue curve is better than green curve because the

feed-forward equalizer (FFE) with 16 taps is utilized here. Figure 4-2 (c) and (d) shows the optical spectrums and constellations when the OPR is 0. And the same, figure 4-2 (e) and (f) shows the optical spectrums and constellations when the OPR is 1. In optical spectrum, the left sideband is single carrier QPSK signal and the right sidband is clock signal. The original optical carrier is removed by FBG. In the figure, the best receiver sensitivity is obtained when the OPR is 0 and the constellation is very clear.

4.3.2 Transmission results

-16 -15 -14 -13 -12

10 9 8 7 6 5 4 3 2

QPSK_BTB_with FFE QPSK_BTB_w/o FFE QPSK_50km_With FFE QPSK_50km_w/o FFE

-L o g (B E R )

Power(dBm)

Figure 4-3 BER curves of single carrier QPSK

Figure 4-3 shows the curve of receiver power versus the BER. The blue curve is back-to-back (BTB) case, and the red curve is transmission length with 50-km SSMF. The full symbol represents the FFE is utilized, and the hollow symbol represents the FFE is not used. In figure 4-3, when the receiver sensitivity is -12 dBm, the order of BER can achieve to -10. The order of BER is improved from 10⁻¹⁰ to 10⁻⁴ with -8 dBm optical power. The receiver power penalty at BER of 10⁻¹⁰ with FFE (10⁻⁴ without FFE) is less than 0.2 dB at BTB case and 50-km SSMF transmission case.

BTB With FFE Without FFE

50km With FFE Without FFE

1 5 5 4 .6 1 5 5 4 .8 1 5 5 5 .0 1 5 5 5 .2 1 5 5 5 .4

Figure 4-4 optical spectrums and constellations of single carrier QPSK

Figure 4-4 shows optical spectrums and constellations with receiver power -12 dBm. For (a), it is BTB case. The left is optical spectrums, and the middle and the right are constellations. In optical spectrums, the left sideband is QPSK signal, the right sideband is clock signal, the original optical carrier is removed by FBG. The middle constellations is with FFE compensated the uneven frequency responses. The right constellations is without FFE. For (b), it is 50-km SSMF transmission case. The 50-km SSMF transmission case is similar to the BTB case. The left is optical spectrums, and the middle and the right are constellations. In optical spectrums, the left sideband is single carrier QPSK signal, the right sideband is clock signal, the original optical carrier is removed by FBG. The middle constellations is with FFE, and The right constellations is

After AWG Upconvert to 30G

27 28 29 30 31 32 33

Figure 4-5 electrical spectrums of single carrier QPSK

Figure 4-5 shows electricdal spectrums. (a) is the electrical spectrum after an AWG. (b) is the QPSK signal up-converted to 30-GHz with a electrical mixer. (c) is the RF QPSK signal down-converted to 2.5-GHz with a electrical mixer.

4.4 Experimental results of TSSB using single carrier 8QAM 4.4.1 Optimal condition of single carrier 8QAM

-6 -4 -2 0 2 4 6 8QAM_Without FFE

EVM(%)

OPR

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55 1554.4 1554.6 1554.8 1555.0 1555.2 1555.4

-55

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55 1554.4 1554.6 1554.8 1555.0 1555.2 1555.4

-55

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55

Figure 4-6 OPR curve, optical spectrums and constellations of optimal condition with single carrier 8QAM

In this part , the signal is single carrier 8QAM with data rate 2.34375Gb/s.

The OPR is power of clock over 8QAM signal. The OPR is a very important factor which effects the receiver sensitivity of signal. Figure 4-6 shows the error vector magnitude (EVM) versus OPR curve, optical spectrums and constellations. The EVM is defined as

2 1 2

4-6, the EVM of blue curve is better than green curve because the FFE with 16 taps is utilized here. Figure 4-6 (c) and (d) shows the optical spectrums and constellations when the OPR is -1. In optical spectrum, the left sideband is single carrier 8QAM signal and the right sidband is clock signal. The original optical carrier is removed by FBG. And the same, figure 4-6 (e) and (f) shows the optical spectrums and constellations when the OPR is -5. In optical spectrum, the left sideband is single carrier 8QAM signal and the right sidband is clock signal. The original optical carrier is removed by FBG. In the figure, the best receiver sensitivity is obtained when the OPR is 0 and the constellation is very clear.

4.4.2 Transmission results

-20 -18 -16 -14 -12 -10 -8 10

15 20 25 30 35 40

45

BTB with FFE

BTB w/o FFE 50km with FFE 50km w/o FFE

E V M (% )

POWER(dBm)

Figure 4-7 EVM curves of single carrier 8QAM

Figure 4-7 illustrates the curve of receiver power versus the EVM. The blue curve is BTB case, and the red curve is transmission length with 50-km.

SSMF. The full symbol represents the FFE is utilized, and the hollow symbol represents the FFE is not used. In figure 4-7, Before FFE, there is an EVM fluctuation due to receiver saturation with the increasing of the received optical power. However, the FFE can compensate not only the uneven frequency response but also the receiver saturation. After the FFE, the EVM is improved from 18.9% to 15.7 % with -8 dBm optical power. The power penalty at EVM of 15% with FFE (18% without FFE) is less than 0.2 dB at BTB case and 50-km SSMF transmission case.

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -55

1554.4 1554.6 1554.8 1555.0 1555.2 1555.4 -70

Figure 4-8 optical spectrums and constellations of single carrier 8QAM

Figure 4-8 shows optical spectrums and constellations with receiver power -8 dBm. For (a), it is BTB case. The left is optical spectrums, the middle and the right are constellations. In optical spectrums, the left sideband is 8QAM signal, the right sideband is clock signal, the original optical carrier is removed by FBG. The middle constellations is with FFE compensated the uneven frequency responses. The right constellations is without FFE. For (b), it is 50-km SSMF transmission case. The 50-km SSMF transmission case is similar to the BTB case. The left is optical spectrums, and the middle and the right are constellations. In optical spectrums, the left sideband is 8QAM signal, the right sideband is clock signal, the original optical carrier is removed by FBG. The middle constellations is with FFE, and The right constellations is without FFE.

After AWG Upconvert to 30G Downconvert to 2.5G

1 2 3 4

Figure 4-9 electricdal spectrums of single carrier 8QAM

Figure 4-9 shows electricdal spectrums. (a) is the electrical spectrum after an AWG. (b) is the 8QAM signal up-converted to 30-GHz with a electrical mixer. (c) is the RF 8QAM signal down-converted to 2.5-GHz with a electrical mixer.

4.5 Experimental results of TSSB for uplink with OOK using single carrier QPSK as downlink signal

4.5.1 Uplink with OOK using single carrier QPSK as downlink signal

Downlink_50km-Uplink_0km (BTB)

Downlink_50km-Uplink_50km (50km)

PD_Input_-18.5dBm_130mv/div;200ps/div

-29 -28 -27 -26 -25 109

8 7 6 5 4 3

2 BTB

50km

-Log(BER)

Power(dBm)

(b) (c)

(a)

Figure 4-10 BB BER curves and eye diagrams of uplink with OOK using single carrier QPSK as downlink signal

Figure 4-10 shows BB BER curves and eye diagrams of uplink 1.25-Gb/s OOK signal for downlink single carrier QPSK signal. In (a), the full symbol means BTB case. The hollow symbol means 50-km SSMF case. The order of BER can achieve to 10⁻¹⁰ when the receiever optical power is -25.2 dBm.

After 50-km SSMF transmission, there is no significant receiver power penalty of the OOK uplink signal. Figure 4-10 (b) is BTB case, and (c) is 50-km SSMF transmission case. These two eye diagrams are very clear. The eye diagrams are catched by digital communications analyzer (DCA) when the input power is -18.5 dBm. The vertical scale of these two eye diagrams is 130mv/div, and the horizontal scale of these two eye diagrams is 200ps/div.

Before Circulator After FBG

Original optical carrier without uplink signal

Original optical carrier with uplink signal

Figure 4-11 optical spectrums of uplink with OOK using single carrier QPSK as downlink signal

Figure 4-11 shows optical spectrums. In (a), it is at the output of the MZM case. The left sideband is single carrier QPSK signal, the middle sideband is original optical carrier, and the right sideband is clock signal. In (b), The left sideband is single carrier QPSK signal, and the right sideband is clock signal.

The original optical carrier is removed by FBG. In (c), the middle sideband is original optical carrier without 1.25Gb/s OOK signal. The original optical carrier is separated from the other optical sidebands by using circulator and FBG. In (d), it is original optical carrier with 1.25Gb/s OOK signal for uplink application using RSOA.

4.6 Experimental results of TSSB for uplink with OOK using single carrier 8QAM as downlink signal

4.6.1 Uplink with OOK using single carrier 8QAM as downlink signal

Downlink_50km-Uplink_0km (BTB)

Downlink_50km-Uplink_50km (50km)

PD_Input_-18.5dbm_130mv/div;200ps/div

-29 -28 -27 -26 -25 109

8 7 6 5 4 3

2 BTB

50km

-Log(BER)

Power(dBm)

(a) (b) (c)

Figure 4-12 the BB BER curves and eye diagrams of uplink with OOK using single carrier 8QAM as downlink signal

Figure 4-12 shows BB BER curves and eye diagrams of uplink 1.25-Gb/s OOK signal for downlink single carrier 8-QAM signal. In (a), the full symbol means BTB case. The hollow symbol means 50-km SSMF case. The order of BER can achieve to 10⁻¹⁰ when the receiever optical power is -25.4 dBm.

After 50-km SSMF transmission, there is no significant receiver power penalty of the OOK uplink signal. Figure 4-10 (b) is BTB case, and (c) is 50-km SSMF transmission case. These two eye diagrams are very clear. The eye diagrams are catched by DCA when the input power is -18.5 dBm. The vertical scale of these two eye diagrams is 130mv/div, and the horizontal scale of these two eye diagrams is 200ps/div.

Before Circulator After FBG

Original optical carrier without uplink signal

Original optical carrier with uplink signal

Figure 4-13 optical spectrums of uplink with OOK using single carrier 8QAM as downlink signal

Figure 4-13 shows optical spectrums. In (a), it is at the output of the MZM case. The left sideband is single carrier 8QAM signal, the middle sideband is original optical carrier, and the right sideband is clock signal. In (b), The left sideband is single carrier 8QAM signal, and the right sideband is clock signal.

Chapter 5 Experimental Results of Proposed System for OFDM

5.1 Experimental results of Proposed System for OFDM QPSK using tunable laser

5.1.1 Optimal condition of OFDM QPSK using tunable laser

-2 0 2 4 6 8 10 12

4 3

-Log(BER)

OPR

1554.6 1554.8 1555.0 1555.2 1555.4 -65

Figure 5-1 shows OPR curve and optical spectrum for OFDM QPSK

In this part , the signal is OFDM QPSK and the laser source is tunable laser. The OFDM signal with 88 carriers, every subcarrier is encoded with 78.125MHz QPSK symbol. The IFFT size is 256. Because the sampling rate is 20GHz, so the total bandwidth of OFDM signal is (20GHz/256)*88=6.875GHz (about 7GHz). In this case, the modulation intensitity (MI) is V_PP over 2V_π (V_π is about 5 , V_PP is power of clock signal), and the MI is 0.11. The OPR is

power of clock over OFDM QPSK signal. The OPR is a very important factor which effects the receiver sensitivity of signal. Figure 5-1 shows the BER versus OPR curve, optical spectrums and constellations. Figure 5-1 (a) shows the optical spectrums and constellations when the OPR is -1. Figure 5-1 (b) shows the optical spectrums and constellations when the OPR is 6. And the same, figure 5-1 (c) shows the optical spectrums and constellations when the OPR is 11. As the figure depicts, the left sideband of the optical spectrum is OFDM QPSK signal. The right sideband of the optical spectrum is clock signal.

The original optical carrier is removed by FBG. In the figure, the best receiver sensitivity is obtained when the OPR is 6.

5.1.2 Transmission results

- 8 .0 - 7 .5 - 7 .0 - 6 .5 - 6 .0 5

4 3 2

B T B 2 5 k m 5 0 k m

-Log(BER)

P o w e r ( d B m )

BTB

25km

50km

(a) (b)

(c)

(d)

Figure 5-2 the BER curves and constellations for OFDM QPSK

The figure 5-2 shows BER curve of the OFDM QPSK signal using tunable laser.

In (a), the BER can achieve to 10⁻⁵ at both BTB case, 25km and 50km SSMF transmission case when optical power is at -6 dBm. The receiever power at 50km SSMF transmission is less then 0.2 dBm. There is no significant penalty of receiver power after 50km SSMF transmission. The figure 5-1 (b) is constellation of BTB case.

The figure 5-1 (b) and (c) are 25km and 50km SSMF transmission case. The constellations of BTB, 25km, 50km case are clear.

Figure 5-3 shows the electrical spectrum for OFDM QPSK

Figure 5-3 shows the electrical spectrums. Figure 5-3 (a) is electrical spectrum of BB OFDM signal which is generated from the AWG, and the total carriers are 44. The bandwidth is 3.4375-GHz. Figure 5-3 (b) is electrical spectrum of up-converted OFDM signal with a electrical mixer and 30-GHz RF LO signal, and the total carriers are 88. The bandwidth of up-converted OFDM signal is 6.875-GHz. Figure 5-3 (c) is electrical spectrum of down-converted OFDM signal with a electrical mixer and 55-GHz RF LO signal. Due to the performance of the 60-GHz waveguide components, uneven channel response is observed at the down-converted signal. Nevertheless, the bandwidth of each carrier of the OFDM signal is low and one-tap equalization is utilized. The uneven channel response can be easily compensated.

5.2 Experimental results of Proposed System for OFDM QPSK using tunable laser at different MI

5.2.1 Optimal condition of OFDM QPSK using tunable laser at different MI

1554.6 1554.8 1555.0 1555.2 1555.4 -65

Figure 5-4 shows OPR curves and optical spectrums of OFDM QPSK at different MI

Figure 5-4 shows OPR curves and optical spectrums of OFDM QPSK signal for tunable laser at different MI. The MI means V_PP over 2V_π (V_π is about 5 , V_PP is power of clock signal). The OFDM signal with 88 carriers, every subcarrier is encoded with 78.125MHz QPSK symbol. The IFFT size is 256. Because the sampling rate is 20GHz, so the total bandwidth of OFDM QPSK signal is (20GHz/256)*88=6.875GHz (about 7GHz). The OPR is power

of clock over power of OFDM QPSK signal. In figure 5-4 (a) shows the optical spectrum and constellation when the MI is 0.126 and the OPR is-1. In optical spectrum, the left sideband is OFDM QPSK signal and the right sidband is clock signal. The original optical carrier is removed by FBG. As the same, (b) and (c) are optical spectrums and constellations when MI is 0.126 and OPR are

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