Modulation System with Frequency Sextupling

There are three main drawbacks of the frequency doubling method discussed ear-lier. Firstly, the electrical mixer will significantly degrade the signal quality. Secondly the modulator is not operated at linear E-field region. For an OFDM signal which is sensitive to PAPR, this considerably limits the modulation index. Thirdly, at trans-mitter, expensive components with frequency up to 35 GHz are needed. To overcome these issues, a new modulation scheme with frequency sextupling is proposed in this section.

5.3.1 Concept

Figure 5.8 shows the conceptual diagram of the 60-GHz vector signal genera-tion system with frequency sextupling. The proposed system is composed of two up-conversion stages with two dual-parallel Mach-Zehnder modulators (DP-MZM). The first DP-MZM is utilized in the first stage to generate an optical single sideband with carrier suppression (SSB-CS) signal [71]. A local oscillator (LO) and an intermediate frequency (IF) vector data signal are combined to form a composite driving signal.

After the first DP-MZM, an LO optical carrier is generated at the lower sideband and a data-modulated OFDM signal is generated at the upper sideband, as shown in the inset (i) of FIG. 5.8. Notably, frequency doubling is achieved after the first stage.

The second stage is an optical up-conversion system which is composed of the wavelength-independent optical millimeter-wave generation system with frequency quadrupling along with an optical interleaver. After the optical up-conversion, both the LO and data-modulated optical sidebands are up-converted with frequency four times that of the up-conversion system driving signal, as shown in inset (ii) of FIG.

Ch. N

FIG. 5.8. Conceptual diagram of the 60-GHz vector signal generation system with frequency sextupling.

5.8. Then, the optical interleaver is employed to select the desired optical sideband and a modified SSB signal with frequency sextupling is obtained, as shown in inset (iii) of FIG. 5.8. Based on the modified SSB modulation scheme, high spectral efficiency vec-tor signals, such as M-PSK, QAMand OFDM can be transmitted without dispersion-induced fading performance degradation. Since the LO and data-modulated optical sidebands are generated from two individual driving signals, the receiver performance can be optimized by controlling the OPR between two optical sidebands. Because of the frequency sextupling using optical up-conversion system, signal processing can be handled using 10-GHz components with higher reliability and lower costs. Moreover, wavelength-division-multiplexed (WDM) up-conversion can be realized based on the wavelength independent optical up-conversion system.

Figure 5.9 schematically depicts the principle of the RoF system. Since the sub-MZMs in the first stage (ie. MZ1-a, and MZ1-b) are biased at null point, DSB-CS modulation schemes will be generated. The driving signals in the first stage are the combination of LO and OFDM signals, and90^◦ phase delays are added on the lower-arm LO signal and upper-lower-arm OFDM signal. Optical spectra after the modulation of MZ1-a and MZ1-b will be obtained as shown in inset (ii) and (iii) of FIG. 5.9. The

main MZM of the first stage is biased at quadrature point, and a90^◦ phase difference is introduced between the output of MZ1-a and MZ1-b. At the output of the first DP-MZM, a modified SSB signal will be obtained as shown in inset (v) of FIG. 5.9. Then, the 20-GHz modified SSB signal is sent into the second stage. Since the sub-MZMs in the second DP-MZM (ie. MZ2-a and MZ2-b) are biased at the full point, DSB modulation scheme will be obtained. Because of a 90^◦ phase delay which is added on the lower-arm of the second-stage driving signal, optical spectra as shown in inset (vi) and (vii) will be obtained. Since the main MZM of the second DP-MZM is biased at the null point, a180^◦ phase difference is introduced between the output of MZ2-a and MZ2-b. The combined output signal of the second DP-MZM will have the optical spectrum shown in inset (ix) of FIG. 5.9. Then, an optical interleaver is utilized to select the desired optical sidebands. A 60-GHz modified SSB signal as shown in inset (x) of FIG. 5.9 will be obtained at the output of the optical interleaver.

5.3.2 Experimental Setup

Figure 5.10 shows the experimental setup of the proposed 60-GHz RoF system with frequency sextupling. A distributed-feedback (DFB) laser is utilized as the

opti-Laser ^/2

MZ1-a

MZ1-b Interleaver

(i)

(iii) (ii)

(iv)

(v)

(ix)

(x)

60 GHz (vii)

(viii) (vi)

 MZ2-a MZ2-b

(x) 20 GHz

FIG. 5.9. The principle of the proposed RoF system with frequency sextupling.

cal source. In the first stage, the sub-MZMs (MZ1-a and MZ1-b) of the first DP-MZM are biased at the null point, while the main MZM is biased at the quadrature point. The OFDM signals are generated at base band (BB) using an arbitrary waveform generator (AWG, TektronixAWG7102) with the following parameters: the digital to analog^R converter (DAC) sampling rate is 20 GHz; inverse-fast-Fourier transform (IFFT) size is 256; subcarrier frequency separation is 78.125 MHz; 44 subcarriers are generated at BB and the bandwidth is 3.4375 GHz. The BB OFDM signal is then up-converted us-ing an electrical mixer driven with a 10-GHz LO signal, resultus-ing in a 6.875-GHz-wide OFDM signal with 88 sub-carriers centered at 10-GHz. Restricted by the bandwidth of the AWG, the 7-GHz OFDM signals are emulated using a DSB OFDM signal. Al-though both sidebands of the OFDM signals are related after electrical up-conversion, the OFDM sub-carriers are transmitted independently (un-correlated) and they are de-modulated independently at the receiver. Each sub-carrier experiences different am-plitude and phase response from the transmission. Therefore, there was no correlation between sub-carriers in two sidebands of the recovered OFDM signal.

10GHz

1547.5 1547.6 1547.7 1547.8 1547.9 -70

1547.2 1547.4 1547.6 1547.8 1548.0 1548.2 -40

1547.2 1547.4 1547.6 1547.8 1548.0 1548.2 -70

FIG. 5.10. Experimental setup of the frequency sextupling system.

As a result the 88 sub-carriers occupying the 6.875-GHz band all together repre-sent the total throughput of the system, which was 13.75 and 20.625 Gb/s for OPSK-OFDM and 8QAM-OPSK-OFDM, respectively. The 10-GHz LO signal was generated by an RF signal generator. Both the 10-GHz OFDM signal and the 10-GHz LO signal are divided into two paths (upper path and lower path). To generate the modified SSB signal using the SSB-CS modulation scheme, a 90-deg phase delay is added on the upper path of the OFDM signal and the lower path of the 10-GHz sinusoid. The upper path and lower path signals are combined and fed into the DP-MZM as the driving signals. At the output of the first stage, a 20-GHz modified SSB signal is obtained as shown in inset (a) of FIG. 5.10.

Then, the 20-GHz modified SSB signal is fed into the second stage for optical up-conversion. After the second DP-MZM, both the LO and data-modulated optical side-bands of the 20-GHz modified SSB signal are up-converted as shown in the optical spectrum in inset (b) of FIG. 5.10. To generate the 60-GHz modified SSB OFDM signal, an optical interleaver is utilized to select the desired sidebands, as shown in inset (c) of FIG. 5.10. After transmission over 25-km SSMF, the optical signal is received using a V-band photo diode. The received 60-GHz OFDM signal is down-converted to 5-GHz using an electrical mixer driven by a 55-GHz local LO signal.

The down-converted IF frequency of the signal was chosen to fit the bandwidth of IF amplifier, the bandwidth of the real-time digital oscilloscope and the IFFT size of the OFDM signal. The down-converted signal waveforms are captured by a real-time digital oscilloscope for off-line demodulation and analysis using the Matlab program.

5.3.3 Experimental Results and Discussion

By controlling the intensity of the driving signals, the OPR can be freely adjusted in the proposed modified SSB system. FIG. 5.11 shows the bit error rate (BER) versus OPR curve and related constellation diagrams of the QPSK-OFDM signal. The OPR

-2 0 2 4 6 8 10 12

11 10 9 8 7 6

-log(BER)

OPR (dB) (a)

(b)

(c)

(a)

(b) (c)

FIG. 5.11. Bit error rate versus optical power ratio curve and constellation diagrams of the QPSK-OFDM signal,and constellations with (a) -1-dB , (b) 6-dB , and (c) 9-dB OPR.

here is defined as the LO optical sideband power to data-modulated optical sideband power ratio. The OPR was measured using an optical spectrum analyzer with 0.01-nm resolution.

For QPSK-OFDM modulated signals the best receiver performance was obtained when the OPR was equal to +6 dB shown in FIG. 5.11. FIG. 5.12 shows the EVM versus OPR curve and related constellation diagrams of the 8QAM-OFDM signal.

The optimal OPR for 8-QAM-OFDM modulated signals was found to be +5-dB as shown in FIG. 5.12. Compared with the conventional single carrier system which has 0-dB optimal OPR [70], the optimal OPR in the proposed OFDM systems was found to be higher.

Figure 5.13 shows the BER curves of the transmission result of the QPSK-OFDM signal. After transmission over 25-km SSMF, the received power penalty was negligi-ble. The constellation diagrams at back to back (BTB) and after fiber transmission are also shown in insets of FIG. 5.13. After fiber transmission, no significant distortion is observed in the constellation diagrams. FIG. 5.14 shows the EVM curves of the transmission result of the 8QAM-OFDM signal. Similar to the QPSK-OFDM

sys-0 2 4 6 8 10 6

7 8 9 10 11

EVM (%)

OPR (dB)

(a)

(b) (c)

(a)

(c)

(b)

FIG. 5.12. Error vector magnitude versus optical power ratio curve and constellation diagrams of the 8QAM-OFDM signal, and constellations with (a) 0-dB , (b) 5-dB , and 10-dB OPR.

tem, negligible receiver penalty is observed after transmission over 25-km of standard single-mode fiber. In addition there is no significant distortion of the constellation diagrams observed, as shown in insets of FIG. 5.14.

5.3.4 Summary

A signal up-conversion and transmission system for 60-GHz RoF links was pro-posed in this section. Based on the modified SSB modulation scheme, high spectral efficiency OFDM modulation format can be transmitted with no performance degra-dation due to dispersion-induced fading. Since frequency sextupling was employed, only highly reliable 10-GHz components were utilized in the transmitter. Frequency-quadrupling WDM optical up-conversion can also be realized using the proposed sys-tem. The generation and transmission of 13.75-Gb/s QPSK-OFDM and 20.625-Gb/s 8QAM-OFDM signals occupying the full 7-GHz license-free band at 60 GHz was ex-perimentally demonstrated. After transmission over 25-km of standard single-mode fiber, no significant received power penalty was observed with both OFDM signal formats.

-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 10

9 8 7 6 5 4 3

BTB 25 km

-log(BER)

Level(dBm)

25km BTB

FIG. 5.13. Bit error rate curves of the QPSK-OFDM signal fiber transmission results.

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 5

10 15 20 25

EVM (%)

Level(dBm) BTB

25km

BTB

25km

FIG. 5.14. Error vector magnitude curves of the 8QAM-OFDM signal fiber transmission results.

5.4 Intensity Modulation Direction Detection Systems Using a Mach-Zehnder