W-band OFDM photonic vector signal generation employing a single Mach-Zehnder modulator and precoding

Jiangnan Xiao; Xinying Li; Yuming Xu; Ziran Zhang; Long Chen; Jianjun Yu

doi:10.1364/OE.23.024029

1. Introduction

Recently, with the increasing demand of broadband wireless communications, W-band (75-110 GHz) has attracted a lot of interest, due to its larger bandwidth and capacity [1–6 ]. But the limitation of bandwidth requirement electronic bottleneck of traditional wireless communication makes electrical transmission system in W-band difficult to implement, which, however, can be overcome by radio-over-fiber (RoF) technique. Orthogonal frequency division multiplexing (OFDM) has been widely used in current transmission system, due to its robustness against chromatic dispersion and polarization mode dispersion effects in optical fiber channels and wireless multipath fading in wireless channels. So far, several W-band RoF systems have been proposed and experimentally demonstrated, and these RoF systems are based mainly on three techniques. The first one is remote up-conversion with photonic transmitter-mixer [7–11 ]. However, because the frequency and phase shift of two free-running lightwaves coupled as the optical carrier are inconsistent, photonic transmitter-mixer is not stable. Meanwhile complex digital signal processing (DSP) is needed. The second one is self-coherent heterodyning technique [12]. However, self-coherent heterodyning needs several modulators to generate millimeter-wave, which increases the cost of the RoF system. The last but not the least one is direct-detection (DD) technique [13–16 ]. DD technique based on optical carrier suppression (OCS) scheme uses only one laser and one external modulator. Moreover, stable high-frequency carrier generation can be realized based on low-frequency RF signal. OFDM-RoF employing DD technique is proposed, not only to reduce the complexity of RoF system but also enhance the frequency utilization efficiency of transmit signals. Recently, we have experimentally demonstrated single carrier vector signal generation and transmission [17–19 ]. OFDM signal has some advantages relative to signal carrier signal [20]. Thus, it is interesting to investigate OFDM vector signal generation and transmission.

In this paper, we propose a novel and simple OFDM-RoF system at W-band employing direct detection and precoding technique. This system uses one Mach-Zehnder modulator (MZM) based on OCS scheme to generate W-band OFDM photonic vector signals. The experimental results show that the bit-error ratios (BERs) of the transmission system are less than the forward-error-correction (FEC) threshold of 3.8 × 10⁻³. The experimental results demonstrate that the photonic OFDM vector signal generation at W-band based on our proposed scheme can be employed in OFDM-RoF architecture.

2. Principle of photonic OFDM vector signal generation at W-band

Because of the ‘square-law’ characteristic of the photodetector (PD), the amplitudes and the phase of the driving radio-frequency (RF) OFDM signal are changed after PD detection. In order to ensure that the output signal of the PD is regular OFDM signal, the amplitudes and phase of the driving RF OFDM signal need to be precoded.

Figure 1 shows the principle of the electrical qudrature-phase-shift-keying (QPSK) OFDM vector signal generation. The input pseudo-random binary sequence (PRBS) is first converted into many parallel data pipes (P/S), and then QPSK mapped. The digital time domain signal is obtained by using inverse Fast Fourier Transformation (IFFT). The schematic constellation of the original OFDM signal after IFFT is showed in Fig. 1(a). After IFFT, the OFDM signal is precoded. Firstly, the OFDM signal on each sub-carrier is phase precoded, and then amplitude precoded. The phase and amplitude precoding technology follows certain rules, and these rules will be explained in detail below. Figures 1(b) and 1(c) show the schematic constellations after phase precoding and further amplitude precoding, respectively. After precoding, guard interval is inserted to prevent intersymbol-interference (ISI) due to channel dispersion. Next, the I and Q branches of the QPSK OFDM precoding signal are up-converted into intermediate-frequency (IF) signals at f_s by mixing with two sinusoidal RF signals with quadrature phase at f_s, respectively. The summation of the two IF signals is the desired electrical OFDM vector signal at f_s.

Fig. 1 The principle of electrical QPSK OFDM vector signal generation. (a) Constellation of regular QPSK OFDM signal after IFFT, (b) Constellation of precoding QPSK OFDM signal after phase precoding, and (c) Constellation of precoding QPSK OFDM signal after amplitude precoding.

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The photonic OFDM vector signal is generated by a MZM driven with the aforementioned electrical OFDM vector signal at f_s. Figure 2 shows the principle of the photonic OFDM vector signal generation at W-band.

Fig. 2 The principle of photonic QPSK OFDM vector signal generation at W-band. Optical spectra: (a) before MZM, (b) after MZM, and (c) after WSS. CW: continuous-wavelength, MZM: Mach-Zehnder modulator, WSS: wavelength selective switch, PD: photodiode.

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The continuous-wavelength (CW) lightwave as a carrier is generated by one laser, and is defined as

E_{i n} (t) = E_{1} \exp (2 π f_{c} t) .

where E₁ and f_c are the amplitude and frequency of the optical carrier, respectively. The baseband electrical OFDM signal can be described as

S_{B B_O F D M} (t) = \frac{1}{\sqrt{N}} \sum_{n = 0}^{N} a_{n} \exp (j 2 π \frac{n}{T} t + φ_{n}) .

where t is the discrete time index, N is the number of the sub-carrier, and T is the symbol duration. a_n and φ_n are the amplitude and phase of data symbol modulated on the nth sub-carrier, respectively. The baseband electrical OFDM signal is subcarrier modulated with RF carrier at f_s. The modulated electrical OFDM vector signal at f_s can be expressed as

S_{O F D M_d r i v e} (t) = S_{B B_O F D M} (t) * \exp (j 2 π f_{s} t) = \frac{1}{\sqrt{N}} \sum_{n = 0}^{N} a_{n} \exp (j 2 π (\frac{n}{T} + f_{s}) t + φ_{n}) .

Then the optical carrier is intensity-modulated via one MZM by the RF OFDM vector signal. The output of the MZM is expressed as

\begin{matrix} E_{M Z M} (t) = \frac{1}{2} E_{1} [\exp (j π \frac{V_{O F D M_d r i v e r} + V_{D C}}{V_{π}}) + \exp (- j π \frac{V_{O F D M_d r i v e r} + V_{D C}}{V_{π}})] \\ = E_{1} \cos ϕ \sum_{k = - \infty}^{+ \infty} \sum_{n = 0}^{N} j^{2 k} J_{2 k} (a_{n} A) \exp (j 4 k π (\frac{n}{T} + f_{s}) t + j 2 k φ_{n}) \\ + E_{1} \sin ϕ \sum_{k = - \infty}^{+ \infty} \sum_{n = 0}^{N} j^{2 k - 1} J_{2 k - 1} (a_{n} A) \exp (j (4 k - 2) π (\frac{n}{T} + f_{s}) t + j (2 k - 1) φ_{n}) . \end{matrix}

where V_π is the half-wave voltage of the MZM, and ϕ = πV_DC /V_π is the optical phase variation caused by the driving voltage. A = π/(sqrt(N)V_π) is the modulation index of the MZM. When V_DC = 0, that is, the MZM is biased at its maximum transmission point, odd-order optical subcarriers and optical carrier are suppressed. In this case, the optical signal after MZM can be expressed as

E_{M Z M_e v e n} (t) = E_{1} \sum_{k = - \infty}^{+ \infty} \sum_{n = 0}^{N} j^{2 k} J_{2 k} (a_{n} A) \exp (j 4 k π (\frac{n}{T} + f_{s}) t + j 2 k φ_{n}) .

When two identical m-th even-order sidebands are selected as optical carrier by a wavelength selective switch (WSS), the output of the WSS is expressed as

\begin{matrix} E_{W S S} (t) = E_{1} [\sum_{n = 0}^{N} j^{m} J_{m} (a_{n} A) \exp (j 2 m π (\frac{n}{T} + f_{s}) t + j m φ_{n}) + \sum_{n = 0}^{N} j^{m} J_{- m} (a_{n} A) \exp (- j 2 m π (\frac{n}{T} + f_{s}) t - j m φ_{n})] \\ = 2 E_{1} J_{m} (a_{n} A) \sum_{n = 0}^{N} {(- 1)}^{k} \cos (j 2 m π (\frac{n}{T} + f_{s}) t + j m φ_{n}) \end{matrix}

where m = 2k, and J_m is the first kind Bessel function of order m. After the detection by a PD, the output current of the PD is expressed as

\begin{matrix} i_{P D} (t) = R E_{P D}^{2} (t) = 4 E_{1}^{2} J_{m}^{2} (a_{n} A) {(\sum_{n = 0}^{N} \cos (j 2 m π (\frac{n}{T} + f_{s}) t + j m φ_{n}))}^{2} \\ = 2 R E_{1}^{2} J_{m}^{2} (a_{n} A) \sum_{n = 0}^{N} \cos (j 2 m \times 2 π (\frac{n}{T} + f_{s}) t + j 2 m φ_{n}) \end{matrix}

where R is the conversion efficiency of the PD. Based on the comparison of the phase and amplitudes of the driving electrical OFDM vector signal on each sub-carrier from Eq. (3) with those of the OFDM signal on each sub-carrier after PD from Eq. (7), we can see that the phase and amplitudes of OFDM signal on each sub-carrier after PD are changed relative to those of the driving OFDM RF signal on corresponding sub-carrier. The phase of OFDM signal on each sub-carrier after PD is 2m times that of the electrical OFDM vector signal on corresponding sub-carrier for the drive of the MZM, and the amplitude of OFDM signal on each sub-carrier after PD is J2 m(a_nA) times that of the electrical OFDM vector signal on corresponding sub-carrier for the drive of the MZM. In order to ensure that the output signal of the PD is regular OFDM signal, the amplitude and phase of the driving RF OFDM signal on each sub-carrier need to be precoded. The precoding rule is that the phase of phase-precoding OFDM signal on each sub-carrier after IFFT should be 1/2m of that of original OFDM signal on corresponding sub-carrier after IFFT, and the amplitude of amplitude-precoding OFDM signal on each sub-carrier after IFFT should be 1/J2 m(a_nA) of that of regular OFDM signal on corresponding sub-carrier after IFFT, which ensures that the OFDM signal after PD can be restored to regular OFDM signal. In our experimental system, we select two fourth-order modes as carrier. Therefore the phase of phase-precoding OFDM signal on each sub-carrier is 1/8 of that of original OFDM signal on corresponding sub-carrier after IFFT, and the amplitude of precoding OFDM signal on each sub-carrier is 1/J2 4(a_nA) of that of original OFDM signal on corresponding sub-carrier after IFFT. The schematic constellations of the original OFDM signal after IFFT and precoding OFDM signal are showed in Figs. 1(a), 1(b) and 1(c), respectively. The schematic optical spectra before MZM, after MZM and after WSS are showed in Figs. 2(a), 2(b) and 2(c), respectively.

3. Experimental results and discussions

The experimental setup of the photonic OFDM vector signal transmission at W-band is shown in Fig. 3 .

Fig. 3 The experimental setup for photonic QPSK OFDM vector signal transmission at W-band. (a) The photo of the transmission system. ECL: external cavity laser, MZM: Mach-Zehnder modulator, AWG: arbitrary waveform generator, EA: electrical amplifier, EDFA: erbium-doped fiber amplifier, WSS: wavelength selective switch, PD: photodiode, OSC: oscilloscope.

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In the experiment, the electrical OFDM vector signal is generated offline in MATLAB and then loaded into an Arbitrary Waveform Generator (AWG) operating at a sampling rate of 24 GS/s. The process for the generation of the OFDM vector signal is showed in Fig. 1. The OFDM vector signal is up-converted into 10-GHz IF signal after both phase and amplitude precoding. Then the generated OFDM vector signal is amplified by an electrical amplifier (denoted by EA1) with 30-dB gain and 12-GHz 3-dB bandwidth. The CW lightwave from an external cavity laser (ECL) is modulated by a MZM driven by the electrical OFDM vector signal, to generate photonic OFDM vector signal. The ECL at 1565.36 nm has less than 100-kHz linewidth and output power of 14.5 dBm. Then the CW lightwave from the ECL is amplified by an Erbium-doped fiber amplifier (denoted by EDFA1), and the power of the CW lightwave after EDFA1 is 19.36 dBm. The MZM is biased at its maximum transmission point, and only even-order optical subcarriers are generated. The MZM has a 3-dB bandwidth of ~36 GHz, 2.8-V half-wave voltage (V_π) and 5-dB insertion loss. The power of the light after the MZM is about 9.18 dBm. The optical spectra with 0.02-nm resolution after MZM is shown as Figs. 4(a) . In the experiment, we selected the two fourth-order sidebands as optical mm-wave carrier by a 1x4 WSS with a 10-GHz grid and 7-dB insertion loss. The frequency of optical mm-wave carrier is 80GHz, and the power of the light after the WSS is about −6.6dBm. Then the optical mm-wave signal is amplified by an Erbium-doped fiber amplifier (denoted byEDFA2), and the power of the mm-wave signal after EDFA2 is 11.64dBm. The optical spectrum with 0.02-nm resolution after WSS is shown as Fig. 4(b). Then the optical mm-wave signal is converted into electrical mm-wave OFDM signal via a high-speed PD (90-GHz 3-dB bandwidth). The photodetected OFDM signal at 80GHz is amplified by EA2 with a narrowband bandwidth of 100GHz centered at 90GHz, 23-dB gain and 4-dBm saturation output power, and then fed to a rectangular horn antenna in the 75-110 GHz band with 24-dBi gain and 10-degree half-power-beamwidth. After 1-m wireless transmission, the mm-wave OFDM signal is received by an identical rectangular horn antenna. An electrical mixer driven by a local oscillator (LO) RF signal at 75 GHz is employed for frequency down-conversion. The down-converted signal is amplified by a low-noise electrical amplifier (denoted by EA3). The 5-GHz down-converted signal is sampled by a digital oscilloscope (OSC) with 40-GSa/s sampling rate and 16-GHz electrical bandwidth. After the analog-to-digital conversion, the DSP is performed to retrieve the transmitted data with the details shown in Fig. 3, and OFDM vector signal includes frequency down-conversion, time synchronization, frequency and channel estimation, pilot-based phase estimation, data mapping and BER- calculation. The time and frequency offset estimation (FOE) is applied to retrieve signal using the training sequence. The carrier phase estimation (CPE) is based on the pilots. Figure 3(a) shows the photo of the transmission system.

Fig. 4 (a) Optical spectrum (0.02-nm resolution) after MZM. (b) Optical spectrum (0.02-nm resolution) after WSS. (c) BER versus launched optical power into PD.

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Figure 4(c) shows the BER versus the launched optical power into PD and the corresponding constellations for QPSK OFDM vector signal at the net bit rates of 1.33, 2.66 and 4 Gb/s, respectively. The BER performance becomes worse with the increase of transmission rate, because of the limited bandwidth of the AWG, other electrical devices and the WSS. Because the AWG used in our experiment is Tektronix AWG7122C with about 5-GHz 3-dB bandwidth and 24-GSa/s maximum sample rate, which can only generate RF/microwave signals with a maximum frequency of 12-GHz. The frequency of electrical OFDM vector signal in our experiment is 10GHz, for which only 2-GHz AWG bandwidth is remained, so the maximum transmission rate of vector signal is 2-Gband. When the net bit rate is 4Gb/s, the signal has reached the limit of AWG bandwidth, and the generated signal quality is poorer. So if the bandwidth of the AWG could be wider and the filtering effect of the WSS could be smaller, the transmission system could transmit higher rate signals and have better BER performance. The BER of the OFDM at the rate of 4 Gb/s cannot reach the FEC threshold of 3.8x10⁻³, which is mainly due to the increase of side lobe power and the decrease of main lobe power.

4. Conclusion

The photonic OFDM vector signal generation and wireless transmission in W-band has been experimentally demonstrated using a single MZM and precoding technique. The OFDM RF signal used to drive MZM is precoded, which includes phase precoding and amplitude precoding. The proposed OFDM vector signal generation scheme can provide a cost- and power-efficient solution for RoF system. The experimental results show that the QPSK OFDM vector signals at 1.33- and 2.66-Gb/s transmission rates have BERs less than the FEC threshold of 3.8x10⁻³. The results show that this proposed system provides for us a promising solution to simplify the architecture and reduce the cost of the OFDM-RoF system. This work was partially supported by NNSF of China (61325002), and Key Program of Shanghai Science and Technology Association (13JC1400700).

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W-band OFDM photonic vector signal generation employing a single Mach-Zehnder modulator and precoding

Abstract

1. Introduction

2. Principle of photonic OFDM vector signal generation at W-band

3. Experimental results and discussions

4. Conclusion

References and links

Cited By

Figures (4)

Equations (7)

Optics Express