Misaligned optic carrier polarization division multiplexing SSB-OOFDM system based on a beat interference cancellation receiver

Zhiqi Hu; Jianxin Ma

doi:10.1364/OSAC.2.002935

1. Introduction

With the advent of the broadband Internet era and the rise of technologies such as cloud computing and big data, the rapidly growing bandwidth demands have made the capability of communication networks face great challenges. In recent years, optical orthogonal frequency division multiplexing (OFDM) has been preferred more and more due to its high spectral efficiency (SE) and strong ability to resist inter-symbol interference (ISI) introduced by chromatic dispersion (CD) [1–4]. In optical communication, both coherent optical OFDM (CO-OFDM) and direct detection optical OFDM (DD-OOFDM) systems have been studied more and more [5–8]. CO-OFDM is being applied in long-haul transmission due to its excellent performance. However, the transmitter of the CO-OFDM system is complex and expensive since it requires a very narrow linewidth laser and the receiver requires complex equipment including local oscillator (LO), two 90° optical hybrids, and balanced photodiodes (PDs) to deal with the frequency offset and phase noise. In contrast, the receiver of the DD-OOFDM is simple because the OFDM signal can be recovered by heterodyne beating the optical carrier and the optical OFDM (OOFDM) in a square-law photodiode (PD) [9,10]. Estimation and correction of the frequency and phase offsets are not necessary because the optical carrier and the OOFDM signal come from the same laser with completely synchronized frequency and phase offset fluctuation.

Traditional DD-OOFDM system requires only one PD to convert the OOFDM signal to the RF one. However, due to the power fading effect caused by chromatic dispersion, DD-OOFDM scheme is limited to be used for short-distance transmission [11]. Fortunately, SSB-OOFDM signal can avoid the power fading caused by chromatic dispersion with the DSB one. Indeed, DD-SSB-OOFDM system has proven to be simpler than CO OFDM system. Whereas, if the guard band between the optical carrier and the OOFDM signal is small, the RF-OFDM signal we desired will be polluted by the signal-signal beat interference (SSBI) after DD. Currently, several digital signal processing algorithms have been applied to eliminate SSBI [12,13]. Apart from this, the K-K receiver method has also been proposed as a direct-detection receiver to cancel the SSBI. Combing the efficient spectral efficiency of coherent transmission and the cost-effectiveness of direct detection, K-K receiver can fully reconstruct the received complex OFDM signal from the detected photocurrent as the minimum phase condition is satisfied [14,15]. In [16], a beat interference cancellation receiver is proposed to eliminate the SSBI based on balanced detection with an optical interleaver (IL), a 2$\times$2 three-decibel optical coupler (OC), and a balanced photodiode pair (BPD). Ying Zhang and Jianxin Ma proposed a beat interference cancellation receiver (BICR) structure with an optical IL, a 3×3 OC and three PDs to enhance SE efficiently by eliminating the SSBI [17]. However, the above methods can only eliminate SSBI for the single polarization system.

Polarization-division multiplexing (PDM) technology uses two orthogonal polarization states of lightwave to transmit two independent modulated signals to double the capacity of the communication system. Nonetheless, in the PDM fiber link, the polarization mode dispersion (PMD) degrades the system performance because of the random rotation and coupling of the two polarization multiplexed optical signals. Complex and expensive equipment is required to deal with the PMD. Recently, multi-input-multi-output (MIMO) technology is proposed to deal with the PMD with the training sequences to estimate the channel transmission matrix for the PDM CO-OOFDM systems [18–20]. Then PDM DD-OOFDM system with MIMO technology was proposed to simplify the receiver in [21–27]. Whereas a sufficient guard band (GB) is required for the PDM DD-OOFDM signal to avoid the SSBI, and so the SE is sacrificed. In order to solve both PMD and SSBI in PDM DD-OOFDM system, Yang proposed a PDM-SSB-OOFDM scheme with two BICRs [28]. Nonetheless, digital signal processing (DSP) is required for implementing the MIMO technology, which increases the calculation complexity.

In this paper, we have proposed a MOC PDM-SSB-OOFDM scheme based on BICR. At the transmitter, two polarization orthogonal optical carriers at different frequencies are modulated by two radio frequency (RF) OFDM signals with different center frequencies, which generates SSB-OOFDM signal with overlapped optical spectra of the orthogonal sideband OOFDM signal. At the receiver, the two polarization orthogonal MOCs and the OOFDM signals received are recombined and beat with each other respectively in a single BICR to produce the RF OFDM signals at different frequencies with non-overlapped RF spectra. At the same time, the SSBIs are eliminated with only a small GB between the OOFDM signals and their respective optical carriers. Because both optical carriers and the OOFDM signals are rotated synchronously, the two SSB-OOFDM signals always maintain polarization-orthogonal as transmitted over the fiber at each instant. Our proposed scheme has almost identical SE with the conventional dual-polarization SSB-OOFDM transmission system, but the proposed scheme has a simple receiver with only one BICR and does not require the polarization demultiplexing. Moreover, since no PBS is used to separate the two polarization modes of lightwave, our proposed scheme is not sensitive to PMD so that the complex MIMO technology to deal with the PMD can be avoided. To demonstrate our proposed scheme, a simulation link is built with a dual-40Gb/s 16-ary quadrature amplitude modulation (16-QAM) PDM-SSB-OOFDM signal, and the error vector magnitude (EVM) and constellations of the 16-QAM OFDM signal after fiber transmission are obtained. The simulation results show that the EVMs are kept below 16.3% even after 120km fiber transmission, especially below 7.68% after 30km fiber transmission.

The remainder of the paper is organized as follows. In the Section 2, the principle of MOC PDM-SSB-OOFDM scheme is described, including a detailed analysis of SSBI elimination and the separation of two completely overlapping optical band signals. Section 3 presents a simulated link of dual-40 Gb/s MOC PDM-SSB-OOFDM 16-QAM signals to demonstrate the feasibility of our proposed scheme, and the simulation results are analyzed. Finally, conclusions are drawn in Section 4.

2. Principle of the MOC PDM-SSB-OOFDM link

Figure 1 shows the principle diagram of our proposed MOC PDM-SSB-OOFDM link. At the transmitting end, firstly, two optical carriers at different center frequencies f_CX and f_CY are generated by using a Mach-Zehnder modulator (MZM) and separated by an optical interleaver (IL), which can be expressed as E_inX(t)=E_C exp(j2πf_CXt) and E_inY(t)=E_C exp(j2πf_CYt). Here, E_C is the amplitude of the lightwave field and the MZM works at the minimum transmission point to implement the optical carrier suppression (OCS) modulation. The complex envelopes S_X(t) and S_Y(t) of the two baseband OFDM signals are carried by the RF carriers at frequency of f_RFX and f_RFY, and can be expressed as

(1)$${S_k}(t) =\sum\limits_{i = 0}^{i = \infty } {\left[ {\sum\limits_{n ={-} \frac{N}{2}}^{n = \frac{N}{2} - 1} {C_{ni}^k} \Pi (t - iT)\exp (j2\pi {f_n}t)} \right]} $$

\Pi (t) = \left\{ \begin{array}{l} 1,0 \lt t \lt = T\\ 0 ,{otherwise} \end{array}\right.

where k = X, Y; i is the index of the OFDM symbol; N is the number of subcarriers; $C_{ni}^k$ is the complex symbols carried on the n^th carrier of the i^th OFDM symbol on k polarization mode, respectively; Π(t) is the pulse shaping function; T is the OFDM symbol duration; f_n=n/T is the frequency of the n^th subcarrier of the baseband OFDM signal; Since −N/2$\le $n < N/2, the bandwidth of the baseband OFDM signal is W_S=N/T and GB is the guard band between the optical carrier and OOFDM signal W_G=f_RF−W_S/2 [16]

Fig. 1. the simulated link diagram of our proposed MOC PDM-SSB-OOFDM scheme. LD, laser diode; MZM: Mach-Zehnder modulator; IL: interleaver; PC: Polarization control; OFDM Modulator: orthogonal frequency division multiplexing modulator; I/Q mod: I/Q modulator; OBPF: optical band-pass filter; PBC: polarization beam combiner; SSMF: standard single mode fiber; EDFA: erbium-doped fiber amplifier; VOA: variable optical Attenuator; OC: optical coupler; PD: photodiode.

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Here, to assure the spectra of the OOFDM signals overlapping with each other, the frequencies of optical carrier and RF carrier meet the condition f_CX+f_RFX=f_CY+f_RFY. Then, the two optical carriers at f_CX and f_CY are modulated by the RF OFDM signals via two MZMs, respectively. Here, the optical signal output from the one MZM in the upper branch, as shown in Fig. 1, can be expressed as:

(2)$${E_{OUTX}}(t) = \frac{\alpha }{2}{E_{inX}}(t)\left\{ {\exp \left[ {j\pi \frac{{{V_{1X}}(t) + {V_{bias1}}}}{{{V_\pi }}}} \right] + \exp \left[ { - j\pi \frac{{{V_{1X}}(t) + {V_{bias2}}}}{{{V_\pi }}}} \right]} \right\}$$

where α is the amplitude attenuation coefficient caused by MZM; V_π is the half-wave voltage of MZM. If the RF OFDM signal driving the MZM can be expressed as

(3)$$\begin{aligned} {V_{1X}}(t) &= {\mathop{\textrm {Re}}\nolimits} [{{S_X}(t)} ]{V_{RF}}\cos (2\pi {f_{RFX}}t) - {\mathop{\textrm {Im}}\nolimits} [{{S_X}(t)} ]{V_{RF}}\sin (2\pi {f_{RFX}}t)\\ &= {V_m}{a_X}(t)\cos (\textrm{2}\pi {f_{RFX}}t + {\varphi _X}(t)) \end{aligned}$$

where V_ma_X(t)=|V_RFS_X(t)| and φ_X(t)=arg(S_X(t)) represent the amplitude and phase of the RF-OFDM signal, respectively; a_X(t) represents the normalized amplitude of the RF-OFDM signal. If V_bias₁=0 and V_bias₂=V_π−Δ, with Jacobi-Anger expansion, Eq. (2) becomes

(4)$$\begin{aligned} {E_{OUTX}}(t) \approx \frac{\alpha }{2}{E_c} &\left\{ { \left[ {1 - \exp ( - j\frac{{\pi \Delta }}{{{V_\pi }}})} \right]{J_0}({{m_{hX}}{a_X}(t)} )\exp [{j2\pi {f_{CX}}t} ]} \right.\\ & + \left[ {1 + \exp ( - j\frac{{\pi \Delta }}{{{V_\pi }}})} \right]j{J_1}({{m_{hX}}{a_X}(t)} )\exp [{j(2\pi ({{f_{CX}} + {f_{RFX}}} )t + {\varphi_X}(t))} ]\\ &\left. { + \left[ {1 + \exp ( - j\frac{{\pi \Delta }}{{{V_\pi }}})} \right]j{J_1}({{m_{hX}}{a_X}(t)} )\exp [{j(2\pi ({{f_{CX}} - {f_{RFX}}} )t - {\varphi_X}(t))} ]} \right\}\\ \\ \approx \frac{\alpha }{2}{E_c}&\left\{ {(j\frac{{\pi \Delta }}{{{V_\pi }}})\exp [{j2\pi {f_{CX}}t} ]} \right.\\ & + j{m_{hX}}{a_X}(t)\exp [{j(2\pi ({{f_{CX}} + {f_{RFX}}} )t + {\varphi_X}(t))} ]\\ &\left. { + j{m_{hX}}{a_X}(t)\exp [{j(2\pi ({{f_{CX}} - {f_{RFX}}} )t - {\varphi_X}(t))} ]} \right\} \end{aligned}$$

where m_hX=πV_m/V_π is the modulation depth of MZM and Δ is the deviated bias voltage relative to the minimum transmission point and is small relative to V_π. Since the MZM outputs a perfect OCS OOFDM signal without optical carrier only as Δ=0, the MZM outputs the OOFDM signal in the DSB pattern spectrum but with a partly suppressed optical carrier as the MZM is a little deviated from the minimum transmission point (Δ≠0). So the carrier-to-signal power ratio (CSPR) can be tuned to 0 dB by adjusting the bias voltage of Δ. In order to obtain a SSB- OOFDM signal, an optical bandpass filter (OBPF) is used to suppress one of their two first-order sidebands, and the generated SSB-OOFDM signals can be expressed as

(5)$$\begin{aligned}{E_X}(t) &= \frac{\alpha }{2}{E_C}\left\{ {j\frac{{\pi \Delta }}{{{V_\pi }}}\exp [j2\pi {f_{CX}}t]} \right.\\ &\qquad\left. { + j\frac{{\pi {V_m}}}{{{V_\pi }}}{a_X}(t)\exp [j(2\pi ({f_{CX}} + {f_{RFX}})t + {\varphi_X}(t))]} \right\}\\ & = {{E}_{{CX}}}({t} ){ + }{{E}_{{SX}}}({t} )\end{aligned}$$

where E_CX(t) and E_SX(t) represent optical carrier and OOFDM signal, respectively. In the same way, the other SSB-OOFDM signal output from the lower branch can be expressed as

(6)$$\begin{aligned} {E_Y}(t) & = \frac{\alpha }{2}{E_C}\left\{ {j\frac{{\pi \Delta }}{{{V_\pi }}}\exp [{j2\pi {f_{CY}}t} ]} \right.\\ &\qquad\left. { + j\frac{{\pi {V_m}}}{{{V_\pi }}}{a_Y}(t)\exp [j(2\pi ({f_{CY}} + {f_{RFY}})t + {\varphi_Y}(t))]} \right\}\\ & = {{E}_{CY}}({t} ){ + }{{E}_{SY}}({t} )\end{aligned}$$

where E_CY(t) and E_SY(t) represent optical carriers and OOFDM signals, respectively. Then the two SSB-OFDM optical signals are combined by a polarization beam combiner (PBC) after adjusting their polarization states by the PCs to produce the PDM-SSB-OOFDM signal, which can be expressed as

(7)$$\begin{aligned} \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over E} (t) &= {{\hat{e}}_x}{E_X}(t) + {{\hat{e}}_y}{E_Y}(t)\\& = \left( {\begin{array}{{cc}} {{{\hat{e}}_x}}&{{{\hat{e}}_y}} \end{array}} \right)\left( {\begin{array}{{cc}} {{E_X}(t)}\\ {{E_Y}(t)} \end{array}} \right)\\& = \left( {\begin{array}{{cc}} {{{\hat{e}}_x}}&{{{\hat{e}}_y}} \end{array}} \right)\left( {\begin{array}{{c}} {{E_{CX}}(t) + {E_{SX}}(t)}\\ {{E_{CY}}(t) + {E_{SY}}(t)} \end{array}} \right) \end{aligned}$$

As shown in Fig. 1(c), the spectra of E_SX(t) and E_SY(t) overlap with each other but with orthogonal polarization states while E_CX(t) and E_CY(t) have misaligned frequency, which can assure the two RF spectra do not overlap after optical-electrical conversion.

After transmitted over the standard single mode fiber (SSMF), the generated PDM-SSB-OOFDM signal is received by BICR without polarization demultiplexing in the receiving end and then converted to electrical OFDM signal [16]. In the BICR, the PDM-SSB-OOFDM signal is firstly divided into the optical carrier part ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over E} _C}(t) = {\hat{e}_x}{E_{CX}}(t) + {\hat{e}_y}{E_{CY}}(t)$ and OOFDM signal part ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over E} _S}(t) = {\hat{e}_x}{E_{SX}}(t) + {\hat{e}_y}{E_{SY}}(t)$ by an IL since one edge of the IL is located at the middle of the gap between the lower boundary of OOFDM signals and the optical carrier at f_CX in frequency domain. Then the optical carriers ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over E} _C}(t)$ and OOFDM signals ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over E} _S}(t)$ are recombined by a 2×2, three-decibel optical coupler (OC) with the transmission matrix

(8)$$T = \frac{1}{{\sqrt 2 }}\left[ {\begin{array}{{cc}} 1&j\\ j&1 \end{array}} \right]$$

Since their polarization states maintain orthogonal, the output light wave fields of the OC become

(9)$$\begin{aligned} \left[ {\begin{array}{{c}} {{E_1}(t)}\\ {{E_2}(t)} \end{array}} \right] &= T\left[ {\begin{array}{{c}} {{E_C}(t)}\\ {{E_S}(t)} \end{array}} \right]\\ &= \frac{1}{{\sqrt 2 }}\left[ {\begin{array}{{cc}} 1&j\\ j&1 \end{array}} \right]\left[ {\begin{array}{{c}} {{{\hat{e}}_x}{E_{CX}}(t) + {{\hat{e}}_y}{E_{CY}}(t)}\\ {{{\hat{e}}_x}{E_{SX}}(t) + {{\hat{e}}_y}{E_{SY}}(t)} \end{array}} \right]\\ &= \frac{1}{{\sqrt 2 }}\left[ {\begin{array}{{c}} {{{\hat{e}}_x}[{{E_{CX}}(t) + j{E_{SX}}(t)} ]+ {{\hat{e}}_y}[{{E_{CY}}(t) + j{E_{SY}}(t)} ]}\\ {{{\hat{e}}_x}[{j{E_{CX}}(t) + {E_{SX}}(t)} ]+ {{\hat{e}}_y}[{j{E_{CY}}(t) + {E_{SY}}(t)} ]} \end{array}} \right] \end{aligned}$$

It can be seen that, in each output of the OC, the two SSB-OOFDM signals with the RF carriers at f_RFX and f_RFY keep orthogonal polarizations; for each SSB-OOFDM signal, the optical carrier and OOFDM signal have identical polarization state, but a ± 90° phase difference is introduced between the optical carrier and OOFDM signal for the two recombined SSB-OOFDM signal output from the two ports of the OC. Then, they are injected to the balanced photodetector consisting of PD1 and PD2 with identical responsivity of μ for photoelectric conversion. The photocurrents of the two PDs can be expressed as

(10)$$\begin{aligned}\left[ {\begin{array}{{c}} {{I_1}(t )}\\ {{I_2}(t )} \end{array}} \right] &= \mu \left[ {\begin{array}{{c}} {{{|{{E_1}(t )} |}^2}}\\ {{{|{{E_2}(t )} |}^2}} \end{array}} \right]\\& = \frac{\mu }{2}\left[ {\begin{array}{{c}} \begin{array}{l} {|{{E_{CX}}(t )} |^2} + {|{{E_{SX}}(t )} |^2} + \textrm{j}[{{E_{SX}}(t )E_{CX}^ \ast (t )- E_{SX}^ \ast (t ){E_{CX}}(t )} ]+ \\ {|{{E_{CY}}(t)} |^2} + {|{{E_{SY}}(t )} |^2} + \textrm{j}[{{E_{SY}}(t )E_{CY}^ \ast (t )- E_{SY}^ \ast (t ){E_{CY}}(t )} ]\end{array}\\ \begin{array}{l} {|{{E_{CX}}(t )} |^2} + {|{{E_{SX}}(t )} |^2} - \textrm{j}[{{E_{SX}}(t )E_{CX}^ \ast (t )- E_{SX}^ \ast (t ){E_{CX}}(t )} ]+ \\ {|{{E_{CY}}(t )} |^2} + {|{{E_{SY}}(t )} |^2} - \textrm{j}[{{E_{SY}}(t )E_{CY}^ \ast (t )- E_{SY}^ \ast (t ){E_{CY}}(t )} ]\end{array} \end{array}} \right] \end{aligned}$$

It can be seen that since the two SSB-OOFDM signals with the RF carriers at f_RFX and f_RFY are polarization-orthogonal, there is no cross-beating term between the two SSB-OFDM signals but only the self-beating of each SSB-OOFDM signal in the photocurrent of each PD. In fact, the self-beating of each SSB-OFDM signal in each PD consists of the homodyne beating of the optical carrier and that of the OOFDM signal (SSBI), and heterodyne beating between the optical carrier and the OOFDM signal. For each SSB-OOFDM signal in the two PDs, both the homodyne beating of the optical carrier and the OOFDM signal have equal positive photocurrents, while the heterodyne beating photocurrents between the optical carrier and the OOFDM signal have equal amplitude but are antiphase. So, the photocurrent output from the differentiate circuit of the BICR becomes

(11)$$\begin{aligned}{I_{OUT}}(t )&= {I_1}(t )- {I_2}(t )\\& = \textrm{j}\mu [{{E_{SX}}(t )E_{CX}^ \ast (t )- E_{SX}^ \ast (t ){E_{CX}}(t )+ {E_{SY}}(t )E_{CY}^ \ast (t )- E_{SY}^ \ast (t ){E_{CY}}(t )} ]\\& = 2\mu {\mathop{\textrm {Im}}\nolimits} \{{E_{CX}^ \ast (t ){E_{SX}}(t )\textrm{ + }E_{CY}^ \ast (t ){E_{SY}}(t )} \}\\& = \frac{{\mu {\alpha ^2}{\pi ^\textrm{2}}E_C^2\Delta {V_\textrm{m}}}}{{2V_\pi ^2}}{\mathop{\textrm {Im}}\nolimits} \{{{a_X}(t)\exp [j(2\pi {f_{RFX}}t + {\varphi_X}(t))] + {a_Y}(t)\exp [j(2\pi {f_{RFY}}t + {\varphi_Y}(t))]} \}\end{aligned}$$

As can be seen, the SSBI and direct current (DC) components in the photocurrents of two PDs are cancelled since they are in phase, while the desired RF OFDM signals are doubled by constructive addition for they are antiphase. By properly setting the RF carrier frequency to meet the condition f_RFX>W_S/2 and f_RFY−f_RFX>W_S, the spectrum overlapping of the two RF OFDM signals can be avoided. In addition, since the optical carrier and the OOFDM signal are derived from the same linearly polarized laser and are transmitted over the same optical link, the frequency and phase offsets of the optical carrier and the OOFDM signal keep synchronized, and so they can be eliminated by beating with each other, thereby reducing the linewidth requirement of the laser diode [16]. Moreover, in the fiber link, since all the components of the PDM-SSB-OOFDM signal suffer from the polarization rotation synchronously, the SSB-OOFDM signals remain orthogonal polarization states, and so there is no beating interference between the two polarization-orthogonal SSB OOFDM signals in the photocurrent when they are photodetected with only one BICR. This means that our proposed optical link for the PDM-SSB-OOFDM signal is insensitive to PMD of the fiber.

In our proposed scheme, since the BICR eliminates DC and SSBI, the GB between optical carrier and OOFDM signal is theoretically no longer needed. In fact, due to the non-ideality of the filtering edge of the IL, a bit of GB is reserved for accommodating roll-off edge of the IL to avoid spectrum leakage of the optical carrier and the OOFDM signal. On the other hand, the increase of GB leads to the reduction of the spectrum efficiency. So there is a trade-off between the SE and the link performance degradation caused by the spectrum leakage in real system. In addition, by using PDM, the channel capacity of the system are doubled without MIMO technology and polarization demultiplexing at the receiving end, the receiver has relative simple configuration and is easy to implement since only one BICR is used.

3. Simulation setup and results

In order to demonstrate the feasibility of our proposed MOC PDM-SSB-OOFDM scheme, we have built a simulation link based on optisystem associated with matlab, as shown in Fig. 2.

Fig. 2. Simulation link diagram of the MOC PDM-SSB-OOFDM scheme with W_G2 of 1 GHz. The 7 illustrations in the lower part of the Fig. 2 are the spectra of different locations of MOC. The illustrations (a) and (b) represent the spectrum of the RF-OFDM signal at the X and Y polarization directions of the transmitting end, respectively; (c) represent Optical carrier spectrum generated by CW LD; (d) and (e) represent SSB-OOFDM signal and Optical carrier spectrum at the X and Y polarization directions; (f) represents the spectrum of the optical signal transmitted in the fiber; (g) represents the spectrum of the RF-OFDM signal after BICR.

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At the transmitting end, two different baseband OFDM signals with 10 GHz bandwidth are firstly generated. For each one, a pseudo-random bit sequence (PRBS) with the bit rate of 40 Gb/s and word length of 2¹⁶−1 = 65535 is mapped into 16384 16-QAM symbols. Then, by serial-to-parallel(S/P) conversion, inverse fast Fourier transform (IFFT), cyclic prefix and training sequence adding, parallel-serial (S/P) conversion and digital to analog conversion (DAC), the baseband OFDM signal with 128 OFDM symbols is obtained. Here, only 128 subcarriers of the total 256 carriers are used for data transmission, and the other 128 subcarriers at the edges are padded with zeros for oversampling. In order to avoid crosstalk between symbols caused by the fiber chromatic dispersion, a cyclic prefix with 32 sampling points is added to each OFDM symbol. Two training symbols are also added in front of the OFDM sequence in each polarization for channel estimation and equalization. After the DAC, the I and Q components of the baseband OFDM signal are up-converted to an intermediate frequency (IF) with a center frequency at the range from 6 to 10 GHz through IQ modulation, resulting in a variable GB of 1 to 5 GHz. Figure 2(a) and (b) show the spectra of the RF OFDM signals to be modulated on the optical carrier in the X and Y polarization directions, respectively. MZM1 works at the DC bias voltage of 0.5V_π to realize the optical carrier suppression (OCS) modulation and its output consists mainly of the two first-order sidebands as the optical carrier for the two RF OFDM signals, as shown in Fig. 2(c). The two tones are separated by an IL and then are modulated by two RF OFDM signals via MZM2 and MZM3 in push-pull pattern with V_π=4 V. Here Δ is set to 0.42 V, so V_bias₁=0 V, V_bias₂=3.58 V. The generated two DSB-OOFDM signals have CSPR about 0 dB due to part optical carrier suppression. To avoid the power fading effect as transmitted over the chromatic dispersive fiber, the DSB-OOFDM signals are converted to the SSB ones by optically band-pass filtering. Figure 2(d) and (e) show the optical spectra of the generated SSB-OOFDM signals. The two SSB-OOFDM signals are combined by a PBC to form the MOC PDM-SSB-OOFDM signal. Figure 2(f) shows the optical spectrum of MOC PDM-SSB-OOFDM signal. There is no change in the power of the two optical carriers, but the power of the OOFDM signal is increased by about 3 dB due to their optical spectral overlapping. The MOC PDM-SSB-OOFDM signal is then injected to a standard single mode fiber (SSMF) with the loss coefficient of 0.2 dB/km, chromatic dispersion coefficient of 16 ps/nm/km, PMD coefficient of 0.5 ps/km^1∕2 and nonlinear coefficient of 2.6×10⁻²¹ m²/W, for transmission. An Erbium-doped fiber amplifier (EDFA) with the noise figure of 4 dB is used to enhance the launch power of the optical signal.

At the receiving end, the MOC PDM-SSB-OOFDM signal is received by a single BICR. In the BICR, an IL is firstly used to separate optical signals into MOCs and polarization multiplexed OOFDM signals, and then they are recombined by a 2×2 three-decibel OC to produce two equal-power output signals but with relative phase shifts of 90° and −90° between the optical carriers and the OOFDM signals, respectively. Next, the two recombined MOC PDM-SSB-OOFDM signals are injected into a balanced photodiode consisting of two PDs with identical responsivity of 1 mA/mW and thermal noise of 1 mA/mW and 1×10⁻²²W/Hz, respectively, for photoelectric conversion. Figure 2(g) shows the spectrum of the RF OFDM signal after photocurrent conversion. It can be seen that the photocurrent mainly includes two RF OFDM signals with different center frequencies, while the SSBIs are completely cancelled by the subtraction of the two photocurrents. Then, two RF OFDM signals are respectively down-converted to the baseband OFDM signal and processed by Matlab. The processing includes serial-to-parallel conversion, cyclic prefix removal, fast Fourier transform (FFT), channel estimation and equalization, parallel-to-serial conversion, 16QAM demodulation in sequence, and finally the EVMs are calculated based on the constellations for evaluating the link performance.

In order to find the optimal value of CSPR, the link performance was tested at different fiber length when CSPR is equal to −10 dB, −7 dB, −3 dB, 0 dB, 3 dB, 7 dB and 10 dB, respectively, as shown in Fig. 3. From Fig. 3(a), we can see that when the CSPR is set to 0 dB, the EVMs of both X- and Y- polarization directions get smallest at different signal transmission distance. The variation of EVMs of the MOC PDM-SSB-OOFDM signal versus the transmission distance with the CSPR of 0 dB and GB of 1 GHz, as shown in Fig. 3(b). It can be seen that although the EVMs of the two polarizations increase with the fiber length increasing, they always keep below the forward error correction (FEC) limit of 16.3% when the MOC PDM-SSB-OOFDM signal transmission distance is less than 120 km.

Fig. 3. (a) EVMs of X-Polarization direction and Y-Polarization direction versus CSPR in this scheme at the GB of 1 GHz. 16.3% is the forward error correction (FEC) limit. (b) EVMs versus transmission distance of MOC PDM-SSB-OOFDM signal at the GB of 1 GHz when CSPR is equal to 0 dB.

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In order to emphasis the advantage of MOC PDM-SSB-OOFDM scheme with BICR at the reduced GB, the simulation of the DD PDM-SSB-OOFDM system was conducted for comparison. The dual-40 Gb/s MOC 16-QAM SSB-OOFDM signal is respectively received by two PDs after being transmitted through a SSMF of 30 km, where the W_G1 changes from 0 to 10 GHz corresponding to the center frequency f_RFX of the RF OFDM signal transmitted in the X-polarization direction changes from 6 to 15 GHz. Figure 4(a) shows the curve of EVM versus W_G1 of MOC 16-QAM PDM-SSB-OOFDM signal. It can be seen from Fig. 4(a) that when W_G1 is less than the RF OFDM signal bandwidth of 10 GHz, the EVM of MOC PDM-SSB-OOFDM system with BICR keeps below 20%, while the EVM of the DD PDM-SSB-OOFDM system is always higher than 20% as W_G1 increases until 9 GHz. This is consistent with the previous theoretical analysis. We also check the influence of the guard band between the two RF OFDM signals (W_G2), which can be adjusted by tuning the frequency of RF local oscillators. As can be seen from Fig. 4(b), when W_G2 increases from 0 to 2 GHz, the performance is improved obviously with the EVM reduction of about 27%, while it keeps almost constant as W_G2 goes beyond 2 GHz.

Fig. 4. (a) EVMs of X-Polarization direction versus the W_G1 when transmission distance of MOC PDM-SSB-OOFDM signal is 30km. (b) EVM versus the W_G2 at GB of 1 GHz when transmission distance of MOC PDM-SSB-OOFDM signal is 30km.

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In order to investigate the influence of the launch optical power on the link performance at different transmission distance, the EVM versus the launch optical power curves for the dual-40 Gb/s MOC 16-QAM PDM-SSB-OOFDM signals are measured at the optical back-to-back case and after different transmission distances when W_G1=1GHz, W_G2=2GHz, the receiving optical power is controlled by optical attenuator at 0 dBm and CSPR = 0dB, as shown in Fig. 5. It can be seen from Fig. 5 that the EVM declines rapidly as the optical power is gradually increased from −10dBm since the signal to noise ratio of the optical signal improves with the increase of the launch optical power. When the optical power changes from 4dBm to 20dBm, the EVM remains almost at the minimum value. However, when the optical power further increases, the EVM begins to rise rapidly. It attributes to the fact that the nonlinearity distortion of the fiber link increases rapidly after the launch optical power goes beyond the nonlinearity threshold. Moreover, as the transmission distance of the MOC 16-QAM PDM-SSB-OOFDM signal increases, the range of optical power required to keep the EVM at a minimum is getting smaller and smaller, and at the same time, the minimum optical power required for the EVM to reach below the EVM FEC limit of 16.3% increases. It can be seen from the variation of the curve in Fig. 5 that although increasing the optical power can improve the system performance by increasing the optical signal to noise ratio (OSNR), excessive increase of optical power results in serious nonlinear performance degradation.

Fig. 5. EVM versus the optical launch power of MOC PDM-SSB-OOFDM signal at GB of 1 GHz in case of different transmission distance of 0km, 30km, 60km, and 90km,120km.

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4. Conclusion

In this paper, we have proposed and demonstrated an MOC PDM-SSB-OOFDM scheme with BICR. The receiver requires only one BICR based on balanced detection. This scheme is easy to implement because it avoids the complex MIMO technology to separate the signals on both X and Y polarization directions. The scheme not only doubles channel capacity with PDM technology but also simplifies receiver with BICR eliminating SSBI caused by the square-law photodetection. The proposed scheme is validated by a simulation link of dual-40 Gb/s 16-QAM MOC PDM-SSB-OOFDM signals transmission over 120km SSMF. When the transmission distance of the MOC PDM-SSB-OOFDM signal is below 120km, EVM can maintain below the FEC limit of 16.3%, especially when the transmission distance is less than 30km, EVM can keep below 7.68%. The simulation results agree well with our theoretical analysis.

Funding

National Natural Science Foundation of China (61690195); Fund of State Key Laboratory of IPOC (IPOC2018ZT09).

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Misaligned optic carrier polarization division multiplexing SSB-OOFDM system based on a beat interference cancellation receiver

Abstract

1. Introduction

2. Principle of the MOC PDM-SSB-OOFDM link

3. Simulation setup and results

4. Conclusion

Funding

References

Cited By

Figures (5)

Equations (12)

OSA Continuum