We introduce and simulate a technique enabling to utilize the polarization dimension in direct-detection optical transmission, supporting polarization multiplexing (POL-MUX) over direct-detection (DD) methods previously demonstrated for a single polarization such as direct-detection OFDM. POL-MUX is currently precluded in self-coherent DD with remotely transmitted pilot, as signal x pilot components may randomly fade out. We propose POL-MUX transmission of advanced modulation formats, such as 16-QAM and higher, by means of a novel low-complexity photonic integrated optical front-end and adaptive 3x2 MIMO DSP. The principle of operation is as follows: an additional X x Y cross-polarizations signal is generated, providing three projections onto an over-complete frame of three dependent vectors. This enables to resiliently reconstruct the received state of polarization even when the remotely transmitted pilot fades along one of the received polarization axes.
© 2013 Optical Society of America
Short-reach access and metro networks (e.g., point-to-point interconnects between data centers at ~100 km or long-reach PONs) are emerging cost-sensitive transmission applications wherein capacity requirements are high, yet coherent detection is prohibitive. Improvements in the capacity-complexity (cost) tradeoff would be welcome in this field; in particular cost-effective increase in the data rate over direct-detection (DD) links would be a laudable goal.
A family of self-coherent systems, mainly for OFDM DD [1,2], targets this application by coherent multiplexing of the I and Q field quadratures, based on remote pilot transmission and low-cost DD, without requiring a local oscillator (LO) laser in the receiver (Rx). The remote pilot is effectively used as a local oscillator, emulating coherent detection in the direct detection process. The remotely transmitted Pilot (P) and Data (D) signals (Fig. 1) beat in the DD photo-current, forming the desired DxP product, plus spurious DxD and PxP products which are mitigated by various methods. The relative spectral locations of the P and D components set a tradeoff between performance, complexity, spectral efficiency and whether just band-pass filtering or more advanced DSP is required to mitigate the DxD spurious signal (possibly also mitigation of transmitter-side quadratic non-linearities ).
The challenge of enhancing self-coherent systems with polarization (POL) multiplexing (MUX) capability still eludes a satisfactory solution. The difficulty is that the remotely transmitted pilot POL randomly rotates in the fiber, and once this random POL signal reaches the Polarization Beam Splitter (PBS) in the Rx front-end (Fig. 2), the P signal at either of the PBS branches may (nearly) vanish. The received PxD terms then fades out, causing outages in either of the two X and Y POL components. To achieve POL-MUX DD despite the “pilot POL-fading” effect, various approaches have been explored [4–7], e.g., launching two orthogonally polarized pilots at different frequencies along with sophisticated large-rank MIMO Rx DSP.
Another POL-MUX approach by Infinera  features an optical pre-processing front-end comprising a relatively complex Photonic Integrated Circuit, 8 ADCs (if equalization is done digitally rather than in analog fashion) and 4x2 complex-valued MIMO processing.
We propose a far simpler Rx structure mitigating pilot POL-fading, endowing the DD self-coherent Rx with POL-MUX capability, with 3 ADCs and 3x2 MIMO processing. Our concept was briefly introduced in . The novel principle is based on 2x3 optical front-end, redundantly projecting onto a non-orthogonal 3-dimensional basis, followed by DSP reconstruction. Here we elaborate on the theory of operation and present more extensive simulation results.
For ease of introduction, the concept is initially presented in sections 2-4 in continuous-time (all signals functions of t), under the assumption of a memory-less (frequency-flat) fiber channel free of Inter-Symbol Interference (ISI). Section 5 extends the treatment to more realistic optical channels which are not frequency-flat, e.g. in the presence of Polarization Mode Dispersion (PMD), Chromatic Dispersion(CD) and electrical analog filtering, which impart memory to the channel. POL-demultiplexing by frequency-domain 3x2 MIMO equalization is introduced for a (DFT-Spread) OFDM system and simulation results are presented, indicating that the POL fading impairment is effectively mitigated with smallSignal to Noise Ratio SNR) penalty and even with a small SNR gain, depending on the incoming POL state.
2. Review of conventional self-heterodyne direct-detection Rx – the POL fading problem
Let us consider the spectral allocation of Fig. 1 for a single channel in direct-detection link with pilot and spectral gap, referred to here as self-heterodyne (self-HET). The Transmitter (Tx) transmits a pilot at optical frequency with POL components Px, Py of equal amplitude, along with independent or dependent data signals, Dx, Dy, falling in the optical passband . The system requires a bandwidth of 2W (the spectral support of the overall transmitted signal is ), just half of which is used to carry the modulated signal. A conventional DD POL-Diversity (POL-DIV, as opposed to POL-MUX, as explained in caption of Fig. 2) Rx for such transmitted signal features received signals with spectra shown in Fig. 1(a). This structure will be henceforth referred to as “reference POL-DIV self-HET Rx. It essentially comprises a PBS feeding two photo-diodes (PD) on the X and Y POL arms. The PDs are used to opto-electronically mix the data (D) and pilot (P) components.
The spectral gap introduced in the electrical domain is used to absorb the spurious and mixing products generated in the quadratic photo-detection process. The useful PxD signal is generated in the electrical band [W,2W] and is electrically demodulated to baseband, by either analog or digital means. This is well-known but it is reviewed in the caption of Fig. 1 and generalized further below for two polarizations. If the modulation format for the D signal is OFDM, then the self-heterodyne scheme is referred to as direct-detection OFDM, however, the D signal may have any modulation format (e.g., Nyquist single-carrier), as long as it is confined to the same spectral band.
The analysis to follow points to a key impairment arising in this reference POL-MUX self-HET Rx; this limitation is subsequently mitigated by the proposed Rx of Fig. 2(b). The key impairment precluding POL-MUX is Pilot Polarization Fading: the pilot SOP may randomly transform into or close to either the X or Y linear POLs, depleting the other POL axis of LO, precluding detection along the other POL axis. To analyze this effect, setting the motivation for our improved Rx, we introduce the following model.
The analytic-signal of the band-pass optical field for the respective X and Y POLs is expressed as,Eq. (4) is precluded, as either of the two components of the pilot LO (Eq. (5)) may randomly fade away, nulling out either of the two PxD products in Eq. (4). Thus, self-coherent reception is severely degraded as either may occasionally (nearly) vanish. This fading occurs whenever approaches , nulling out one of the two pilot components in Eq. (5). This is unlike in a coherent receiver, wherein the LO components along the two polarizations are fixed, whereas here the two LOs trade power with each other.
To mitigate the pilot POL fading impairment, one possible solution is to give up POL-multiplexing, i.e., not transmit independent data at all over the two complex envelopes , losing a factor of two in spectral efficiency. POL-DIV operation may be adopted, launching the same complex data , over both polarizations: . The two signals in Eq. (4) now become linearly dependent:10], a 2x1 Dual-Input Single-Out (DISO) adaptive filter may be used in the Tx for coherent combining of the two signals of Eq. (6) with optimal combining coefficients, , yielding a recovered signal:
We next propose a novel Rx structure, enabling to recover the second polarization, even when the two polarizations carry independent data, at the expense of modest investment in extra opto-electronic hardware and DSP.
3. Overview of the novel self-heterodyne direct-detection Rx supporting POL-MUX
The proposed self-coherent Rx with POL-MUX (Fig. 2(b)) is quite efficient in hardware. It comprises a PBS, two optical splitters, an optical directional coupler (ODC), 4 PDs, 3 Trans-Impedance Amplifiers (TIA) and 3 ADCs. Relative to the conventional POL-DIV Rx of Fig. 1(a), which supports just half the data-rate, we have just added a third optical path comprising an ODC, the balanced PD pair, a TIA and an ADC. The only change in DSP is to use a 3x2 POL-demux MIMO filter, rather than the conventional 2x2 MIMO. This entails using two extra complex-multipliers (CM) (increasing the CMs count from 4 to 6). The front-end optical elements are further amenable to photonic integration over a cost-effective passive photonic integrated circuit structure comprising just the PBS (e.g., a grating coupler) 2 OSs and ODC. However, even discrete-component realizations may be cost-effective, as the PBS, OS and ODC + balanced PD components are readily available with fiber pigtails in discrete form.
This relatively simple receiver structure enables doubling the spectral efficiency by concurrently transmitting independent data on the X and Y orthogonal polarizations.
We briefly overview here the principle of operation of the novel POL-MUX DD Rx (the explanation is further elaborated and quantified in the next section).
As already illustrated in Fig. 2(b) we have added a new path, featuring a hybrid, comprising a 3 dB coupler combining the signals from the X and Y POL arms, feeding a photo-diode (PD) balanced pair, a TIA and an extra ADC. This extra path generates amixing product enabling to mitigate pilot POL-fading. The ADC outputs yield baseband signals, after some pre-DSP. The 2x2 MIMO POL-demux DSP of a conventional coherent receiver is replaced here by 3x2 MIMO processing, as shown in Fig. 2(b), yielding two decoupled POL streams, optionally processed in a conventional Rx DSP back-end by any of the scalar self-coherent methods [1,2], applied to each of the two decoupled POLs. The new 3x2 MIMO equalizer has its coefficients adaptively adjusted, e.g. by means of a Least-Mean-Square (LMS) data-aided algorithm, further simulated below.
The key idea is that if the pilot fades out along one of the two polarization axes, it must then become strong along the other polarization axis, thus it is now going to be the new path, that will generate a reliable signal. E.g. if the pilot fades along X (precluding detection of the X-data via ) then the pilot must be strong along Y (i.e., is strong), then now contains a strong term. Thus, rather than appearing on the X output, the reconstructed data, , now appears on new output. Similarly, if P fades along Y (precluding detection of the Y-data via ) then must be strong, then the path contains a term, enabling to reliably detect . These have been limiting cases wherein the pilot completely faded along one of the two POL axes. In these cases the 3x2 MIMO equalizer routes the output to the X output port whereas the Y output is routed to the Y output port. In contrast, when the pilot has nearly equal powers in its two polarization components, the three X, Y and measurements generated by the optical front-end are redundantly used to reliably recover received POL. The next section analyzes the process of reliable extraction of two POL components from the three inputs, irrespective of the data and pilot SOPs, establishing that in certain cases, it is possible to even improve the SNR relative to a 2x2 conventional system (which would only function well when the pilot has nearly equal powers in the two orthogonal received polarization).
4. Theory of operation
Our novel solution to mitigating POL-fading in direct detection with pilot + guardband is to add a hybrid yielding a third POL measurement electro-optically mixing signals from the two polarizations (Fig. 2(b)).
The 50/50 ODC outputs generate the sum and difference of the two optical fields fed into the ODC input ports. The new XxY branch output cross-term,, is given by the difference of the two PD photo-currents, reducing to the conjugate product of the two CE signals fed into the two hybrid ports:Eq. (4) to provide a complete set of three projections,
The 3 rows of the analysis matrix, P are three vectors onto which the received Jones 2-vector, , is projected, yielding the three “measurements”, . The three vectors are referred to in modern linear algebra as a frame , a redundant representation, as in 2-dimensional space, two orthogonal projections would suffice (whereas we have here three vectors, namely the three rows of . Frames are sets of over-complete (redundant) dependent vectors, generalizing the concept of linear algebra base (here the three rows of are not independent, but they span ). Given a properly specified frame, a vector to be measured may always be reconstructed from the frame coefficients, namely the projections of the given vector onto the frame vectors. In the current finite-dimensional context, thematrix multiplication projects -vector onto the three frame vectors, yielding the three frame coefficients . Here is a new measurement, further to the two orthogonal components, , which are usually measured.
The full column rank of (having its two columns independent) is necessary in order to have its three rows form a frame, enabling a unique inverse transformation reconstructing the vector from the projection measurements. The independence of the two columns of implies that there is a unique vector, , yielding the measured frame coefficients , thus the frame projection may be always inverted from these three inner products of the vector with the three rows of .
Back to the matrix-oriented view, the PI is readily evaluated in terms of the Moore Penrose formula of Eq. (11). We first evaluate the Gram matrix and its inverse, which are invertible here, since has full column rank:Eq. (15)). As is a left-inverse of , namely , we obtain ideally perfect reconstruction:
An intuitive sense why the third middle branch (the W-signal) resolves the remote-LO POL fading may be obtained by inspecting pathological POL-fading cases, such as ,() in Eqs. (5), (17). In this case the X-branch peaks out, , whereas the Y-branch completely fades out, , but the new third branch (Eq. (9)) comes to the rescue, recovering the lost Y-data:Eq. (14), yielding:
A unique feature of our solution is that the measurement frame is not fixed but it varies with the SOP of the pilot. Assume, without loss of generality, that the pilot is received linearly polarized, . Then, the analysis matrix, P, (Eq. (10)), becomes real-valued and the three frame vectors (rows of the P matrix) reduce to,Fig. 3 for nine pilot tilt angles, ranging from 0 to . The extreme frames on the diagonal are seen to degenerate from proper three-vectors frames to orthogonal bases, each containing just two vectors. Following the color code, the U (V)-measurement and its associated vector is red (blue), while the new W-measurement is green. At the Y (X) pilot fades out thus the blue (red) vector is gone from the frame, but the missing vector (be it red or blue) is replaced by the green vector, as generated by the third measurement. The 3-vectors frame now degenerates to an orthogonal basis of two vectors, in these extreme cases. The system is invertible at anyvalue (SOP of the pilot). It is just that in the extreme cases, the three vectors frame degenerates to a two-vector orthogonal base, whereas in all other cases, projections onto proper frames are generated.
The pseudo-inversion procedure described above is able to reconstruct the received Jones vector, , with components which are linear mixtures of the two transmitted polarizations:
The two outputs of the 3x2 MIMO equalizers are estimates of the two independently transmitted X and Y signals. In the proposed system, there is no need to separately evaluate the pseudo-inverse and the Jones inverse and take their product. The 2x3 MIMO equalizer matrix is generated at once, adaptively estimated using a data-aided LMS algorithm, directly acquiring and tracking the six elements of the matrix by adaptively adjusting the 3x2 MIMO equalizer taps such that they effectively form a matrix reconstructing a vector close (in the mean-square sense) to the transmitted training data-vector, .
5. Numeric simulations
Extension to optical channels with memory: The analysis heretofore assumed for simplicity a frequency-flat (memoriless) optical channel. Over a frequency-colored OFDM link (e.g. for long-haul transmission, in the presence of CD and PMD effects, which induce channel memory) one should resort to a frequency-domain version of proposed scheme, whereby 3x2 MIMO processing is separately applied onto each OFDM sub-carrier. Each of the three X,Y and XxY paths are fed into to M-point FFTs. Corresponding index sub-carriers from the three FFT outputs are then input into 3x2 MIMO modules, yielding POL-demultiplexed X and Y signals for each sub-carrier. The 6 complex taps of each 3x2 MIMO module per sub-carrier are adjusted by a data-aided LMS algorithm. An OFDM cyclic prefix (CP) of proper length is assumed. This provides the complete specification of the OFDM POL-demux algorithm.
In the simulations below for the optical amplified POL-MUX self-HET OFDM link of Fig. 4, we assume OFDM with M = 128 subcarriers (128 samples OFDM symbol), and a CP of 2 samples. The simulation results are presented in Figs. 5 and 6 numerically demonstrating that the proposed Rx of Fig. 2(b), based on the novel 2x3 photonic FE and 3x2 MIMO DSP with LMS data-aided tracking, is capable of robustly carrying POL-MUX double rate transmission of two independent 16-QAM OFDM streams, while mitigating POL fading over arbitrarily rotated SOPs in the fiber link.
Figure 5 presents the various IQ plane constellations for the worst case that the SOP rotation is the fiber is such that the pilot, launched at relative to the principal axes of the Tx, is received aligned with the X-axis. In this case, the POL measurement along the Y-axis completely fades out, but the third POL measurement, , enables recovering the faded POL as explained in the last section. The detected SNR at the output of the 3x2 MIMO stage, as measured in our simulation, is 17.9 dB, lower by just 0.4 dB than a reference SNRref = 18.3 dB. The reference system is defined as follows: A POL-DIV self-HET Tx transmitting the same signal on both POLs (such that the Tx signal POL is at deg with respect to the PBS principal axes, i.e., aligned with the transmitted pilot). The energy transmitted per bit is normalized to be the same in our system as in the reference system, therefore, since our POL-MUX system carries twice the data rate, then its power is twice as large as that of the reference system (which is set at half the power of the system under test).
The worst case then occurs when the received pilot SOP is aligned with one of the Rx PBS axes (Fig. 5), yielding SNR lower by just 0.4 dB than the reference SNR, a small penalty worth paying for doubling data throughput.
The most favorable case (Fig. 6(g)-6(m), corresponding to null SOP rotation angle in the fiber, , such that the received SOP is identical to the transmitted SOP) occurs when the pilot SOP is received at with respect to the PBS principal axes, In this case SNR attains its optimal value over all pilot POL angles, at 19.21 dB, exceeding by 0.91 dB the reference case with same energy per bit. The source of the SNR improvement is that the redundant frame projection onto the frame of three vectors provides processing gain. An interim case is shown in Fig. 6(a)-6(f). In this case, the fiber SOP rotation angle is , such that the received SOP makes an angle of with the X-axis and the Y-signal partially fades out. Nevertheless the redundant 3x2 processing recovers “clean” 16-QAM constellations for the X and Y components with SNR >18.3dB, just a 0.9 dB degradation relative to that for to the optimal SOP.
For comparison, we also present in Fig. 7 the received constellations for the naïve 2x2 MIMO Rx of Fig. 2(a) based on straightforward detection on the two PBS outputs (lacking the proposed XxY cross-POL signal). Surprisingly, for an ideal SOP of whereby both X and Y pilot POLs are detected with equal pilot powers, the 3x2 Rx is seen to provide 1.0 dB SNR gain relative to the 2x2 Rx (cf. Fig. 6(l), 6(m) vs. Figure 7(g), 7(h)). We conjecture that this SNR gain stems from the third XxY cross signal, averaging out the signal x ASE noise terms. A thorough signal vs. noise propagation analysis is deferred to future work. Comparing Fig. 6(e), 6(f), vs. Figure 7(c), 7(d) for the case of pilot SOP at , corresponding to partial pilot fading, the 3x2 Rx yield a 2.8 dB advantage vs. the 2x2 Rx, consistent with the pilot fading visible in Fig. 6(b).
6. Comparisons with alternative schemes
Adding the third XxY path in the Rx in Fig. 2(b) may be deemed a reasonable price to pay (relative to the naïve Rx of Fig. 2(a), which is affected by POL-fading) enabling spectral efficiency doubling by signaling over both polarizations with direct-detection, making this proposed Rx particularly useful in optical access applications which may not tolerate the higher cost of a fully coherent Rx. The enhanced spectral efficiency may be alternatively applied to reduce electrical bandwidth rather than transmitting higher data rates, hence cutting Rx costs for cost-sensitive short-reach links.
Traditionally, for direct-detection receivers, polarization multiplexing has been contemplated to be performed by active polarization controllers, which have attained very high performance, see [12,13], however such devices are currently not yet cost-effective, hence the polarization control approach has not been adopted in practical use.
Briefly reviewing other approaches for POL-MUX over direction detection, Refs [4,6,7]. use either 4x4 MIMO or a simplified version of block diagonal 4x4 MIMO or alternatively switch among various MIMO levels depending on the received states of polarization. Ref . incurs some reduced spectral efficiency due to pilot carriers and requires some higher rank MIMO processing. These schemes are affected by higher DSP complexity than the proposed scheme.
A similar principle enabling to decode the data irrespective of the POL alignment of the Rx, features in , restricted to DQPSK direct-detection with POL-MUX (namely mixing X and Y POLs to generate new redundant terms as in our scheme), however based on a more complex optical front-end based on 8-way rather than 3-way redundancy. Another characteristic of our scheme (not enjoyed by the scheme of ) is that our photonic integrated circuit tolerances are relaxed - there is no need to accurately control the optical path lengths on the waveguides –slacks in couplings and phase-shifts are adaptively corrected in DSP.
Note that the spectral efficiency of our scheme may be further improved by resorting to various DSP methods [1,2] in order to separate out the DxD term while reducing or eliminating the wasteful spectral gap.
DSP complexity: The 3x2 MIMO equalizer requires 6 complex multipliers per sample, for POL-demultiplexing a frequency-flat optical link, which is also the case treated in . If implemented with 4 real multipliers per complex multiplier, as was done in Table 1 of , then our 3x2 MIMO equalizer would require 24 real multipliers which is more favorable than the other entries in Table 1 of  (the minimal value there being 32 real multipliers).
Over a frequency-colored link (e.g. for long-haul transmission, in the presence of CD and PMD which induce channel memory) one should use the frequency-domain version of proposed scheme, whereby 3x2 MIMO processing is applied onto each sub-carrier. In this case the 6 complex multipliers per sample direct contribution of the MIMO processing remains effectively valid (as the number of 3x2 MIMO modules equals the number M of subcarriers in the OFDM symbol, however each of these 3x2 MIMO modules operates M times slower). However, the complexity count should now also comprise a new FFT to be added in the third XxY branch in addition to the two FFTs already present on the X and Y branches for conventional OFDM detection. In our system, as M = 128, the complexity per sample of the 128-FFT is 0.5M log(2,M) / M = 3.5 complex multipliers. However, this extra FFT complexity should not be tallied in the comparison vs , as the extra FFT is associated with frequency-colored channels, which are outside the scope of  (the complexity in  would further increase if that scheme were to be adapted to channels with memory, which has not been done there).
In this paper we enabled POL-MUX advanced modulation formats over direct-detection, without active POL control, at very low extra complexity. The direct-detection receiver features a simple optical front-end and just slightly more complex DSP than a reference self-het receiver with dual polarization paths (e.g. as used in polarization diversity). The proposed scheme is modulation format-transparent e.g., supports POL-MUX m-QAM for higher capacity at short-reach, either (DFT-Spread) OFDM or Nyquist-shaped single-carrier.
The system is DSP-compatible with prior-art system variants [1,2] which aim at reducing or eliminating the wasteful spectral gap. Our solution may be useful in addressing the need for low-cost high-capacity links in access networks and short-reach optical interconnects in the range of ~10-100 km. It would also be interesting to explore comparisons with coherent detection over longer metro links, and in particular relation of the self-heterodyne technique with simplified coherent heterodyne technique in .
We acknowledge the kind support of the Israel Science Foundation (ISF) and of the Piano + OTONES trans-national EU photonics program.
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