Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (2024)

CunLiang MaSchool of Information Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, China  WeiGuang ZhouSchool of Information Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, China  Zhoujian Cao111corresponding authorzjcao@amt.ac.cnInstitute of Applied Mathematics, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190, ChinaSchool of Fundamental Physics and Mathematical Sciences, Hangzhou Institute for Advanced Study, UCAS, Hangzhou 310024, China

Abstract

Future GW observatories, such as the Einstein Telescope (ET), are expected to detect gravitational wave signals, some of which are likely to overlap with each other. This overlap may lead to misidentification as a single GW event, potentially biasing the estimated parameters of mixture GWs. In this paper, we adapt the concept of speech separation to address this issue by applying it to signal separation of overlapping GWs. We show that deep learning models can effectively separate overlapping GW signals. The proposed method may aid in eliminating biases in parameter estimation for such signals.

I Introduction

The field of gravitational-wave (GW) detection has witnessed remarkable progress since the first direct detection [1, 2, 3, 4, 5, 6, 7]. The third observing run (O3) of GW detection ended in spring 2020, boosting the total number of confident events to above 90, with an event rate currently standing at 1.5 per week [8, 9, 10, 11]. However, the upcoming third-generation (3G) detectors such as Einstein Telescope (ET) [12, 13] and Cosmic Explorer (CE) [14, 15], envisioned in the 2030s, promise a significant leap forward. This enables the detection rate of above 105superscript10510^{5}10 start_POSTSUPERSCRIPT 5 end_POSTSUPERSCRIPT per year at cosmological distances. The surge in detection rate, along with the remarkable enhancement of sensitivity across both lower and higher frequency ranges in 3G detectors, will significantly extend the duration of signals within the sensitivity band. As a consequence, the probability of GW signals overlapping in these 3G detectors will become significant [16], posing potential challenges for the GW search and parameter estimation.

As early as 2009, T. Regimbau and Scott A. Hughes delved into the effects of binary inspiral confusion on the sensitivity of ground-based GW detectors [16]. They emphasized the necessity for rigorous data analysis to disentangle mixture signals. Since then, numerous studies have focused on analyzing the strain with mixture signals. Y. Himemoto et al., utilizing Fisher matrix analysis, explored the statistical ramifications of mixture GWs on parameter estimation [17]. Their findings revealed that mixture signals can introduce notable statistical errors or systematic biases, especially when the coalescence times and redshifted chirp masses of the mixture GWs are closely matched. A realistic distribution analysis further indicated that mergers occurring within a second of each other are common occurrences over a year in 3G detectors [18].

Modern data analysis techniques for parameter estimation typically assume the presence of a single signal amidst background noise. However, when two or more GWs are simultaneously detected, their signals overlap, creating a distorted, non-physical waveform. This leads the sampling software to identify parameter sets aligned with this composite waveform, rather than the individual signals [19]. Experimental results from P. Relton et al. demonstrated that, in most instances, current parameter estimation methods can accurately assess the parameters of one of the mixture events [19]. Notably, if one signal is at least three times stronger than the other, the louder signal’s source parameters remain unaffected [19]. By applying a narrow prior on the coalescence time, obtained during the GW detection phase, it may be feasible to accurately recover both posterior parameter distributions [20]. Experiments conducted by E. Pizzati et al. showed that parameter inference remains robust as long as the coalescence time difference in the detector frame exceeds 1 second [20]. Conversely, when this time difference is less than 0.5 seconds, significant biases in parameter inference are likely to emerge [20]. Upon comparing the effects of mixture signals on coefficients at various post-Newtonian (PN) orders, it has been determined that, overall, the 1PN coefficient experiences the greatest impact. The findings further indicate that, although a significant proportion of mixture signals introduce biases in PN coefficients, which individually might suggest deviations from General Relativity (GR), collectively, these deviations occur in random directions. As a result, a statistical aggregation of these effects would still tend to align with GR [21]. Quantifying source confusion within a realistic neutron star binary population reveals that parameter uncertainty generally rises by less than 1%, except in cases where overlapping signals exist with a detector-frame chirp mass difference of 0.01Mless-than-or-similar-toabsent0.01subscript𝑀direct-product\lesssim 0.01\,M_{\odot}≲ 0.01 italic_M start_POSTSUBSCRIPT ⊙ end_POSTSUBSCRIPT and an overlap frequency of 40Hzgreater-than-or-equivalent-toabsent40Hz\gtrsim 40\,\text{Hz}≳ 40 Hz [22]. Among 1×1061superscript1061\times 10^{6}1 × 10 start_POSTSUPERSCRIPT 6 end_POSTSUPERSCRIPT simulated signals, only 0.14% fall within this specific range of detector-frame chirp mass differences, yet their overlap frequencies are usually below 40 Hz [22].

Apart from the task of parameter estimation, several studies focus on exploring the impact of overlapping signals on gravitational wave detection. Within the CWB framework for GW searching, most signals resulting from closely merged events will only be detected as a single trigger [23]. In the context of the PyCBC framework and the search for binary black hole (BBH) events, it has been noted that when the relative merger time exceeds 1 second, the search efficiency diminishes by approximately 1% [23]. In cases where the relative merger time is less than 1 second, the search efficiency drops by 26% because most paired signals are either detected by a single trigger or not detected at all [23]. The biases in the estimation of the PSD will negatively impact the sensitivity of the 3G ground-based GW detectors, especially considering the large population of overlapping signals [24]. The confusion noise’s contribution to the signal-to-noise ratio (SNR) is considerably lesser than that of the instrumental noise [24].

Certain studies focus on refining data processing techniques to address the challenge posed by overlapping signals. J. Janquart et al. analyze the overlapping binary black hole merger with hierarchical subtraction and joint parameter estimation [25]. They find that joint parameter estimation is usually more precise but comes with higher computational costs. J. Langendorff et al. first utilize normalizing flows for the parameter estimation of overlapping GW signals [26]. Compared to the traditional Bayesian method, the normalizing flow results in broader posterior distributions, whereas the Bayesian-based approach tends to become overconfident, potentially overlooking the injection [26].

Recently, we have proposed a novel framework (MSNRnet) aimed at accelerating the matched filtering process for GW detection [27]. This is achieved by incorporating deep learning techniques for waveform extraction and discrimination. However, as the waveform extraction stage solely captures one waveform, in scenarios where multiple signals overlap, there is a possibility that the MSNRnet framework may overlook one of the overlapping signals.

Real-world speech communication frequently takes place in vibrant, multi-speaker settings [28]. To function effectively in these environments, a speech processing system must possess the capability to distinguish and separate speeches from various speakers. While this endeavor comes naturally to humans, it has been exceedingly challenging to replicate in machines. However, in recent years, deep learning strategies have notably pushed the boundaries of this problem [29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40], surpassing traditional techniques like independent component analysis (ICA) [41] and semi-nonnegative matrix factorization (semi-NMF) [42]. The mixed speech can be compared to mixed GW signals. Drawing inspiration from the task of speech separation, this study marks the first attempt to apply deep learning to GW separation. The proposed method for GW signal separation holds potential for future applications in GW search and parameter estimation. Furthermore, this work serves as a complement to the existing tasks of deep learning applied to GW data processing, including end-to-end GW signal search [43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61], parameter estimation [62, 63, 64, 65, 66], waveform or envelope extraction [67, 68, 69, 70], GW source localization [71, 72, 73, 74], and glitch classification [75, 76, 77, 78]. Since the GW components buried in noise, the GW separation task is more challenging than speech separation.

In this work, we first explored the potential of utilizing deep learning for GW separation. We find that the mixture strain with noise and multi-signals can be separated.

II METHOD FOR GW SEPARATION

In the early stages of applying deep learning to speech separation, the preprocessing phase typically involved converting mixed sound into a time-frequency representation [79, 80, 81, 82], isolating source bins via time-frequency masks, and synthesizing waveforms via invert time-frequency transform. However, challenges arose, including the optimality of Fourier decomposition and the need to handle both magnitude and phase in the complex STFT domain. This often led to methods that only adjusted the magnitude, ultimately limiting separation performance. In 2018, Luo et al. introduced the Time-domain Audio Separation Network (TasNet) [28]. This neural network was designed to directly model the time-domain mixture waveform through an encoder-separation-decoder framework, where the actual separation occurred at the encoder’s output. The following year, they further refined TasNet, evolving it into Conv-TasNet [29]. The key innovation of Conv-TasNet was the use of a Temporal Convolutional Network (TCN) for the separation component, consisting of stacked one-dimensional dilated convolutional blocks. In 2020, the same team proposed DPRNN [32], which incorporated a dual-path RNN for the separation phase. Later that year, J. Chen et al. enhanced DPRNN, giving birth to DPTNet [34]. This advancement replaced the dual-path RNN module with a dual-path transformer module. We have utilized all three iterations of TasNet—Conv-TasNet, DPRNN, and DPTNet—for the task of GW separation. Among these, we find that DPRNN has proven to be superior to the other two methods. So, in this work, we focus on DPRNN for GW separation.

Suppose that the strain captured by the interferometer, denoted as d(t)𝑑𝑡d(t)italic_d ( italic_t ), can be regarded as a combination of a noise component, n(t)𝑛𝑡n(t)italic_n ( italic_t ), and the GW component, h(t)𝑡h(t)italic_h ( italic_t ).

d(t)=n(t)+h(t)𝑑𝑡𝑛𝑡𝑡d\left(t\right)=n\left(t\right)+h\left(t\right)italic_d ( italic_t ) = italic_n ( italic_t ) + italic_h ( italic_t )(1)

For the GW separation task, the GW component comprises multiple signals, represented as h(t)=i=1Nhi(t)𝑡superscriptsubscript𝑖1𝑁subscript𝑖𝑡h\left(t\right)=\sum_{i=1}^{N}{h_{i}(t)}italic_h ( italic_t ) = ∑ start_POSTSUBSCRIPT italic_i = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_N end_POSTSUPERSCRIPT italic_h start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_t ), where hi(t)subscript𝑖𝑡h_{i}(t)italic_h start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_t ) signifies each individual GW signal, and N𝑁Nitalic_N signifies the overall count of GW signals existing in the analyzed data segment. In this work, we will solely focus on the scenario where there are two signals hA(t)subscript𝐴𝑡h_{A}(t)italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) and hB(t)subscript𝐵𝑡h_{B}(t)italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ) present in the data. So

d(t)=n(t)+hA(t)+hB(t).𝑑𝑡𝑛𝑡subscript𝐴𝑡subscript𝐵𝑡d\left(t\right)=n\left(t\right)+h_{A}\left(t\right)+h_{B}\left(t\right).italic_d ( italic_t ) = italic_n ( italic_t ) + italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) + italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ) .(2)

We aim to directly estimate hA(t)subscript𝐴𝑡h_{A}\left(t\right)italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) and hB(t)subscript𝐵𝑡h_{B}\left(t\right)italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ) from d(t)𝑑𝑡d\left(t\right)italic_d ( italic_t ). The TasNet-like framework decomposes the signal separation task into three stages: Encoder, Separation, and Decoder, and the overall framework for GW separation is shown in Fig.1. During the Encoder stage, the input signal is encoded into a hidden layer feature F𝐹Fitalic_F. In the Separation stage, masks (MAsubscript𝑀𝐴M_{A}italic_M start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT and MBsubscript𝑀𝐵M_{B}italic_M start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT) for each signal component are evaluated. Subsequently, the Decoder stage utilizes these masked features to obtain the separated output as follows:

h~Asubscript~𝐴\displaystyle\widetilde{h}_{A}over~ start_ARG italic_h end_ARG start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT=Decoder(MAF),absentDecoderdirect-productsubscript𝑀𝐴𝐹\displaystyle=\text{Decoder}\left(M_{A}\odot F\right),= Decoder ( italic_M start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ⊙ italic_F ) ,(3)
h~Bsubscript~𝐵\displaystyle\widetilde{h}_{B}over~ start_ARG italic_h end_ARG start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT=Decoder(MBF).absentDecoderdirect-productsubscript𝑀𝐵𝐹\displaystyle=\text{Decoder}\left(M_{B}\odot F\right).= Decoder ( italic_M start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ⊙ italic_F ) .(4)

where direct-product\odot denotes the Hadamard product. The Encoder, Separation, and Decoder stages can be likened to the STFT, time-frequency masking, and inverse STFT stages respectively, of signal separation utilized by the short-time Fourier transform. In the subsections that follow, we will elaborate on the three stages of GW separation.

Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (1)

II.1 Encoder stage

Suppose the Encoder receives an input signal s1×L𝑠superscript1𝐿s\in\mathbb{R}^{1\times L}italic_s ∈ blackboard_R start_POSTSUPERSCRIPT 1 × italic_L end_POSTSUPERSCRIPT, where L𝐿Litalic_L denotes the number of time samples of the input strain. Through the Encoder stage, we get the signal feature FC×L𝐹superscript𝐶𝐿F\in\mathbb{R}^{C\times L}italic_F ∈ blackboard_R start_POSTSUPERSCRIPT italic_C × italic_L end_POSTSUPERSCRIPT by

F=ReLU(Conv1D(s)),𝐹ReLUConv1D𝑠F=\text{ReLU}\left(\text{Conv1D}\left(s\right)\right),italic_F = ReLU ( Conv1D ( italic_s ) ) ,(5)

where in the 1D convolutional layer, C=256 filters are used, and the filter size is configured to 2.

II.2 Separation stage

The input of the separation stage is signal feature F𝐹Fitalic_F and the output generates two feature masks namely MAsubscript𝑀𝐴M_{A}italic_M start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT and MBsubscript𝑀𝐵M_{B}italic_M start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT. The signal feature F𝐹Fitalic_F is initially passed through Layer Normalization and a Conv1D layer, undergoing transformation into a tensor representation having a shape of N×Lsuperscript𝑁𝐿\mathbb{R}^{N\times L}blackboard_R start_POSTSUPERSCRIPT italic_N × italic_L end_POSTSUPERSCRIPT where N=64𝑁64N=64italic_N = 64 represents the number of Conv1D filters. Afterward, the tensor sequentially undergoes a segmentation operation, followed by processing through four DPRNN blocks, and concludes with an overlap-add operation. In the segmentation step, the 2D tensor undergoes a transformation into a 3D tensor through sub-frame alternation. This transformed tensor is then relayed to a stack of DPRNN blocks, where both local and global modeling are alternately and interactively employed. Upon completion of DPRNN processing, the output from the final layer is conveyed to a 2D convolutional layer and subsequently reverted to two 2D tensors via the Overlap-Add operation. These tensors are then simultaneously processed through two distinct convolutional modules equipped with different activation functions: Tanh and Sigmoid. Following this, the tensors are combined and subjected to a ReLU activation function, ultimately yielding two masks, designated as MAsubscript𝑀𝐴M_{A}italic_M start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT and MBsubscript𝑀𝐵M_{B}italic_M start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT.

II.2.1 Segmentation and Overlap-Add

Fig.2 shows the flow chart of the Segmentation and Overlap-Add step in the separation stage. Let the input of the segmentation is a 2D tensor F𝐹Fitalic_F and the output of the segmentation is a 3D tensor T𝑇Titalic_T. For the segmentation stage, we first split the 2D tensor to S𝑆Sitalic_S small tensors (DiN×Ksubscript𝐷𝑖superscript𝑁𝐾D_{i}\in\mathbb{R}^{N\times K}italic_D start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_K end_POSTSUPERSCRIPT, i{1,2,,S}𝑖12𝑆i\in\{1,2,\ldots,S\}italic_i ∈ { 1 , 2 , … , italic_S }). Then concatenate all the small 2D tensors together to form a 3D tensor T=[D1,D2,,DS]N×K×S𝑇subscript𝐷1subscript𝐷2subscript𝐷𝑆superscript𝑁𝐾𝑆T=[D_{1},D_{2},\ldots,D_{S}]\in\mathbb{R}^{N\times K\times S}italic_T = [ italic_D start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_D start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_D start_POSTSUBSCRIPT italic_S end_POSTSUBSCRIPT ] ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_K × italic_S end_POSTSUPERSCRIPT. In this work K=250𝐾250K=250italic_K = 250 and S=134𝑆134S=134italic_S = 134.

Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (2)

Suppose the output of the last DPRNN block as TB+1N×K×Ssubscript𝑇𝐵1superscript𝑁𝐾𝑆T_{B+1}\in\mathbb{R}^{N\times K\times S}italic_T start_POSTSUBSCRIPT italic_B + 1 end_POSTSUBSCRIPT ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_K × italic_S end_POSTSUPERSCRIPT, then the Overlap-Add step can be seen as the inverse process of the Segmentation step. It applies the S𝑆Sitalic_S 2D tensors to form output QN×L𝑄superscript𝑁𝐿Q\in\mathbb{R}^{N\times L}italic_Q ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_L end_POSTSUPERSCRIPT. Initially, we split the 3D tensor into S𝑆Sitalic_S 2D tensors and aligned according to real-time. Following this, we added the S𝑆Sitalic_S 2D tensors up and got one 2D tensor.

II.2.2 DPRNN block

The segmentation output T𝑇Titalic_T is subsequently forwarded to a stack consisting of 4 DPRNN blocks. Each block maps a 3D tensor into another 3D tensor of the same shape. Let’s take the map TiTi+1subscript𝑇𝑖subscript𝑇𝑖1T_{i}\rightarrow T_{i+1}italic_T start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT → italic_T start_POSTSUBSCRIPT italic_i + 1 end_POSTSUBSCRIPT as an example to illustrate the calculation process of a DPRNN block. The flow chart depicting the DPRNN block is illustrated in Fig.3. Initially, the input tensor is processed through a local modeling block, followed by a global modeling block. The key distinction between these two blocks lies in their approach to signal slicing. Specifically, the local modeling block slices the 3D tensor based on the third indicator, whereas the global modeling block performs slicing using the second indicator. For brevity, we only detail the mathematical expression pertaining to local modeling in this context.

Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (3)

Suppose the input of the local modeling is Tisubscript𝑇𝑖T_{i}italic_T start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT and the output is T^isubscript^𝑇𝑖\hat{T}_{i}over^ start_ARG italic_T end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT. We first put each divided chunk to a bidirectional LSTM block and concatenate them together to get a tensor UiH×K×Ssubscript𝑈𝑖superscript𝐻𝐾𝑆U_{i}\in\mathbb{R}^{H\times K\times S}italic_U start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∈ blackboard_R start_POSTSUPERSCRIPT italic_H × italic_K × italic_S end_POSTSUPERSCRIPT. In this work, we set H𝐻Hitalic_H to 256.

Ui=Concatenate𝑗BiLSTM(Ti[:,:,j]),subscript𝑈𝑖𝑗ConcatenateBiLSTMsubscript𝑇𝑖::𝑗U_{i}=\underset{j}{\operatorname*{Concatenate}}\text{ BiLSTM}(T_{i}[:,:,j]),italic_U start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = underitalic_j start_ARG roman_Concatenate end_ARG BiLSTM ( italic_T start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT [ : , : , italic_j ] ) ,(6)

where Ti[:,:,j]N×Ksubscript𝑇𝑖::𝑗superscript𝑁𝐾T_{i}[:,:,j]\in\mathbb{R}^{N\times K}italic_T start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT [ : , : , italic_j ] ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_K end_POSTSUPERSCRIPT is the sequence defined by chunk j𝑗jitalic_j. We then apply a fully connected layer to the tensor Uisubscript𝑈𝑖U_{i}italic_U start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT and obtain U^iN×K×Ssubscript^𝑈𝑖superscript𝑁𝐾𝑆\widehat{U}_{i}\in\mathbb{R}^{N\times K\times S}over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_K × italic_S end_POSTSUPERSCRIPT as follows:

U^i=Concatenate𝑗GUi[:,:,j]subscript^𝑈𝑖𝑗Concatenate𝐺subscript𝑈𝑖::𝑗\widehat{U}_{i}=\underset{j}{\operatorname*{Concatenate}\,}GU_{i}[:,:,j]over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = underitalic_j start_ARG roman_Concatenate end_ARG italic_G italic_U start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT [ : , : , italic_j ](7)

where GN×H𝐺superscript𝑁𝐻G\in\mathbb{R}^{N\times H}italic_G ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × italic_H end_POSTSUPERSCRIPT. Then Layer normalization is applied to U^isubscript^𝑈𝑖\widehat{U}_{i}over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT as follows:

LN(U^i)=U^iμ(U^i)σ(U^i)+ϵz+r𝐿𝑁subscript^𝑈𝑖direct-productsubscript^𝑈𝑖𝜇subscript^𝑈𝑖𝜎subscript^𝑈𝑖italic-ϵ𝑧𝑟LN(\widehat{U}_{i})=\frac{\widehat{U}_{i}-\mu(\widehat{U}_{i})}{\sqrt{\sigma(%\widehat{U}_{i})+\epsilon}}\odot z+ritalic_L italic_N ( over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) = divide start_ARG over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_μ ( over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) end_ARG start_ARG square-root start_ARG italic_σ ( over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) + italic_ϵ end_ARG end_ARG ⊙ italic_z + italic_r(8)

where z,rN×1𝑧𝑟superscript𝑁1z,r\in\mathbb{R}^{N\times 1}italic_z , italic_r ∈ blackboard_R start_POSTSUPERSCRIPT italic_N × 1 end_POSTSUPERSCRIPT are the rescaling factors, ϵitalic-ϵ\epsilonitalic_ϵ is a small positive number for numerical stability, and μ()𝜇\mu(\cdot)italic_μ ( ⋅ ) and σ(U^i)𝜎subscript^𝑈𝑖\sigma({\widehat{U}}_{i})italic_σ ( over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) represent the mean and standard deviation operators, respectively. Then we get T^isubscript^𝑇𝑖\widehat{T}_{i}over^ start_ARG italic_T end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT as follows:

T^i=Ti+LN(U^i).subscript^𝑇𝑖subscript𝑇𝑖𝐿𝑁subscript^𝑈𝑖\widehat{T}_{i}=T_{i}+LN(\widehat{U}_{i}).over^ start_ARG italic_T end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = italic_T start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_L italic_N ( over^ start_ARG italic_U end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) .(9)

Put the 3D tensor T^isubscript^𝑇𝑖\widehat{T}_{i}over^ start_ARG italic_T end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT to the global modeling block, we then get the output of the DPRNN block Ti+1subscript𝑇𝑖1T_{i+1}italic_T start_POSTSUBSCRIPT italic_i + 1 end_POSTSUBSCRIPT.

II.3 Decoder stage

The Decoder stage maps the masked Encoded feature FMi=MiFC×Lsubscript𝐹subscript𝑀𝑖direct-productsubscript𝑀𝑖𝐹superscript𝐶𝐿F_{M_{i}}=M_{i}\odot F\in\mathbb{R}^{C\times L}italic_F start_POSTSUBSCRIPT italic_M start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_POSTSUBSCRIPT = italic_M start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ⊙ italic_F ∈ blackboard_R start_POSTSUPERSCRIPT italic_C × italic_L end_POSTSUPERSCRIPT to separated signal. Each element in FMisubscript𝐹subscript𝑀𝑖F_{M_{i}}italic_F start_POSTSUBSCRIPT italic_M start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_POSTSUBSCRIPT (which can be likened to feature values) may be viewed as a component of a hidden vector (comparable to feature vectors) at a specific time.

h~i=ConvTranspose1d(FMi),subscript~𝑖𝐶𝑜𝑛𝑣𝑇𝑟𝑎𝑛𝑠𝑝𝑜𝑠𝑒1𝑑subscript𝐹subscript𝑀𝑖\tilde{h}_{i}=ConvTranspose1d(F_{M_{i}}),over~ start_ARG italic_h end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = italic_C italic_o italic_n italic_v italic_T italic_r italic_a italic_n italic_s italic_p italic_o italic_s italic_e 1 italic_d ( italic_F start_POSTSUBSCRIPT italic_M start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_POSTSUBSCRIPT ) ,(10)

The hidden vectors can be regarded as the adjustable parameters of the transposed convolutional layer. This layer accepts N𝑁Nitalic_N input channels and outputs a single channel. Its purpose is to decrease the channel count of the masked encoded features from C𝐶Citalic_C to 1111. By configuring the kernel size as 2222, stride as 1111, and padding as 00, the transposed convolution preserves the length of the time series at L𝐿Litalic_L. As a result, the masked encoded features are reconstituted into a one-dimensional time series, denoted as h~i1×Lsubscript~𝑖superscript1𝐿\widetilde{h}_{i}\in\mathbb{R}^{1\times L}over~ start_ARG italic_h end_ARG start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∈ blackboard_R start_POSTSUPERSCRIPT 1 × italic_L end_POSTSUPERSCRIPT.

III DATA FOR TRAINING AND TESTING

In this paper, we concentrate on the Einstein Telescope, which could potentially consist of three detectors arranged in a triangular configuration. For simplicity, we limit our analysis to just one of these detectors. We utilize the PyCBC package [83, 84, 85, 86] for synthesizing data, which aids in training, validation, and testing processes. The strain captured by the detector can be represented as a combination of noise and two mixture signals: n(t)+hA(t)+hB(t)𝑛𝑡subscript𝐴𝑡subscript𝐵𝑡n(t)+h_{A}(t)+h_{B}(t)italic_n ( italic_t ) + italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) + italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ), where n(t)𝑛𝑡n(t)italic_n ( italic_t ) signifies the noise component. This noise is generated using the power spectrum density (PSD) linked to the Einstein Telescope, which offers insights into the detector’s sensitivity at various frequencies. Specifically, we use EinsteinTelescopeP1600143 to simulate this noise.

Both hA(t)subscript𝐴𝑡h_{A}(t)italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) and hB(t)subscript𝐵𝑡h_{B}(t)italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ) are generated through a linear combination of h+(t)subscript𝑡h_{+}(t)italic_h start_POSTSUBSCRIPT + end_POSTSUBSCRIPT ( italic_t ) and h×(t)subscript𝑡h_{\times}(t)italic_h start_POSTSUBSCRIPT × end_POSTSUBSCRIPT ( italic_t ), which are accurately modeled by SEOBNRv4. In our waveform simulation, the masses of the two black holes range from (10M,80M)10subscript𝑀direct-product80subscript𝑀direct-product(10M_{\odot},80M_{\odot})( 10 italic_M start_POSTSUBSCRIPT ⊙ end_POSTSUBSCRIPT , 80 italic_M start_POSTSUBSCRIPT ⊙ end_POSTSUBSCRIPT ). The dimensionless spin is randomly sampled within the interval (0,0.998)00.998(0,0.998)( 0 , 0.998 ). Additionally, the declination and right ascension are uniformly sampled across the entire sphere. During the simulation of h+(t)subscript𝑡h_{+}(t)italic_h start_POSTSUBSCRIPT + end_POSTSUBSCRIPT ( italic_t ) and h×(t)subscript𝑡h_{\times}(t)italic_h start_POSTSUBSCRIPT × end_POSTSUBSCRIPT ( italic_t ), the luminosity distance from the astrophysical source to Earth is fixed at 4000 Mpc.

In the training phase, the amplitudes of hA(t)subscript𝐴𝑡h_{A}(t)italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) and hB(t)subscript𝐵𝑡h_{B}(t)italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ) undergo random rescaling to align with two randomly generated signal-to-noise ratios (SNRs) falling between 5 and 20. Furthermore, the peak amplitude times of hA(t)subscript𝐴𝑡h_{A}(t)italic_h start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT ( italic_t ) and hB(t)subscript𝐵𝑡h_{B}(t)italic_h start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ( italic_t ) are randomly positioned between 50% and 95% of the designated time window, which spans a duration of 4 seconds. The entire simulation operates at a sampling frequency of 4096 Hz.

IV PERFORMANCE OF THE GW SEPARATION NETWORK

Previous studies examining data processing of overlapping gravitational wave (GW) strains have primarily focused on how GW overlapping affects traditional GW data processing methods, such as matched filtering for GW detection [23] and Bayesian posterior sampling for parameter estimation [20]. Recently, the normalizing flow has emerged as a new technique for parameter estimation of overlapping GW strains [26, 84]. In our study, we propose the utilization of signal separation via deep learning for the analysis of overlapping GW strains.

The gravitational wave (GW) separation network can be considered a parameterized system. The network’s output includes the waveforms of the estimated clean gravitational wave signals. To optimize the performance of the proposed model, we train it using utterance-level permutation invariant training (uPIT) [87], aiming to maximize the scale-invariant signal-to-noise ratio (SI-SNR) [28]. SI-SNR is defined as:

stargetsubscript𝑠𝑡𝑎𝑟𝑔𝑒𝑡\displaystyle s_{target}italic_s start_POSTSUBSCRIPT italic_t italic_a italic_r italic_g italic_e italic_t end_POSTSUBSCRIPT=h~,hhh2absent~superscriptnorm2\displaystyle=\frac{\langle\tilde{h},h\rangle h}{\|h\|^{2}}= divide start_ARG ⟨ over~ start_ARG italic_h end_ARG , italic_h ⟩ italic_h end_ARG start_ARG ∥ italic_h ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG(11)
enoisesubscript𝑒𝑛𝑜𝑖𝑠𝑒\displaystyle e_{noise}italic_e start_POSTSUBSCRIPT italic_n italic_o italic_i italic_s italic_e end_POSTSUBSCRIPT=h~stargetabsent~subscript𝑠𝑡𝑎𝑟𝑔𝑒𝑡\displaystyle=\tilde{h}-s_{target}= over~ start_ARG italic_h end_ARG - italic_s start_POSTSUBSCRIPT italic_t italic_a italic_r italic_g italic_e italic_t end_POSTSUBSCRIPT(12)
SI-SNR:=10log10starget2enoise2assignabsent10subscript10superscriptnormsubscript𝑠𝑡𝑎𝑟𝑔𝑒𝑡2superscriptnormsubscript𝑒𝑛𝑜𝑖𝑠𝑒2\displaystyle:=10\log_{10}\frac{\|s_{target}\|^{2}}{\|e_{noise}\|^{2}}:= 10 roman_log start_POSTSUBSCRIPT 10 end_POSTSUBSCRIPT divide start_ARG ∥ italic_s start_POSTSUBSCRIPT italic_t italic_a italic_r italic_g italic_e italic_t end_POSTSUBSCRIPT ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG start_ARG ∥ italic_e start_POSTSUBSCRIPT italic_n italic_o italic_i italic_s italic_e end_POSTSUBSCRIPT ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG(13)

where h~1×L~superscript1𝐿\tilde{h}\in\mathbb{R}^{1\times L}over~ start_ARG italic_h end_ARG ∈ blackboard_R start_POSTSUPERSCRIPT 1 × italic_L end_POSTSUPERSCRIPT and h1×Lsuperscript1𝐿{h}\in\mathbb{R}^{1\times L}italic_h ∈ blackboard_R start_POSTSUPERSCRIPT 1 × italic_L end_POSTSUPERSCRIPT are the estimated and target clean sources respectively, L𝐿Litalic_L denotes the length of the signals, and h~~\tilde{h}over~ start_ARG italic_h end_ARG and hhitalic_h are both normalized to have zero-mean to ensure scale-invariance. During the training phase, the Adam method is used. A learning rate of 105superscript10510^{-5}10 start_POSTSUPERSCRIPT - 5 end_POSTSUPERSCRIPT is established. The system undergoes 20 epochs of training. During the training stage, we assume that the peak time of signal A lags behind that of signal B. In other words, typically, signal A is only disrupted by the inspiral stage of signal B, whereas signal B experiences interference from the entire signal process, encompassing the inspiral, merger, and ringdown stages.

In this section, we explore the performance of the GW separation network. Prior researches [19, 20] have established that the accuracy of parameter estimation for the two sources can be notably influenced by both the peak time difference and the SNR difference. Our study examines how these two factors specifically affect GW separation.

Fig.4 illustrates an example of overlapping signal shapes, considering variations in peak time differences (a) and signal-to-noise ratio (SNR) differences (b). In subsequent sub-sections, we will introduce noise to these waveforms to produce simulated strain data, and then evaluate the performance of the GW separation model using this simulated data. From this figure, it is evident that, in most scenarios, the near merger and ringdown stages of signal A remain unaffected, whereas all stages of signal B appear blurred.

The following subsections will demonstrate that despite the blurring of signal B and the inspiral stage of signal A, in most cases, the waveforms of both signal A and signal B can often be accurately reconstructed.

Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (4)
Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (5)
Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (6)
Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (7)
Gravitational Wave Mixture Separation for Future Gravitational Wave Observatories Utilizing Deep Learning (8)

IV.1 Impact of peak time difference on the GW separation

In this subsection, we elaborate on the influence of peak time disparities on GW separation. We produce three elements constituting a single strain: noise, signal A, and signal B. The source parameters of signal A and signal B are the same as the waveform shown in Fig.4. With signal A peaking at 3.7 seconds within the entire strain window, we adjust the peak time of signal B to generate eight distinct waveforms. These waveforms exhibit time differences between the peaks of signal A and signal B ranging from -0.7 s to 0 s. By combining these three components, we synthesize eight unique strains. Subsequently, we subject these strains to the GW separation network and analyze the outputs. Fig.5 displays the individual outputs corresponding to each of the eight strains. To measure the separation performance, we utilize the overlap between the two separated signals and the two original signals. The overlap of signal hhitalic_h and h~~\widetilde{h}over~ start_ARG italic_h end_ARG can be written as

overlap(h,h~)=h(t)h~(t)𝑑th2(t)𝑑th~2(t)𝑑t𝑜𝑣𝑒𝑟𝑙𝑎𝑝~𝑡~𝑡differential-d𝑡superscript2𝑡differential-d𝑡superscript~2𝑡differential-d𝑡overlap\big{(}h,\tilde{h}\big{)}=\frac{\int h(t)\tilde{h}(t)dt}{\sqrt{\int h^{%2}(t)dt\int\tilde{h}^{2}(t)dt}}italic_o italic_v italic_e italic_r italic_l italic_a italic_p ( italic_h , over~ start_ARG italic_h end_ARG ) = divide start_ARG ∫ italic_h ( italic_t ) over~ start_ARG italic_h end_ARG ( italic_t ) italic_d italic_t end_ARG start_ARG square-root start_ARG ∫ italic_h start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( italic_t ) italic_d italic_t ∫ over~ start_ARG italic_h end_ARG start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( italic_t ) italic_d italic_t end_ARG end_ARG(14)

From Fig.5 we can see that all eight strains have been successfully separated. Surprisingly, in extreme situations where the peak time of signal A and signal B are the same, the overlaps of both signal A and signal B are greater than 0.95.

Fig.5 presents a single case study demonstrating the effect of peak time difference on GW separation. Here, we undertake a comprehensive statistical analysis to investigate the broader influence of peak time disparities on the process of GW separation. To this end, we have generated eleven sub-test-datasets, with the sole difference among them being the peak time disparities, specifically {-1.0 s, -0.9 s, -0.8 s, -0.7 s, -0.6 s, -0.5 s, -0.4 s, -0.3 s, -0.2 s, -0.1 s, 0 s}. Each of these sub-datasets comprises 1000 samples, ensuring consistency in noise distribution and other parameter distribution across all datasets. The stack plot in Fig.6 illustrates the distribution of separated signals based on their relative merger time (TBTAsubscript𝑇𝐵subscript𝑇𝐴T_{B}-T_{A}italic_T start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT - italic_T start_POSTSUBSCRIPT italic_A end_POSTSUBSCRIPT). Please note that if the overlap between the isolated signal and the actual injected signal exceeds 0.9, we consider the signal to be successfully isolated. This figure reveals that in most scenarios, both signal A and signal B are effectively separated. Notably, even in the most extreme circ*mstance, where the merger time of signal A and signal B coincide, over 80% of the samples are still accurately separated, while approximately 10% of the samples yield successful separation of only one of the two injections. Approximately 5% of the samples show unsuccessful separation for both signal A and signal B. These results further underscore the exceptional performance of our model in denoising and separating mixed signals. The model effectively distinguishes overlapped signals under different peak time difference conditions, achieving high-quality separation results in the majority of cases. This highlights its robustness and capability in signal-processing tasks.

IV.2 Impact of SNR difference on the GW separation

In the preceding section, we discussed the impact of peak time differences on the separation of gravitational wave signals. In practical scenarios, the amplitudes of the individual components within the entangled signals exhibit diversity. Herein, we delve into the influence of signal strength on GW disentanglement. Signal strength can be quantified by the matched signal-to-noise ratio (SNR). To be specific, we maintain an SNR of 10 for signal A while adjusting the SNR differential between signal B and signal A in increments of 2, spanning from -4 to 10. Consequently, the SNRs for signal B are adjusted to the following values: {6, 8, 10, 12, 14, 16, 18, 20}.

We configure the parameters identically to those presented in Fig.4. Specifically, we establish the peak time of signal A at 3.7 seconds within the strain window and set the peak time of signal B at 3.5 seconds, resulting in a peak time difference of -0.2 seconds. We then adjusted the SNR of signal B, varying it from 6 to 20. After superimposing signal A, signal B, and noise, we input the combined signal into the Gravitational Wave (GW) separation network and obtained the output. Fig.7 illustrates the separated and injected waveforms for both signal A and signal B.

Here, we analyze the influence of signal A on the GW separation performance of signal B by the right column of Fig.7. By changing the SNR of signal B from 6 to 20, the separation results of signal A almost unchanged. All the separation overlaps of signal A are greater than 0.98.

When the Signal-to-Noise Ratio (SNR) of signal B is 6, we can see that the overlap between the separated signal B and the buried signal is approximately 0.82. We hypothesize that there may be two primary factors influencing the separation performance of signal B. Firstly, the SNR of signal B is significantly low, causing noise to interfere with the separation process. Secondly, both signal A and noise contribute to the decrease in separation performance. To gain a deeper understanding of the reasons behind the incorrect separation, we subtract signal A and preserve only signal B and the noise in the strain data. This modified data is then inputted into the separation model to observe the impact on the separation of signal B. We verified that the overlap of signal B is equal to 0.80, which is nearly identical to 0.82. The results suggest that the underwhelming performance observed in the separation of signal B in Fig.7 (a) is unrelated to the overlapping signal B, but is instead impacted by the intensity of noise.

To further investigate the impact of SNR differences on separation performance and identify potential shortcomings of our model, we prepared 1,000 samples for each SNR difference value. Fig.8 illustrates the four separation scenarios under different SNR differences, with the x-axis representing SNR differences ranging from -4 to 10. Note that the SNR of signal A is set to 10. We set the SNR of signal B to {6, 8, 10, 12, 14, 16, 18, 20} corresponding to the SNR difference {-4, -2, 0, 2, 4, 6, 8, 10}. From the area chart in Figure 8, it is evident that the orange region, indicating the successful separation of both signals, occupies the majority of the area. The red region, representing scenarios where neither signal was successfully separated, remains very small. Specifically, when the SNR of Signal A is fixed at 10 and the SNR of Signal B is 6 or 8, the instances where only Signal A is successfully separated significantly outnumber the instances where only Signal B is successfully separated. This indicates that, in scenarios with smaller SNR differences, the model is more likely to successfully separate the signal with the higher SNR. These results suggest that further optimization is needed to enhance the model’s performance in separating overlapping signals with low SNR parts. At the same time, they also confirm the robustness of the current model in most cases.

V Conclusion

In this paper, we attempt to address the challenge posed by overlapping GW signals, which is an emerging issue as future GW observatories. We have demonstrated the feasibility of adapting speech separation techniques to the domain of GW signal separation, employing deep learning models for this task. Our findings reveal that the proposed approach can effectively disentangle overlapping GW signals, even when they exhibit different peak time differences. This capability ensures robust signal identification and accurate extraction of individual GW events from a complex signal mixture. Additionally, we observed that the method performs remarkably well across a range of SNRs. Even in low SNR scenarios, where noise levels are relatively high, the model demonstrates its ability to separate and identify GW signals with reasonable accuracy.

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