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Low noise optical multi-carrier generation using optical-FIR filter for ASE noise suppression in re-circulating frequency shifter loop

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Abstract

In this paper, an improved multi-carrier generation scheme based on single-side-band recirculating frequency shifter with optical finite impulse response (FIR) filter for amplified spontaneous emission (ASE) noise suppression is proposed and experimentally demonstrated. The carrier-to-noise-ratio (CNR) instead of tone-to-noise-ratio (TNR) is introduced to more reasonably and exactly evaluate the signal-to-noise-ratio of a multi-carrier source with non-flat noise floor. We have experimentally attain the worst case CNR of 22.5dB and 19.1dB for generated 50 and 69 flat low noise carriers, which has shown significant improvement than the previous cited works based on recirculating frequency shifter.

© 2014 Optical Society of America

1. Introduction

Multi-carrier source generation is crucial for achieving higher data rates optical communication systems with multi-carrier modulation formats, such as coherent dense wavelength division multiplexing (Co-DWDM) [1,2], coherent optical orthogonal frequency division multiplexing (Co-OFDM) [35] and Nyquist-WDM [6] based superchannel systems. Optical multi-carrier sources with high quality and large number of carriers are good candidates to serve as a laser array at transmitter end [15] or a local oscillator array at receiver end [7] for ultra-high speed and spectrum efficiency (SE) optical communication systems. On the other hand, these frequency locked carriers are very important to super-channel systems as small frequency drift of laser source would immediately lead to inter-channel crosstalk. Many endeavors have been made on generation of optical multi-carrier source such as multi-wavelength erbium-dropped fiber laser [8], cascaded modulators [5,911] and re-circulating frequency shifter (RFS) [3,4,1113]. Among these technologies, single-side-band (SSB) modulation based RFS loop has attracted much attention due several advantages of relative simple structure, flexibility on frequency spacing control, low driving voltages, less sensitive to phase noise and ability to generate large number of flat carriers. Whereas, in most of SSB-RFS multi-carrier source applied terabits long reach transmission experiments, only 20~40 carriers (having a worst TNR of 20~25dB) are generated for signal loading. One of the reason is the noise nature of SSB-RFS method that ASE noise accumulates round by round, resulting limited number of available carriers, especially for cases of large desired carrier number (>50) and multi-EDFAs deployed in the loop [1,3,4]. Therefore, further investigations are required for low noise RFS based multi-carrier generation schemes. There are many improved RFS implementations techniques have been proposed to achieve better performance by halving required circulating times, such as multi-frequency shifting (MFS) method [14], double RFS structure [15] and complementary frequency shifter (CFS) loop [16,17], while increasing the complicity of the structures, doubling optical components, and limited noise characteristic improvements.

In this paper, we propose and experimentally demonstrate an improved SSB-RFS optical multi-carrier generation configuration with an ASE noise suppression scheme using an optical FIR filter. By applying this scheme, notable carrier-to-noise ratio (CNR) performance improvement is achieved. With proposed optical FIR ASE noise suppression scheme deployed in SSB-RFS loop, 50 and 69 stable and flat carriers with high CNR are generated.

2. Proposed scheme and noise characteristic analysis

2.1 Proposed ASE noise suppression scheme

The proposed low noise SSB-RFS multi-carrier generator with ASE noise suppression scheme is illustrated in Fig. 1(a). This configuration includes a basic SSB-RFS loop and an optical FIR filter for noise suppression. In the basic SSB-RFS loop [3,4,12,13], the seed carrier of frequency f0 is provided by CW laser, and an I/Q modulator driven by two RF signals of frequency fs is used to implement carrier frequency shift. The exact polarization alignments are ensured by the polarization controllers (PCs) and the number of generated carriers is controlled by a band pass filter (BPF). The role of EDFA is to compensate the total loss suffered in one round trip (RT) with inevitable ASE noise accumulation that could result in a great system performance degradation. In our proposed scheme, an N-tap optical FIR structured notch filter is placed after EDFA to further reduce accumulated ASE noise, which ensures a significant improvement in overall system performance.

 figure: Fig. 1

Fig. 1 (a) Schematic of proposed low noise SSB-RFS multi-carrier generation scheme; (b) parallel implementation of optical FIR filter for ASE noise suppression; (c) serial implementation of optical FIR filter for ASE noise suppression. PC: polarization controller; BPF: band pass filter; EDFA: erbium doped fiber amplifier; OSA: optical spectrum analyzer; RF: radio frequency; PS: phase shifter; EA: electrical amplifier.

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In this work, two different structures of optical FIR filter implementations are taken into consideration, one is N-tap parallel structured FIR (direct implementation) and the other is N cascaded 2-tap one as illustrated in Figs. 1(b) and 1(c). The theoretical analysis of noise accumulation and reduction will be given out in the next part, and then the system performance improvement and experiment results.

2.2 Analysis of noise accumulation and CNR definition

It is convenient to treat the output of SSB-RFS loop in frequency domain in the presence of ASE noise, and a recursive expression can be represented by Eq. (1).

E1(f)=E0(f)+[g1lT(f)E0(f)+n1(f)]HBPF(f)E2(f)=E0(f)+[g2lT(f)E1(f)+n2(f)]HBPF(f),En(f)=E0(f)+[gnlT(f)En1(f)+nn(f)]HBPF(f)
where E0(f) is the seed laser centered at f0, En(f) is the output of nth (n = 1,2,…N) RT. l is the total loop loss including modulation loss and insertion loss, and gn is EDFA gain in nth RT. A stable and flat output is always ensured by EDFA condition of gnl1 .The transfer functions of IQ modulator is denoted by T(f)FFT(ej2πfst)=δ(ffs). However, 3rd-order crosstalk is neglected for simplicity as this work mainly focus on processes of ASE noise accumulation and reduction. denotes convolution operator and the BPF transfer function HBPF(f) is assumed ideal rectangle window here and represented by Eq. (2).
Hrect(f)={1f0-fs/2<f<f0+(N+1/2)fs0other.
nn(f) is Fourier transform of nth RT induced random ASE noise nn(t), that can be treated as bandwidth limited additive white Gaussian noise process obeying nn(t)~(0,σ2). The statistical average quantity of E[|nn(f)|2] accounts for the noise power spectrum density (PSD) S(f), and can be derived from EDFA parameters of noise figure Fn and power gain G using Eq. (3).
S(f)=E[|n(f)|2]=Fn(G1)hf2.
where f is optical frequency and h is Planck’s constant. The variance σ2 of ASE noise within bandwidth B can be calculated by σ2=PASE=S(f)B. In order to have a better understanding of noise accumulation process, nN(f) is rewritten by summing series of narrow-band noise block nBs,n(f)(centered at f0 with bandwidth Bs=fs) i.e. Equation (4) as shown in Fig. 2.
nn(f)=k=nBs,n(fkfs).
Therefore, the corresponding output of multi-carrier source can be written as Eq. (5).
EN(f)k=0NE0(fkfs)Ndiscrectcarrierlines+m=1Nk=Nm+1NnBs,m(fkfs)residualASEnoiseinducedinmthRTtotalASEnoiseafterNthRT(a)=k=0NE0(fkfs)Ndiscrectcarrierlines+k=1Nm=Nk+1NnBs,m(fkfs)ASEnoiseinkthchanneltotalASEnoiseafterNthRT(b).
Here, Eq. (5) separates generated carriers and accumulated noise and provides a very convenient way to investigate the noise characteristic from different perspectives. The term k=Nm+1NnBs,m(fkfs) in Eq. (5a) describes the variation of ASE noise induced in each RT as circulating time increases, which can be easily understood with the help of Fig. 2. After BPF, the random wide-band noise nm(f) becomes band-limited having a spectrum range of [f0+fs/2,f0+(N+1/2)fs], and will be shifted right by fs on the spectrum in next cycle. Moreover, the part of noise that shifted out of BPF window will be filtered out and results in a reduced noise spectrum range of [f0+3fs/2,f0+(N+1/2)fs]. The overlapped PSD in Nth channel can be calculated by Eq. (6).

 figure: Fig. 2

Fig. 2 Illustration of ASE noise accumulation as circulating time increases.

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Stot,N(f)=E[|m=1NnBs,m(fNfs)|2]=m=1NE[|nBs,n(fNfs)|2]=m=1NSBs,m(f)=NSBs(f).

Thus, the power of accumulated ASE noise in Nth channel is represented by Eq. (7).

PASEN=Stot,N(f)Bs=NFn(G1)hfBs2=NPASE,Bs.
Equation (7) indicates that ASE noise power accumulation in each channel seems linear with required circulating time N. Where, N times noise power accumulated on Nth generated carrier tone compared to first tone, would result in a great degradation on signal loading process. In this work, carrier-to-noise ratio (CNR) is defined and used to have a quick and accurate evaluation on noise characteristic of each channel instead of tone-to-noise ratio (TNR, defined as10lg(Pcarrier/Pnoisefloor), Pcarrierand Pnoisefloor are the readout powers of carrier tone and noise floor from OSA) in other works [13]. Although, TNR is suitable for the cases with flat noise power spectrum, but not for non-flat noise power spectrum as exploited in the present work. CNR of nth channel with 0.1nm reference noise bandwidth likewise OSNR is defined as Eq. (8).
CNRn=10lg(Pcarrier/PASE,n)+10lg(Bs/Br).
where PASE,n is the ASE noise power in nth channel within channel spacing Bs, and Br is 0.1nm reference noise bandwidth. In a flat noise power spectrum case, CNR and TNR have a relationship of Eq. (9).
CNRn=TNRn+10lg(Bres/Br).
where Bres is resolution of optical spectrum analyzer (OSA). The CNR curve of ordinary SSB-RFS loop is depicted in Fig. 3 (blue line). It can be deduced that CNR decreases as required circulating times increases and 100th generated carrier tone is degraded 20dB in contrary to first tone. The simulation results of loaded Nyquist-16QAM signal degradation caused by CNR reduction are also shown in Fig. 3. In our simulation, the generated first carrier has a CNR value of 32dB that is consistent with the TNR value of first carrier observed from the experimental results in Fig. 8(a) and converted with the relationship of Eq. (6). The constellation of signal loaded on 1st-carrier has a very clear QAM pattern having an error vector magnitude (EVM) of 5.4% while blurred constellation is observed on 100th carrier having an EVM of 19.8% as depicted in Fig. 3. Moreover, the constellation points at the range of outer circles have a more scattered distribution than inner ones which implies that the impairments induced by accumulated ASE noise would act like Gaussian distribution phase noise.

 figure: Fig. 3

Fig. 3 (a) Carrier-to-noise-ratio per channel in SSB-RFS loop with/without optical FIR and constellations of loaded signal on different carriers; Output spectrum (b)without and (c)with 2-tap optical FIR filter applied in the loop (simulation resolution of 24.4MHz).

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2.3 Analysis of ASE noise power reduction

Before investigating CNR performance improved by proposed noise suppression scheme, the reduced ASE noise power by optical FIR filter should be analyzed first.

CASE I: Parallel FIR implementation

The power transfer function of N-tap FIR filter could be represented by Eq. (10).

GFIRP(f)=|FP(f)|2=|FFT(1Nn=0N1δ(tnτ))|2=2N2n=0N1(Nn)cos(fnτ)1.
where τ is the delay unit. A free spectrum range (FSR) equals channel spacing is expected here so that τ has a value of 1/fs. The simulation results of output spectrums of SSB-RFS loop with and without 2-tap FIR filter is illustrated in Figs. 3(b) and 3(c), and significantly notched noise floor is observed in 3(c) compared to a linearly increased noise floor in 3(b). The output power when ASE noise passes N-tap optical FIR filter can be calculated by solving Eq. (11).
Pout,ASE=12πffs/2f+fs/212Sin,ASE(f)GFIR(f)df.
and we will have
Pout,ASE=1NPin,ASE.
Equation (12) implies that only 1/N of the noise power remains when ASE noise passes N-tap optical FIR filter and this would have a considerable contribution to CNR performance improvement.

CASE II Serial FIR implementation

The serial FIR structured implementation can be treated as N cascade 2-tap FIR (both tap coefficients are 1/2) or N-tap FIR with mth tap coefficient equals (1/2)NCNm. Hence, the power transfer function is Eq. (13).

GFIRS(ω)=|FS(ω)|2=|(12)N[1+exp(jωτ)]N|2=(12)N[m=0NCNmcosm(ωτ)].
Corresponding, output ASE noise power can be calculated as mentioned above and has a form of Eq. (14).

Pout,ASE=ffs/2f+fs/212Sin,ASE(f)GFIRS(f)df=1+m=1N/2[CN2m(2m1)(2m3)312m(2m2)42]2NPin,ASE.

In Fig. 4, the power transfer functions of two structured optical FIR filters are illustrated, considering tap number of 2, 8 and stage number of 8. Apparently, with same tap number, more noise power would be filtered as parallel structured FIR employed in contrary to serial structured FIR, but the later maybe easier to implement.

 figure: Fig. 4

Fig. 4 Power transfer function of optical FIR filter with different tap coefficients.

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The principle of proposed scheme on improving system performance can be easily understood. In existent application, the generated sub-carriers are firstly separated by a de-multiplexer, such as arrayed waveguide grating (AWG) which has a relatively flat window with bandwidth equal to channel spacing. Then the desired carrier, together with accumulated ASE noise (within bandwidth of channel spacing) will be sent into the transmitter. The signal constellation will thus be blurred while being loaded onto a “noisy” carrier. This scheme works because the carrier bandwidth without modulation is quite small, which allows us to “clear” the generated carriers using a notch filter with narrow 3dB bandwidth, such as N-tap optical FIR filter, resulting less noise accumulates within channel. The “cleared” carrier is selected and modulated, contributing an improved signal quality after modulation. And the detailed system performance (CNR, EVM and EDFA efficiency) improvements will be discussed in the following section.

2.4 System performance improvements with the proposed ASE noise suppression scheme

CNR improvement

When N-tap optical FIR structured ASE noise suppression scheme employed in the loop, the output of SSB-RFS after N RTs could be denoted by Eq. (15).

EN(f)=k=0NE0(fkfs)Ndiscrectcarrierlines+k=1Nm=Nk+1Nnm,Bs(fkfs)FFIRNm+1(f)residualASEnoiseinkthchanneltotalresidualASEnoiseafterNthRT.
And corresponding PSD of ASE noise in Nth channel can be represented by Eq. (16).
SASEN(f)=Ε[|m=1Nnm,Bs(fNfs)FFIRNm+1(f)|2]=m=1NSASEm(f)GFIRNm+1(f).
There are two observations can be inferred from Eq. (16), which are: 1) total noise in Nth channel has been overlapped N times and 2) ASE noise induced in mth trip has passed FIR filter N-m + 1 times. Therefore, the power of residual ASE noise after FIR filtering in Nth channel can be represented by
PN,resASE=ωsSASEN(f)df=m=1NωsSASEm(f)GFIRNm+1(f)df.
According to the above definition, the CNR of Nth channel with 0.1nm equivalent reference noise bandwidth is
CNRNFIR=10lg(Ps/PN,resASE)+10lg(fs/Br).
It is hard to find analytic solution from Eq. (18) since PN,resASE has a very complicated form of Eq. (17). Therefore, the simulation results of Eq. (18) are given out in Fig. 3 as indicated by lines of SSB-RFS-2-tap, 4-tap and 8-tap with different colors. In the case of 2-tap ASE noise suppression, we can find about 9.86dB CNR improvement in the 100th channel (the worst channel), having an equal CNR value with 10th carrier in the loop without applying ASE noise suppression scheme. A 16.4dB CNR incensement will be found in the 8-tap case. The improved constellation of loaded 16 QAM signal on 100th carrier is also shown in Fig. 3, where, an EVM has increased from 19.8% to 10.2% (2-tap) and 6.6% (8-tap), and the detailed EVM analysis will come in next section. Furthermore, the CNR curve by applying multi-frequency-shifting (MFS) (2 frequencies for instance) [14] is also denoted by the green line in Fig. 3, which suggests that limited contribution on CNR improvement will be achieved by halving required circulating times. Moreover, optical FIR filter based ASE noise suppression can be combined with MFS or CFS loop for additional CNR performance improvement.

EVM improvement of loaded signals

As revealed above, CNR evaluation for the naked carrier tone is in close relationship with error vector magnitude (EVM) evaluation for the quality of loaded signal, which also implies the performance of whole transmission system. In Fig. 5, this relationship is illustrated in detail for different SSB-RFS schemes, including ordinary SSB-RFS, SSB-RFS with N-tap (N = 2, 4, 8) FIR ASE noise suppression, SSB-RFS with 2-frequency shifting (2-FS), and combined scheme of N-tap optical FIR and 2-FS. In order to load the signal on the carrier, we must select it with a WDM de-multiplexer, which in our simulation a 10-order super-Gaussian filter of 28GHz bandwidth is used (almost the MUX and DEMUX window). The filtered carrier is loaded with 28Gbaud Nyqusit-16QAM signal and detected with coherent detection. The linewidth of both signal laser and local oscillator is set to 0kHz. A match filter is applied on received signal without phase recovery algorithm for digital signal processing (DSP). The EVM of signals loaded on ordinary SSB-RFS carriers significantly increases with number of carriers as denoted by the blue line, having 15 carriers with EVM values better than 11% and about 25 carriers better than 13% (BER≈3.8e-3). Apparently, the last several tens carriers cannot support higher-order modulation format systems. In the results of SSB-RFS with 2-FS, a limited performance improvement is achieved as only a small part of the noise is reduced. Whereas an outstanding improvement can be found as optical FIR filter based ASE noise scheme deployed in the loop. Even a 2-tap case can help improve the EVM performance of generated 100th carrier from 19.7% to around 10%, and additional EVM improvement could be achieved when the 2-tap case combined with 2-FS. Notably, when 8-tap optical FIR deployed in the loop, EVMs of all 100 generated carriers have values of less than 6.5%, which is comparable with first several carrier tones generated by ordinary scheme. This is very meaningful especially for multi-carrier source based super-channel transmission systems since higher order modulation formats such as 16QAM and 32QAM could be loaded on large amounts of carriers. We can also conclude that among all the improved SSB-RFS multi-carrier generation schemes, optical FIR structured ASE noise suppression scheme would contribute most significant improvement to system performance.

 figure: Fig. 5

Fig. 5 EVM performances of loaded Nyquist-16QAM signal when applying proposed ASE noise scheme with different tap number.

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Amplification efficiency improvement of EDFA

Another contribution of proposed ASE noise suppression scheme is improvement of amplification efficiency of EDFA. In our previous analysis, EDFA is modeled as constant gain coefficient with adequate saturation output power [12]. In reality, EDFA is specified with a parameter of saturation output powerPs, but the different carrier number requires different saturation output powers. Considering saturation output power, the EDFA power gain G is given by the implicit expression of Eq. (19) [18].

G=G0exp((G1)PinPs).
where G0 is the peak gain and Pin is input power. In Fig. 6, the required saturation power is shown against number of carriers, considering ASE suppressed SSB-RFS scheme with different tap numbers. In the ordinary scheme, when generating 100 carriers, a saturation power of 29dBm is required. For SSB-RFS with 2-tap ASE noise suppression scheme, the required saturation power is reduced by 1dB. By analyzing all curves in Fig. 6, it can be concluded that 1dB saturation power will be saved as tap number doubles, which means that, by applying optical FIR filter based noise suppression scheme in the loop, more carriers can be generated with a certain EDFA saturation output power.

 figure: Fig. 6

Fig. 6 Required EDFA saturation output power when different tap number employed.

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3. Experimental setup and results

Experimental setup is shown in Fig. 7(a). In this experiment, a 2-tap optical FIR filter case is carried out, which is implemented by Mach-Zehnder delay interferometer (MZ-DI). The seed laser is centered at 1559.5nm with linewidth of several tens kilohertz and the output power is set to be 16dBm. An I/Q modulator with insertion loss of 7dB is used here to implement frequency shifting, and both of its arms are biased at null points. A phase shift of π/2 between I and Q branches is ensured by adjusting bias voltage of phase modulator. The frequency of sine and cosine RF driven signals applied to I/Q modulator is 10.7GHz. Both amplitudes and phases of the driven signals are carefully adjusted by electrical amplifier and phase shifter, resulting that, as shown in Fig. 7(b), only 1st order line (the desired shifted frequency carrier tone) remains on the spectrum (about 40dB power larger than the seed tone, −1st, ± 2nd and −3rd order tone) when the loop is set open, which indicates a minimum harmonic crosstalk remained and helps to achieve good fatness of the generated multi-carriers [12]. After I/Q modulator, a tunable band pass filter (BPF) with insertion loss of 6dB is used to limit the number of generated carriers. The role of EDFA is to compensate the total loop loss suffered in every round trip.

 figure: Fig. 7

Fig. 7 (a) Experiment setup for proposed low noise multi-carrier generation scheme; (b) The optical spectrum of first tone generation when the loop is open.

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In this experiment, the EDFA works at automatic power control (APC) mode and has being applied with setting saturation output power of 25dBm. The noise figure of EDFA is less than 5.5dB at −10dBm input power and 25dBm output power. Exactly polarization alignments between I/Q modulator and EDFA are ensured by the two PCs. In the first experiment, 3dB bandwidth of BPF is set to about 4.4nm to allow 50 carriers pass. The generated 50 carriers without applying MZ-DI (2-tap FIR filter) in the loop are shown in Fig. 8(a), compared to 1st carrier, the noise floor of last one has been raised about 18dB that is consistent with our simulation results in Fig. 3, having an equivalent CNR with 0.1nm noise bandwidth of 14dB (the value of transferred from TNR of 42 dB calculated through Eq. (9)). To improve the CNR, a MZ-DI (2-tap FIR filter) with parameters of fixed FSR = 10.7GHz, insertion loss of 1.54dB and extinction ratio of 36dB is inserted after EDFA in our experimental setup. The phase arm of MZ-DI is adjusted to align the middle of FSR window to the generated carrier tones, and corresponding output optical spectrum is shown in Fig. 8(b), where the CNR of 50th tone has been increased to 22.2dB.

 figure: Fig. 8

Fig. 8 Generated 50 tones without (a) and with (b) ASE noise suppression scheme and zoomed version of last carrier.

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Then in the second experiment, the 3dB bandwidth of BPF is set to 6nm (the maximum value of our instrument) to generate 69 tones with EDFA saturation output power of 26dBm. The corresponding output spectrum is shown in Fig. 9, and both the measured and theoretical CNR values of 69 carriers are all depicted in Fig. 10(a). The measured CNR values of SSB-RFS with and without 2-tap filter decrease with the number of generated carriers in the trend of theoretical results. The measured CNR values of 67th carrier in two cases are about 17.9dB and 9.2dB respectively, showing 8.7dB CNR increment by using proposed scheme. All these results are observed using the optical spectrum analyzer APEX AP2440A with a resolution of 0.16pm (20MHz). The measured ΔCNRn values are also shown in Fig. 10(b), where we can find that the measured ΔCNRn has a small difference from the theoretical value, thus the improved CNR can be derived using Eq. (20), without resorting to integration.

CNRnFIR=10lg(Pcarrier_n/Pnoisefloor)+10lg(Bres/Br)+ΔCNRntheory.
In the equation above, ΔCNRntheory represents the theoretical value of how much ASE power is reduced by N-tap FIR filter in nth channel and is read from Fig. 3. Moreover, in our previous experiment results of 50 carrier tones with 12.5GHz spacing having a worst TNR of 20dB observed with a resolution of 0.02nm OSA [13] (corresponding TNR of 41dB at 20MHz OSA resolution and equivalent CNR of 13dB at Br = 12.5GHz), EDFA saturation output power is about 29dBm while in this work only 25dBm output power is needed to generate 50 flat carriers but with much higher CNR. It is consistent with theoretical analysis above that, not only ASE noise power is reduced by 2-tap optical FIR filter but also the EDFA amplification efficiency is improved.

 figure: Fig. 9

Fig. 9 The generated 69 low noise flat carriers covering 6nm spectrum range and details of first five and last 5 carriers.

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 figure: Fig. 10

Fig. 10 (a) Measured and theoretical CNR values of SSB-RFS with/without 2-tap optical-FIR filter for ASE noise suppression; (b) Theoretical and measured value of CNR increment by using SSB-RFS with 2-tap ASE suppression.

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4. Conclusion

In this paper, ASE noise accumulation of SSB-RFS multi-carrier generation is analyzed. To improve CNR performance of generated multi-carrier, a low noise SSB-RFS multi-carrier generation configuration with optical FIR filter based ASE noise suppression scheme is proposed and corresponding analysis of amount of noise reduction is given out. Theoretically, the worst CNR can be improved about 9.5dB (2-tap) and 16.4dB (8-tap) with the proposed noise suppression scheme when 100 carriers are generated, which would enable all generated 100 carriers supporting 16QAM or higher modulation formats. An experimental demonstration is also carried out, and 50 flat carriers with and without proposed noise suppression scheme are generated, having a worst CNR of 14dB and 22.2dB respectively. After that, 69 carriers covering 6nm range with 10.7GHz frequency spacing and high flatness (<1dB) are generated, having a high CNR about 17dB for the worst carrier tone. The achieved results will pave the way to support higher order modulation formats and long-distance transmission.

Acknowledgments

This work is partly supported by the National Natural Science Foundation of China (Grant No.61205065), Open Fund of State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, China (Grant No. 2013G- ZKF031310), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110005110014), and Open Fund of IPOC (BUPT) (IPOC2013B005).

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13. F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011). [CrossRef]  

14. J. Zhang, J. Yu, N. Chi, Z. Dong, Y. Shao, L. Tao, and X. Li, “Theoretical and experimental study on improved frequency-locked multi-carrier generation by using recirculating loop based on multi-frequency shifting single-side band modulation,” IEEE Photon. J. 4(6), 2249–2261 (2012). [CrossRef]  

15. J. Zhang, J. Yu, N. Chi, Y. Shao, L. Tao, J. Zhu, and Y. Wang, “Stable Optical Frequency-Locked Multicarriers Generation by Double Recirculating Frequency Shifter Loops for Tb/s Communication,” J. Lightwave Technol. 30(24), 3938–3945 (2012). [CrossRef]  

16. J. Li and Z. Li, “Frequency-locked multicarrier generator based on a complementary frequency shifter with double recirculating frequency-shifting loops,” Opt. Lett. 38(3), 359–361 (2013). [CrossRef]   [PubMed]  

17. J. Li, C. Yu, and Z. Li, “Complementary frequency shifter based on polarization modulator used for generation of a high-quality frequency-locked multicarrier,” Opt. Lett. 39(6), 1513–1516 (2014). [CrossRef]  

18. G. P. Agrawal, Applications of Nonlinear Fiber Optics (Academic Press, 2001), Chap. 4.

References

  • View by:

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    [Crossref]
  2. D. Hillerkuss, R. Schmogrow, M. Meyer, S. Wolf, M. Jordan, P. Kleinow, and J. Leuthold, “Single-laser 32.5 Tbit/s Nyquist WDM transmission,” J. Opt. Commun. Netw. 4(10), 715–723 (2012).
    [Crossref]
  3. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express 17(11), 9421–9427 (2009).
    [Crossref] [PubMed]
  4. S. Chandrasekhar, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” European Conference on Optical Communications, paper PD2.6, Vienna, Austria (2009).
  5. X. Liu, S. Chandrasekhar, X. Chen, P. J. Winzer, Y. Pan, T. F. Taunay, B. Zhu, M. Fishteyn, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “1.12-Tb/s 32-QAM-OFDM superchannel with 8.6-b/s/Hz intrachannel spectral efficiency and space-division multiplexed transmission with 60-b/s/Hz aggregate spectral efficiency,” Opt. Express 19(26), B958–B964 (2011).
    [Crossref] [PubMed]
  6. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011).
    [Crossref]
  7. N. K. Fontaine, “Spectrally-sliced coherent receivers for THz bandwidth optical communications.” European Conference on Optical Communications, paper Mo.3.C.1, London, UK (2012).
    [Crossref]
  8. M. A. Mirza and G. Stewart, “Multi-wavelength operation of erbiumdoped fiber lasers by periodic filtering and phase modulation,” J. Lightwave Technol. 27(8), 1034–1044 (2009).
    [Crossref]
  9. J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, J. Yu, and L. Tao, “Flattened comb generation using only phase modulators driven by fundamental frequency sinusoidal sources with small frequency offset,” Opt. Lett. 38(4), 552–554 (2013).
    [Crossref] [PubMed]
  10. X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
    [Crossref]
  11. J. Zhang, N. Chi, J. Yu, Y. Shao, J. Zhu, B. Huang, and L. Tao, “Generation of coherent and frequency-lock multi-carriers using cascaded phase modulators and recirculating frequency shifter for Tb/s optical communication,” Opt. Express 19(14), 12891–12902 (2011).
    [Crossref] [PubMed]
  12. J. Li, X. Zhang, F. Tian, and L. Xi, “Theoretical and experimental study on generation of stable and high-quality multi-carrier source based on re-circulating frequency shifter used for Tb/s optical transmission,” Opt. Express 19(2), 848–860 (2011).
    [Crossref] [PubMed]
  13. F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011).
    [Crossref]
  14. J. Zhang, J. Yu, N. Chi, Z. Dong, Y. Shao, L. Tao, and X. Li, “Theoretical and experimental study on improved frequency-locked multi-carrier generation by using recirculating loop based on multi-frequency shifting single-side band modulation,” IEEE Photon. J. 4(6), 2249–2261 (2012).
    [Crossref]
  15. J. Zhang, J. Yu, N. Chi, Y. Shao, L. Tao, J. Zhu, and Y. Wang, “Stable Optical Frequency-Locked Multicarriers Generation by Double Recirculating Frequency Shifter Loops for Tb/s Communication,” J. Lightwave Technol. 30(24), 3938–3945 (2012).
    [Crossref]
  16. J. Li and Z. Li, “Frequency-locked multicarrier generator based on a complementary frequency shifter with double recirculating frequency-shifting loops,” Opt. Lett. 38(3), 359–361 (2013).
    [Crossref] [PubMed]
  17. J. Li, C. Yu, and Z. Li, “Complementary frequency shifter based on polarization modulator used for generation of a high-quality frequency-locked multicarrier,” Opt. Lett. 39(6), 1513–1516 (2014).
    [Crossref]
  18. G. P. Agrawal, Applications of Nonlinear Fiber Optics (Academic Press, 2001), Chap. 4.

2014 (2)

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

J. Li, C. Yu, and Z. Li, “Complementary frequency shifter based on polarization modulator used for generation of a high-quality frequency-locked multicarrier,” Opt. Lett. 39(6), 1513–1516 (2014).
[Crossref]

2013 (3)

J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, J. Yu, and L. Tao, “Flattened comb generation using only phase modulators driven by fundamental frequency sinusoidal sources with small frequency offset,” Opt. Lett. 38(4), 552–554 (2013).
[Crossref] [PubMed]

J. Li and Z. Li, “Frequency-locked multicarrier generator based on a complementary frequency shifter with double recirculating frequency-shifting loops,” Opt. Lett. 38(3), 359–361 (2013).
[Crossref] [PubMed]

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

2012 (3)

2011 (5)

2009 (2)

Bosco, G.

Carena, A.

Chandrasekhar, S.

Chang, G. K.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

Chen, S.

Chen, X.

Chi, N.

Curri, V.

Dimarcello, F. V.

Dong, Z.

J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, J. Yu, and L. Tao, “Flattened comb generation using only phase modulators driven by fundamental frequency sinusoidal sources with small frequency offset,” Opt. Lett. 38(4), 552–554 (2013).
[Crossref] [PubMed]

J. Zhang, J. Yu, N. Chi, Z. Dong, Y. Shao, L. Tao, and X. Li, “Theoretical and experimental study on improved frequency-locked multi-carrier generation by using recirculating loop based on multi-frequency shifting single-side band modulation,” IEEE Photon. J. 4(6), 2249–2261 (2012).
[Crossref]

Fini, J. M.

Fishteyn, M.

Forghieri, F.

Hillerkuss, D.

Huang, B.

Jordan, M.

Kleinow, P.

Leuthold, J.

Li, J.

Li, X.

J. Zhang, J. Yu, N. Chi, Z. Dong, X. Li, Y. Shao, J. Yu, and L. Tao, “Flattened comb generation using only phase modulators driven by fundamental frequency sinusoidal sources with small frequency offset,” Opt. Lett. 38(4), 552–554 (2013).
[Crossref] [PubMed]

J. Zhang, J. Yu, N. Chi, Z. Dong, Y. Shao, L. Tao, and X. Li, “Theoretical and experimental study on improved frequency-locked multi-carrier generation by using recirculating loop based on multi-frequency shifting single-side band modulation,” IEEE Photon. J. 4(6), 2249–2261 (2012).
[Crossref]

Li, Z.

Liu, X.

Ma, Y.

Meyer, M.

Mirza, M. A.

Monberg, E. M.

Pan, Y.

Poggiolini, P.

Ralph, S. E.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

Schmogrow, R.

Shao, Y.

Shieh, W.

Stark, A.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

Stewart, G.

Tang, Y.

Tao, L.

Taunay, T. F.

Tian, F.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

J. Li, X. Zhang, F. Tian, and L. Xi, “Theoretical and experimental study on generation of stable and high-quality multi-carrier source based on re-circulating frequency shifter used for Tb/s optical transmission,” Opt. Express 19(2), 848–860 (2011).
[Crossref] [PubMed]

F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011).
[Crossref]

Wang, Y.

Wen, H.

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

Winzer, P. J.

Wolf, S.

Xi, L.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

J. Li, X. Zhang, F. Tian, and L. Xi, “Theoretical and experimental study on generation of stable and high-quality multi-carrier source based on re-circulating frequency shifter used for Tb/s optical transmission,” Opt. Express 19(2), 848–860 (2011).
[Crossref] [PubMed]

F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011).
[Crossref]

Yan, M. F.

Yang, Q.

Yu, C.

Yu, J.

Zhang, H.

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

Zhang, J.

Zhang, X.

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

F. Tian, X. Zhang, J. Li, and L. Xi, “Generation of 50 stable frequency-locked optical carriers for Tb/s multicarrier optical transmission using a recirculating frequency shifter,” J. Lightwave Technol. 29(8), 1085–1091 (2011).
[Crossref]

J. Li, X. Zhang, F. Tian, and L. Xi, “Theoretical and experimental study on generation of stable and high-quality multi-carrier source based on re-circulating frequency shifter used for Tb/s optical transmission,” Opt. Express 19(2), 848–860 (2011).
[Crossref] [PubMed]

Zheng, X.

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

Zhou, B.

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

Zhou, X.

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

Zhu, B.

Zhu, J.

IEEE Photon. J. (1)

J. Zhang, J. Yu, N. Chi, Z. Dong, Y. Shao, L. Tao, and X. Li, “Theoretical and experimental study on improved frequency-locked multi-carrier generation by using recirculating loop based on multi-frequency shifting single-side band modulation,” IEEE Photon. J. 4(6), 2249–2261 (2012).
[Crossref]

J. Lightwave Technol. (4)

J. Opt. Commun. Netw. (1)

Opt. Commun. (1)

X. Zhou, X. Zheng, H. Wen, H. Zhang, and B. Zhou, “Generation of broadband optical frequency comb with rectangular envelope using cascaded intensity and dual-parallel modulators,” Opt. Commun. 313, 356–359 (2014).
[Crossref]

Opt. Eng. (1)

F. Tian, X. Zhang, L. Xi, A. Stark, S. E. Ralph, and G. K. Chang, “Experiment of 2.56-Tb/s, polarization division multiplexing return-to-zero 16-ary quadrature amplitude modulation, 25 GHz grid coherent optical wavelength division multiplexing, 800 km transmission based on optical comb in standard single-mode fiber,” Opt. Eng. 52(11), 116103 (2013).
[Crossref]

Opt. Express (4)

Opt. Lett. (3)

Other (3)

N. K. Fontaine, “Spectrally-sliced coherent receivers for THz bandwidth optical communications.” European Conference on Optical Communications, paper Mo.3.C.1, London, UK (2012).
[Crossref]

S. Chandrasekhar, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” European Conference on Optical Communications, paper PD2.6, Vienna, Austria (2009).

G. P. Agrawal, Applications of Nonlinear Fiber Optics (Academic Press, 2001), Chap. 4.

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Figures (10)

Fig. 1
Fig. 1 (a) Schematic of proposed low noise SSB-RFS multi-carrier generation scheme; (b) parallel implementation of optical FIR filter for ASE noise suppression; (c) serial implementation of optical FIR filter for ASE noise suppression. PC: polarization controller; BPF: band pass filter; EDFA: erbium doped fiber amplifier; OSA: optical spectrum analyzer; RF: radio frequency; PS: phase shifter; EA: electrical amplifier.
Fig. 2
Fig. 2 Illustration of ASE noise accumulation as circulating time increases.
Fig. 3
Fig. 3 (a) Carrier-to-noise-ratio per channel in SSB-RFS loop with/without optical FIR and constellations of loaded signal on different carriers; Output spectrum (b)without and (c)with 2-tap optical FIR filter applied in the loop (simulation resolution of 24.4MHz).
Fig. 4
Fig. 4 Power transfer function of optical FIR filter with different tap coefficients.
Fig. 5
Fig. 5 EVM performances of loaded Nyquist-16QAM signal when applying proposed ASE noise scheme with different tap number.
Fig. 6
Fig. 6 Required EDFA saturation output power when different tap number employed.
Fig. 7
Fig. 7 (a) Experiment setup for proposed low noise multi-carrier generation scheme; (b) The optical spectrum of first tone generation when the loop is open.
Fig. 8
Fig. 8 Generated 50 tones without (a) and with (b) ASE noise suppression scheme and zoomed version of last carrier.
Fig. 9
Fig. 9 The generated 69 low noise flat carriers covering 6nm spectrum range and details of first five and last 5 carriers.
Fig. 10
Fig. 10 (a) Measured and theoretical CNR values of SSB-RFS with/without 2-tap optical-FIR filter for ASE noise suppression; (b) Theoretical and measured value of CNR increment by using SSB-RFS with 2-tap ASE suppression.

Equations (20)

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E 1 ( f )= E 0 ( f )+[ g 1 lT( f ) E 0 ( f )+ n 1 ( f ) ] H BPF ( f ) E 2 ( f )= E 0 ( f )+[ g 2 lT( f ) E 1 ( f )+ n 2 ( f ) ] H BPF ( f ) , E n ( f )= E 0 ( f )+[ g n lT( f ) E n1 ( f )+ n n ( f ) ] H BPF ( f )
H rect ( f )={ 1 f 0 - f s /2 <f< f 0 +( N+1/2 ) f s 0 other .
S( f )=E[ | n( f ) | 2 ]= F n ( G1 )hf 2 .
n n ( f )= k= n Bs, n ( fk f s ) .
E N ( f ) k=0 N E 0 ( fk f s ) N discrect carrier lines + m=1 N k=Nm+1 N n Bs,m ( fk f s ) residual ASE noise induced in mth RT total ASE noise after Nth RT (a) = k=0 N E 0 ( fk f s ) N discrect carrier lines + k=1 N m=Nk+1 N n Bs,m ( fk f s ) ASE noise in kth channel total ASE noise after Nth RT (b) .
S tot,N ( f )=E[ | m=1 N n Bs, m ( fN f s ) | 2 ]= m=1 N E[ | n Bs, n ( fN f s ) | 2 ] = m=1 N S Bs,m ( f ) =N S Bs ( f ).
P ASEN = S tot,N ( f ) B s = N F n ( G1 )hf B s 2 =N P ASE, B s .
CN R n =10lg( P carrier / P ASE,n )+10lg( B s / B r ).
CN R n =TN R n +10lg( B res / B r ).
G FIRP ( f )= | F P ( f ) | 2 = | FFT( 1 N n=0 N1 δ( tnτ ) ) | 2 = 2 N 2 n=0 N1 ( Nn )cos( fnτ ) 1.
P out,ASE = 1 2π f f s /2 f+ f s /2 1 2 S in,ASE ( f ) G FIR ( f )df .
P out,ASE = 1 N P in,ASE .
G FIRS ( ω )= | F S ( ω ) | 2 = | ( 1 2 ) N [ 1+exp( jωτ ) ] N | 2 = ( 1 2 ) N [ m=0 N C N m cos m ( ωτ ) ].
P out,ASE = f f s /2 f+ f s /2 1 2 S in,ASE ( f ) G FIRS ( f )df = 1+ m=1 N/2 [ C N 2m ( 2m1 )( 2m3 )31 2m( 2m2 )42 ] 2 N P in,ASE .
E N ( f )= k=0 N E 0 ( fk f s ) N discrect carrier lines + k=1 N m=Nk+1 N n m,Bs ( fk f s ) F FIR Nm+1 ( f ) residual ASE noise in kth channel total residual ASE noise after Nth RT .
S ASEN ( f )=Ε[ | m=1 N n m,Bs ( fN f s ) F FIR Nm+1 ( f ) | 2 ]= m=1 N S ASEm ( f ) G FIR Nm+1 ( f ) .
P N,resASE = ω s S ASEN ( f )df = m=1 N ω s S ASEm ( f ) G FIR Nm+1 ( f )df .
CN R NFIR =10lg( P s / P N,resASE )+10lg( f s / B r ).
G= G 0 exp( ( G1 ) P in P s ).
CN R nFIR =10lg( P carrier_n / P noisefloor )+10lg( B res / B r )+ΔCN R ntheory .

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