The wired and wireless hybrid access network based on optimized self-homodyne and heterodyne technology

A novel wired and wireless hybrid access system based on self-homodyne and heterodyne technology is presented in this paper. With the 16 Quadrature Amplitude Modulation(16QAM) and optimized self-homodyne mechanism, the system achieves 224Gbit/s wired access. Through optical heterodyne technology, the 30GHz wireless covering is implemented. The performance of the system is further improved by using the Pre-Compensation module and Post-Compensation module. Self-homodyne system improves the Optical Signal Noise Ratio(OSNR) threshold by 1.7 dB compared to the conventional coherent system, while reducing the system device requirements. The adoption of the Pre-Compensation module will save 1.7dB of OSNR tolerance even further. DSP modules of low computational complexity suitable for self-homodyne system and heterodyne system are redesigned.


INTRODUCTION
The development of cloud computing and Data Center (DC) requires a higher transporting bit rate and channel capacity.According to the 2019 Cisco Cloud Index, by 2021, the number of hyper-scale Data Centers will double and global Data Centers' IP traffic will grow 3-fold [1].For a time in the past, the traditional intensity modulation-direct detection (IM-DD) combined with wavelength-division multiplexing (WDM) technology has been the main means to enhance the transmission capacity of optical fiber.At present, 50Gbaud with 4-pulse amplitude modulation (4-PAM) and direct detection are unable to meet the DC connection requirements [2]- [4].To improve the capacity of the interconnection links, three solutions including increasing the symbol rate, increasing the number of parallel channels, and using higher order modulation formats.Increasing symbol rate and parallel channels suffer the bandwidth limitation of opto-electrical components and system cost [5] .
Coherent detection techniques have the potential to improve link transmission performance compared to IM-DD [6].However, due to the constraints of cost, bandwidth, power consumption, and integration, the digital coherent optical communication technology developed for long-distance and high speed optical communication systems was not considered suitable for short-distance optical communication systems.A wired and wireless hybrid access system based on self-homodyne and heterodyne technology is proposed.The system has been redesigned to lower the system device requirements and simplify the DSP.
If the local oscillation light and signal light are of the same origin, it can effectively simplify the DSP module [7]- [10].At the same time, it makes it possible to use wide linewidth, low-cost lasers in coherent optical communication systems.The transmission delay will be processed using the pilot information instead of using the traditional Blind Phase Search(BPS) algorithm, which helps to reduce computational complexity significantly.
At the same time, heterodyne detection can be realized by the difference in frequency between the signal light and local oscillation light [11]- [12].Compared with the traditional Radio over Fiber (RoF), the heterodyne detection technology has better anti-dispersion performance, higher spectral efficiency, and higher sensitivity [13].The wired and wireless hybrid access network takes advantage of homodyne detection's high sensitivity and stability and realizes flexible wireless access by heterodyne detection.
The structure of this paper is organized as follows.In the second part of the article, we will introduce wired and wireless hybrid access network architecture.The third part will focus on pre-compensation to compensate for bandwidth limitations.In fourth part, we discuss the advantages of self-homodyne coherent optical network in simplifying the DSP module.In the fifth section, we conclude the paper.

THE WIRED AND WIRELESS HYBRID ACCESS NETWORK
The architecture of the hybrid wired and wireless access network is shown in Figure 1.The downlink laser is used not only as the signal light but also as the local oscillation light at the receiving end for self homodyne detection.The receiver side uses the uplink laser for heterodyne detection of the received signal to obtain an electrical intermediate frequency (IF) signal for wireless coverage.2) ( ) ( ) cos( 2) Where AS(t) and ALO(t) are the amplitudes of the signal light and the local oscillation light, fS and fLO are the frequency of the signal light and the local oscillation light respectively, and φS and φLO are the phases of the signal light and the local oscillation light.The phase noise of a laser is generally modeled as a Wiener process which satisfies Eq.3.
( ) , (0, 2 ) In Eq.3, εk is an increment at each sampling moment.The phase noise increment at each sampling moment conforms to a Gaussian distribution with a mean of zero and a variance proportional to the product of the linewidth ΔϑS and the sampling interval TS.
The local oscillation light is coupled to the signal light and then enters the photodetector.Let the responsiveness of the photodetector be R, we can get the photodetector output I(t) as In Eq.4,PS is the power of signal light and PLO is the power of local oscillation light.If the IF difference is 0, this gives a direct baseband signal.If the IF is not 0, then the signal originally carried on the signal light is successfully moved to the IF using the heterodyne detection.

PRE-COMPENSATION FOR BANDWIDTH LIMITATION
To compensate for signal impairment caused by bandwidth limitation of the devices, such as Digital To Analog Converter (DAC), the Pre-Compensation module is designed.In this paper, the bandwidth limitation of the device is compensated by Digital pre-emphasis(DPE).
The Effective Number of Bits (ENOB) is used to describe the effect of noise and loss in DACs on DAC performance.In this paper, the ENOB is set to 6.35 [14].Then the frequency response function of the DAC is a Bessel function with a 3dB bandwidth of 25GHz, and the frequency response function of the Analog To Digital Converter (ADC) is a Bessel function with a 3dB bandwidth of 35Ghz [15].
The flow for optimizing the DAC bandwidth limitation using digital pre-emphasis is shown in Figure 2 (a), and the parameters for the system simulation are shown in Table 1.We conduct a simulation in a back-to-back system with 224Gbps, 12% FEC overhead.We test the BER of the conventional homodyne system, the self-homodyne system, and the self-homodyne system with the Pre-Compensation module under different OSNR respectively, as shown in Figure 3.It can be seen from the results that at the FEC threshold, compared with the traditional homodyne system, the OSNR of the self-homodyne system saves 1.7dB.When the pre-compensation module is adopted, the OSNR of the system saves 3.4dB at the same threshold.

DSP-Lite self-homodyne receiver
In today's optical receiver modules, DSP power consumption accounts for a very high percentage of the overall receiver power consumption.For the 400G-ZR-QSFP-DD, nearly half of the overall power consumption is allocated to the DSP processing module [16].The composition of the simplified DSP module suitable for self-homodyne coherent system is shown in Figure 4, which requires only three components: low-pass filter, Constant modulus algorithm (CMA) and Radius Detected (RD) algorithm and phase rotation based on pilot sequence.For self-homodyne coherent system, both the local oscillation light and the signal light experience the same carrier phase, and if the transmission delay can be matched well, the compensation of the phase noise and frequency offset of the signal light can be completed well, which will make the subsequent DSP processing significantly simplified.
Transmission delay is a problem where there is a delay between the signal light and the local oscillation light, which ends up with optical signal phase noise.In this paper, instead of the traditional BPS algorithm,we can use pilot sequence to compensate for the phase noise due to the transmission delay.
The approach of recovering the phase information based on the pilot sequence that for the transmitted signal, a specific signal is added every N signal and then the actual phase rotation angle of this specific signal is calculated.After calculating the phase rotation angle for all pilot sequence, we calculate the average of the phase rotation angles ∆ϕ as shown in Eq.6., 1, 2,3..., We can then use the mean of the phase shifts obtained from the calculation to compensate for the effects caused by phase noise.The advantage of utilizing the method of pilot sequence for the compensation of phase signals over the conventional BPS is the substantial computational savings.For the BPS algorithm, the average computational effort to estimate the phase rotation value for a single point is proportional to the number of tested phases B. For the pilot sequence, the amount of computation for estimating the phase rotation value for a single point is related to the interval N. If only multiplication, addition and look-up table (LUT) are considered, the computational complexity for N symbol points is shown in Table 2.For a complex index of a given angle, two LUT are required.In this paper, the value of B is 16, while N is 40.

BPS Pilot
Real Additions The complexity of the pilot algorithm is at least one order of magnitude lower than that of the BPS, which results in a significant reduction in the complexity of the DSP at the receiver side.

Low-power DSP heterodyne receiver for wireless access
After the optical coupler, the signal and LO are received using two photodiodes (PD).The difference between the LO frequency fLO and the signal light frequency fS is 30 GHz.Through the optical heterodyne technology, the 30Ghz electric IF signal is obtained after the photodetector.Then it passes through the electric amplifier to realize the wireless coverage.After the IQ demodulation module, the received electric IF signal is downconverted to a baseband signal.The specific flow is shown in Figure 6.We use the blind phase search and maximum likelihood estimation (BPS/ML)algorithm to replace BPS algorithm to compensate phase noise.In BPS/ML algorithm, the computational overhead for each additional cascade of a maximum likelihood phase estimator is equivalent to the computational overhead required to add two additional test phases in the BPS algorithm [17].Nevertheless, in the first level rough estimation process, the computational complexity is reduced by nearly 50% due to the significant reduction of the number of test phases.Compared with BPS, BPS/ML can effectively reduce computational complexity while ensuring performance and effectiveness.The constellation points are not distinguishable after the CMA+RD compensation, indicating that there is a significant frequency offset in the system.We use the QPSK partitioning algorithm to compensate for this, and the compensation effect, Finally, the BPS/ML algorithm is used to compensate for the phase deviation, as shown in Figure 7.(b).

CONCLUSION
The proposed wired and wireless hybrid access scheme has achieved 224Gbps per wavelength.This architecture has the advantages of both homodyne and heterodyne systems, which can utilize the stability of the homodyne system and also realize wireless access by using heterodyne, greatly improving the flexibility of the system.Moreover, the selfhomodyne coherent system implements DSP-lite, which greatly simplifies the DSP module.At the same time, the BPS/ML algorithm replaces the BPS algorithm, which reduces the computational complexity.

Figure 1 .
Figure 1.Conceptual diagram of wired and wireless hybrid access scheme.The principle of coherent detection is to mix the signal light with the local oscillation light and finally recover the signal in the electrical domain.The signal light and local oscillation light can be expressed as ( ) ( ) cos(2 )

Figure 2 .
Figure 2. (a) Compensation process of the Pre-Compensation module.(b)The spectral response of the Pre-Compensation module.(c)The spectral response of the DAC.Where N(f) is the Nyquist filter and n(f) is the Gaussian noise.The idea of the Pre-Compensation model is to enhance the high-frequency component of the signal,which can compensate for the DAC bandwidth limitation.We realize the Pre-Compensation circuit by designing a filter.Its response function is shown as Eq.5.
the filter cutoff frequency.Figure2.(b)shows the filter's spectral response.Figure2.(c) represents the spectral response of the low pass cosine roll-down filter which is used to simulate the effect of DAC bandwidth limitation.

Figure 3 .
Figure 3.The effects of the self-homodyne mechanism and the Pre-Compensation module on the system performance under different OSNR are compared.

Figure 4 .
Figure 4.The simplified DSP for Self-Homodyne Coherent Optical Network.

Figure 6 .
Figure 6.Architecture of the wireless access.

Figure 7 .
Figure 7. (a)Constellation map compensated by the CMA+RD algorithm.(b) Constellation map after BPS/ML algorithm.

Figure 7 .
Figure 7.(a) shows the results of the constellation diagram after the CMA+RD algorithm has been compensated.The constellation points are not distinguishable after the CMA+RD compensation, indicating that there is a significant frequency offset in the system.We use the QPSK partitioning algorithm to compensate for this, and the compensation effect, Finally, the BPS/ML algorithm is used to compensate for the phase deviation, as shown in Figure7.(b).

Funding:
National Key Research and Development Program of China (2022YFB2802403) ，National natural science foundation (62220106002), Natural Science Foundation of Shandong Province(ZR2021MF018), and Open Fund of IPOC 2021(BUPT).

Table 1 .
Parameters of the system.

Table 2 .
Comparison of computational complexity.