We theoretically study the stability of single-sideband-modulator based recirculating frequency shifter (RFS) used for Tb/s multi-carrier optical transmission, in the presence of modulator nonlinearity and optical amplifier noise. Optimum operation conditions are found for various scenarios.
Bit-interleaved coded modulation with iterative decoding (BICM-ID), which can improve the BER performance within limited bandwidth, is getting growing international attention. Symbol mapping, defined by the signal constellation and the bit labeling is the crucial design parameter to achieve a high coding gain for BICM-ID. It has been found that BICM-ID can yield a better coding gain by taking full advantage of the signal constellation. In this paper, an improved symbol mapping scheme, the combination of two QPSK with different radius and phases, called Cross Equalization-8PSK-Quasi-MSP (CE-8PSK) is proposed for BICM-ID. Simulation results show that, under the premise of same average power, the proposed scheme can improve the receiving performance of BICM-ID compared with the conventional symbol mapping schemes. At BER=10−4, the improved scheme can achieve more than 0.4dB code gains over Rayleigh fading channels.
A microwave photonic single bandpass filter is proposed and successfully demonstrated. The principle is based on the Hi-Bi FBG-FP cavity transmission to break the amplitude balance of PM sidebands and achieve effective PM-AM conversion.
For the digital subcarriermultiplexing (DSM) transmission, it is compulsory to realize the frequency offset estimation (FOE) and compensation (FOC) before the subcarrier de-multiplexing and the chromatic dispersion (CD) compensation (CDC). Here, we demonstrate a blind FOE method for the spectral-efficient DSM signals with a sharper root raised cosine (RRC) spectral shaping, with the capabilities of the transparent modulation format operation, a wide FOE range, and a high FOE accuracy. By reusing the spectral information for the CDC, the proposed FOE method is hardware efficient. Moreover, the proposed FOE method is numerically identified robust to transmission impairments, including the amplified spontaneous emission (ASE) noise, the optical filtering effect, and the accumulated CD, with an estimation error of less than 35 MHz under a FFT size of 2048. The correct function of the proposed FOE method is finally experimentally verified for 25 Gbaud 16QAM DSM signals under scenarios of both back-to-back (B2B) and standard single-mode fiber (SSMF) transmissions.
Hybrid in-band on-channel (IBOC) broadcasting system as a scheme of digital audio radio could transmit analog FM and digital audio simultaneously. In IBOC systems, broadcasters transmit signals within the allocated channel bands, thus the bandwidth assigned for digital audio transmission is very challenging. BICM-ID can improve BER performance over Rayleigh fading channels without bandwidth expansion, thus it is used in FM IBOC and has been verified it can yield a better coding gain. However, conventional IBOC systems apply Gray mapping as symbol mapping scheme, which can not take full advantage of iterative effects compared with larger diversity mappings such as SSP and MSP mappings. In this paper, various symbol mappings based on transfer functions and harmonic mean of the minimum Euclidean distance are compared and analyzed. Simulation results show that advanced mapping scheme can effectively improve the performance of BICM-ID FM IBOC. When BER is below 10 -4 , SSP labeling scheme can achieve more than 3dB coding gains over Rayleigh fading channels compared with the conventional mapping scheme.
This paper proposes a new stopping criterion for BICM-ID (bit-interleaved coded modulation with iterative decoding) system based on the CE (cross-entropy) stopping criterion. Unlike the conventional CE stopping criterion, the new scheme only computes and compares the cross-entropy value of the odd bits of the entire frame bits. We name the proposed new criterion as Partial-CE stopping criterion. The new criterion can reduce about 50% computation complexity of the BICM-ID receiver. Simulations comparing the new criterion with the original CE stopping criterion show that the proposed Partial-CE scheme can achieve similar performances in terms of BER and the average iteration numbers.
A few-mode erbium-doped fiber amplifier with two few-mode EDFs of uniformly doped step-index profile is proposed and experimentally demonstrated, employing two optical circulators to control the routings of different modes. In order to minimize the differential modal gain (DMG), amplifier parameters are discussed, including the lengths of two few-mode EDFs, core pump modes, and power ratio. In simulations, the gain performance of the proposed FM-EDFA is investigated, combining the conditions of the EDFs' lengths and pump configurations. Additionally, we construct an experimental setup of a four-mode EDFA and measured results indicate a DMG of 0.2 dB at 1550 nm, with less than 2.57 dB DMG and an average gain of 19.56 dB across the C-band.
Bit-interleaved coded modulation with iterative decoding (BICM-ID) is a better scheme than BICM through applying iterative decoding algorithm at the receiver. It can get excellent performances both in AWGN and in Rayleigh fading channels. However, the iterative algorithm increases the complexity of the receiver and leads to much decoding delay. Thus, the stopping criterion, which can timely terminate the unnecessary decoding iterations, is put forward. In this paper, based on the cross-entropy (CE) concept, we propose a new method named LLR (log-likelihood-ratio) stopping criterion. Simulation results show that without any exponential computation, the proposed LLR scheme can take a better trade off between the average iteration numbers and the bit-error-rate (BER) performance degradation. Although with a little more iterative numbers than CE at high signal-to-noise ratio (SNR) sections, it obtains excellent BER performance as compensation. More importantly, besides the less iteration numbers, the LLR scheme performs even better than the fixed scheme at high SNR sections in terms of BER performance.
The dynamic gain of a few-mode erbium-doped fiber amplifier (FM-EDFA) is vital for the long-haul mode division multiplexing (MDM) transmission. Here, we investigate the mode-dependent dynamic gain of an FM-EDFA under various manipulations of the pump mode. First, we numerically calculate the gain variation with respect to the input signal power, where a mode-dependent saturation input power occurs under different pump modes. Even under the fixed intensity profile of the pump laser, the saturation input power of each spatial mode is different. Moreover, high-order mode pumping leads to a compression of the linear amplification region, even though it is beneficial for the mitigation of the differential modal gain (DMG) arising in all guided modes. Then, we develop an all-fiber 3-mode EDFA, where the fundamental mode of the pump laser can be efficiently converted to the LP11 mode using the all-fiber mode-selective coupler (MSC). In comparison with the traditional LP01 pumping scheme, the DMG at 1550 nm can be mitigated from 1.61 dB to 0.97 dB under the LP11 mode pumping, while both an average gain of 19.93 dB and a DMG of less than 1 dB can be achieved from 1530 nm to 1560 nm. However, the corresponding signal input saturation powers are reduced by 0.3 dB for the LP01 mode and 1.6 dB for the LP11 mode, respectively. Both theoretical and experimental results indicate that a trade-off occurs between the DMG mitigation and the extension of the linear amplification range when the intensity profile of pump laser is manipulated.
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