Single Sideband Signals for Phase Noise Mitigation in Wireless THz-Over-Fibre Systems

Abstract

In future photonic-based THz backhaul links, integrating the optical local oscillator (LO) in a remote antenna unit can be beneficial in terms of optical bandwidth efficiency and higher compatibility with passive optical networks. In such a scenario, two approaches can be used to reduce the high phase noise associated with free-running lasers: 1) baseband (BB) signals & carrier recovery, and 2) single sideband (SSB) signals & envelope detection. In this paper, we compare the performance of the two approaches for various optical LO linewidths using 5 GBd 16-QAM signals. We find that, for a total linewidth wider than 0.55 MHz, the SSB approach yields better results. The superior performance, however, comes at the expense of reducing the net information spectral density (ISD) of the SSB signal by 39% compared to that of the BB signal. However, using signal-signal beat interference-mitigation algorithms, an ISD only 15% lower than the BB signal ISD was sufficient to meet the FEC requirement. Given these results, we believe that the envelope detection of SSB signals is a promising solution to mitigate the phase noise problem of THz links based on free-running lasers, without excessively compromising the spectral efficiency of the system. Index Terms-Broadband communication, digital signal processing, envelope detectors, microwave photonics, millimeter wave communication, optical mixing, photonic integrated circuits, semiconductor lasers.This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see http://creativecommons.org/licenses/by/3.0/

Relaxation of the Radio-Frequency Linewidth for Coherent-Optical Orthogonal Frequency-Division Multiplexing Schemes by Employing the Improved Extreme Learning Machine

“…Consequently, the OFDM symbol periods correspond to 11 × 10 −9 s for BPSK, 22 × 10 −9 s for QPSK, and 44 × 10 −9 s for 16QAM. By considering a hard-decision forward error correction (FEC) threshold of 3.8 × 10 −3 [2], the RF-linewidth values are adopted according to the subcarrier modulation format in order to properly evaluate the system performance. For practical purposes, the RF linewidth is utilized in the rest of manuscript.…”

“…Taking into account the 16QAM signaling and 50-kHz mixed linewidth, for instance, the log 10 (BER) goes from −2.6 with 10 hidden neurons and without regularization parameter to −3.2 subject to 18 hidden nodes and a positive constant equals to 2 5 . Namely, there is a log 10 (BER) improvement of −0.6, but the dimension of the hidden layer output matrix (H) and its Moore-Penrose generalized inverse (H † ) possesses an increase of 8 rows and 8 columns and additionally the summation of the C rectangular matrix (C/I 18×18 ) must be computed in the learning procedure, refer to expression (2). Note that in our previous study (ELMs to reduce phase error in radio-over-fiber OFDM schemes with direct detection [24]), H † does not consider the regularization parameter and, consequently, its inclusion could be explored in a next paper.…”

“…These relationships tend to reduce as the BER reaches the FEC limit, which can be attributed to the ELM behavior as the constellation alphabet and phase-noise level augment. Because the system performance of phase-uncorrelated OFDM signals depends on the relationship between the RF linewidth and OFDM symbol period for a certain subcarrier modulation format [2,3,7], this outcome and the rest of observations along the paper are useful for the design and implementation of CO-OFDM schemes.…”

“…For optical fiber transmissions, intersymbol interference originated by polarization-mode dispersion and chromatic dispersion becomes relevant as the bit rate achieves the order of various tens of Gbps [1]. As an efficient alternative to mitigate this issue, coherent optical (CO) orthogonal frequency division multiplexing (OFDM) systems have attracted the attention of the scientific community because intersymbol interference may be eliminated as long as the guard interval occupies enough space of the OFDM symbol period [2]. Additionally, next-generation Tbps, bandwidth variable elastic optical systems can be a direct consequence of the CO-OFDM technology by its high receiver sensitivity as well as its effective transmission rate.…”

Pilot-Tone Assisted 16-QAM Photonic Wireless Bridge Operating At 250 GHz

“…In this paper we demonstrate a photonic wireless bridge operating at 250 GHz and using a carrier recovery scheme based on the transmission of a reference pilot tone. Compared to our previous works in [8] and [9] there are two fundamental differences: the first one is that envelope detection is no longer used for demodulation, and the second that the transmission reported here incorporates the second portion of optical fiber (after THz reception) that characterizes wireless bridges.…”