Comparison of Optical Single Sideband Techniques for THz-Over-Fiber Systems

Abstract

The use of single sideband (SSB) signals and envelope detection is a promising approach to enable the use of economic free-running lasers in photonic THz communications. To combat the signal-signal beat interference (SSBI) associated with envelope detection, broad guard bands (GBs) may be used given the large unregulated spectrum available at THz frequencies (100 GHz-10 THz). In this scenario, the conventional way of generating SSB signals through a digital SSB filter (here referred to as the CSSB scheme) would require quite high analog digital-to-analog converter (DAC) bandwidths. Digital virtual SSB (DVSSB) and analog virtual SSB (AVSSB) have been proposed in directdetection optical systems for relaxing the DAC bandwidth requirements. In this paper, we compare the three techniques through simulations and implement them, for the first time, in a THz-over-fiber (ToF) system operating at 250 GHz. For the transmission experiments we employ 5 GBd 16-QAM signals with three different GBs (5.5 GHz, 4.75 GHz and 3.5 GHz). The simulations show that the best performance is obtained with the AVSSB technique, while the worst is obtained with the DVSSB scheme, where the quality of the generated sideband degrades with carrier-to-sideband power ratio. In the experimental transmissions, where receiver noise was the main source of noise, similar behavior was found between the three techniques. At the 3.5 GHz GB, however, the DVSSB exhibited a penalty of 1 dB with respect to the other two. This is likely to be due to nonlinear distortions caused by the increase in the virtual tone power.

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.…”

“…(3) offset and a very large averaging block was used. The fixed phase offset results from the biasing points of the I-and Qcomponents in the optical IQ modulator as detailed in [8]. In a practical system, to reduce digital complexity, this offset may be removed by simply adding the proper phase value to the filtered tone [22].…”

Carrierless Amplitude and Phase Modulation for Terahertz Communications

“…Remarkably, supporting the generation and detection of such a highorder modulation scheme complicates the terahertz transceiver architecture. As an example, in [33] and [34], the transmitters are based on digital as well as optical I-Q mixing to generate 16-QAM signals. At the receiver side, the 16-QAM terahertz signals are down-converted to the IF band using SHM.…”

Pilot-tone assisted 16-QAM photonic wireless bridge operating at 250 GHz

“…The latter, on the other hand, is only used to compensate a fixed phase offset and a very large averaging block is used. The fixed phase offset results from the biasing points of the I-and Q-components in the optical IQ modulator as detailed in [16]. In a practical system, this offset may be removed with a single-phase rotation without the Viterbi-Viterbi algorithm to reduce digital complexity [17].…”