Objective With the advantages of large available bandwidth and minimum interference in existing wireless services, millimeter-wave (mm-wave) technology can be widely used in future space communication or wireless communication. In previous research, the generation schemes of photon-assisted mm waves have mainly included: direct modulation technology, external modulation technology, and optical heterodyne technology. In order to overcome the bandwidth limitation of devices and meet the requirements of low-cost, the generation of vector mm waves with low radio frequency (RF) signals and intensity modulators has become a research hotspot. However, this method suffers from two main drawbacks, namely the phase multiplication and the cost of the system. To improve the above-mentioned problems, we generate a carrier suppression mm-wave signal with an intensity modulator. We also consider the carrier suppression vector mm-wave signal for achieving four-wave mixing (FWM) in a highly non-linear dispersion-shifted fiber to increase the frequency of the mm-wave. We use a balanced precoding algorithm to demodulate the distortion of the signal during transmission. That is, the complex and expensive I/Q modulator, as well as the bandwidth requirements of the optoelectronic devices is avoided. Furthermore, a single side band (SSB) vector mm-wave signal that can be delivered over relatively long fiber transmission distances is generated. Based on our scheme, we propose and experimentally demonstrate a 2-Gbaud QPSK vector mm-wave signal generation at 72 GHz by six-fold frequency. A mm-wave signal transmission over a 15-km fiber and with 1-m wireless for a bit error rate (BER) below the hard decision-forward error correction (HD-FEC) threshold of 3.8×10 -3 is also realized.
Methods We propose a new scheme to generate a six-fold requency vector mm-wave signal based on FWM and balanced precoding. To achieve six-fold frequency, we use carrier suppression modulation with a single intensity modulator and FWM with a 1-km highly non-linear dispersion-shifted fiber. Combined with balanced precoding, a 72-GHz carrier frequency quadrature phase shift keying (QPSK) vector mm-wave signal is generated. Maintaining the signal quality is essential for reducing the system cost. We demonstrate, via experiments, the generation of a 2-Gbaud 72-GHz QPSK vector mm-wave signal, and discuss the effects of baud rate and fiber transmission length of the QPSK signal on the signal.
Results and Discussions The BER versus photodetector (PD) input power curves after back-to-back (BTB) and 15-km fiber transmission are almost the same (Fig. 7), indicating a lack of dispersion penalty after the 15-km fiber transmission. To further explore the transmission performance of the signal in the fiber, we determine (via experiments) the maximum receiver optical power required for achieving the BER at 3.8×10 -3 under different fiber lengths (Fig. 9). As shown in Fig.9, the optical power is remained at -6.3 dBm after <15 km of fiber transmission. However, the required optical power increases after the aforementioned transmission. After 30-km fiber transmission, the required optical power is 3.3 dBm with a considerable power penalty. The motivation for changing the baud rate of the QPSK signal is to further investigate the influence of the signal baud rate during the transmission experiment. We measured the BER versus the baud rate of a 72-GHz QPSK signal (Fig. 10). As shown in Fig.10, the BER performance of the signal improves with decreasing baud rate of the signal. When the baud rate of the QPSK signal is lower than 2.5 Gbaud, the BER is considerably lower than the HD-FEC threshold. The result shows that after 15 km of fiber and 1 m of wireless transmission, the BER of the signal is lower than the HD-FEC threshold. This occurs when the optical power input to the photodiode is >-6.3 dBm and the QPSK signal baud rate is ≤2.5 Gbaud.
Conclusions We demonstrate to the generation of a six-fold frequency vector mm-wave signal using precoding. In our scheme, we generate a 72-GHz V-band vector mm-wave signal with oscilloscope (OSC) and FWM using our designed electrical signals. We demonstrate (via experiments) the generation of a 72-GHz 2-Gbaud QPSK signal. After 15-km fiber transmission and 1-m wireless transmission, when the baud rate of the signal is ≤2.5 Gbaud, the HD-FEC algorithm can achieve error-free transmission. The results revealed that the vector mm-wave signal based on this scheme exhibits excellent transmission performance. As indicated above, this scheme overcomes the bandwidth limitation of devices, meets the requirements for low-cost devices, and reduces the frequency of the RF signal. Our proposed scheme with six-fold frequency combines the advantages of our two aforementioned categories of photonic vector mm-wave generation schemes. The authors believe that expensive electronics operating at high carrier frequencies with a bandwidth limitation can be avoided. That is, photonics-aided mm-wave technology has been widely applied to the generation and processing of mm waves. The millimeter ROF delivery based on highly efficient spectrum modulation will be a promising method of developing larger capacity links than currently available links.
