We have carried out research and development of next generation (5G) wireless communication systems in dense user environments utilizing advanced photonic technologies. Especially, we have focused on a heterogeneous wireless communication system, which includes 4G, 3G, WiFi and millimeter-wave wireless links, with broadband, low latency, low-power consumption and low-cost. In this report, photonic-based millimeter-wave wireless links of ultra-small cells (atto-cells) in a big football stadium are discussed. Advantages of an asymmetric millimeter-wave link combined with terminal localization techniques are also pointed out. Some basic experimental results on the millimeter-wave wireless links in an actual big football stadium are also reported.
We describe some of our recent results on DWDM mm-wave radio-on-fiber (RoF) technology for future broadband wireless systems. A supercontinuum (SC) light source is a promising multiwavelength light source for the system with photonic up-conversion. Multiplexing and demultiplexing schemes for optical frequency interleaving in order to extend the number of antenna base stations can be constructed using a properly designed arrayed-waveguide grating (AWG). We demonstrate a full-duplex WDM mm-wave RoF system using a SC light source. The periodic nature of AWG, where the period is free spectral range (FSR), is used for utilizing the full bandwidth of the SC light source. Half of the SC output modes are used for downlink transmission with photonic upconversion. Another half of the SC output modes are used for uplink transmission with photonic downconversion. These techniques are effective to use a wealth of optical frequency resources from the SC light source. Two-channel downlink and one-channel uplink 60- GHz band RoF signals were simultaneously transmitted over 25-km standard single-mode fiber with error-free and no noticeable power penalty.
We experimentally demonstrate a demultiplexing scheme for the frequency-interleaved dense wavelength division multiplexing (DWDM) radio-on-fiber system using an arrayed-waveguide grating (AWG). We first show the principle of the frequency-interleaved DWDM radio-on-fiber and possible configurations of multiplexing and demultiplexing (DEMUX) schemes using an AWG. In the experiment, 25-GHz separated 2-channel optical double sideband signals modulated by a 60-GHz millimeter-wave carrying a 156 Mbps data are optically multiplexed by frequency interleaving. The power penalty after DEMUX, which was due to interchannel interference, was less than 0.5 dB. We also made a transmission experiment in 25-km standard single mode fiber (SMF). No noticeable power penalty in the received data due to transmission was observed. This is because carrier and only one of the sidebands are detected in the proposed DEMUX scheme.
When closely temporally separated two optical pulses of identical wavelength are transmitted in optical fiber, the pulse separation could be changed due to nonlinear effect. We evaluate the pulse-to-pulse interaction in dense dispersion management by simulation and experiment. First, we evaluate the influence of third order dispersion on the pulse-topulse interaction by numerical simulation of transmission. The interaction can be effectively reduced when dispersion shifted fiber for dispersion slope tuning is placed at input of EDFA repeaters. Then we performed a recirculating loop transmission experiment of 3.5-ps optical RZ pulses with 12.5-ps initial separation in dispersion flattened dispersionmanaged fiber. The interaction can be greatly reduced by employing dense dispersion managed fiber, which is favorable for ultra-high speed OTDM transmission systems.