Contents I. Introduction
Recently, mobile data traffic has drastically increased, along with demand for high-speed radio communication technologies for next-generation mobile services to enhance user experience [1]. To increase the total capacity of radio access networks (RANs), the introduction of small-cell architectures will be indispensable [2]–[5]. In small-cell architecture, simple configuration of equipment at the cell site is necessary for cost-effective coverage over wide service areas. Therefore, a radio transceiver at a base station can be functionally separated into two parts: a modulation/demodulation unit (M/dMU) and a radio antenna unit (RAU). The M/dMU serves as the primary signal processor, and the RAU is the radio air interface. The M/dMU can be located at a network site, whereas an RAU should be located at each remote cell site. Thanks to low transmission losses and the extremely wide bandwidth of optical fibers, fiber-optic transmission of radio signals for mobile services is a promising candidate for connecting M/dMU to RAUs; this approach is called radio over fiber (RoF) [6]. If RoF technology is used to support RANs, the simplest equipment at the cell site would basically consist of an RAU, optical-to-electrical (O/E) converter, electrical-to-optical (E/O) converter, and amplifiers. In general, without any amplification, the introduction of a fiber-optic link between M/dMU and RAU merely causes loss of signal. When losses cannot be compensated using only electrical amplifiers, optical amplifiers (OAs), such as an erbium-doped fiber amplifier (EDFA) and a semiconductor optical amplifier, are also needed. In such RoF links, the transmission quality of RoF signals depends on the characteristics of the OA because the OA's behavior is affected by input optical power. In general, radio signals used for mobile services are transmitted only in a specified time. In addition, after power recovery, a low-cost light source for simple cell-site equipment may suddenly turn on. Therefore, there is a possibility that, especially for the uplink, an RoF signal could rise sharply in response to a burst of in-coming radio signals or to the turn-on timing of a light source; both these cases could cause failure of the photo detector because of the optical surge generated at the OA. For such sharply rising RoF signals, we have shown the effectiveness of a burst-mode EDFA (BM-EDFA), which is a type of transient-suppressing EDFA with additional gain stabilization [7]. Moreover, the output power range of the OA is another important factor because it determines the operational power range in the optical domain. Hence, we have also investigated the performance of a BM-EDFA for a sharply rising RoF signal using the long-term evolution (LTE) format [8]. The larger the maximum optical output power is just before optical saturation of an EDFA, the larger the power of the photodetected RF signal is under the operable power range of the photo detector. To further improve performance, we have developed a new BM-EDFA that has a wider range for output power.
