Contents I. Introduction
The significant growth in the number of mobile subscribers has led to the need for increased speed and capacity in mobile networks. In recent years, the compound annual growth rate of mobile communications traffic exceeded 60% [1]. Globally, the application of third-generation (3G) mobile communication technologies such as wideband code-division-multiple-access (WCDMA) has become widespread, and the technology has matured with a connection speed of 50 Mb/s. In the USA, South Korea, Japan, and several other countries, mobile services involving Long-Term Evolution (LTE) have been developed. LTE has a capacity greater than 100 Mb/s, and is referred to as 4G commercially [2]. In addition, the number of radio bands assigned for mobile services has increased in order to realize higher capacity transmissions. Moreover, in a small cell scenario designed to realize high-speed radio access including next-generation mobile services, equipment at the cell site should be able to cover wider areas while remaining cost effective. In general, a base station (BS) can be functionally separated into a modulation/demodulation unit (M/dMU) and a radio antenna unit (RAU). To meet the above demand, the M/dMU is commonly located on the network side, while the RAU is distributed at each cell site. The connection between the M/dMU and RAU in the BS is generally called the “mobile fronthaul.” In the standard of the Common Public Radio Interface (CPRI), which is a kind of digital interface for signal transmission inside a BS, the M/dMU and RAU are called the baseband unit (BBU) and remote radio head (RRH), respectively [3]. When the CPRI standard is used, the mobile fronthaul is realized by transmitting a digitized radio signal. In future mobile technologies including fifth-generation (5G) mobile, either cloud-based or centralized radio access network (C-RAN) architecture would be implemented; centralized M/dMUs in the cloud are connected to RAUs at remote sites via optical fiber networks [4], [5], [6], [7], [8]. However, in such architecture, there is a problem with the latency due to signal processing and configuring the packet structure for digital signal transmission, such as serialization/de-serialization and signal optimization for radio transmission. In addition, the time frame defined by the conventional layer 2 standard, such as a passive optical network (PON), may limit the transmission delay. The latency would be larger with an increase in the degree of signal processing. In LTE-Advanced, which is the next LTE standard with a capacity of 1 Gb/s, the latency of a one-way trip in the user plane should be less than 5 ms [9]. Therefore, it is important to realize a reduction in the latency on the mobile fronthaul link in order to ensure consistency in the standard of mobile services as well as the enhancement of the user experience.
