**1 Overview**
As the demand for timely, accurate, and convenient communication continues to grow, it has become increasingly important to access information quickly and easily from anywhere. In 2002, the IEEE introduced the 10 Gb/s Ethernet standard—IEEE 802.3ae [1]. This marked a major advancement in traditional Ethernet technology, increasing the transmission speed tenfold compared to Gigabit Ethernet, thereby meeting the growing needs of mobile communication services.
In January 2009, China issued its first 3G license, signaling the beginning of the 3G era. To meet the demands of mobile communication development, major telecom operators launched large-scale 3G network construction. The core of this infrastructure is the base station, which represents the highest cost in the project. Additionally, the performance of these stations directly affects the quality of mobile services. Therefore, while operators strive to improve service quality, they also look for ways to reduce network construction costs. Distributed base stations, with their low cost, strong environmental adaptability, and ease of installation, are seen as the future direction of next-generation base stations.
The key feature of distributed base stations is separating the baseband processing unit (BBU) and the remote radio unit (RRU), connecting them via fiber optics. During deployment, the BBU is placed centrally, offering large capacity and enabling efficient coverage. Meanwhile, the RRU is installed on-site, allowing for flexible placement and shared BBU resources among multiple units, reducing baseband investment. To ensure seamless integration between base stations and repeaters, companies like Ericsson, Huawei, NEC, Nortel, and Siemens developed the CPRI interface protocol, which has since become a widely accepted industry standard.
**2 CPRI Protocol Overview [2]**
The CPRI (Common Public Radio Interface) protocol consists of two layers: the physical layer (L1) and the data link layer (L2). It serves as an internal data interface between the Radio Equipment Controller (REC) and the Radio Equipment (RE), as well as between two REs. The CPRI interface supports three types of data streams: user platform data stream (SAPIQ), control and management platform data stream (SAPCM), and synchronization platform data stream (SAPS).
**3 FPGA-Based CPRI Protocol Transmission Scheme**
**3.1 Basic Schemes**
Implementing the CPRI protocol using FPGA can be done through two main approaches.
**(1) Option 1**
This approach uses the RockeTIO transceiver found in Xilinx Virtex2Pro and later chips. RockeTIO is a dedicated serial communication module that doesn't consume other FPGA resources. In the Virtex5 series, it is referred to as GTP. The advantage of this method is its compact design, which simplifies PCB layout and improves system anti-interference capability. Each Virtex5 FPGA includes multiple GTP transceivers, and by using four of them, high-speed 10 Gb/s transmission can be achieved. Each GTP core includes both a receive and transmit link [4–5].
**(2) Option 2**
This option involves using dedicated chips such as TI’s TLK2501 or National Semiconductor’s SCAN25100. Among these, the SCAN25100 offers the most comprehensive features, including 8b/10b encoding, high-speed serialization/deserialization, lock detection, and CPRI signal/frame loss detection. It also includes precise delay calibration, clock management, and signal conditioning capabilities.
**3.2 Detailed Implementation**
The SCAN25100 supports data rates of 2.4576 Gb/s, 1.2288 Gb/s, and 0.6144 Gb/s, while the TLK2501 supports 1.5 to 2.5 Gb/s. To achieve a 10 Gb/s rate using dedicated chips, four of them would be required, increasing complexity and PCB size, which is not ideal for design.
This paper adopts the first design scheme to implement 10 Gb/s CPRI high-speed data transmission. As shown in Figure 1, the Ethernet optical interface comprises four parts: a 10GE optical interface, a PHY transceiver, a clock module, and an FPGA. The 10GE optical interface and PHY transceiver are hardware components responsible for 10G Ethernet optical transmission, while the FPGA is the core of the design, using Xilinx’s Virtex6 chip.
**10GE Optical Interface**: This fiber optic module includes optoelectronic components, functional circuits, and optical interfaces, divided into transmit and receive sections. The transmit section converts electrical signals into modulated light signals, while the receive section converts incoming optical signals back to electrical signals. The design uses an XFP optical module, which is small, low-power, and suitable for multi-port integration.
**PHY Transceiver**: This physical layer chip facilitates Ethernet access. It transmits four 3.125 Gb/s data channels from the FPGA to the optical module and receives a 12.5 Gb/s data stream from the optical module, splitting it into four 3.125 Gb/s channels. With 8b/10b encoding, the effective data rate reaches 10 Gb/s, meeting the paper's design requirements.
**Clock Module**: A 61.44 MHz active crystal oscillator is used as the system clock. Given the 3.125 Gb/s data rate, the GTP reference clock requires high precision. Instead of using DCM, an external differential crystal oscillator is preferred to avoid jitter. The GTP generates internal clocks for RXUSRCLK, RXUSRCLK2, TXUSRCLK, and TXUSRCLK2, ensuring stable and synchronized data transmission.
**FPGA Part**: The FPGA handles data encoding, high-speed serialization, framing, de-framing, synchronization, and data transmission. It also controls the optical and PHY modules, monitoring their status and enabling real-time diagnostics through the XFP's two-wire interface and the SMI interface for the PHY module.
**4 Design Verification**
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