The baseband fiber technology enhances the digital microwave relay system by relocating the intermediate frequency section from the indoor unit to the outdoor unit. By using optical fiber as the transmission medium and sending baseband digital signals, the system can achieve transmission distances of several kilometers or more. This approach eliminates the need for traditional feeder cables between the indoor and outdoor units, reducing signal loss, lowering the power requirements for the power amplifier, and allowing greater flexibility in antenna placement. As a result, system installation becomes more efficient and adaptable to various environmental conditions. This paper explores the functionality of the 88E1111 and presents a design solution for implementing a baseband fiber-optic remote connection in a digital microwave relay system, addressing the complexity and challenges typically associated with such designs.
1. Introduction to 88E1111
1.1 Features of 88E1111
The 88E1111 is a high-performance, single-chip Gigabit Ethernet physical layer (PHY) chip developed by Marvell. It supports the entire IEEE 802.3 protocol suite and includes an integrated 1.25 Gbps serial deserializer for high-speed optical transmission applications. It supports multiple MAC interfaces, including GMII, TBI, RGMII, and RTBI, and provides automatic detection for 10/100/1000BaseT. The chip is fabricated using 0.13 μm CMOS technology, operates at low voltages (2.5 V and 1.2 V), and has a maximum power consumption of 0.75 W. It also features automatic power reduction for energy efficiency.
1.2 Interface of 88E1111
1) The GMII interface connects the 88E1111 to the MAC layer, as shown in Table 1.
2) The management interface consists of two signals: MDC (clock) and MDIO (data). MDC runs at up to 8.3 MHz, while MDIO is synchronized with it. A "0 1" pattern in the data stream indicates the start of an operation, followed by the opcode (read or write), the physical address, register address, and data. The CPU controls and monitors the chip by accessing these registers.
3) The LED/Configuration interface includes LED_Link10, LED_Link100, LED_Link1000, LED_TX, LED_RX, LED_Duplex, VDDO, and VSS. The configuration interface uses Config[6:0] to set the operating mode. For example, in 1000BaseX full-duplex mode, specific pins are connected accordingly, as shown in Table 2.
4) The high-speed serial interface uses three pairs of differential signals, operating at CML level. S_IN± is the input, S_OUT± is the output, and SD± is the optical power detection signal.
1.3 88E1111 Registers
The 88E1111 contains 32 16-bit control registers, with addresses ranging from 00H to 1FH. These registers are used for functions like chip reset, rate setting, and duplex mode selection, as detailed in Table 3.
2. Program Design
Based on the features of the 88E1111 and the requirements for a baseband fiber-optic remote connection, this paper proposes an interface design for a digital microwave relay system. The block diagram of the design is shown in Figure 1, which includes an indoor and outdoor unit. In the transmit direction, the service code stream from the indoor unit is processed by an FPGA complex resolver, encapsulated into an IEEE 802.3 compliant data frame, and sent to the 88E1111 via the GMII interface. The 88E1111 then converts the data into a serial signal and sends it through a 1.25 Gbps optical transceiver. The optical signal is transmitted to the outdoor unit, where it is converted back to electrical form and processed again before being modulated and transmitted over the air via the intermediate frequency radio unit. The reverse path follows the same logic.
3. Hardware Design
Figure 2 shows the circuit schematic of the 1.25 Gbps optical transceiver SSFF315l. The RD± and TD± pins are connected to the high-speed serial interface of the 88E1111. Figure 3 illustrates the 88E1111 circuit. Key connections include the GMII interface (connected to the FPGA), the management interface (MDIO and MDC connected to the microprocessor), the configuration interface mapped according to Table 2, and the clock input (XTAL1). The RSET pin is connected to ground through a 5 kΩ resistor, and SEL_FREQ is set low to select the 125 MHz clock.
The 88E1111 operates in full compliance with the IEEE 802.3 standard. TX_CLK is the transmit clock, and TX_EN enables the transmission. When active, data is sent on the rising edge of TX_CLK. RX_CLK is the receive clock, and RX_DV enables data reception. Data is received on the rising edge of RX_CLK when RX_DV is active.
4. Issues to Consider in the Design
4.1 Electrical Interface Matching
The 88E1111 uses a CML interface, while the optical transceiver uses an LVPECL interface. To ensure compatibility, a matching circuit must be added. AC-coupled circuits require biasing resistors (R9 and R10, 142–200 Ω) on the LVPECL outputs and a 100 Ω resistor (R5) between the inputs.
4.2 GMII Interface Design
The GMII interface operates at 125 Mbps, so careful attention must be paid to signal timing. TXD[7:0], CTX_CLK, and TX_EN form one group, and RXD[7:0], RX_CLK, and RX_DV form another. Both groups must have equal propagation delays to avoid phase errors.
4.3 PCB Layout Design
The board contains LVTTL, LVPECL, and CML signals. To minimize interference, differential pairs should be kept close together, with consistent spacing to maintain differential impedance and common-mode rejection.
5. Conclusion
Using the 88E1111, the baseband fiber-optic remote interface achieves a data rate of 800 Mb/s in gigabit full-duplex mode and supports transmission distances of up to 20 km over single-mode fiber, meeting all design requirements for microwave relay systems. This solution has been successfully implemented in numerous digital microwave products, offering a simple, stable, and cost-effective alternative to traditional designs. Compared to conventional methods, it introduces two key innovations: first, the use of IEEE 802.3-compliant data frames ensures standardization and reliability; second, the adoption of optical fiber eliminates the need for feeder cables, significantly reducing system costs and installation complexity.
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