Interface design scheme for baseband fiber optic remote extension of digital microwave relay system based on 88E1111

The baseband fiber is extended by moving the intermediate frequency portion of the digital microwave relay system from the indoor unit to the outdoor unit. Since the physical medium that the signal is far away is optical fiber and the baseband digital signal is transmitted, the transmission distance is generally several kilometers or more. The baseband fiber pull-out avoids the inconvenience of the feeder connection between the indoor unit and the outdoor unit of the system, reduces the feeder loss, reduces the power requirement of the power amplifier, and most importantly, the position adjustment of the antenna is no longer restricted by the indoor unit. According to the characteristics of the surrounding environment, it is possible to select a suitable location to reduce the difficulty of opening the device. This paper introduces the functions and features of the 88E1111, and gives a design scheme for the baseband fiber-optic remote connection of the digital microwave relay system using 88E1111, which solves the problem that the design of the baseband fiber-optic remote interface is complicated and difficult to implement.

1.1 Features of 88E1111

The 88E1111 is a single-chip integrated high-performance Gigabit Ethernet physical layer chip from Marvell. It has the following functions: full support for the IEEE802.3 protocol cluster; built-in 1.25 G serial deserializer for gigabit optical transmission applications; Support GMII, TBI, RGMII, RTBI and other MAC layer interfaces; support 10/100/1000BaseT adaptive detection; use 0.13 μm CMOS technology, support 2.5 V, 1.2 V low voltage power supply, maximum power consumption O. 75W, and support automatic power reduction.

1.2 Interface of 88E1111

1) The data interface between the GMII interface 88E1111 and the MAC layer is shown in Table 1.

2) The management interface consists of two signals: MDC and MDIO. The MDC is the clock signal with a maximum rate of 8.3 MHz. The MDIO is the data signal and is synchronized with the MDC. The occurrence of "0 1" in the data stream indicates the start of the operation; followed by the opcode, "10" indicates the read operation, "01" indicates the write operation; then the physical address, register address, and register data. The CPU controls and monitors the chip by accessing the corresponding physical address and register address.

3) LED/ConfiguraTIon interface LED interface consists of LED_Link10, LED_Link100, LED_Link1000, LED_TX, LED_RX, LED-Duplex, VDDO, VSS. The Co-nfiguraTIon interface consists of Config[6:0]. The chip can be configured to the appropriate operating mode by connecting Config[6:0] to different signals on the LED interface. The typical 1000Ba-seX and full-duplex working mode configuration mapping relationship is shown in Table 2.

4) The high-speed serial signal interface consists of 3 pairs of differential signals. The interface level is CML, where S_IN± is the serial data input, S_OUT± is the serial data output, and SD± is the optical power effective input.

1.3 88E1111 registers

The 88E1111 has 32 control registers, each with 16 bits, and the address offset is OOH~1FH. The functions are reset chip, set rate, duplex mode, etc., and their description is shown in Table 3.

According to the functional characteristics of the 88E1111 and the design requirements of the baseband fiber optic cable, this paper proposes the interface design of the baseband fiber optic cable for the digital microwave relay system with 88E1111. The block diagram of the interface design scheme is shown in Figure 1. It consists mainly of an indoor unit and an outdoor unit. In the direction of transmission, the indoor unit service code stream is input into the FPGA complex resolver, and the service data is packaged and packaged into a data frame structure conforming to the IEEE802.3 standard. The data frame structure is transmitted to the 88E1111 through the GMII interface, and the data is serially and serially converted by the 88E1111 through the high-speed serial signal. The interface sends the signal to the 1.25 G optical transceiver, which transmits the 1.25 G optical signal to the outdoor unit after the electro-optical conversion. The outdoor unit 1.25 G optical transceiver receives the optical signal, completes the photoelectric conversion, inputs the high-speed electric signal into the 88E1111 through the high-speed serial signal interface, completes the data string conversion by 88Ellll, and sends the parallel data to the FPGA modem through the GMII interface, completing After the data is deframed and modulated, the radio frequency signal is transmitted to the air through the intermediate frequency radio unit. The direction of return is the reverse flow of the direction of transmission.

Figure 2 shows the circuit schematic of the 1.25 G optical transceiver SSFF315l. The transceiver pins RD± and TD± are connected to the high-speed serial signal interface S_IN± and S_0UT± signals of the 88E1111. Figure 3 shows the circuit schematic of 88E1111. The main pin connections are as follows: GMII interface signal (see Table 1 for connection to FPGA; Management interface signals MDl0, MDC are connected to the microprocessor; Config interface signals are mapped according to Table 2. The relationship is connected to the LED interface; the XTALl pin inputs a 125 MHz clock signal with a frequency stability of ±50 ppm; RSET is the chip reference voltage input pin, connected to ground through a 5 kΩ precision resistor; SEL_FREQ is the clock input select pin, which is low At level, select the 125 MHz clock input.

The 88E1111 works in full compliance with the IEEE802.3 protocol. TX_CLX is the transmit clock, TX_EN is the transmit enable signal, and when TX_EN is active, the data is transmitted on the rising edge of the transmit clock TX_CLK, TXD[7:0] to 88E1111, and the transmit operation is completed. RX_CLK is the receive clock and RX_DV is the receive data enable signal. When RX_DV is active, data RXD[7:0] is received from 88E1111 on the rising edge of the receive clock RX_CLK, and the receiving operation is completed.

4.1 Electrical interface matching

The high-speed serial signal interface of the 88E111l is a CML interface, and the signal interface of the optical transceiver is an LVPECL interface. Therefore, the CML to LVPECL electrical interface matching circuit should be added between the interfaces. When the AC-coupled interface matching circuit is used, the transmitting end adds a biasing resistor to ground on the two output signals of the IVPECL, that is, R9 and R10 in FIG. 2, and the resistance value is selected from the range of 142 to 200 Ω. The input bridges a resistor between the two input signals of LVPECL, that is, R5, and the resistance is 100 Ω.

4.2 GMII interface design

The GMII interface has a data rate of 125 Mb/s and a high rate. In order to avoid phase errors caused by different propagation delays on the PCB, TXD[7:0], CTX_CLK, and TX_EN are a group of signals. RXD[7:0], RX_CLK, and RX_DV are a group of signals, and the two sets of signals must be strictly equal.

4.3 PCB layout design

The baseband fiber optic remote interface board has various signals such as LVTTL, LVPECL, and CML. To avoid mutual interference, PCB layout should be noted that within the differential pair, the distance between the two lines should be as short as possible to maintain the receiver's common-mode rejection capability. On the PCB, between the two differential lines. The distance should be as consistent as possible to avoid discontinuity in differential impedance.

Based on the 88E1111 baseband fiber optic remote interface design, the interface data transmission rate can reach 800 Mb/s in gigabit and full-duplex operation mode; the transmission distance can reach 20 km in single-mode fiber transmission, fully reaching the number. Design requirements for microwave relay systems. This solution has been applied in many digital microwave relay products, and has the advantages of simple design and stable performance. Compared with the traditional scheme, there are two innovations: 1) The data transmission structure adopts the data frame structure conforming to the IEEE802.3 protocol, the interface standard and reliability; 2) The optical fiber is used as the transmission medium, and the feeder between the indoor and outdoor units of the system is avoided. The inconvenience of connection greatly reduces the cost of the system and the difficulty of opening the device.