Detailed description of Wireless Access Network Technology USB Interface Design

At present, the wireless access network technology is still developing very rapidly. Many people may not understand the development of the wireless access network technology. It does not matter. After reading this article, you will certainly have a lot of GAINS, I hope this article will teach you more things.

Introduction

Currently, the wireless access network technologies used for indoor computer data communication mainly include Bluetooth, infrared, and HomeRF. From the transmission rate, the Bluetooth value is 1 Mbps, and the FIR standard infrared value can reach 4 Mbps (the VFIR standard infrared value will reach 16 Mbps in the future); The HomeRF transmission rate is only 1 Mbps ~ 2 Mbps (8 Mbps to FCC recommended ~ 11 Mbps ). In addition, there is a large gap between their actual testing speed and the theoretical value, which can only meet the Wireless Access Network Technology with low speed requirements. However, in order to adapt to the high-speed wireless access network technology and meet the high-speed data transmission requirements of large capacity, experts and scholars are studying Wireless Access Networks with higher transmission rates. The combination of computer USB interface and wireless optical communication technology will provide high-speed wireless access for computers.

Visible Light Wireless Access System

Wireless Optical Communication is an extension of higher rf frequency based on radio communication. Compared with radio communication, Wireless Optical Communication has more obvious advantages, such as wider bandwidth, larger capacity, higher security and confidentiality, and no interference from radio frequency electromagnetic waves, and no need to use frequency licenses for spectrum resources. Due to the above advantages, coupled with the requirements for high-speed wireless networks, Wireless Optical Communication has received more and more attention and attention. Because USB is fast, convenient, and simple, the USB interface has become the highest priority when connecting external devices to a computer. High-speed wireless access network technology can be achieved using wireless optical communication through USB interfaces. Figure 1 shows the principle of a visible-light Wireless Communication System Based on USB interfaces. The system consists of three parts:

1. the interface circuit connects the computer and optical transceiver, converts the USB signal from the computer into a signal suitable for transmitting circuit modulation, and converts the signal sent from the receiving circuit into a USB signal to the computer, implement high-speed wireless duplex communication.

2. the optical emission circuit performs encoding and modulation of signals processed by the USB interface circuit. The modulated signals are then amplified and sent to the modulated light source-visible light laser (LD ), make it emit visible light pulse signal modulated by the encoded electrical signal (this process is called electro-optic conversion, that is, E/O conversion), and then transmit it to the atmospheric channel through the optical antenna.

3. the optical receiving circuit collects the optical pulse signal sent from the sending end to the photoelectric detector (APD) and converts the received optical signal into an electrical signal (this process is called photoelectric conversion, that is, O/E conversion), the converted weak electrical signal is amplified and demodulated and sent to the USB interface circuit.

Circuit System Design

Bridging Circuit

The purpose of a bridge circuit is to coordinate the computer with the subsequent receiving and receiving systems. In Figure 1, the pulse code modulation/Demodulation Circuit and interface circuit can be integrated into a chip called a bridge. The USB interface signal is a bus signal. This data level is not the pulse waveform required in communication, that is, it is not the baseband signal in communication. Therefore, it must be converted to a base band signal such as a TTL level before it can be used as a modulation signal. The main task of a bridge circuit is to complete this conversion. When a computer receives data, the main function of the bridge circuit is opposite to that of the data transmission.

During high-speed data transmission, the transmission rate is closely related to the bridge adapter's conversion rate. Therefore, it is important to select and design the bridge and its peripheral circuit, use hardware and software controls to work with different USB speed formats (USB2.0 has three speed formats: low speed, full speed, and high speed ). Here, the STIR4220 bridging chip is used, and the conversion rate can reach 16 Mbps. It is a low-cost, low-power, USB-to-infrared (can be used for visible light signal transmission) Bridge controller. It completes the conversion between the pc usb interface data and the optical signal, and enables the optical modulation signal to communicate with the computer wirelessly through the USB port. It mainly solves the following three problems: conversion of signal form-conversion of differential signal/TTL level signal; conversion of encoding method-conversion of reverse NRZI Differential Code of PPM encoding/USB; transfer Rate Conversion-the conversion of the corresponding USB speed/16 Mbps rate, which is a bridge to the implementation of high-speed wireless optical communication.

Transmit Circuit

A transmitter is an important part of Wireless Access Network technology communication. It is used to convert an electrical signal into an optical signal and send it out. It is mainly composed of a light source and a driving circuit. The driver selected in the system design is the MAX3905 optical communication driver chip of Maxim, and its transmission rate is 8 Mbps ~ 150 Mbps. It has two input buffer circuits, one is the input (IN + and IN-) designed for differential data, and the other is the buffer for single-ended input of TTL-level signals. The two are controlled by the DIFF level. When the DIFF side is suspended (high level), select the TTL single-ended input. When the DIFF side is grounded (low level), select the differential input, in this design, select TTL single-ended input. The chip also contains input buffer, buffer input controller, signal detector, thermostat, modulation current generator, offset current generator, output driver and other circuits. MAX3905 is an excellent type of Related Devices in terms of circuit design, layout, size (small), performance, and functions.

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