Design of Direct Sequence Spread Spectrum Wireless Communication System Based on 89C52 Single Chip Microcomputer and Z2000 Chip

In today’s era of rapid transmission of various information, more communication functions and communication resources are required, and the frequency band is now very limited. In order to improve the utilization rate of the frequency band, it is required that the unit band can have the largest possible information transmission capacity. Mobile communication has developed from frequency division multiple access (FDMA) to time division multiple access (TDMA) to today’s code division multiple access (CDMA). The technical basis of this kind of code division multiple access is spread spectrum communication (Spread Spectrum Communication). Spread spectrum communication technology can provide point-to-point or point-to-multipoint data, voice, image services, wide area network, local area network interconnection, and Internet access services.This article introduces

In today’s era of rapid transmission of various information, more communication functions and communication resources are required, and the frequency band is now very limited. In order to improve the utilization rate of the frequency band, it is required that the unit band can have the largest possible information transmission capacity. Mobile communication has developed from frequency division multiple access (FDMA) to time division multiple access (TDMA) to today’s code division multiple access (CDMA). The technical basis of this kind of code division multiple access is spread spectrum communication (Spread Spectrum Communication). Spread spectrum communication technology can provide point-to-point or point-to-multipoint data, voice, image services, wide area network, local area network interconnection, and Internet access services. This article introduces a direct sequence spread spectrum wireless communication system that can be used as a part of a CDMA wireless multimedia communication system, or it can operate independently as a separate system.

1. Spread spectrum communication technology and its characteristics

The so-called spread spectrum communication is simply expressed as follows: Spread spectrum communication technology is a method of information transmission. The frequency bandwidth occupied by the signal is much larger than the minimum bandwidth necessary for the transmitted information. The expansion of the frequency band is achieved through coding and modulation. , And has nothing to do with the transmitted information data; at the receiving end, the same spreading code is used for relative demodulation to despread and restore the transmitted information data. The principle of the system is shown in Figure 1.

 Design of Direct Sequence Spread Spectrum Wireless Communication System Based on 89C52 Single Chip Microcomputer and Z2000 Chip

Spread spectrum communication has a series of excellent characteristics, such as strong anti-interference, good concealment, confidential information transmission, arbitrary site selection, low bit error rate, anti-multipath interference, etc. There are many kinds of existing spread spectrum communication systems (direct spread, frequency hopping, time hopping), this project uses direct sequence spread spectrum (DS). Direct sequence spreading is to directly use a spreading code sequence with a high code rate to spread the spectrum of the signal at the transmitting end, and use the same spreading code for despreading at the receiving end. Restore the expanded spread spectrum signal to the original information.

2. System block diagram and functions

The system block diagram is shown in Figure 2. The operator inputs text, voice, video and other signals into the computer, and the system uses the improved NETMEETING platform as the man-machine dialogue software. NETMEETING sends the signal to the defined RS-232 communication port, and then the spread-spectrum digital transceiver ① connected to the RS-232 communication port will spread and modulate it into a pseudo-random sequence code (the spectrum is greatly expanded) and then transmit , Establish the key circuit through microwave (or satellite); in receiving turbulence, the antenna will receive the signal to the letter machine ② despread, demodulate, and convert the signal into text, voice, and video. In this way, a video conference with multiple participants can be realized.

Design of Direct Sequence Spread Spectrum Wireless Communication System Based on 89C52 Single Chip Microcomputer and Z2000 Chip

3. Design of spread spectrum digital transceiver (referred to as letter machine)

(1) A point-to-multipoint transceiver is designed using the Z2000 spread spectrum chip of Zilog Company, which has an adaptive baud rate (up to 2048kb/s) interface for communication with the RS-232 port of the PC. It can be achieved from a bandwidth of 2.48MHz to a bandwidth of 20.48MHz. The hardware structure block diagram is shown as in Fig. 3.

Design of Direct Sequence Spread Spectrum Wireless Communication System Based on 89C52 Single Chip Microcomputer and Z2000 Chip

(2) The machine uses 89C52 as the core processor of the system, Z2000 as the main functional chip (to achieve spread spectrum function), and cooperates with other chips to complete the baseband processing part.

When data is to be transmitted, the PC detects the RS-232 communication port, and the communication machine implements flow control by the RS-232 port (the specific communication control is in the fifth part). When the PC receives a valid signal, it sends out the data, which is sent to 89C52 through the RS-232 port, sampled with a signal with a clock of 2048kb/s, and processed by Z2000 (first differential encoding, and then 20.48MHz pseudo-random sequence pair Data is spread spectrum), and then use MAX2452 to generate 70MHz carrier frequency for modulation to produce a digitized intermediate frequency QPSK output signal. After MAX436 and single-chip amplification, the signal with intermediate frequency of 70MHz is obtained. After DA conversion, it is sent via intermediate frequency cable (through A multiplexer) RF part up-conversion, RF modulation and transmission.

When receiving, the antenna receives the spread-spectrum signal, the radio frequency module demodulates it, down-converts it, amplifies it and sends it to the transmitter (the transmitter is in the automatic search state after it is turned on), and the PN matched filter of the main chip Z2000 filters the received Once the matching data is detected, it will start the receiving program and enter the receiving state. After automatic gain control, A/D conversion is performed to convert the analog signal into two (I and Q channels) digital signals, which are then processed by Z2000 to obtain 2.048 The synchronous data of MHz is sent to the RS-232 port.

(3) The function of the main chip

89C52: host interface, frame formatting, setting the control register, buffer memory, communication protocol, and control circuit of the spread spectrum chip;

MAX233: Conversion from RS-232 port level to TTL level;

MAX497: video cache;

MAX436: video operational amplifier;

MAX2452: is an integral regulator, the input amplitude is 1.35V, the bandwidth is 15MHz differential I and Q baseband signals; the output is 70MHz differential signal. Here as the first interrupt;

tp3067: Duty codec (voice processing);

sn75108: line driver;

cd4053: is a triple dual-channel analog multiplexer, as an analog switch, switch the working state of the phone (duty or data);

AD9059: is a dual 8-bit D/A converter;

QL12X16B: Field programmable device, used for rate control, tap synchronization, clock peripherals, etc.;

CLC520; AGC, namely automatic gain control;

Multiplexer: There are mainly four functions (shooting, receiving, sending, and remote control).

4. Computer part

The basic idea of ​​the whole system is to establish an Ethernet simulation connection on the serial port. After completion, the serial port is a network card, which is almost the same as a normal network card, but the speed is slow. Another point is that the RTS line can be controlled according to whether the packet is sent or not (the author has compiled a program to control, in which the method of controlling the RTS/CTS response timing is to control the sending waiting time, if there is a packet to send, first raise the RTS and wait for the CTS If it is high or pass the time-out, it will be sent out. There is also idle waiting time. If there is no packet sending for a certain period of time, the RTS will be lowered to make the channel machine stop sending and give up the channel. Parameter value 1~ 255, the length of time is related to the speed of the machine). As long as the driver that can achieve these two points can be used (just to simplify the program to be written).

Design of Direct Sequence Spread Spectrum Wireless Communication System Based on 89C52 Single Chip Microcomputer and Z2000 Chip

The program flow chart is shown as in Fig. 4.

Packet driver is a simple and open Ethernet driver interface. slx_rzk is a complete serial port Ethernet simulation program (not a simple point-to-point slip or ppp), which can be used for any point-to-multipoint. To change the flow control mode of the packet driver, you only need to plug a shell. This shell is sliprts (of course this program can be used on slip or ppp). However, the packet driver cannot be used directly in windows. Therefore, the bridge can only be built through ODI. This bridge is pdether, but it is not 100% compatible with windows. You have to use your own net.cfg. Therefore, one more sentence copy net_cfg.cfg must be added to autoexec.bat to deal with the overbearing behavior of windows. Configure these, then configure the IP settings on the existing ODI adapter, and connect the serial ports of the two PCs with a NULL MODEM cable (crossed RS-232 cable).

In addition, the improved NETMEETING of windows is also used as the man-machine dialogue software and main communication software for multimedia communication.

5. Modification of communication port control

(1) The communication port is designed according to the principle of computer serial communication. The main communication control lies in the handshake control of the RS-232C interface (Recommended Standard). This project uses its seven control signals, two of which are short-circuited at their own terminals (DTR and DCD). Only when DCD is detected are valid, those serial communication programs will work, and DTR is valid after the PC is turned on. Its RTS and CTS control and PC work (flow control). (2) The signal description of the RS-232C interface is shown in Table 1.

(3) Communication control in the process of sending and receiving. The signal machine detects RTS, if it is low, it only receives the signal; if it is high, the sending program is started and is in the sending state, and the PC must detect that the CTS is valid (indicating that the signal machine is ready to send, if the CTS is invalid, it can prevent the PC from sending data , To prevent the phone from losing data) before the data is transferred to the phone.

(4) Point-to-multipoint realization. The omnidirectional antenna is used as the main antenna, and the directional antenna is the slave antenna. A control software in the main PC (modified the PN code in 89C52) is used to achieve the purpose of selecting multiple points.

6. RF part

The schematic diagram of the radio frequency part is shown as in Fig. 5.

Design of Direct Sequence Spread Spectrum Wireless Communication System Based on 89C52 Single Chip Microcomputer and Z2000 Chip

Its working principle is as follows:

・Transmitting end: The 70MHz intermediate frequency spread spectrum signal output by the channel machine is mixed and modulated to a radio frequency of 2410MHz by 2340MHz, and then transmitted by the antenna after passing through a bandpass filter and LPA.

・Receiving end: The 2450MHz signal received by the antenna is demodulated by 2340MHz to an intermediate frequency signal of 110MHz after the LNA and bandpass filter, and then transmitted to the channel machine via the intermediate frequency cable.

·Performance:

Master: Tx: 2410MHz Slave: Tx: 2450MHz

Rx: 2450MHz Rx: 2410MHz

Bandwidth: 20.48MHz

Sensitivity of receiving antenna:[email protected]

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