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Wireless communication especially the links between computers have become an important part of today's computing world. People are now looking for wireless alternatives for transferring data between several computers in the home as well as outside. This is currently being done widely with the use of WIFI routers. However there are very few options available for direct wireless links. This would be useful in the event where a network or router is unavailable.
Current WIFI, Bluetooth and Infra-Red links or networks are highly saturated and either provide low bandwidth or are affected greatly by interference thus reducing their range. This project therefore presents a method of a simple direct wireless link with long range (500-1000ft) and concurrent communication with enough bandwidth (56kbps) for basic data transfer. This is achieved by creating transceivers connected to each PC.
The wireless link achieved was ideal for simple text communication and the work can be carried on further in terms of achieving greater bandwidth as well as the use of a single antenna and minimizing the circuit size for easy portability. Work can also be done on communication between multiple (more than two) devices at the same time.
Modules Selection and Design
Hardware Design Detail
Complete Transceiver Circuit
Software Design Detail
Basic Software functions
Serial Coding in Windows
Code snippets with explanations
Individual parts testing
Transmitter and Receiver
Complete Transceiver Test
Direct Serial Link Test
Test with transceiver circuits
Final Testing and Results
The purpose of the project was to design a wireless link between two PC's. In order to achieve this several options were initially reviewed, and these will be discussed under the background reading section of the report.
A set of targets were put down that were to be achieved in the design of the wireless link. These were as follows:
The project was then divided into two major parts: Software and Hardware. The Hardware part of the project was then split further into the transmitter and receiver. These parts were then carefully selected through research, based on calculations to achieve the required targets.
The Transmitter and Receiver Circuits were then put together to form Transceivers which would be connected to the individual PC's and would allow two way wireless communications.
The Software was also further split into two sections: a) To allow communication between the computers serial port and the transceiver, and b) To allow communication between the transceivers in terms of the wireless protocol which will be discussed further in later parts of this report.
1.1. Background Theory:
A lot of previous work has been done on this topic. And several means of wireless communications are now widely available. These include:
WIFI - This uses 802.11 Technology in the range of 2.4GHz. WIFI is the most common mode of wireless communication between two PC's, it generally requires routers that form WLAN(Wireless Local Area Networks)
Bluetooth - This is a short range wireless communication standard mostly used on mobile devices and it creates PAN's (personal area networks).
Infra-Red - This is the use of electromagnetic radiation with a wavelength greater than that of visible light for communication, it is mostly used for short range communication and requires line of sight.
Laser - This is the transmission of a beam through free space, similar to fiber optics however it requires line of sight.
Microwave Radio - This is a system that uses radio waves of a particular frequency within the microwave range to transmit data over short or long distances and is mainly used for video or audio such as radio and television stations. It can be fixed or moving depending on the application.
Antennas - These are a necessary part of all radio equipment and are used to transmit and receive the waves. They turn current into electromagnetic waves for transmission and vice versa. There are several different types of antenna such as the dipole, yagi, parabolic and many more. The lengths of the antenna are also an important consideration for the implementation of a wireless link. The radiation pattern of an antenna is also taken into consideration as the direction in which the maximum signal strength is required is very important.
2.1 Parts Selection
Transmitter and Receiver:
After deciding that a radio link will be used with frequencies of around 900MHz, a fair amount of research was done on the transmitters and receivers available in the market. Carefully reviewing all the products available it was decided that the LINX ES series modules would be used. Two frequencies, that is 869MHz and 916MHz were chosen for the modules so as to allow two way concurrent communication. The modules and their details are shown below:-
Transmitter module 916Mhz
Receiver module 916MHz
The modules function at 5V and have a range of 500-1000 feet. They utilize FSK modulation thus giving better performance. These modules have a 56kbps data rate which is adequate for simple text data communication. This module falls under the microwave category and is within the legal unlicensed range of operation in the UK.
Four antennas of the same frequency as the radio chips were used. The antennas used were ¼ wave whip antenna. The reason for using these antennas is the fact that they are omnidirectional and therefore will allow communication in all directions. Their length was also ideal for this purpose from the following formula:
L = 234/F MHz
Where: L = length in feet of quarter-wave length , F = operating frequency in megahertz
A directional antenna was not used due to the fact that it would have a high gain which is not legally permitted for transmitters, it also would only allow data to be transmitted in one direction which would not be useful for data communication between multiple PC's in different locations/directions.
The antenna gain was calculated by the following formula
Gant = (4. Ï€ . Ae)/ Î»2
The optimum length to achieve an antenna gain of 1(0dB) was calculated from the formula:
C=f Î» where c= speed of light(3x108 m/s) , f= frequency, Î»= wavelength
In the case of the 869MHz frequency the antenna length required would be:
Î»= (3x108 ) / (869x106) = 0.345m
The size and effective area from the above calculation would be impractical as it would be too large for our application; we therefore use a quarter wave whip antenna whose length was Î»/4 as shown below:
Î»/4 = 0.345/4 = 8.7 cm
quarter wave whip antenna with radiation pattern
2.2 Hardware Design
The basic hardware design consists of 2 black boxes; each box is connected to a PC via a SERIAL port. The boxes are basically radio transceivers; that is they contain a set of transmitters and receivers, these transmitters and receivers operate at two different frequencies (868 and 916 MHZ) so as to allow concurrent two-way communication. The boxes also have two antennas each attached to their own transmitter and receiver.
( DIAGRAM TO SHOW ABOVE)
2.3 Software Design
The software code was written in Microsoft visual basic and was used to communicate data between the user and the computer as well as the computers serial port and the transmitter and receiver modules. The basic protocol is as follows:
If port is found wait for signals from child threads else output "Port not found" in text input box and disable text input.
-SEND button is pressed
-A variable is created which is the buffer of size 140 characters and this is then filled with the text from the input window.
-If the buffer variable is found, it is written to the port, ELSE nothing is done.
CREATE BUFFER VARIABLE
SEND BUTTON PRESSED
Store input into buffer
Write to Port
3. Hardware Design Detail
The LINX 868 and 916MHz Transmitters are hybrid circuits that have been mounted onto small boards that form the modules. They therefore had to be put onto PCB's in order to provide optimum performance. Giving the modules enough ground space as well as avoiding interference to components on the underside of the module from conducting material. The modules were therefore soldered together with their respective antennas onto a custom made PCB.
The linx transmitters require a 3-5v source that is input to the DATA pin. The input required is a square wave of either 3V or 5V giving an average deviation of 75 and 115KHz respectively. As we are using an RS-232 we need to perform buffering and conversion of the logic levels. This is achieved by using the MC1489 Line Driver chip which converts the wave from the RS-232 pin to an inverted TTL wave with amplitude of 5V which is required by the transmitter, the wave is then passed through a SN7404 Hex inverter chip which inverts the TTL wave without changing its amplitude, this wave is then passed to the DATA pin of the Transmitter modules. The Transmitter modules then take this digital wave and modulate the crystal, which produces a wave that is then amplified and filtered after which it is passed on to the Antenna pin to be transmitted.
(circuit diags and wave showing conversion)
The Linx 868 and 916 MHz receiver modules were also placed onto PCBs jus as the transmitter modules. The receivers function by filtering the received signal from the antenna, this signal is then amplified after which it is mixed with a LO frequency signal in order to convert it to a 10.7MHz IF which is then amplified and filtered. This is then demodulated to recover the analog baseband, which is then squared and passed on to the DATA pin. The signal from the DATA pin is a square wave of amplitude 5v, this is not understandable by the 12v magnitude RS-232 input pin. The wave from the Receiver data pin is therefore passed through the SN7404 hex inverter that inverts the wave that is passed through the mc1488 line driver, which converts it to an RS-232 wave and inverts it again.
The Receiver does not provide squelching of the data line, this therefore requires additional circuitry which is shown later. Data hysteresis/squelching is required in order to avoid the data line switching randomly in the absence of a valid signal. The RSSI pin on the receiver is used for hysteresis.
The antennas used for this project were introduced earlier in the report. These antennas were selected specifically for the modules. They are mounted on the grounded area of the PCB with their relevant transmitter or receiver modules. As per the project proposal the antennas were to be shared between the modules by using an antenna switch, the switch was however difficult to locate and therefore for added simplicity a separate antenna was purchased for each of the radio modules. The antennas were mounted at right angle to the PCB, which was the ground plane, for optimum performance. The surface area of the ground plane was made as close as possible to the antennas length for greater performance.
(diagram showing antenna ground plane and connection)
3.4. Additional Circuitry
Powering up the modules:
The transmitter and receiver modules require 5v to operate. In order to provide a constant clean and regulated voltage additional circuitry is added to power up the module, this is due to the fact that the module does not have its own voltage regulator and in order to reduce noise from the power source. In addition to this other modules in the circuit require different voltages. All of this need to be taken into account. The regulation is done by using a 7805 voltage regulator together with the 10 ohm resistor and 10 microFarad capacitor as suggested by LINX to power up the modules.
The circuits are shown below.
(Diagram of second circuit)
This is the additional circuitry added to the receivers in order to avoid it randomly switching as explained in the receiver section of this report.
3.5. Complete Transceiver Circuit
5. Hardware Testing
All the different pieces of hardware used for this project need to be tested in order to ensure that they all function to the requirement. The testing methods are discussed in the next part of this report.
5.1 Individual Parts Testing
The parts were first tested individually in order to compare with their respective data sheets. This was in order to ensure not only that the results were as per the information given in the modules' data sheets however to also see if they fulfilled the requirements of the project. Overall it was noted that the results were not identical to those provided by the manufacturer and this was due to the fact that the conditions were near optimal and not the same as those used by the manufacturer during testing.
5.1.1. Transmitter and Receiver
The transmitters were powered up and connected to a function generator and the receivers were powered up and connected to the oscilloscope. The output wave was then compared with the one fed into the oscilloscope in order to ensure that they were the same.
The signal generator was set to output a wave between 3-5v with frequency from 250Hz- 1kHz and steps of 250Hz. Each wave was compared to that seen on the oscilloscope at the various frequencies. The waves were seen to be similar with a little distortion.
The Transmitter and receivers were connected with their respective antennas and the RSSI pin connected to the oscilloscope. The test for the Transmitters and Receivers was then repeated with the signal strength being observed at distances of 100-1000 feet with steps of 100 feet, it was observed that as the transmitters and receivers were moved further apart the signal strength dropped at every stage.
This was then repeated for eight different directions to test for the signal strength in various directions. It was noted that at some points the signal strength dropped even at the same distance showing the directional pattern of the antenna was not a perfect circle, the results were compared to the radiation pattern provided by the manufacturer.
The test was then repeated with one antenna kept horizontally rather than vertically to compare the antennas horizontal and vertical polarization. It was noted that there was better performance when the antenna was vertical at 90 degrees to the ground plane. The signal strength dropped due to cross-polarisation rejection. The antennas were also rotated around the horizontal axis for detecting the point of maximum signal strength.
5.2. Complete Transceiver Test
After the individual testing, the complete circuit was built. The complete circuit was then tested to see if it functioned correctly and gave the optimal performance. The test was carried out as follows:
The signal generator was set to output a wave of +12 to -12v and the same procedure as that for the individual modules was followed. The performance was not optimum however after making a few changes to the circuit a near optimum was obtained and there was not much time to make more changes.
Two completed transceivers were then used to transmit text between two computers using the HyperTerminal program built into windows. It was observed that text was being transmitted however it was slightly inaccurate due to possible interference.
The test was repeated with two computers in separate rooms to test the effect of both distance and a wall between the transceivers. A communication link was still achievable and the hardware testing was complete.
It was concluded that the project was a success as the projects goals were achieved in terms of obtaining a simple wireless link that would work without the need of a network and that would allow two way communication over a fairly large distance. It was however noticed that the speed of transmission was very low for modern applications and that the design was too bulky for portability. It was also concluded that the software part of the project could be improved upon to allow for multiple devices to communicate as well as having a better communication protocol in order to avoid errors and improve the links performance. In terms of cost the device would be very competitive in the current market due to its low material and building cost. Further work can therefore be done on the limitations of the devices built. It was noted as well that many more features could be added to the product with the use of a microprocessor or an FPGA board, in utilizing features such as the clock and power down, these would improve the product further.
Overall the aim of obtaining a wireless link between two PC's that would be able to transmit text data directly, was achieved making the project an overall success.