A random bit generator was employed as data source. These data were first mapped to +1 and-1 to be multiplied by the CDMA code bits. The CDMA codebits were programmed in a repetitive sequence block. The two bit streams were multiplied in next step. The resulting coded data stream is split up into two branches, with even numbered bits in upper branch , odd numbered bits in lower branch. Each branch was then modulated by a carrier wave of 2.4GHz. As the two waveforms have a phase shift of 90degrees to each other, the upper branch is represented by cosine wave and lower sine wave. Both branches were then added together.For simulation RF transmission and receiving sections were assumed to be without losses. The transmission path was modeled to simulate the time delayed transmissions, superpositions, and interface within an enclosed machine environment.
The received signal was demodulated on the receiver end. The demodulation causes a time delay of one-bit duration.As time delay is known in simulation, decoding of CDMA spreading was correlated to coding bit sequence.
EXPERIMENTAL EVALUATION
The designed system was experimentally evaluated in two aspects. First the signal strength of the transmitted signal with respect to the transmission distance was investigated to establish the maximum possible transmission range. Second actual power consumption of transmitter was measured.
Experiments have been conducted in lab environment .The signal strength was measured for a transmitter –receiver distance that varied from 15 to 300cm. Two test configurations were used :
1) Transmitter unobstructed on a workbench
2) Transmitter enclosed within a metal casing.
In the latter the casing has a nonmetallic opening of 2*30cm that was covered by a plastic plate of 3cm. The opening was not directed towards the receiver. This situation can be considered as a typical arrangement for a transmitter embedded within a machine environment. In fig 9 both the open space and shielded transmission have shown.
Increasing the distance further will not cause any appreciable signal strength decrease in both cases. From the signal trend analysis it is concluded that the current design is able to ensure reliable transmission within a few tens of meters,which is acceptable for many practical applications. As for the power consumption it is estimated that the transmitter should be able to operate on a set of batteries for a minimum period of three months. The power consumption of transmitter in power-down mode was measured as 4mA. When actively transmitting at 100MHz,the total power consumption of transmitter was 150mA. Reducing clock frequency and transmission speed will reduce power consumption, but lead to prolonged transmission time. For a full cycle of measurements the active transmission period for transmitter is less than 90s. If10 data transmissions were scheduled for each working day of 8hours, atotal 7.5 h active on time would be needed for a period of three months. Such batteries are commercially available in small packages suited for machine embedded applications. For applications requiring extremely small space, the RF amplifier in the present design can be replaced by other low power models at the price of reduced transmission range.
ADVANTAGES
· Power tuning
· Voice quality.
· User density.
· Cost at a suitable level ie, less cost.
· Wireless increases utility and accessibility.
· Increased mobility and scalability: more portable, half the size of credit card.
· Extended range with CDMA : it allows for multiple transmitters to share the same frequency band, as the receiver can distinguish and identify every specific transmitter by the code, ie, unique.
· Since the energy of transmitted data is spread over a broader frequency band than required, even if a portion of frequency band is distorted due to noise, only a part of energy on transmitted data is lost. So the entire data can be reconstructed correctly.
· Software implementation allows further modification.
· CDMA allows multiple transmitters to share the same frequency band.
DISADVANTAGES
· When transmitted is embedded with in a machinery the signal strength decreases as distance increases than transmitter in free space.
· Ensure reliable transmission only into few lines of meters. It can be increased by designing efficient amplifiers.
· Reducing clock frequency and transmission speed for reduced power consumption will lead to a prolonged transmission time.
· It reported initial difficulty in market introduction.
· WCDMA has higher data speed than CDMA.
FUTURE SCOPE
· Can be used in mobile communication with a speed up to 2mbps for voice, video data and image transmission with WCDMA.
APPLICATIONS
· Design of a wide range of electronic instruments such as data loggers, data acquisition cards, hand-held metering devices
· Machine health monitoring to machine components
· Systems that are difficult to access or not suitable for wired sensor data acquisition.
· A rice sized chip called Verichip embedded in body ,made of biocompatible materials, stores entire medical history of implantee.
CONCLUSION
A compact, low power digital wireless data transmitter based on CDMA coding technique has been designed, simulated, prototyped and experimentally tested .The design demonstrates the feasibility of of employing a sophisticated transmission scheme in an embedded sensor for machine health monitoring. Focusing on constraints and power efficiency during the design phase.
Has reduced the number of components to a minimum. All the employed components of prototype are available on IC dies and can be integrated to a multichip module that fits on a centimeter sized substrate. The software implementation of CDMA coding allowed for easy extensibility of present design.More systematic experiments will be conducted in future to investigate the utility of such an integrated sensing approach for condition monitoring of bearing in a realistic machine set up.
REFERENCES
1. Robert. X . Gao, Philipp Hunerberg- design of a CDMA-Based wireless Data Transmitter for Embedded serving. IEEE TRANSACTION ON INSTRUMENTATION AND MEASSUREMENT Vol:5 p.p. 1259, Dec. 2002
2. Halsall.F,’ “ Wireless Local Area Networks”, Data communications , Computer networks and open systems. p.p. 317-334
3. Thomasi, “Digital Communication” Electronic Common Systems p.p. 480- 489
4. EFY, “Breakthrough in embedded systems”, Electronics for you , NOV’02, pp 65- 69.
5. http://www.techonline/ .COM
6. http://www.google/ .COM
7. Dr.Kamilo Feher, Wireless digital communications, Upper saddle river, NJ:Prentice Hall, 2002, pp 297-307
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