SIGNAL MODULATION TECHNIQUES
MULTIPLE ACCESS METHODS.
TRANSMITTER DESIGN
SIMULATION
EXPERIMENTAL EVALUATION
ADVANTAGES
DISADVANTAGES
FUTURE SCOPE
APPLICATIONS
CONCLUSION
REFERENCES
INTRODUCTION
The increasing demand on productivity and quality has led widespread automation of manufacturing processes and equipment. Cost effective and efficient maintenance is needed to prevent costly machine failure related downtime and ensure product quality. For the past decade the state of technology for machine health monitoring has been continuously improving. But majority of monitoring systems employed at the factory floor still require maintenance engineers to manually collect data and analyse them off-line. Most of these systems require wire connections for data acquisition and transmission ,which limit their accessibility and utility in many situations where space restrictions do not allow for such connections.
Wireless data transmission for embedded sensing and machine condition monitoring has been developed in recent years to overcome the restrictions of wired data connections. Because of the constraints concerning power supply, most of these systems operate on batteries and transmit data over a short distance to a data logging station nearby. A challenging issue in wireless data transmission is to design for low power, less circuitry complexity, and high reliability.
A low power ,compact digital wireless data transmitter employing code division multiple access scheme has been introduced designed simulated, prototyped and bench tested in lab environment. The design provides a generic wireless solution for embedded sensing and measurement. Salient features of the transmitter include an interface to multiple sensors ,and sharing of the save data receiver by multiple transmitters. Using surface mount packaging ,all the components of the transmitter could be accommodated within an area of about half the size of credit card,making transmitter well suited for structural integration into machines. Since the components are all available in the form of ICdies, the entire design can be further miniaturized into a single hybrid chip.
SIGNAL MODULATION TECHNIQUES
Inorder to transmit data wirelessly in an air channel ,amodulation scheme is needed inorder to translate data symbols to variations of acarrier wave of specified transmission frequency .Three major techniques exists for digital data modulation are: ASK, FSK, and PSK .The ASK technique uses amplitude variations of the carrier wave to represent the symbols transmitted, FSK uses different frequencies for each symbol, and PSK uses the phase shift of the carrier for symbol representation.
The robustness of wireless data transmission is measured by the probability of a symbol error, which is dependent on SNR that measures the strength of transmitted signal with respect to that of background noise.
A. Amplitude Shift Keying
The ASK technique modulates the data by assigning each symbol a different amplitude level, eg: two levels for binary data. For application in a metallically sealed machine environment such as a bearing housing where signal reflections may superimpose each other, an ASK receiver may not be able to distinguish between the original signal and the reflected one ,leading to data misinterpretation. Hence the ASK was not considered for the present application.
B. Frequency Shift Keying
This technique employs different frequencies for different symbols transmitted. To ensure reliable transmission, the difference between two frequencies used has to be at least
where T is the duration of the symbol.FSK is a low performance type of digital modulation. Binary FSK is a form of constant amplitude angle modulation. The general expression for binary FSK is
V fsk(t) = Vc COS { 2 [ fc+ Vm(t) ft}
Where V fsk(t) = binary FSK waveform
Vc = peak carrier amplitude ( volts )
fc = carrier centre frequency ( hertz )
f = peak frequency deviation ( hertz )
Vm(t) = binary input modulating signal with binary FSK, the carrier frequency is shifted by binary input signal.
For binary data transmission, the probability of error is
where 
Employing symbols that comprise more bits will reduce error probability, but will increase complexity in filtering and decoding hardware on receiver end. Such complexity would further lead to increased power consumption, which is detrimental to the bearing condition monitoring applications so the FSK technique was not considered for the present design.
C. PHASE SHIFT KEYING
Here , the carrier wave by itself represent a symbol, whereas all other symbols are defined by phase shift from the phase of carrier. Phase shift keying (PSK) is a form of angle-modulated, constant-amplitude digital modulation.
With binary phase shift keying (BPSK), two output phases are possible for a single carrier frequency (“binary” meaning “2”). One output phase represents a logic I and the other a logic 0. As the input digital signal changes state, the phase of the output carrier sifts between two angles that are 1800 out of phase.
For a binary signal the phase shift for second symbol will be180 degree. In fig 1, a binary data signal 110101 is illustrated, together with the carrier wave and the modulated signal. The logic level 0 is represented by -1, since the receiver cannot differentiate no transmission and transmission of 0. whenever the logic level of data changes, a phase shift of 180 degree occurs in the modulated signal. So in PSK the carrier wave by itself represents a sybol and all other symbols are defined by the phase shift of carrier phase
Fig. 1. Datatransmission of a string 110.101using PSK modulation
The probability of a symbol error for transmitting a binary data using PSK is
If the quadrature phase shift key is used that transmit two bits at the same time, then the error probability is
For an assumed SNR of 8 dB, the probability of error transmitting a symbol using QPSK is 0.02% and for PSK is 0.01%.To compare them on a bit error basis the symbol error probability has to be translated into bit error probability. The relationship between symbol and bit error probability can be approximated as
Pebit =1/2 Pe
This means that with QPSK it is possible to transmit twice as much data within the same period of time as with ordinary BPSK with same low probability of error. This result cannot be transferred simply to other modulation variations that combine more than two bits in one symbol.
Quaternary phase shift keying (QPSK), or quadrature PSK as it is sometimes called, is an other form of angle-moduled, constant-amplitude digital modulation. With QPSK four output phases are possible for a single carrier frequency. Because there are four different output phases, there must be four different input conditions. Because the digital input to a QPSK modulator is a binary (base2) signal, to produce four different input conditions, it takes more than a single input bit. With two bits, there are four there are four possible conditions: 00, 01, 10 and 11. There fore, with QPSK, the binary input data are combined into groups of two bits called dibits. . Therefore, the rate of change at the output (baud rate) is one-half of the input bit rate.
QPSK transmitter
A block diagram of a QPSK modulator is shown in figure 2. Two bits (a dibit) are clocked in to the bit splitter. After both bits have been serially inputted, they are simultaneously parallel outputted. One bit is directed to the I channel and the other to the Q channel. The I bit modulates a carrier that is in phase with the reference oscillator (hence, the same “I” for “in phase” channel), and the Q bit modulates a carrier that is 900 out of phase or in quadrature with the reference carrier (hence, the name “Q” for “quadrature” channel).
It can be seen that once a dibit has been split in to the I and Q channels, the operation is the same as in a BPSK modulator. Essentially a QPSK modulator is two BPSK modulators combined in parallel. Again, for a logic 1 = + 1V and a logic ) = - 1 V, two phase are possible at the output of the I balanced modulator and two phases are possible at the output of the Q balanced modulator When the linear summer combines the two quadrature (900 out of phase) signals, there are four possible resultant phasors given by these expressions:
With QPSK each of the four possible output phasors has exactly the same amplitude. Therefore, the binary information must be encoded entirely in the phase of the output signal. This constant amplitude characteristic is the most important characteristic of PSK that distinguishes it from QAM. Also, from figure 2 b it can be seen that the angular separation between any two adjacent phasors in QPSK is 900 . therefore, a QPSK signal can undergo almost a+450 or a- 450 shift in phase during transmission and still retain the correct encoded information when demodulated at the receiver. Figure 12-21 shows the output phase-versus-time relationship for QPSK modulator.
Each dibit code generates one of the four possible output phases. Therefore, for each two-bit dibit clocked into the modulator a single output change occurs.Employing QPSK instead of BPSK requires additional operation steps, either by adding specialized h/w components or a software implementation .
--------------------------------------------------------------------------------
.jpg)
(8)
No comments:
Post a Comment