Frequency Shift Keying Fsk

1. Frequency shift keying FSK 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) Of]t}

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.

2. Phase Shift Keying ( PSK ) 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.

QPSK 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. 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. FDMA The frequency ranges can be transmitted simultaneously. The frequencies should be nonoverlapping.

If

overlapping one of the overlapping range is shifted to another range

with same bandwidth.

CDMA It is a digital cellular technology. It uses spread spectrum technique. CDMA doesn’t design a specific frequency to each user. Each channel use fully available spectrum. Individual conversions encoded with pseudo random digital sequence. It is a military technology. It was used in second world war in jamming transmission. Using this all stations are permitted to transmit over the entire frequency all the time. Multiple transmissions are separated using coding which makes an assumption that when multiple signals combined together they will not get gargled. They add linearly.

Each bit is sub-divided into ‘ m’ short intervals called chips. Each station is assigned to a unique m – bit code called chip sequence. To transmit a 1-bit the station will be transmitting the chip sequence and for a o-bit the station will be transmitting 1’s compliment of chip sequence. No other patterns are permitted. We use bipolar **** signals with 1-bit being +1 and 0-bit being -1.

Suppose a station is having a chip sequences. An important assumption to be noted is that all chip sequences must be pair wise orthogonal is the normalized inner product of any two different chip sequences must be zero ie, S.T=0 and S.T=0. where T another stations chip sequence.

S.T

=

1/m * m

Recovery of data: to recover the data being transmitted the receiver must know, the chip sequence of the sender in advance. So for recovery process, the normalized inner product of transmitter and receiver is found. ***

is the received chip sequence and C is the

chip sequence of sender, three cases are. i.

S.C = 0

( C has not transmitted anything )

ii.

S.C = 1

( C has transmitted a 1-bit )

iii.

S.C = -1

( C has transmitted a 0-bit )

DESIGN OF AN EFFICIENT RF POWER AMPLIFIER Advantages and disadvantages

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 broades frequency band than required, even if a portioin 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.

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. APPLICATIONS Design of a wide range of electronic instruments such as data loggers, data acquisition cards, hand-held metering devices. Computer peripherals, hand-held pads, Lap tops etc. targetted application sectors. Machine health monitoring to machine components Systems that are difficult to access or not suitable for wired sensor data acquisition.

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. Halsa U.F, “ Wireless Local Area Networks” Data Common, Computer networks and open systems. p.p. 317-334

3. Thomasi, “Digital Communication” Electronic Common Systems p.p. 480-489

Contents 1.

INTRODUCTION

2.

SIGNAL MODULATION TECHNIQUES A. A.S.K B. F.S.K C. CDMA

3.

MULTIPLE ACCESS METHODS. A. FDMA B. TDMA C. CDMA

4.

TRANSMITTER DESIGN

5.

WORKING

6.

SIMULATION

7.

EXPERIMENTAL EVALUATION

8.

ADVANTAGES AND DISADVANTAGES

9.

FUTURE SCOPE

10.

APPLICATIONS

11.

CONCLUSION

12.

REFERENCES

PHASE SHIFT KEYING Phase shift keying (PSK) is a form of angle-modulated. Constant-amplitude digital modulation. PSK is similar to conventional phase modulation except that with PSK the input signal is a binary digital signal and a limited number output phases are possible. Binary Phase Shift Keying. 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 1 and the other 2 logic 0. as the input digital signal changes state, the phase of the output carrier shifts between two angles that are 1800 out of phase. Quaternary Phase Shift Keying Quaternary phase shift keying (QPSK), or quadrature PSK as it is sometimes called, is another form of angle-modulated, 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 possible conditions: 00,01,10 and 11. therefore, with QPSK, the binary input data are combined into groups of two bits called dibits. 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. 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 12-19. 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 (+ sin are possible at the output of the Q balanced modulator (

) and two phases ). 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, which is explained latter in this chapter. Also, from figure 12-20b 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. CDMA Code- division multiple access (CDMA) is used specifically with spread spectrum radio systems. As described in section 6.4.2, both direct sequence and frequency-hopping use a unique pseudorandom spreading/hopping sequence as the basis of their operating modes. In such systems, therefore, a different pseudorandom sequence can be allocated to each node and the complete set of sequences may be known by all nodes. To communicate with another node, the transmitter simply selects and uses the pseudorandom sequence of the intended recipient. In this way, multiple communications between different pairs of nodes can take place concurrently.

In practice, as figure 6.32 shows, this is possible only with frequency-hopping systems since, with direct sequence, a phenomenon known as the near far effect can occur. This is experienced when a second transmitter for example node X in the figure- is operating that is physically nearer to the intended receiver- node A-than the other communicating partner- node B. although the transmissions form node X are suppressed by the dispreading process in the node A, because of its closer proximity, the (spread) interference signal may have more power than the required signal from node B which, in turn causes the receiver in node A to miss the transmission. This is also known as the hidden terminal effect. In contrast, with frequency-hopping, since the two transmitters are constantly changing frequency channels, the probability of both operating in the same channel at the same time is very law. This can be reduced even further by careful planning of the hopping sequences. The disadvantage of both schemes, however, is the need for all nodes to know the pseudorandom sequence of all other nodes which, in a wireless LAN, is difficult to administer.

.