Title
RECENT TRENDS IN DIGITAL COMMUNICATION (ANTIJAMMING FEATURE OF CDMA)
Presented By : Sagar S Deshpande
Ratan V Gawai
Email ID : al alwayssaga agar@y
[email protected]
Email ID: ID: ama amar_r r_rata
[email protected]
( Department Of Electronics ) Rajiv Gandhi College Of Engg. Research And Technology Chandrapur.
ABSTRACT ANTIJAMMING FEATURE OF CDMA
In conventional system a specific band is allotted to each user. He has to have his transmission within that allotted band only. Any unauthorized user can intentionally disturb this transmission by transmitting high power noise in the same frequency band. This is called Jamming. In our conventional analog communication and digital communication system as certain bands are allotted to each user because of this reason this signal are proned to get jammed. CDMA, Code Division Multiple Access provides number of user to use entire transponders bandwidth all the time. Each receiving station is allotted by a CDMA code; any transmitting station that wants to send data to the earth’s station must use a correct code. This actually widens its spectrum in proportional to the length of the code. That’s why it is also called as Spread Spectrum. With the help of spread spectral method the transmitted signal gets spread over the entire frequency band. This signal itself appears as a noise and at the same time for the jammers it becomes difficult to sent the jamming signals. Thus Spread Spectrum or in other words CDMA provides Antijamming capability.
ANTIJAMMING FEATURE OF CDMA
CDMA
The two most common multiple access techniques for satellite communication are FDMA( Freq. Division Multiple Access) and TDMA ( Time Division Multiple Access). In FDMA all users access the satellite channel by transmitting simultaneously but by using disjoint frequency bands. In TDMA, all users occupy the same RF bandwidth of the satellite channel, but they transmit sequentially in time. When, however, all users are permitted to transmit simultaneously and also occupy the same RF bandwidth of satellite channel, then some other method must be provided for separation of individual signals at the receiver. Code Division Multiple Access (CDMA) is the method that makes it possible to perform this separation. CDMA is a scheme in which a number of user can occupy entire transpond bandwidth all the time. CDMA signals are encoded such that the information from the individual transmitter can be recovered by receiving station that knows the code being used, in the presence of all the CDMA signals in same bandwidth. This provides the decentralized satellite network as only the pair of earth station that are communicating need to co-ordinate their transmission. In Code Division Multiple Access uplink stations are identified by uniquely separable address code embedded within the carrier waveform. Each uplink station use the entire satellite bandwidth and transmit through the satellite whenever desired, with all active stations superimposing their waveforms on the downlink. Thus no frequency or time separation is required. Carrier separation is achieved at an earth station by identifying the carrier with the proper address. These addresses are usually in the form of periodic binary sequences that either modulate the carrier directly or change the frequency
state of the carrier. Address identification is accomplished by carrier correlation operations. CDMA carrier crosstalk occurs only in the inability to correlate out the undesired address while properly synchronizing to the correct address for decoding. As in TDMA, CDMA carriers have the use of the entire satellite bandwidth for their total activity period, and CDMA has the advantage that no controlled uplink transmission time is required, and no uniformity over station bit rates is imposed.
However, system performance depends quite heavily on
the ability to recognize addresses, which often becomes difficult if the number of stations in the system is large. Digital addresses are obtained from code generators that produce periodic sequences of binary symbols. A station’s address generator continually cycles through its address sequence, which is superimposed on the carrier along with the data. If the address is modulated directly on the carrier, the format is referred to as Direct Sequence CDMA ( DS - CDMA ). If the digital address is used to continually change the frequency of the carrier, the system is referred to as Frequency Hopped CDMA ( FH - CDMA ). Superimposing address on modulated uplink carriers generally produces a larger carrier bandwidth than that which will be generated by the modulation alone. This spreading of the carrier spectrum has caused CDMA systems to be referred to also as Spread - Spectrum Multiple Access ( SSMA ) systems. Spreading of the carrier spectrum has an important application in military satellite systems since it produces inherent antijam advantages. For this reason the designation SSMA is generally used in conjunction with military systems, while CDMA is usually reserved for commercial usage.
The use of CDMA offers three attractive features over TDMA ; i)
CDMA doesn’t require synchronization network, which is an essential feature of TDMA.
ii)
CDMA offers a gradual degradation in performance as the number of users is increased.
iii)
CDMA offers an external interference rejection capability i.e Antijamming capability.
DIRECT SEQUENCE SPREAD SPECTRUM (DS-SS) SIGNALS
In this method, the data sequence directly modulates the pseudo-noise sequence. Let the data sequence be represented by b k . This data sequence is converted to bipolar NRZ waveform b(t). That is, when,
bk = 1,
and
bk = 0,
b(t) = +1 b(t) = - 1
Let ck denote the pseudo-noise sequence. And c(t) represent the bipolar NRZ signal generated by c k , that is, when,
ck = 1,
c(t) = +1
and
ck = 0,
c(t) = - 1
The data signal b(t) and pseudo-noise signal c(t) are applied to the product
modulator or modulator. The output of the modulator is the wide
spectrum signal. The spectrum of this signal is quite high compared to that of narrowband data signal b(t). Fig.(a) shows this simplified direct sequence modulator.
Lets consider that the pseudo-noise sequence generated is used to generate the pseudo-noise signal. Let this signal be modulated by some data signal b(t). Fig.(b) shows the waveform of arbitrary data signal and the waveform of the pseudo-noise signal generated by sequence 0011101... . The third waveform i.e. the modulate signal m(t). The one bit period of data signal is ‘T b’. The pseudo-noise signal contains integer number of bits in one bit period of data signal. As shown in waveform of fig (b) there are 7 bits of pseudo-noise sequence in one bit period of data signal. The pseudo-noise sequence is also repeats after 7 bits. The message m(t) can be written as, m(t) = c(t) b(t).
The pseudo-noise signal c(t) is wideband signal. It is multiplied with the narrowband data signal b(t). The modulated message signal m(t) also has wide spectrum as large as that of c(t). Thus the narrow band data signal can be transmitted directly using baseband transmission. Fig.(a) shows this baseband channel. During the transmission through channel, the noise i(t) interferes with the message signal to generate noisy signal r(t), i.e., r(t) = m(t) + i(t) By putting values of m(t) in above equation we obtain, r(t) = C(t) b(t) + i(t)
Since the message signal is not modulated on any bandpass carrier, the transmission system shown in fig.(a) is the bandpass transmission. Fig.(c) shows the block diagram of spread spectrum receiver of decoder for baseband transmission. The receiver consists of a multiplier and integrator. As shown in fig.(c), a locally generated pseudo-noise signal is applied to the multiplier. The signal is an exact replica of that used in the transmitter. The output of the multiplier is equal to the received noisy signal r(t) and pseudo-noise signal c(t) i.e,
z(t) = c(t) r(t) Putting value of r(t) in above equation we obtain, z(t) = c2(t) b(t) + c(t) i(t)
Observe in the waveform of c(t) in fig.(b) that the value of c(t) is either +1 or -1. Therefore the squared signal c 2(t) will be, c2(t) = + 1 Thus,
always
z(t) = b(t) + c(t) i(t) Thus at the output of multiplier, the data signal b(t) is reproduced with
noise c(t) i(t). We know that c(t) is wide band signal. When it is multiplied with noise i(t), the product c(t) it is also a wide band signal. The bandwidth of c(t) i(t) is larged compared to that of b(t). The signal z(t) is then passed through the integrator, which integrates over one bit period T b. This integrator acts as a low pass filter and removes the wide band noise c(t) i(t). The decision device compares the output of an integrator with the threshold of zero. It takes decision in favor of symbol ‘0’ of v < 0 and symbol ‘1’ if v > 0.
This is an essentially synchronous system. The receiver requires an exact knowledge of pseudo-noise sequence for proper detection of the data signal b(t). Therefore the receivers operates in perfect synchronism with the transmitter. If the receivers doesn’t know the pseudo-noise sequence, then it will not be able to detect the data signal b(t). Thus because of spread spectrum modulation, unwanted receivers cannot detect the data signal.
FREQUENCY HOPPED CDMA The alternative to direct sequence CDMA is to use the digital sequence to produce frequency hopping. In this mode the available bandwidth is partitioned into frequency bands, and the transmission time partitioned into time slots, as shown in fig.(d). A hopping pattern in this frequency time matrix is defined as a sequence of specific frequency bands, one for each time slot, as shown. A transmitter assigned a particular hopping pattern jump from one band to the next according to the pattern, readjusting its carrier frequency from one time slot to the next. Such a frequency hopping transmitter is shown in fig.(e). During each band transmission, the transmitter sends some form of modulated carrier that occupies only the designated band. Thus a station with a hopping patterns appears to utilize the entire bandwidth when observe over a long time although transmitting only within a specified band at any one time. In a military environment, it is often further required that the hopping patterns appear random, so that future values of the pattern cannot be predicted.
Hopping patterns can be obtained from periodic binary sequences similar to address codes. If such a sequence is partitioned into blocks, each block can designate a particular frequency band in a frequency - time matrix. Thus a particular binary sequence specifies a specific hopping pattern, and as the code sequence periodically repeats, the hopping pattern will likewise repeat.
WHAT IS JAMMING ? In the convolution system certain frequency bands are allotted to each station to operate. These systems have to operate on these frequency bands which they have been allotted. They cannot transmit their signal outside the allotted band frequencies. This will violates the rules o international communication i.e it may enter in other users band. So interface may takes place or we can say he may be interfacing in other users bandwidth. That will disturbs his transmission as well as his adjacent user bandwidth. In this case if somebody else transmit the signal intentionally in the same frequency band that too with high power in order to disturb the transmission from unknown location then this can be easily done. This effect is called Jamming effect. In this case it is difficult to detect the required signal. Such type of problems occurs in military applications. in such cases enemies or ravel parties should know the frequency bands in which one is operating. This can be very easily done in conventional AM, FM and digital systems like PSK, FSK.
HOW IT CAN BE AVOIDED ? With the help of spread spectral method the transmitted signal gets spread over the entire frequency band as the user continuously hops from one frequency band to another during his entire transmission interval. Infact every instant he is transmitting a certain frequency and to the next instant he is switching to another frequency which is again randomly selected. With this process every user is actually using the entire frequency band allotted for that transmission. Because of this reason jamming is not possible. This signal itself appears as a noise and at the same time for the jammers it becomes difficult to sent the jamming signals. Thus Spread Spectrum or in other words CDMA, Code Division Multiple Access provides Antijamming.
