15-03-2011, 12:09 PM
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CODE DIVISION MULTIPLE ACCESS
INTRODUCTION:-
Mobile communications are rapidly becoming more and more necessary for everyday activities. With so many more users to accommodate, more efficient use of bandwidth is a priority among cellular phone system operators. Equally important is the security and reliability of these calls. One solution that has been offered is a CODE DIVISION MULTIPLE ACCESS SYSTEM.
CDMA is one method for implementing a multiple access communication system. MULTIPLE ACCESS is a technique where many subscribers or local stations can share the use of the use of a communication channel at the same time or nearly so despite the fact originate from widely different locations. A channel can be thought of as merely a portion of the limited radio resource, which is temporarily allocated for a specific purpose, such as someone’s phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how the channels are allocated to the many users of the system.
Since there are multiple users transmitting over the same channel, a method must be established so that individual users will not disrupt one another. There are essentially three ways to do this.
1. FREQUENCY DIVISION MULTIPLE ACCESS
In this technique, the available bandwidth is split up into non-overlapping frequency bands and these disjoint sub bands of frequency are allocated to the different users on a continuous time basis. In order to reduce interference between users allocated adjacent channel bands, channel bands are used to act as buffer zones, as illustrated in figure(1). These guard bands are necessary because of the impossibility of achieving ideal filtering for separating the different users. It could be compared to AM or FM broadcasting radio where each station has a frequency assigned.
1. TIME DIVISION MULTIPLE ACCESS
In this technique, each user is allocated the full spectral occupancy of
The channel, but only for a short duration of time called time slot. Buffers zones are in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arises due to system imperfections, especially in synchronization scheme.
DRAWBACKS:
In both FDMA and TDMA, only one subscriber at a time is assigned to a channel. No other conversion can access this channel until the subscriber’s call is finished or until that original call to handed off to a different channel by the system. Voice data tends to be burst in nature. So much of the time, no data is being sent over the channel. This inefficiency tends to limit the capacity of the system.
3. CODE DIVISION MULTIPLE ACCESS
The above drawbacks are overcome in this third technique in which the users are spread across both frequency and time in the same
Channel. This is a hybrid combination of FDMA and TDMA. For example, frequency hopping may be employed to ensure during each successive time slot, the frequency bands assigned to the users are recorded in random manner. During time slot 1, user 1 occupies frequency band 1, user 2 occupies frequency band 2, user 3 occupies band 3 and so on. During time slot 2, user 1 hops to frequency band 3, user 2 hops to band 1, user 3 hops to band 2, and so on. An important advantage of CDMA over FDMA and TDMA is that it can provide for secure communication.
MEANING OF CDMA:
Here, the users are spread across both frequency and time in the same channel. Here, unique digital codes, rather than separate RF frequencies or channels are used to differentiate subscribers. The codes are shared by both the mobile stations (cellular phone) and the base station, and are called “pseudo random code sequences” or “pseudo-noise code sequences”.
PN – SEQUENCE:
A PN – sequence is a periodic binary sequence with a noise like waveform that is usually generated by means of a feedback shift register. ”pseudo” word is used, as these are not real noise. These are noise like.
BASIS OF CDMA:
Basis of CDMA is the spread spectrum technology.
SPREAD SPECTRUM is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. Spread spectrum is accomplished before transmission through the use of a code that is independent of the data sequence (PN).
It can provide secure communication in hostile environment such that the transmitted signal is not easily detected or recognized by unwanted listeners. It can reject interference whether it is the unintentional interference by another user simultaneously attempting to transmit through the channel, or the intentional interference by a hostile transmitter attempting to jam the transmission. Another application is in multiple access communication in which a number of independent users can share a common channel without an external synchronizing mechanism.
TYPES OF SPREAD SPECTRUM
1. DIRECT SEQUENCE SPREAD SPECTRUM
DS sequence allows each station to transmit over the entire frequency
Spectrum all the time. Multiple simultaneous transmissions are separated using some sort of coding technique that is each user is assigned a chip sequence. The sender and receiver synchronize by the receiver locking into the chip sequence and the sender and receiver locking into the chip sequence of the sender. All the other (unsynchronized) transmission is then seen as random noise. So with CDMA each user uses the full frequency spectrum.
They employ a high speed code sequence along with the basic information being sent, to modulate their RF carriers. The high speed code sequence is used directly setting the transmitted RF bandwidth.
Binary phase shift keying (BPSK) is the most common technique used in DS system. Direct sequence is, in essence, multiplication of a more conventional communication waveform by PN sequence in the transmitter.
2. FREQUENCY HOPPING SPREAD SPECTRUM
FH – CDMA is a kind of spread spectrum technology that enables many users to share the same channel by employing a unique hopping pattern to distinguish different users’ transmission. The type of spread spectrum in which the carrier hops randomly from one frequency to another is called FH spread spectrum. A common modulation format for FH system is that of M-ary frequency shift keying (MFSK).the combination is referred to as FH/MFSK.
A major advantage of frequency hopping is that it can be implemented over a much larger frequency band than it is possible to implement DS- spreading, and the band can be noncontiguous. Another major advantage is that frequency hopping provides resistance to multiple – access interference while not requiring power control to prevent near – far problems. In DS – systems , accurate power control is crucial but becomes less effective as the carrier frequency is increased.
Frequency hopping does not cover the entire spread spectrum
Instantaneously, we are led to consider the rate at which the hops occur. So, we may identify two basic characterizations of frequency hopping.
1. Slow frequency hopping, in which the symbol rate Rs of MFSK signal is an integrator multiple of the hop rate Rh . that is, several symbols are transmitted on each frequency hop
2. Fast frequency hopping, in which the hop rate Rh is an integrator multiple of the MFSK symbol rate Rs. that is, the carrier frequency will change or hop several times during the transmission of one symbol.
CODE DIVISION MULTIPLE ACCESS
INTRODUCTION:-
Mobile communications are rapidly becoming more and more necessary for everyday activities. With so many more users to accommodate, more efficient use of bandwidth is a priority among cellular phone system operators. Equally important is the security and reliability of these calls. One solution that has been offered is a CODE DIVISION MULTIPLE ACCESS SYSTEM.
CDMA is one method for implementing a multiple access communication system. MULTIPLE ACCESS is a technique where many subscribers or local stations can share the use of the use of a communication channel at the same time or nearly so despite the fact originate from widely different locations. A channel can be thought of as merely a portion of the limited radio resource, which is temporarily allocated for a specific purpose, such as someone’s phone call. A multiple access method is a definition of how the radio spectrum is divided into channels and how the channels are allocated to the many users of the system.
Since there are multiple users transmitting over the same channel, a method must be established so that individual users will not disrupt one another. There are essentially three ways to do this.
1. FREQUENCY DIVISION MULTIPLE ACCESS
In this technique, the available bandwidth is split up into non-overlapping frequency bands and these disjoint sub bands of frequency are allocated to the different users on a continuous time basis. In order to reduce interference between users allocated adjacent channel bands, channel bands are used to act as buffer zones, as illustrated in figure(1). These guard bands are necessary because of the impossibility of achieving ideal filtering for separating the different users. It could be compared to AM or FM broadcasting radio where each station has a frequency assigned.
1. TIME DIVISION MULTIPLE ACCESS
In this technique, each user is allocated the full spectral occupancy of
The channel, but only for a short duration of time called time slot. Buffers zones are in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arises due to system imperfections, especially in synchronization scheme.
DRAWBACKS:
In both FDMA and TDMA, only one subscriber at a time is assigned to a channel. No other conversion can access this channel until the subscriber’s call is finished or until that original call to handed off to a different channel by the system. Voice data tends to be burst in nature. So much of the time, no data is being sent over the channel. This inefficiency tends to limit the capacity of the system.
3. CODE DIVISION MULTIPLE ACCESS
The above drawbacks are overcome in this third technique in which the users are spread across both frequency and time in the same
Channel. This is a hybrid combination of FDMA and TDMA. For example, frequency hopping may be employed to ensure during each successive time slot, the frequency bands assigned to the users are recorded in random manner. During time slot 1, user 1 occupies frequency band 1, user 2 occupies frequency band 2, user 3 occupies band 3 and so on. During time slot 2, user 1 hops to frequency band 3, user 2 hops to band 1, user 3 hops to band 2, and so on. An important advantage of CDMA over FDMA and TDMA is that it can provide for secure communication.
MEANING OF CDMA:
Here, the users are spread across both frequency and time in the same channel. Here, unique digital codes, rather than separate RF frequencies or channels are used to differentiate subscribers. The codes are shared by both the mobile stations (cellular phone) and the base station, and are called “pseudo random code sequences” or “pseudo-noise code sequences”.
PN – SEQUENCE:
A PN – sequence is a periodic binary sequence with a noise like waveform that is usually generated by means of a feedback shift register. ”pseudo” word is used, as these are not real noise. These are noise like.
BASIS OF CDMA:
Basis of CDMA is the spread spectrum technology.
SPREAD SPECTRUM is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. Spread spectrum is accomplished before transmission through the use of a code that is independent of the data sequence (PN).
It can provide secure communication in hostile environment such that the transmitted signal is not easily detected or recognized by unwanted listeners. It can reject interference whether it is the unintentional interference by another user simultaneously attempting to transmit through the channel, or the intentional interference by a hostile transmitter attempting to jam the transmission. Another application is in multiple access communication in which a number of independent users can share a common channel without an external synchronizing mechanism.
TYPES OF SPREAD SPECTRUM
1. DIRECT SEQUENCE SPREAD SPECTRUM
DS sequence allows each station to transmit over the entire frequency
Spectrum all the time. Multiple simultaneous transmissions are separated using some sort of coding technique that is each user is assigned a chip sequence. The sender and receiver synchronize by the receiver locking into the chip sequence and the sender and receiver locking into the chip sequence of the sender. All the other (unsynchronized) transmission is then seen as random noise. So with CDMA each user uses the full frequency spectrum.
They employ a high speed code sequence along with the basic information being sent, to modulate their RF carriers. The high speed code sequence is used directly setting the transmitted RF bandwidth.
Binary phase shift keying (BPSK) is the most common technique used in DS system. Direct sequence is, in essence, multiplication of a more conventional communication waveform by PN sequence in the transmitter.
2. FREQUENCY HOPPING SPREAD SPECTRUM
FH – CDMA is a kind of spread spectrum technology that enables many users to share the same channel by employing a unique hopping pattern to distinguish different users’ transmission. The type of spread spectrum in which the carrier hops randomly from one frequency to another is called FH spread spectrum. A common modulation format for FH system is that of M-ary frequency shift keying (MFSK).the combination is referred to as FH/MFSK.
A major advantage of frequency hopping is that it can be implemented over a much larger frequency band than it is possible to implement DS- spreading, and the band can be noncontiguous. Another major advantage is that frequency hopping provides resistance to multiple – access interference while not requiring power control to prevent near – far problems. In DS – systems , accurate power control is crucial but becomes less effective as the carrier frequency is increased.
Frequency hopping does not cover the entire spread spectrum
Instantaneously, we are led to consider the rate at which the hops occur. So, we may identify two basic characterizations of frequency hopping.
1. Slow frequency hopping, in which the symbol rate Rs of MFSK signal is an integrator multiple of the hop rate Rh . that is, several symbols are transmitted on each frequency hop
2. Fast frequency hopping, in which the hop rate Rh is an integrator multiple of the MFSK symbol rate Rs. that is, the carrier frequency will change or hop several times during the transmission of one symbol.
