HISTORY OF CDMA:
Somewhere close to the Second World War, Hollywood actress-turned-inventor, Hedy Lamarr and co-inventor George Antheil, co-patented a way for controlling torpedoes by sending signals over multiple radio frequencies using random patterns. They called this “frequency hopping”.
After some hue and cry, the US Navy discarded their work as architecturally unfeasible. In 1957, Sylvania Electronic System Division, in Buffalo, New York , took up the same idea. After the expiry of the inventor’s patent, they used the same technology to secure communications for the US military.
In the mid-80s, the US military declassified what is now called CDMA technology, a technique based on spread-spectrum technology, for use in wireless communication. The spread-spectrum technology works by digitizing multiple conversations, attaching a code(known only to the sender and receiver), and then breaking the signals into bits and reassembling them.
Qualcomm, which patented CDMA, and other telecommunication companies, were attrached to the technology because it enabled many simultaneous conversations, rather than the limited stop-and-go transmissions of analogue technology and the previous digital option.
EVOLUTION OF CDMA:
1940s and 1950s Spread Spectrum technique for military anti-jam applications.
1949 Claude Shannon and Robert Pierce develop basic ideas of CDMA
1970s Several CDMA developments for military systems (e. g. GPS)
In March 1992, the TIA (Telecommunications Industry Association) established the TR-45.5 subcommittee with the charter of developing a spread spectrum digital cellular standard. In July of 1993, the TIA gave its approval for the CDMA Technology standard.
1993 IS-95 CDMA standard finalized
1995 Commercial operation of N-CDMA system (IS-95) in Hong Kong/Korea
October 1, 2000 SK Telecom of Korea launches the first commercial cdma2000 network
April 17, 2001 Ericsson and Vodafone UK claim to have made the world's first WCDMA voice call over commercial network.
October 1, 2001 NTT DoCoMo launched the first commercial WCDMA 3G mobile network.
January 28, 2002 SK Telecom in Korea launched the world's first commercial CDMA2000 1xEV-DO.October 1, 2002 Qualcomm announces world's first Bluetooth WCDMA (UMTS) and GSM Voice Calls.
INTRODUCTION:
This paper is intended to provide an introduction to CDMA use in wireless telephone systems. The focus is on explaining, in generally non-technical language, both the key aspects of CDMA technology, and the primary benefits the technology offers to wireless communication system operators and their subscribers. There is a tremendous amount of detailed technical information which is intentionally not covered in this forum.
It has been necessary, though, to assume at least a rudimentary familiarity with cellular telephone systems, including the basic characteristics of radio and the RF spectrum, as well as fundamental system design concepts such as frequency re-use.
Motorola welcomes your comments and feedback on this paper.
What is CDMA?
One of the most important concepts to any cellular telephone system is that of "multiple access", meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of the limited radio resource which is temporary 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 channels are allocated to the many users of the system.
The CDMA Cellular Standard
With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo-Random Code Sequences." All users share the same range of radio spectrum.
For cellular telephony, CDMA is a digital multiple access technique specified by the Telecommunications Industry Association (TIA) as "IS-95".
In March 1992, the TIA established the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave its approval of the CDMA IS-95 standard.
IS-95 systems divide the radio spectrum into carriers which are 1,250 KHz (1.25 MHz) wide. One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed number. Rather, the capacity of the system will be dependent on a number of different factors. This will be discussed in later sections.
CDMA - Code Division Multiple Access
IS-95 uses a multiple access spectrum spreading technique called Direct Sequence (DS) CDMA.
Each user is assigned a binary, Direct Sequence code during a call. The DS code is a signal generated by linear modulation with wideband Pseudorandom Noise (PN) sequences. As a result, DS CDMA uses much wider signals than those used in other technologies. Wideband signals reduce interference and allow one-cell frequency reuse.
There is no time division, and all users use the entire carrier, all of the time.
CDMA Technology
Though CDMA application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many military applications, such as anti-jamming (because of the spread signal, it is difficult to jam or interfere with a CDMA signal), ranging (measuring the distance of the transmission to know when it will be received), and secure communications (the spread spectrum signal is very hard to detect).
Spread Spectrum
CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.
The standard data rate of a CDMA call is 9600 bits per second (9.6 kilobits per second). This initial data is "spread," including the application of digital codes to the data bits, up to the transmitted rate of about 1.23 megabits per second. The data bits of each call are then transmitted in combination with the data bits of all of the calls in the cell. At the receiving end, the digital codes are separated out, leaving only the original information which was to be communicated. At that point, each call is once again a unique data stream with a rate of 9600 bits per second.
Traditional uses of spread spectrum are in military operations. Because of the wide bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in contrast to technologies using a narrower bandwidth of frequencies. Since a wideband spread spectrum signal is very hard to detect, it appears as nothing more than a slight rise in the "noise floor" or interference level. With other technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect.
Increased privacy is inherent in CDMA technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band.
Synchronization
In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code. In order to assure that the time offsets used remain unique from each other, CDMA stations must remain synchronized to a common time reference.
The primary source of the very precise synchronization signals required by CDMA systems is the Global Positioning System (GPS). GPS is a radio navigation system based on a constellation of orbiting satellites. Since the GPS system covers the entire surface of the earth, it provides a readily available method for determining position and time to as many receivers as are required.
The Balancing Act
CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics - Coverage, Quality and Capacity - must be balanced off of each other to arrive at the desired level of system performance.
In a CDMA system these three characteristics are tightly inter-related. Even higher capacity might be achieved through some degree of degradation in coverage and/or quality. Since these parameters are all intertwined, operators can not have the best of all worlds: three times wider coverage, 40 time capacity, and "CD" quality sound. For example, the 13 kbps vocoder provides better sound quality, but reduces system capacity as compared to an 8 kbps vocoder.
Motorola is using system simulation and real world testing to identify and implement the correct balances in CDMA system application. Operators will have the opportunity to balance these parameters to best serve a particular area. The best balance point may change from cell site to cell site. Sites in dense downtown areas may trade off coverage for increased capacity. Conversely, at the outer edges of a system, capacity could be sacrificed for coverage area.
Motorola's system expertise, as demonstrated by its winning of the 1995 NCSA Industrial Grand Challenge Award for system simulation and testing achievements, is especially beneficial to operators in their efforts to balance system parameters.
MAIN TYPES OF CDMA
CDMAONE:
This is the older version of the CDMA technology and now it is now known as cdmaone as well as IS-95.
CDMA 2000:
We now have cdma2000 and its variants like 1X EV, 1XEV-DO, and MC 3X. The reffer to variants of usage of a 1.25MHz channel. 3X uses a 5 MHz channel.
This first phase of cdma2000 - variously called 1XRTT, 3G1X, or just plain 1X - is designed to double current voice capacity and support always-on data transmission speeds 10 times faster than typically available today, some 153.6 kbps on both the forward and reverse links.
CDMA2000 Technical Detail:
Frequency band: Any existing band.
Minimum frequency band required: 1x: 2x1.25MHz, 3x: 2x3.75
Chip rate: 1x: 1.2288, 3x: 3.6864 Mcps
Maximum user data rate: 1x: 144 kbps now, 307 kbps in the future 1xEV-DO: max 384 kbps - 2.4 Mbps, 1xEV-DV: 4.8 Mbps.
WCDMA:
Wideband CDMA that forms the basis of 3G networks, Developed originally by Qualcomm, CDMA is characterized by high capacity and small cell radius, employing spread-spectrum technology and a special coding scheme. WCDMA uses 5 MHz bandwidth.
CDMA Phones at Glance:
Samsung SCH-N191
LG RD2030
LG-Elect-TM910
LG Electronics TM510
THE Tata Indicom CDMA Mobile Cost Table
Activation Cost
Rs.1050
Monthly Rental
Rs.450
Deposit
Rs.3,000
Handset
Hyundai HGC-310E Rs.9,800
Samsung SCH-620 Rs.10,800
CDMA PRINCIPLE
If we change our communication topology from point-to-point to point-to-multipoint, we have hanged the communication environment from single-link to a multiple-access link. The multiple-access scheme in a spread-spectrum system is termed code-division multiple-access (CDMA).
Each access to a common channel needs some form of orthogonality. For frequency-division multiple-access (FDMA), we achieve orthogonality in the frequency domain by selecting nonoverlapping unique frequency bands to each user. We achieve orthogonality in the time domain by selection nonoverlapping unique time segments to each user; this process is referred to as time-division multiple-access (TDMA). The spread-spectrum form of multiple access exploits the orthogonality in the code domain and is termed code-division multiple-access (CDMA).
The multiuser environment in the spread-spectrum case is set up for each user in assigning each user a unique spreading sequence out of a family of orthogonal sequences. Each user in a CDMA network occupies the same channel bandwidth.
A CDMA system is clearly not a collision avoidance system like FDMA and TDMA.The opposite is true and explains the differences in the behavior of CDMA systems compared to FDMA and TDMA. In general, the collisions at the channel is a disadvantage of CDMA system and can be mitigated by careful selection of the sequence and power control that is close to perfect.
WORKING OF CDMA
The CDMA uses the spread spectrum technology. The spread spectrum refers to any system that satisfies the following conditions :
1. The spread spectrum may be viewed as a kind of modulation scheme in which the modulated(spread spectrum) signal bandwidth is much greater than the message(baseband) signal bandwidth. Thus, spread spectrum is a wideband scheme.
2.The spectral spreading is performed by a code that is independent of the message signal. This same ode is also used at the receiver to despread the received signal in order to recover the message signal (from spread spectrum signal). In secure communication, this code is known only to the person(s) for whom the message is intended.
The spread spectrum increases the bandwidth of the message signal by a factor N, called the processing gain. If the message signal bandwidth is B Hz and the corresponding spread spectrum signal bandwidth is Bss Hz, then Processing gain N = Bss / B
Thus, the key to CDMA is to be able to extract the desired signal while rejecting everything else as random noise. A somewhat simplified description of CDMA follows:
In CDMA each bit time is subdivided into m short intervals called chips. Typically, there are 64 or 128 chips per bit, but in the example given below we will use 8 chips/bit for simplicity.
Each station is assigned a unique m-bit code or chip sequence. To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the one’s complement of its chip sequence. No other patterns are permitted. Thus for m = 8, if a station A is assigned the chip sequence 00011011, it sends a 1 bit by sending 00011011 and 0 bit by sending 11100100.
If we have 1-MHz band available for 100 stations, with FDM each one would have 10 kHz and could send at 10 kbps (assuming 1 bit per Hz). With CDMA, each station uses the full 1 MHz, so the chip rate is 1 Megachip per second. With fewer than 100 chips per bit, the effective bandwidth per station is higher for CDMA than FDMA, and the channel allocation problem is also solved.
It is more convenient to use a bipolar notation, with binary 0 being –1 and binary 1 being +1. We will show chip sequences in parentheses, so a 1 bit for station A now becomes (-1-1-1+1+1-1+1+1). In Fig. (1), we show the binary chip sequence assigned to four example stations. In Fig. (2), we show them in our bipolar notation.
A: 0 0 0 1 1 0 1 1 A: (-1-1-1+1+1-1+1+1)
B: 0 0 1 0 1 1 1 0 B: (-1-1+1-1+1+1+1-1)
C: 0 1 0 1 1 1 0 0 C: (-1+1-1+1+1+1-1-1)
D: 0 1 0 0 0 0 1 0 D: (-1+1-1-1-1-1+1-1)
Fig. (1)Binary chip Fig. (2)Bipolar chip sequence
Sequence for 4 stations
Six Examples:
_ _1_ C S1= ( -1 +1 –1 +1 +1 +1 –1 -1)
_ 11_ B+C S2= ( -2 0 0 0 +2 +2 0 -2)
1 0_ _ A+B S3= ( 0 0 –2 +2 0 -2 0 +2)
1 0 1 _ A+B+C S4= ( -1 +1 –3 +3 –1 –1 –1 +1)
1 1 1 1 A+B+C+D S5= ( -4 0 -2 0 +2 0 +2 -2)
1 1 0 1 A+B+C+D S6= ( -2 –2 0 –2 0 –2 +4 0 )
Fig. (3) Six example of Transmission
S1lC = (1+1+1+1+1+1+1+1)/8 = 1
S2lC = (2+0+0+0+2+2+0+2)/8 = 1
S3lC = (0+0+2+2+0-2+0-2)/8 = 0
S4lC = (1+1+3+3+1-1+1-1)/8 = 1
S5lC = (4+0+2+0+2+0-2+2)/8 = 1
S6lC = (2-2+0-2+0-2-4+0)/8 = -1
Fig. (4) Recovery of station C’s signal
Each station has its own unique chip sequence. Let’s use symbol S to indicate the m-chip vector for station S , and S for its negation. All chip sequences are pairwise orthogonal, by which we mean that the normalized inner product of any two distinct chip sequences, S and T (SlT) is 0. In mathematical terms,
m
SlT = 1/m å Si * Ti = 0
i=1
in plain, English, as many pairs are same as are different. This orthogonality property will prove crucial. Note that if SlT = 0 then SlT= 0. The normalized inner product of any chip sequence with itself is 1:
m m
SlS = 1/m å Si * Si = 1/m å(+1)²=1
i=1 i=1
This follows because each of the m terms in the inner product is 1, so the sum is m. Also note that SlS = -1.
During each bit time, a station can transmit a 1 by sending its chip sequence, it can transmit a 0 by sending negative of its chip sequence, or it can be silent and transmit nothing. For the moment, we assume that all stations are synchronized in time, so all chip sequence begin at the same instant.
When two or more station transmit simultaneously, their bipolar signals add linearly. For example, if in one chip period three stations output +1 and one station outputs –1, the result is +2. One can think of this as adding voltages: three stations outputting +1 volts and 1 station outputting –1 volts gives 2 volts.
In Fig.(3), we see six examples of one or more stations transmitting at the same time. In the first example, C transmits a 1 bit, so we just get C’s chip sequence. In the second example, both B and C transmit 1 bits, so we get the sum of their bipolar chip sequences.
In the third example, station A sends 1 and station B sends a 0. The others are silent. In the fifth example, all four stations sends 1 bit. Finally, in the last example A, B, and D sends a 1 bit, while C sends a 0 bit. Note that each of the six sequences S1 through S6 given in Fig. (3) represents only one bit time.
Somewhere close to the Second World War, Hollywood actress-turned-inventor, Hedy Lamarr and co-inventor George Antheil, co-patented a way for controlling torpedoes by sending signals over multiple radio frequencies using random patterns. They called this “frequency hopping”.
After some hue and cry, the US Navy discarded their work as architecturally unfeasible. In 1957, Sylvania Electronic System Division, in Buffalo, New York , took up the same idea. After the expiry of the inventor’s patent, they used the same technology to secure communications for the US military.
In the mid-80s, the US military declassified what is now called CDMA technology, a technique based on spread-spectrum technology, for use in wireless communication. The spread-spectrum technology works by digitizing multiple conversations, attaching a code(known only to the sender and receiver), and then breaking the signals into bits and reassembling them.
Qualcomm, which patented CDMA, and other telecommunication companies, were attrached to the technology because it enabled many simultaneous conversations, rather than the limited stop-and-go transmissions of analogue technology and the previous digital option.
EVOLUTION OF CDMA:
1940s and 1950s Spread Spectrum technique for military anti-jam applications.
1949 Claude Shannon and Robert Pierce develop basic ideas of CDMA
1970s Several CDMA developments for military systems (e. g. GPS)
In March 1992, the TIA (Telecommunications Industry Association) established the TR-45.5 subcommittee with the charter of developing a spread spectrum digital cellular standard. In July of 1993, the TIA gave its approval for the CDMA Technology standard.
1993 IS-95 CDMA standard finalized
1995 Commercial operation of N-CDMA system (IS-95) in Hong Kong/Korea
October 1, 2000 SK Telecom of Korea launches the first commercial cdma2000 network
April 17, 2001 Ericsson and Vodafone UK claim to have made the world's first WCDMA voice call over commercial network.
October 1, 2001 NTT DoCoMo launched the first commercial WCDMA 3G mobile network.
January 28, 2002 SK Telecom in Korea launched the world's first commercial CDMA2000 1xEV-DO.October 1, 2002 Qualcomm announces world's first Bluetooth WCDMA (UMTS) and GSM Voice Calls.
INTRODUCTION:
This paper is intended to provide an introduction to CDMA use in wireless telephone systems. The focus is on explaining, in generally non-technical language, both the key aspects of CDMA technology, and the primary benefits the technology offers to wireless communication system operators and their subscribers. There is a tremendous amount of detailed technical information which is intentionally not covered in this forum.
It has been necessary, though, to assume at least a rudimentary familiarity with cellular telephone systems, including the basic characteristics of radio and the RF spectrum, as well as fundamental system design concepts such as frequency re-use.
Motorola welcomes your comments and feedback on this paper.
What is CDMA?
One of the most important concepts to any cellular telephone system is that of "multiple access", meaning that multiple, simultaneous users can be supported. In other words, a large number of users share a common pool of radio channels and any user can gain access to any channel (each user is not always assigned to the same channel). A channel can be thought of as merely a portion of the limited radio resource which is temporary 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 channels are allocated to the many users of the system.
The CDMA Cellular Standard
With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called "pseudo-Random Code Sequences." All users share the same range of radio spectrum.
For cellular telephony, CDMA is a digital multiple access technique specified by the Telecommunications Industry Association (TIA) as "IS-95".
In March 1992, the TIA established the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave its approval of the CDMA IS-95 standard.
IS-95 systems divide the radio spectrum into carriers which are 1,250 KHz (1.25 MHz) wide. One of the unique aspects of CDMA is that while there are certainly limits to the number of phone calls that can be handled by a carrier, this is not a fixed number. Rather, the capacity of the system will be dependent on a number of different factors. This will be discussed in later sections.
CDMA - Code Division Multiple Access
IS-95 uses a multiple access spectrum spreading technique called Direct Sequence (DS) CDMA.
Each user is assigned a binary, Direct Sequence code during a call. The DS code is a signal generated by linear modulation with wideband Pseudorandom Noise (PN) sequences. As a result, DS CDMA uses much wider signals than those used in other technologies. Wideband signals reduce interference and allow one-cell frequency reuse.
There is no time division, and all users use the entire carrier, all of the time.
CDMA Technology
Though CDMA application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many military applications, such as anti-jamming (because of the spread signal, it is difficult to jam or interfere with a CDMA signal), ranging (measuring the distance of the transmission to know when it will be received), and secure communications (the spread spectrum signal is very hard to detect).
Spread Spectrum
CDMA is a "spread spectrum" technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.
The standard data rate of a CDMA call is 9600 bits per second (9.6 kilobits per second). This initial data is "spread," including the application of digital codes to the data bits, up to the transmitted rate of about 1.23 megabits per second. The data bits of each call are then transmitted in combination with the data bits of all of the calls in the cell. At the receiving end, the digital codes are separated out, leaving only the original information which was to be communicated. At that point, each call is once again a unique data stream with a rate of 9600 bits per second.
Traditional uses of spread spectrum are in military operations. Because of the wide bandwidth of a spread spectrum signal, it is very difficult to jam, difficult to interfere with, and difficult to identify. This is in contrast to technologies using a narrower bandwidth of frequencies. Since a wideband spread spectrum signal is very hard to detect, it appears as nothing more than a slight rise in the "noise floor" or interference level. With other technologies, the power of the signal is concentrated in a narrower band, which makes it easier to detect.
Increased privacy is inherent in CDMA technology. CDMA phone calls will be secure from the casual eavesdropper since, unlike an analog conversation, a simple radio receiver will not be able to pick individual digital conversations out of the overall RF radiation in a frequency band.
Synchronization
In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code. In order to assure that the time offsets used remain unique from each other, CDMA stations must remain synchronized to a common time reference.
The primary source of the very precise synchronization signals required by CDMA systems is the Global Positioning System (GPS). GPS is a radio navigation system based on a constellation of orbiting satellites. Since the GPS system covers the entire surface of the earth, it provides a readily available method for determining position and time to as many receivers as are required.
The Balancing Act
CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics - Coverage, Quality and Capacity - must be balanced off of each other to arrive at the desired level of system performance.
In a CDMA system these three characteristics are tightly inter-related. Even higher capacity might be achieved through some degree of degradation in coverage and/or quality. Since these parameters are all intertwined, operators can not have the best of all worlds: three times wider coverage, 40 time capacity, and "CD" quality sound. For example, the 13 kbps vocoder provides better sound quality, but reduces system capacity as compared to an 8 kbps vocoder.
Motorola is using system simulation and real world testing to identify and implement the correct balances in CDMA system application. Operators will have the opportunity to balance these parameters to best serve a particular area. The best balance point may change from cell site to cell site. Sites in dense downtown areas may trade off coverage for increased capacity. Conversely, at the outer edges of a system, capacity could be sacrificed for coverage area.
Motorola's system expertise, as demonstrated by its winning of the 1995 NCSA Industrial Grand Challenge Award for system simulation and testing achievements, is especially beneficial to operators in their efforts to balance system parameters.
MAIN TYPES OF CDMA
CDMAONE:
This is the older version of the CDMA technology and now it is now known as cdmaone as well as IS-95.
CDMA 2000:
We now have cdma2000 and its variants like 1X EV, 1XEV-DO, and MC 3X. The reffer to variants of usage of a 1.25MHz channel. 3X uses a 5 MHz channel.
This first phase of cdma2000 - variously called 1XRTT, 3G1X, or just plain 1X - is designed to double current voice capacity and support always-on data transmission speeds 10 times faster than typically available today, some 153.6 kbps on both the forward and reverse links.
CDMA2000 Technical Detail:
Frequency band: Any existing band.
Minimum frequency band required: 1x: 2x1.25MHz, 3x: 2x3.75
Chip rate: 1x: 1.2288, 3x: 3.6864 Mcps
Maximum user data rate: 1x: 144 kbps now, 307 kbps in the future 1xEV-DO: max 384 kbps - 2.4 Mbps, 1xEV-DV: 4.8 Mbps.
WCDMA:
Wideband CDMA that forms the basis of 3G networks, Developed originally by Qualcomm, CDMA is characterized by high capacity and small cell radius, employing spread-spectrum technology and a special coding scheme. WCDMA uses 5 MHz bandwidth.
CDMA Phones at Glance:
Samsung SCH-N191
LG RD2030
LG-Elect-TM910
LG Electronics TM510
THE Tata Indicom CDMA Mobile Cost Table
Activation Cost
Rs.1050
Monthly Rental
Rs.450
Deposit
Rs.3,000
Handset
Hyundai HGC-310E Rs.9,800
Samsung SCH-620 Rs.10,800
CDMA PRINCIPLE
If we change our communication topology from point-to-point to point-to-multipoint, we have hanged the communication environment from single-link to a multiple-access link. The multiple-access scheme in a spread-spectrum system is termed code-division multiple-access (CDMA).
Each access to a common channel needs some form of orthogonality. For frequency-division multiple-access (FDMA), we achieve orthogonality in the frequency domain by selecting nonoverlapping unique frequency bands to each user. We achieve orthogonality in the time domain by selection nonoverlapping unique time segments to each user; this process is referred to as time-division multiple-access (TDMA). The spread-spectrum form of multiple access exploits the orthogonality in the code domain and is termed code-division multiple-access (CDMA).
The multiuser environment in the spread-spectrum case is set up for each user in assigning each user a unique spreading sequence out of a family of orthogonal sequences. Each user in a CDMA network occupies the same channel bandwidth.
A CDMA system is clearly not a collision avoidance system like FDMA and TDMA.The opposite is true and explains the differences in the behavior of CDMA systems compared to FDMA and TDMA. In general, the collisions at the channel is a disadvantage of CDMA system and can be mitigated by careful selection of the sequence and power control that is close to perfect.
WORKING OF CDMA
The CDMA uses the spread spectrum technology. The spread spectrum refers to any system that satisfies the following conditions :
1. The spread spectrum may be viewed as a kind of modulation scheme in which the modulated(spread spectrum) signal bandwidth is much greater than the message(baseband) signal bandwidth. Thus, spread spectrum is a wideband scheme.
2.The spectral spreading is performed by a code that is independent of the message signal. This same ode is also used at the receiver to despread the received signal in order to recover the message signal (from spread spectrum signal). In secure communication, this code is known only to the person(s) for whom the message is intended.
The spread spectrum increases the bandwidth of the message signal by a factor N, called the processing gain. If the message signal bandwidth is B Hz and the corresponding spread spectrum signal bandwidth is Bss Hz, then Processing gain N = Bss / B
Thus, the key to CDMA is to be able to extract the desired signal while rejecting everything else as random noise. A somewhat simplified description of CDMA follows:
In CDMA each bit time is subdivided into m short intervals called chips. Typically, there are 64 or 128 chips per bit, but in the example given below we will use 8 chips/bit for simplicity.
Each station is assigned a unique m-bit code or chip sequence. To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the one’s complement of its chip sequence. No other patterns are permitted. Thus for m = 8, if a station A is assigned the chip sequence 00011011, it sends a 1 bit by sending 00011011 and 0 bit by sending 11100100.
If we have 1-MHz band available for 100 stations, with FDM each one would have 10 kHz and could send at 10 kbps (assuming 1 bit per Hz). With CDMA, each station uses the full 1 MHz, so the chip rate is 1 Megachip per second. With fewer than 100 chips per bit, the effective bandwidth per station is higher for CDMA than FDMA, and the channel allocation problem is also solved.
It is more convenient to use a bipolar notation, with binary 0 being –1 and binary 1 being +1. We will show chip sequences in parentheses, so a 1 bit for station A now becomes (-1-1-1+1+1-1+1+1). In Fig. (1), we show the binary chip sequence assigned to four example stations. In Fig. (2), we show them in our bipolar notation.
A: 0 0 0 1 1 0 1 1 A: (-1-1-1+1+1-1+1+1)
B: 0 0 1 0 1 1 1 0 B: (-1-1+1-1+1+1+1-1)
C: 0 1 0 1 1 1 0 0 C: (-1+1-1+1+1+1-1-1)
D: 0 1 0 0 0 0 1 0 D: (-1+1-1-1-1-1+1-1)
Fig. (1)Binary chip Fig. (2)Bipolar chip sequence
Sequence for 4 stations
Six Examples:
_ _1_ C S1= ( -1 +1 –1 +1 +1 +1 –1 -1)
_ 11_ B+C S2= ( -2 0 0 0 +2 +2 0 -2)
1 0_ _ A+B S3= ( 0 0 –2 +2 0 -2 0 +2)
1 0 1 _ A+B+C S4= ( -1 +1 –3 +3 –1 –1 –1 +1)
1 1 1 1 A+B+C+D S5= ( -4 0 -2 0 +2 0 +2 -2)
1 1 0 1 A+B+C+D S6= ( -2 –2 0 –2 0 –2 +4 0 )
Fig. (3) Six example of Transmission
S1lC = (1+1+1+1+1+1+1+1)/8 = 1
S2lC = (2+0+0+0+2+2+0+2)/8 = 1
S3lC = (0+0+2+2+0-2+0-2)/8 = 0
S4lC = (1+1+3+3+1-1+1-1)/8 = 1
S5lC = (4+0+2+0+2+0-2+2)/8 = 1
S6lC = (2-2+0-2+0-2-4+0)/8 = -1
Fig. (4) Recovery of station C’s signal
Each station has its own unique chip sequence. Let’s use symbol S to indicate the m-chip vector for station S , and S for its negation. All chip sequences are pairwise orthogonal, by which we mean that the normalized inner product of any two distinct chip sequences, S and T (SlT) is 0. In mathematical terms,
m
SlT = 1/m å Si * Ti = 0
i=1
in plain, English, as many pairs are same as are different. This orthogonality property will prove crucial. Note that if SlT = 0 then SlT= 0. The normalized inner product of any chip sequence with itself is 1:
m m
SlS = 1/m å Si * Si = 1/m å(+1)²=1
i=1 i=1
This follows because each of the m terms in the inner product is 1, so the sum is m. Also note that SlS = -1.
During each bit time, a station can transmit a 1 by sending its chip sequence, it can transmit a 0 by sending negative of its chip sequence, or it can be silent and transmit nothing. For the moment, we assume that all stations are synchronized in time, so all chip sequence begin at the same instant.
When two or more station transmit simultaneously, their bipolar signals add linearly. For example, if in one chip period three stations output +1 and one station outputs –1, the result is +2. One can think of this as adding voltages: three stations outputting +1 volts and 1 station outputting –1 volts gives 2 volts.
In Fig.(3), we see six examples of one or more stations transmitting at the same time. In the first example, C transmits a 1 bit, so we just get C’s chip sequence. In the second example, both B and C transmit 1 bits, so we get the sum of their bipolar chip sequences.
In the third example, station A sends 1 and station B sends a 0. The others are silent. In the fifth example, all four stations sends 1 bit. Finally, in the last example A, B, and D sends a 1 bit, while C sends a 0 bit. Note that each of the six sequences S1 through S6 given in Fig. (3) represents only one bit time.
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