In some communication systems, CDMA is used as an access method that permits carriers from different stations to use the same transmission equipment by using a wider bandwidth than the individual carriers. On reception, each carrier can be distinguished from the others by means of a specific modulation code, thereby allowing for the reception of signals that were originally overlapping in frequency and time. Thus, several transmissions can occur simultaneously within the same bandwidth, with the mutual interference reduced by the degree of orthogonality of the unique codes used in each transmission .
Prepared By: Sapan M. Raval (6th I.T.) Rishin S. Kumar (6th I.T.) C.U.SHAH COLLEGE OF ENGG. & TECH. WADHWAN CITY. Code Division Multiple Access WADHWAN CITY DIST: SURENDRANAGAR CERTIFICATE This is to certify that Mr. / Ms. SAPAN M. RAVAL Mr. / Ms. RISHIN S. KUMAR is / are studying in Sem – VI of B.E. Information Technology having Roll No. 40 & 41 has / have completed his / her / their seminar on the following topic successfully. Topic Name: CDMA TECHNOLOGY Staff – Incharge Head of Dept. (Miss Saroj Bodar) Date : ___________ C.U.Shah College of Engg. & Tech. -2- Code Division Multiple Access ACKNOWLEDGEMENT We take this opportunity to thank our respected H.O.D. Saroj Bodar and Mr. H. D Kothari without whose support and encouragement, this paper could not have been completed. We also take this opportunity to thankfull to our other faculty members and friends who have helped us in gathering and organizing the required information. C.U.Shah College of Engg. & Tech. -3- Code Division Multiple Access ABSTRACT The world is demanding more from wireless communication technologies than ever before. More people around the world are subscribing to wireless services and consumers are using their phones more frequently. Add in exciting Third-Generation (3G) wireless data services and applications - such as wireless email, web, digital picture taking/sending and assisted-GPS position location applications - and wireless networks are asked to do much more than just a few years ago. And these networks will be asked to do more tomorrow. This is where CDMA technology fits in. CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built. CDMA permits a more uniform distribution of energy in the emitted bandwidth Short for Code Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. CDMA is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space. As its name implies, CDMA assigns unique codes to each communication to differentiate it from others in the same spectrum. In a world of finite spectrum resources, CDMA enables C.U.Shah College of Engg. & Tech. -4- Code Division Multiple Access many more people to share the airwaves at the same time than do alternative technologies. CDMA has greater voice quality and call clarity than other technologies because it filters out background noise, cross-talk and interference. CDMA has fewer dropped calls because it increases voice and date transmission reliability. CDMA has greater spectral efficiency because it packs more transmissions into the same space, resulting in fewer cell sites, which lowers operators' costs. Add to this increased capacity, enhanced privacy, and improved coverage… why would you use any other wireless technology? The CDMA air interface is used in both 2G and 3G networks. 2G CDMA standards are branded cdmaOne and include IS-95A and IS-95B. CDMA is the foundation for 3G services: the two dominant IMT-2000 standards, CDMA2000 and WCDMA, are based on CDMA. CDMAONE: THE FAMILY OF IS-95 CDMA TECHNOLOGIES cdmaOne describes a complete wireless system based on the TIA/EIA IS-95 CDMA standard, including IS-95A and IS-95B revisions. It represents the end-to-end wireless system and all the necessary specifications that govern its operation. cdmaOne provides a family of related services including cellular, PCS and fixed wireless (wireless local loop). CDMA 2000: Leads To 3G revolution CDMA2000 represents a family of ITU-approved, IMT-2000 (3G) standards and includes CDMA2000 1X and CDMA2000 1xEV technologies. They deliver increased network capacity to meet growing demand for wireless services and high-speed data services. CDMA2000 1X was the world's first 3G technology commercially deployed (October 2000). CDMA Deployments CDMA is the fastest growing wireless technology and it will continue C.U.Shah College of Engg. & Tech. -5- Code Division Multiple Access to grow at a faster pace than any other technology. It is the platform on which 2G and 3G advanced services are built. INDEX 1. INTRODUCTION 2 1.1 Definition. 1.2 Overview. 2. TYPES OF CDMA TECHNOLOGY. 3 2.1 CDMA one. 2.2 CDMA 2000. 2.3 Technical details. 2.4 Wide band CDMA. 3. BIRTH OF CDMA 4 3.1 CDMA at world war-2. 3.2 History of CDMA. 4. EVOLUTION OF CDMA 5 5. SPREAD-SPECTRUM AND CDMA PRINCIPLE 6 6. WORKING OF CDMA 7 7. FEATURES OF CDMA 11 7.1 CDMA and WLL. 7.2 3G Technology. 8. APPLICATIONS OF CDMA 16 9. COMPARISON OF CDMA AND GSM 17 9.1 Introduction. 9.2 The Basics. 9.3 Voice Encoding. 9.4 Spectral efficiency. 9.5 In building coverage. 9.6 Final Observations. 10. ADVANTAGES OF CDMA TECHNOLOGY 22 11. DISADVANTAGES OF CDMA TECHNOLOGY 24 12. CONCLUSION 25 13. BIBLIOGRAPHY 26 C.U.Shah College of Engg. & Tech. -6- Code Division Multiple Access 1.INTRODUCTION 1.1 DEFINITION : A coding scheme, used as a modulation technique, in which multiple channels are independently coded for transmission over a single wideband channel. 1.2 SOME IMPORANT NOTES: Note 1: In some communication systems, CDMA is used as an access method that permits carriers from different stations to use the same transmission equipment by using a wider bandwidth than the individual carriers. On reception, each carrier can be distinguished from the others by means of a specific modulation code, thereby allowing for the reception of signals that were originally overlapping in frequency and time. Thus, several transmissions can occur simultaneously within the same bandwidth, with the mutual interference reduced by the degree of orthogonality of the unique codes used in each transmission . Note 2: CDMA permits a more uniform distribution of energy in the emitted bandwidth Short for Code Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. Because Qualcomm Inc. created communications chips for CDMA technology, it was privy to the classified information. Once the information became public, Qualcomm claimed patents on the technology and became the first to commercialize it. Code Division Multiple Access (CDMA) is the ultimate in inside communications technology. The fastest growing wireless technology, CDMA already has tens of millions of users throughout the world. The reasons for this are simple. CDMA supports current voice and data needs while providing a quick, cost- efficient evolutionary path to tomorrow’s systems. CDMA has greater voice quality and call clarity than other technologies because it filters out background noise, cross-talk and interference. CDMA has fewer dropped calls because it increases voice and date transmission reliability. CDMA has greater spectral efficiency because it packs more transmissions into the same space, resulting in fewer cell sites, which lowers operators' costs. Add to this increased capacity, enhanced privacy, and improved coverage… why would you usee for any other wireless technology? C.U.Shah College of Engg. & Tech. -7- Code Division Multiple Access 2.Main types of CDMA technology: 2.1 cdmaOne This is the older version of the CDMA technology and now it is now known as cdmaOne as well as IS-95. 2.2 cdma2000 We now have cdma2000 and its variants like 1X EV, 1XEV-DO, and 3X. The refer to variants of usage of a 1.25Mhz channel. 3X uses a 5 Mhz channel.MC 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. 2.3 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: 14.4 kbps now, 115 kbps in the future 1xEV-DO: max 307 kbps - 2.4 Mbps, 1xEV-DV: 4.8 Mbps. 2.4 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. 2.5 CDMA Phones at Glance Samsung SCH-N191 LG RD2030 LG-Elect-TM910 LG Electronics TM510 C.U.Shah College of Engg. & Tech. -8- Code Division Multiple Access 3. BIRTH OF CDMA 3.1 CDMA At World War II CDMA is a military technology first used during World War II by the English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal. 3.2 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. C.U.Shah College of Engg. & Tech. -9- Code Division Multiple Access 4. 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. C.U.Shah College of Engg. & Tech. - 10 - Code Division Multiple Access 5. SPREAD SPECTRUM AND CDMA PRICIPLE 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. CDMA2000 systems are currently being deployed in the 450 MHz, 800 MHz, 1700 MHz, 1900 MHz and 2100 MHz bands. C.U.Shah College of Engg. & Tech. - 11 - Code Division Multiple Access 6.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 preson(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. (a), we show the binary chip sequence assigned to four example stations. In Fig. (b), 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.U.Shah College of Engg. & Tech. - 12 - Code Division Multiple Access 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. (a)Binary chip Fig. (b)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) 101_ A+B+C S4= ( -1 +1 –3 +3 –1 –1 –1 +1) 1111 A+B+C+D S5= ( -4 0 -2 0 +2 0 +2 -2) 1101 A+B+C+D S6= ( -2 –2 0 –2 0 –2 +4 0 ) Fig. (c) Six example of Transmission S1C = (1+1+1+1+1+1+1+1)/8 = 1 S2C = (2+0+0+0+2+2+0+2)/8 = 1 S3C = (0+0+2+2+0-2+0-2)/8 = 0 Fig. (d) Recovery of station S4C = (1+1+3+3+1-1+1-1)/8 = 1 C’s signal S5C = (4+0+2+0+2+0-2+2)/8 = 1 S6C = (2-2+0-2+0-2-4+0)/8 = -1 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 (ST) is 0. In mathematical terms, m ST = 1/m 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 ST = 0 then ST= 0. The normalized inner product of any chip sequence with itself is 1: m m SS = 1/m Si * Si = 1/m (+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 SS = -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. C.U.Shah College of Engg. & Tech. - 13 - Code Division Multiple Access 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.(c), 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. (c) represents only one bit time. To recover the bit stream of an individual station, the receiver must know that station’s chip sequence in advance. It does the recovery by computing the normalized inner product of the received chip sequence (the linear sum of all the stations that transmitted) and the chip sequence of the station whose bit stream it is trying to recover. If the received chip sequence is S and the receiver is trying to listen to a station whose chip sequence is C, it just computes the normalized inner product, SC. To see why this works, imagine the two stations, A and C, both transmit a 1 bit at the same time that B transmit a 0 bit. The receiver sees the sum: S = A+B+C and computes SC = AC+ BC+ CC =0+0+1 = 1 C.U.Shah College of Engg. & Tech. - 14 - Code Division Multiple Access The first two terms vanish because all pairs of chip sequence have been carefully chosen to be orthogonal. Now it should be clear why this property must be imposed on the chip sequence. To make the decoding process more concrete, let us consider the six examples of fig.(d) again. Suppose that the receiver is interested in extracting the bit sent by station C from each of the six sums S1 through S6. It calculates the bit by summing the pairwise products of the received S and C vector of Fig.(b), and then taking 1/8 of the result (since m=8 here). As shown, each time the correct bit is decoded. ASSUMPTIONS IN THE ABOVE EXAMPLE First, we assumed that all the chips are synchronized in time. In reality, doing so is impossible. What can be done is that the sender and receiver synchronize by having the sender transmit a long enough known chip sequence that the receiver can lock onto. All other (unsynchronized) transmissions are then seen as random noise. An implicit assumption in the above example is that the power levels of all stations are the same as perceived by the receiver. CDMA is typically used for wireless systems with a fixed base station and many mobile stations at varying distances from it. The power levels received at the base station depends on how far away the transmitters are. A good heuristic here is for each mobile station to transmit to the base station at the inverse of the power level it receives from the base station, so a mobile station receiving a weak signal from the base will use more power than one getting a strong signal. The base station can also give explicit commands to the mobile stations to increase or decrease their transmission power. We have also assumed that the receiver knows who the sender is. In principle, given enough computing capacity, the receiver can listen to all the senders at once by running the decoding algorithm for each of them in parallel. In real life, suffice it to say that this is easier than done. C.U.Shah College of Engg. & Tech. - 15 - Code Division Multiple Access 7.FEATURES OF CDMA 7.1 CDMA AND WLL For many years now, India has been a GSM subscriber. In 1999, when MTNL decided to provide the CDMA-based WiLL(Wireless in Local Loop) service in India, quite a few eyebrows were raised. The biggest reason why mobile operators opposed the entry of WiLL is that it is uncertain to allow mobility in the local loop. CDMA is restricted to a short distance charging area(SDCA). Currently, there are 2600 SDCAs within the country. A CDMA-based phone can thus ‘roam’ only within its SDCA. This is NOT a technological restriction. In India, Reliance Infocom and Tata Indicom use CDMA technology to provide WiLL services. In remote rural areas, where installing cables is diffucult as well as expensive, CDMA-based WiLL networks can be deployed quickly. 7.2 3G (3rd Generation) 3G, as it is popularly called, refers to the 3rd generation of wireless networks. The 3rd generation provides higher frequency bands (of 2Ghz and more) and a bandwidth of around 5 MHz. The Bandwidth and frequency is matched by speeds of 384 Kbps in a mobile environment. Will CDMA be the path towards 3G” The world seems to be divided on this. While the standard choosen by Reliance-CDMA2000 1x-is the 3G avatar of C.U.Shah College of Engg. & Tech. - 16 - Code Division Multiple Access CDMA, the restrictions imposed by the TRAI(Telecom Regulatory Authoriy of India) doesn’t let it explore the 3G realms. Plus, some Wide CDMA supporters(W- CDMA) aren’t helping the situation by claiming CDMA 1x is not 3G. Third-generation applications includes WCDMA, 1x and High Data Rate (HDR). 7.2.1 3G-MIGRATION CDMA2000 3G offers a viable solution for any existing cellular and PCS as well as new 3G licensed operator. CDMA2000 was designed so that any wireless carrier, regardless of existing air interface, frequency or core network standards can benefit from its spectrum efficiencies and data capabilities 188.8.131.52 CCDMA ONE TO CDMA2000 MIGRATION CDMA2000 is the natural 3G evolution for cdmaOne operators, requiring only minor upgrades to the network and small capital investment. CDMA2000 handsets are backward compatible with the legacy cdmaOne infrastructure. Because of this, the transition from cdmaOne to CDMA2000 1X is relatively easy for operators and transparent for consumers This inherent advantage of cdmaOne gives operators significant market advantage. cdmaOne operators lead deployment of 3G across markets in Asia, the Americas and Europe. C.U.Shah College of Engg. & Tech. - 17 - Code Division Multiple Access The transition from cdmaOne to CDMA2000 requires channel card and software upgrades to cdmaOne base stations (older base stations may require some hardware upgrades) and introduction of new handsets. 184.108.40.206 TDMA to CDMA2000 Migration Lacking a technological migration path to enable higher voice capacity and bandwidth-intense mobile data services, TDMA has reached its limit. Thus, TDMA operators are faced with a difficult decision to choose a technology that will assure their future in the wireless market. CDMA2000 is an extremely attractive solution for TDMA operators. It offers them a direct path to3G, preserves their investment in the existing core network, and allows them flexibility to migrate to 3G over time, as the market for advanced service evolves. Advantages of CDMA2000 as a migration path for TDMA: Offers a direct path to 3G via a single evolutionary step rather than two or three upgrades required for an alternative path Reuses IS-41 elements of the existing core network, providing for substantial cost savings and enabling service transparency for end users Increases capacity of the existing TDMA air interface by 4 to 8 times to meet future demand for voice services Requires only a small amount of spectrum (1.25 MHz) for advanced services while preserving capacity to service existing customers Does not require new spectrum. CDMA2000 is designed to operate in all existing cellular and PCS bands, including 800 MHz and 1900 MHz where TDMA currently is deployed. A wide range of competitively priced devices are commercially available A number of infrastructure manufacturers offer solutions today For TDMA operators in Latin America, CDMA2000 also opens significant roaming markets in the Americas. Because CDMA2000 is backward compatible with cdmaOne, the subscribers would be able to roam in 15 countries in South America, as well as with the U.S. and Canada. Already a number of TDMA operators have selected CDMA2000 as a migration path either through cdmaOne or directly to CDMA2000. 220.127.116.11 GSM1X : GSM TO CDMA 2000 MIGRATION The CDMA2000 family of standards, including CDMA2000 1x and CDMA2000 1x EV-DO, can also be a 3G solution for GSM operators. GSM1x, is an overlay solution which enables the coexistence of CDMA2000 1X radio access C.U.Shah College of Engg. & Tech. - 18 - Code Division Multiple Access networks with GSM-MAP core service networks in existing or new spectrum. With GSM1x, GSM operators can seamlessly leverage their existing GSM core network and services while enhancing the data capabilities and spectral efficiencies of their radio access with CDMA2000 infrastructure. GSM1x Components A GSM1x network has four primary components: 1. The service core network is a standard GSM core network. This enables existing network equipment to be utilized. And further offers GSM service transparency for both basic and supplementary service. 2. The radio access network is standard CDMA2000 radio infrastructure which is IOS4 compliant. This enables CDMA2000 capacity of 35 calls per sector per 1.25 MHz channel, and peak data rates up to 153 kbps with CDMA2000 1x and 2.4 Mbps with CDMA2000 1x EVDO 3. The handsets are standard SIM-enabled CDMA2000 handsets, the same SIM as GSM handsets. 4. The mobile switching node (MSN) interfaces with both the unmodified core network and the radio access network concurrently, integrating both voice and data traffic. GSM1x Overview GSM1x takes the concept of network convergence to a new level. It combines the feature-rich services of GSM with the spectral efficiency of the CDMA2000 family of air interfaces. GSM1x is a unique solution that takes full advantage of an operator's investment in its existing GSM core network. It has also been designed to overlay an existing GSM network with no impact to current services and features. GSM1x C.U.Shah College of Engg. & Tech. - 19 - Code Division Multiple Access provides a seamless integration to the GSM core network and services, by maintaining all existing infrastructure. The GSM1x MSN (Mobile Switching Node) interfaces with both the unmodified GSM core network and the CDMA20001x and/or 1x EVDO radio access networks concurrently, integrating both voice and data traffic. By utilizing CDMA2000 radio access to improve efficiency of existing spectrum (800, 900, 1800, 1900 and 2100 MHz), GSM1x will increase an operator's voice and data capacity. And service transparency between the GSM and the GSM1x network is maintained. As with CDMA20001x, GSM1x provides peak rates of 307 kbps per sector and peak data rates up to 2.4 Mbps with 1xEV-DO in a 1.25 MHz channel. This allows GSM operators to offer advanced capabilities, in a cost effective manner, while maintaining the inherent strength of the core network. Furthermore, an operator can provide simultaneous voice and data through the enhanced capacity of this solution. GSM1x uses a standard CDMA2000 handset with a CDMA SIM interface and a software upgrade to accept GSM SIM cards. Dual-mode, single-mode, and dual- band handsets will be available, to meet the diverse needs of subscribers. In conclusion, GSM1x provides a unique combination of both technical and economic advantages to the carrier using existing GSM technology (please see below). GSM1x further enables new profitable service models such as landline replacement, high-speed VPN, and dispatch services. By utilizing this dynamic and revolutionary technology, carriers will be able to grow their business with minimal outlay of additional capital. ` C.U.Shah College of Engg. & Tech. - 20 - Code Division Multiple Access 8. APPLICATIONS OF CDMA TECHNOLOGY. 8.1 DAILY APPLICATIONS POSSIBLE WITH CDMA 8.1.1 Daily Downloads : ringers characters images horoscopes 8.1.2 Real time stock quotes : of different stock exchanges 8.1.3 Text Communication : Chat instant messaging SMS e-mail message board member search 8.1.4 Sending photos over the air : MMS messages 8.1.5 Position Location Services : navigation assistance friend finder 8.1.6 Games and Entertainment : magazine comic book store All these services are already being offered in South Korea and Japan. C.U.Shah College of Engg. & Tech. - 21 - Code Division Multiple Access 9. CDMA Vs GSM (The Old Horse) 9.1 INTRODUCTION In India, clutching a cell phone is still sometimes a status symbol. And if the phone uses technology that is said to be far superior to the one used in America, well, that calls for a celebration. That is how it was with GSM technology. But just as the drinks were being served, Reliance entered the party, bringing with it ‘outclassed technology’. Suddenly, the phones stopped ringing. Because the ‘outclassed technology’ had become ‘the’ technology. The same people who had said CDMA had no takers were suddenly fascinated with it. What happened? Why did GSM lose its appeal? Or, did it? 9.2 BASICS Let’s begin by learning what these two acronyms stand for. TDMA stands for "Time Division Multiple Access", while CDMA stands for "Code Division Multiple Access". Three of the four words in each acronym are identical, since each technology essentially achieves the same goal, but by using different methods. Each strives to better utilize the radio spectrum by allowing multiple users to share the same physical channel. You heard that right. More than one person can carry on a conversation on the same frequency without causing interference. This is the magic of digital technology. Where the two competing technologies differ is in the manner in which users share the common resource. TDMA does it by chopping up the channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in a round-robin fashion. In reality, only one person is actually using the channel at any given moment, but he only uses it for short bursts. He then gives up the channel momentarily to allow the other users to have their turn. This is very similar to how a computer with just one processor can seem to run multiple applications simultaneously. CDMA on he hand really does let everyone transmit at the same time. Conventional wisdom would lead you to believe that this is simply not possible. Using conventional modulation techniques, it most certainly is impossible. What makes CDMA work is a special type of digital modulation called "Spread Spectrum". This form of modulation takes the user's stream of bits and splatters them across a very wide channel in a pseudo-random fashion. The "pseudo" part is very important here, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order. If you are still having trouble understanding the differences though, perhaps this analogy will help you. This my own version of an excellent analogy provided by Qualcomm: Imagine a room full of people, all trying to carry on one-on-one conversations. In TDMA each couple takes turns talking. They keep their turns short by saying only one sentence at a time. As there is never more than one person speaking in the room at any given moment, no one has to worry about being heard C.U.Shah College of Engg. & Tech. - 22 - Code Division Multiple Access over the background din. In CDMA, each couple talk at the same time, but they all use a different language. Because none of the listeners understand any language other than that of the individual to whom they are listening, the background din doesn't cause any real problems. 9.3 VOICE ENCODING At this point many people confuse two distinctly different issues involved in the transmission of digital audio. The first is the WAY in which the stream of bits is delivered from one end to the other. This part of the "air interface" is what makes one technology different from another. The second is the compression algorithm used to squeeze the audio into as small a stream of bits as possible. This latter component is known at the "Voice Coder", or Vocoder for short. Another term commonly used is CODEC, which is a similar word to modem. It combines the terms "COder" and "DECoder". Although each technology has chosen their own unique CODECs, there is no rule saying that one transmission method needs to use a specific CODEC. People often lump a technology's transmission method with its CODEC as though they were single entities. We will discuss CODECs in greater detail later on in this article. Voice encoding schemes differ slightly in their approach to the problem. Because of this, certain types of human voice work better with some CODECs than they do with others. The point to remember is that all PCS CODECs are compromises of some sort. Since human voices have such a fantastic range of pitch and tonal depth, one cannot expect any single compromise to handle each one equally well. This inability to cope with all types of voice at the same level does lead some people to choose one technology over another. All of the PCS technologies try to minimize battery consumption during calls by keeping the transmission of unnecessary data to a minimum. The phone decides whether or not you are presently speaking, or if the sound it hears is just background noise. If the phone determines that there is no intelligent data to transmit, it blanks the audio and reduces the transmitter duty cycle (in the case of TDMA) or the number of transmitted bits (in the case of CDMA). When the audio is blanked, your caller would suddenly find themselves listening to "dead air", and this may cause them to think the call has dropped. To avoid this psychological problem, many service providers insert what is known as "Comfort Noise" during the blanked periods. Comfort Noise is synthesized white noise that tries to mimic the volume and structure of the real background noise. This fake background noise assures the caller that the connection is alive and well. However, in newer CODECs such as EVRC (used exclusively on CDMA systems), background noise is generally suppressed even while the user is talking. This piece of magic makes it sound as though the cell phone user is not in a noisy environment at all. Under these conditions, Comfort Noise is neither necessary, nor desirable. You can read my article on EVRC by clicking here. C.U.Shah College of Engg. & Tech. - 23 - Code Division Multiple Access 9.4 SPECTRAL EFFICIENCY Channel capacity in a TDMA system is fixed and indisputable. Each channel carries a finite number of "slots", and you can never accommodate a new caller once each of those slots is filled. Spectral efficiency varies from one technology to another, but computing a precise number is still a contentious issue. For example, GSM provides 8 slots in a channel 200 kHz wide, while IS-136 provides 3 slots in a channel only 30 kHz wide. GSM therefore consumes 25 kHz per user, while IS-136 consumes only 10 kHz per user. One would be sorely tempted to proclaim that IS-136 has 2.5 times the capacity of GSM. In a one-cell system this is certainly true, but once we start deploying multiple cells and channel reuse, the situation becomes more complex. Due to GSM's better error management and frequency hopping, the interference of a co- channel site is greatly reduced. This allows frequencies to be reused at closer range without a degradation in the overall quality of the service. Capacity is measured in "calls per cell per MHz". An IS-136 system using N=7 reuse (this means you have 7 different sets of frequencies to spread out around town) the figure is 7.0. In GSM we get figures of 5.0 for N=4 and 6.6 for N=3. It was hoped that IS-136 could use tighter reuse than N=7, but its inability to cope with interference made this impossible. Computing this figure for CDMA requires that certain assumptions are made. Formulas have been devised, and using very optimistic assumptions, CDMA can provide a whopping 45 users per cell per MHz. However, when using more pessimistic (and perhaps more realistic) assumptions, the value is 12. That still gives CDMA an almost 2:1 advantage over the TDMA competition. 9.5 IN BUILDING COVERAGE Now let's deal with another issue involving CDMA and TDMA. In- building coverage is something that many people talk about, but few people properly understand. Although CDMA has a slight edge in this department, due to a marginally greater tolerance for weak signals, all the technologies fair about the same. This is because the few dB advantage CDMA has is often "used up" when the provider detunes the sites to take advantage of this process gain. Buildings come in many configurations, but the most important aspect to their construction is the materials used. Steel frame buildings, or those with metal siding, shield their interiors more thoroughly than building made of wood. Large window openings allow signals to penetrate more deeply into buildings, so malls with glass roofs will generally provide better service than fully enclosed ones. More important than the type of building however, is the proximity of the nearest site. When a site is located just outside a building, it can penetrate just about any building material. When a site is much further away however, the signals have a much harder time of getting past the walls of a structure When it comes to distance, remember that signals are subject to the "distance squared law". This means that C.U.Shah College of Engg. & Tech. - 24 - Code Division Multiple Access signals decrease by the square of the distance. A site at 0.25 kilometers away will have 4 times the signal strength of a site at 0.50 kilometers away, and 16 times that of a site 1.0 kilometers away. Distance squared however, is the rate of signal reduction in free space. Recent studies have shown that terrestrial communications are usually subject to rates as high as "Distance cubed", or even "Distance to the 4th". If the latter is true, then a site 1.0 kilometers away will actually be 256 times weaker than a site 0.25 kilometers away. In-building penetration is therefore less a technology issue than it is an implementation issue. Service providers who have sites close to the buildings you commonly visit will inevitably look better those who don't. Never use someone else's in-building experiences unless you expect to go in the same buildings as they do. You cannot make useful generalizations about in-building coverage based upon one person's experience. CDMA does however have one peculiarity concerning in-building penetration that does not affect TDMA. When the number of users on a channel goes up, the general level of signal pollution goes up in tandem. To compensate for this the CDMA system directs each phone to transmit with slightly more power. However, if a phone is already at its limit (such as might be the case inside a building) it cannot do anything to "keep up with the pack". This condition is known as "the shrinking coverage phenomenon" or "site breathing". During slow periods of the day you might find coverage inside a specific building quite good. During rush hour however, you might find it exceedingly poor (or non-existent). 9.6 Some Final Observations CDMA really comes into its element when you are out in the countryside with few sites covering large expanses of land. Under these conditions CDMA provides extremely stable audio with few frame errors to mess things up. This is because Channel Pollution is almost unknown in these situations. Under similar conditions TDMA suffers too readily from interference and it will often blank the audio. Many people who use CDMA systems in sparsely populated areas have given this technology extremely high marks. TDMA systems also have great difficulties in open regions just outside densely populated areas. In this situation your phone is exposed to signals coming from countless sites in the densely populated areas, but there are no dominant signals from a close-by site. CDMA can suffer under these conditions too (due to channel pollution), but not quite so badly. Valleys don't present a big problem for TDMA, but high ground is a killer. You can experience choppiness in the audio even when your signal indicator is reading 2 or 3 bars. So in the end, can we really proclaim a winner in the CDMA vs TDMA war? For the time being I think not. Perhaps in the future when newer technologies built around the W-CDMA standard (wideband CDMA) come into existence, the issue will warrant another look. By that time, even GSM will have moved to CDMA as its air interface of choice, but don't let that fool you into believing that C.U.Shah College of Engg. & Tech. - 25 - Code Division Multiple Access they think the current TDMA air interface is inadequate for its purpose. Future standards are being built around high speed data. If you are presently in the market for a new phone, my advise to you is to ignore the hype surrounding the technologies and look at service provider instead. Judge each with an eye to price, phone choice, coverage, and reputation. Technology should play a very small roll in your choice. If you follow this advice, you'll probably be much happier with the phone and service you inevitably wind up with. C.U.Shah College of Engg. & Tech. - 26 - Code Division Multiple Access 10.ADVANTAGES OF CDMA No SIM card is required. Improved call quality: CDMA provides better and more consistent sound quality than systems based on other technologies. Enhanced privacy when compared to systems using other technologies. Increased talk time and standby time for mobiles. They are difficult to intercept for an unauthorized person. They are easily hidden. For an unauthorized person, it is difficult to ever detect their presence in many cases. They are resistant to jamming. Capacity increases of 8 to 10 times that of an AMPS Analog system, and 4 to 5 times GSM , because of CDMA’s unique spread spectrum technology. Many users can share the same carrier frequency, and without time-sharing. This means that mobile phone Service providers can handle more customers on a CDMA network than on a GSM network. Improved call quality, with better and more consistent sound , CDMA systems use precise power control—that is, the base station sends commands to every mobile phone currently involved in a call, turning down the power on the nearby ones, and increasing the power of those further away. The result is a nice, even noise level across the carrier, with lower overall power levels and no spiky interference. In this civilized atmosphere, each station can easily pick out its own coded data frames, decode them and deliver a clean end result. Dropped calls are minimized by CDMA's unique ability to keep every sector of every cell on the same frequency, so handoffs are "soft" as the mobile phone moves from one area to the next. (There is no hole in the signal as one cell is dropped and another is acquired.) CDMA decoders interpret constant sounds, such as road noise, as having no useful content, and ignore them as much as possible. Simplified system planning through the use of the same frequency in every sector of every cell. Other types of systems (analog, GSM, etc.) need to break up their frequency spectrum allotments so that each cell uses a different frequency. And since no two adjoining cells can use the same frequency, a given cell has to be surrounded by a circle of six other cells, all of which have to be on different frequencies. This translates to frequency re-use of only 1 in 7, and if you change one (by adding a cell for example), the effects ripple through the system. C.U.Shah College of Engg. & Tech. - 27 - Code Division Multiple Access To an eavesdropper, the call looks like unintelligible noise. CDMA was originally developed by the military for this very reason. CDMA providers have no such planning earaches, since every sector of every cell uses the same frequency. Enhanced Privacy is inherent in the way CDMA works. Each call is spread over the entire 1.25 MHz carrier—much wider bandwidth than is needed for a single call. The data bits used to convey real information are mixed with digital coding that is known only to the base station and the individual mobile phone. Improved coverage characteristics, allowing for the possibility of fewer cell sites This comes from the accurate power control of all mobile phones using the site, and the fact that individual sites don't interfere with each other, since they are all on the same frequency. C.U.Shah College of Engg. & Tech. - 28 - Code Division Multiple Access 11. DISADVANTAGE 11.1 COLLISIONS 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. 11.2 ROAMING Since most countries have chosen the GSM standard, “roaming” on CDMA is limited. 11.3 M-COMMERCE A CDMA doesn’t have a SIM card, which makes m-commerce difficult. C.U.Shah College of Engg. & Tech. - 29 - Code Division Multiple Access 12. CONCLUSION Where CDMA scores Where CDMA needs to scores 12.1 WHERE CDMA SCORES Voice Quality CDMA reduces background noise and cross talk, ensuring better voice quality, which is further enhanced by the microprocessors inside the phones. Call Security By design, CDMA is more secure against evasdropping. Talk Time A CDMA phone consumes very little power, and has a longer talk time. Bandwidth CDMA 2000 1x offers 144kbps, which makes it capable for multimedia tasks. Weight CDMA phones due to their low-power requirements can do with smaller-sized batteries, which decrease the overall weight of a CDMA phone. 12.2 WHERE CDMA NEEDS TO SCORE Roaming Since most countries have chosen the GSM standard, “roaming” on CDMA is limited. M-commerce A CDMA doesn’t have a SIM card, which makes m-commerce difficult. C.U.Shah College of Engg. & Tech. - 30 - Code Division Multiple Access BIBLIOGRAPHY REFERENCE BOOKS 1. CRESPO, P.M., HONIG, M.L., and SALEHI, J.A. : “Spread- Time Code-Division Multiple Access” IEEE Trans. On Commun., vol 43, pp. 2139- 2148, June 1995. 2. COMPUTER NETWORKS, 3rd Ed. - By Andrew S. Tanenbaum 3. VITERBI, A.J.: CDMA Principle of Spread Spectrum Communications, Reading ,MA: Addison-Wesley, 1995. Websites www.cdg.org www.umts.org www.palowireless.com www.ieee.org www.yahoo.com www.google.com C.U.Shah College of Engg. & Tech. - 31 -
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