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IS-95 Overview

CDMA, a spread-spectrum technology, revolutionized wireless communication by increasing system capacity and quality. Learn about its history, architecture, services, and modulation techniques. Explore CDMA's impact on cellular systems and key features in this comprehensive overview.

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IS-95 Overview

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  1. IS-95 Overview

  2. CDMA Overview • Code Division Multiple Access (CDMA) is a radically new concept in wireless communications. It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. • CDMA is a form of spread-spectrum, a family of digital communication techniques that have been used in military applications for many years. • Use of noise-like carrier waves • Bandwidths much wider than the bandwidth of traditional narrow band signal.

  3. CDMA Overview (Cont.) • Originally there were two motivations • Either to resist enemy efforts to jam the communications (anti-jam, or AJ). Or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). • It has a history that goes back to the early days of World War II. • CDMA for civilian mobile radio applications is was proposed theoretically in the late 1940's. • Commercial applications became possible because of two evolutionary developments of Integrated Circuit and effective access method.

  4. Market Coverage CDMA CDMA

  5. HLR VLR EIR AuC Network Architecture • Five Levels • Mobile Station (MS) • Base Station System(BSS) • Switching System(SS) • Global Positioning System • Control Network (HLR, VLR, EIR, AuC). Public Network GPS Switching System Base Station Base Station

  6. Public Network GPS Switching System Base Station Base Station Logical Architecture • Logical Channels • Pilot Channel • Sync Channel • Paging Channel • Access Channel

  7. Services & Capabilities • Multiple Access using the same Frequency band for multiple users. • Simplicity in Frequceny Planning and efficiency. Reuse factor of one with no cell sectorizaation. • Comprehensive telephone services (voice, text, and low speed data communication). Compatible with other protocols ISDN CODE TIME User n User 2 User 1 FREQUENCY CDMA:Spread Spectrum

  8. Services & Capabilities • CDMA frequency based is the same as AMPS • Channel Bandwidth is 1.25MHz • Modulation is based on QPSK and OQPSK • Channel Bite Rate is based on 1.2288 Mb/s • Channel data rate from 1200 b/s to 9600 b/s • Channel uplink frequency is 824 to 849MHz; down link is between 869 to 894 MHz; with frequency spacing of 45 MHz. • Simplicity in Frequceny Planning and efficiency. One (1) reuse factor with no cell sectorizaation. • Challenge is SIR… need to add more details here. • Compherise telephone services (voice, text, and low speed data communication). Compatible with other protocol sISDN

  9. Channel Structure

  10. Radio Link Overview • Channel Structure • Forward Channel • Reverse Channel • Different modulation techniques for uplink and down link • Two Stage Modulation • Variable Bit Rate • Reuse Factor of 1

  11. Bandwidth • Bandwidth of a CDMA channel is 1.23MHz which is equivalent to 41 contiguous AMPS Channels or about 20 AMPS channels per CDMA carrier • Can be used along side AMPS sharing the same FCC assigned spectrum by giving CDMA a section of the frequency

  12. Processing Gain • Early Implementations use R=9600 b/s for a Processing gain of 128 ch/b • Later Implementations use R=14400 b/s for a processing gain of 85.3 ch/b G = Processing Gain W = Switching Rate Ch/s R = Source Information Rate b/s

  13. Spectrum Efficiency • C = Conversation/base station • 29.8 conversations/cell ≤ C ≤ 111 conversations per cell

  14. PN Sequence • Signal is spread using a pseudorandom noise sequence • Low cross correlation • Feedback shift register with 42 stages • Long code repeats after 242-1 ch or 41.4 days

  15. Reverse Channel

  16. Reverse Channel Modulation • Offset Quadrature Phase Shift Keying (OQPSK) • Modulates in-phase and quadrature components of the carrier-frequency • Modulates at 1,228,800 ch/s and has a periodicity of 215 chips • Terminal synchronizes to base station sync channel

  17. Data Rates Reverse Channel • Frame Length 20 ms

  18. Reverse Channel Digital Modulation Digital Carrier Long Code Random Number Generator 42-Stage Shift Register Digital Logic 1.2288 Mch/s Long Code Mask Channel Identifier (42 bits) Convolutional Code 307,200 b/s 28,800 b/s Rate 1/3 Repeat Interleave 20 ms Walsh Modulator Digital Modulator variable rate 1.2288 Mch/s

  19. OQPSK I-Channel Binary sequence Period = 2/75 seconds In-Phase + 1.288 Mch/s Radio Signal Digital Modulator Radio Carrier Mixer Delay (1/2 Chip) + Quadrature Q-Channel Binary sequence Period =2/75 seconds

  20. Forward Channel

  21. Forward Link Radio TransmissionOverview • 1 Physical channel contains 64 chips • Quadrature Phase Shift Keying (QPSK) • Each basestation has 64 physical channels (One Row of Walsh Hadamard Matrix) numbered 0 to 63 • Base station reduces power to maintain energy per bit when operating at lower bit rates

  22. Data Rates Forward Channel • Frame Length 20 ms

  23. Forward Channel Digital Modulation (Paging and Forward Traffic) Digital Carrier Long Code Random Number Generator 42-Stage Shift Register Digital Logic Decimate 64:1 1.2288 Mch/s Long Code Mask 19.2 kb/s Channel Identifier (42 bits) 64-bit carrier Convolutional Code 19.2 kb/s 19.2 kb/s x Rate 1/2 Repeat Interleave 384 bits 20 ms Digital Modulator pseudorandom scrambling variable rate 1.2288 Mch/s

  24. Forward Channel Digital Modulation (Sync) 64-bit carrier Walsh 32 Convolutional Code 19.2 kb/s 2,400 b/s 4,800 b/s 4,800 b/s Rate 1/2 Repeat 2:1 Interleave 128 bits 26.67 ms Repeat 4:1 Digital Modulator 1.2288 Mch/s 1,200 b/s

  25. Forward Radio Modulation I-Channel Binary sequence Period = 2/75 seconds Base Station Time Delay PN_OFFSET In-Phase + 1.288 Mch/s Radio Signal Digital Modulator Radio Carrier Mixer + Quadrature Base Station Time Delay PN_OFFSET Q-Channel Binary sequence Period =2/75 seconds

  26. CDMA Logical Channels Pilot Sync Forward Variable-Bit-Rate User Information Paging Power Control Traffic Signaling Messages Access Reverse Variable-Bit-Rate User Information Traffic Signaling Messages

  27. Logical Channels Forward Link

  28. Pilot • Transmit a continuous sequence of 0s • Rate 1.2288 Mch/s • Used as references of carrier phase and timing • Also, used for Handoff signal indications

  29. Sync • Rate = 1,200 b/s ; Digital Carrier Walsh 32 • Transmits one Message • System Time • Time delay found in Pseudorandom sequence • Identifiers • Base station • Network

  30. Paging Channel • Up to 7 paging channels • Walsh 1 to Walsh 7 • Rate – 4,800 b/s or 9,600 b/s • Sleep mode

  31. Traffic Channels • Carries voice, control, and power data • Physical channel can change when moving through new cells

  32. Logical Channels Reverse Link

  33. Access Channels • A terminal without a call in progress uses the access channels to communicate with the base station • Originate call, respond to paging message, register location • Transmission rate is 4800 b/s • 88bit or 20 ms frames • Message length of 4-26 frames • Terminal will wait for acknowledgement and retransmit if necessary

  34. Traffic Channels • Carries both voice, control

  35. Messages Traffic Channel • Call Management Messages • Authentication and Privacy Messages • Radio Resource Management Messages • Mobility Management Messages • Operation, Administration, and Maintnence Messages

  36. Messages Paging/Access Channel • Broadcast Messages • Call Management Messages • Authentication and Privacy Messages • Mobility Management Messages • Operations, Administration, and Maintenance Messages

  37. QCELP • Qualcomm Code Excited Linear Prediction • 20ms speech frames • Variable bit rate speech encoder • 4 bit rates; 8,000 4,000 2,000 and 800 b/s

  38. Network Operations • Radio Resources Management • Power Control • Handoffs • Mobility Management • Authentication and Security • Operations and Maintenance

  39. Radio Resources Management • A CDMA system has to maintain a narrow range of received power levels. • Strong received signals interfere excessively with weaker signals. • For power control, IS-95 uses: • Open-loop operations • Closed-loop operations

  40. Power Control Open-loop power control: • Mobile unit measures strength of received pilot signal. • Strong signal = close to base station • Mobile transmits at low power. • Weak signal = far from base station • Mobile transmits at high power. • The power level of all received signals at the base • station should be equal. • Popen(dBm) = -Preceive(dBm) + Ptarget(dB) • Ptarget(dB) = -73 dB + Pcontrol(dB)

  41. Power Control Closed-loop power control: • Ptransmit (dBm) = Popen (dBm) + Pclosed (dB) • Pclosed (dB) = Pclosed (dB) + 1 dB, 0 received* • Pclosed (dB) = Pclosed (dB) - 1 dB, 1 received* • * Base station transmits either 0 or 1 in power • control subchannel, based on received • power level at base station.

  42. Soft Handoff • As mobile moves from cell to cell, • it communicates with both cells. • Handoffs are mobile initiated and switch controlled. • The switch makes handoff decisions and • assigns new physical channels. • Handoffs use the correlators of the RAKE • receiver to communicate with multiple • base stations at once.

  43. Soft Handoff Region Same Frequency Band Soft Handoff • Base station lists: • Active list • Candidate list • Neighbor list • Remaining list • Base stations are • assigned to a list • based on signal • strength.

  44. Switch Soft Handoff Old Base New Base Mobile conversation neighbor > threshold Pilot Strength Measurement Flow chart of a soft handoff: select channel at new base new active set Handoff Direction program correlators Handoff Complete conversation select signal active < threshold Pilot Strength Measurement Handoff Direction Handoff Complete release correlators conversation

  45. Other Handoffs • Analog Handoff • Moving to region without CDMA • Hard Handoff • Change frequency band • Change switch

  46. Mobility Management The mobile sends registration messages to the system. Registration messages help the system to locate the mobile when a call is directed to the mobile.

  47. Authentication and Security • Authentication and security in IS-95 are the same as in GSM. • Added security feature for IS-95: • Private Long Code Mask

  48. Operations and Maintenance • Signals in all cells cover a bandwidth of 1.23 MHz: • System is vulnerable to malfunctions • in mobile units. • A mobile unit radiating excessive power • causes interference to other users. • System prevents malfunctioning units from • accessing the system.

  49. IS-95 Conclusions • Uses CDMA technology: • Reduces cell reuse factor • Efficient bandwidth use • PN sequences to spread signal • Soft handoffs

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