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IS95 CDMA Technology
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IS95 CDMA Technology

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Brief overview of CDMA tech..!!

Brief overview of CDMA tech..!!

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  • 1. CDMA Technology & IS-95
  • 2. What is CDMA Both an access method and air-interface Rest of the network is very similar Radio resource management, mobility management, security are similar Power control and handoffs are different Uses DSSS and ECC Frequency reuse factor is 1 3 systems IS-95 2G, W-CDMA, and CDMA2000
  • 3. Advantages of CDMA Cellular Higher capacity Improves voice quality (new coder) Soft-handoffs Less power consumption (6-7 mW) Choice for 3G systems
  • 4. Advantages of CDMA Cellular Frequency diversity – frequency-dependent transmission impairments have less effect on signal Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals Graceful degradation – system only gradually degrades as more users access the system
  • 5. Drawbacks of CDMA Cellular Self-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes
  • 6. Drawbacks of CDMA Cellular Air-interface is the most complex Not symmetrical (unlike TDMA) Forward and reverse channels are different Forward channel (1 Many) synchronized Forward channel uses orthogonal spreading codes Reverse channel transmissions are not synchronized Orthogonal codes are used for orthogonal waveform coding
  • 7. Mobile Wireless CDMA DesignConsiderations RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them This method achieves better performance than simply recovering dominant signal and treating remaining signals as noise Soft Handoff – mobile station temporarily connected to more than one base station simultaneously
  • 8. Principle of RAKE Receiver
  • 9. IS-95 CDMA Forward Channel The forward link uses the same frequency spectrum as AMPS (824-849 Mhz) Each carrier 1.25MHz 4 types of logical channel: A pilot, a synchronization, 7 paging, and 55 traffic channels Channels are separated using different spreading codes QPSK is the modulation scheme Orthogonal Walsh codes are used (64 total) After orthogonal codes, they are further spread by short PN spreading codes Short PN spreading codes are M sequences generated by LFSRs of length 15 with a period of 32768 chips.
  • 10. Forward channel-2 Why we have two spreading codes? The orthogonal codes are used to differentiate between the transmissions within a cell The PN spreading codes are used to isolate different cells (BSs) that are using the same frequencies. The same PN sequence is used in all BSs. The offset for each BS is different. Of course, this requires synchronization Synchronization is achieved by GPS.
  • 11. One Forward CDMA Link, 1.25 MHz in the 824 – 849 MHz bands PCH PCH Code Code Code Code CodePilot Synch 1 7 1 N P S 55W0 W32 W1 W7 W8 W63 Fundamental Mobile Power Fundamental Mobile Power Supplementary Code Channel Control Code Channel Control Code Channel Data Subchannel Data Subchannel Data Figure 8.4: IS-95 Forward Channel
  • 12. I Pilot PN at Walsh Code 1.288 Mcps 1.2288 Baseband Filter Channel McpsDependent Symbols Baseband Filter Q Pilot PN at 1.288 Mcps Figure 8.5: Basic Spreading Procedure on the Forward Channel in IS-95
  • 13. The pilot channel Provide a reference signal for all MSs that provides the phase reference for COHERENT demodulation 4-6 dB stronger than all other channels Used to lock onto other channels Obtained using all zero Walsh code; i.e., contains no information except the RF carrier Spread using the PN spreading code to identify the BS. (512 different BS*64 offsets) No power control in the pilot channel
  • 14. I Pilot PN at Walsh Code W0 1.2288 Mcps 1.2288 Baseband Filter All 0s Mcps To QPSK Modulator Baseband Filter Q Pilot PN at 1.2288 Mcps (a) I Pilot PN at Walsh Code W32 1.2288 Mcps SynchChannel 1.2288 BBF 2.4 4.8Message Convolutional ksps Symbol ksps Block Mcps Encoder Repetition Interleaver1.2 ksps 4.8 Code Modulated ksps BBF Rate 1/2 Symbol Symbol (b) Q Pilot PN at 1.2288 Mcps Figure 8.6: (a) Pilot and (b) Sync Channel Processing in IS -95
  • 15. Sync channel Used to acquire initial time synchronization Synch message includes system ID (SID), network ID (NID), the offset of the PN short code, the state of the PN-long code, and the paging channel data rate (4.8/9.6 Kbps) Uses W32 for spreading Operates at 1200 bps
  • 16. I Pilot PN at Walsh Code W0 1.2288 Mcps 1.2288 Baseband Filter All 0s Mcps To QPSK Modulator Baseband Filter Q Pilot PN at 1.2288 Mcps (a) I Pilot PN at Walsh Code W32 1.2288 Mcps SynchChannel 1.2288 BBF 2.4 4.8Message Convolutional ksps Symbol ksps Block Mcps Encoder Repetition Interleaver1.2 ksps 4.8 Code Modulated ksps BBF Rate 1/2 Symbol Symbol (b) Q Pilot PN at 1.2288 Mcps Figure 8.6: (a) Pilot and (b) Sync Channel Processing in IS -95
  • 17. Paging channels Used to page the MS in case of an incoming call, or to carry the control messages for call set up Uses W1-W7 There is no power control Additionally scrambled by PN long code, which is generated by LFSR of length 42 The rate 4.8 Kbps or 9.6Kbps
  • 18. I Pilot PN at Walsh Code W1-7 1.2288 Mcps Paging 19.2 1.2288 BBFChannel 9.6 or 19.2 19.2Message ksps ksps ksps Mcps Convolutional Symbol Block Encoder Repetition Interleaver4.8 or 9.6 Code ksps Symbol Modulated BBF Rate 1/2 Symbol 19.2 64:1 ksps Long Code Mask Long Code Long Code Q Pilot PN at For Paging Channel Generator Decimator 1.2288 Mcps 1.2288 Mcps Figure 8.7: Paging Channel Processing in IS -95
  • 19. The traffic channels Carry user information Two possible date rates RS1={9.6, 4.8, 2.4, 1.2 Kbps} RS2={14.4, 7.2, 3.6, 1.8 Kbps} RS1 is mandatory for IS-95, but support for RS2 is optional Also carry power control bits for the reverse channel
  • 20. Rate 1/2 Voice Traffic Convolutional I Pilot PN at Encoder Power Walsh Code Wi 1.2288 Mcps Control Bits 800 bps 19.2 1.2288 BBF ksps Mcps Symbol Block Repetition Interleaver MUX 19.2 ksps BBF 19.2 ksps 800 bps Q Pilot PN at 64:1 24:1 1.2288 McpsLong Code Mask Long Code Long Code Long Code Generator Decimator Decimator 1.2288 Mcps Figure 8.8: Forward Traffic Channel Processing in IS –95 (Rate Set 1)
  • 21. Rate 1/2 Voice Traffic Convolutional Symbol Encoder Repetition I Pilot PN at Power Walsh Code Wi 1.2288 Mcps Control Bits 800 bps 19.2 1.2288 BBF ksps Mcps Puncture 2 of Block Every 6 inputs Interleaver MUX 19.2 ksps BBF 19.2 ksps 800 bps Q Pilot PN at 64:1 24:1 1.2288 McpsLong Code Mask Long Code Long Code Long Code Generator Decimator Decimator 1.2288 Mcps Figure 8.9: Forward Traffic Channel Processing in IS –95 (Rate Set 2)
  • 22. IS-95 CDMA Reverse Channel Fundamentally different from the forward channels Uses OQPSK for power efficiency QPSK demodulation is easy 869-894 MHz range. No spreading of the data using orthogonal codes Same orthogonal codes are used for WAVEFORM encoding Two types of logical channels: The access channels and the reverse traffic channels
  • 23. One Reverse CDMA Link, 1.25 MHz in the 869 – 894 MHzAccess Access Access Access Traffic TrafficChannel Channel Channel Channel Channel Channel PCH1 1 PHCP PHCP 1 T Fundamental Supplementary Supplementary Supplementary Supplementary Code Channel Code Channel Code Channel Code Channel Code Channel Data Data Data Data Data Figure 8.10: IS-95 Reverse Channel
  • 24. 000 001 010 011 100 101 110 111W0 W4W1 W5W2 W6W3 W7 Figure 8.11: Mapping data bits to Walsh encoded symbols
  • 25. I Pilot PN at 1.2288 McpsAccessMessage 14.4 28.8 1.2288 BBF ksps ksps 64-ary Mcps Convolutional Symbol Block Orthogonal Encoder Repetition Interleaver4.8 ksps Modulator Rate 1/3 BBF Long Code Mask Long Code Q Pilot PN at Generator 1.2288 Mcps Figure 8.12: Access Channel Processing in IS-95