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C.D.M.A

C.D.M.A. CONTENTS Principals of Spread spectrum technology. The codes used in CDMA The Channels used in CDMA Power controls used in CDMA CDMA 2000 1x & 3x 3G Systems. CDMA : Code Division Multiple Access

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C.D.M.A

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  1. C.D.M.A

  2. CONTENTS • Principals of Spread spectrum technology. • The codes used in CDMA • The Channels used in CDMA • Power controls used in CDMA • CDMA 2000 1x & 3x • 3G Systems

  3. CDMA : Code Division Multiple Access CDMA is a method in which users are combined in time and frequency and channelized by unique assigned codes.

  4. C.D.M.A • CDMA uses spread spectrum technology. • There are codes for the cells, codes for the sectors & also codes for the Channels . • The channels are separated & isolated from one another by different codes. • This technology is in use since 1995

  5. Features of S.S technology • Simplified Multiple Access, no Co ordination among users. • Selective address capability, chip code provides authentication. • Retreival of signals at the receiver end when S/N ratio is –15 to –25 dBs

  6. CDMA stations separated by unique code sequence. • It has ability to secure from eavesdropping. • Interference Rejection, SS Receiver suppresses NB interference. • The Receiver used in CDMA technology is called RAKE Receiver. • Rake receiver is the combination of 4to 6 Receiver fingers

  7. Multi-path Propagation in cellular System

  8. RAKE RECEIVER

  9. RAKE RECEIVER PROCESSING • Rake receiver Allows combined reception of multi paths • Provides search receiver to identify changes in path characteristics/new cells • Provides both path diversity and frequency diversity CDMA phones use rake receivers. • Each finger then demodulates the signal corresponding to a strong multi path. • The result are then combined together to make the signal stronger.

  10. TECHNICAL DATA • Freq. Of operation : 824-849 MHz 869-894MHZ • Duplexing Method : FDD • R F Spacing : 1..25 MHz • Coverage : 5 KMs with hand held Telephone. : 20 KMs with fixed units • Grade of service : 1 %

  11. Frequency Re-use arrangement in Cellular System

  12. E D F F C B E G B C C D A G F A D D B E F C E B B A G C Frequency Reuse of 7 (For FDMA)

  13. Frequency Reuse of 1 ( In CDMA) A A A A A A A A A A A A A A A A A A A A A A A A A A A A

  14. CODES used in CDMA WALSH CODE • 64 codes of 64 Bits Length. • Forward Traffic channel Codes. • All codes are orthogonal to each other. • These codes provide isolation between multiple signals Transmitted by Base station.

  15. Long Code • 242 Bits polynomials • Forward channels Data (Traffic and paging channels) scrambled. • Provides channelization for the reverse channels • This Code is unique for every user. • It is known as user address mask or user identification. • Subscribers are differentiated from one another as no two same codes are used.

  16. SHORT CODE • This PN sequence is based on 215 characteristics polynomial. • Differentiates cells and sectors. • Identifies cells and sectors. • Consists of codes for 1 to Q channels. • Each cell uses different codes.

  17. Forward Direction Code Channels Base Station to Mobile

  18. Reverse Direction Code Channels Mobile to Base Station (1) Traffic Channels (2) Access Channels

  19. PILOT CHANNEL • Has All Zeros. • Uses no WALSH CODE. • Used for receive level comparison for power control. • Gives O/H information for locking with sync. SYNC CHANNEL • Used during system Acquisition stage. • To be re-used when the systems is powered up again. i.e one time use during conversation.

  20. SYNC CHANNEL (Contd.) • After finding pilot carrier mobile switches to sync. Channel. • Sync channel uses predetermined Walsh code. • Initialises Power control loop. • Transmits all information needed to uplink • Transmits cell/sector ID

  21. PAGING CHANNEL • Means of communication between Base to Mobile station when traffic channel is not in use. • Paging CHL Data rates can be 2.4 , 4.8 or 9.6 KBPS. • Slotted paging also introduced. • CDMA assignment has 7 paging CHLS @ 180 pages per channel per second.

  22. ACCESS CHLS • Provides communication from mobile to base station when mobile is not using traffic channel. • Each access channel uses long PN code. • Base station responds to transmission on a particular Access CHL. • Base station replies on paging CHL. • Mobile responds to base station message by transmitting on Access CHL.

  23. BASE STATION TRANSCEIVER SUBSYSTEM TECHNICAL TR.. RECEIVER P S T N BASE STATION CONTROLLER PLUS SWITCH FAX BASE STATION TRANSCEIVER SUBSYSTEM SYSTEM ARCHITECTURECDMA

  24. S(t) S(n) E(n) A/D Speech Encoder CDMA Modem R/F Microphone S(t): input analogue speech S(n): input digitised speech blocked into 20 msec frames. E(n): Encoded packets representing parameters of speech S(n): reconstructed speech in the Digital format. S(t): reconstructed analogue speech Mobile Radio Channel S(n) E(n) S(t) D/A Speech Decoder CDMA Modem R/F Speaker TRANSMITTER RECEIVER

  25. VOCODER • Variable Rate Vocoder • Telephone quality speech is band limited to 4 KHz. • When digitising with PCM, bandwidth can rise to 64 KHz. Vo-coding can compress to a lower bit rate to reduce bandwidth. • Slight signal degradation occurs, but bandwidth required is reduced.

  26. Vocoders used in CDMA

  27. CDMA TRANSMITTER

  28. CDMA RECEIVER

  29. PROBLEMS IN RADIO PROPEGATION

  30. Power Control • Power is to be regulated & More power is provided to mobile who are (1) Far away from the base station. (2) In the location experiencing more difficult environment. (3) Mobiles coming under the shades of the high rise buildings.

  31. POWER CONTROL METHOD (Contd) • Provide high quality communication. • Minimise interference to other mobile stations. • More no. of mobiles are able to use the link without further degradation. • Reduces RF pollution & Provides satisfactory communication.

  32. Power Control Open loop Fast closed loop The system requires fast closed loop power control for Rayleigh fading, wide dynamic range open loop power control to handle variations in path losses in different location.

  33. POWER CONTROL METHODS • Subscribers use entire BW of 1.25 MHZ. • Frame freq. Of all subscribers is the same. • Subscribers separated by different codes. • Noise contributed is added up at the receive direction

  34. It results in interference. • To minimise the interference, very powerful power control methods are adopted. • CDMA IS-95 uses power controls in the forward & reverse directions

  35. Reverse link • (1) Open loop power control. • (2) Closed loop power control. • In the forward link there is only one power control i.e Forward link power control. • Reverse link open loop P.C. • Mobile adjusts its trans power according to change in its received power from the base station. • Ensures same signal strength at B.S from different mobiles.

  36. CLOSED LOOP POWER CONTROL • Base station provides rapid correction to mobile. • B.S measures received signal strength from mobiles. • Compares with the threshold value .

  37. Takes decision whether to increase or decrease the power. • Commands the mobile to adjust the o/p power accordingly. • Freq. 800 HZ per sec or after every 1.25ms .

  38. FORWARD LINK POWER CONTROL • Power measurement is provided by the mobile station. • Cell adjusts its power in the forward link for the subscriber.

  39. Call processing in CDMA

  40. (a)Initialization State • Mobile acquires System via Pilot code channel. • Mobile synchronizes with system via Sync code channel. • (b) Idle State • Mobile gets the overhead information via the paging channel • Paging messages are received when traffic channel is not in use. • (c)Access state • Mobile accesses the network via Access CHL

  41. Mobile responds to base Station on Access channel. This process continues till the traffic Channel is used Types of calls (1) Land to mobile call. (2)Mobile to land Call. (3) Mobile to Mobile Call

  42. Hand off arrangement in CDMA In CDMA we have soft hand off arrangement ( i.e make before break arrangement)

  43. Advantages of CDMA These advantages are given as 6 C’s i.e (1) Coverage. (2) Capacity. (3) Clarity (4) Cost (5) Compatibility (6) Customer Satisfaction

  44. COVERAGE

  45. CAPACITY • CDMA system Capacity is much higher than FDMA or TDMA • We have 6 Calls per sector in FDMA , 18 Calls per sector in TDMA where as in CDMA IS-95 we have 22 calls per sector & 45 calls per sector when WLL technology is used

  46. CLARITY • CDMA Messages are more clear than FDNA or TDMA • This is due to Rake Receiver, Variable rate vocoder, Soft handoff, Power control,Wide band Signal, Encoding & interleaving

  47. Cost comparison. CDMA technology is made cheaper due to large Cell coverage , more capacity &less no of cell sites involved

  48. CDMA 2000 FAMILY Evolution from IS-95 to CDMA 20001x Ev

  49. CDMA 2000 represents • (1) CDMA 2000 1x • (2) CDMA 20001X EV • CDMA 2000 1x doubles the voice capacity of cdmaOne network. & delivers peak DATA at 307.2 Kbps in mobile environment • CDMA 2000 1x EV DO delivers peak Data speed of 2.4 Mbps. • CDMA 20001x EVDV provides voice & simultaneous high speed packet data service upto 3.09 Mbps

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