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Second Generation (2G) Cellular

Second Generation (2G) Cellular. Dr. A. Chockalingam Assistant Professor Indian Institute of Science, Bangalore-12 achockal@ece.ucsd.edu http://www.ece.iisc.ernet.in/~achockal. 2G Cellular Systems. GSM European Digital Cellular Standard TDMA Access Technology IS-54/IS-136

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Second Generation (2G) Cellular

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  1. Second Generation (2G) Cellular Dr. A. Chockalingam Assistant Professor Indian Institute of Science, Bangalore-12 achockal@ece.ucsd.edu http://www.ece.iisc.ernet.in/~achockal

  2. 2G Cellular Systems • GSM • European Digital Cellular Standard • TDMA Access Technology • IS-54/IS-136 • North American Digital Cellular Standard • TDMA Access Technology • IS-95A • North American Digital Cellular Standard • CDMA Access Technology WW'99, Dept of ECE, IISc, Bangalore

  3. GSM • Global System for Mobile communications OMC BTS BSC PSTN MSC BTS MS ISDN BTS Data Networks BSC AUC HLR VLR BTS Public Networks Network and Switching Subsystem (NSS) BTS MS: Mobile Station MSC: Mobile Switching Center BTS: Base Transceiver Station HLR: Home Location Register BSC: Base Station Controller VLR: Visitor Location Register AUC: Authentication Center OMC: Operation Maintenance Center MS Base Station Subsystem (BSS) WW'99, Dept of ECE, IISc, Bangalore

  4. GSM - Interfaces BTS MSC BSC PSTN BTS MS A Interface (standardized) BTS SS7 Abis Interface (standardized) GSM Radio Air Interface (standardized) WW'99, Dept of ECE, IISc, Bangalore

  5. GSM: Services & Features • Teleservices • standard mobile telephony, both mobile originated and mobile terminated • Data Services • computer to computer traffic (e.g., Async Data) • Digital Fax • Supplementary Services • Caller ID, Short Messaging Service (SMS) • Subscriber Identity Module (SIM) • user memory device to activate service from any GSM phone WW'99, Dept of ECE, IISc, Bangalore

  6. GSM: Air Interface • TDMA access technology • 25 MHz BW spectrum on both directions • 890 to 915 MHz Reverse link (mobile-to-base) • 935 to 960 MHz Forward link (base-to-mobile) • 200 KHz RF carriers • 8 TDMA slots on each carrier (i.e., each 200 KHz carrier can support 8 simultaneous calls) • (25 MHz / 200 KHz) * 8 = 1000 traffic channels • 13 Kbps vocoder rate (half rate vocoder can double capacity) WW'99, Dept of ECE, IISc, Bangalore

  7. GSM: Air Interface • Slow frequency hopping to mitigate fading effects • Channel data rate : 270.833 Kbps • GMSK modulation with 0.3 BT product • Channel types • Traffic Channels (TCH) - carry traffic signals • Full Rate TCH, Half Rate TCH • Control Channels (CCH) - carry call control signals • Broadcast Channel (BCH) • Common Control Channel (CCH): Paging and Random Access Channels • Dedicated Control Channel (DCCH) WW'99, Dept of ECE, IISc, Bangalore

  8. TDMA Frame in GSM Time Slot 577 microseconds 0 1 2 3 4 5 6 7 GSM TDMA Frame (8 time slots = 4.615 milliseconds) WW'99, Dept of ECE, IISc, Bangalore

  9. GSM: Normal Traffic Burst 0 1 2 3 4 5 6 7 148 BITS T A I L F L A G T A I L F L A G DATA DATA Training Sequence (e.g. encoded voice) (e.g. encoded voice) 26 BITS 57 BITS 57 BITS 3 BITS 1 BIT 1 BIT 3 BITS WW'99, Dept of ECE, IISc, Bangalore

  10. GSM: Air Interface • Downlink Control Signaling • Frequency Correction: Sends a frequency reference. • Synchronization: Sends a timing reference. • Broadcasting: Broadcasts general information about the Base Station. • Paging: Notifies the mobile of incoming calls. Assigns a traffic channel to a mobile. • Uplink Signaling • Random Access Channel: used by mobiles to request a traffic channel WW'99, Dept of ECE, IISc, Bangalore

  11. GSM: Channel Interaction • Mobile - Base Station Initialization • Mobile identifies a Frequency Correction burst on the Frequency Correction Channel. • Mobile synchronizes timing using a synchronization burst on the Synchronization Channel. • Mobile obtains general system information over the Broadcast channel. • Mobile - Base Station Communication • Mobile and Base communicate over Random Accessand Pagingchannels when not involved in a call. • Mobile and Base communicate over Traffic channels while involved in a call. WW'99, Dept of ECE, IISc, Bangalore

  12. IS-54 • IS-54 • also known as “Digital AMPS” (D-AMPS) • upgrade AMPS analog technology to a digital technology • same spectrum and frequency spacing (30 KHz) like AMPS • supports 3 or 6 users on a single single 30 KHz carrier using TDMA scheme with 6 slots • control channels are identical to analog AMPS control channels, but twice as many control channels as AMPS WW'99, Dept of ECE, IISc, Bangalore

  13. IS-54 Radio Interface WW'99, Dept of ECE, IISc, Bangalore

  14. IS-54 vs IS-136 • IS-54 vs IS-136 • IS-54 uses 10 Kbps FSK modulated control channels • IS-136 uses 48.6 Kbps digital modulated control channels • IS-136 does not support 10 Kbps FSK control channel. • So IS-136 user terminals are not compatible with IS-54 • IS-136 provides a host of new features and services, including • short messaging capabilities • private user group features (suited for wireless PBX and paging applications) • “Sleep Mode” to conserve battery power WW'99, Dept of ECE, IISc, Bangalore

  15. GSM / IS-54 Summary WW'99, Dept of ECE, IISc, Bangalore

  16. IS-95 CDMA • Direct sequence spread spectrum signaling on reverse & forward links • Each channel occupies 1.25 MHz • Fixed chip rate 1.2288 Mcps • Variable user data rate - depends on voice activity • Universal frequency reuse • fast power control to overcome near-far problem • RAKE receiver to take advantage of multipath • Soft handoffs WW'99, Dept of ECE, IISc, Bangalore

  17. CDMA Channels & Frequencies • CDMA frequencies assigned through a 11-bit CDMA Channel number, N • At Mobile MHz MHz • At Base Station MHz MHz WW'99, Dept of ECE, IISc, Bangalore

  18. CDMA Channel & Frequency Reverse CDMA Channel Forward CDMA Channel 1.25MHz 1.25MHz Frequency CDMA Channel Frequency 847.74 MHz 45 MHz 892.74 MHz WW'99, Dept of ECE, IISc, Bangalore

  19. A A A D F E A A A A F B E C A A A C B G A A A A A C G D A A A A D F A A A A D B F E A A A E C B A A A A A C B G Frequency Reuse CDMA • Freq Reuse Plan in CDMA 7 cell Freq Reuse Plan WW'99, Dept of ECE, IISc, Bangalore

  20. Spreading Codes in IS-95 CDMA • Two types of spreading codes are used in IS-95 • Walsh codes of length 64 are used on the forward link (base-to-mobile link) e.g., c1 = 0 0 0 0 c2 = 0 1 0 1 c3 = 0 0 1 1 c4 = 0 1 1 0 • used to separate one user from another • PN codes are used on both forward and reverse (mobile-to-base) links WW'99, Dept of ECE, IISc, Bangalore

  21. IS-95 CDMA Forward Link • Pilot Channel(Code Channel 0) • provides phase reference for coherent demodulation • pilot strength measurement for handoffs • Paging Channel(up to 7 channels - Code Channels 1 to 7) • sends control messages and page messages • Walsh Code Channels 1 through 7 • Sync Channel(Code Channel 32) • broadcasts system timing messages • Traffic Channel(up to 63 channels - remaining code channels) • supports variable data rates at 9600, 4800, 2400, or 1200 bps WW'99, Dept of ECE, IISc, Bangalore

  22. Forward Link Channel Structure Forward CDMA Channel (1.25 MHz Chl. Tx by Base Stn Paging Chl. 7 Traffic Chl. 1 Traffic Chl. 55 Paging Chl. 1 Traffic Chl. 2 Sync Chl Pilot Chl W8 W0 W7 W63 W1 W9 W32 Power Control Sub channel Traffic data WW'99, Dept of ECE, IISc, Bangalore

  23. FL Modulation Structure W0 1.2288 Mcps To Quadrature Spreading Pilot Chl: all 0’s W32 Sync Chl To Quadrature Spreading 1.2288 Mcps 1200 bps 4800 bps Block Interleaver Convol. Encoder/ Repetition Wp To Quadrature Spreading 1.2288 Mcps Paging Chl 19.2 Kbps Block Interleaver Convol. Encoder/ Repetition 9600 bps 4800 bps 2400 bps 1.2288 Mcps 19.2 Kbps Long Code Generator Paging Chl p Long code Mask Decimator WW'99, Dept of ECE, IISc, Bangalore

  24. FL Modulation Structure Power Control Bit Walsh Code M U X Convolutional Encoder and Repetition r=1/2, K = 9 19.2 k User data 1.2288 Mcps Block Interleaver 9600 bps 4800 bps 2400 bps 1200 bps Symbol cover Scrambling Quadrature Spreader 800 Hz Long Code generator Decimator Decimator Q-Chl Pilot PN Seq I-Chl Pilot PN Seq 1.2288 Mcps Baseband filter Baseband filter Long code for nth user Forward CDMA Traffic Channel Structure Note: Pilot PN Offset identifies the base station WW'99, Dept of ECE, IISc, Bangalore

  25. FL Modulation Parameters Sync Channel WW'99, Dept of ECE, IISc, Bangalore

  26. FL Modulation Parameters Paging Channel WW'99, Dept of ECE, IISc, Bangalore

  27. FL Modulation Parameters Forward Traffic Channel WW'99, Dept of ECE, IISc, Bangalore

  28. IS-95 CDMA Reverse Link Reverse CDMA Channel (1.25 MHz Chl. Rx by Base Stn Traffic Chl. m Access Chl. 2 Traffic Chl. 1 Access Chl. 1 Access Chl. n Traffic Chl. 3 Traffic Chl. 2 Addressed by long code PNs WW'99, Dept of ECE, IISc, Bangalore

  29. IS-95 CDMA Reverse Link • Access Channels • enables mobile to communicate non-traffic information (e.g., call request) in random access mode • fixed data rate at 4.8 kbps • identified by a distinct access channel long code sequence offset • a paging channel number is associated with access channel • Traffic Channels • identified by long distinctuser code offset • data rate 9.6, 4.8, 2.4, 1.2 Kbps • data is convolutionally encoded, block interleaved, 64-ary orthogonal modulated, and direct sequence spread before transmission WW'99, Dept of ECE, IISc, Bangalore

  30. RL Modulation Structure Long code Mask for user n Long Code generator 1.2288 Mcps PN chip Code symbol Walsh chip Code symbol Information bit Zero offset Pilot PN Seq Q Chl Convolutional Encoder and Repetition r=1/3, K = 9 64-1ry Orthogonal Modulator Data burst randomizer Block Interleaver 9600 bps 4800 bps 2400 bps 1200 bps Zero offset Pilot PN Seq I Chl 28.8 Ksps 307.2 Kcps 1/2 PN chiip delay=406.9 ns D Baseband filter Baseband filter Reverse CDMA Traffic Channel Structure WW'99, Dept of ECE, IISc, Bangalore

  31. RL Modulation Parameters Reverse Traffic Channel WW'99, Dept of ECE, IISc, Bangalore

  32. Power Control • To combat the effect of fading, shadowing and distance losses • Transmit only the minimum required power to achieve a target link performance (e..g, FER) • Minimizes interference • Increases battery life • FL Power Control • To send enough power to reach users at cell edge • RL Power Control • To overcome “near-far” problem in DS-CDMA WW'99, Dept of ECE, IISc, Bangalore

  33. Power Control • Types of Power Control • Open Loop Power Control • Closed loop Power Control • Open Loop Power Control (on FL) • Channel state on the FL is estimated by mobile • RL Transmit power made proportional to FL channel Loss • Works well if FL and RL are highly correlated • which is generally true for slowly varying distance and shadow losses • but not true with fast multipath Rayleigh fading • So open loop power control can effectively compensate for distance and shadow losses, and not for multipath fading WW'99, Dept of ECE, IISc, Bangalore

  34. Power Control • Closed Loop Power Control (on RL) • Base station measures the received power • Compares it with the desired received power (target Eb/No) • Sends up or down command to mobile asking it to increase or decrease the transmit power • Must be performed fast enough a rate (approx. 10 times the max. Doppler BW) to track multipath fading • Propagation and processing delays are critical to loop performance WW'99, Dept of ECE, IISc, Bangalore

  35. Power Control in IS-95 • At 900 MHz Carrier frequencyand 120 km/h mobile speed, Doppler = 100 Hz • In IS-95A, closed loop power control is operated at 800 Hz update rate • Power control bits are punctured into the traffic data stream • Closed loop power control step size is +/- 1 dB • Power control bit errors do not affect performance much • Coding and interleaving has effect on CLPC performance • Both open (outer) and closed (inner) loops drive the transmit power to ensure a target FER of 1% WW'99, Dept of ECE, IISc, Bangalore

  36. RAKE Receiver • 4 RAKE fingers are used in the Mobile Receiver • 3 fingers for tracking and demodulating multipath components of the FL CDMA channel • 1 finger is used for searching and estimating the signal strength on different pilots • used to select the desired (strongest) base station in idle mode • for generating pilot strength information messages during traffic mode to enable Handoff WW'99, Dept of ECE, IISc, Bangalore

  37. Handoffs in IS-95 CDMA • Types of Handoff • Soft Handoff • Mobile commences commun with a new base station without interrupting commun with old base station • same freq assignment between old and new base station • provides different site selection diversity • Softer handoff • Handoffs between sectors in a cell • CDMA-to-CDMA Hard Handoff • Mobile transmits between two base stations with different frequency assignment WW'99, Dept of ECE, IISc, Bangalore

  38. Soft Handoff Architecture Switch Diversity: MSC selects the bit stream with lower error rate To other switch MSC R BSC BSC R R BTS BTS BTS BTS Old Link New Link R - Handoff request sent to the old cell on the degrading link Energy measurements are made at the mobile Mobile WW'99, Dept of ECE, IISc, Bangalore

  39. Handoff Procedure • Pilot Sets • Active Set • Pilot associated with FL traffic channels assigned to the mobile • Candidate Set • Pilots that are not in Active Set but are received by the mobile with sufficient strength • Neighbor Set • Pilots not in Active or Candidate Set but are likely candidates for handoff • Remaining Set • Set in the current system on current freq assignment, excluding the above 3 sets WW'99, Dept of ECE, IISc, Bangalore

  40. Handoff Example Pilot Strength T_ADD T_DROP (7) (6) (5) (1) (4) (2) (3) Time Neighbor Set Neighbor Set Candidate Set Active Set T_TDROP WW'99, Dept of ECE, IISc, Bangalore

  41. Handoff Example (..contd) (1) Pilot strength exceeds T_ADD. Mobile sends a PilotStrength Measurement Message (PSMM) to base station and transfers pilot to the Candidate Set (2) Base station sends a Handoff Direction Message (HDM) (3) Mobile transfers pilot to Active Set and sends s Handoff Completion Message (HCM) (4) Pilot strength drops below T_DROP. Mobile starts handoff drop timer (5) Handoff drop timer expires. Mobile sends a PSMM (6) Base station sends a HDM (7) Mobile moves pilot from Active Set to Neighbor Set and sends a HCM WW'99, Dept of ECE, IISc, Bangalore

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