Second Generation (2G) Cellular Dr. A. Chockalingam Assistant Professor Indian Institute of Science, Bangalore-12

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore2 2G Cellular Systems u GSM –European Digital Cellular Standard –TDMA Access Technology u IS-54/IS-136 –North American Digital Cellular Standard –TDMA Access Technology u IS-95A –North American Digital Cellular Standard –CDMA Access Technology

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore3 GSM u Global System for Mobile communications BTS MS BSC MSC PSTN ISDN Data Networks OMC HLR VLR AUC Base Station Subsystem (BSS) Network and Switching Subsystem (NSS) Public Networks 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore4 GSM - Interfaces BTS MS BSC MSC PSTN SS7 A Interface (standardized) Abis Interface (standardized) GSM Radio Air Interface (standardized)

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore5 GSM: Services & Features u Teleservices –standard mobile telephony, both mobile originated and mobile terminated u Data Services –computer to computer traffic (e.g., Async Data) –Digital Fax u Supplementary Services –Caller ID, Short Messaging Service (SMS) u Subscriber Identity Module (SIM) –user memory device to activate service from any GSM phone

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore6 GSM: Air Interface u TDMA access technology u 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) u 200 KHz RF carriers u 8 TDMA slots on each carrier (i.e., each 200 KHz carrier can support 8 simultaneous calls) u (25 MHz / 200 KHz) * 8 = 1000 traffic channels u 13 Kbps vocoder rate (half rate vocoder can double capacity)

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore7 GSM: Air Interface u Slow frequency hopping to mitigate fading effects u Channel data rate : Kbps u GMSK modulation with 0.3 BT product u 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)

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore8 TDMA Frame in GSM GSM TDMA Frame 577 microseconds (8 time slots = milliseconds) Time Slot 3

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore9 GSM: Normal Traffic Burst DATA 57 BITS (e.g. encoded voice ) Training Sequence FLAGFLAG FLAGFLAG TAILTAIL TAILTAIL 3 BITS1 BIT 3 BITS 148 BITS 26 BITS

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore10 GSM: Air Interface Downlink Control Signaling l Frequency Correction: Sends a frequency reference. l Synchronization: Sends a timing reference. l Broadcasting: Broadcasts general information about the Base Station. l Paging: Notifies the mobile of incoming calls. Assigns a traffic channel to a mobile. l Uplink Signaling Random Access Channel: used by mobiles to request a traffic channel

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore11 GSM: Channel Interaction l Mobile - Base Station Initialization u Mobile identifies a Frequency Correction burst on the Frequency Correction Channel. u Mobile synchronizes timing using a synchronization burst on the Synchronization Channel. u Mobile obtains general system information over the Broadcast channel. l Mobile - Base Station Communication u Mobile and Base communicate over Random Access and Paging channels when not involved in a call. u Mobile and Base communicate over Traffic channels while involved in a call.

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore12 IS-54 u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore13 IS-54 Radio Interface

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore14 IS-54 vs IS-136 u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore15 GSM / IS-54 Summary

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore16 IS-95 CDMA u Direct sequence spread spectrum signaling on reverse & forward links u Each channel occupies 1.25 MHz u Fixed chip rate Mcps u Variable user data rate - depends on voice activity u Universal frequency reuse u fast power control to overcome near-far problem u RAKE receiver to take advantage of multipath u Soft handoffs

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore17 CDMA Channels & Frequencies u CDMA frequencies assigned through a 11-bit CDMA Channel number, N u At Mobile MHz u At Base Station MHz

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore18 CDMA Channel & Frequency 1.25MHz 1.25MHz Reverse CDMA Channel Forward 45 MHz Frequency CDMAChannelFrequency MHz MHz

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore19 Frequency Reuse CDMA 7 cell Freq Reuse Plan Freq Reuse Plan in CDMA

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore20 Spreading Codes in IS-95 CDMA u 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 = c2 = c3 = c4 = »used to separate one user from another –PN codes are used on both forward and reverse (mobile-to-base) links

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore21 IS-95 CDMA Forward Link u Pilot Channel (Code Channel 0) –provides phase reference for coherent demodulation –pilot strength measurement for handoffs u Paging Channel (up to 7 channels - Code Channels 1 to 7) –sends control messages and page messages –Walsh Code Channels 1 through 7 u Sync Channel (Code Channel 32) –broadcasts system timing messages u Traffic Channel (up to 63 channels - remaining code channels) –supports variable data rates at 9600, 4800, 2400, or 1200 bps

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore22 Forward Link Channel Structure Forward CDMA Channel (1.25 MHz Chl. Tx by Base Stn Pilot Chl W0 Sync Chl W32 Paging Chl. 1 W1 Paging Chl. 7 W7 W8 W9 W63 Traffic Chl. 1 Traffic Chl. 2 Traffic Chl. 55 Traffic data Power Control Sub channel

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore23 FL Modulation Structure W0 W32 Pilot Chl: all 0’s Convol. Encoder/ Repetition Block Interleaver Mcps Sync Chl 1200 bps 4800 bps Convol. Encoder/ Repetition Block Interleaver 9600 bps 4800 bps 2400 bps 19.2 Kbps Mcps Wp Paging Chl Paging Chl p Long code Mask Long Code Generator Decimator Mcps 19.2 Kbps To Quadrature Spreading To Quadrature Spreading To Quadrature Spreading

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore24 FL Modulation Structure Convolutional Encoder and Repetition r=1/2, K = 9 User data Block Interleaver Long Code generator Decimator MUXMUX Baseband filter Baseband filter Long code for nth user Mcps 19.2 k Power Control Bit 800 Hz Walsh Code Symbol cover Scrambling I-Chl Pilot PN Seq Q-Chl Pilot PN Seq Forward CDMA Traffic Channel Structure 9600 bps 4800 bps 2400 bps 1200 bps Quadrature Spreader Note: Pilot PN Offset identifies the base station

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore25 FL Modulation Parameters Sync Channel

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore26 FL Modulation Parameters Paging Channel

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore27 FL Modulation Parameters Forward Traffic Channel

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore28 IS-95 CDMA Reverse Link Reverse CDMA Channel (1.25 MHz Chl. Rx by Base Stn Access Chl. 1 Access Chl. 2 Access Chl. n Traffic Chl. 1 Traffic Chl. 2 Traffic Chl. m Traffic Chl. 3 Addressed by long code PNs

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore29 IS-95 CDMA Reverse Link u 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 u Traffic Channels –identified by long distinct user 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore30 RL Modulation Structure Convolutional Encoder and Repetition r=1/3, K = 9 Information bit Block Interleaver Long Code generator Baseband filter Baseband filter Long code Mask for user n Mcps Zero offset Pilot PN Seq I Chl Reverse CDMA Traffic Channel Structure 9600 bps 4800 bps 2400 bps 1200 bps Code symbol 28.8 Ksps 64-1ry Orthogonal Modulator Zero offset Pilot PN Seq Q Chl Data burst randomizer Code symbol Walsh chip Kcps PN chip D 1/2 PN chiip delay=406.9 ns

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore31 RL Modulation Parameters Reverse Traffic Channel

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore32 Power Control u To combat the effect of fading, shadowing and distance losses u Transmit only the minimum required power to achieve a target link performance (e..g, FER) –Minimizes interference –Increases battery life u FL Power Control –To send enough power to reach users at cell edge u RL Power Control –To overcome “near-far” problem in DS-CDMA

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore33 Power Control u Types of Power Control –Open Loop Power Control –Closed loop Power Control u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore34 Power Control u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore35 Power Control in IS-95  At 900 MHz Carrier frequency and 120 km/h mobile speed, Doppler = 100 Hz u In IS-95A, closed loop power control is operated at 800 Hz update rate u Power control bits are punctured into the traffic data stream u Closed loop power control step size is +/- 1 dB u Power control bit errors do not affect performance much u Coding and interleaving has effect on CLPC performance u Both open (outer) and closed (inner) loops drive the transmit power to ensure a target FER of 1%

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore36 RAKE Receiver u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore37 Handoffs in IS-95 CDMA u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore38 Soft Handoff Architecture Mobile BSC BTS New Link Old Link R R MSC To other switch R R - Handoff request sent to the old cell on the degrading link Energy measurements are made at the mobile Switch Diversity: MSC selects the bit stream with lower error rate

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore39 Handoff Procedure u 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

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore40 Handoff Example Time Pilot Strength (1) T_ADD T_DROP (2) (3) (4) (5) (6) (7) Neighbor Set Candidate Set Active Set T_TDROP Neighbor Set

Dr. A. ChockalingamWW'99, Dept of ECE, IISc, Bangalore41 Handoff Example (..contd) (1) Pilot strength exceeds T_ADD. Mobile sends a Pilot Strength 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