Chapter 8: GSM & CDAMA Systems

Chapter 8: GSM & CDAMA Systems Global System for Mobile Communication (GSM) Second Generation (Digital) Cellular System Operated in 900 MHz band GSM is also operated in 1800 MHz band and this version of GSM is better known as DCS 1800 Because of excessive crowding in the above bands, GSM has also started to operate in 2 GHz band GSM Services Telephone Services Bearer Services or Data Services Packet Switched Protocols Data rate: 300 bps to 9.6 kbps Supplementary ISDN Services Call Diversion Caller ID Closed User Groups SMS Call Restrictions etc. GSM Features Subscriber Identification Module (SIM) On-the-air privacy Memory Device that stores information such as the subscriber s identification number, the networks and countries where the subscriber is entitled to service, privacy keys, and other user- specific information SIM gives subscriber units their identity Encryption of bit stream transmitted by GSM transmitter Secret cryptographic key that changes with time for each user Unlike analog FM cellular phone systems, virtually impossible to monitor and listen other s conversation 1

GSM System Architecture Three major interconnected subsystems: Base Station Subsystem (BSS) (includes MS) Network and Switching Subsystem (NSS) Operation Support Subsystem (OSS) BSS Provides and manages radio transmission paths between MS and MSC Consists of many BSCs connected to MSC Many BTS connected to each BSC Handoff between two BTSs under same BSC is conducted by the BSC, MSC informed Digital Signal Processing Trans coding and rate adaptation Channel coding and decoding Frequency hopping and power control Radio Resource Control Configuration, Selection, Allocation, De-allocation, Monitoring, Encryption of radio channels NSS Switching of GSM calls Connects MSC with other networks: PSTN, ISDN, and other MSC etc. MSC is the central unit Contains three different databases: Home Location Register( HLR) Visitor Location Register( VLR) Authentication Center( AuC) 2

HLR Functions of MSC Call setup, supervision and release Call routing Billing Information Collection Mobility Management Network Registration Location updating Inter BSS and Inter MSC handovers Manage connections to BSS, other MSCs and other networks (PSTN, ISDN etc.) Contains identities of mobile subscribers IMSI( International Mobile Subscriber Identity) SSI( Service Subscription Information) Service Restrictions (if any) Supplementary Services Billing/Accounting Information Location Information VLR AuC Temporarily stores certain information of roaming users IMSI Temporary Mobile Subscriber Identity Location of registration Once registered in the VLR, certain data from HLR is copied to the VLR for proper routing of the roaming user s call Strongly Protected Database Maintains authentication and encryption keys for each subscriber in the HLR and VLR Equipment Identity Register( EIR) International Mobile Equipment Identity( IMEI) May be valid, suspect or fraudulent OSS Contains several Operation Maintenance Centers( OMCs) Monitoring and maintaining of BSS and MSC units Three functions:- Maintain telecom hardwares and networks Manage charging and billing procedures Manage all mobile equipments in the system 3

GSM Radio Subsystem Originally employed in 900 MHz band but nowadays used in many bands Uplink ( Reverse Link) : 890-915 MHz ( 25 MHz band) Downlink ( Forward Link) : 935-960 MHz (25 MHz band) A 20 MHz guard band : 915-935 MHz Tx-Rx freq spacing : 45MHz Tx-Rx Time spacing : 3 TS Available forward and reverse bands divided into 200 KHz channels :ARFCN ( Absolute Radio Frequency Channel Numbers) Each ARFCN time shared by as many as 8 users A certain time slot of certain ARFCN forms the physical channel Radio transmission rate( Modulation data rate) : 270.833 kbps Effective channel transmission rate per user: 270.833/8 = 33.854 Actual user data rate considering overhead( maximum): 24.7 kbps Bit duration: 3.692 µs Modulation Scheme : BT=0.3 GMSK Each TS has an equivalent time allocation of 156.25 bits Duration of each TS= 576.92 µs Frame duration= 4.615 ms Total no. of channel within a 25 MHz band With no guard band= 125 With guard band of 100 KHz on either side= 124 GSM Channel Types Two types: Traffic Channels ( TCH) Control Channels (CCH) Traffic Channels carry digitally encoded speech or user data TCH have similar format in both forward and reverse link Some CCH may just be defined for forward or reverse link Control Channels carry signaling and synchronizing information between the BS and the MS GSM Traffic Channels (TCHs) Full rate TCH User data transmitted in one TS per frame Half rate TCH User data transmitted in same TS in alternate frames TCH data not sent in TS0 in certain ARFCN Frames of TCH data broken every 13th frame by either Slow associated control channel( SACCH) or idle frames Each group of 26 frames forms a Speech Multiframe 4

Full Rate TCH Full-Rate Speech Channel( TCH/FS) Carries user speech digitized at a data rate of 13 kbps After channel coding 22.8 kbps Full Rate Data Channel for 9600 bps( TCH/F9.6) Carries user data sent at 9600 bps After error correction coding, user data sent at 22.8 kbps Full Rate Data Channel for 4800 bps( TCH/F4.8) Carries user data sent at 4800 bps After error correction coding, user data sent at 22.8 kbps Full Rate Data Channel for 2400 bps( TCH/F2.4) Carries user data sent at 2400 bps After error correction coding, user data sent at 22.8 kbps Half-Rate TCH Half-Rate Speech Channel( TCH/HS) Carries user speech digitized at a data rate of 6.5 kbps After channel coding 11.4 kbps Half Rate Data Channel for 4800 bps( TCH/H4.8) Carries user data sent at 4800 bps After error correction coding, user data sent at 11.4 kbps Half Rate Data Channel for 2400 bps( TCH/H2.4) Carries user data sent at 2400 bps After error correction coding, user data sent at 11.4 kbps GSM Control Channels (CCH) Broadcast Channels (BCH) Operates in forward link of a specific ARFCN Transmits data only in TS0 of certain frames of broadcast ARFCNs Provides synchronization for all mobiles within a cell 5

Common Control Channels (CCCH) Forward channels occupy TS0 of every GSM frame that is not used my BCH or Idle frame in any broadcast ARFCN Reverse channels occupy TS0 of every frame of broadcast ARFCN Pages specific MS, assign signaling channels, receive mobile requests for service Dedicated Control Channel (DCCH) Bidirectional and may exist in any TS and on any ARFCN except TS0 of Broadcast ARFCN Used for signaling services reqd. by users and to supervise data transmission between MS and BS during a call Broadcast Channels (BCH) 1. Broadcast Control Channel (BCCH) Forward Channel Transmits cell and network identity Broadcasts info about channels used, channels available etc. Frames 2-5 in a control multiframe (51 frames) 2. Frequency Correction Channel (FCCH) Transmitted at TS0 and repeated every 10 frames Allows subscriber units to synchronize with the frequency of the BTS 3. Synchronization Channel (SCH) Transmitted at TS0 of a frame immediately following a FCCH frame Allows each mobile to frame synchronize with the BS Transmits Frame Number (FN) along with Base Station Identity Code (BSIC) Issues coarse timing advancements commands to MS so as to synchronize the received signal with the BS clock Common Control Channels (CCCH) 1. Paging Channel (PCH) Provides paging signal to all mobiles and notifies a specific mobile about an incoming call Transmits IMSI of target subscriber and requests for ACK in the RACH 6

2. Random Access Channel (RACH) Reverse link channel Used by subscriber to send ACK for a page or to initiate a call Uses TS0 of all frames of the broadcast ARFCN 3. Access Grant Channel (AGCH) Forward Link Channel Instructs MS to operate in certain physical channel Used by BS to respond to a RACH signal Dedicated Control Channel (DCCH) 1. Stand-alone Dedicated Control Channel (SDCCH) Carries Signaling Data during connection of MS and BS Ensures connection between MS and BS while MSC verifies the subscriber unit and allocate resources 2. Slow Associated Control Channel (SACCH) Maps into the same physical channel as the traffic channel of each user Used to transmit slow but regularly changing control information such as transmit power level instructions and timing advance instructions in the forward link Reverse SACH carries info about received signal quality and strength of neighboring cells 3. Fast Associate Control Channel (FACCH) Carries urgent but similar kind of info as SDCCH Assigned when there is urgent message such as handoff Gains access by stealing frames from the traffic channel If the two stealing bits are set in certain TS then it is considered FACCH rather than TCH 7

Frame Structure for GSM Each of the normal speech frames are grouped into larger structures called multiframes which in turn are grouped into superframes and hyperframes. One multiframe contains 26 TDMA frames, and one superframe contains 51 multiframes, or 1326 TDMA frames. A hyperframe contains 2048 superframes, or 2,715,648 TDMA frames. 8

CDMA (Code Division Multiple Access) Frequency and Channel specification: IS 95 (Interim Standard 95) 2G standard Reverse link: 824-849 MHz Forward link: 869-894 MHz Each carrier is 1.25 MHz wide Duplexing: FDD Multiple Access: CDMA Maximum user data rate: 9.6 kbps User data spread to a channel chip rate of 1.2288 Mchip/s Modulation and Spreading techniques are different in forward and reverse link CDMA Standards for 3G CDMA 2000 1xRTT First 3G CDMA standard Recognized by ITU as IMT-2000 standard in 1999 One times the original cdma One channel bandwidth Supports user data rate up to 307kbps, throughput rates up to 144kbps Improvement through rapidly adaptable signaling rate No additional RF equipment needed to enhance performance, all changes made in software or in baseband hardware CDMA2000 1xEV (Evolutionary) Provided CDMA carriers with option of installing radio channels with data only ( cdma2000 1xEV-DO) or with data and voice (cdma2000 1xEV-DV) Cdma2000 1xEV-DO dedicates radio channels strictly to data users, and supports greater than 2.4Mbps of instantaneous high-speed packet throughput Cdma2000 1xEV-DV supports both voice and data users and can offer usable data rates up to 144kbps with about twice as many voice channels as IS 95B MC 3G CDMA Cdma2000 3xRTT Uses 3 adjacent 1.25MHz radio channels that are used together to form a 3.75MHz channel that provides instantaneous packet data throughput speed in excess of 2Mbps per user 3 non-adjacent channels may also be used together 3G GSM Standards TD-SCDMA Developed by China as a IMT-2000 3G standard Combines TDMA /TDD technology with CDMA Supports up to 384kbps of packet data rate Radio channels divided into 1.6 MHz bands Each frame divide into 7 TS which may be provided to a high data rate user or several slow rate users Supports Asynchronous Traffic Demand Uplink Synchronization: Precise tuning of transmission timing of individual terminals with the BTS 9

Smart Antennas, Joint Detection, Terminal Synchronization, Dynamic Channel Assignment Low intracellular and intercellular interference, High Spectral Efficiency Operated in 1900 and 2000 MHz band Each physical channel is identified by a particular time slot and a particular code on a particular carrier frequency WCDMA (UMTS: Universal Mobile Telecommunication System) 3G standard for GSM, PDC and IS-136 Network structure and bit level packaging similar to GSM standards Fundamental radio air interface technology similar to cdmaone and cdma2000 Requires minimum spectrum allocation of 5 MHz Supports packet data rate upto 2.048 Mbps Fast closed loop power control in both uplink and downlink BS uses two transmitting antennas for phase and amplitude diversity Supports high quality data, multimedia, videoconferencing, telemedicine etc. Requires complete change of RF equipments at the BS. 10

Pseudo Noise Code (PN Code) The PN code is pseudo-random sequences They are deterministic, periodic sequences which mimic randomness properties. Randomness or noise-like characteristics include 1 s and 0 s appear randomly in sequence. The number of 1 and 0 are (almost) the same in any long segment of sequence Half of the total no. of run lengths of 0s or 1s are of length 1, then almost 50% of remaining are of length 2, again 50% of remaining are of length 3 and so on. Use of PN sequences in CDMA system: To spread the bandwidth of the signal to larger transmission bandwidth To distinguish between different user signals by utilizing the same transmission bandwidth in multiple access scheme. Generated using LFSRs and modulo-2 adders N shift registers are used to generate 2N-1 length long PN sequences A PN sequence and a phase shifted version of itself are nearly orthogonal i.e, no. of Agreements- no. of Disagreements is nearly equal to zero CP X1 X2 X3 X4 X5 Outpu C 1=0 C 2=0 C 3=1 C 4=0 C 5=1 ttt PN-Code Generation Mode (N=5) Walsh Codes Perfectly orthogonal to each other. Different codes have zero cross correlation IS-95 uses 64 Walsh codes on forward link Used for call identification Reserved codes for pilot, paging and sync IS-95 uses Walsh Codes in the reverse link for modulation Generated using Hadamard Matrix/Array 11

Spreading In CDMA, the user data is multiplied i.e, modulo-2 added with a spreading sequence that has much wider bandwidth than that of the user s signal. So the resultant signal has a bandwidth in the range of that of the spreading sequence. So the information carried by a narrowband signal is spread over a wider bandwidth. This is called spreading. Two types of codes are used for spreading the user s signal:- PN Codes and Walsh Codes Forward CDMA channel 64 Walsh Coded channels available for use Consists of Pilot Channel, Synchronization Channel, up to 7 Paging Channels and remaining and remaining forward traffic channels Pilot Channel: Provides phase reference to each MS for coherent demodulation Transmitted with a signal strength 4-6 db higher than traffic channels Allows MS to compare strengths of signals coming from different BS to determine when to handoff Always coded with Walsh Code 0 Need to lock onto other logical channels on the same RF Carrier 12

Synchronization Channel Used to broadcast time synchronization messages Transmits information such as System ID, Network ID, offset from the system time, paging channel data rate etc. Operates at a fixed rate of 1200 bps Always assigned Walsh Code 32 Paging Channel Used to send control information and paging messages from the base station to the mobiles Assigned Walsh codes 1 through 7 Operates in different data rates 9600,4800,2400 or 1200 bps Control information include no. of paging channels, base station identifier, access parameter message, channel list message etc Traffic Channels: Supports different speech/data rate:- 9600,4800,2400 or 1200 bps (RS1) 13

Convolution Encoder Near Far problem Near Far Problem MS near to BS increases the noise floor for the far users Two Power Control Schemes:- Open Loop Control MS adjusts its transmission power automatically by analyzing the pilot signal Closed Loop Control The BS measures the SIR from the MS and then sends command to MS so that it adjusts its transmission power accordingly 14

Reverse CDMA Channel Consists of Access Channels and Reverse Traffic Channels Up to 32 Access Channels associated with each Paging Channel Access Channels used to respond to a page or to initiate a call Modulation scheme different from that in forward channel 15