Wireless Communications Technology Lesson 5: GSM Bellevue Community College Bob Young, Instructor

GSM Global System for Mobile Communications

New Features Support for international roaming (this was the design driver) Distinction between user and device identification SIM Card (Subscriber Identification Module) All of the interfaces are standardized PCS 1900 MHz band (in North America)

System Architecture Mobile Switching Center (MSC) Base Station Controller (BSC) Base Transceiver Station (BTS) Home Location Register (HLR) Visitor Location Register (VLR) Equipment Identity Register (EIR) Authentication Center (AuC)

SIM Contains: International Mobile Subscriber Identity (IMSI) Authentication Key (Ki) Service Plan Personal Directory (speed dial)

Mobile Device Contains: International Mobile Equipment Identifier (IMEI) Temporary Mobile Subscriber Identity (TMSI) Mobile Station Classmark (Mobile SCM)

Base Station Contains: Base Station Identity Code (BSIC) Location Area Identity

GMSK Gaussian Minimum Shift Keying –"Slides" through the phase transitions +90 degrees = degrees = 1

Frequencies Downlink Frequency: 200 KHz bandwidth Uplink Frequency: 200 KHz Bandwidth Uplink and downlink are divided into a similar "slot" structure Uplink delayed, referenced to downlink –Eliminates need for duplexer in mobile station

Power Control See chart on page 112 Only the mobiles have power adjusted Memory aid: top of chart: 2, 1, 39

Two Channel Types Control channels Traffic channels

Control Channels Broadcast Control Channel Common Control Channel Dedicated Control Channels

Broadcast Control Channel  Frequency Correction Channel (FCCH)  148 zeros. In GMSK, this creates a sine wave. It is used for preliminary synchronization.  Synchronization Channel  Broadcast Control Channel (BCCH)  Contains info for setting up calls

Common Control Channel Paging Channel (PCH) –Alerts the mobile Access Grant Channel –Directs the mobile to the dedicated control channel

Random Access Channel The mobile can use any time slot on the common control channel as the Random Access Channel (RACH) Used by the mobile when not on a call: –To originate a call –To initiate registration –etc.

Dedicated Control Channels Stand Alone Dedicated Control Channel (SDCCH) –Two way logical channel: same time slot used on both the uplink and downlink for functions like registration and call setup Slow Associated Control Channel (SACCH) –Different from SACCH on traffic channel

Traffic Channels 8 time slots (voice channels) per frame

Vocoders 1)Linear Prediction Coding with Regular Pulse Excitation (LPC-RPE) 2)Enhanced Full Rate (EFR) ACELP for GSM 3)Half Rate ACELP for GSM

Frequency Hopping Frequency Hopping Spread Spectrum (FHSS) Resistant to fading Minimizes the effects of co-channel interference 217 hops/sec, equal to 1200 bits/hop

Handoffs All handoffs in GSM are "hard" handoffs Like TDMA, GSM supports Mobile Assisted Handoff (MAHO)

Multipath Equalization Receiver uses extensive synchronization information to select the best incoming signal (in a multipath environment)