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

identification and

device

• 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 = 0 • -90 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)