ECE442: Wireless Communications - Lecture 10: Existing ...

ECE442: Wireless CommunicationsLecture 10: Existing Cellular Systems - GSM and IS-95

Prof. Sudharman K. JayaweeraDepartment of Electrical and Computer Engineering

University of New Mexico.

Prof. Sudharman K. Jayaweera ECE442: Wireless Communications

Existing Wireless Technologies

Cellular telephony

Cordless phones

Wireless Local Loop (WLL) systems

Wireless Local Area Networks (WLAN’s)

Wireless Personal Digital Assistants (PDA’s)

Pagers

Bluetooth


Emerging Wireless Technologies

Mobile satellite services (MSS)

Wireless geo-location systems (E-911)

Third Generation (3G) and beyond (4G etc.) cellular

WiMax

Wireless Personal Area networks (WPAN’s)

Wireless Internet

Mobile ad-hoc networks (MANET’s) and Wireless SensorNetworks (WSN’s)


GSM: Global System for Mobile Communications

1982: The Conference of European Posts and Telegraphs(CEPT) formed the study group named Groupe SpecialMobile (GSM) to develop a pan-European public and landmobile system

1989: GSM responsibility was transferred to EuropeanTelecommunications Standards Institute (ETSI)

1990: GSM Phase I specifications were published

1991: Commercial GSM service was started in mid-1991

Although GSM was standardized in Europe it was quicklyadapted worldwide becoming a global standard

As a result now GSM is used to mean Global System forMobile communications

In US, PCS1900 (also called the GSM1900) system is basedon a derivative of the GSM standard

In other parts of the world DCS1800 is a GSM-based system


GSM Network Architecture

A GSM network consists of three main parts:

Mobile Station (MS): carried by the subscriberBase Station Subsystem (BSS): controls the radio link withthe MSNetwork and Switching Subsystem (NSS): main component isthe Mobile Switching Center (MSC) that controls theswitching of calls between mobile users, between mobile andfixed network users, and handles the mobility management

An Operations Support Subsystem (OSS) provided solely forthe system engineers of the operating company to monitor,diagnose and troubleshoot the GSM system


Mobile Station (MS)

The MS consists of two main parts:1 Mobile equipment (the terminal)

The mobile equipment is uniquely identified by theInternational Mobile Equipment Identity (IMEI).

2 Subscriber Identity Module (SIM)

The SIM is a smart card that provides personal mobility suchthat the user can have access to subscriber services from anyterminal.The SIM contains a unique identifier called InternationalMobile Subscriber Identity (IMSI) that identifies the subscriberto the systemIt also contains a secret authentication and other informationThe IMEI and IMSI are independent of each other (for theflexibility in mobility)


Architecture of a GSM Network


Base Station Subsystem (BSS)

The BSS is also called the radio subsystem. It consists of two mainparts:

1 Base Transceiver Station (BTS)The BTS contains the radio transceivers that define a cellIt also handles the radio-link protocols between the MSIn any given area there might be a large number of BTS’s

2 Base Station Controller (BSC)A BSS consists of many BSC’s that connect to a single MSC.The BSC manages the radio resources for one or more BTS’s(usually a BSC controls up to several hundred BTS’s).BSC handles radio-channel set-up and frequency hopping.When a handoff is between two BTS’s under the control of thesame BSC, the BSC will also handle the handoff without goingthrough the MSC.The BSC is the connection between the MS and the MSC

3 The BTS and BSC communicate across a standardizedinterface called Abis interface allowing operation betweencomponents made by different manufacturers


Network and Switching Subsystem (NSS)

,;/~Ji_

MS

Base Station Subsystem

I

I

I~I~~r~tion SupportSubsystem

I

Network Switching Subsystem I Public Networks

NSS handles the switching of GSM calls between externalnetworks and the BSC’s of the GSM systemsThe main component of the network subsystem is the MobileSwitching Center (MSC)

MSC is very much like a normal switching node in a PSTN, butalso provides the additional functionalities needed to supportsubscriber mobility such as registration, authentication,location updating, handoffs and call routing for roamingThe MSC connects the GSM network to the fixed networkssuch as PSTN or ISDN


Databases and Registers in NSS

The NSS contains several databases that connect to the MSC.

Home Location Register (HLR) and the Visitor LocationRegister (VLR) supports call routing

The HLR contains all the administrative information of eachsubscriber registered in the corresponding GSM network andthe current location of each of those mobilesThere is only one logical HLR in a GSM network (although itmay be implemented as a distributed data base)

The Equipment Identity Register (EIR) is a database thatcontains a list of all valid mobile equipment in the network

each mobile station is identified by its unique IMEI

A protected database called the Authentication Center (AuC)stores a copy of the secret key stored in each subscriber’s SIMcard


GSM Call Routing and Roaming: HLR and VLR

Call routing and roaming capability of GSM is managed bythe Home Location Register (HLR) and the Visitor LocationRegister (VLR) in conjunction with the MSC

The VLR contains selected administrative information fromthe HLR that are necessary for call control and provision ofthe subscribed services for each of the mobiles currentlylocated in the geographical area controlled by VLR

Usually the VLR is implemented together with the MSC(though not necessary) so that the area controlled by an MSCcorresponds to the area controlled by the VLR

The MSC contains no information about particular mobilestations. This information is stored in location registers


GSM Interfaces

Um interface: MS and BSS (specifically the BTS)communicate across the Um interface

also called the air interface or the radio link

A interface: The BSS (specifically BSC) communicates withthe MSC across the A interface

Abis interface: The BTS communicates with the BSC acrossthe Abis interface


GSM Radio Subsystem

In Europe, GSM systems operate in the ITU allocated bandsof 890-915 MHz (uplink) and 935-960 MHz (downlink)

As GSM was adopted worldwide the operating frequency bandsvary in in different markets

Each 25MHz bandwidth (uplink or downlink) is divided firstinto 124 channels of 200KHz each

Since a guard band of 100kHz is left at each end of thespectrum band

Each of these carriers, called the Absolute Radio FrequencyChannel Numbers (ARFCN), are then divided in time viaTDMA


Physical Channels in GSM

Multiple-access capability is a hybrid of time and frequencydivision multiple-access

Each TDMA frame is made of 8 so-called burst periods ortime slots

A time slot (TS) is the fundamental unit of time in GSMTDMA scheme and is equal to 15/26 ms (≈ 0.577 ms or576.92 µ s)Hence a TDMA frame is equal to 120/26 ms (≈ 4.615 ms)

One physical channel corresponds to one time slot per TDMAframe

i.e. a physical channel constitutes the combinations of oneARFCN and one TS number

Each physical channel can be mapped into various logicalchannels at any given time


Logical Channels in GSM

Two basic types of logical channels in GSM are:

1 Traffic channels (TCH)2 Control Channels (CCH)

There are three different types of control channels also:Broadcast channels (BCH)Common Control Channels (CCCH)Dedicated Control Channels (CCCH)


GSM Traffic Channels (TCH)

156.25 bits

576.92/.ls.....--.

TSo

TS1TS2TS3TS4TS5TS6TS7

"

4.615 ms •

TSn: nth Time Slot(Normal) Speech Multiframe = 26 TDMA frames

120 ms

To

T1T2 .........TlO

TnT12S T13T14T15. ........T22

T23T24IIS

Tn: nth TCH frameS: Slow Associated Control Channel frameI: Idle frame

GSM traffic channels can carry either digitized speech or dataat full-rate or half-rate

In full rate, data is transmitted in one TS per each frameIn half-rate, user data is transmitted in one TS per every twoframe (i.e. in alternating frames)

After every 13 consecutive frames of TCH data, GSM insertseither a Slow Associated Control Channel (SACCH) data oran idle frame

Each group of 26 consecutive TDMA frames is called amultiframe


GSM Frame Structure

Superframe

Multiframe

Frame

~/ "-

/ "-

~/ "

/ "/ ""

~~/' --

51 Multiframes

26 Frames

8 Time slots

/' --576.92 Jls

--

3

57126157 38.25

Time slot 156.25 bits

Tail Coded Stealing Midamble Stealing Coded Tail Guardbit Data flag flag data bit Period


Control Channels in GSM: Broadcast channels (BCH)

Operates only in the forward link of a specific ARFCN withineach cell, and only in the first time slot (TS0) of certain GSMframes

Serves as a TDMA beacon and provides synchronization to allmobiles within the cell

There are three different types of BCH’s: Broadcast ControlChannels (BCCH), Frequency Correction Channels (FCCH),and Synchronization Channels (SCH)

These 3 types of BCH’s are given access to TS0 duringvarious frames of a 51-length frame sequence


Control Channels in GSM: Common Control channels(CCCH)

On the same ARFCN used for BCH, the Common ControlChannels (CCCHs) occupy the TS0 of every GSM frame thatis not used by the BCH or an Idle frame

There are three different types of CCCH’s as well:1 Paging Channels (PCH): Only in forward link. Used by the BS

to notifies a MS of an incoming call2 Random Access Channels (RACH): Only in reverse link. Used

by the MS to either originate a call or to acknowledge a pagefrom the BS on a PCH. Uses slotted-ALOHA protocol.

3 Access Grant Channels (AGCH): Only in Forward link. Used bythe BS to provide the MS with information on which physicalchannel to be used (i.e. the ARFCN and the TS number), inconjunction with which dedicated control channel.


Control Channels in GSM: Dedicated Control Channels(DCCH)

Dedicated Control Channels (DCCHs) are bidirectional andtheir functionality is the same in both forward and reverse links

DCCH can be operated in any time slot (except for the TS0)of any ARFCN

There are three different types of DCCH channels:1 Stand-alone Dedicated Control Channels (SDCCH): Provide

signalling services required by the MS2 Slow Associated Control Channels (SACCH): Always

associated with a traffic channel (TCH) or a SDCCH and mapsonto the same physical channel as them. On the forward linkSAACH is used to provide slowly changing control informationsuch as transmit power level to the MS.

3 Fast Associated Control Channels (FACCH): Carries urgentmessages whenever a SDCCH is not yet dedicated. Anexample is a handoff request.


GSM Speech Coding

The 64 kbps PCM voice output stream in toll-quality voice istoo high for wireless communications

GSM implements a speech coding algorithm called RegularPulse Excited Linear Predictive Coding (RPE-LPC) to reducethe required bit rate

The RPE-LPC algorithm uses the past samples to linearlypredict the current sampleThen the coefficients of the linear predictor and an encodedversion of the prediction error is send to the receiver

Speech is divided into 20 ms samples and is encoded using260 bits

Thus the total bit rate of GSM voice signals is 13 kbps (this iscalled the full-rate speech coding)


Channel Coding in GSM

To protect against man-made as well as naturalelectromagnetic interference, GSM uses convolutionalencoding and block interleaving

The exact methods used for speech and data signals aredifferent

For speech signals, the 260 bits in each 20 ms block aredivided first into three classes based on their perceivedimportance in determining the speech quality:

Class Ia: most sensitive to bit error (50 bits)Class Ib: moderately sensitive to bit error (132 bits)Class II: least sensitive to bit error (78 bits)

Each class is encoded differently to provide different levels ofprotection


Channel Coding for Speech Signals

First a 3-bit cyclic Redundancy Check (CRC) code is added tothe Class Ia bits for error detection (this gives 53 bits)

Next, these 53 bits together with 132 Class Ib bits areencoded using a rate 1/2 convolutional encoder withconstraint length 4.

The 78 Class II bits are then added (unprotected) to the 378bits from the convolutional encoder output resulting in 456bits (in a 20 ms block)

This gives a bit rate of 22.8 kbps

To further protect against burst errors, each sample isinterleaved before transmission


Modulation in GSM

The resulting digital signal is modulated onto the analogcarrier signal using so-called Gaussian Minimum Shift Keying(GMSK)

GMSK provides a compromise among spectral efficiency,transmitter complexity and limited spurious emissions

Note that,

Reduced transmitter complexity is important to reduce thepower consumption at the mobile terminalsIn order to reduce the co-channel interference it is importantthat the spurious emissions outside the allocated bandwidth isminimized


Power Control in GSM

Based on the peak transmitter power, the GSM systemdefines 5 classes of mobile stations:

- rated at 20, 8, 5, 2 and 0.8 Watts

In order to minimize the co-channel interference, both the MSand the BTS operate at the minimum power level required tomaintain an acceptable signal quality

Power levels can be stepped up or down in steps of 2dB fromthe peak power for the class down to a minimum of 13dBm(i.e. down to 20mW)

The MS measures the signal strength and signal quality(based on the BER) and passes that information to the BSC

The BSC then decides if and when the power level needed tobe changed based on that information


Global Market Comparisons (Figures from 1998 onward areprojections

Source: Ojanpera and Prasad, Wideband CDMA for ThirdGeneration Mobile Communications.


GSM Standard Parameter Summary

Parameter

Reverse Channel Frequency

Forward Channel Frequency

ARFCN Number

TxlRx Frequency Spacing

TxIRx Time Slot Spacing

Modulation Data Rate

Frame Period

Users per Frame (Full Rate)

Time Slot Period

Bit Period

Modulation

ARFCN Channel Spacing

Interleaving (max. delay)

Voice Coder Bit Rate

Specifications

890-915 MHz

935-960 MHz

o to 124 and 975 to 1023

45 MHz

3 Time slots

270.833333 kbps

4.615 ms

8

576.91ls

3.6921ls

0.3 GMSK

200 kHz

40ms

13.4 kbps

Source: Rapppaport.


CDMA IS-95 Systems

Interim Standard 95 (IS-95) defines 10 CDMA bands (each of1.25 MHz) in the already allocated 25 MHz of AMPSfrequency bands

uplink: 869-894 MHzdownlink: 824-849 MHz

The multiple-access capability is provided using DS-CDMA:

In each frequency band 64 orthogonal Walsh codes (W0

through W63) are used to identify downlink channelsIn each frequency band 64 long PN codes are used to identifyuplink channels

Each physical IS-95 channel is identified by specifying a carrierfrequency as well as a code sequence

A PCS version of IS-95 is also available for international usein the 1.8GHz-2GHz band


IS-95 Systems

Modulation: QPSK

Nominal data rate: 9600 bps

Spreading factor: 128

Chip rate: 1.2288 Mcps

Filtered bandwidth: 1.23 MHz

Coding: convolutional coding with a Viterbi decoder

Interleaving: in 20 ms blocks

IS-95 uses a RAKE receiver to combine multipath energy


Logical Channels in IS-95

Variable-bit-rate

< user information

Logical channels

[-S-l-'g-n-a-li-n-g-m-es-s-a-g-e-

Variable-bit-rateuser information

Power control

I Signaling messages

Pilot channels

Sync channels

Paging channels

Traffic channels

Logical channels

Forwardchannels

Logicalchannels

Source: Textbook.


IS-95 Forward Link

Forward link (downlink) includes:

one pilot channel, one sync. channel, up to seven pagingchannels, and a set of forward traffic channels

Information on each channel is modulated by an appropriateWalsh code at a fixed chip rate of 1.2288 Mcps

Each user in a cell is assigned a different Walsh codeTo reduce co-channel interference among cells, all signals inone cell are also scrambled by a length 215 PN sequence

The pilot and sync. channels are always assigned to codechannel number 0 and 32, respectively (i.e. W0 and W32)

Forward traffic channels are grouped into rate-sets:

RS1: contains rates 9.6, 4.8, 2.4 and 1.2 kbpsRS2: contains rates 14.4, 7.2, 3.6 and 1.8 kpbs


IS-95 Reverse Link

Reverse link (uplink) channels are only access channels ortraffic channels

there are 62 traffic channelsthere can be up to 32 access channels

The access channel operates at the fixed rate of 4.8 kbps

The spreading method used in the reverse link is differentfrom that in forward link:

The reverse link traffic channel can operate at any RS1 rate(i.e. 9.6, 4.8, 2.4 or 1.2 kbps)


IS-95 Speech Coding

In IS-95 speech is encoded using a variable-rate vocoder,named the Code Excited Linear Predictive (CELP) coder, thatdepends on the voice activity

Since frame duration is fixed at 20 ms, this results in a variablenumber of bits per frameRate can be changed from 2400 to 19,200 symbols per second

downlink: The resulting bits are channel encoded using a rate1/2 convolutional encoder and then interleaved

uplink: The resulting bits are channel encoded using a rate1/3 convolutional encoder and then interleaved

Both convolutional encoders have a constraint length of 9


IS-95 Froward Link Modulation Process

I-Channel Pilot PN Sequence

Q-Channel Pilot PN Sequence

User data19.2

from base Convolutionalstation

Encoder andBlockkbps

RepetitionInterleaverr=1I2 K=99600 bps 4800 bps2400 bps

Long CodeLong Code1200 bpsfor nth userGenerator

Walsh code

41.2288

Mcps

+SymbolCover

Source: Rapppaport.


IS-95 Reverse Link Modulation Process

PN chip

Q(t)

Baseband let)Filter

Zero-offset Pilot PN

Sequence I-channel

Data Burst

RandomizerWalsh

chip

307.2 kcps

1.2288 Mcps

64-ary

OrthogonalModulator

Code

Symbol

Long Code

Maskfor user n

Code

Symbol Block

In terleaver

28.8 ksps

Convolutional

Encoder and

Repetitionr=1I3K=99600 bps

4800 bps2400 bps1200 bps

Zero-offset Pilot PN

Sequence Q-channel

Source: Rapppaport.


Power Control in IS-95

Power control is an important aspect in IS-95 CDMA systems

Goal is to ensure that the received power at the BS due to allthe MS’s are equal

Power control is applied at both the MS as well as the BS

Open loop power control at the MS: The mobile senses thestrength of the pilot channel and adjusts its power accordingto that

Closed-loop power control at the MS: Mobiles received thepower control information from the BS (at a rate of800b/sec).

Open-loop power control at the BS: the BS decreases itspower level gradually and waits to hear the frame error rate(FER) from the MS. If the FER is high the BS will againincrease its power


References

D. P. Agrawal and Q. Zeng, Introduction to Wireless andMobile Systems, Second Edition, Thomson, 2006. Chapter 10.

T. S. Rappaport, Wireless Communications: Principles andPractices, Prentice-Hall, Second edition, 2006. Chapter 11.


Next time...

Existing Wireless Technologies: WLLs, WLANs, and PagingSystems

References: D. P. Agrawal and Q. Zeng, Introduction toWireless and Mobile Systems, Second Edition, Thomson,2006. Chapter 10.


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