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18-759: Wireless NetworksLecture 17: Cellular

Peter Steenkiste

Peter A. Steenkiste, CMU 1

Departments of Computer Science andElectrical and Computer Engineering

Spring Semester 2010http://www.cs.cmu.edu/~prs/wirelessS10/

Outline

Cellular landscapepAMPSGSM

» HSCSD» GPRS» EDGE

CDMA


OFDM

Some slides provided by Rui AguiarUniversity of Aveiro

The Cellular Landscape

FDMA

0.15bps/HzMax.rate 64KbpsTDMA &CDMA

0.30 bps/HzMax.rate 2 Mbps

TDMA CDMA d WCDMA

5-10 bps/HzMax. rate ~

100Mbps/1Gbs

1GAnalog

2GDigital ModulationConvolution coding

Power Control

2.6G/3GHierarchical cell structure

Turbo-coding

4GSmart antennas?

MIMO?Adaptive Systems

OFDM Modulation

FDMA TDMA &CDMA TDMA,CDMA and WCDMA pWCDMA


AMPSTACSNMTC-450

PDCGSMHSCSDGPRSIS-54/IS-136IS-95/IS-95A/IS-95BPHS

EDGECdma2000WCDMA/UMTS3G 1x EV-DO3G 1X EV-DV

Cellular Standards

2G systems: digital voice y g» GSM - FDMA/TDMA, most widely deployed, 200 countries,

a billion people » IS-95 - first CDMA-based cellular standard, developed by

Qualcomm » IDEN - TDMA, Nextel, merged with Sprint, being phased

out for CDMA2000 » IS-136 - uses FDMA/TDMA, North America, Cingular and

US Wireless, being phased out for GSM, CDMA2000


US Wireless, being phased out for GSM, CDMA2000

2.5G systems: voice and data channels » GPRS - evolved from GSM, packet-switched, 170 kbps

(30-70 in practice) » CDMA2000 1xRTT - evolved from IS-95, 144 kbps

Cellular Standards

2.75G - almost 3G in speed p» EDGE - another enhancement of GSM, 384 kbps, 2.75G » Thanks to new modulation scheme (8PSK) – may coexist

with GMSK

3G: voice (circuit-switched) and data (packet-switched)

» UMTS - W-CDMA, successor to GSM networks, 384 kbps - 2 Mbps, European, some Japan, Cingular in U.S.


» CDMA2000 1xEV - CDMA2000 with high data rates - 3.1 Mbps up, 1.8 Mbps down, U.S., Japan, Korean, Canada –Verizon, Sprint

4G: 10 Mbps and up, seamless mobility between different cellular technologies, mesh, etc.

GSM Evolution

Messages» SMS – Short Message Service» News» USSD – Unstructured Supplementary Service Data

Data:» HSCSD – High Speed Circuit-Switched Data» GPRS – General Packet Radio Service» Edge – Enhanced Data Rate for GSM Evolution


» Edge – Enhanced Data Rate for GSM Evolution» UMTS – Universal Mobile Telecommunication System

HSCSD

Based on fast circuit switching, introduced in phase 2+ (1997)2+ (1997).

» Non optimum solution for packets (cost/capacity)Same GSM layers

» Same interoperation function in MSC, » same transport network

Uses multiple time slots per user (max 6)


» Changes link level protocol» Bitrates: 19.2 ; 28.8 ; 28.4 ; 48 ; 56 and 64 kb/s.» Asymmetric configurations (n slots on the uplink and m

slots on the downlink).» Increases blocking probability of the system

GPRS

General Packet Radio ServicePacket-oriented transport service, for data network connections (Internet)GPRS features:

» Better transmission bit rates(max 150kbps).» Allows burst communications (“immediate”: connections in <1s)» New network applications» New billing mechanisms (user-oriented: by traffic p ex )


» New billing mechanisms (user-oriented: by traffic, p.ex.)

GPRS principlesGPRS principles

Transmission of non-periodic and bursty data (e.g.: mails),F t ll k t ( t l ti billi d i t )Frequent small packets (e.g.: telematic, billing and micro-payments),

» Large but unfrequent packets (e.g.: ftp).

Higher bit-rates per TCH (9.05 ; 13.4 ; 15.6 ; 21.4 kb/s),Higher bit-rates with up to 8 time slots per user,Channel sharing by active terminals,Separate allocation of uplink and downlink channels,


Separate packet transmission network between the BSC and external packet transmission networks: GSS(based on SGSN and GGSN).

GSS: GPRS SubSystem

GPRS Architecture

New entities are definedSGSN – serving GPRS support nodeSGS g G S ppGGSN – gateway GPRS support nodeInterfaces between entities GPRS, GSM, core, e PSTN

Transmission planeData packets are transmitted by a tunnel mechanisms

Control planeGTP: a protocol for tunnel management (create remove etc )


GTP: a protocol for tunnel management (create, remove, etc..)GPRS Tunnel Protocol

Radio interfaceChanges the logical channels and how they are managedKeeps the concept of “master-slave”

GPRS Architecture

SGSNOther GPRSnetworks

BTS

BTSMT

BSC

Gb

SGSN

Gf

GsGr

DEIR

Gc

Gn

GGSN GiPDN

Gp GGSN

networks


D

MSC/VLR

HLR

SGSN – serving GPRS support nodeGGSN – gateway GPRS support node

GPRS introduction in a GSM network

.. < > ^ ... . . .

BTS

BTS

BSC

TRAU

MSC/VLR

PSTN

HLR A

Abis

IN Plate-form

BTS

PCU

Gb Gs

SGSN Gr, Gd, Gf

Gn

BorderGPRS

backbone S i WAP

SS7 Network Gf

EIR

Gr

MSC/VLR


Border Gateway

GGSN

Inter-operator

backbone

GPRS backbone

Internet PDN

Router LAN

Serviceplate-form

WAP, WWW, ...

Gc

GSM to GPRS Evolution

BSS Evolution:• Replace/Upgrade existing elements: BTS, BSC,Replace/Upgrade existing elements: BTS, BSC,

O&M, Network planning, Links (Abis, Ater, …).• New element: PCU (Packet Controller Unit).

NSS Evolution• A new core network (GSS) dedicated to GPRS:

IP/ATM based, network packet nodes


(SGSN,GGSN), Internet equipment (DNS servers, Firewalls, …).

• Evolution of the network elements: HLR, MSC/VLR, SS7.

GGSN (Gateway GPRS Support Node)Functions

Gateway:All th ti t th IP GPRS t k» Allows the connection to other IP or GPRS networks.

Routing:» IP router which supports dynamic or static routing,

Mobility management:» Use of routing areas.» Handover management between the BSCs and other SGSNs.

All th ti f th k t t d th SGSN


» Allows the routing of the packets towards the users SGSNs,according to their mobility.

Sessions management:» At each session, the SGSN activates a PDP (Packet Data

Protocol) context, and allocates an IP address to the MT.

Security:» Ciphers the communications towards or from the

GGSN (Gateway GPRS Support Node) Functions

» Ciphers the communications towards or from themobiles.

» Includes firewalls for filtering the packets comingfrom external IP networks.

Authentication:» At Attach and inter-SGSN RA updates.

Billing:


» Production of the CDRs according to the quantity ofinformation and the session duration (attachment,duration of active PDP context).

SMS:» Supports the Gd interface for the communications

with the SMS-GMSC and the SMS-IWMSC.

MT Registration

There is an explicit registration of the MT in the network:network:

» GPRS attach» GPRS detach – can be started by the MT or by the network» Location packets are periodically sent

HLR (modified!) keeps information on the MT status, including:

» GPRS state (ready, standby, idle)


» GPRS state (ready, standby, idle)» QoS profile (priority 3, delay 4, reliability 5, throughput

peak 9 and media 19)» Context PDP (Packet Data Protocol)

– Also stored in GS and in the GGSN and SGSN

Connection Management

After attach: receive a packet with a PDP identifierActs as an addressActs as an address

PDP identifier: per session.– static: allocated by the MT home networks– Dinamic: allocated by the GGSN

PDP profile:TypePDP identifier


de t eRequested QoScorrespondent GGSN address

PDP context activation

MT SGSN GGSN

Activate PDP Context Request

Security FunctionsCreate PDP Context Request

PDP type,PDP AddressQoS Requested,Access Point,…

PDP type,PDP AddressQ S N ti t d A P i t


Activate PDP Context Accept Create PDP Context Response

QoS Negotiated,Access Point,…

PDP type,QoS Negotiated,…PDP type,PDP AddressQoS Negotiated,…

Time Slot

GPRS Radio Interface

0 1 2 43 5 6 7 0 1 2 3 40 1 2 43 5 6 7 0 1 2 3 4

0 1 2 43 5 6 7 0 1 2 3 40 1 2 43 5 6 7 0 1 2 3 4

UplinkF1

F2

F3

F4

F1Carrierfrequency


Downlink

User1 Voice

User2 Voice

User3 GPRS

User4 GPRS

User5 GPRS

F2

F3

F4

frequency

GPRS: logical channels

Group Channel Function Directionp

Packet data Traffic channel

PDTCH Data Traffic MS BSS

Packet broadcast control channel

PBCCH Broadcast Control MS BSS

Packet commonControl Channel

PRACH

PAGCH

Random AccessAccess Grant

MS BSSMS BSS


Control Channel(PCCCH) PPCH

PNCH

PagingNotification

MS BSSMS BSS

Packet DedicatedControl Channels

PACCH

PTCCH

Associated Control

Timing Advance Control

MS BSSMS BSS

Data Transfer (Uplink)

MT BSS

PRACH or RACH

PAGCH or AGCH

T

PACCH

PACCH

Packet Immediate assignment

Packet resource Request

Packet resource assignment

F T i i

Packet channel Request


Transmission

PDTCH

PACCH

PDTCH

PACCH

Frame Transmission

Negative Acknowledgement

Retransmission of blocks in error

Acknowledgement

Data Transmission (downlink)

Packet paging requestMT BSS

PRACH or RACH

PAGCH or AGCH

Paging

Packet channel Request

PACCH

PACCH or PAGCH

Packet Immediate assignment

Packet paging response

Packet resource assignment

PPCH or PCH


Transmission

PDTCH

PACCH

PDTCH

PACCH

Negative Acknowledgement

Retransmission of blocks in error

Frame Transmission

Acknowledgement

Enhanced Data rates for GSM Evolution (EDGE)

Objective:Increase the data bitrates (GPRS EGPRS)Increase the data bitrates (GPRS EGPRS).

Bitrates:- 473 kb/s for the terminals of 100 km/h maximum.- 80-130 kb/s on average.- 144 kb/s for the terminals of 250 km/h maximum.

Means:


-1- New modulation (8-PSK).-2- Link adaptation

New mobiles, upgrade/replacement of TRXs and capacityenhancement (Abis, …).

EDGE

Enhanced Data Rates for GSM EvolutionEnhanced Data Rates for GSM Evolution» Announced as low-cost 3G (marketing…)» 2.5G evolution to GSM» Improved GPRS structure, but retaining basic

structure» Improved data rates (144kbps a 470kbps)» Improved spectrum efficiency (2-6x)


» EDGE supports GMSK & (new) 8-PSK» Requires lots of changes in transceiver design!

Goals of 3G?

Greater system capacity both in terms of users and bandwidthusers and bandwidthGood support for mobility at high data rates at high speeds

cdma2000 (Qualcomm) vs. W-CDMA» W-CDMA is the air interface for Universal Mobile

Telecommunication system (UMTS), successor of GSM


– Uses two 5MHz bands for up and down link– UMTS sometimes also uses single band (TDD)

» cdma2000 is successor of Qualcomm’s cdma– Uses two 1.25 MHz bands for up and down link– Adds 64 orthogonal channels relative to IS95– Evolution Data Optimized (EV-DO) adds TDMA

Code Division Multiple Access (CDMA)

CDMA uses codes to convert between analog gvoice signals and digital signals. It then uses codes to divide voice and control data into data streams called “channels”


CDMA Signal Generation

Analog to digital conversion (Pulse Code g g (Modulation)VocodingEncoding and InterleavingChannelizationConversion of the digital signal to a RF signal (modulation)


(modulation)

Variable Rate Vocoder

Human speech is full of pauses (thinking, iti t h b k t )waiting to hear back, etc.)

CDMA vocoder varies compression of the voice signal into one of four data rates, based on the user’s speech activityThe four rates are: Full, 1/2, 1/4, 1/8Full rate when the person talks very fast


1/8 when the person is silent or nearly soCDMA systems can use either a 8 Kbps or a 13 Kbps vocoder

» Extended Variable Rate Coding (EVRC) vocoder produces the quality of the 13 Kbps vocoding, with a 8 Kbps rate

Encoding and Interleaving

Builds redundancy into the signal to recover y ginformation lossEncoding relies on convolutional encodingSimplified scheme is the repetition code: every bit is repeated three timesThe encoded bits are called symbolsThe decoder at the receiver uses a majority


The decoder at the receiver uses a majority logic ruleCombat burst errors

(built in BTS and phones)

Interleaving

Reduce the effect of burst errors and recover lost bitsSymbols are said to be interleaved or scrambled in a pattern that the receiver knowsDe-interleaving at the receiver unscrambles the bits, spreading any burst errors that occur d i t i i


during transmission

Channelization

The encoded voice is further encoded to separate it from other encoded voice dataThe encoded symbols are spread over the entire bandwidth of the CDMA channelThe receiver knows the code and uses it to recover the data


Code channels in CDMA

Code channel is a stream of data designated gfor a specific use of personThis channel may be voice data or overhead control dataChannels are separated by codesThe forward and reverse links use different types of channels


yp

Two types of codes

BTS to mobile» Walsh codes » Nearly “orthogonal” codes» Unique enough that the voice data can only be

recovered by a receiver applying the same Walsh code

Mobile to BTSP d d N i (NS) d


» Pseudorandom Noise (NS) codes» Appears to be random but is not» 4.4 trillion combinations of code for CDMA» Less computationally intensive» (assigned during setup, hardwired set of codes for

discovery)

At the receiver…

RF to digital signalg gDespreading of the signalDe-interleaving and decodingVoice decompressionDigital to Analog voice recovery


Forward Link Channels

Pilot» The BTS constantly transmits here. » The mobile uses this channel to acquire the system. » Mobile uses the pilot signal to monitor and adjust its

power

Sync» The BTS constantly transmits here. The mobile uses this

channel for time synchronization – system time and


identification number of the cell site. The mobile ignores the sync channel after it is synchronized.

Forward Link Channels

Pagingg g» CDMA uses up to seven paging channels» It transmits overhead information such as commands and

pages to mobiles» Traffic channel assignment during call set-up» Mobile ignores paging channel after a traffic channel is

established

Traffic


» CDMA uses 55-61 forward traffic channels to send both voice and overhead control data during a call

» When the call is completed, the mobile tunes back into the paging channel

Reverse Link Channels

Access» Register with the network» Originate calls» Respond to pages and commands » Transmit overhead messages

Traffic» Only used when there is a call

Transmits voice data to the BTS


» Transmits voice data to the BTS» Transmits the overhead control information during the

call

Call processing stages

Initialization» Acquires the system via the Pilot code channel » Synchronizes with the system via the Sync code channel

Idle mode» Mobile and base station communicate over the access

and paging code channels» The mobile obtains overhead information via the paging

code channel


Access mode» Call origination» Use of access and paging channels for call set up until a

traffic channel has been established

Call Overhead Messaging

Uses “Dim and Burst” or “Blank and Burst” signaling, which replaces part of the voice traffic with system messagesStrong data recovery schemes prevent the user from detecting this


cdma2000 vs W-CDMA

cdma2000 W-CDMA

Chip Rate 3.6864 Mbps 4.096 Mbps

Downlink Pilot for Channel Estimation

CDM common pilot

TDM dedicated Pilot


Antenna Beamforming

Aux. Pilot TDM dedicated Pilot

BS Synchronization

Synchronous Asynchronous

cdma2000/W-CDMA similarities

Coherent forward link (FL) and reverse link (RL)( ) ( )Fast power control in FL and RLVariable length orthogonal Walsh sequences for FL channelizationComplex QPSK spreading on FL and RLIdentical Polynomials for Convolutional CodesP ll l t b d f hi h d t t


Parallel turbo codes for higher data rates

cdma2000/W-CDMA similarities

Variable spreading factors for higher data p g gratesMobile assisted inter-frequency hard handoff proceduresVariable rate operation with blind rate estimation for simple services (voice)Continuous reverse link operation


p

What is Next?

OFDMWiMAXLong Term EvolutionCellular Landscape


References

http://www.youtube.com/watch?v=EDDEsX7vp yaIIhttp://www.youtube.com/watch?v=bur9hq_abog


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