Page 1
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
Peter A. Steenkiste, CMU 2
OFDM
Some slides provided by Rui AguiarUniversity of Aveiro
Page 2
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
Peter A. Steenkiste, CMU 3
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
Peter A. Steenkiste, CMU 4
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
Page 3
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.
Peter A. Steenkiste, CMU 5
» 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
Peter A. Steenkiste, CMU 6
» Edge – Enhanced Data Rate for GSM Evolution» UMTS – Universal Mobile Telecommunication System
Page 4
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)
Peter A. Steenkiste, CMU 7
» 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 )
Peter A. Steenkiste, CMU 8
» New billing mechanisms (user-oriented: by traffic, p.ex.)
Page 5
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,
Peter A. Steenkiste, CMU 9
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 )
Peter A. Steenkiste, CMU 10
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”
Page 6
GPRS Architecture
SGSNOther GPRSnetworks
BTS
BTSMT
BSC
Gb
SGSN
Gf
GsGr
DEIR
Gc
Gn
GGSN GiPDN
Gp GGSN
networks
Peter A. Steenkiste, CMU 11
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
Peter A. Steenkiste, CMU 12
Border Gateway
GGSN
Inter-operator
backbone
GPRS backbone
Internet PDN
Router LAN
Serviceplate-form
WAP, WWW, ...
Gc
Page 7
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
Peter A. Steenkiste, CMU 13
(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
Peter A. Steenkiste, CMU 14
» 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.
Page 8
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:
Peter A. Steenkiste, CMU 15
» 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)
Peter A. Steenkiste, CMU 16
» 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
Page 9
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
Peter A. Steenkiste, CMU 17
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
Peter A. Steenkiste, CMU 18
Activate PDP Context Accept Create PDP Context Response
QoS Negotiated,Access Point,…
PDP type,QoS Negotiated,…PDP type,PDP AddressQoS Negotiated,…
Page 10
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
Peter A. Steenkiste, CMU 19
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
Peter A. Steenkiste, CMU 20
Control Channel(PCCCH) PPCH
PNCH
PagingNotification
MS BSSMS BSS
Packet DedicatedControl Channels
PACCH
PTCCH
Associated Control
Timing Advance Control
MS BSSMS BSS
Page 11
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
Peter A. Steenkiste, CMU 21
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
Peter A. Steenkiste, CMU 22
Transmission
PDTCH
PACCH
PDTCH
PACCH
Negative Acknowledgement
Retransmission of blocks in error
Frame Transmission
Acknowledgement
Page 12
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:
Peter A. Steenkiste, CMU 23
-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)
Peter A. Steenkiste, CMU 24
» EDGE supports GMSK & (new) 8-PSK» Requires lots of changes in transceiver design!
Page 13
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
Peter A. Steenkiste, CMU 25
– 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”
Peter A. Steenkiste, CMU 26
Page 14
CDMA Signal Generation
Analog to digital conversion (Pulse Code g g (Modulation)VocodingEncoding and InterleavingChannelizationConversion of the digital signal to a RF signal (modulation)
Peter A. Steenkiste, CMU 27
(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
Peter A. Steenkiste, CMU 28
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
Page 15
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
Peter A. Steenkiste, CMU 29
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
Peter A. Steenkiste, CMU 30
during transmission
Page 16
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
Peter A. Steenkiste, CMU 31
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
Peter A. Steenkiste, CMU 32
yp
Page 17
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
Peter A. Steenkiste, CMU 33
» 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
Peter A. Steenkiste, CMU 34
Page 18
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
Peter A. Steenkiste, CMU 35
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
Peter A. Steenkiste, CMU 36
» 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
Page 19
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
Peter A. Steenkiste, CMU 37
» 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
Peter A. Steenkiste, CMU 38
Access mode» Call origination» Use of access and paging channels for call set up until a
traffic channel has been established
Page 20
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
Peter A. Steenkiste, CMU 39
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
Peter A. Steenkiste, CMU 40
Antenna Beamforming
Aux. Pilot TDM dedicated Pilot
BS Synchronization
Synchronous Asynchronous
Page 21
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
Peter A. Steenkiste, CMU 41
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
Peter A. Steenkiste, CMU 42
p
Page 22
What is Next?
OFDMWiMAXLong Term EvolutionCellular Landscape
Peter A. Steenkiste, CMU 43
References
http://www.youtube.com/watch?v=EDDEsX7vp yaIIhttp://www.youtube.com/watch?v=bur9hq_abog
Peter A. Steenkiste, CMU 44