Cellular Networks and Mobile Computing COMS 6998-8, Spring 2012

Cellular Networks and Mobile ComputingCOMS 6998-8, Spring 2012

Instructor: Li Erran Li ([email protected])

http://www.cs.columbia.edu/~coms6998-8/1/30/2012: Cellular Networks: UMTS and LTE

Cellular Networks and Mobile Computing (COMS 6998-8)

2

Outline• Wireless communications basics

– Signal propagation, fading, interference, cellular principle

• Multi-access techniques and cellular network air-interfaces– FDMA, TDMA, CDMA, OFDM

• 3G: UMTS– Architecture: entities and protocols– Physical layer– RRC state machine

• 4G: LTE– Architecture: entities and protocols– Physical layer– RRC state machine

1/23/12


3

Basic Wireless Communication

Transmitter

Information is embedded in electromagnetic radiation

Receiver

Lossy signal and interference

Noise

Recover information

1/23/12 Courtesy: Harish Vishwanath


4

Noise & Interference

• Thermal Noise– Generated due to random motion of electrons in the conductor and

proportional to temperature

– No= KoT dBm/Hz where Ko is Boltzmann’s constant– Receiver Noise Figure – extent to which thermal noise is enhanced

by receiver front end circuitry ~ 10 dB• Interference – signals transmitted by other users of the

wireless network• Signal transmitted by other wireless devices from different

wireless networks– Example: Microwave ovens near 802.11 network



5

Impact of White Gaussian Noise

-10 -5 0 5 10 15 20 25 300

1

2

3

4

5

6

7

8

9

10

SNR (dB)

Cap

acity

(bits

/sec

/Hz) SNR =

Signal Power

Noise Power

Shannon Capacity

C = log (1 + SNR)



6

Scattering of Signals - Multipath Fading

21( ) ( ) cj f ts t x t e

2 ( / )2 ( ) ( / ) cj f t d c js t x t d c e

ReflectionDiffractionAbsorption

Multiple paths with random phases and gains combine constructively and destructively to cause significant amplitude variations



7

Impact of Mobility

v

21( ) ( ) cj f ts t x t e

2 cos( ) ( )

2 ( ) ( )cvj f t t j

s t a x t t e

Doppler Shift =

cos( )v

Signal Amplitude

time

Multipath Fading



8

Flat & Frequency Selective Fading• When the multipath delay is small compared to

symbol duration of the signal, fading is flat or frequency non-selective

• Happens when signal bandwidth is small

• Urban macro-cell delay spread is 10 micro seconds • When signal bandwidth is large different bands

have different gains – frequency selective fading

max( ) ( )x t t x t

max

1xB t=

1-1 -1

1Symbol



9

Typical Pathloss1.0 10.0 100.0

-50

-70

-90

-110

Free space : -20 dB/decade

Urban Macro cell -40 dB/decade

Shadow fadingLog-normal with std ~ 8 dB

A decade : transmitter and receiver distance increase 10 times



10

Spectrum Reuse

Sa

I b

Sb

I a

a and b can receive simultaneously on the same frequency band if SINRa and SINRb are above required threshold

This happens if the respective transmitters are sufficiently far apart

SINRa = Sa

I a+ N

A B



11

The Cellular Principle

• Base stations transmit to and receive from mobiles at the assigned spectrum– Multiple base stations use the same spectrum (spectral

reuse)• The service area of each base station is called a cell• The wireless network consists of large number of cells

– Example – The network in Northern NJ is about 150 base stations for a given operator

• Cells can be further divided into multiple sectors using sectorized antennas

• Each terminal is typically served by the “closest” base station(s)



12

Fixed Frequency PlanningEach base is assigned a fixed frequency band

Reuse of 7 – nearest co-channel interferer is in the second ring

Reuse of 3

g1

g2

g1

g2

g3

g1

g2

g3

g2

g3

g1

g3

g2

g7

g1

g6

g3

g4

g5

g6

g4

g2

g7

g5

g3

g1

g4

g5

g7

g3

g2

g1

g6

g6

Reuse of 1/3



13

Cellular Network Evolution

IS- 136

GSMTDMA

EDGETDMA

UMTSCDMA

LTEANALOGFDMA

IS-95

TDMA

CDMA

CDMA 2000

1X-DOTDMA/CDMA

OFDM



14

The Multiple Access problem• The base station has to transmit to all the mobiles in its cell

(downlink or forward link)– Signal for user a is interference for user b

– Interference is typically as strong as signal since a and b are relatively close

– How to avoid interference?• All mobiles in the cell transmit to the base station (uplink or

reverse link)– Signal from a mobile near by will swamp out the signal from a

mobile farther away

– How to avoid interference?



15

Meeting Room AnalogySimultaneous meetings in different rooms (FDMA)

Simultaneous meetings in the same room at different times (TDMA)

Multiple meetings in the same room at the same time (CDMA)



16

Frequency Division Multiple Access

Each mobile is assigned a separate frequency channel for the duration of the call

Sufficient guard band is required to prevent adjacent channel interference

Mobiles can transmit asynchronously on the uplink

Guard Band



17

Time Division Multiple AccessTime is divided into slots and only one mobile transmits during each slot

FRAME j FRAME j + 1 FRAME j+2

SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6

Guard time – Signal transmitted by mobiles at different locations do not arrive at the base at the same time



18

TDMA Characteristics

• Discontinuous transmission with information to be transmitted buffered until transmission time– Possible only with digital technology– Transmission delay

• Synchronous transmission required – Mobiles derive timing from the base station signal

• Guard time can be reduced if mobiles pre-correct for transmission delay– More efficient than FDMA which requires significant

guard band



19

Orthogonality in TDMA/FDMAEvery information signal lasts a certain duration of time and occupies a certain bandwidth and thus corresponds to a certain region in the time-frequency plane

Granularity is determined by practical limitations

Time division and frequency division are invariant under transformation of the channel and retain the orthogonality

Any orthogonal signaling scheme for which orthogonality is preserved will be a useful multiple access technique

time

frequency



20

Code Division Multiple Access

• Use of orthogonal codes to separate different transmissions• Each symbol or bit is transmitted as a larger number of bits using the user

specific code – Spreading• Spread spectrum technology

– The bandwidth occupied by the signal is much larger than the information transmission rate

– Example: 9.6 Kbps voice is transmitted over 1.25 MHz of bandwidth, a bandwidth expansion of ~100



21

Spread Spectrum systems

frequency

time

code

time

Code orthogonality is preserved under linear transformations and hence near orthogonality is preserved under signal propagation



22

Orthogonal Walsh Codes1 1 1 11 -1 1 -11 1 -1 -11 -1 -1 1

Spread factor 4 Walsh Array

chip

Transmitter ReceiverWalsh Code

Information SpreadingDe-spreading

bit



23

Power Control is critical

• The dynamic range of the pathloss for a typical cell is about 80 dB

• The signal received from the closest mobile is 80 dB stronger than the farthest mobile without power control– Code orthogonality is not sufficient to separate the signals -

Near-far problem in CDMA– Strict orthogonality in TDMA/FDMA makes power control not

critical• Power Control – Mobiles adjust their transmit power

according to the distance from the base, fade level, data rate



Why CDMA?• Simplified frequency planning

– Universal frequency reuse with spreading gain to mitigate interference

– Interference averaging allows designing for average interference level instead of for worst case interference

TDMA / FDMA CDMA



25

Why CDMA?• Variable rate Vocoder with Power Control

– Advanced data compression technology is used to compress data according to content

– Typical voice activity is 55% - CDMA reduces interference by turning down transmission between talk spurts

– Reduced average transmission power increases capacity through statistical multiplexing

– Compensate for fading through power control - transmit more power only under deep fades avoiding big fade margins



26

Why CDMA?

• Simple multipath combining to combat fadingEach signal arriving at a different time can be recovered separately and combined coherently

The resulting diversity gain reduces fading

21( ) ( ) cj f ts t x t e

2 ( )2 ( ) ( ) c cj f t T j

cs t x t T e Spreading sequence in is offset by one chip compared to spreading sequence in

( )x t

( )cx t T



27

Why CDMA?

• Soft Handoff - Make-before-break handoff

Mobile can transmit and receive from multiple base stations because all base stations use the same frequency

Signals from different bases can be received separately and then combined because each base uses a unique spreading code



28

What is OFDM ?

OFDM invented in Bell Labs by R.W. Chang in ~1964 and patent awarded in 1970

Widely used: Digital audio and Video broadcasting, ADSL, HDSL, Wireless LANs

Orthogonal Frequency Division Multiplexing is block transmission of N symbols in parallel on N orthogonal sub-carriers

Guard Band

Traditional Multi-carrier

Frequency

1T

Frequency

OFDM

Implemented digitally through FFTs



29

High Spectral Efficiency in Wideband Signaling

Closely spaced sub-carriers without guard band

Each sub-carrier undergoes (narrow band) flat fading

- Simplified receiver processing

Frequency or multi-user diversity through coding or scheduling across sub-carriers

Dynamic power allocation across sub-carriers allows for interference mitigation across cells

Orthogonal multiple access

Frequency

Narrow Band (~10 Khz)

Wide Band (~ Mhz)

T large compared to channel delay spread

Sub-carriers remain orthogonal under multipath propagation

T1



30

Reverse link Orthogonal Frequency Division Multiple Access

User 1

User 2

User 3

Efficient use of spectrum by multiple users

Sub-carriers transmitted by different users are orthogonal at the receiver

- No intra-cell interference

CDMA uplink is non-orthogonal since synchronization requirement is ~ 1/W and so difficult to achieve

Users are carrier synchronized to the base

Differential delay between users’ signals at the base need to be small compared to T

W



31

Typical Multiplexing in OFDMA Each color represents a user Each user is assigned a frequency-time

tile which consists of pilot sub-carriers and data sub-carriers Yellow color indicates pilot sub-

carriers Channel is constant in each tile

Block hopping of each user’s tile for frequency diversity

Time

Freq

uenc

y

Typical pilot ratio: 4.8 % (1/21) for LTE for 1 Tx antenna and 9.5% for 2 Tx antennas



32






1/23/12


33

UMTS System Architecture

USIM

ME

Node B

Node BRNC

Node B

Node BRNC

MSC/VLR GMSC

SGSN GGSN

HLR

UTRAN CNUE

Exte

rnal

Net

wor

ks

Cu

Uu Iu

IubIur

1/23/12

UMTS Control Plane Protocol Stacks

IuUuSGSN

Signaling Bearer

SCCP

RANAP

GMM/SM/SMS

AAL5ATM

UE

UMTS RF

MAC

RLC

RRC

GMM/SM/SMS

RNS

RRC

Signaling Bearer

MAC

AAL5ATM

SCCP

RANAP

Relay

UMTS RF

RLC

UMTS User Plane Protocol Stacks

Iu Uu Gn GiISP

IP

SGSN

GTP-UGTP-U

Relay

AAL5

ATM L1

L2

UDP/IPUDP/IP

UTRAN

GTP-UPDCP

Relay

UMTS RF

UDP/IPRLC

AAL5

ATM

MAC

UE

IP, PPP,OSP

Appli-cation

PDCP

UMTS RF

RLC

Appli-cation

I P

GTP-U

IP

IP

GGSN

L1

L2 IP

UDP/IP

IP, PPP,OSP UDP/

TCP

Relay

MAC


36

UTRAN UE UTRAN CN

Node B

Node BRNC

Node B

Node BRNC

IubIur

UTRAN

RNS

RNS

Two Distinct Elements :

Base Stations (Node B)Radio Network Controllers (RNC)

1 RNC and 1+ Node Bs are group together to form a Radio Network Sub-system (RNS)

Handles all Radio-Related Functionality

Soft Handover Radio Resources Management Algorithms

Maximization of the commonalities of the PS and CS data handling

UMTS Terrestrial Radio Access Network Overview

1/23/12


37

UTRAN UE UTRAN CN

Node B

Node BRNC

Logical Roles of the RNC

Controlling RNC (CRNC)Responsible for the load and congestion control of its own cells

CRNC

Node B

Node B SRNC

Serving RNC (SRNC)Terminates : Iu link of user data, Radio Resource Control SignallingPerforms : L2 processing of data to/from the radio interface, RRM operations (Handover, Outer Loop Power Control)

Drift RNC (DRNC)Performs : Macrodiversity Combining and splitting

Node B

Node BDRNC

Node B

Node BSRNC

Node B

Node BDRNC

UE

UE

Iu

Iu

Iu

Iu

Iur

Iur

1/23/12


38

Radio Resources Management

• Network Based Functions– Admission Control (AC)

• Handles all new incoming traffic. Check whether new connection can be admitted to the system and generates parameters for it.

– Load Control (LC)• Manages situation when system load exceeds the threshold and some counter

measures have to be taken to get system back to a feasible load.– Packet Scheduler (PS): at RNC and NodeB (only for HSDPA and HSUPA)

• Handles all non real time traffic, (packet data users). It decides when a packet transmission is initiated and the bit rate to be used.

• Connection Based Functions– Handover Control (HC)

• Handles and makes the handover decisions.• Controls the active set of Base Stations of MS.

– Power Control (PC)• Maintains radio link quality.• Minimize and control the power used in radio interface, thus maximizing the call

capacity.

UE UTRAN CN

1/23/12


39

Connection Based FunctionPower Control

Prevent Excessive Interference and Near-far Effect

Fast Close-Loop Power Control Feedback loop with 1.5kHz cycle to

adjust uplink / downlink power to its minimum

Even faster than the speed of Rayleigh fading for moderate mobile speeds

Outer Loop Power Control Adjust the target SIR setpoint in base

station according to the target BER Commanded by RNC

Fast Power ControlIf SIR < SIRTARGET, send “power up” command to MS

Outer Loop Power ControlIf quality < target, increases SIRTARGET

UE UTRAN CN

1/23/12


40

Connection Based FunctionHandover Softer Handover

A MS is in the overlapping coverage of 2 sectors of a base station

Concurrent communication via 2 air interface channels

2 channels are maximally combined with rake receiver

Soft Handover A MS is in the overlapping coverage of 2

different base stations Concurrent communication via 2 air interface

channels Downlink: Maximal combining with rake

receiver Uplink: Routed to RNC for selection combining,

according to a frame reliability indicator by the base station

Hard handover HSDPA Inter-system and inter-frequency

UE UTRAN CN

1/23/12


41

HSDPAHigh Speed Downlink Packet Access Improves System Capacity and User Data Rates in the Downlink Direction

to 10Mbps in a 5MHz Channel

Adaptive Modulation and Coding (AMC) Replaces Fast Power Control :

User farer from Base Station utilizes a coding and modulation that requires lower Bit Energy to Interference Ratio, leading to a lower throughput

Replaces Variable Spreading Factor :Use of more robust coding and fast Hybrid Automatic Repeat Request (HARQ, retransmit occurs only between UE and BS)

HARQ provides Fast Retransmission with Soft Combining and Incremental Redundancy Soft Combining : Identical Retransmissions Incremental Redundancy : Retransmits Parity Bits only

Fast Scheduling Function which is Controlled in the Base Station rather than by the RNC

UE UTRAN CN

1/23/12


42

Core Network UE UTRAN CN

MSC/VLR GMSC

SGSN GGSN

HLR

Exte

rnal

Net

wor

ks

Iu-cs

Core Network

CS Domain : Mobile Switching Centre (MSC)

Switching CS transactions Visitor Location Register (VLR)

Holds a copy of the visiting user’s service profile, and the precise info of the UE’s location

Gateway MSC (GMSC) The switch that connects to

external networks

PS Domain : Serving GPRS Support Node (SGSN)

Similar function as MSC/VLR Gateway GPRS Support Node (GGSN)

Similar function as GMSC

Register :

Home Location Register (HLR) Stores master copies of users

service profiles Stores UE location on the

level of MSC/VLR/SGSN

Iu-ps

1/23/12


43

WCDMA Air Interface UE UTRAN CN

Direct Sequence Spread Spectrum

User 1

User N

Spreading

SpreadingReceived

Despreading

Narrowband

Code Gain

Þ Frequency Reuse Factor = 1

Wideband

Wideband

Þ 5 MHz Wideband Signal Allows Multipath Diversity with Rake Receiver

Wideband

Narrowband

f

f

ff

f

f

t

t

Multipath Delay Profile

Variable Spreading Factor (VSF)

User 1

Spreading : 256

Widebandf f

User 2

Spreading : 16

Widebandf fÞ VSF Allows Bandwidth on Demand. Lower Spreading Factor requires Higher SNR, causing Higher Interference in exchange.

1/23/12


44

WCDMA Air Interface (Cont’d)

Multiple Access Method DS-CDMADuplexing Method FDD/TDDBase Station Synchronization Asychronous OperationChannel bandwidth 5MHzChip Rate 3.84 McpsFrame Length 10 msService Multiplexing Multiple Services with different QoS

Requirements Multiplexed on one Connection

Multirate Concept Variable Spreading Factor and MulticodeDetection Coherent, using Pilot Symbols or Common

Pilot

1/23/12

UE UTRAN CN


45

• Channel concepts

UE UTRAN CN

1/23/12

WCDMA Air Interface (Cont’d)


46

WCDMA Air Interface (Cont’d) UE UTRAN CN

Mapping of Transport Channels and Physical ChannelsBroadcast Channel (BCH)

Forward Access Channel (FACH)

Paging Channel (PCH)Random Access Channel

(RACH)Dedicated Channel (DCH)

Downlink Shared Channel (DSCH)

Common Packet Channel (CPCH)

Primary Common Control Physical Channel (PCCPCH)Secondary Common Control Physical Channel (SCCPCH)Physical Random Access Channel (PRACH)Dedicated Physical Data Channel (DPDCH)Dedicated Physical Control Channel (DPCCH)Physical Downlink Shared Channel (PDSCH)Physical Common Packet Channel (PCPCH)Synchronization Channel (SCH)Common Pilot Channel (CPICH)Acquisition Indication Channel (AICH)Paging Indication Channel (PICH)CPCH Status Indication Channel (CSICH)

Collision Detection/Channel Assignment Indicator Channel (CD/CA-ICH)

Highly Differentiated Types of Channels enable best combination of Interference Reduction, QoS and Energy Efficiency

1/23/12


47

WCDMA Air Interface (Cont’d) UE UTRAN CN

• Code to channel allocation

1/23/12

48

Codes in WCDMA

• Channelization Codes (=short code)– Used for

• channel separation from the single source in downlink• separation of data and control channels from each

other in the uplink– Same channelization codes in every cell / mobiles and

therefore the additional scrambling code is needed

• Scrambling codes (=long code)– Very long (38400 chips = 10 ms =1 radio frame), many

codes available– Does not spread the signal– Uplink: to separate different mobiles– Downlink: to separate different cells– The correlation between two codes (two

mobiles/Node Bs) is low• Not fully orthogonal

1/23/12

UE UTRAN CN



49

• IDLE: procedures based on reception rather than transmission– Reception of System Information messages – PLMN selection Cell selection Registration

(requires RRC connection establishment) – Reception of paging Type 1 messages with a DRX

cycle (may trigger RRC connection establishment) Cell reselection

– Location and routing area updates (requires RRC connection establishment)

RRC State Machine

1/23/12

UE UTRAN CN


50

• CELL_FACH: need to continuously receive (search for UE identity in messages on FACH), data can be sent by RNC any time– Can transfer small PS data– UE and network resource required low– Cell re-selections when UE mobile– Inter-system and inter-frequency handoff possible– Can receive paging Type 2 messages without a

DRX cycle

RRC State Machine (Cont’d)

1/23/12

UE UTRAN CN


51

• CELL_DCH: need to continuously receive, and sent whenever there is data– Possible to transfer large quantities of uplink and downlink

data – Dedicated channels can be used for both CS and PS

connections – HSDPA and HSUPA can be used for PS connections – UE and network resource requirement is relatively high– Soft handover possible for dedicated channels and HSUPA

Inter-system and inter-frequency handover possible – Paging Type 2 messages without a DRX cycle are used for

paging purposes

1/23/12

RRC State Machine (Cont’d) UE UTRAN CN

RRC State Machine (Cont’d)

• State promotions have promotion delay• State demotions incur tail times

Tail Time

Tail Time

Delay: 1.5s

Delay: 2s

Channel Radio Power

IDLE Not allocated

Almost zero

CELL_FACH Shared, Low Speed

Low

CELL_DCH Dedicated, High Speed

High

Courtesy: Feng Qian

Cellular Networks and Mobile Computing (COMS 6998-8)1/23/12

UE UTRAN CN


53






1/23/12


54

LTE Technical Objectives and Architecture

• User throughput [/MHz]:– Downlink: 3 to 4 times Release 6 HSDPA – Uplink: 2 to 3 times Release 6 Enhanced Uplink

• Downlink Capacity: Peak data rate of 100 Mbps in 20 MHz maximum bandwidth

• Uplink capacity: Peak data rate of 50 Mbps in 20 MHz maximum bandwidth

• Latency: Transition time less than 5 ms in ideal conditions (user plane), 100 ms control plane (fast connection setup)

• Cell range: 5 km - optimal size, 30km sizes with reasonable performance, up to 100 km cell sizes supported with acceptable performance

1/23/12


55

• Mobility: Optimised for low speed but supporting 120 km/h– Most data users are less mobile!

• Simplified architecture: Simpler E-UTRAN architecture: no RNC, no CS domain, no DCH

• Scalable bandwidth: 1.25MHz to 20MHz: Deployment possible in GSM bands.

LTE Technical Objectives and Architecture (Cont’d)

1/23/12


56

LTE Architecture

• Entities and functionalities

1/23/12

Mobility anchoring

UE IP address allocationPacket filtering

Radio bearer controlInter-cell RRMConnection mobility ControlRadio admission control

NAS securityIdle state mobility handlingEPS bearer control


57

LTE Control Plane Protocol Stack

1/23/12


58

LTE Data Plane Protocol Stack

1/23/12


59

Functions of eNodeB

• Terminates RRC, RLC and MAC protocols and takes care of Radio Resource Management functions– Controls radio bearers– Controls radio admissions– Controls mobility connections– Allocates radio resources dynamically (scheduling)– Receives measurement reports from UE

• Selects MME at UE attachment• Schedules and transmits paging messages coming from MME• Schedules and transmits broadcast information coming from MME &

O&M• Decides measurement report configuration for mobility and scheduling• Does IP header compression and encryption of user data streams

1/23/12

UE eNodeB CN


60

Functions of MME

• Mobility Management Entity (MME) functions– Manages and stores UE context– Generates temporary identities and allocates

them to UEs– Checks authorization– Distributes paging messages to eNBs– Takes care of security protocol– Controls idle state mobility– Ciphers & integrity protects NAS signaling

1/23/12

UE eNodeB CN


61

Session Establishment Message Flow

1/23/12


62

Session States

1/23/12


63

LTE vs UMTS

GGSN

SGSN

RNC

Node B eNodeB

RNC functions moved to eNodeB.• No central radio controller node• OFDM radio, no soft handover• Operator demand to simplify

Mobility Management EntityMME(not user plane functions)

Control plane/user plane split for better scalability• MME control plane only• Typically centralized and pooled

PGWSGW

PDN GateWay Serving GateWay

PGW/SGW • Deployed according to traffic demand• Only 2 user plane nodes (non-roaming case)

• Functional changes compared to the current UMTS Architecture

1/23/12


64

LTE PHY Basics

• Six bandwidths– 1.4, 3, 5, 10, 15, and 20 MHz

• Two modes – FDD and TDD

• 100 Mbps DL (SISO) and 50 Mbps UL• Transmission technology

– OFDM for multipath resistance– DL OFDMA for multiple access in frequency/time– UL SC-FDMA to deal with PAPR ratio problem

1/23/12

UE eNodeB CN


65

Frame StructureFrame Structure Type 1 (FDD)

Frame Structure Type 2 (TDD)

1/23/12

UE eNodeB CN


66

Resource Grid

• 6 or 7 OFDM symbols in 1 slot

• Subcarrier spacing = 15 kHz

• Block of 12 SCs in 1 slot = 1 RB– 0.5 ms x 180 kHz– Smallest unit of allocation

6 or 7 OFDM symbols

One downlink slot, Tslot

:

:

Transmission BW

Resource block

Resource element

l=0 l=6

12 subcarriers

1/23/12

UE eNodeB CN


67

2-D time and Frequency Grid

Time

Frequency

1 radio fra

me = 10 msec (3

07200 x Ts)

#0#1

#2#3

#4#5

#19#18

#17#16

Sub-fr

ame

NBWDL subcarriers

NscRB subcarriers (=12)

Pow

er

1 slot =

0.5

mse

c

1/23/12

UE eNodeB CN


68

DL PHY Channels and Signals

• Signals: generated in PHY layers– P-SS: used for initial sync– S-SS: frame boundary determination– RS: pilots for channel estimation and tracking

• Channels: carry data from higher layers– PBCH: broadcast cell-specific info– PDCCH: channel allocation and control info– PCFICH: info on size of PDCCH– PHICH: Ack/Nack for UL blocks– PDSCH: Dynamically allocated user data

1/23/12

UE eNodeB CN


69

DL Channel Mapping

64QAM16QAM QPSK

Frequency

Time

P-SCH - Primary Synchronization SignalS-SCH - Secondary Synchronization SignalPBCH - Physical Broadcast ChannelPDCCH -Physical Downlink Control ChannelPDSCH - Physical Downlink Shared ChannelReference Signal – (Pilot)

1/23/12

UE eNodeB CN


70

UL PHY Signals and Channels

• Signals: generated in the PHY layer– Demodulation RS : sync and channel estimation– SRS: Channel quality estimation

• Channels: carry data from higher layers– PUSCH: Uplink data– PUCCH: UL control info– PRACH: Random access for connection

establishment

1/23/12

UE eNodeB CN


71

UL Channel MappingPUSCHDemodulation Reference Signal(for PUSCH)PUCCHDemodulation Reference Signal(for PUCCH format 0 or 1, Normal CP)

64QAM16QAMQPSK

QPSKBPSK

Frequency

Time

1/23/12

UE eNodeB CN


72

RRC State Machine

• Much simpler than UMTS

1/23/12

UE eNodeB CN


73

Summary

• Cellular networks are very different from WiFi– Cannot be based on carrier sensing due to large coverage

area– Path must be setup dynamically due to mobility– Need to handle charging functions and QoS

• Different physical layer technologies have very different overhead during inactivity– Dedicated channels prevent others from using the channel

• Frequent RRC state transitions in UE can result in high network overhead and UE battery power consumption

1/23/12

Questions?

Cellular Networks and Mobile Computing COMS 6998-8, Spring 2012

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