Cellular networks and mobile computing coms 6998 8 spring 2012
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Cellular Networks and Mobile Computing COMS 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. Outline. Wireless c ommunications b asics

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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


Outline

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Basic wireless communication

Information is embedded in electromagnetic radiation

Lossy signal and interference

Noise

Recover information

Transmitter

Receiver

Basic Wireless Communication

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Noise interference

Noise & Interference

  • Thermal Noise

    • Generated due to random motion of electrons in the conductor and proportional to temperature

    • No= KoTdBm/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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Impact of white gaussian noise

Impact of White Gaussian Noise

Shannon Capacity

10

Signal Power

9

SNR =

Noise Power

8

7

Capacity (bits/sec/Hz)

6

5

C = log (1 + SNR)

4

3

2

1

0

-10

-5

0

5

10

15

20

25

30

SNR (dB)

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Scattering of signals multipath fading

Scattering of Signals - Multipath Fading

Reflection

Diffraction

Absorption

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Impact of mobility

Doppler Shift =

Signal Amplitude

Multipath Fading

time

Impact of Mobility

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Flat frequency selective fading

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

1

1

-1

-1

Symbol

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Typical pathloss

Shadow fading

Urban Macro cell -40 dB/decade

Log-normal with std ~ 8 dB

Typical Pathloss

1.0

10.0

100.0

-50

A decade : transmitter and receiver distance increase 10 times

-70

Free space : -20 dB/decade

-90

-110

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Spectrum reuse

Spectrum Reuse

A

B

I

b

S

I

a

a

SINRa =

I

+ N

a

S

b

S

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


The cellular principle

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)

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Fixed frequency planning

g1

g1

g4

g3

g3

g6

g7

Reuse of 3

g2

g2

g2

g6

g1

g1

g5

g1

g5

g3

g3

g3

g7

g2

g2

g4

g3

g2

g6

g7

g2

g6

g1

g3

g5

g1

g4

Reuse of 1/3

Fixed Frequency Planning

Each base is assigned a fixed frequency band

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Cellular network evolution

CDMA 2000

IS-95

CDMA

1X-DO

ANALOG

TDMA/CDMA

FDMA

TDMA

LTE

GSM

EDGE

UMTS

TDMA

TDMA

CDMA

Cellular Network Evolution

IS- 136

OFDM

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


The multiple access problem

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?

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Meeting room analogy

Meeting Room Analogy

Simultaneous 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)

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


F requency d ivision m ultiple a ccess

Frequency Division Multiple Access

Guard Band

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 transmitasynchronously on the uplink

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


T ime d ivision m ultiple a ccess

Time Division Multiple Access

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

FRAME j+2

FRAME j

FRAME j + 1

SLOT 2

SLOT 3

SLOT 1

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Tdma characteristics

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Orthogonality in tdma fdma

Orthogonality in TDMA/FDMA

Every 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

frequency

Granularity is determined by practical limitations

time

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


C ode d ivision m ultiple a ccess

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Spread spectrum systems

Spread Spectrum systems

frequency

time

time

code

Code orthogonality is preserved under linear transformationsand hence near orthogonality is preserved under signal propagation

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Orthogonal walsh codes

De-spreading

Information

Spreading

bit

chip

Walsh Code

Receiver

Transmitter

Orthogonal Walsh Codes

Spread factor 4 Walsh Array

1 1 1 1

1 -1 1 -1

1 1 -1 -1

1 -1 -1 1

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Power control is critical

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 – Mobilesadjust their transmit power according to the distance from the base, fade level, data rate

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Why cdma

CDMA

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Why cdma1

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Why cdma2

Each signal arriving at a different time can be recovered separately and combined coherently

The resulting diversity gain reduces fading

Spreading sequence in is offset by one chip compared to spreading sequence in

Why CDMA?

  • Simple multipath combining to combat fading

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Why cdma3

Why CDMA?

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

  • Soft Handoff - Make-before-break handoff

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


What is ofdm

1

Traditional Multi-carrier

T

Guard Band

What is OFDM ?

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

OFDM

Implemented digitally through FFTs

Frequency

Frequency

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

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


High spectral efficiency in wideband signaling

High Spectral Efficiency in Wideband Signaling

T large compared to channel delay spread

  • 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

1

T

Frequency

Narrow Band (~10 Khz)

Wide Band (~ Mhz)

Sub-carriers remain orthogonal under multipath propagation

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Reverse link orthogonal frequency division multiple access

Reverse link Orthogonal Frequency Division Multiple Access

  • Users are carrier synchronized to the base

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

User 1

W

  • 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

User 2

User 3

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Typical multiplexing in ofdma

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

Frequency

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

Time

Cellular Networks and Mobile Computing (COMS 6998-8)

Courtesy: Harish Vishwanath


Outline1

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Umts system architecture

UMTS System Architecture

Uu

Iu

Node B

MSC/VLR

GMSC

RNC

Node B

USIM

Cu

Iur

HLR

Iub

External Networks

ME

Node B

RNC

SGSN

GGSN

Node B

UE

UTRAN

CN

Cellular Networks and Mobile Computing (COMS 6998-8)


Cellular networks and mobile computing coms 6998 8 spring 2012

GMM/SM/SMS

GMM/SM/SMS

Relay

RRC

RANAP

RANAP

RRC

SCCP

RLC

SCCP

RLC

Signaling Bearer

Signaling Bearer

MAC

MAC

AAL5

AAL5

UMTS RF

UMTS RF

ATM

ATM

UE

RNS

SGSN

U

I

u

u

UMTS Control Plane Protocol Stacks


Umts user plane protocol stacks

Appli-

cation

Appli-

cation

Relay

IP, PPP,

OSP

IP, PPP,

OSP

I P

GTP-U

Relay

Relay

PDCP

PDCP

GTP-U

GTP-U

GTP-U

UDP/IP

UDP/IP

UDP/IP

UDP/IP

UDP/

TCP

IP

IP

MAC

IP

L2

L2

IP

UMTS RF

UMTS RF

L1

ATM

ATM

L1

UE

UTRAN

SGSN

GGSN

ISP

U

I

G

G

u

u

n

i

RLC

RLC

AAL5

AAL5

UMTS User Plane Protocol Stacks

MAC


Utran

UTRAN

UE

UTRAN

CN

UMTS Terrestrial Radio Access NetworkOverview

  • 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

Node B

RNC

Node B

RNS

Iur

Iub

Node B

RNC

Node B

RNS

UTRAN

Cellular Networks and Mobile Computing (COMS 6998-8)


Utran1

UTRAN

UE

UTRAN

CN

Logical Roles of the RNC

Controlling RNC (CRNC)

Responsible for the load and congestion control of its own cells

CRNC

Node B

RNC

Node B

Iu

Node B

Serving RNC (SRNC)

Terminates : Iu link of user data, Radio Resource Control Signalling

Performs : L2 processing of data to/from the radio interface, RRM operations (Handover, Outer Loop Power Control)

SRNC

Node B

Iur

UE

Iu

Node B

DRNC

Node B

Iu

Node B

SRNC

Node B

Drift RNC (DRNC)

Performs : Macrodiversity Combining and splitting

Iur

Iu

Node B

UE

DRNC

Node B

Cellular Networks and Mobile Computing (COMS 6998-8)


Radio resources management

UE

UTRAN

CN

Radio Resources Management

  • Network Based Functions

    • Admission Control (AC)

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

    • Load Control (LC)

      • Manages situation when system load exceeds the threshold and some counter measures have to betaken 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 isinitiated 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.

Cellular Networks and Mobile Computing (COMS 6998-8)


Connection based function

Connection Based Function

UE

UTRAN

CN

Power 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

Outer Loop Power Control

If quality < target, increases SIRTARGET

Fast Power Control

If SIR < SIRTARGET, send “power up” command to MS

Cellular Networks and Mobile Computing (COMS 6998-8)


Connection based function1

Connection Based Function

UE

UTRAN

CN

Handover

  • 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

Cellular Networks and Mobile Computing (COMS 6998-8)


Hsdpa

HSDPA

UE

UTRAN

CN

High 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

Cellular Networks and Mobile Computing (COMS 6998-8)


Core network

Core Network

UE

UTRAN

CN

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

MSC/VLR

Iu-cs

GMSC

HLR

External Networks

Iu-ps

SGSN

GGSN

  • Register :

    • Home Location Register (HLR)

      • Stores master copies of users service profiles

      • Stores UE location on the level of MSC/VLR/SGSN

Cellular Networks and Mobile Computing (COMS 6998-8)


Wcdma air interface

WCDMA Air Interface

UE

UTRAN

CN

Direct Sequence Spread Spectrum

Spreading

f

f

Code Gain

User 1

Wideband

Despreading

Spreading

f

f

Received

Narrowband

f

f

User N

Wideband

  • Frequency Reuse Factor = 1

Variable Spreading Factor (VSF)

Multipath Delay Profile

Spreading : 256

f

f

t

User 1

Wideband

Narrowband

Spreading : 16

f

f

t

User 2

Wideband

Wideband

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

  • 5 MHz Wideband Signal Allows Multipath Diversity with Rake Receiver

Cellular Networks and Mobile Computing (COMS 6998-8)


Wcdma air interface cont d

WCDMA Air Interface (Cont’d)

UE

UTRAN

CN

Cellular Networks and Mobile Computing (COMS 6998-8)


Wcdma air interface cont d1

UE

UTRAN

CN

WCDMA Air Interface (Cont’d)

  • Channel concepts

Cellular Networks and Mobile Computing (COMS 6998-8)


Wcdma air interface cont d2

WCDMA Air Interface (Cont’d)

UE

UTRAN

CN

Mapping of Transport Channels and Physical Channels

Broadcast Channel (BCH)

Primary Common Control Physical Channel (PCCPCH)

Forward Access Channel (FACH)

Secondary Common Control Physical Channel (SCCPCH)

Paging Channel (PCH)

Random Access Channel (RACH)

Physical Random Access Channel (PRACH)

Dedicated Physical Data Channel (DPDCH)

Dedicated Channel (DCH)

Dedicated Physical Control Channel (DPCCH)

Downlink Shared Channel (DSCH)

Physical Downlink Shared Channel (PDSCH)

Physical Common Packet Channel (PCPCH)

Common Packet Channel (CPCH)

Synchronization Channel (SCH)

Common Pilot Channel (CPICH)

Acquisition Indication Channel (AICH)

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

Paging Indication Channel (PICH)

CPCH Status Indication Channel (CSICH)

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Cellular networks and mobile computing coms 6998 8 spring 2012

WCDMA Air Interface (Cont’d)

UE

UTRAN

CN

  • Code to channel allocation

Cellular Networks and Mobile Computing (COMS 6998-8)


Codes in wcdma

UE

UTRAN

CN

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Rrc state machine

UE

UTRAN

CN

RRC State Machine

  • 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)

Cellular Networks and Mobile Computing (COMS 6998-8)


Rrc state machine cont d

UE

UTRAN

CN

RRC State Machine (Cont’d)

  • 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

Cellular Networks and Mobile Computing (COMS 6998-8)


Cellular networks and mobile computing coms 6998 8 spring 2012

RRC State Machine (Cont’d)

UE

UTRAN

CN

  • 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

Cellular Networks and Mobile Computing (COMS 6998-8)


Rrc state machine cont d1

UE

UTRAN

CN

RRC State Machine (Cont’d)

  • State promotions have promotion delay

  • State demotions incur tail times

Courtesy: FengQian

Tail Time

Delay: 1.5s

Delay: 2s

Tail Time

Cellular Networks and Mobile Computing (COMS 6998-8)


Outline2

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte t echnical o bjectives and a rchitecture

LTE Technical Objectivesand Architecture

  • User throughput [/MHz]:

    • Downlink: 3 to 4 times Release 6 HSDPA

    • Uplink: 2 to 3 times Release 6 EnhancedUplink

  • DownlinkCapacity: Peak data rate of 100 Mbps in 20 MHz maximumbandwidth

  • Uplinkcapacity: Peak data rate of 50 Mbps in 20 MHz maximumbandwidth

  • Latency: Transitiontimelessthan 5 ms in idealconditions (userplane), 100 ms controlplane (fastconnectionsetup)

  • Cell range: 5 km - optimal size, 30km sizes with reasonable

    performance, up to 100 km cell sizes supported with

    acceptable performance

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte t echnical o bjectives and architecture cont d

LTE Technical Objectivesand Architecture (Cont’d)

  • 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.

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte architecture

LTE Architecture

  • Entities and functionalities

NAS security

Idle state mobility handling

EPS bearer control

UE IP address allocation

Packet filtering

Radio bearer control

Inter-cell RRM

Connection mobility Control

Radio admission control

Mobility

anchoring

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte control plane protocol stack

LTE Control Plane Protocol Stack

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte data plane protocol stack

LTE Data Plane Protocol Stack

Cellular Networks and Mobile Computing (COMS 6998-8)


Functions of enodeb

UE

eNodeB

CN

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Functions of mme

UE

eNodeB

CN

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Session establishment message flow

Session Establishment Message Flow

Cellular Networks and Mobile Computing (COMS 6998-8)


Session states

Session States

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte vs umts

PDN GateWay

Serving GateWay

PGWSGW

MME

Mobility Management Entity

(not user plane

functions)

Node B

  • PGW/SGW

  • Deployed according to traffic demand

  • Only 2 user plane nodes (non-roaming case)

eNodeB

  • RNC functions moved to eNodeB.

  • No central radio controller node

  • OFDM radio, no soft handover

  • Operator demand to simplify

  • Control plane/user plane split for better scalability

  • MME control plane only

  • Typically centralized and pooled

LTE vs UMTS

  • Functional changes compared to the current UMTS Architecture

GGSN

SGSN

RNC

Cellular Networks and Mobile Computing (COMS 6998-8)


Lte phy basics

UE

eNodeB

CN

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Frame structure

UE

eNodeB

CN

Frame Structure

Frame Structure Type 1 (FDD)

Frame Structure Type 2 (TDD)

Cellular Networks and Mobile Computing (COMS 6998-8)


Resource grid

:

:

UE

eNodeB

CN

Resource Grid

One downlink slot, Tslot

  • 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

Resource block

Transmission BW

Resource element

12subcarriers

l=0

l=6

Cellular Networks and Mobile Computing (COMS 6998-8)


2 d time and frequency g rid

UE

eNodeB

CN

2-D time and Frequency Grid

#19

#18

#17

#16

1 radio frame = 10 msec (307200 x Ts)

Time

#5

#4

#3

#2

Sub-frame

#1

NscRB subcarriers (=12)

#0

Power

1 slot = 0.5 msec

Frequency

NBWDL subcarriers

Cellular Networks and Mobile Computing (COMS 6998-8)


Dl phy channels and signals

UE

eNodeB

CN

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Dl channel mapping

QPSK

16QAM

64QAM

P-SCH - Primary Synchronization Signal

S-SCH - Secondary Synchronization Signal

PBCH - Physical Broadcast Channel

PDCCH -Physical Downlink Control Channel

PDSCH - Physical Downlink Shared Channel

Reference Signal – (Pilot)

Time

Frequency

UE

eNodeB

CN

DL Channel Mapping

Cellular Networks and Mobile Computing (COMS 6998-8)


Ul phy signals and channels

UE

eNodeB

CN

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Ul channel mapping

PUSCHDemodulation Reference Signal(for PUSCH)

PUCCH

Demodulation Reference Signal(for PUCCH format 0 or 1, Normal CP)

UE

eNodeB

CN

UL Channel Mapping

64QAM16QAMQPSK

QPSK

BPSK

Time

Frequency

Cellular Networks and Mobile Computing (COMS 6998-8)


Rrc state machine1

UE

eNodeB

CN

RRC State Machine

  • Much simpler than UMTS

Cellular Networks and Mobile Computing (COMS 6998-8)


Summary

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

Cellular Networks and Mobile Computing (COMS 6998-8)


Questions

Questions?


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