Mobility and Handover Issues. Mobile Communication Two aspects of mobility: –user mobility: users communicate (wireless) anytime, anywhere, with anyone.

Mobility and Handover Issues

Mobile Communication

• Two aspects of mobility:– user mobility: users communicate (wireless) “anytime, anywhere,

with anyone”– device portability: devices can be connected anytime, anywhere

to the network

• Wireless vs. mobile Examples stationary computer notebook in a hotel wireless LANs in historic buildings Personal Digital Assistant (PDA)

What is Mobility?

• A device that moves– Between different geographical locations– Between different networks

• A person who moves– Between different geographical locations– Between different networks– Between different communication devices– Between different applications

Device mobility• Plug in laptop at home/work on Ethernet

– Occasional long breaks in network access– Wired network access only (connected => well-connected)– Network address changes– Only one type of network interface– May want access to information when no network is available:

hoard information locally

• Cell phone with access to cellular network– Continuous connectivity– Phone # remains the same (high-level network address)– Network performance may vary from place to place

Device mobility, continued• Can we achieve best of both worlds?

– Continuous connectivity of wireless access– Performance of better networks when available

• Laptop moves between Ethernet, WLAN and Cellular networks– Wired and wireless network access– Potentially continuous connectivity, but may be breaks in service– Network address changes– Radically different network performance on different networks

People mobility• Phone available at home or at work

– Multiple phone numbers to reach me– Breaks in my reachability when I’m not in

• Cell phone– Only one number to reach me– Continuously reachable– Sometimes poor quality and expensive connectivity

• Cell phone, networked PDA, etc.– Multiple numbers/addresses for best quality connection– Continuous reachability– Best choice of address may depend on sender’s device or

message content

Mobility means changesHow does it affect the following?• Hardware

– Lighter– More robust– Lower power

• Wireless communication– Can’t tune for stationary access

• Network protocols– Name changes– Delay changes– Error rate changes

Changes, continued• Fidelity

– High fidelity may not be possible

• Data consistency– Strong consistency no longer possible

• Location/transparency awareness– Transparency not always desirable

• Names/addresses– Names of endpoints may change

• Security– Lighter-weight algorithms– Endpoint authentication harder– Devices more vulnerable

Changes, continued, again• Performance

– Network, CPU all constrained– Delay and delay variability

• Operating systems– New resources to track and manage: energy

• Applications– Name changes– Changes in connectivity– Changes in quality of resources

• People– Introduces new complexities, failures, devices

Mobile and wireless services – Always Best Connected

UMTS,DECT2 Mbit/s

UMTS, GSM384 kbit/s

LAN100 Mbit/s,WLAN54 Mbit/s

UMTS, GSM115 kbit/s

GSM 115 kbit/s,WLAN 11 Mbit/s

GSM 53 kbit/sBluetooth 500 kbit/s

GSM/EDGE 384 kbit/s,WLAN 780 kbit/s

LAN, WLAN780 kbit/s

Example changes• Addresses

– Phone numbers, IP addresses

• Network performance– Bandwidth, delay, bit error rates, cost, connectivity

• Network interfaces– PPP, eth0, strip

• Between applications– Different interfaces over phone & laptop

• Within applications– Loss of bandwidth triggers change from color to B&W

• Available resources– Files, printers, displays, power, even routing

Effects of device portability• Power consumption

– limited computing power, low quality displays, small disks due to limited battery capacity

– CPU: power consumption ~ CV2f• C: internal capacity, reduced by integration• V: supply voltage, can be reduced to a certain limit• f: clock frequency, can be reduced temporally

• Loss of data– higher probability, has to be included in advance into the design (e.g.,

defects, theft)• Limited user interfaces

– compromise between size of fingers and portability– integration of character/voice recognition, abstract symbols

• Limited memory– limited value of mass memories with moving parts– flash-memory or ? as alternative

Mobile devices

performanceperformance

Pager• receive only• tiny displays• simple text messages

Mobile phones• voice, data• simple graphical displays

PDA• simpler graphical displays• character recognition• simplified WWW

Palmtop• tiny keyboard• simple versions of standard applications

Sensors,embeddedcontrollers

Summing upGenerally, mobility stresses all resources further:• CPU• Power• Bandwidth• Delay tolerance• Radio spectrum• Human attention• Physical size• Constraints on peripherals and GUIs (modality of

interaction)• Locations (body parts!) for device placement

Mobility and handover issues

• Mobility allows the possibility for the mobile subscriber of being reachable anywhere and at anytime.

• Managing the mobile terminal mobility is one of the most essential parts of cellular system functionality.

• In a radio communication system Paging, Location Update and Handover Operations provide the User mobility.

Mobility and handover issues

• Handover mechanisms guarantees that whenever the mobile is moving from one base station area (Cell) to another, the radio signal is handed over to the target Base Station.

• Location update and Paging mechanisms guarantee that the mobile station can be reached even though there is no continues active radio link between the mobile and the corresponding Base Station.– The Paging mechanism is always initiated by the Network.– The Location Update procedure is always initiated by the Mobile

Station.

Mobility and handover issues - Handover Control

• Handover is one of the essential means to guarantee the User Mobility in a mobile communications network

• The basic concept of handover control is that when the subscriber moves from the coverage area of one cell to another, a new connection with the new target cell has to be set-up and the connection with the old cell may be released.

Mobility and handover issues - Handover Control

Frequency F1

Frequency F1

Frequency F1

Frequency F1

Frequency F1

Frequency F1

Cell 1

Cell 2

Mobility and Handover Issues -Reasons behind the Handover

• The basic reason behind the handover is that the air interface connection does not fulfil the desired criteria set for it any more resulting in unacceptable QoS and thus either the Mobile Station or the Network initiates Handover in order to improve the procedure.

• The decision of whether the handover should be performed or not is based on handover criteria.

• Handover may occur due to Signal Quality, User Mobility, Traffic Distribution etc.

Mobility and Handover Issues -Reasons behind the Handover

• Signal Quality handover – occurs when the quality or the strength of the radio signal falls below

certain parameters specified in handover criteria. – deterioration of the signal is detected by constant signal

measurements carried out by both the Mobile Terminal and the Base Station.

– The signal quality reason handover may be applied both for the uplink and downlink radio links.

• Traffic handover – occurs when the traffic capacity of a cell has reached its maximum

or is approaching it– the UE near the edges of the cell with high load may be handed over

to neighbouring cells with less traffic load. – the system load can be distributed more uniformly.

Mobility and Handover Issues -Reasons behind the Handover

• The number of handovers depends on the degree of mobility.

• It is obvious that the faster the mobile node (MN) is moving, the more handovers it causes to the Network.

• To avoid undesirable handovers the MN with high motion speed may be handed over from micro-cells to macro-cells.

• On the other hand, if the MN moving slowly or not at all, it can be handed over from macro-cells to micro-cells to improve the radio signal strength and avoid consuming its battery.

Mobility and Handover Issues -Reasons behind the Handover

Macro Cell

Micro Cell 2

Micro Cell 1Frequency F1

UE Moving from Micro Cell 1 to Macro Cell

Frequency F2

Macro Cell

Micro Cell 2

Micro Cell 1Frequency F1

UE Moving from Micro Cell 1 to Macro Cell

Frequency F2

Overlay Networks - The Global Goal

regional

metropolitan area

campus-based

in-house

verticalhandover

horizontalhandover

integration of heterogeneous fixed andmobile networks with varyingtransmission characteristics

Mobility and Handover Issues – Handover Process

• A basic handover process consists of three main phases:

Mobility and Handover Issues – Handover Process

• Handover measurement provision is a very important task for the system performance. This is because the signal strength of the radio channel may vary drastically due to fading and signal path loss, resulting from the cell environment (e.g. buildings, mountains) and user mobility.

• Decision phase consists of assessment of the overall QoS of the connection and comparing it with the requested QoS attributes and estimates measured from neighboring cells. Depending on the outcome of this comparison, the handover procedure may or may not be trigger.

• The Network checks whether the values indicated in the measurement reports meet the QoS specified for the end-user service. If not, then it allows executing the handover.

Mobility and Handover Issues – Handover Decision

Receive LevelBTSold

Receive LevelBTSold

MS MS

HO_MARGIN

BTSold BTSnew

Mobility and Handover Issues – Handover Procedure

HO access

BTSold BSCnew

measurementresult

BSCold

Link establishment

MSCMSmeasurementreport

HO decision

HO required

BTSnew

HO request

resource allocation

ch. activation

ch. activation ackHO request ackHO commandHO commandHO command

HO completeHO completeclear commandclear command

clear complete clear complete

Mobility and Handover Issues – GSM: 4 Types of Handover

MSC MSC

BSC BSCBSC

BTS BTS BTSBTS

MS MS MS MS

12 3 4

Mobility and Handover Issues – UMTS Radio Access Network Architecture

• UTRAN comprises several RNSs

• Node B can support FDD or TDD or both

• RNC is responsible for handover decisions requiring signaling to the UE

• Cell offers FDD or TDD

RNC: Radio Network Controller

RNS: Radio Network Subsystem

RNC

Iub

RNS

CN

RNC

Iub

RNS

Iur

Iu

Node B

Node BNode B

Node BNode B

Node B

Node B

UE

Mobility and Handover Issues – Support of mobility in UTRAN

• From and to other systems (e.g., UMTS to GSM)– This is a must as UMTS coverage will be poor in the beginning

• RNS controlling the connection is called SRNS (Serving RNS)• RNS offering additional resources (e.g., for soft handover) is called Drift RNS (DRNS)

• End-to-end connections between UE and CN only via Iu at the SRNS

– Change of SRNS requires change of Iu

– Initiated by the SRNS

– Controlled by the RNC and CN

SRNC

UE

DRNC

Iur

CN

Iu

Node BIub

Node BIub

Mobility and Handover Issues – Example Handover Types in UMTS/GSM

RNC1

UE1

RNC2

Iur

3G MSC1

Iu

Node B1

IubNode B2

Node B3 3G MSC2

BSCBTS 2G MSC3

AAbis

UE2

UE3

UE4

Mobility and Handover Issues – Support of mobility in UTRAN: Macrodiversity

• Multicasting of data via several physical channels

– Enables soft handover– FDD mode only

• Uplink– Simultaneous reception of UE data

at several Node Bs– Reconstruction of data at Node B,

SRNC or DRNC

• Downlink– Simultaneous transmission of data

via different cells– Different spreading codes in

different cells

CNNode B RNC

Node BUE

Mobility and Handover Issues – Soft Handover Algorithm (Macrodiversity)

Mobility and Handover Issues – Soft Handover Algorithm example

• By the term Soft Handover we mean that the mobile node is maintaining connections with more than one base stations.

• The Active Set includes the cells that form a soft handover connection to the mobile station.

• The Neighbor/Monitored Set is the list of cells that the mobile station continuously measures, but their signal strength is not powerful enough to be added to the Active Set.

SHO Algorithm

• The algorithm samples the signal strength of the surrounding base stations every 1 sec

• Uses 3dB as the threshold for soft handover and

• Uses 6dB as the threshold for hard handover.

• The size of the Active Set is 3 signals.

1. Each UE is connected to its Primary_BS, and keeps an Active_ Set (2 “closest” cells)

2. Each UE measures the SIR received from the surrounding cells.

3. If (AS1_SIR – Pr_BS_SIR) >3dB OR (AS2_SIR – Pr_BS_SIR) > 3dB

i. UE enters Soft Handover

ii. UE keeps a simultaneous connection to the Primary_BS and one or both of the Active_Set cells

4. i. If (AS1_SIR – Pr_BS_SIR) > 6dB for three

measurements in a row: AS1 becomes the Primary_BS

ii. If (AS2_SIR – Pr_BS_SIR) > 6dB for three measurements in a row: AS2 becomes the Primary_BS

5. Neighboring cells replace the cells in the Active_Set if their SIR exceeds the Active_Set cells’ SIR by 6dB.

SHO in UMTS

Cell 6

Cell 7

Cell 11

Cell 10

Power Control during Handover