Chapter 9�Mobile Communication Systems

Prof. Chih-Cheng Tseng

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tsengcc@niu.edu.tw

Chih-Cheng Tseng

1

EE of NIU

Wireless Communication Network Lab.

Wireless Communication Network Lab.

Cellular System Infrastructure

Chih-Cheng Tseng

2

EE of NIU

Authentication Center

Visitor Location register

Home Location register

Equipment Identity Register

Wireless Communication Network Lab.

HLR/VLR/AUC/EIR

• HLR is located at the MSC where the MS is initially registered
• VLR contains information about all visiting MSs in that particular area of MSC
• AUC provides authentication and encryption parameters
• EIR contains identity of equipment that prevents service to unauthorized MSs

Chih-Cheng Tseng

3

EE of NIU

Wireless Communication Network Lab.

Classical Mail Forwarding Technique

Chih-Cheng Tseng

4

EE of NIU

Wireless Communication Network Lab.

Automatic Location Update

Chih-Cheng Tseng

5

EE of NIU

Wireless Communication Network Lab.

Automatic Call Forwarding using HLR-VLR

Chih-Cheng Tseng

6

EE of NIU

Wireless Communication Network Lab.

Redirection of Call to MS at a Visiting Location

Chih-Cheng Tseng

7

EE of NIU

BS

MS

Cell where MS is currently located

Through backbone

Wireless Communication Network Lab.

Registration

• Wireless system needs to know whether MS is currently located in its home area or some other area (routing of incoming calls)
• This is done by periodically exchanging signals between BS and MS known as Beacons
• BS periodically broadcasts beacon signal (1 signal per second) to determine and test the MSs around
• Each MS listens to the beacon, if it has not heard it previously then it adds it to the active beacon kernel table
• This information is used by the MS to locate the nearest BS
• Information carried by beacon signal: cellular network identifier, timestamp, gateway address ID of the paging area, etc.

Chih-Cheng Tseng

8

EE of NIU

Wireless Communication Network Lab.

Steps for Registration

• MS listens to a new beacon, if it’s a new one, MS adds it to the active beacon kernel table
• If MS decides that it has to communicate through a new BS, kernel modulation initiates handoff process.
• MS locates the nearest BS via user level processing
• The visiting BS performs user level processing and decides:
• Who the user is?
• What are its access permissions?
• Keeping track of billing
• Home site sends appropriate authentication response to the current serving BS
• The BS approves/disapproves the user access

Chih-Cheng Tseng

9

EE of NIU

Wireless Communication Network Lab.

Using a Mobile Phone Outside the Subscription Area

Chih-Cheng Tseng

10

EE of NIU

Wireless Communication Network Lab.

Applications and Characteristics of Beacon Signals

Chih-Cheng Tseng

11

EE of NIU

Wireless Communication Network Lab.

Handoff Parameters (1/3)

• Change of radio resources from one cell to an adjacent one
• Handoff depends on cell size, boundary length, signal strength, fading, reflection, etc.
• Handoff can be initiated by MS or BS and could be due to
• Radio link
• Network management
• Service issues

Chih-Cheng Tseng

12

EE of NIU

Wireless Communication Network Lab.

Handoff Parameters (2/3)

• Radio link handoff is due to mobility of MS
• It depends on:
• Number of MSs in the cell
• Number of MSs that have left the cell
• Number of calls generated in the cell
• Number of calls transferred from the neighboring cells
• Number and duration of calls terminated in the cell
• Number of calls that were handoff to neighboring cells
• Cell dwell time

Chih-Cheng Tseng

13

EE of NIU

Wireless Communication Network Lab.

Handoff Parameters (3/3)

• Network management may cause handoff if there is drastic imbalance of traffic in adjacent cells and optimal balance of resources is required
• Service related handoff is due to the degradation of QoS (quality of service)

Chih-Cheng Tseng

14

EE of NIU

Wireless Communication Network Lab.

Time for Handoff

• Factors deciding right time for handoff:
• Signal strength
• Signal phase
• Combination of above two
• Bit error rate (BER)
• Distance
• Need for handoff is determined by:
• Signal strength
• CIR (carrier to interference ratio)

Chih-Cheng Tseng

15

EE of NIU

Wireless Communication Network Lab.

Handoff Region (1/2)

• By looking at the variation of signal strength from either base station, it is possible to decide on the optimum area where handoff can take place

Chih-Cheng Tseng

16

EE of NIU

BS_i

Signal strength due to BS_j

Ε

X₁

Signal strength due to BS_i

X₃

X₅

MS

P_min

P_i(x)

P_j(x)

X₄

X_th

X₂

BS_j

Wireless Communication Network Lab.

Handoff Region (2/2)

• Region X₃-X₄ indicates the handoff area, where depending on other factors, the handoff needs to be performed
• One option is to do handoff at X₅ where the two signal strengths are equal
• If MS moves back and forth around X₅, it will result in too frequent handoffs (ping-pong effect)
• Therefore MS is allowed to continue with the existing BS till the signal strength decreases by a threshold value E
• Different cellular systems follow different handoff procedure

Chih-Cheng Tseng

17

EE of NIU

Wireless Communication Network Lab.

Types of Handoff

• Hard Handoff (break before make)
• Releasing current resources from the prior BS before acquiring resources from the next BS
• FDMA,TDMA follow this type of handoff
• Soft Handoff (make before break)
• In CDMA, since the same channel is used, we can use the same if orthogonal to the codes in the next BS
• Therefore, it is possible for the MS to communicate simultaneously with the prior BS as well as the new BS

Chih-Cheng Tseng

18

EE of NIU

Wireless Communication Network Lab.

Hard Handoff (FDMA and TDMA)

Chih-Cheng Tseng

19

EE of NIU

Wireless Communication Network Lab.

Soft Handoff (CDMA only)

Chih-Cheng Tseng

20

EE of NIU

Wireless Communication Network Lab.

Roaming Support (1/2)

• To move from a cell controlled by one MSC area to a cell connected to another MSC
• Beacon signals and the use of HLR-VLR allow the MS to roam anywhere provided the same service provider using that particular frequency band, is there in that region

Chih-Cheng Tseng

21

EE of NIU

Wireless Communication Network Lab.

Roaming Support (2/2)

Chih-Cheng Tseng

22

EE of NIU

Wireless Communication Network Lab.

Handoff Scenarios with Different Degree of Mobility

Chih-Cheng Tseng

23

EE of NIU

Paging Area 1

MSC₂

c

MSC₄

Paging Area 2

e

MSC₃

d

Wireless Communication Network Lab.

Possible Handoff Situations

• Assume MSC₁ to be the home of the MS for registration, billing, authentication, etc.
• When handoff is from position “a” to “b”, the routing can be done by MSC₁ itself
• When handoff is from position “b” to “c” , then bi-directional pointers are set up to link the HLR of MSC₁ to VLR of MSC₂
• When handoff occurs at “d” or “e”, routing of information using HLR-VLR may not be adequate (“d” is in a different paging area)
• Concept of Backbone network

Chih-Cheng Tseng

24

EE of NIU

Wireless Communication Network Lab.

Information Transmission Path when MS Hands Off from “b” to “c”

Chih-Cheng Tseng

25

EE of NIU

Wireless Communication Network Lab.

MSC Connections to Backbone Network & Routing/ Rerouting

Chih-Cheng Tseng

26

EE of NIU

Wireless Communication Network Lab.

Backbone Network

• Routing done according to the topology and connectivity of the backbone network
• The dotted lines show the possible paths for a call headed for different MS locations
• One option is to find a router along the original path, from where a new path needs to start to reach the MSC along the shortest path

Chih-Cheng Tseng

27

EE of NIU

Wireless Communication Network Lab.

Home Agents (HA), Foreign Agents (FA) and Mobile IP

• Two important software modules are associated with routers, home agent (HA) and foreign agent (FA)
• MS is registered with a router, mostly a router closest to the home MSC can be used to maintain its HA
• A router other than closest one could also serve as an HA
• Once a MS moves from the home network, a software module in the new network FA assists MS by forwarding packets for the MS
• This functionality is somewhat similar to HLR-VLR

Chih-Cheng Tseng

28

EE of NIU

Wireless Communication Network Lab.

Home MSC and Home Agent (HA) for the Previous Network

Chih-Cheng Tseng

29

EE of NIU

Wireless Communication Network Lab.

Call Establishment using HA-FA (1/2)

• Whenever a MS moves to a new network, it still retains its initial HA
• The MS detects the FA of the new network, by sensing the periodic beacon signals which FA transmits
• MS can also itself send agent solicitation messages to which FA responds
• When FA detects a new MS, it allocates a CoA (care of address) to the MS, using dynamic host configuration protocol (DHCP)
• Once MS receives CoA, it registers its CoA with its HA and the time limit binding for its validity
• Such registration is initiated either directly by MS to the HA of the home router or indirectly through FA

Chih-Cheng Tseng

30

EE of NIU

Wireless Communication Network Lab.

Call Establishment using HA-FA (2/2)

• HA confirms its binding through a reply to the MS
• A message sent from an arbitrary source to the MS at the home address is received by the HA
• Binding is checked, the CoA of the MS is encapsulated in the packet and forwarded to the network
• If CoA of the FA is used, then packet reaches FA, it decapsulates packet and passes to MS at the link layer
• In an internet environment, it is called Mobile IP
• After binding time, if MS still wants to have packets forwarded through HA, it needs to renew its registration
• When MS returns to its home network, it intimates its HA

Chih-Cheng Tseng

31

EE of NIU

Wireless Communication Network Lab.

Chih-Cheng Tseng

32

EE of NIU

Registration Process Btw FA, MS, and HA When the MS Moves to a Paging area

Wireless Communication Network Lab.

Message Forwarding using HA-FA Pair

Chih-Cheng Tseng

33

EE of NIU

Wireless Communication Network Lab.

Routing in Backbone Routers

• How FA finds HA of the MS?
• One approach is to have a global table at each router of each MSC so that the route from FA to HA for that MS can be determined
• Disadvantages: Information too large, one network might not like to give out information about all its routers to any external network (only gateways information is provided)
• Use of Distributed Routing Scheme

Chih-Cheng Tseng

34

EE of NIU

Wireless Communication Network Lab.

Illustration of Paging Areas (PAs) and Backbone Router Interconnect

Chih-Cheng Tseng

35

EE of NIU

Wireless Communication Network Lab.

Distributed Routing Table and Location PAs

Chih-Cheng Tseng

36

EE of NIU

Table at router

W

Table at router

X

Table at router

Y

Table at router

Z

Wireless Communication Network Lab.

Multicasting

• Process of transmitting messages from a source to multiple recipients by using a group address for all hosts that wish to be the members of the group
• Reduces number of messages to be transmitted as compared to multiple unicasting
• Useful in video/audio conferencing, multi party games
• Multicasting can be performed either by building a source based tree or core based tree

Chih-Cheng Tseng

37

EE of NIU

Wireless Communication Network Lab.

Source-Based Tree Multicasting

• In source based tree, for each source of the group a shortest path is maintained, encompassing all the members of the group, with the source being the root of the tree

Chih-Cheng Tseng

38

EE of NIU

Source

0

Wireless Communication Network Lab.

Core-Based Tree Multicasting

• In core based tree, a particular router is chosen as a core and a tree is maintained with the core being the root.
• Every source forwards the packet to a core router, which then forwards it on the tree to reach all members of the multicast group

Chih-Cheng Tseng

39

EE of NIU

Tree

Wireless Communication Network Lab.

Multicasting over Mobile IP (1/3)

• Bi-directional Tunneling (BT) and Remote Subscription approaches have been proposed by IETF for providing multicast over Mobile IP
• In BT approach, whenever a MS moves to a foreign network, HA is responsible for forwarding the multicast packets to the MS via FA
• In Remote Subscription protocol, whenever a MS moves to a foreign network, the FA (if not already a member of multicast group) sends a tree join request

Chih-Cheng Tseng

40

EE of NIU

Wireless Communication Network Lab.

Multicasting over Mobile IP (2/3)

• Remote Subscription based approach is simple and prevents packet duplication and non optimal path delivery
• It can cause data interruption till the FA is connected to the tree
• It results in a number of tree join and tree leave requests when MS are in continuous motion
• In contrast, in the BT approach, the HA creates a bi-directional tunnel to FA and encapsulates the packets for MS
• FA then forwards the packets to the MS

Chih-Cheng Tseng

41

EE of NIU

Wireless Communication Network Lab.

Multicasting over Mobile IP (3/3)

• BT approach prevents data disruption due to the movement of MS
• But causes packet duplication if several MSs of the same HA, that have subscribed to the same multicast group move to same FA
• Also causes Tunnel Convergence Problem, where one FA may have several MSs subscribed to the same group, belonging to different HAs and each HA may forward a packet for its MSs to the same FA

Chih-Cheng Tseng

42

EE of NIU

Wireless Communication Network Lab.

Packet Duplication in BT Tunnel Approach

Chih-Cheng Tseng

43

EE of NIU

Wireless Communication Network Lab.

Tunnel Convergence Problem

Chih-Cheng Tseng

44

EE of NIU

Wireless Communication Network Lab.

MoM Protocol

• To overcome Tunnel Convergence Problem, MoM protocol is proposed wherein the FA selects one of the HAs, called the Designated Multicast Service Provider (DMSP), from the HA List for a particular group
• The remaining HAs do not forward packets to FA

Chih-Cheng Tseng

45

EE of NIU

Wireless Communication Network Lab.

Illustration of MoM Protocol

Chih-Cheng Tseng

46

EE of NIU

Wireless Communication Network Lab.

Ultra-Wideband Technology (UWB)

• UWB Radio: Radio System having large bandwidth.
• Bandwidth > 25% of center frequency or > 1 GHz.
• Wide bandwidth makes it possible to share spectrum with other users with certain co-coverage sense.
• Wide band signals are naturally suited for location determination applications.

Chih-Cheng Tseng

47

EE of NIU

Wireless Communication Network Lab.

UWB Basics - TM-UWB

• Basic element: Ultra short monocycle wavelet
• Wavelet pulse width between 0.2 and 1.5 nsecs
• Center frequency between 5 GHz and 600 MHz
• Pulse-to-pulse interval between 25 and 1000 nsecs
• System uses pulse position modulation
• No Intermediate frequency stage
• Reduces complexity
• Single bit of information is spread over multiple monocycels

Chih-Cheng Tseng

48

EE of NIU

Wireless Communication Network Lab.

TM-UWB Modulation and DSC-UWB

• Pulse position modulation
• Positions signal one quarter cycle early or late relative to the nominal PN coded location or pulse polarity
• Modulation further smoothes the spectrum of the signal, thus making the system less detectable
• DSC-UWB
• Wavelet pulse trains at duty cycles approaching that of a sine wave carrier are direct sequence modulated to spread the signal. A PN sequence provides spectrum spreading, channelization and modulation

Chih-Cheng Tseng

49

EE of NIU

Wireless Communication Network Lab.

UWB Signal Propagation

• Generally follow a free space propagation law
• Millions of coded pulses transmitted per second
• Emissions below conventional receiver noise floor and across an ultra wide bandwidth
• Very low extant RF signature , providing intrinsically secure transmissions
• Low probability of detection and interception

Chih-Cheng Tseng

50

EE of NIU

Wireless Communication Network Lab.

UWB Applications

• Communications:
• Noise like spectral characteristics of UWB signals enables secure communication with less detection
• Suitable for robust in-building communications
• Advanced Radar Sensing:
• Through the wall radar, terrain mapping radar, ground penetrating radar
• Precision Location and Tracking:
• Remote, secure and real time tracking system

Chih-Cheng Tseng

51

EE of NIU

Wireless Communication Network Lab.

Difference Between UWB and Spread Spectrum Techniques

• UWB uses an extremely wide band of RF spectrum to transmit more data in a given period of time, which is a time-domain concept

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• Spread spectrum technique, including direct sequence spread spectrum or frequency hopping spread spectrum, is a transforming technique in frequency domain

Chih-Cheng Tseng

52

EE of NIU

Wireless Communication Network Lab.

UWB Advantages & Limitations

• UWB radio systems have large bandwidth (>1 GHz).
• UWB has potential to address today’s “spectrum drought”
• Emissions below conventional level
• Single technology with 3 distinct capabilities
• Secure transmission, low probability of interception or detection and anti-jam immunity
• Not appropriate for a WAN (Wide Area Network) deployment such as wireless broadband access
• UWB devices are power limited

Chih-Cheng Tseng

53

EE of NIU

Wireless Communication Network Lab.

Need for Femtocell

• Advanced cellular standards such as 3GPP’s UMTS and LTE; 3GPP2’s CDMA2000, 1x, EVDO and WiMAX
• High data rate and seamless coverage important objective
• Signal strength is weak inside buildings
• More than 50% voice call and more than 70% data traffic start from an indoor environment
• Easier solution is to deploy some indoor devices serving only the indoor users
• Femtocell Network is an effective way to remedy coverage holes

Chih-Cheng Tseng

54

EE of NIU

Wireless Communication Network Lab.

Femtocell

Chih-Cheng Tseng

55

EE of NIU

Wireless Communication Network Lab.

Femtocell Characteristics (1/2)

• Femtocell Base Station (F-BS)
• Wireless Interface
• Internet Interface
• Internet Link
• Femtocell Gateway (FGW)
• Benefits of Femtocell Network
• Better and seamless coverage
• Enhanced capacity
• Lower transmit power
• Prolong handset battery life
• Higher signal-to-interference-noise ratio (SINR)
• Hand off
• Synchronization
• Self-Configuration, Self-Operation and Location Tracking
• Security Issues

Chih-Cheng Tseng

56

EE of NIU

Wireless Communication Network Lab.

Femtocell Characteristics (2/2)

Chih-Cheng Tseng

57

EE of NIU

Wireless Communication Network Lab.

Comparing F-BS with AP

Chih-Cheng Tseng

58

EE of NIU

Wireless Communication Network Lab.

Handoff

• Three hand off categories:
• Hand-Out: user handset handoff from a Femtocell to a macrocell
• Hand-In: user handset handoff from a Femtocell to a macrocell
• Hand off: user handset handoff from a Femtocell to close-by another Femtocell

Chih-Cheng Tseng

59

EE of NIU

Wireless Communication Network Lab.