1. Introduction to Wireless Networking
ECE/CSC 575 – Section 1
Introduction to Wireless Networking
Lecture 9 Dr. Xinbing Wang

2. Current Wireless Systems: Cellular Systems--UMTS
Fundamentals of cellular communications
System capacity
frequency reuse
Cell splitting
Admission control
handoff
Universal mobile telecommunication system (UMTS)
Network architecture
Functional units
Quality of service
Mobility management
In our daily lives, wireless communication technology is used everywhere, from VCR remote control, to satellite weather forecast. The common characteristics of wireless communication systems is that there is no physical (visible) lines between two communication parties.
Therefore, a wireless system is able to support user roaming. For example, we do not have to use a remote control in a particular position to.., we can use our cellular phones almost everywhere.
However, there are many impairments to a wireless channel, causing a lot of limitations to wireless communications system such as geographical.. (signal fading, additional noise, cochannel interference. Wireless systems also suffers from limit usable spectral width, so that the transmission rate is relatively low.
Specifically, wireless cellular systems based on radio propagation has been evolving from narrow band (1G, late 170s) to wide-band(3G).
With their geographical coverage limitation, wireless systems need a backbone network to extend their geographical coverage to enable global communications.
The interoworking of a wireless network as the front-end and the Internet as the backbone has received much attention in recent years.
So we will first take a look at the network architecture of current wireless systems,…, Then we will talk about the evolution from 2G to 3G systems.
Dr. Xinbing Wang

3. Worst Case Cochannel Interference
We need to modify our assumption, i.e., assume Di=D.
The S/I ratio can be expressed as
D+R R
D+R D D
D-R
D-R
Dr. Xinbing Wang

4. Example: Worst Case Cochannel Interference (2)
A cellular system that requires an S/I ratio of 18dB. (a) if frequency reuse ratio is 7, what is the worst-case S/I? (b) Is a frequency reuse factor of 7 acceptable in terms of cochannel interference? If not, what would be a better choice of frequency reuse ratio?
Dr. Xinbing Wang

5. Cell Capacity
Average number of MSs requesting service (Average arrival rate): 
Average length of time MS requires service (Average holding time): T
Offered load: a = T
e.g., in a cell with 100 MSs, on an average 30 requests are generated during an hour, with average holding time T=360 seconds.
Then, arrival rate =30/3600 requests/sec.
A channel kept busy for one hour is defined as one Erlang (a), i.e.,
Dr. Xinbing Wang

6. Cell Splitting
A method to increase the capacity of a cellular system by dividing one cell into more smaller cells.
Cell splitting reduces the call blocking probability. But it increases the handoff rate, i.e., more frequent crossing of borders between the cells.
Again we repeat the formula in calculating path loss, we have
Pr(dBW) = P0(dBW) - 10 log10(d/d0)
Where d0 is the distance from the reference point to the transmitter, and P0 is the power received at the reference point. R
R/2
Dr. Xinbing Wang

7. Example – Cell Splitting
Suppose each BS is allocated 60 channels regardless of the cell size. Find the number of channels contained in a 3x3 km2 area without cell splitting, i.e., R= 1km and with cell splitting, R/2 = 0.5km.
Dr. Xinbing Wang

8. Directional Antennas (Sectoring)
The basic form of antennas is omnidirectional. Directional antennas can increase the system capacity.
If we sectorize the cell with
120o in each sector, the S/I
becomes
The handoff between sectors is managed by the BS. 1 2 1 3 2 3
Dr. Xinbing Wang

9. Worst-Case Scenario in 120o Sectoring
Let D be the distance between the adjacent cochannel cells.
With the distance approximation and use path loss component , the signal-to-interference ratio is
D+0.7R 1 2 3 D
End of Capacity
Dr. Xinbing Wang

10. Signal Strength Signal strength (in dB) Cell i Cell j -60 -60 -70 -70
-80
-80
-90
-90
-100
-100
Select cell j on right of boundary
Select cell i on left of boundary
Ideal boundary
Dr. Xinbing Wang

11. Signal Strength (2) Cell j Cell i -60 -70 -60 -80 -70 -90 -80 -90 -100
(in dB)
Cell j
Cell i
-60
-70
-60
-80
-70
-90
-80
-90
-100
-100
Signal strength contours indicating actual cell tiling. This happens because of terrain, presence of obstacles and signal attenuation in the atmosphere.
Dr. Xinbing Wang

12. Signal strength due to BSj Signal strength due to BSi
Handoff Region
Signal strength due to BSj
Signal strength due to BSi
Pi(x)
Pj(x) E
Pmin
BSi MS
BSj X1 X3 X5
Xth X4 X2
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.
Dr. Xinbing Wang

13. Handoff Procedures (1)
Mobile Switching Center (e.g, MSTO in the textbook)
MSC
MSC
Dr. Xinbing Wang

14. Handoff Procedure (2)
MSC
MSC
MSC
MSC
Dr. Xinbing Wang

15. Handoff Management
Initial Phase: make decision based on the measures of received signal strength
Without hysteresis: a handoff is initiated as soon as the average signal level from the new base station exceeds that from the current BS.
With hysteresis: a handoff is initiated when the average signal level from the new BS exceeds that from the current BS by a threshold amount specified by the hystersis level.
Execution phase
Channel assignment
Exchange of control messages
Dr. Xinbing Wang

16. Handoff Strategies
Mobile controlled handoff (MCHO): the MT initiates and controls the procedure of handoff.
Network controlled handoff (NCHO): the network (e.g., MSC) makes decision and initiates handoff based on the measurements informed by BSs/APs.
MT assisted handoff (MAHO): the MT measures the network (e.g., MSC) makes decision and initiates handoff based on the measurements collected by the MT.
Dr. Xinbing Wang

17. Types of Handoff
Hard handoff is characterized by an MT having a radio link with only one BS/AP at any time.
This mode of operation is referred to as break (old connection) before make (new connection).
Soft handoff in which an MT can simultaneously communicate with more than one BS/AP during the handoff.
This mode of operation is referred to as make (new connection) before break (old connection).
Backward handoff: The handoff is predicted ahead of time and initiated via the existing radio link.
May be affected by a sudden loss or rapid deterioration of radio links
Forward handoff: The handoff is initiated via the new radio link.It may result in large delay when combined with hard handoff.
Dr. Xinbing Wang

18. Handoff Performance
Call dropping probability – probability that a call is terminated due to a handoff
Handoff blocking probability – probability that a handoff cannot be successfully completed
Handoff probability – probability that a handoff occurs before call termination
Rate of handoff – number of handoffs per unit time
Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station
Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur
Dr. Xinbing Wang

19. Current Wireless Systems: Cellular Systems--UMTS
Fundamentals of cellular communications
System capacity
frequency reuse
Cell splitting
Admission control
handoff
Universal mobile telecommunication system (UMTS)
Network architecture
Functional units
Quality of service
Mobility management
In our daily lives, wireless communication technology is used everywhere, from VCR remote control, to satellite weather forecast. The common characteristics of wireless communication systems is that there is no physical (visible) lines between two communication parties.
Therefore, a wireless system is able to support user roaming. For example, we do not have to use a remote control in a particular position to.., we can use our cellular phones almost everywhere.
However, there are many impairments to a wireless channel, causing a lot of limitations to wireless communications system such as geographical.. (signal fading, additional noise, cochannel interference. Wireless systems also suffers from limit usable spectral width, so that the transmission rate is relatively low.
Specifically, wireless cellular systems based on radio propagation has been evolving from narrow band (1G, late 170s) to wide-band(3G).
With their geographical coverage limitation, wireless systems need a backbone network to extend their geographical coverage to enable global communications.
The interoworking of a wireless network as the front-end and the Internet as the backbone has received much attention in recent years.
So we will first take a look at the network architecture of current wireless systems,…, Then we will talk about the evolution from 2G to 3G systems.
Dr. Xinbing Wang

20. Introduction to UMTS
UMTS (Universal Mobile Telecommunications System) is the European version of a 3rd Generation (3G) mobile communication system.
It is proposed by 3GPP (3rd generation partnership project).
It includes two parts: UTRAN (Universal Terrestrial Radio Access Network) and the Core network inherited from GSM (Global System for Mobile Communications).
UMTS is a wideband, circuit- and packet-based transmission systems of text, digitized voice, video, and multimedia with data rates up to 2 Mbps (possibly higher).
Dr. Xinbing Wang

21. UMTS Services and Their Relationship to the Internets
Service Category
Session Type
Protocols
Internet Elements
Location-based info- and entertainment
WWW
HTTP, WML,
cHTML, xHTML
ISP, portal, servers
Intranet access (mobile VPN), mobile office, mobile commerce
All types-transparent tunnel
IP, higher layers transparent
ISP, firewall server, corporate portal
Internet access
ISP, portal
Multimedia messaging
SMS, , downloading
SMTP, SMS, IP
ISP, , SMS-server
Audio, video, download
File transfer, streaming
MP3, MPEG-4, FTP, IP-based SIP
ISP, portal, database server
Voice, real-time audio, video
Interactive/dialog streaming/one-way
SIP
Media gateway
Dr. Xinbing Wang

22. Data rate and Spectrum
Maximum data rate and maximum speed for different hierarchical layer
Macrolayer: 144 kbps with max. speed of 500km/h.
Microlayer: 384 kbps with max speed of 120km/h
Picolayer: 2Mbps with 10km/h
Bit Error Rate (BER)
Real-time applications: 10-3 to 10-7 with maximum constant delay: 20ms to 300 ms
No real-time applications: 10-5 to 10-8 with maximum delay >= 150ms.
Spectrum: 1900 MHz-2025 MHz, and MHz
FDD (macro- and micro- cells: uplink is from 1920 MHz to 1980 MHz, downlink is from 2110 MHz to 2170 MHz
TDD (pico- cells: not divided by use of different frequency carriers (not suitable for large prop delays).
Dr. Xinbing Wang

23. Network Architecture CN UTRAN VLR MSC GMSC HLR SGSN GGSN PSTN ISDN
Node B
Radio
Network
controller
MSC
UMTS
Subscriber
Identity
module
GMSC
ISDN
Node B
HLR
User
equipment
Radio
Network
controller
Node B
Mobile
equipment
SGSN
GGSN
Internet
Dr. Xinbing Wang

24. Radio Network Controller (RNC)
One RNC controls one or more Node Bs.
It may be connected via Iu interface to an MSC (IuCS), or to an SGSN via Iu (IuPS).
The interface between RNCs (Iur) is logical interface, and a direct physical connection does not necessarily exist.
An RNC is comparable to a base station controller (BSC) in GSM networks.
Dr. Xinbing Wang

25. RNC Functions Iub (Node B and RNC) transport resources management
Control of Node B logical O&M resources
System information management and scheduling
Traffic management of common channels
Soft handover
Power control for uplink and downlink
Admission control
Traffic management of shared channels
Macro diversity combining/splitting of data streams transferred over several Node Bs.
Dr. Xinbing Wang

26. Node B
Node B is the UMTS equivalent of a base station transceiver. It may support one or more cells, although in general only one cell one Node B.
It is a logical terminal and the base station is often used for physical entity.
Functions
Mapping of Node B logical resources onto hardware resources
Uplink power control
Reporting of uplink interference measurements and downlink power information
Contains the air interface physical layer, it has to perform many functions such as RF processing, modulations, coding, and so on.
Dr. Xinbing Wang

27. WCDMA Air Interface
In UMTS, the UTRAN is used to keep the mobility management (MM) and connection management (CM) layers independent of the air interface radio technology
This idea is realized as the concepts of access stratum (AS) and nonaccess stratum (NAS)
AS: functional entity that includes radio access protocols between the user equipment (UE) and the UTRAN (terminate here).
NAS: includes core network (CN) protocols between the UE and the CN itself.
The NAS protocols can be kept the same, thus, the GSM’s MM and CM resources are used almost unchanged in 3G NAS.
Dr. Xinbing Wang

28. UMTS Architecture UE UTRAN CN Non-access Stratum Core network
protocols
Core network
protocols
Access Stratum
Radio
Protocols
Radio
Protocols lu
Protocols lu
Protocols
Uu-interface
Iu-interface
Dr. Xinbing Wang

29. Layered Architecture There are three protocol layers in the AS
Physical layer (L1)
Data link layer (L2)
Medium access control (MAC)
Radio link control (RLC)
Broadcast/multicast control (BMC)
Packet data convergence protocol (PDCP)
Network layer (L3)
Radio resource control (RRC)
There is one layer (L3) in the NAS
Mobility management
Call management
Dr. Xinbing Wang

30. RLC Services
These functions are provided to upper layers:
Segmentation and reassembly of higher-layer PDUs (Protocol Data Unit) into/from smaller RLC payload units
Padding
Transfer of user data
Error corrections
In-sequence delivery of higher-layer PDUs
Ciphering
Sequence number check
Dr. Xinbing Wang

31. RLC Functions
These functions (for itself) are supported by the RLC:
Segmentation and reassembly of higher-layer PDUs (Protocol Data Unit) into/from smaller RLC payload units
Padding
Transfer of user data
Error corrections
In-sequence delivery of higher-layer PDUs
Flow control
Ciphering
Sequence number check
Dr. Xinbing Wang

32. RRC Services
General control: this is an information broadcast service. The information transferred in unacknowledged, and it is broadcast to all mobiles within a certain area.
Notification: This includes paging and notification broadcast services.
The paging services broadcasts paging information in a certain geographical area, but it is addressed to a specific UE or UEs.
The notification broadcast service is defined to provide information broadcast to all UEs in a cell or cells.
Dedicated control: This service includes the establishment and release of a connection and transfer of messages using this connection.
Dr. Xinbing Wang

33. RRC Functions These functions (for itself) are supported by the RRC:
Initial cell selection and cell reselection
Broadcast of information
Reception of paging and notification messages
Establishment, maintenance, and release of RRC connections
Establishment, reconfiguration, and release of radio bearers
Assignment, reconfiguration, and release of radio resources for the RRC connection
Handover
Measurement control
Power control
Security mode control
QoS control
Dr. Xinbing Wang

34. Logical Channels in UTRAN
Control Channel (CCH)
Broadcast Control Channel (BCCH)
Paging Control Channel (PCCH)
Dedicated Control Channel (DCCH)
Common Control Channel (CCCH)
Shared Channel Control Channel (SHCCH)
ODMA Dedicated Control Channel (ODCCH)
ODMA Common Control Channel (OCCCH)
Traffic Channel (TCH)
Dedicated Traffic Channel (DTCH)
ODMA Dedicated Traffic Channel (ODTCH)
Common Traffic Channel (CTCH)
Dr. Xinbing Wang

35. After Class Reading materials Exercises Chapter 10.2
Chapter skip TDMA format, Speech coding, data coding
Chapter 10.5
Reading materials for UMTS
Exercises
What is handoff?
What is the Impact of cluster size on cochannel interference? How to find the worst-case cochannel interference
Why cell splitting and sectoring
Describe the architecture of UMTS.
What is the main difference between UMTS and 2G systems?
List 3 functions of RNC and Node B, respectively
Dr. Xinbing Wang