Wireless WANS and MANS

Chapter 3

Cellular Network Concept

• Use multiple low-power transmitters (100 W or less)

• Areas divided into cells

– Each served by its own antenna

– Served by base station consisting of transmitter, receiver,

and control unitand control unit

– Band of frequencies allocated

– Cells set up such that antennas of all neighbors are

equidistant (hexagonal pattern)

2009/10/19 J. P. Sheu 2

2009/10/19 J. P. Sheu 3

Frequency Reuse

• Adjacent cells assigned different frequencies to avoid

interference or crosstalk

• Objective is to reuse frequency in nearby cells

– 10 to 50 frequencies assigned to each cell

– Transmission power controlled to limit power at that – Transmission power controlled to limit power at that

frequency escaping to adjacent cells

– The issue is to determine how many cells must intervene

between two cells using the same frequency

• N: reuse factor

– N= I2 + J2 + (I x J)

2009/10/19 J. P. Sheu 4

Frequency Reuse

2009/10/19 J. P. Sheu 5

2009/10/19 J. P. Sheu 6

Capacity Enhancement

• Adding new channels

• Frequency borrowing – frequencies are taken from adjacent cells by congested cells

• Cell splitting – cells in areas of high usage can be split into smaller cellssplit into smaller cells

• Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels

• Microcells – antennas move to buildings, hills, and lamp posts

2009/10/19 J. P. Sheu 7

Macro-cell

Micro-cell

Pico-cell

Figure 3.2. Cell-splitting

2009/10/19 J. P. Sheu 8

Sectorization

• Space Division Multiple Access (SDMA)

2009/10/19 J. P. Sheu 9

Channel Allocation Algorithms

• Fixed channel allocation

– Allowed to borrow some channels from neighbors

• Dynamic channel allocation

– Require a centralized arbitrator to allocate channels– Require a centralized arbitrator to allocate channels

• Hybrid channel allocation

– A set of local channels and another set of borrowable

channels

2009/10/19 J. P. Sheu 10

Cellular System Overview

2009/10/19 J. P. Sheu 11

Cellular Systems Terms

• Base Station (BS) – includes an antenna, a controller, and a number of transceivers

• Mobile telecommunications switching office (MTSO) – connects calls between mobile units

• Two types of channels available between mobile unit • Two types of channels available between mobile unit and BS

– Control channels – used to exchange information having to do with setting up and maintaining calls

– Traffic channels – carry voice or data connection between users

2009/10/19 J. P. Sheu 12

Steps in an MTSO Controlled Call

Between Mobile Users

• Mobile unit initialization

– MH scans and select the strongest setup control channel

• Mobile-originated call

– MH sending the called unit on the pre-selected setup channelchannel

• Paging

– The MTSO sends a paging message to certain BSs

• Call accepted

• Ongoing call

• Handoff

2009/10/19 J. P. Sheu 13

2009/10/19 J. P. Sheu 14

Handoff Performance Metrics

• Handoff delay – delay time in the transfer an on-

going call from the current cell to the new cell

• Duration of interruption – the duration of time

during a handoff which is not connected to either BSduring a handoff which is not connected to either BS

• Handoff success probability – probability that a

handoff is successful

• Probability of unnecessary handoff – probability of

the pin-pong effect

2009/10/19 J. P. Sheu 15

Improved Handoff Strategies

• Prioritization: a certain number of channels are reserved for

handoff

• Relative signal Strength: a minimum time for which an MT

must be in a cell before it can request a handoff

• Soft handoffs: a period of time when more than one BS • Soft handoffs: a period of time when more than one BS

handles a call can be allowed

• Predictive handoffs: predict the mobility pattern of mobile

users

• Adaptive handoffs: users may have to be shifted across

different layers, from micro- to macro- cellular

2009/10/19 J. P. Sheu -16

Handoff Strategies Used to

Determine Instant of Handoff

• Relative signal strength (at L1 )

• Relative signal strength with threshold (at L2 for Th2)

• Relative signal strength with hysteresis (at L3 )

• Relative signal strength with hysteresis and threshold

(at L3

for Th1

or Th2 , at L4 for Th3 )

• Prediction techniques: based on the expected future

value of the received signal strength

2009/10/19 J. P. Sheu 17

2009/10/19 J. P. Sheu 18

Power Control

• Design issues making it desirable to include

dynamic power control in a cellular system

– Received power must be sufficiently above the

background noise for effective communication

– Desirable to minimize power in the transmitted signal – Desirable to minimize power in the transmitted signal

from the mobile

• Reduce co-channel interference, alleviate health concerns, save

battery power

– In SS systems using CDMA, it’s desirable to equalize the

received power level from all mobile units at the BS

2009/10/19 J. P. Sheu 19

Types of Power Control

• Open-loop power control

– Depends solely on mobile unit

– No feedback from BS

– Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength fluctuations in signal strength

• Closed-loop power control

– Adjusts signal strength in reverse channel based on metric of performance

– BS makes power adjustment decision and communicates to mobile on control channel

2009/10/19 J. P. Sheu 20

2009/10/19 J. P. Sheu 21

First-Generation Analog

• Advanced Mobile Phone Service (AMPS)

– In North America, two 25-MHz bands allocated

to AMPS

• One for transmission from base to mobile unit• One for transmission from base to mobile unit

• One for transmission from mobile unit to base

– Each band split in two to encourage

competition

– Frequency reuse exploited

2009/10/19 J. P. Sheu 22

Differences Between First and

Second Generation Systems

• Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital

• Encryption – all second generation systems provide encryption to prevent eavesdropping

• Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception

• Channel access – second-generation systems allow channels to be dynamically shared by a number of users

2009/10/19 J. P. Sheu 23

Mobile Wireless TDMA Design

Considerations

• Global System for Mobile Communications (GSM)

– Number of logical channels (number of time slots in TDMA frame): 8

– Maximum cell radius (R): 35 km

– Frequency: region around 900 MHz,1800 MHz, or 1900 – Frequency: region around 900 MHz,1800 MHz, or 1900 MHz

– Maximum vehicle speed (Vm

):250 km/hr

– Maximum coding delay: approx. 20 ms

– Maximum delay spread (∆m

): 10 µs (mutipath delay)

– Bandwidth: Not to exceed 200 kHz (25 kHz per channel)

• Maximum data rate for each channel is 34Kbps

2009/10/19 J. P. Sheu 24

Control Channels

• Broadcast Control Channel (BCCH)

– A downlink channel that contains the BS’s idendity and

channel status. All MTs monitor the BCCH to detect if the

have moved to a new cell.

• Dedicated Control Channel (DCCH)• Dedicated Control Channel (DCCH)

– Used for call-setup, locations updates, and all call-

management related information exchange. Every call has

its own allotted DCCH

• Common Control Channels (BCCH)

– Consists of the downlink paging channel to page any MT

– MT to BS for call-initiation, and the access grant channel

2009/10/19 J. P. Sheu 25

2009/10/19 J. P. Sheu 26

Speech Coding

• Traditional speech coding use pulse code

modulation (PCM)

– The data rate of PCM: 64 kbps

– This rate is undesirably high for use in cellular radio– This rate is undesirably high for use in cellular radio

• With current technology, 12kbps is enough

– Due to the maximum coding delay is 20 ms it is

reasonable to form the encoded speech into blocks of

20 ms duration, or speech samples of 240 bits

2009/10/19 J. P. Sheu 27

ITU’s View of Third-Generation

Capabilities

• Voice quality comparable to the public switched telephone network

• 144 kbps data rate available to users in high-speed motor vehicles over large areas

• 384 kbps available to pedestrians standing or • 384 kbps available to pedestrians standing or moving slowly over small areas

• Support for 2.048 Mbps for office use

• Symmetrical / asymmetrical data transmission rates

• Support for both packet switched and circuit switched data services

2009/10/19 J. P. Sheu 28

ITU’s View of Third-Generation

Capabilities (IMT 2000)

• An adaptive interface to the Internet to reflect

efficiently the common asymmetry between

inbound and outbound traffic

• More efficient use of the available spectrum in • More efficient use of the available spectrum in

general

• Support for a wide variety of mobile equipment

• Flexibility to allow the introduction of new

services and technologies

2009/10/19 J. P. Sheu 29

Table 3.1. Evolution plan to 3G standards

Country Existing 2G

Standard

3G Standard

Europe GSM W-CDMA Europe GSM W-CDMA

(UMTS)

Japan PDC W-CDMA

(DoCoMo)

USA IS-95 / cdma one Cdma2000

USA IS-136 UWC-136

2009/10/19 J. P. Sheu 30

Table 3.2. IMT-2000 Service Types

Service Upstream Downstrea

m

Example Switching

Interactive

Multimedia

256 Kbps 256 Kbps Video

conference

Circuit

High

Multimedia

20 Kbps 2 Mbps TV Packet

Multimedia

Medium

Multimedia

19.2 Kbps 768 Kbps Web

surfing

Packet

Switched

Data

43.2 Kbps 43.2 Kbps Fax Circuit

Simple

Messaging

28.8 Kbps 28.8 Kbps E-mail Packet

Speech 28.8 Kbps 28.8 Kbps Telephony Circuit

2009/10/19 J. P. Sheu 31

The Problems with 3G Systems

• Difficult to find a common slice of spectrum to

enable global roaming

• Disappointing performance of CDMA in

practicepractice

• It is hard to find suitable applications

2009/10/19 J. P. Sheu 32

Claims Reality

Capacity of 20 times that of

AMPS

Only 3-4 times that of AMPS

Table 3.3 CDMA – The debate

AMPS

No more dropped calls

No problem of interference

Quality if speech promised at 8

Kbps

40 percent dropped calls when

loaded

Interference from existing AMPS

Had to change to 13 Kbps

2009/10/19 J. P. Sheu 33

Wireless in Local Loop

• Local loop: the last hop connectivity between

the subscriber and PSTN

• Advantages:

– Easy deployment, high scalability– Easy deployment, high scalability

– Low investment cost, cost effective

2009/10/19 J. P. Sheu 34

FSU

Telephone

FSU

Telephone

FSU

Telephone

Figure 3.6. WLL architecture

2009/10/19 J. P. Sheu 35

BTS

IEEE 802.16 Standard

• Metropolitan area networks (MANs) span several km and cover large parts of cities

• MANs much larger in size than LANs and their functionalities differ from those LANsfunctionalities differ from those LANs

• IEEE 802.16 is called air interface for fixed broadband wireless access systems

– Based on OSI model, specifies air interface including data link layer and physical layer

2009/10/19 J. P. Sheu 36

Differences between 802.11 and

802.16

• IEEE 802.16 was designed for broadband data

such as digital video and telephony

• The number of users and BW usage per user is

much higher than IEEE 802.11much higher than IEEE 802.11

• IEEE 802.16 is completely connection-oriented

and QoS guarantees

2009/10/19 J. P. Sheu 37

Physical Layer

• Uses traditional narrow-band radio (10-66 GHz) with conventional modulation scheme

• Two new protocols attempt to close to IEEE 802.11– 802.16a operates in the 2 – 11 GHz and 802.16b operates in

5 GHz ISM band

• Since the signal strength falls off sharply with distance • Since the signal strength falls off sharply with distance from the BS, the following three modulation scheme are used– QAM-64 used by subscribers located near the BS

– QAM-16 used by subscribers located at intermediate distance from the BS

– QPSK used by subscribers located far away from the BS

2009/10/19 J. P. Sheu 38

Data Link Layer

• DLL of IEEE 802.16 can be subdivided into

three sublayers

– Security sublayer: only the payloads are encrypted

– MAC sublayer: deals with channel management – MAC sublayer: deals with channel management

and slot allocation to stations

– Services specific convergence sublayer: interface

to the network layer

2009/10/19 J. P. Sheu 39

MAC Sublayer

– On the downlink, data to the SS is TDM and on

the uplink, the medium is shared by the SSs

using TDMA

– Each uplink connection has four classes of – Each uplink connection has four classes of

services:

• Constant bit rate service (voice)

• Real-time variable bit rate service (multimedia)

• Non-real-time variable bit rate service (file transfer)

• Best effort service (others)

2009/10/19 J. P. Sheu 40

Table 3.4. A brief comparison among IEEE 802.11b WLANs,

IEEE 802.16 WMANs, and GSM WWANs

Feature IEEE 802.11B

WLANs

IEEE 802.16 WMANs GSM WWANs

Range Few hundred meters Several Km Few tens of Km

Frequency 2.4 GHz ISM band 10-66 GHz 900 or 1800 MHz

Physical Layer CCK, BPSK, QPSK QAM-64, QAM16,

QPSK

GMSK

Maximum Data Rates 11 Mbps 60–180 Mbps 9.6 Kbps/userMaximum Data Rates 11 Mbps 60–180 Mbps 9.6 Kbps/user

Medium Access CSMA/CA TDM/TDMA FDD/TDMA

QoS Support DCF-No

PCF-Yes

Yes Yes

Connectivity DCF-connectionless

PCF-connection

oriented

Connection oriented Connection oriented

Typical Applications Web browsing, e-mail Multimedia, digital

TV broadcasting

Voice

2009/10/19 J. P. Sheu 41

Homework

• 3, 4, 9

2009/10/19 J. P. Sheu 42