Wireless WANS and MANS

Chapter 3

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Cellular Network Concept

Use multiple low-power transmitters (100 W or less)Areas divided into cells

Each served by its own antennaServed by base station consisting of transmitter, receiver, and control unitBand of frequencies allocatedCells set up such that antennas of all neighbors are equidistant (hexagonal pattern)

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Frequency Reuse

Adjacent cells assigned different frequencies to avoid interference or crosstalkObjective is to reuse frequency in nearby cells

10 to 50 frequencies assigned to each cellTransmission power controlled to limit power at that frequency escaping to adjacent cellsThe issue is to determine how many cells must intervene between two cells using the same frequency

N: reuse factorN= I2 + J2 + (I x J)

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Frequency Reuse

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Capacity Enhancement

Adding new channelsFrequency borrowing – frequencies are taken from adjacent cells by congested cellsCell splitting – cells in areas of high usage can be split into smaller cellsCell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channelsMicrocells – antennas move to buildings, hills, and lamp posts

Macro-cell

Micro-cell

Pico-cell

Figure 3.2. Cell-splitting

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Sectorization

Space Division Multiple Access (SDMA)

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(a)Omnidirectional (b)Sectorized

Figure 3.3. Omnidirectional and sectorized antennas

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Channel Allocation Algorithms

Fixed channel allocationAllowed to borrow some channels from neighbors

Dynamic channel allocationRequire a centralized arbitrator to allocate channels

Hybrid channel allocationA set of local channels and another set of borrowable channels

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Cellular System Overview

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Cellular Systems Terms

Base Station (BS) – includes an antenna, a controller, and a number of transceiversMobile telecommunications switching office (MTSO) – connects calls between mobile unitsTwo types of channels available between mobile unit and BS

Control channels – used to exchange information having to do with setting up and maintaining callsTraffic channels – carry voice or data connection between users

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Steps in an MTSO Controlled Call Between Mobile Users

Mobile unit initializationMH scans and select the strongest setup control channel

Mobile-originated callMH sending the called unit on the pre-selected setup channel

PagingThe MTSO sends a paging message to certain BSs

Call acceptedOngoing callHandoff

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Handoff Performance Metrics

Handoff delay – delay time in the transfer an on-going call from the current cell to the new cellDuration of interruption – the duration of time during a handoff which is not connected to either BSHandoff success probability – probability that a handoff is successfulProbability of unnecessary handoff – probability of the pin-pong effect

Improved Handoff Strategies

Prioritization: a certain number of channels are reserved for handoffRelative signal Strength: a minimum time for which an MT must be in a cell before it can request a handoffSoft handoffs: a period of time when more than one BS handles a call can be allowedPredictive handoffs: predict the mobility pattern of mobile usersAdaptive handoffs: users may have to be shifted across different layers, from micro- to macro- cellular

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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 L4 for Th3 )Prediction techniques: based on the expected future value of the received signal strength

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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 communicationDesirable 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

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Types of Power Control

Open-loop power controlDepends solely on mobile unitNo feedback from BSNot as accurate as closed-loop, but can react quicker to fluctuations in signal strength

Closed-loop power controlAdjusts signal strength in reverse channel based on metric of performanceBS makes power adjustment decision and communicates to mobile on control channel

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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 unitOne for transmission from mobile unit to base

Each band split in two to encourage competitionFrequency reuse exploited

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Differences Between First and Second Generation Systems

Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digitalEncryption – all second generation systems provide encryption to prevent eavesdroppingError detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice receptionChannel access – second-generation systems allow channels to be dynamically shared by a number of users

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Mobile Wireless TDMA Design Considerations

Global System for Mobile Communications (GSM)Number of logical channels (number of time slots in TDMA frame): 8Maximum cell radius (R): 35 kmFrequency: region around 900 MHz,1800 MHz, or 1900 MHzMaximum vehicle speed (Vm):250 km/hrMaximum coding delay: approx. 20 msMaximum delay spread (Δm): 10 μsBandwidth: Not to exceed 200 kHz (25 kHz per channel)

Maximum data rate for each channel is 34Kbps

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)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 MTMT to BS for call-initiation, and the access grant chnnel

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Speech Coding

Traditional speech coding use pulse code modulation (PCM)

The data rate of PCM: 64 kbpsThis 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

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ITU’s View of Third-Generation CapabilitiesVoice quality comparable to the public switched telephone network144 kbps data rate available to users in high-speed motor vehicles over large areas384 kbps available to pedestrians standing or moving slowly over small areasSupport for 2.048 Mbps for office useSymmetrical / asymmetrical data transmission ratesSupport for both packet switched and circuit switched data services

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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 trafficMore efficient use of the available spectrum in generalSupport for a wide variety of mobile equipmentFlexibility to allow the introduction of new services and technologies

Table 3.1. Evolution plan to 3G standards

Country Existing 2G Standard

3G Standard

Europe GSM W-CDMA (UMTS)

Japan PDC W-CDMA (DoCoMo)

USA IS-95 / cdma one Cdma2000

USA IS-136 UWC-136

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Table 3.2. IMT-2000 Service Types

Service Upstream Downstream

Example Switching

Interactive Multimedia

256 Kbps 256 Kbps Video conference

Circuit

High Multimedia

20 Kbps 2 Mbps TV Packet

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 Circuit2008/10/24 31J. P. Sheu

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The Problems with 3G Systems

Difficult to find a common slice of spectrum to enable global roamingDisappointing performance of CDMA in practiceIt is hard to find suitable applications

Claims Reality

Capacity of 20 times that of AMPSNo more dropped callsNo problem of interferenceQuality if speech promised at 8 Kbps

Only 3-4 times that of AMPS40 percent dropped calls when loadedInterference from existing AMPSHad to change to 13 Kbps

Table 3.3 CDMA – The debate

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Wireless in Local Loop

Local loop: the last hop connectivity between the subscriber and PSTNAdvantages:

Easy deployment, high scalabilityLow investment cost, cost effective

FSU

Telephone

FSU

Telephone

FSU

Telephone

Figure 3.6. WLL architecture

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BTS

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IEEE 802.16 Standard

Metropolitan area networks (MANs) span several km and cover large parts of citiesMANs much larger in size than LANs and their functionalities differ from those LANsIEEE 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

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Differences between 802.11 and 802.16

IEEE 802.16 was designed for broadband data such as digital video and telephonyThe number of users and BW usage per user is much higher than IEEE 802.11IEEE 802.16 is completely connection-oriented and QoS guarantees

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Physical LayerUses traditional narrow-band radio (10-66 GHz) with conventional modulation schemeTwo 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 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 BSQPSK used by subscribers located far away from the BS

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Data Link Layer

DLL of IEEE 802.16 can be subdivided into three sublayers

Security sublayer: only the payloads are encryptedMAC sublayer: deals with channel management and slot allocation to stationsServices specific convergence sublayer: interface to the network layer

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MAC Sublayer

On the downlink, data to the SS is TDM and on the uplink, the medium is shared by the SSs using TDMAEach 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)

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/user

Medium Access CSMA/CA TDM/TDMA FDD/TDMA

QoS Support DCF-NoPCF-Yes

Yes Yes

Connectivity DCF-connectionlessPCF-connection oriented

Connection oriented Connection oriented

Typical Applications Web browsing, e-mail Multimedia, digital TV broadcasting

Voice

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Home Work

2, 4, 7, 9