Cellular Wireless Networks

ICS 620

Fall 2003

Week #9

Importance of Wireless

Freedom of movement No loss of connectivity Increase in productivity

Cellular Network Organization 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 unit Band of frequencies allocated Cells set up such that antennas of all neighbors are

equidistant (hexagonal pattern)

Cellular Spectrum

A band10 MHz

333 channels30khz

B band10 MHz

333 channels30khz

825 835 845

870 880 890

824 846.5 849

869

A band10 MHz

333 channels30khz

B band10 MHz

333 channels30khz

Phone Transmit

Base Transmit891.5 894

A” b

and

A’ b

and

A” b

and

A’ b

and

B’ b

and

B’ b

and

1 MHz33 chan

1.5 MHz50 chan

2.5 MHz83 chan

1 MHz33 chan

1.5 MHz50 chan

2.5 MHz83 chan

20 MHz Guard

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

frequency escaping to adjacent cells The issue is to determine how many cells must

intervene between two cells using the same frequency

Approaches to Cope with Increasing Capacity 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 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

Cellular System Overview

Cellular Systems Terms Base Station (BS) – includes an antenna, a

controller, and a number of receivers Mobile telecommunications switching office

(MTSO) – connects calls between mobile units 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

Steps in an MTSO Controlled Call between Mobile Users

Mobile unit initialization Mobile-originated call Paging Call accepted Ongoing call Handoff

Additional Functions in an MTSO Controlled Call

Call blocking Call termination Call drop Calls to/from fixed and remote mobile

subscriber

Mobile Radio Propagation Effects Signal strength

Must be strong enough between base station and mobile unit to maintain signal quality at the receiver

Must not be so strong as to create too much co-channel interference with channels in another cell using the same frequency band

Fading Signal propagation effects may disrupt the signal and

cause errors

Handoff Performance Metrics Cell blocking probability – probability of a new

call being blocked Call dropping probability – probability that a call

is terminated due to a handoff Call completion probability – probability that an

admitted call is not dropped before it terminates Probability of unsuccessful handoff – probability

that a handoff is executed while the reception conditions are inadequate

Handoff Performance Metrics 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

Handoff Strategies Used to Determine Instant of Handoff Relative signal strength Relative signal strength with threshold Relative signal strength with hysteresis Relative signal strength with hysteresis and

threshold Prediction techniques

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

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

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 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

Traffic Engineering Ideally, available channels would equal

number of subscribers active at one time In practice, not feasible to have capacity

handle all possible load For N simultaneous user capacity and L

subscribers L < N – non-blocking system L > N – blocking system

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 mobile unit to base

Each band split in two to encourage competition

Frequency reuse exploited

Frequency Division Multiple Access

Definition - FDMA is a multiple access method in which users are assigned specific frequency bands. The user has sole right of using the frequency band for the entire call duration. (Qualcomm, 1997)

FDMA

Frequency Division Multiple Access

Fre

quen

cyF

requ

ency

TimeTime

Chan A

Chan B

Chan C

Chan D

AMPS Operation Subscriber initiates call by keying in phone number

and presses send key MTSO verifies number and authorizes user MTSO issues message to user’s cell phone

indicating send and receive traffic channels MTSO sends ringing signal to called party Party answers; MTSO establishes circuit and

initiates billing information Either party hangs up; MTSO releases circuit, frees

channels, completes billing

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

Time Division Multiple Access Definition - TDMA is an assigned

frequency band shared among a few users. However, each user is allowed to transmit in predetermined time slots. Hence, channelization of users in the same band is achieved through separation in time. (Qualcomm, 1997)

TDMA

Time Division Multiple Access

Fre

quen

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requ

ency

TimeTime

Chan A

Chan B

Mobile Wireless TDMA Design Considerations Number of logical channels (number of time slots

in TDMA frame): 8 Maximum cell radius (R): 35 km Frequency: region around 900 MHz Maximum vehicle speed (Vm):250 km/hr Maximum coding delay: approx. 20 ms Maximum delay spread (m): 10 s Bandwidth: Not to exceed 200 kHz (25 kHz per

channel)

GSM Network Architecture

Mobile Station Mobile station communicates across Um interface

(air interface) with base station transceiver in same cell as mobile unit

Mobile equipment (ME) – physical terminal, such as a telephone or PCS ME includes radio transceiver, digital signal processors

and subscriber identity module (SIM) GSM subscriber units are generic until SIM is

inserted SIMs roam, not necessarily the subscriber devices

Base Station Subsystem (BSS) BSS consists of base station controller and

one or more base transceiver stations (BTS) Each BTS defines a single cell

Includes radio antenna, radio transceiver and a link to a base station controller (BSC)

BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging

Network Subsystem (NS) NS provides link between cellular network and

public switched telecommunications networks Controls handoffs between cells in different BSSs Authenticates users and validates accounts Enables worldwide roaming of mobile users

Central element of NS is the mobile switching center (MSC)

Mobile Switching Center (MSC) Databases Home location register (HLR) database – stores

information about each subscriber that belongs to it Visitor location register (VLR) database –

maintains information about subscribers currently physically in the region

Authentication center database (AuC) – used for authentication activities, holds encryption keys

Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station

TDMA Format – Time Slot Fields Trail bits – allow synchronization of transmissions

from mobile units Encrypted bits – encrypted data Stealing bit - indicates whether block contains data or

is "stolen" Training sequence – used to adapt parameters of

receiver to the current path propagation characteristics Strongest signal selected in case of multipath propagation

Guard bits – used to avoid overlapping with other bursts

GSM Speech Signal Processing

GSM Signaling Protocol Architecture

Functions Provided by Protocols Protocols above the link layer of the GSM

signaling protocol architecture provide specific functions: Radio resource management Mobility management Connection management Mobile application part (MAP) BTS management

Code Division Multiple Access

Definition - CDMA is a method in which users occupy the same time and frequency allocations, and are channelized by unique assigned codes. The signals are separated at the receiver by using a correlator that accepts only signal energy from the desired channel. Undesired signals contribute only to noise. (Qualcomm, 1997)

CDMA Code Division Multiple Access

Frequency

FrequencyTimeTime

CodeCode

Capacity

CDMA has the ability to deliver 10 to 20 times the capacity as FDMA for the same bandwidth. CDMA also has a capacity advantage over TDMA by 5 to 7 times.

Advantages of CDMA Cellular Frequency diversity – frequency-dependent

transmission impairments have less effect on signal

Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation

Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals

Graceful degradation – system only gradually degrades as more users access the system

Drawbacks of CDMA Cellular Self-jamming – arriving transmissions from

multiple users not aligned on chip boundaries unless users are perfectly synchronized

Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away

Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

Mobile Wireless CDMA Design Considerations RAKE receiver – when multiple versions of a

signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them This method achieves better performance than simply

recovering dominant signal and treating remaining signals as noise

Soft Handoff – mobile station temporarily connected to more than one base station simultaneously

Principle of RAKE Receiver

Types of Channels Supported by Forward Link Pilot (channel 0) - allows the mobile unit to acquire

timing information, provides phase reference and provides means for signal strength comparison

Synchronization (channel 32) - used by mobile station to obtain identification information about cellular system

Paging (channels 1 to 7) - contain messages for one or more mobile stations

Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels

Forward Traffic Channel Processing Steps Speech is encoded at a rate of 8550 bps Additional bits added for error detection Data transmitted in 2-ms blocks with forward

error correction provided by a convolutional encoder

Data interleaved in blocks to reduce effects of errors

Data bits are scrambled, serving as a privacy mask

Forward Traffic Channel Processing Steps (cont.) Power control information inserted into traffic

channel DS-SS function spreads the 19.2 kbps to a rate of

1.2288 Mbps using one row of 64 x 64 Walsh matrix

Digital bit stream modulated onto the carrier using QPSK modulation scheme

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 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

ITU’s View of Third-Generation Capabilities 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 general

Support for a wide variety of mobile equipment Flexibility to allow the introduction of new

services and technologies

Alternative Interfaces

CDMA Design Considerations Bandwidth – limit channel usage to 5 MHz Chip rate – depends on desired data rate, need for

error control, and bandwidth limitations; 3 Mcps or more is reasonable

Multirate – advantage is that the system can flexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the capacity required for each service

Paging & SMS

Evolution of Paging Tone Boy, early 1960’s Tone-Voice, late 1960’s

Digital Pagers, 1970s Numeric Paging Systems Alpha/Numeric Paging Systems

Paging

Larger coverage area in each site Signal, Numeric, Alpha-numeric Marketed by coverage area.

Features--Web messaging, modem messaging

Paging

Current Applications Fax Forwarding E-Mail Forwarding Voice Mail Notification Automated Problem Notification Two-way Paging

Wireless Local Loop Wired technologies responding to need for reliable,

high-speed access by residential, business, and government subscribers ISDN, xDSL, cable modems

Increasing interest shown in competing wireless technologies for subscriber access

Wireless local loop (WLL) Narrowband – offers a replacement for existing telephony

services Broadband – provides high-speed two-way voice and data

service

WLL Configuration

Advantages of WLL over Wired Approach Cost – wireless systems are less expensive due to

cost of cable installation that’s avoided Installation time – WLL systems can be installed

in a small fraction of the time required for a new wired system

Selective installation – radio units installed for subscribers who want service at a given time With a wired system, cable is laid out in anticipation of

serving every subscriber in a given area

Propagation Considerations for WLL Most high-speed WLL schemes use millimeter

wave frequencies (10 GHz to about 300 GHz) There are wide unused frequency bands available above

25 GHz At these high frequencies, wide channel bandwidths

can be used, providing high data rates Small size transceivers and adaptive antenna arrays can

be used

Propagation Considerations for WLL Millimeter wave systems have some

undesirable propagation characteristics Free space loss increases with the square of the

frequency; losses are much higher in millimeter wave range

Above 10 GHz, attenuation effects due to rainfall and atmospheric or gaseous absorption are large

Multipath losses can be quite high

Fresnel Zone How much space around direct path between

transmitter and receiver should be clear of obstacles? Objects within a series of concentric circles around the line

of sight between transceivers have constructive/destructive effects on communication

For point along the direct path, radius of first Fresnel zone:

S = distance from transmitter D = distance from receiver DS

SDR

Atmospheric Absorption Radio waves at frequencies above 10 GHz

are subject to molecular absorption Peak of water vapor absorption at 22 GHz Peak of oxygen absorption near 60 GHz

Favorable windows for communication: From 28 GHz to 42 GHz From 75 GHz to 95 GHz

Effect of Rain Attenuation due to rain

Presence of raindrops can severely degrade the reliability and performance of communication links

The effect of rain depends on drop shape, drop size, rain rate, and frequency

Estimated attenuation due to rain:

A = attenuation (dB/km) R = rain rate (mm/hr) a and b depend on drop sizes and frequency

baRA

Effects of Vegetation Trees near subscriber sites can lead to multipath

fading Multipath effects from the tree canopy are

diffraction and scattering Measurements in orchards found considerable

attenuation values when the foliage is within 60% of the first Fresnel zone

Multipath effects highly variable due to wind

Multipoint Distribution Service (MDS) Multichannel multipoint distribution service

(MMDS) Also referred to as wireless cable Used mainly by residential subscribers and small

businesses Local multipoint distribution service (LMDS)

Appeals to larger companies with greater bandwidth demands

Advantages of MMDS MMDS signals have larger wavelengths and

can travel farther without losing significant power

Equipment at lower frequencies is less expensive

MMDS signals don't get blocked as easily by objects and are less susceptible to rain absorption

Advantages of LMDS Relatively high data rates Capable of providing video, telephony, and

data Relatively low cost in comparison with

cable alternatives

802.16 Standards Development Use wireless links with microwave or millimeter

wave radios Use licensed spectrum Are metropolitan in scale Provide public network service to fee-paying

customers Use point-to-multipoint architecture with

stationary rooftop or tower-mounted antennas

802.16 Standards Development Provide efficient transport of heterogeneous traffic

supporting quality of service (QoS) Use wireless links with microwave or millimeter

wave radios Are capable of broadband transmissions (>2

Mbps)

Protocol Architecture Physical and transmission layer functions:

Encoding/decoding of signals Preamble generation/removal Bit transmission/reception

Medium access control layer functions: On transmission, assemble data into a frame with

address and error detection fields On reception, disassemble frame, and perform address

recognition and error detection Govern access to the wireless transmission medium

Protocol Architecture Convergence layer functions:

Encapsulate PDU framing of upper layers into native 802.16 MAC/PHY frames

Map upper layer’s addresses into 802.16 addresses

Translate upper layer QoS parameters into native 802.16 MAC format

Adapt time dependencies of upper layer traffic into equivalent MAC service

IEEE 802.16.1 Services Digital audio/video multicast Digital telephony ATM Internet protocol Bridged LAN Back-haul Frame relay

IEEE 802.16.3 Services Voice transport Data transport Bridged LAN

IEEE 802.16.1 Frame Format

IEEE 802.16.1 Frame Format Header - protocol control information

Downlink header – used by the base station Uplink header – used by the subscriber to convey

bandwidth management needs to base station Bandwidth request header – used by subscriber to

request additional bandwidth Payload – either higher-level data or a MAC

control message CRC – error-detecting code

MAC Management Messages Uplink and downlink channel descriptor Uplink and downlink access definition Ranging request and response Registration request, response and acknowledge Privacy key management request and response Dynamic service addition request, response and

acknowledge

MAC Management Messages Dynamic service change request, response,

and acknowledge Dynamic service deletion request and

response Multicast polling assignment request and

response Downlink data grant type request ARQ acknowledgment

Physical Layer – Upstream Transmission Uses a DAMA-TDMA technique Error correction uses Reed-Solomon code Modulation scheme based on QPSK

Physical Layer – Downstream Transmission Continuous downstream mode

For continuous transmission stream (audio, video) Simple TDM scheme is used for channel access Duplexing technique is frequency division duplex (FDD)

Burst downstream mode Targets burst transmission stream (IP-based traffic) DAMA-TDMA scheme is used for channel access Duplexing techniques are FDD with adaptive modulation,

frequency shift division duplexing (FSDD), time division duplexing (TDD)

Wireless LAN Technology

Wireless LAN Applications LAN Extension Cross-building interconnect Nomadic Access Ad hoc networking

LAN Extension Wireless LAN linked into a wired LAN on

same premises Wired LAN

Backbone Support servers and stationary workstations

Wireless LAN Stations in large open areas Manufacturing plants, stock exchange trading

floors, and warehouses

Multiple-cell Wireless LAN

Cross-Building Interconnect Connect LANs in nearby buildings

Wired or wireless LANs Point-to-point wireless link is used Devices connected are typically bridges or

routers

Nomadic Access Wireless link between LAN hub and mobile

data terminal equipped with antenna Laptop computer or notepad computer

Uses: Transfer data from portable computer to office

server Extended environment such as campus

Ad Hoc Networking Temporary peer-to-peer network set up to

meet immediate need Example:

Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting

Wireless LAN Requirements Throughput Number of nodes Connection to backbone LAN Service area Battery power consumption Transmission robustness and security Collocated network operation License-free operation Handoff/roaming Dynamic configuration

Wireless LAN Categories Infrared (IR) LANs Spread spectrum LANs Narrowband microwave

Strengths of Infrared Over Microwave Radio Spectrum for infrared virtually unlimited

Possibility of high data rates Infrared spectrum unregulated Equipment inexpensive and simple Reflected by light-colored objects

Ceiling reflection for entire room coverage Doesn’t penetrate walls

More easily secured against eavesdropping Less interference between different rooms

Drawbacks of Infrared Medium Indoor environments experience infrared

background radiation Sunlight and indoor lighting Ambient radiation appears as noise in an

infrared receiver Transmitters of higher power required

Limited by concerns of eye safety and excessive power consumption

Limits range

IR Data Transmission Techniques Directed Beam Infrared Ominidirectional Diffused

Directed Beam Infrared Used to create point-to-point links Range depends on emitted power and

degree of focusing Focused IR data link can have range of

kilometers Cross-building interconnect between bridges or

routers

Ominidirectional Single base station within line of sight of all

other stations on LAN Station typically mounted on ceiling Base station acts as a multiport repeater

Ceiling transmitter broadcasts signal received by IR transceivers

IR transceivers transmit with directional beam aimed at ceiling base unit

Diffused All IR transmitters focused and aimed at a

point on diffusely reflecting ceiling IR radiation strikes ceiling

Reradiated omnidirectionally Picked up by all receivers

Spread Spectrum LAN Configuration Multiple-cell arrangement (Figure 13.2) Within a cell, either peer-to-peer or hub Peer-to-peer topology

No hub Access controlled with MAC algorithm

CSMA Appropriate for ad hoc LANs

Spread Spectrum LAN Configuration Hub topology

Mounted on the ceiling and connected to backbone

May control access May act as multiport repeater Automatic handoff of mobile stations Stations in cell either:

Transmit to / receive from hub only Broadcast using omnidirectional antenna

Narrowband Microwave LANs Use of a microwave radio frequency band

for signal transmission Relatively narrow bandwidth Licensed Unlicensed

Licensed Narrowband RF Licensed within specific geographic areas

to avoid potential interference Motorola - 600 licenses in 18-GHz range

Covers all metropolitan areas Can assure that independent LANs in nearby

locations don’t interfere Encrypted transmissions prevent eavesdropping

Unlicensed Narrowband RF RadioLAN introduced narrowband wireless

LAN in 1995 Uses unlicensed ISM spectrum Used at low power (0.5 watts or less) Operates at 10 Mbps in the 5.8-GHz band Range = 50 m to 100 m