Charts from Stallings modified and added to

1

Cellular Architecture and Basics

Session 2

Nilesh Jha

Mobile Communications Space

Subscribers Terminal Price, Size/Wt., Service Price and Model

(Minutes/Free), Range of Services (call forward, email, etc), Roaming charges

Performance: % dropped calls, Voice quality, Outage areas and coverage, Battery life, Availability sp. during busy hours

Operating companies Performance and Cost: Infrastructure technology and cost,

capacity, technology spectral efficiency cell location and sizes, deployment schedules, technology and deployment evolution, Impact of data/IP

Business bottom line: ARPU (revenues/user), penetration and market share, Quality of service (total), churn (lost customers -- can be 30%/year, costs $400 to recover), Financing

Mobile Communications Space Equipment and Technology

Providers Technology, Selling existing

infrastructure and terminals, owning the interface to the terrestrial PSTN switches, Developing and Selling new infrastructure and terminals, Financing, Impact of data, Impact of IP, New innovative competitors

Cellular Network Organization Use multiple transmitters Areas divided into cells

Each served by its own antenna Served by base station consisting of antennas,

transmitters, receivers, and control unit Band of frequencies allocated to carrier, carrier

allocates portions to each base station

Cellular System Overview

Wireless Infrastructure

From iec.org

Cellular Systems Elements Base Station (BS) – includes an antenna, a controller, and a

number of receivers Made up of Base Transceiver Station (BTS) and Base Station

Controller (BSC) --- often one BSC controls multiple BTS’s Mobile telecommunications switching office (MTSO) –

connects calls between mobile units --Also called Mobile Switching Center (MSC)

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

Cellular Architecture and Mobility

Cellular Architectures Include

Mobiles Base Stations

In Adjacent Cells Switching Centers: MSC’s Interfaces to the PSTN With Networked MSC’s

A Key Factor is Mobility Requires Handoff as User

Moves from Cell to Cell Requires Databases to Track

User Mobility -- Used by MSC’s for Mobility Management

Figure adapted from IEC cellular and wireless tutorials. Iec.org

OtherMSC’s

Steps in an MSC Controlled Call between Mobile Users

Mobile unit initialization (power on) Mobile signals Base Station it’s there, Base Station and

MSC’s update location registers Mobile-originated call (push send button) Base Station and MSC accept & route call Paging -- to called user via his Base Station

Or call routed to PSTN phone Call accepted by called user Ongoing call (traffic channels with controls) Handoff as mobile unit moves to adjacent cell

System Operations Information Transport

Air Interface Defines the Protocol -- Frequency Band(s), Uplink/Downlink, Multiple Access Scheme, Modulation/ Demodulation and Coding/Decoding, Bandwidths and Data Rates

Call Management Call setup, release, maintenance Uses control channel signaling, backend I/F to PSTN

Mobility Management Registers/tracks user location, handoff, handles roaming

Authentication/Encryption Authentication in network, encryption options

Radio Resource Management Manages power, assigns channels, supports other functions

How can a wireless system (e.g. cellular) have users in the same geographic area not interfere with one another?

Wireless System

U: Users Throughout Area

Multiple Access

UU

U U

U

U

Answer: Multiple Access Technique

Multiple Access

1 2 3 …. N

•N channels, shared by M users•Multiple Access technique defines the N channels and assigns them to M users•One example is frequency channels, each assigned to a TV station (nearly) permanently -- the M users are the M TV stations and a license gives each a channel

• Here N=M --- in fact some UHF channels can remain unlicensed, unassigned•In cellular: the M users are usually many more than the N channels -- users ask for channels and later complete phone calls so the assignments are dynamically changing

Multiple Access Techniques Multiple Access is the scheme used to allow

multiple users to operate transceivers without interfering with each other

It defines physical channel separations based on some physical signal characteristic

Common schemes FDMA -- based on frequency separation TDMA -- based on time separation CDMA -- based on code separation SDMA -- based on spatial separation

Frequency Division Multiple Access --- FDMA

Time Division Multiple Access -- TDMA

Time

Multiple Access Schemes

From Goodman

Multiple Access Efficiency The M.A. ‘technologies’ driven by critical features & efficiencies FDMA is oldest, used most with analog modulations

Used in analog AMPS cellular systems, power hungry In AMPS provides 2.25 calls/MHz/cell or 56 calls/cell

TDMA was next, used in 2G cellular/PCS, digital phones Digital modulations and technology, less power hungry In US TDMA does 7 calls/MHz/cell or 210 calls/cell

CDMA introduced for 2G cellular/PCS, also digital Spread spectrum modulations and codes, also less power hungry IP/chipsets initially Qualcomm controlled Does about 15-30 calls/MHz/user (soft limit), or 450-900 calls/cell Broadband version is 3G, coming slowly, first in Europe, Japan Also used in WLAN (802.11), no Qualcomm IP

Channels Physical Channels

The physical resource unit assigned to carry information eg, a wire, a frequency and bandwidth (for FDMA, like for radio and

TV channels), a time slot (for TDMA), a code (for CDMA) ---- defined at layer 1 of the ISO stack

Logical Channels Defined by the nature of the information they carry and

the format ---- defined at layer 2 of the ISO stack eg, paging channel, or sync channel, or pilot channel, or traffic

channel

‘Circuits’ often used interchangeably with ‘channels’ or ‘links’

Air Interface Layers

From Goodman

Links Multiple use term Refers to point to point electromagnetic path

between transmitter and receiver Also refers to the physical connection

established between transmitter and receiver

Forward or Downlink refers to Base Station to user link

Reverse or Uplink refers to user to Base Station link

Channel Types Logical channels are mapped to physical channels by

assigning specific physical parameters for transmission Channels can be traffic channels (carries user data) or

control channels (controls the communications link) Traffic channels typically want be be as high a data rate as

possible so the link is used effectively to transfer the user data

Control channels typically want to be as low a data rate as possible so this overhead does not waste channel capacity

In cellular control channels are used to make calls, synchronize the different radios and clocks for transmissions, handoff as you move to another cell, and hang-up

Traffic channels carry the voice

Mobile Radio Propagation Effects -- The Challenge of Wireless

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 cochannel interference with channels in another cell

using the same frequency band Distance based signal attenuation -- often not line of sight (ie, worse than free space)

Fading and ISI Signal propagation effects such as multipath cause fading of signal strength and some distortion Fast and slow fading

Slow fading by interposed physical structures as the user moves in and out of sight ---- requires a margin for shadowing to be used in planning needed power

Fast fading due to interference from a number of multi-path signals adding and subtracting together.

Also causes signal shape distortion and (ISI) inter-symbol interference Requires significant signal processing to mitigate effect

Traffic Engineering Ideally, available channels would equal

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

handle all possible loads For N simultaneous user capacity and L

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

Blocking System Performance Questions Probability that call request is blocked? What capacity is needed to achieve a certain

upper bound on probability of blocking? What is the average delay? What capacity is needed to achieve a certain

average delay?

Traffic Intensity in Erlangs Load presented to a system, defined in Erlangs:

= mean rate of calls attempted per unit time h = mean holding time per successful call, call duration A = average number of calls arriving during average holding period Example 1: One call lasting 10 minutes made every hour is a traffic

load of (1/hour)*(one sixth of an hour)=1/6 Erlangs Example 2: a channel used for twenty minutes during an hour is

passing .33 Erlangs of traffic

hA

Factors that Determine the Nature of the Traffic Model Manner in which blocked calls are handled

Lost calls delayed (LCD) – blocked calls put in a queue awaiting a free channel

Blocked calls rejected and dropped Lost calls cleared (LCC) – user waits before another attempt Lost calls held (LCH) – user repeatedly attempts calling

Number of traffic sources Whether number of users is assumed to be finite or

infinite

Probability of Blocking

C

k

k

C

kA

CA

blockingofyprobabilit

ServiceofGradeGOS

CAP

0!

!

)(

),(

C= number of channels neededA=total traffic offered in Erlangs

For Erlang type B, LCC and infinite sourcesNote: this is what is normally used in traffic analysis

Source:Rappaport p.49

Blocking Example Traffic Intensity per user

2 calls/hour, 3 minutes each Au= .1 Erlangs (traffic intensity per user)

If 20 channels available, how many users can be supported with blocking probability of .5% A=UxAu so U=A/Au -- A is total Traffic Intensity From figure, A=11 So U=110

Issues in Wireless -- 1 Operating Companies --- Critical

ARPU=Revenues/user >>> services Quality of Service (Total): voice

quality/BER, dropped calls, calls blocked

Spectrum Owned or Trying to Get/Buy Deployment schedules and coverage Churn: % of customers lost ---

expensive

Issues in Wireless -- 2 Operating Companies --- Other

Infrastructure Technology -- Multiple Access Technique, Cost

Evolution from One to the Next One 2G to 2.5G to 3G

Capacity and Coverage Spectral Efficiency -- Spectrum is

Expensive Calls/MHz/sq.km or Calls/MHz/cell

Issues in Wireless -- 3 Users

Terminal size/weight, price, features, looks Battery life Service price and model (flat, weekend,

long distance, roaming) Quality of Service (as in Issues #1) and

Coverage Anytime, anywhere --- Including busy hour

calls, busy intersections, out in boonies

Prevalent Cellular/PCS Protocols

Protocols require (inter)national standardization Air Interface and Backbone or Core Network

AMPS --- analog, 50 MHz in 824-895 MHz in US, FM, FDMA, 30 KHz channels European band is 50 MHz in 890-960 MHz, used for GSM and AMPS

US TDMA (IS-54/136) --- digital, AMPS bands+PCS band, 30 KHz carries 3 TDMA channels, DQPSK, multiple 30 KHz carriers (so, called FDMA/TDMA also)

CDMA (IS-95) --- digital, AMPS bands+PCS band, 1.25 MHz carrier uses CDMA channelization, PSK

PCS/DCS bands --- 1850-1910 and 1930-1990 MHz (PCS) in US, 1710-1785 and 1805-1885 MHz (DCS) in Europe

GSM --- digital, in 890-960 MHz band + DCS band, TDMA, GMSK, 200 KHz carries 8 TDMA channels, multiple 200 KHz carriers (so, called FDMA/TDMA also)

3G --- Two prevalent forms, WCDMA and CDMA2000, both CDMA but first also TDMA, 5 MHz carriers uses CDMA for channelization in WCDMA and CDMA200 3x, CDMA2000 1X is like CDMA above. 3-4 other standards incl. TDMA versions.

Backend networks fall into two types: GSM/European, and IS-41 in US