Fundamentals of Networking & Telecommunications

Class # 12Mobile & Cellular Communications

635.411 – Class #12 Fall 2009

Mobile & Cellular Communications• This area has become very important with

the explosive growth in demand for mobility

• Different types of mobile communications Satellite Paging Cordless Telephony Mobile & Trunked Radio Enterprise Wireless Cellular/PCS

• While these can (and do) overlap, we will concentrate on Cellular systems

635.411 – Class #12 Fall 2009

The Basic Cellular Concept• Because of attenuation

frequencies can be reused• Break areas up into cells;

assign seven frequencies to a cluster of cells

• Each cell representing a geographic area gets one of the frequencies at low power

• Frequencies are only re-used two or more cells away

• Capacity can be increased by adding more towers and decreasing power

• Variable power levels allowed cell sites to be sized based on usage. (cell splitting)

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635.411 – Class #12 Fall 2009

Increasing Capacity (1)• Add new channels

Not all channels used to start with

• Frequency borrowing Taken from adjacent cells by congested cells Or assign frequencies dynamically

• Cell splitting Non-uniform distribution of

topography and traffic Smaller cells in high use

areasOriginal cells 6.5 – 13 km1.5 km limit in generalMore frequent handoffMore base stations

635.411 – Class #12 Fall 2009

Increasing Capacity (2)• Cell Sectoring

Cell divided into wedge shaped sectors

3 – 6 sectors per cell Each with own channel set

Subsets of cell’s channels Done with directional antennas or

adaptive antenna arrays

• Microcells Move antennas from tops of hills and

large buildings to tops of small buildings and sides of large buildings

Reduced power Good for city streets, along roads and

inside large buildings

635.411 – Class #12 Fall 2009

Cellular System Components

• Mobile Telecommunications Switching Office (MTSO) Or Mobile Switching Center (MSC)

• Base Station• Mobile Station• PSTN PSTN

Mobile Telecommunications Switching Office

Control Channel

Traffic Channel

635.411 – Class #12 Fall 2009

Operation of Cellular Systems• Base station (BS) at center of each cell

Antenna, controller, transceivers Controller handles call processing between mobile unit

and the rest of the network Number of mobile units may in use at a time

• BS connects to a MTSO/MSC One MTSO serves multiple BS MTSO to BS link by wire or wireless

• MTSO/MSC Functions Connects calls between mobile units and from mobile to

fixed telecommunications network (PSTN) Assigns voice channels Performs handoffs Monitors calls (billing)

635.411 – Class #12 Fall 2009

A More Detailed Diagram

Cell Tower

Cell Tower

BSC

MSC

Cell Tower

Cell Tower Cell Tower

BSC

MSC

MSCSS7 STP

Cellular CarrierSS7 Network

PSTN SS7Network

HLR VLR

Mobile SubscriberDatabase

SS7 STP

TypicalCellSite

Landline PSTNNetwork

Cell Tower

Cell Tower

BSC

TypicalCellSite

Other CellularCarrier Network

MSC: Mobile Switching CenterBSC: Base Station ControllerHLR: Home Location RegisterVLR: Visitor Location Register

635.411 – Class #12 Fall 2009

Typical Call in a Single MTSO (1)

• Mobile unit initialization Scan and select strongest

set up control channel Automatically selected BS

antenna of cell Usually but not always

nearest (propagation anomalies)

Handshake to identify user and register location

Scan repeated to allow for movement

Change of cell if needed Mobile unit monitors for

pages (see below)

635.411 – Class #12 Fall 2009

Typical Call in a Single MTSO (2)

Mobile originated call Check to see if set up

channel is free Monitor forward

channel (from BS) and wait for idle

Send number on pre-selected channel

BS sends request to MTSO

635.411 – Class #12 Fall 2009

Typical Call in a Single MTSO (3)• Paging

MTSO attempts to connect to the CALLED mobile unit

Paging message sent to Base Stations depending on called mobile number

Paging signal transmitted on set up channel

Some phones save power by shutting down all receiving circuitry except for paging during “sleep” mode

635.411 – Class #12 Fall 2009

Typical Call in a Single MTSO (4)

• Call accepted Mobile unit recognizes

number on set up channel

Responds to BS which sends response to MTSO

MTSO sets up circuit between calling and called BSs

MTSO selects available traffic channel within cells and notifies BSs

BSs notify mobile unit of channel

635.411 – Class #12 Fall 2009

Typical Call in a Single MTSO (5)• Ongoing call

Voice/data exchanged through respective BSs and MTSO

• Handoff Mobile unit moves out of range of cell into range of another

cell Traffic channel changes to one assigned to new BS

Without interruption of service to user

635.411 – Class #12 Fall 2009

Mobile Radio Propagation Effects • Two issues that must be dealt with!• Signal strength

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

But not so strong it creates too much co-channel interference Varies as function of distance from BS (which changes) Noise constantly varies

Automobile ignition noise greater in city than in suburbs Other signal sources vary

• Fading: The variation of a received signals power over time caused by changes in the medium and/or path Even if signal strength is good, this can cause disruption Example: atmospheric conditions (rain) Movement of (mobile) antennas significantly complicates this

635.411 – Class #12 Fall 2009

Multipath Propagation• Reflection

Surface large relative to signal wavelength

May have phase shift from original May cancel out original or increase it

• Diffraction Edge of impenetrable body that is large

relative to wavelength Edge of body becomes source of signal May receive signal even if no line of

sight (LOS) to transmitter• Scattering

Obstacle size on order of wavelength or less

Breaks signal into several weaker signals

• If LOS, diffracted and scattered signals not significant Though reflected signals may be!

• If no LOS, diffraction and scattering are primary means of reception

635.411 – Class #12 Fall 2009

Effects of Multipath Propagation• Signals may cancel out due to phase differences• Intersymbol Interference (ISI)

Send pulse at given frequency between fixed antenna & mobile unit Channel may deliver multiple copies at different times Delayed pulses act as noise making recovery of bit information

difficult Timing changes as mobile unit moves

Harder to design signal processing to filter out multipath effects

635.411 – Class #12 Fall 2009

Error Compensation Mechanisms (1)

• Forward error correction Applicable in digital transmission applications Typically, ratio of total bits sent to data bits

between 2 and 3 There is a big overhead penalty

Capacity one-half or one-thirdReflects difficulty of mobile wireless environment

• Adaptive equalization Used to combat inter-symbol interference Gathering the dispersed symbol energy back

together into its original time interval

635.411 – Class #12 Fall 2009

Error Compensation Mechanisms (2)• Diversity

Based on fact that individual channels experience independent fading events

Provide multiple logical channels between transmitter and receiver

Send part of signal over each channel Doesn’t eliminate errors but reduce error rate Space diversity

Multiple nearby antennas receive message or collocated multiple directional antennas

Frequency Diversity Signal is spread out over a larger frequency bandwidth or

carried on multiple frequency carriers E.g. spread spectrum

635.411 – Class #12 Fall 2009

Evolution of Cellular Systems

• Generations of Cellular Systems 1G = Analog Systems (AMPS) 2G = Digital System (GSM, TDMA, CDMA) 2.5G = Bridge to 3G (GSM/GPRS/EDGE, IS-95B) 3G = Broadband Capability (W-CDMA)

Full packet-based infrastructure; voice, high-speed data, & multi-media apps equally supported

4G = All IP Packet Switched (Stupid Fast!)Wi-Fi and/or Wi-MAX integration?

635.411 – Class #12 Fall 2009

1G - Advanced Mobile Phone Service (AMPS) (1)

• First standard cellular system Developed by Bell Labs in the late 1970s First deployment in the U.S. in 1983

• Band is split in two to encourage competition (one given to the local carrier in each market region)

• Original Advanced Mobile Phones Service (AMPS) standard (1983) 40MHz of RF spectrum allocated (two 20MHz bands) 30kHz channels used for FDMA Total of 666 Channels (624 for analog voice, 42 for control)

• Standard extended in 1986 (E-AMPS) RF spectrum expanded to 50MHz (two 25MHz bands)

Downlink 824MHz 849MHz Uplink 869MHz 894MHz

832 Total Channels (790 for voice, 42 for control)

635.411 – Class #12 Fall 2009

1G - Advanced Mobile Phone Service (AMPS) (2)• Narrowband AMPS (N-AMPS) implemented in 1991

Uses 10KHz analog voice channels Tripled cluster capacity to 2,496 channels (2370 voice, 126 control)

• Full-duplex control channels 30-kHz (AMPS) or 10-kHz (N-AMPS) Transmit digital data using FSK Data transmitted in frames

• Control information can be transmitted over voice channel during conversation Mobile unit or the base station inserts burst of data

Turn off voice FM transmission for about 100 ms Replacing it with an FSK-encoded message

Used to exchange urgent messages Change power level Initiate Handoff

635.411 – Class #12 Fall 2009

1G - Advanced Mobile Phone Service (AMPS) (3)• AMPS is a good solid standard but as the demand

for mobile voice and data grew its deficiencies became harder to live with Did not accommodate digital voice Could not support data or

converged services easily

Had security issues and difficultly providing enhanced services

1G-AMPS = GrAMPS!

635.411 – Class #12 Fall 2009

2G - Global System for Mobile Communication (GSM) (1)• Developed & first deployed in Europe (1991) –

now under deployment in U.S. (e.g. – AT&T)• Uses a combination of FDMA and TDMA

200kHz channels divided into 8 time slots Time slots can be used for ‘traffic’ (user voice or

data) or control signaling Voice digitized at either 8-kbps or 13-kbps

• Spectrum 900MHz (50-MHz of bandwidth available) 1800MHz (150-MHz of bandwidth available) 1900MHz (120-MHz of bandwidth available) – used

in the U.S.

635.411 – Class #12 Fall 2009

2G - Global System for Mobile Communication (GSM) (2)

• GSM is more than a radio interface standard for cellular voice

• Other new features/services SIM: subscriber identity module Data Services

SMS (Short Message Service): one and two-way pagingBearer Data Services: low-speed circuit switched data

channel

Authentication & Encryption Services MAP (Mobile Access Protocol)

Provide roaming & location management (similar to IS-41 standard used in AMPS and TDMA for the same purpose)

635.411 – Class #12 Fall 2009

Code Division Multiple Access (1)• Spans both the 2G and 3G “Generations”

Developed by Qualcomm “Narrowband” CDMA used in many 2G deployments Becoming the worldwide standard for cellular modulation -- all

versions of 3G utilize some version of CDMA

• Basic Characteristics Users share a common frequency channel (½ upstream, ½

downstream) Each user gets a unique Pseudo-random Supports digital voice and data Utilizes Direct Sequence Spread Spectrum (DS-SS)

• Current CDMA Based Standards 2G: IS-95 in North America (800-MHz & 1900-MHz PCS) 3G: ‘pure’ standard is called “wideband” CDMA 3G variants: cdma2000 & TD-CDMA

635.411 – Class #12 Fall 2009

Code Division Multiple Access (2)• Important IS-95 Characteristics

Digital Voice coding at ≈9600-bps; data up to 14.4-kbps Uses 1.228-MHz upstream & downstream channels – specific

operation is different in each direction

• Forward Channel 64 logical channels (pilot, synch, paging, user traffic) 55 user channels (coding, FEC, control bring channel rate to 19.2-

kbps QPSK modulation of spread stream onto channel

• Reverse Channel 94 logical channels (32 access, 64 user traffic) Orthogonal coding used to improve reception, but not for chipping The mobile unit’s 42-bit SN is used to generate the chipping code

• “Wideband” CDMA Advancements Move to 5-MHz channel; get data rates up to 2+ Mbps Multi-rate capabilities (TDMA ‘on top’ of CDMA)

635.411 – Class #12 Fall 2009

CDMA Advantages • Frequency diversity

Frequency-dependent transmission impairments (noise bursts, selective fading) have less effect

• Multipath resistance DSSS overcomes multipath fading by frequency diversity Less Intersymbol Interference

• Privacy Inherited from spread spectrum

• Graceful degradation With FDMA or TDMA, fixed number of users can access

system simultaneously With CDMA, as more users access the system

simultaneously, noise level and hence error rate increases

Gradually system degrades

635.411 – Class #12 Fall 2009

CDMA Drawbacks• Self-jamming

Unless all mobile users are perfectly synchronized, arriving transmissions from multiple users will not be perfectly aligned on chip boundaries

Spreading sequences of different users not orthogonal Causing signal degradation

Distinct from either TDMA or FDMA Reasonable time or frequency guardbands provide

sufficient orthogonality

• Near-far problem Signals closer to receiver are received with less

attenuation than signals farther away A “near” phone can act as a jammer to a “far” phone Can be controlled by adjusting power of phones such

that received power at the BS is the same

635.411 – Class #12 Fall 2009

Principle of RAKE Receiver• Receiver

attempts to recover signals from multiple paths and combine them, with suitable delays

• Used to overcome multipath issues in CDMA

635.411 – Class #12 Fall 2009

Reading & Homework

• Covered this Class: Cellular Wireless Networks (Chapter 14)

• Next Class: ATM & Congestion Control• REMINDER: Research Paper is due in

one week! PLEASE try to have someone review it

before you turn it in. Remember to use proper

citations/references Make it representative of your best work!

Would you turn this in to your executive leadership if you wanted a promotion?