UNIT-I

WIRELESS TELECOMMUNICATION NETWORKS& DIGITAL CELLULAR SYSTEM

The Analog Telephone System

The analog system was the first telephone system established worldwide. Currently, telephone systems in a lot of countries are still completely analog. In time however, these systems will become redundant as the world switches to digital telephony.
The components of a telephone system include: Microphone Receiver Transmission System Switching and signaling system
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Telephone system components

Signaling and switching system Transmission system

receiver microphone
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 Microphone Converts the vibrations in the air into an electrical signal Receiver Converts the received electrical signal into sound waves (the reverse action of a microphone) e.g.: loudspeaker Transmission system Conveys the information representing the audio signal from the microphone to the receiver Signaling and switching system Determines and makes appropriate connections among the pieces of the transmission system to create a path from the transmitter to the receiver

The Digital Telephone System

While the description of the analog telephone system provides an accurate overview of the principles of current telephone systems, it is a fact that most telephone calls today are really digital telephone calls In a digital telephone system, the two ends of the call are analog, and the middle section is digital. Conversions from analog to digital (A/D), and back to analog (D/A), are made in such a way that it is essentially impossible for human ear to determine that there was any conversion at all Although the analog telephone system is gradually being converted to digital, the input and output of the system still remains analog because the eventual use is for humans that are only able to process analog information
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 At present, most telephone calls are analog from the telephone at home to the first switching office, so the A/D and D/A conversion is made at this office In the future, as telephone systems become all digital, this conversion from A/D and from D/A will be made within the telephone set at home The A/D conversion process was explained in the previous lectures- The voice signal- an analog waveform was sampled at a sampling frequency, and quantized to a number of levels. These values were then assigned binary codes to complete the conversion process from analog to digital The D/A process was also explained briefly. The bits were decoded into their quantized values, and a waveform similar to the original analog waveform was obtained
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 For voice, we recall that the standard sampling frequency is 8000Hz The standard number of quantization levels for audio signals is 256, requiring 8 bits So, the bit rate for a digital telephone call is: 8,000x8=64,000 bits per second (64 Kbps)
This is the bit rate that would reach the central office if the A/D conversion was being done inside the telephone at home Since many calls arrive at the central office, they can all be combined, and switched to another center to be routed to the destination Combining many channels and sending them simultaneously through a single transmission line is called multiplexing. We will learn more about this in a later chapter
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1/s=Hz

bits

 One advantage of digital transmission, is that after digitization, all types of information are in the form of bits, so a single system, such as a telephone system can be used to carry telephone calls, internet data or any other data at a suitable bit rate

The Cellular Telephone System

The cellular telephone system is different from the previous systems that we discussed, because the major transmission medium is air instead of wires (between the mobile unit and the base station) as in the analog and digital telephone systems In a cellular system, the signal from a mobile unit (cell phone) to a base station is transmitted by radio waves through the air, instead of through metallic wires However, the signal from the base station is sent to a mobile switching center and possibly to a telephone central office through electrical wires where it is switched to the appropriate destination The antenna at the base station converts the radio waves to electrical signals and circuits in the base station send the signal to the appropriate mobile switching center

The Cellular Concept

Base station

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An example cellular telephone system

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How cellular telephone systems work

The area (a city, or a part of town) is divided into a number of cells (typically 2 to 10 miles in size, but can be smaller for more crowded areas) and a base station is positioned within each cell If a user (mobile phone) is within a particular cell, the call is handled by the corresponding base station within that cell The base station transmits the signal to the mobile switching center (also called MTSO), which switches the signal to another base station, or to a Public Switched Telephone Network (PSTN), depending on the destination of the call: whether another mobile unit or a regular telephone As a user moves from one cell to another, the call is handed over to the base station of the other cell-This is called hand-off The handover is (hopefully) transparent to the user

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MTSO
Mobile Telephone Switching Office

PSTN
Public Switched Telephone Network

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 The mobile unit and the base station in a cell communicate at a certain frequency The signal from the mobile unit arrives at the antenna of the base station and is converted into an electrical signal

Base station antenna (3 sector)

1/3rd of cell is covered by 14 each sector of

 Every cell uses a different set of frequencies So how does the phone know what frequency to be on? A Cell-Site Controller handles this process When a cell phone is turned on, it registers with the network and guards a control frequency When a call is placed, the phone requests that a frequency (really two) be assigned When a call is received, the call is set up over the control channel (find the right phone, tell it what frequencies to use, connect the call)
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 Every cell uses a different set of frequencies So how does the phone know what frequency to be on? A Cell-Site Controller handles this process When a cell phone is turned on, it registers with the network and guards a control frequency When a call is placed, the phone requests that a frequency (really two) be assigned When a call is received, the call is set up over the control channel (find the right phone, tell it what frequencies to use, connect the call)
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Principles of Cellular Networks

Underlying technology for mobile phones, personal communication systems, wireless networking etc. Developed for mobile radio telephone
Replace high power transmitter/receiver systems
Typical support for 25 channels over 80km

Use lower power, shorter range, more transmitters

Cellular Network Organization

Multiple low power transmitters
100w or less

Area divided into cells

Each with own antenna Each with own range of frequencies Served by base station
Transmitter, receiver, control unit

Adjacent cells on different frequencies to avoid crosstalk

Shape of Cells
Square
Width d cell has four neighbors at distance d and four at distance 2 d Better if all adjacent antennas equidistant
Simplifies choosing and switching to new antenna

Hexagon
Provides equidistant antennas Radius defined as radius of circum-circle
Distance from center to vertex equals length of side

Distance between centers of cells radius R is Not always precise hexagons

Topographical limitations Local signal propagation conditions Location of antennas

R 3

Cellular Geometries

Frequency Reuse
Power of base transceiver controlled

Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Use same frequency for multiple conversations 10 50 frequencies per cell
N cells all using same number of frequencies K total number of frequencies used in systems Each cell has K/N frequencies Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 frequencies per cell on average

E.g.

Characterizing Frequency Reuse

D = minimum distance between centers of cells that use the same band of frequencies (called cochannels) R = radius of a cell d = distance between centers of adjacent cells (d = R) N = number of cells in repetitious pattern
Reuse factor Each cell in pattern uses unique band of frequencies

Hexagonal cell pattern, following values of N possible

N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3,

Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, D/R= 3N D/d = N

Frequency Reuse Patterns

Increasing Capacity (1)

Add new channels Frequency borrowing
Not all channels used to start with
Taken from adjacent cells by congested cells Or assign frequencies dynamically Non-uniform distribution of topography and traffic Smaller cells in high use areas
Original cells 6.5 13 km 1.5 km limit in general More frequent handoff More base stations

Cell splitting

Cell Splitting

Increasing Capacity (2)

Cell Sectoring
Cell divided into wedge shaped sectors 3 6 sectors per cell Each with own channel set
Directional antennas
Subsets of cells channels

Microcells

Move antennas from tops of hills and large buildings to tops of small buildings and sides of large buildings Form microcells Reduced power Good for city streets, along roads and inside large buildings
Even lamp posts

Frequency Reuse Example

Operation of Cellular Systems

Base station (BS) at center of each cell
Antenna, controller, transceivers

Controller handles call process

Number of mobile units may in use at a time

BS connected to mobile telecommunications switching office (MTSO)

One MTSO serves multiple BS MTSO to BS link by wire or wireless

MTSO:
Connects calls between mobile units and from mobile to fixed telecommunications network Assigns voice channel Performs handoffs Monitors calls (billing)

Fully automated

Overview of Cellular System

Channels
Control channels
Setting up and maintaining calls Establish relationship between mobile unit and nearest BS

Traffic channels
Carry voice and data

Typical Call in Single MTSO Area (1)

Mobile unit initialization
Scan and select strongest set up control channel Automatically selected BS antenna of cell Handshake to identify user and register location Scan repeated to allow for movement
Usually but not always nearest (propagation anomalies) Change of cell

Mobile originated call

Mobile unit monitors for pages (see below) Check set up channel is free
Monitor forward channel (from BS) and wait for idle

Paging

Send number on pre-selected channel

MTSO attempts to connect to mobile unit Paging message sent to BSs depending on called mobile number Paging signal transmitted on set up channel

Typical Call in Single MTSO Area (2)

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

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

Call Stages

Other Functions
Call blocking
During mobile-initiated call stage, if all traffic channels busy, mobile tries again After number of fails, busy tone returned

Call termination
User hangs up MTSO informed Traffic channels at two BSs released

Call drop
BS cannot maintain required signal strength Traffic channel dropped and MTSO informed

Calls to/from fixed and remote mobile subscriber

MTSO connects to PSTN MTSO can connect mobile user and fixed subscriber via PSTN MTSO can connect to remote MTSO via PSTN or via dedicated lines Can connect mobile user in its area and remote mobile user

Mobile Radio Propagation Effects

Signal strength
Strength of signal between BS and mobile unit strong enough to maintain signal quality at the receiver Not strong enough to create too much cochannel interference Noise varies
Automobile ignition noise greater in city than in suburbs Other signal sources vary Signal strength varies as function of distance from BS Signal strength varies dynamically as mobile unit moves

Fading
Even if signal strength in effective range, signal propagation effects may disrupt the signal

Design Factors
Propagation effects Maximum transmit power level at BS and mobile units Typical height of mobile unit antenna Available height of the BS antenna These factors determine size of individual cell Model based on empirical data Apply model to given environment to develop guidelines for cell size E.g. model by Okumura et al refined by Hata
Detailed analysis of Tokyo area Produced path loss information for an urban environment Hata's model is an empirical formulation
Takes into account variety of environments and conditions

Dynamic Hard to predict

Fading
Time variation of received signal Caused by changes in transmission path(s) E.g. atmospheric conditions (rain) Movement of (mobile unit) antenna

Multipath Propagation
Reflection
Surface large relative to wavelength of signal May have phase shift from original May cancel out original or increase it

Diffraction
Edge of impenetrable body that is large relative to wavelength May receive signal even if no line of sight (LOS) to transmitter

Scattering
Obstacle size on order of wavelength
Lamp posts etc.

If LOS, diffracted and scattered signals not significant

Reflected signals may be

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

Reflection, Diffraction, Scattering

Effects of Multipath Propagation

Signals may cancel out due to phase differences

Two Pulses in Time-Variant Multipath

Types of Fading
Fast fading
Rapid changes in strength over distances about half wavelength
900MHz wavelength is 0.33m 20-30dB

Slow fading
Slower changes due to user passing different height buildings, gaps in buildings etc. Over longer distances than fast fading

Flat fading
Nonselective Affects all frequencies in same proportion

Selective fading
Different frequency components affected differently

Error Compensation Mechanisms (1)

Forward error correction Adaptive equalization Diversity

Frequency Diversity
Signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers E.g. spread spectrum (see chapter 9)

First Generation Analog

Original cellular telephone networks Analog traffic channels Early 1980s in North America Advanced Mobile Phone Service (AMPS)
AT&T

Also common in South America, Australia, and China

Spectral Allocation In North America

Two 25-MHz bands are allocated to AMPS
One from BS to mobile unit (869894 MHz) Other from mobile to base station (824849 MHz)

Bands is split in two to encourage competition

In each market two operators can be accommodated

Operator is allocated only 12.5 MHz in each direction Channels spaced 30 kHz apart
Total of 416 channels per operator

Twenty-one channels allocated for control 395 to carry calls Control channels are 10 kbps data channels Conversation channels carry analog using frequency modulation Control information also sent on conversation channels in bursts as data Number of channels inadequate for most major markets For AMPS, frequency reuse is exploited

Operation
AMPS-capable phone has numeric assignment module (NAM) in read-only memory
NAM contains number of phone
Assigned by service provider

Serial number of phone

Assigned by the manufacturer

When phone turned on, transmits serial number and phone number to MTSO (Figure 14.5) MTSO has database of mobile units reported stolen
Uses serial number to lock out stolen units

MTSO uses phone number for billing If phone is used in remote city, service is still billed to user's local service provider

Call Sequence
1. Subscriber initiates call by keying in number and presses send 2. MTSO validates telephone number and checks user authorized to place call
Some service providers require a PIN to counter theft

3. MTSO issues message to user's phone indicating traffic channels to use 4. MTSO sends ringing signal to called party
All operations, 2 through 4, occur within 10 s of initiating call

5. When called party answers, MTSO establishes circuit and initiates billing information 6. When one party hangs up MTSO releases circuit, frees radio channels, and completes billing information

AMPS Control Channels

21 full-duplex 30-kHz control channels
Transmit digital data using FSK Data are 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 Handoff

Second Generation CDMA

Higher quality signals Higher data rates Support of digital services Greater capacity Digital traffic channels

Encryption

Support digital data Voice traffic digitized User traffic (data or digitized voice) converted to analog signal for transmission Simple to encrypt digital traffic (See chapter 6) Very clear voice reception

Error detection and correction

Channel access

Channel dynamically shared by users via Time division multiple access (TDMA) or code division multiple access (CDMA)

Code Division Multiple Access

Each cell allocated frequency bandwidth
Split in two
Half for reverse, half for forward Direct-sequence spread spectrum (DSSS) (see chapter 9)

Code Division Multiple Access Advantages

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

Multipath resistance
DSSS overcomes multipath fading by frequency diversity Also, chipping codes used only exhibit low cross correlation and low autocorrelation Version of signal delayed more than one chip interval does not interfere with the dominant signal as much

Privacy
From spread spectrum (see chapter 9)

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

Code Division Multiple Access

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 Some cross correlation Distinct from either TDMA or FDMA
In which, for reasonable time or frequency guardbands, respectively, received signals are orthogonal or nearly so

Near-far problem
Signals closer to receiver are received with less attenuation than signals farther away Given lack of complete orthogonality, transmissions from more remote mobile units may be more difficult to recover

RAKE Receiver
If multiple versions of signal arrive more than one chip interval apart, receiver can recover signal by correlating chip sequence with dominant incoming signal
Remaining signals treated as noise

Better performance if receiver attempts to recover signals from multiple paths and combine them, with suitable delays Original binary signal is spread by XOR operation with chipping code Spread sequence modulated for transmission over wireless channel Multipath effects generate multiple copies of signal

Each with a different amount of time delay (1, 2, etc.) Each with a different attenuation factors (a1, a2, etc.) Receiver demodulates combined signal Demodulated chip stream fed into multiple correlators, each delayed by different amount Signals combined using weighting factors estimated from the channel

Principle of RAKE Receiver

IS-95
Second generation CDMA scheme Primarily deployed in North America Transmission structures different on forward and reverse links

IS-95 Channel Structure

IS-95 Forward Link (1)

Up to 64 logical CDMA channels each occupying the same 1228-kHz bandwidth Four types of channels:
Pilot (channel 0)
Continuous signal on a single channel Allows mobile unit to acquire timing information Provides phase reference for demodulation process Provides signal strength comparison for handoff determination Consists of all zeros 1200-bps channel used by mobile station to obtain identification information about the cellular system System time, long code state, protocol revision, etc.

Synchronization (channel 32)

IS-95 Forward Link (2)

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

Traffic (channels 8 to 31 and 33 to 63)

55 traffic channels Original specification supported data rates of up to 9600 bps Revision added rates up to 14,400 bps

All channels use same bandwidth

Chipping code distinguishes among channels Chipping codes are the 64 orthogonal 64-bit codes derived from 64 64 Walsh matrix

Forward Link Processing

Voice traffic encoded at 8550 bps Additional bits added for error detection
Rate now 9600 bps

Full capacity not used when user not speaking Quiet period data rate as low as 1200 bps 2400 bps rate used to transmit transients in background noise 4800 bps rate to mix digitized speech and signaling data Data transmitted in 20 ms blocks Forward error correction
Convolutional encoder with rate Doubling effective data rate to 19.2 kbps For lower data rates encoder output bits (called code symbols) replicated to yield 19.2-kbps

Data interleaved in blocks to reduce effects of errors by spreading them

Scrambling
After interleaver, data scrambled Privacy mask Prevent sending of repetitive patterns
Reduces probability of users sending at peak power at same time

Scrambling done by long code

Pseudorandom number generated from 42-bit-long shift register Shift register initialized with user's electronic serial number Output of long code generator is at a rate of 1.2288 Mbps
64 times 19.2 kbps One bit in 64 selected (by the decimator function) Resulting stream XORed with output of block interleaver

Power Control
Next step inserts power control information in traffic channel
To control the power output of antenna Robs traffic channel of bits at rate of 800 bps by stealing code bits 800-bps channel carries information directing mobile unit to change output level Power control stream multiplexed into 19.2 kbps
Replace some code bits, using long code generator to encode bits

DSSS
Spreads 19.2 kbps to 1.2288 Mbps Using one row of Walsh matrix
Assigned to mobile station during call setup If 0 presented to XOR, 64 bits of assigned row sent If 1 presented, bitwise XOR of row sent

Final bit rate 1.2288 Mbps Bit stream modulated onto carrier using QPSK
Data split into I and Q (in-phase and quadrature) channels Data in each channel XORed with unique short code
Pseudorandom numbers from 15-bit-long shift register

Forward Link Transmission

Reverse Link
Up to 94 logical CDMA channels
Each occupying same 1228-kHz bandwidth Supports up to 32 access channels and 62 traffic channels

Traffic channels mobile unique

Each station has unique long code mask based on serial number
42-bit number, 242 1 different masks Access channel used by mobile to initiate call, respond to paging channel message, and for location update

Reverse Link Processing and Spreading

First steps same as forward channel
Convolutional encoder rate 1/3 Tripling effective data rate to max. 28.8 kbps Data block interleaved

Spreading using Walsh matrix

Use and purpose different from forward channel Data from block interleaver grouped in units of 6 bits Each 6-bit unit serves as index to select row of matrix (26 = 64) Row is substituted for input Data rate expanded by factor of 64/6 to 307.2 kbps Done to improve reception at BS Because possible codings orthogonal, block coding enhances decisionmaking algorithm at receiver Also computationally efficient Walsh modulation form of block error-correcting code (n, k) = (64, 6) and dmin = 32 In fact, all distances 32

Data Burst Randomizer

Reduce interference from other mobile stations Using long code mask to smooth data out over 20 ms frame

DSSS
Long code unique to mobile XORed with output of randomizer 1.2288-Mbps final data stream Modulated using orthogonal QPSK modulation scheme Differs from forward channel in use of delay element in modulator to produce orthogonality
Forward channel, spreading codes orthogonal
Coming from Walsh matrix

Reverse channel orthogonality of spreading codes not guaranteed

Reverse Link Transmission

Third Generation Systems

Objective to provide fairly high-speed wireless communications to support multimedia, data, and video in addition to voice ITUs International Mobile Telecommunications for the year 2000 (IMT-2000) initiative defined ITUs view of thirdgeneration capabilities as:
Voice quality comparable to PSTN 144 kbps available to users in vehicles over large areas 384 kbps available to pedestrians over small areas Support for 2.048 Mbps for office use Symmetrical and asymmetrical data rates Support for packet-switched and circuit-switched services Adaptive interface to Internet More efficient use of available spectrum Support for variety of mobile equipment Flexibility to allow introduction of new services and technologies

Driving Forces
Trend toward universal personal telecommunications
Ability of person to identify himself and use any communication system in globally, in terms of single account

Universal communications access

Using ones terminal in a wide variety of environments to connect to information services e.g. portable terminal that will work in office, street, and planes equally well

GSM cellular telephony with subscriber identity module, is step towards goals Personal communications services (PCSs) and personal communication networks (PCNs) also form objectives for thirdgeneration wireless Technology is digital using time division multiple access or codedivision multiple access PCS handsets low power, small and light

Alternative Interfaces (1)

IMT-2000 specification covers set of radio interfaces for optimized performance in different radio environments Five alternatives to enable smooth evolution from existing systems Alternatives reflect evolution from second generation Two specifications grow out of work at European Telecommunications Standards Institute (ETSI)
Develop a UMTS (universal mobile telecommunications system) as Europe's 3G wireless standard Includes two standards
Wideband CDMA, or W-CDMA
Fully exploits CDMA technology Provides high data rates with efficient use of bandwidth

IMT-TC, or TD-CDMA
Combination of W-CDMA and TDMA technology Intended to provide upgrade path for TDMA-based GSM systems

Alternative Interfaces (2)

CDMA2000
North American origin Similar to, but incompatible with, W-CDMA

IMT-SC designed for TDMA-only networks IMT-FC can be used by both TDMA and FDMA carriers
To provide some 3G services Outgrowth of Digital European Cordless Telecommunications (DECT) standard

In part because standards use different chip rates Also, cdma2000 uses multicarrier, not used with WCDMA

IMT-2000 Terrestrial Radio Interfaces

CDMA Design Considerations Bandwidth and Chip Rate

Dominant technology for 3G systems is CDMA
Three different CDMA schemes have been adopted Share some common design issues

Bandwidth
Limit channel usage to 5 MHz Higher bandwidth improves the receiver's ability to resolve multipath But available spectrum is limited by competing needs 5 MHz reasonable upper limit on what can be allocated for 3G 5 MHz is enoughfordata rates of 144 and 384 kHz

Chip rate
Given bandwidth, chip rate depends on desired data rate, need for error control, and bandwidth limitations Chip rate of 3 Mcps or more reasonable

CDMA Design Considerations Multirate

Provision of multiple fixed-data-rate logical channels to a given user Different data rates provided on different logical channels Traffic on each logical channel can be switched independently through wireless fixed networks to different destinations Flexibly support multiple simultaneous applications from user Efficiently use available capacity by only providing the capacity required for each service Achieved with TDMA scheme within single CDMA channel
Different number of slots per frame assigned for different data rates Subchannels at a given data rate protected by error correction and interleaving techniques

Alternative: use multiple CDMA codes

Separate coding and interleaving Map them to separate CDMA channels

Time and Code Multiplexing

Required Reading
Stallings chapter 14 Web search on 3G mobile phones