1

ECE 480

Wireless Systems

Lecture 6

Overview of Wireless Communications

22 Feb 2006

2

History of Wireless Communications

• Most radio systems transmit data through the use of a “data packet”, a burst of digital data

• First established in 1971, “ALOHANET”

• Connected 7 campuses on 4 islands

• Used “star” topology

• Initial LANs using this technology were not successful (only 20 kbps speed and low coverage)

• Wired LANs had 10 Mbps speed

• Today, wireless LANs have a speed in the tens of Mbps

• Wired LANs still faster, 1Gbps

3

Cellular Telephone System

• Not successful early due to lack of capacity

• AT&T developed the cellular concept

• Power falls off at a distance

• Two users separated by a distance can use the same frequency

• Analog systems introduced in 1983 – saturated by 1984

• FCC increased the bandwidth

• Digital systems introduced in early 1990’s

• Higher capacity, lower cost, higher efficiency, and faster speed

• Standards still an issue

4

Satellite Systems• LEO (Low Earth Orbit) – 2000 km altitude

• MEO (Medium Earth Orbit) – 9000 km altitude

• Geosynchronous (GEO) – 40,000 km altitude

• The higher the satellite, the more coverage, BUT the more power required

Suppose that we have a cellular phone with a dipole antenna that delivers 1 mw in a symmetric pattern

S = 2 x 10 – 11 W/m2 for LEO

S = 5 x 10 – 14 W/m2 for LEO

t d = 1.3 S for LEO

t d = 0.27 mS for GEO (Significant)

5

What is Wireless Communications?

• Applications

• Voice

• Internet access

• Sensing and controls

• Data transfer

• LANs

• Text messaging

• Entertainment

6

What is Wireless Communications?

• Systems

• Cellular telephones

• Wireless LANs

• Wide area wireless data systems

• Satellite Systems

Problem: These applications/systems all have different requirements

Result: Fragmentation of standards, services, and products

7

• Voice systems

• Low data rate requirements (20 kbps)

• High Bit Error Rate (10 – 3)

• Low total delay (100 ms)

• Data systems

• High data rates (1 – 100 Mbps)

• Low BER (10 – 8)

• No absolute delay requirement

• Real time video systems

• High data rate requirement

• High bit error rate

• Low total delay

• Paging, Text messaging

• Low data rate requirements

• Low BER

• No absolute delay requirement

8

• The most stringent of these requirements can readily be met by wired systems

• Data rates of several GHz

• BER – 10 – 12

• Wireless systems must be tailored to the application

• More fragmentation

• Different protocols

9

Technical Issues

• Wireless channels are a difficult and capacity-limited broadcast communications medium

• Traffic patterns, user locations, and network conditions are constantly changing

• Applications are heterogeneous with hard constraints that must be met by the network

• Energy and delay constraints change design principles across all layers of the protocol stack

10

• Wireless channels are a difficult and capacity-limited broadcast communications medium

• Spectrum is very crowded and expensive• Bandwidth is usually auctioned to the highest bidder• Spectrum must be reused in the same geographical

area

• Need breakthroughs to enable systems to operate at higher frequencies or to use bandwidth more efficiently

11

• Traffic patterns, user locations, and network conditions are constantly changing

• Mobility is both an advantage and a curse

• Signal experiences random fluctuations in time due to movement, obstacles, or reflection

• Channel characteristics appear to change randomly with time

• Security

12

• Applications are heterogeneous with hard constraints that must be met by the network

• Must locate a user among billions traveling at 100 km/sec

• Must interface with wired networks

• Energy and delay constraints change design principles across all layers of the protocol stack

• Most wired systems are designed in layers

• Layers are designed in isolation with standards to interface between layers

• Wireless systems do not have the same baseline conditions

• Transmission is spotty and may change with time

13

• Energy and delay constraints change design principles across all layers of the protocol stack

• Most wired systems are designed in layers

• Layers are designed in isolation with standards to interface between layers

• Wireless systems do not have the same baseline conditions

• Transmission is spotty and may change with time

• Energy use is also critical – batteries are large, heavy, and expensive

14

• Wireless systems today• 2G Cellular: ~30-70 Kbps.• WLANs: ~10 Mbps.

• Next Generation• 3G Cellular: ~300 Kbps.• WLANs: ~70 Mbps.

• Technology Enhancements • Hardware: Better batteries. Better

circuits/processors.• Link: Antennas, modulation, coding,

adaptivity, DSP, BW.• Network: Dynamic resource allocation.

Mobility support.• Application: Soft and adaptive QoS. (Quality

of Service)

Evolution of Current Systems

15

Rate

Mobility

2G

3G4G

802.11b WLAN

2G Cellular

Other Tradeoffs: Rate vs. Coverage Rate vs. Delay Rate vs. Cost Rate vs. Energy

Consensus among experts:

• Design breakthroughs are needed – not just improvements on present designs

16

Voice VideoDataDelay

Packet LossBER

Data RateTraffic

<100ms - <100ms<1% 0 <1%10-3 10-6 10-6

8-32 Kbps 1-100 Mbps 1-20 MbpsContinuous Bursty Continuous

• One-size-fits-all protocols and design do not work well• Wired networks use this approach with

poor results

Multimedia Design Requirements

17

WIDE AREA CIRCUIT SWITCHING

User Bit-Rate (kbps)

14.4digitalcellular

28.8 modem

ISDN

ATM

9.6 modem

2.4 modem 2.4 cellular

32 kbps PCS

9.6 cellular

wired- wireless bit-rate "gap"

1970 200019901980YEAR

LOCAL AREA PACKET SWITCHING

User Bit-Rate (kbps)

EthernetFDDI

ATM100 M Ethernet

Polling

Packet Radio

1st genWLAN

2nd genWLAN

wired- wirelessbit-rate "gap"

1970 200019901980.01

.1

1

10

100

1000

10,000

100,000

YEAR.01

.1

1

10

100

1000

10,000

100,000

Wireless Performance Gap

18

• QoS refers to the requirements associated with a given application, typically rate and delay requirements

• It is hard to make a one-size-fits all network that supports requirements of different applications

• Wired networks often use this approach with poor results, and they have much higher data rates and better reliability than wireless

• QoS for all applications requires a cross-layer design approach

Quality of Service (QoS)

19

• Adaptive techniques• Link, MAC, network, and application adaptation• Resource management and allocation (power control)

• Diversity techniques• Link diversity (antennas, channels, etc.)• Access diversity • Route diversity• Application diversity• Content location/server diversity

• Scheduling• Application scheduling/data prioritization• Resource reservation• Access scheduling

Crosslayer Techniques

20

Current Wireless Systems

• Cellular Systems

• Wireless LANs

• Satellite Systems

• Paging Systems

• Bluetooth

• Ultrawideband radios

• Zigbee radios

21

BASESTATION

MTSO

Cellular Telephone Systems

Mobile Telephone Switching Office

• Geographic region divided into cells• Frequencies/timeslots/codes reused at spatially-separated

locations.• Co-channel interference between same color cells.• Base stations/MTSOs coordinate handoff and control functions• Shrinking cell size increases capacity, as well as networking

burden

22

BASESTATION

Intercell Interference:

Interference caused by users in different cells operating on the same channel set

Reuse distance:

the spatial separation of cells that reuse the same channel set

23

• Reuse distance cannot be reduced below a minimum value depending on the characteristics of signal propagation within the aggregate of cells

• Early base stations were few and far between, usually on a high spot to cover as much area as possible (macrocells)

• Approximately uniform signal

• Circular cells (approximated by hexagon)

• Present – day base stations are smaller and closer to street level (microcells or picocells)

• Increases capacity

• Lower cost

• More complicated network design

24

• More complicated network design

• Mobile phones change cells more frequently

• Handoffs must be processed more quickly

• Hexagonal shape may no longer be a good approximation

• Location management is more complicated

25

BSBS

MTSOPSTN

MTSO

BS

San Francisco

New YorkInternet

Cellular Phone Networks

Public Switched Telephone Network

• All base stations are connected to a mobile telephone switching office (MTSO) by a high speed link

• MTSO is a central controller

• Allocates channels within cell

• Coordinates handoffs

• Routing calls to and from mobile users

26

BSBS

MTSOPSTN

MTSO

BS

San Francisco

New YorkInternet

Public Switched Telephone Network

• MTSO routes voice calls through PSTN or to the internet

• User request a channel through a separate control channel

• Call handoff occurs when the base station or mobile detects that a signal has fallen below a minimum threshold

• MTSO queries whether a surrounding station can detect the signal

• Call is dropped if the signal strength drops below the minimum threshold

27

• Cellular systems are primarily digital

• Cheaper, faster, smaller, and use less power

• Higher capacity

• More efficient modulation techniques

• Compression techniques

• Encryption techniques

• Data services

28

Spectral Sharing (Multiple Access)

• Divides the signal dimensions along time, frequency, and/or code space axes

• Frequency Division Multiple Access (FDMA)• Total System bandwidth is divided into orthogonal frequency

channels• The subcarrier pulse used for

transmission is chosen to be rectangular. This has the advantage that the task of pulse forming and modulation can be performed by a simple Inverse Discrete Fourier Transform (IDFT) which can be implemented very efficiently as a I Fast Fourier Transform (IFFT).

• Receiver design is simplified

29

• Time Division Multiple Access (TDMA)

• Time is divided orthogonally and each channel occupies the entire frequency band over its assigned timeslot

• More difficult to implement than FDMA – must be time synchronized

• Easier to accommodate multiple data rates

30

• Code – Division Multiple Access (CDMA)

• Implemented using direct – sequence or frequency – hopping

• Direct sequence, each user modulates its data sequence by a different data sequence that is much faster

• In frequency hopping the carrier frequency used to modulate the narrowband data signal is varied by a chip (binary) sequence that may be faster or slower than the data sequence

• Results in a modulated signal that hops over different carrier frequencies

31

• Efficient cellular system designs are "interference limited"

• Interference is higher than random noise

• Methods for improvement

• Cell sectorization

• Directional and smart antennas

• Multiuser detection

• Dynamic resource allocation

32

Second Generation (2G) Standards• Europe uses GSM (Global Systems for Mobile

Communications) standards

• Combination of TDMA and slow frequency hopping with frequency – shift keying for voice modulation

• US has several incompatible standards

• 900 MHz band has 2 standards

• IS-136 uses a combination of TDMA and FDMA and phase - shift keyed modulation

• IS-95 uses direct – sequence CDMA with phase - shift keyed modulation and coding

• 2 GHz PCS (personal communication system) has 3 standards, IS-36, IS-95, and GSM

33

• All 2G standards support high - rate packet data services

• GSM supports data rates up to 140 kbps (GPRS)

• Enhanced Data Rates for GSM Evolution (EDGE) increases data rates up to 384 kbps

• Defines 9 different coding and modulation combinations, each optimized to a different value of S/N ratio (SNR)

• IS - 136 systems use GPRS and EDGE as well with rates up to 384 kbps

• IS - 95 supports data rates up to 115 kbps

34

• Data is bursty, whereas voice is continuous• Typically require different access and routing

strategies • 3G “widens the data pipe”:• 384 Kbps.• Standard based on wideband CDMA• Packet-based switching for both voice and data

• 3G cellular struggling in Europe and Asia• Evolution of existing systems (2.5G,2.6798G):• GSM + EDGE• IS-95 (CDMA)+HDR• 100 Kbps may be enough

• What is beyond 3G? The trillion dollar question

3G Cellular : Voice and Data

35

Wireless Local Area Networks (WLANS)

01011011InternetAccessPoint

0101 1011

• WLANs connect “local” computers (100m range)• Breaks data into packets• Channel access is shared (random access)• Backbone Internet provides best-effort service• Poor performance in some applications (e.g. video)

36

• All wireless LAN standards operate in unlicensed frequency bands

• 900 MHz, 2.4 GHz, 5.8 GHz, and the Unlicensed National Information Infrastructure (U - NII) band at 5 GHz

• No FCC license required – can cause interference with other users

• 1G systems were unsuccessful due to the large number of protocols

37

• 802.11b (Current Generation)• Standard for 2.4GHz ISM band (80 MHz)• Frequency hopped spread spectrum• 1.6-10 Mbps, 500 ft range

• 802.11a (Emerging Generation)• Standard for 5GHz N-II band (300 MHz)• OFDM with time division• 20-70 Mbps, variable range• Similar to HiperLAN in Europe

• 802.11g (New Standard)• Standard in 2.4 GHz and 5 GHz bands• OFDM • Speeds up to 54 Mbps

Wireless LAN Standards

In 200?,all WLAN cards will have all 3 standards

38

Satellite Systems

• Cover very large areas• Different orbit heights• GEOs (39000 Km) versus LEOs (2000 Km)

• Optimized for one-way transmission• Radio (XM, DAB) and movie (SatTV)

broadcasting• Most two-way systems struggling or bankrupt• Expensive alternative to terrestrial system• A few ambitious systems on the horizon

39

• Broad coverage for short messaging• Message broadcast from all base stations• Simple terminals• Optimized for 1-way transmission• Answer-back is hard• Overtaken by cellular

Paging Systems

40

Bluetooth

• Cable replacement RF technology (low cost)• Short range (10m, extendable to 100m)• 2.4 GHz band (crowded)• 1 Data (700 Kbps) and 3 voice channels• Widely supported by telecommunications, PC,

and consumer electronics companies• Few applications beyond cable replacement• Transmitter is imbedded into an IC• Uses frequency hopping• Applications – connection to a printer, etc..

41

IEEE 802.15.4 / ZigBee Radios

• Low-Rate WPAN• Data rates of 20, 40, 250 kbps (slower than Bluetooth)• Star clusters or peer-to-peer operation• Support for low latency devices• CSMA-CA channel access• Very low power consumption (months to years)• Frequency of operation in ISM bands

42

Ultrawideband (UWB) Radios• UWB is an impulse radio: sends pulses of tens of

picoseconds(10-12) to nanoseconds (10-9)

• Duty cycle of only a fraction of a percent

• A carrier is not necessarily needed

• Uses a lot of bandwidth (GHz)

• Low probability of detection

• Excellent ranging capability

• Multipath highly resolvable: good and bad• Can use OFDM to get around multipath problem

43

Why is UWB Interesting?• Unique Location and Positioning propertie• 1 cm accuracy possible

• Low Power CMOS transmitters• 100 times lower than Bluetooth for same range/data

rate• Very high data rates possible• 500 Mbps at ~10 feet under current regulations

• 7.5 Ghz of “free spectrum” in the U.S.• FCC recently legalized UWB for commercial use• Spectrum allocation overlays existing users, but its

allowed power level is very low to minimize interference

• “Moore’s Law Radio”• # of transistors on a chip will double every 18 months• Data rate scales with the shorter pulse widths made

possible with ever faster CMOS circuits

44

Power Dissipation

1 mW

10 mW

100 mW

1 W

10 W

0 GHz 2 GHz1GHz 3 GHz 5 GHz4 GHz 6 GHz

802.11a

UWB

UWBZigBee

Bluetooth

ZigBee

802.11bg3G

45

Data Rate

10 kbits/sec

1 Mbit/sec

10 Mbit/sec

0 GHz 2 GHz1GHz 3 GHz 5 GHz4 GHz 6 GHz

802.11a

UWBZigBee

Bluetooth

ZigBee

802.11b802.11g

3G

UWB

46

Range

1 m

10 m

100 m

1 km

10 km

0 GHz 2 GHz1GHz 3 GHz 5 GHz4 GHz 6 GHz

802.11a

UWB

ZigBee BluetoothZigBee

802.11b,g

3G

UWB

47

The Wireless Spectrum• Licensed

• Government allocates specific frequency bands for specific purposes

• Usually auctioned to highest bidder

• FCC license required

• Unlicensed

• Created to encourage innovation

• Become very crowded very fast

• No FCC license required

48

49

50

Problem 1 - 10

This problem demonstrates the capacity increase associated with a decrease in cell size. Consider a square city of 100 square kilometers. Suppose you design a cellular system for this city with square cells, where every cell (regardless of cell size) has 100 channels and so can support 100 active users. (In practice, the number of users that can be supported per cell is mostly independent of cell size as long as the propagation model and power scale appropriately.)

a. What is the total number of active users that your system can support for a cell size of 1 km 2?

b. What cell size would you use if your system had to support 250,000 active users?

51

Solution

# of cells = Area of city/Area per cellkm# cellskm

2

2

100 1001

# of users = # of cells x users/cell

= 100 x 100 = 10,000

a. What is the total number of active users that your system can support for a cell size of 1 km 2?

b. What cell size would you use if your system had to support 250,000 active users?

# cells = # users/users/cell,# cells

250 000 2500100

kmArea . km / cell 2

21 0 042500

52

Now we consider some financial implications based on the fact that users do not talk continuously. Assume that Friday from 5 – 6 pm is the busiest hour for cell-phone users. During this time, the average user places a single call, and this call lasts two minutes. Your system should be designed so that subscribers need tolerate no greater than a 2% blocking probability during this peak hour. Blocking probability is computed using the Erlang B model:

C

b kC

k

AC !PAk !

0

C = number of Channels

A = U H

U = number of users

= average # of call requests per unit time per user

H = average duration of a call

53

c. How many total subscribers can be supported in the macrocell system (1 km 2 cells) and in the microcell system (with cell size from part (b))?

C

b kC

k

AC !PAk !

0

C = 100 x 100 = 10,000

The unknown is U, the number of users

Iterate U until P b = 0.02

U = 2670 subscribers

Macrocell

Microcell: C = 267,000

U = 6,675,000 subscribers

54

d. If a base station costs $500,000, what are the base station costs for each system

Macrocell: $ 50,000,000

Microcell: $ 1,250,000,000

e. If the monthly user fee in each system is $ 50, what will be the monthly revenue in each case? How long will it take to recoup the infrastructure (base station) cost for each system?

Macrocell: $ 13,350,000/month 3.75 months

Microcell: $ 333,750,000/month 3.75 months