1. Overview of Wireless Communications
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 – W/m2 for LEO
S = 5 x 10 – 14 W/m2 for LEO
t d = S for LEO
t d = 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

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. Evolution of Current Systems
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)

15. Consensus among experts:
Other Tradeoffs:
Rate vs. Coverage
Rate vs. Delay
Rate vs. Cost
Rate vs. Energy
Rate 4G
802.11b WLAN 3G 2G
2G Cellular
Mobility
Consensus among experts:
Design breakthroughs are needed – not just improvements on present designs

16. Multimedia Design Requirements
Voice
Data
Video
Delay
<100ms -
<100ms
Packet Loss
<1%
<1%
BER
10-3
10-6
10-6
Data Rate
8-32 Kbps
1-100 Mbps
1-20 Mbps
Traffic
Continuous
Bursty
Continuous
One-size-fits-all protocols and design do not work well
Wired networks use this approach with poor results

17. Wireless Performance Gap
WIDE AREA CIRCUIT SWITCHING
User
Bit-Rate
(kbps)
14.4
digital
cellular
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
2000
1990
1980
YEAR
LOCAL AREA PACKET SWITCHING
Ethernet
FDDI
100 M
Polling
Packet
Radio
1st gen
WLAN
2nd gen
wired- wireless
bit-rate "gap"
.01 .1 1 10
100
1000
10,000
100,000

18. Quality of Service (QoS)
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

19. Crosslayer Techniques
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

20. Current Wireless Systems
Cellular Systems
Wireless LANs
Satellite Systems
Paging Systems
Bluetooth
Ultrawideband radios
Zigbee radios

21. Cellular Telephone Systems
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
BASE
STATION
MTSO
Mobile Telephone Switching Office

22. 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
BASE
STATION

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. Cellular Phone Networks
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
San Francisco BS BS
Internet
New York
MTSO
MTSO
PSTN BS
Public Switched Telephone Network

26. 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
San Francisco BS BS
Internet
New York
MTSO
MTSO
PSTN BS
Public Switched Telephone Network

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 systems use GPRS and EDGE as well with rates up to 384 kbps
IS - 95 supports data rates up to 115 kbps

34. 3G Cellular : Voice and Data
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

35. Wireless Local Area Networks (WLANS)
1011
0101
Internet
Access
Point
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. Wireless LAN Standards
802.11b (Current Generation)
Standard for 2.4GHz ISM band (80 MHz)
Frequency hopped spread spectrum
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
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. Paging Systems
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

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 / 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
10 W
802.11a 3G
802.11bg
1 W
100 mW
Bluetooth
UWB
ZigBee
10 mW
ZigBee
UWB
1 mW
0 GHz
1GHz
2 GHz
3 GHz
4 GHz
5 GHz
6 GHz

45. Data Rate
UWB
802.11g
802.11a
802.11b
10 Mbit/sec
1 Mbit/sec 3G
Bluetooth
ZigBee
ZigBee
10 kbits/sec
UWB
0 GHz
1GHz
2 GHz
3 GHz
4 GHz
5 GHz
6 GHz

46. Range
10 km 3G
1 km
100 m
802.11b,g
802.11a
Bluetooth
10 m
ZigBee
ZigBee
UWB
1 m
UWB
0 GHz
1GHz
2 GHz
3 GHz
4 GHz
5 GHz
6 GHz

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

50. Problem
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.)
What is the total number of active users that your system can support for a cell size of 1 km 2?
What cell size would you use if your system had to support 250,000 active users?

51. Solution
What is the total number of active users that your system can support for a cell size of 1 km 2?
# of cells = Area of city/Area per cell
# of users = # of cells x users/cell
= x = 10,000
b. What cell size would you use if your system had to support 250,000 active users?
# cells = # users/users/cell

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 = 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))?
Macrocell
C = 100 x 100 = 10,000
The unknown is U, the number of users
Iterate U until P b = 0.02
U = 2670 subscribers
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 months
Microcell: $ 333,750,000/month months