NSS Enriching Knowledge for Information and Communication Technology Curriculum
Series: (5) Latest Technologies on 'Internet Services & Applications' and 'Wireless
Computer Network' (CDI020071828)
28 Feb 2008
Introduction and Several Recent Advances of Wireless Communications
Prof. Jianwei Huang (jwhuang@ie.cuhk.edu.hk)
Acknowledgement
` Minghua Chen (CUHK)
` David Tse (UC-Berkeley)
` Jennifer Hou (UIUC)
` Raj Jain (WUSTL)
` Kaustubh Phanse (LUTH)
Why Wireless?
` Users want to get connected anywhere
` Reality (2006):
` USA: 12%
` Korea: 50%
` Hong Kong: 25%
` Service providers want easy/cheap provision
` No wire or infrastructure node
` Cheap: establishment cost (1/4 of cable),
` Easy installation, low operation cost, robust…
Usage Trend
Technology Trend
` Throughput
` Cellular: 14.4 kbps (modem) Î 144 kbps (1xRTT) Î 384kbps (EDGE) Î 2Mbps (EV-DO) Î? (WiMax) Î…
` Wi-Fi: 2 Mbps (AT&T Wave) Î 11 Mbps(802.11b) Î 54 Mbps (802.11 a/g) Î 74/248 Mbps? (802.11n) Î …
` Catching up the throughput of wired access links…
` Technologies:
` TDMA Î FDMA Î CDMA Î OFDMA Î MIMO+OFDMA Î …
Mobile Internet Access methods
Standard Family Primary Use Radio Tech Downlink (Mbps) Uplink (Mbps) 802.16e WiMAX Mobile Internet MIMO-SOFDMA 70 70
HIPERMAN HIPERMAN Mobile Internet OFDM 56.9 56.9
WiBro WiBro Mobile Internet OFDMA 50 50
iBurst iBurst 802.20 Mobile Internet HC-SDMA 64 64
UMTSW-CDMA
HSDPA+HSUPA UMTS/3GSM Mobile phone CDMA/FDD .384
14.4 .384
5.76
UMTS-TDD UMTS/3GSM Mobile Internet CDMA/TDD 16 16
LTE UMTS UMTS/4GSM General 4G OFDMA/MIMO/SC-
FDMA(HSOPA) >100 >50
1xRTT CDMA2000 Mobile phone CDMA 0.144 0.144
EV-DO1x Rev. 0 EV-DO 1x Rev.A
EV-DO Rev.B CDMA2000 Mobile Internet CDMA/FDD
2.45 3.1 4.9xN
0.15 1.8 1.8xN
802.11 Family
802.11a 802.11b 802.11g 802.11n
Speed 54Mbps 11Mbps 54Mbps 74/248Mbps
Frequency 5 GHz 2.4GHz 2.4GHz 2.4 and/or
5Ghz
Tech. OFDM TDMA OFDM MIMO+OFDM
Indoor Range 25-75 feet 100-200 feet 100-200 feet 200-300 feet Compatibility Incompatible
with b, g, n
Compatible with g and n
Compatible with b and n
Compatible with a, b, and g
Data Rate VS. Range
Spectrum – The Resources
Attenuation (For Fixed Power)
` Transmission range
` High SNR
` Low error rate
` Detection range
` Mid SNR
` Detecting wireless signal
` Interference range
` Low SNR
` Signal adds to the interfered noise
Special Topic I:
Multiplexing Techniques
Multiplexing – Allowing Multiple Access
` Four dimensions
` Time
` Frequency
` Space
` Code
Frequency Division Multiple Access (FDMA)
` Divide frequency band into small sub-band
` Each sub-band is a channel
` Example: 1G
Time Division Multiple Access (TDMA)
` Divide time into small slots
` Each slot is a channel
` Need synchronization
` Example: 2G
Frequency/Time Division Multiple Access (F/TDMA)
` Frequency-time is divided into small F/T block
` Channel is a series of block
` Need synchronization
` Example: GSM
Code Division Multiple Access (CDMA)
` No division into sub-band/slot/block, use the entire frequency band any time
` Divide the signal space into many
orthogonal sub-spaces, using orthogonal codes, assign each device one code
` Project the received signal to your space and decode
` No synchronization
` Example: IS-95, CDMA 2000, WCDMA…
Special Topic II:
Opportunistic Communications
18
Multipath Fading
Classical view: fading channels are unreliable
Modern view: multipath fading can be exploited to increase spectral efficiency.
16dB
19
Traditional Approach to Wireless System Design
Compensates for deep fades via diversity techniques over time, frequency and space.
(Glass is half empty.)
20
Example: GSM/CDMA
` frequency diversity via Rake combining
` time diversity via interleaving and coding
21
Multipath Fading: Another Look
` Multipath fading provides high peaks to exploit.
` Channel capacity is achieved by such an opportunistic strategy.
22
Multiuser Opportunistic Communication
23
Application to CDMA 2000 1x EV-DO
` Multiuser diversity provides a system-wide benefit.
` Challenge is to share the benefit among the users in a fair way.
24
Symmetric Users
Serving the best user at each time is also fair in terms of long term throughputs.
25
Asymmetric Users: Hitting the Peaks
Want to serve each user when it is at its peak.
A peak should be defined with respect to the latency time-scale tc of the application.
Changes of View
` Traditional view: fading is bad, constant is good
` GSM/CDMA (IS-95)
` Modern view: fading can be exploited
` Not enough fading? Î introduce them in purpose
27
Dumb Antennas
The information bearing signal at each of the transmit antenna is multiplied by a time-varying phase.
28
Slow Fading Environment: Before
29
After
30
Dumb Antennas in Action: One User
Most of the time, the beam is nowhere near the user.
31
Many users: Opportunistic Beamforming
•In a large system, there is likely to be a user near the beam at any one time.
•By transmitting to that user, close to true beamforming performance is achieved, without knowing the locations of the users.
Conclusions
` Wireless industry is ever growing
` Better technology with higher throughput
` Many tough and open engineering questions
` We need more good people (including your students!)
