شبکههای بیسیم (628-40)مقدمه دانشکده مهندسی کامپیوتر نیمسال دوّم 98-97 افشین همّتیار
References • A. Kumar, D. Manjunath, and J. Kuri, • Wireless Networking, • Morgan Kaufmann Publishers, 2008. • C.M. Cordeiro and D.P. Agrawal, Ad Hoc and Sensor Networks: Theory and Applications, 2nd Ed, • World Scientific, 2011.
Contents Background Wireless communication: concepts, techniques, and models Application models and performance issues Cellular FDM-TDMA Cellular CDMA Cellular OFDMA-TDMA Random Access and Wireless LANs (WLANs) Mobile Ad-hoc Networks (MANETs) Wireless Mesh Networks (WMNs) Wireless Sensor Networks ( WSNs)
Wired Vs. Wireless Communications Adjacent Channel Interference P P Channel A /Channel A Channel A Channel B f Each cable is a different channel One media (cable) shared by all f Signal attenuation is low Highsignal attenuation Co-Channel Interference No interference High interference noise; co-channel interference;adjacent channel interference
Why Wireless? • Advantages • Sometimes it is impractical to lay cables • User mobility • Cost • Limitations • Bandwidth • Fidelity • Power • Security
Propagation Principle electric field propagation direction magnetic field
ISM band 902 – 928 Mhz 2.4 – 2.4835 Ghz FM radio S/W radio TV TV AM radio cellular 5.725 – 5.785 Ghz VHF UHF SHF EHF LF MF HF 300MHz 30MHz 30GHz 300GHz 3GHz 3MHz 30kHz 300kHz 100mm 10cm 10m 1cm 1m 100m 10km 1km Electromagnetic Spectrum X rays Gamma rays visible UV infrared 1 MHz 1 GHz 1 kHz 1 THz 1 EHz 1 PHz Propagation characteristics are different in each frequency band.
Unlicensed Radio Spectrum(ISM: Industrial, Science, Medicine) 12cm 5cm 33cm 26 Mhz 83.5 Mhz 125 Mhz 902 Mhz 2.4 Ghz 5.725 Ghz 2.4835 Ghz 5.850 Ghz 928 Mhz 802.11a 802.11b Bluetooth Microwave oven cordless phones baby monitors WaveLan
Propagation Mechanisms Line-of-Sight S D Non Line-of-Sight Scattering λ >> D Diffraction λ D Reflection λ << D
Propagation in the “Real World” a wave can be absorbed penetrate reflect bend
Path-loss Models • Path-Loss Exponent Depends on environment: L(d) = L(d0)(d/d0)n Free space n = 2 Urban area cellular n = 2.7 to 3.5 Shadowed urban cell n = 3 to 5 In building LOS n = 1.6 to 1.8 Obstructed in building n = 4 to 6 Obstructed in factories n = 2 to 3
Resource Allocation • Wireline • Static bit-carrier infrastructure • High quality digital transmission over copper or optical media • Bit pipes with a certain bit rate and very small bit error rate • Dynamically reconfigured based on traffic demands • Wireless • Point-to-point Line-of-sight • (same as wireline or higher bit rate) • Time-varying channel impairments • Adaptable PHY layer
Wireless Networking • Our view: • All the mechanisms, procedures, or algorithms for efficient sharing of a portion of the radio spectrum so that all instances of communication between the various devices obtain their desired Quality of Service (QoS).
Fixed Networks • Point to point • Long distance transmission • High gain antennas • Tall masts • Higher bit rate and also higher bit error rate than wireline Point to Point
Mobile and Ad-hoc Networks Access Networks Mesh Network
Mobile Networks: Circuit Multiplexing • GSM (2G) • Narrowband • FDM-TDMA • High SINR • Careful frequency planning to avoid co-channel interference • Call admission control • GSM-GPRS (2.5G) Combining TDM Time slots • GSM-EDGE (2.75G) Combining TDM Time slots and higher order modulation schemes Circuit Multiplexing: resource allocation model Static partitioning of bandwidth in a circuit switched network
Mobile Networks: Centralized Statistical Multiplexing (1) • CDMA (IS-95) • Wideband • CDMA (Spread spectrum) • Correlation receivers • No frequency planning • Interference limited • Call admission control • WCDMA (CDMA-2000) • Most widely adapted standard for 3G 3G CDMA • Current 3G services based on WCDMA • Voice, streaming, high-speed data • Multi-rate service via variable power and spreading • Different services can be mixed on a single code for a user
Mobile Networks: Centralized Statistical Multiplexing (2) • WiMAX(IEEE 802.16 series) • Wireless access to Internet • Fixed subscriber stations • OFDMA • TDD (uplink & downlink) • Specifications now have been extended to include broadband access to mobile users.
Mobile Networks: Distributed Statistical Multiplexing • WLAN (IEEE 802.11 series) • Wireless access to Internet • Limited mobility • Statistical TDMA • Few Mbps (over 100s of meter) • up to 100Mbps (over a few meters) • MIMO-OFDM (enhancement) Statistical TDM Statistical Multiplexing avoids unfilled slots and take less time to send data
Ad hoc Networks: Internet Access and Sensor Networks • No infra-structure • Multi-hop communication • Point-to-point store and forward traffic • Miniature devices for nodes • Low power, low bit rate digital radio transceiver, and small battery VANET (Vehicular Ad hoc NETwork)
Technical Elements • General: • Transport of the user’s bits over the shared radio spectrum • Neighbor discovery, association and topology formation, routing • Transmission scheduling (cross layer) • Only in ad hoc sensor networks: • Location determination • Distributed computation