Elements of a wireless network

1. wireless hosts • laptop, PDA, IP phone • run applications • may be stationary (non-mobile) or mobile • wireless does not always mean mobility network infrastructure Elements of a wireless network

2. base station • typically connected to wired network • relay - responsible for sending packets between wired network and wireless host(s) in its “area” • e.g., cell towers, 802.11 access points network infrastructure Elements of a wireless network

3. network infrastructure Elements of a wireless network wireless link • typically used to connect mobile(s) to base station • also used as backbone link • multiple access protocol coordinates link access • various data rates, transmission distance

4. infrastructure mode • base station connects mobiles into wired network • handoff: mobile changes base station providing connection into wired network network infrastructure Elements of a wireless network

5. Elements of a wireless network ad hoc mode • no base stations • nodes can only transmit to other nodes within link coverage • nodes organize themselves into a network: route among themselves

6. Wireless network taxonomy multiple hops single hop host may have to relay through several wireless nodes to connect to larger Internet: mesh net host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet infrastructure (e.g., APs) no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET no infrastructure no base station, no connection to larger Internet (Bluetooth, ad hoc nets)

7. Wireless Link Characteristics (1) Differences from wired link …. • decreased signal strength: radio signal attenuates as it propagates through matter (path loss) • interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well • multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

8. Wireless Link Characteristics (2) 10-1 • SNR: signal-to-noise ratio • larger SNR – easier to extract signal from noise (a “good thing”) • SNR versus BER tradeoffs • given physical layer: increase power -> increase SNR->decrease BER • given SNR: choose physical layer that meets BER requirement, giving highest thruput • SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 10-2 10-3 10-4 BER 10-5 10-6 10-7 10 20 30 40 SNR(dB) QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)

9. time slots frequency bands Cellular networks: the first hop Two techniques for sharing mobile-to-BS radio spectrum • combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots • CDMA: code division multiple access

11. Cellular standards: brief survey 2G systems: voice channels • IS-136 TDMA: combined FDMA/TDMA (north america) • GSM (global system for mobile communications): combined FDMA/TDMA • most widely deployed • IS-95 CDMA: code division multiple access TDMA/FDMA CDMA-2000 EDGE GPRS UMTS Don’t drown in a bowl of alphabet soup: use this for reference only IS-95 IS-136 GSM

13. Cellular standards: brief survey 2.5 G systems: voice and data channels • for those who can’t wait for 3G service: 2G extensions • general packet radio service (GPRS) • evolved from GSM • data sent on multiple channels (if available) • enhanced data rates for global evolution (EDGE) • also evolved from GSM, using enhanced modulation • data rates up to 384K • CDMA-2000 (phase 1) • data rates up to 144K • evolved from IS-95

14. Cellular standards: brief survey 3G systems: voice/data • Universal Mobile Telecommunications Service (UMTS) • data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 Mbps • CDMA-2000: CDMA in TDMA slots • data service: 1xEvlution Data Optimized (1xEVDO) up to 14 Mbps ….. more (and more interesting) cellular topics due to mobility (stay tuned for details)

16. Characteristics of selected wireless link standards 200 802.11n 54 802.11a,g 802.11a,g point-to-point data 5-11 802.11b 802.16 (WiMAX) 3G cellular enhanced 4 UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO Data rate (Mbps) 1 802.15 .384 3G UMTS/WCDMA, CDMA2000 2G .056 IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km

17. Cellular Concept • Given a propagation environment, increasing transmitted power will increase the service coverage area. • The coverage area can be controlled by using a proper transmitted power level. • In cellular systems, the total service area is divided into a number of smaller areas, each of which is a radio cell. • Advantages: • Low transmitted power • Frequency reuse possible. • Regular polygons may be used to represent the cell coverage.

18. Cellular Concept • Hexagonal cells are popular because • closest to a circle • tight cellular packing • perfect partitioning of the service area. • Frequency reuse is limited by co-channel interference. Cells which use the same frequency channels are called co-channel cells. • Frequency is reused from cell cluster to cell cluster. No frequency channel is reused among cells in the same cell cluster. • Cells in each cell cluster use unique frequency channels.

19. Multiple Access Techniques • Radio cell: a geographical coverage area in which the services of mobile stations (MSs) are supported by a single base station (BS) • Forward link (downlink): BS → multiples MSs (one to many broadcasting) • Reverse link (uplink): MSs →BS (many to one multiple access)

20. Multiple Access Techniques • Multiple MSs want to access the common BS simultaneously • If two or more user signals arrive at the BS at the same time, there will be interferences, unless the signals are orthogonal • How can we achieve the orthogonality?

21. FDMA • The total bandwidth is divided into nonoverlapping frequency bands (channels) • Each user occupies a channel for the duration of the connection • waste of resources • Narrowband transmission • Forward and reverse links use FDD

22. TDMA • Time is partitioned into frames • Each frame consists of Nslot data slots plus a header and a trailer • Each slot is for transmission of one information unit • A user continues to use the same slot in every frame during call connection • waste of resources • TDMA systems require strict time synchronization.

23. TDMA

24. TDMA • W-TDMA: Each user occupies the total frequency bandwidth during its slots • N-TDMA: The total frequency spectrum is divided into frequency subbands (channels); within each frequency channel, TDMA is used. −→Both time and frequency are partitioned.

25. Code Division Multiple Access (CDMA) • used in several wireless broadcast channels (cellular, satellite, etc) standards • unique “code” assigned to each user; i.e., code set partitioning • all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data • encoded signal = (original data) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

26. CDMA

27. d0 = 1 1 1 1 1 1 1 d1 = -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M Di = SZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA Encode/Decode channel output Zi,m Zi,m= di.cm data bits sender slot 0 channel output slot 1 channel output code slot 1 slot 0 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0

28. CDMA: two-sender interference