Microsoft PowerPoint - ch4-dayem.ppt

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1 Wireless Transmission Fundamentals (Dayem s book, Chapter 4) (Nico s book, Chapter 2) Electromagnetic Spectrum Wireless Propagation Models Digital Modulation Techniques Multiple Access Performance Issues Cellular and Ad Hoc Concepts Link Budget Analysis tseng:1

2 Electromagnetic Waves predicted by British physicist James Maxwell in 1865, and observed by German physicist Heinrich Hertz in 1887 These waves are created by the movement of electrons and have the ability to propagate through space. using appropriate antennas, transmission and reception of electromagnetic waves through space becomes feasible. the speed of electron vibration determines the wave s frequency. Hertz: how many times the wave is repeated in 1 sec. (to honor Heinrich Hertz) tseng:2

3 Wavelength and Amplitude l = wavelength, f = frequency, c = speed of light tseng:3

4 Electromagnetic Spectrum spectrum: range of electromagnetic radiation band: spectrum parts tseng:4

5 Radio Waves HF band enables worldwide transmission: HF signals are reflected off the ionosphere and thus can travel very large distances tseng:5

6 Microwaves small wavelengths compared to radio waves easily attenuated by objects tseng:6

7 Infrared emitted by very hot objects such as human body (night vision applications) frequency depends on the temperature of the emitting body line-of-sight, point-to-point of no use outdoors (interfered by heat of sun) short-rang: 10 meters IrDA: Infrared Data Association tseng:7

8 Microwave and Infrared Bands Most wireless networking traffic is in the microwave frequency bands. some licensed, some unlicensed Infrared: for short-range wireless communication tseng:8

9 Spectrum Regulation ITU = Int l Telecommunications Union a worldwide spectrum regulation org. the world is split into 3 parts: American continent Europe, Africa, and former Soviet union rest of Asia and Oceania Rules of assigning spectrum lottery auction comparative bidding such as pricing, technology, etc. tseng:9

10 Licensed Microwave Band Examples: cellular, paging, PCS Use of a license is typically in an order of 10 years. A company can t have the license and not use it. Bandwidth is regarded as a resource that the public wants and needs. tseng:10

11 Unlicensed Microwave Band Also on the same microwave band, but no license required. To avoid interfering primary (licensed) users, spreading spectrum is required. Two types: FHSS: Frequency-hopping spread spectrum DSSS: Direct sequence spread spectrum Also known as ISM band. industrial, scientific, and medical tseng:11

12 Model of Wireless Propagation Free space path loss Doppler shift Slow/fast fading Error modeling tseng:12

13 Shannon s Formula an upper bound on the bit rate W of any channel of bandwidth H Hz: W = H log 2 (1 + S/N) S/N = signal to thermal noise ratio However, in real world, the upper bound is difficult to achieve due to: free space path loss proportional to r -2, where r is the distance between transmitter and receiver (sometimes at higher exponent) Doppler shift a signal transmitter and receiver are moving relative to one another slow/fast fading tseng:13

14 Slow Fading tseng:14

15 Definitions Reflection: when an electromagnetic wave falls on an object with dimension very large compared to the wave s wavelength Scattering: when obstructed by objects with dimensions in the order of the wavelength Diffraction (or shadowing): when the wave falls on an impenetrable object in which case, the secondary waves are formed behind the obstructing body tseng:15

16 Fast Fading: Multipath Effect waves traveling along different paths may be completely out of phase when they reach the antenna (thereby canceling each other) tseng:16

17 Multipath propagation delay can degrade performance in indoor/outdoor environment. When the path length differences are short, the effect is smaller. multipath fading is also referred as fast fading When LOS (line of sight) exists, this kind of fading is known as Ricean Fading When LOS does not exist, this kind of fading is known as Rayleigh Fading tseng:17

Propagation Models We say that the relative strength of signal x, P(x), to that of signal y, P(y), is D db, if D = 10 log 10 (P(x)/P(y)) In free space, the average path loss (PL) at a distance of r

18 Propagation Models We say that the relative strength of signal x, P(x), to that of signal y, P(y), is D db, if D = 10 log 10 (P(x)/P(y)) In free space, the average path loss (PL) at a distance of r is (in db): PL(r) = PL(r 0 ) + 10n log(r/r 0 ) r 0 = reference distance (typically 1 Km for macrocells; and 100 m for microcells) n = environmental factor (typically >= 2) To take into account of the shadowing effect PL(r) = PL(r 0 ) + 10n log(r/r 0 ) + X δ X δ = zero-mean Gaussian random variable with standard deviation δ tseng:18

19 Digital Modulation Techniques Binary Modulation Phase Shift Keying Minimum Shift Keying π/4-shifted QPSK tseng:19

20 Basics Convert digital stream into the analog signal A(t)cos(wt + θ), where w = 2πf. The characteristics in this formulation that may be changed are: amplitude frequency phase Ex: ASK = amplitude shift keying; FSK = frequency shift keying; PSK = phase shift keying tseng:20

21 Most systems modulate the information onto a carrier centered in a (small) allocated spectrum. tseng:21

22 tseng:22

23 Binary Modulation Scheme Amplitude Shift Keying (ASK): using ON/OFF to represent 1/0 keying : like a telegraph key Frequency Shift Keying (FSK): 1/0 represented by two different frequencies separated by some distance tseng:23

24 Binary Phase Shift Keying Binary Phase Shift Keying (BPSK) use alternative sine wave phases to encode bits simple to implement very robust, used extensively in satellite communications tseng:24

25 Quarternary Phase Shift Keying QPSK: multi-level modulation: 2 bits per symbol more spectrally efficient, more complex receiver tseng:25

26 Differential PSK (DPSK) 1 = changing the phase relative to the previous symbol by some amount 0 = having the same phase as the previous symbol adv: self-clocked tseng:26

27 coding by bit pairs varying the phase of the current bit pair to the phase of the previous bit pair by a multiple of π/4 example: π/4-shifted QPSK (Fig. 2.27) (i.e., -π/4, -π/4, +5π/4) tseng:27

28 Hybrid of PSK + ASK QAM = Quadrate Amplitude Modulation mixture of PSK and ASK 3 bits at a time tseng:28

29 Multiple Access defining how nodes in a wireless network to share a common medium tseng:29

30 Objectives MAC layer is to define how a user access a channel when he needs one. Random access: ALOHA and CSMA Ordered access: Token bus and Token Ring Deterministic access: FDMA, TDMA, and CDMA Combinations: TDMA-over-FDMA, TDD- CDMA, and TDMA/CSMA tseng:30

31 FDMA frequency division multiple access ** NMT = nordic Mobile Telephony tseng:31

32 TDMA time division multiple access tseng:32

33 CDMA code division multiple access each station has a station code each bit is encoded by station code code 1 is mapped to 1 code 0 is mapped to -1 tseng:33

34 ALOHA A type of packet-radio network. The first well-known wireless network as well as network system. Very simple, but not efficient! Variations: pure-aloha: whenever desired, send the packet slotted-aloha: further divide time axis into slots tseng:34

35 CSMA Before sending, sense the carrier. tseng:35

36 Persistent and Non-persistent CSMA Persistent CSMA: when the medium is busy, a station can persistently wait for the medium to become idle, and then transmit with a probability p This is called 1-persistent or p-persistent CSMA. Non-persistent CSMA: A station can stop monitoring the wireless medium, and listen to the medium again at predefined time. tseng:36

37 Hidden-Node Problem CSMA has the following problem: when two nodes are too far away, carrier sensing is difficult tseng:37

38 CSMA/CA CA = collision avoidance sender first does carrier sense sender broadcasts RTS (request to send) to receiver receiver broadcasts CTS (clear to send) to sender then send data packet Q: Is CSMA/CD possible in wireless network? tseng:38

39 tseng:39

40 Ordered MAC Techniques Can a token-ring or token-bus protocol be applied to a wireless network? Problems: mobility (nodes joining or leaving the ring) token loss tseng:40

41 Comparison and Summary Random access: CSMA under light load: fast response time under heavy load: throughput declines simplicity Deterministic protocols: TDMA, FDMA guaranteed bandwidth larger average delay small delay variance Hybrid: CSMA/TDMA adaptive, higher overhead tseng:41

42 Spread Spectrum FHSS DSSS tseng:42

43 Spread Spectrum Technology Spread spectrum must be used in ISM band. Two major technologies: Frequency Hopping SS (FHSS) Direct Sequence SS (DSSS) Located at the PHY of the network stack: tseng:43

44 FHSS Most Wireless LANs use the ISM bands as secondary users. They must use SS in order not to interfere with the primary users. FHSS: send info in different frequencies on different time slots. Hopping Pattern In each time slot, the occupied frequencies are separated by some distance to avoid interference. tseng:44

45 FHSS is different from FDM (frequency division multiplexing). Example: (Fig. 4.7) In the 2.4 GHz band of ISM, we have a space of 80 MHz. (2400~2483MHz) A typical bandwidth of the information signal is 1 MHz. Maximum occupancy is 1MHz regulated by FCC. One time slot = 0.1 sec. tseng:45

46 Primary vs. Secondary Users In FHSS, a typical power limit is 1 watt. For primary users, the power limit is much larger. So the interference from FHSS will not be noticeable primary users. For FHSS secondary user, when there is 1 primary user there will be a throughput loss of 1/80 = 1.25%; when there are 2 primary users there will be a throughput loss of 2/80 = 2.5%. fig 4.8 tseng:46

47 primary user primary user tseng:47

48 DSSS The input data stream is transferred to a chip stream that is x times higher by XOR. a chip is 0 or 1, but is called so to distinguish from a bit example: x = 11, 13, 15, 16 chips/bit tseng:48

49 The frequency spectrum is spread out and the spectral energy is x times lower. It s so low that primary users are not interfered. tseng:49

50 Comparison of Interference Degradation due to existence of interference: FHSS: linear to the level of interference DSSS: degraded by half after a certain point (since it typically occupies 50% of the bandwidth) won t work after a certain level tseng:50

51 tseng:51

52 tseng:52

53 Link Budget Analysis Tutorial on Basic Link Budget Analysis Application Note, June 1998, AN9804.1, Intersil Co. tseng:53

54 Communication Basics When evaluating a wireless link, there are 3 most important questions to be answered: How much radio frequency (RF) power is available? How much bandwidth is available? What is the required reliability? evaluated by BER (bit error rate) tseng:54

55 Link Budget Example 1 Wireless Link to Modem required rate: 40 Kbps (28.8 Kbps plus framing, overhead, checksum) range: 5 meters BER: 10-6 tseng:55

56 Choices of Technology: 900 MHz 2.4GHz and 5GHz are not selected since the required rate is low. no spread spectrum since low transmission power is sufficient for 5 meters Orthogonal FSK simplicity: two separated frequencies (one for 1 and the other for 0 ) separated by 40 khz (called orthogonal since frequency-separation/bit-rate = 1) tseng:56

57 Wireless USB Link Budget Example 2 required data rate = 2 Mbps (1.408 Mbps plus framing, overhead, and checksum) range = 30 meters BER = 10-6 tseng:57

58 Selection of Technologies: ISM band in 2.4 GHz (with 83MHz of band to use) DSSS spreading to support long distance transmission will occupy 2 x 11 = 22 MHz of bandwidth due to spreading DQPSK (differential quadrature phase shift keyed) modulation tseng:58

59 Performance Increasing Techniques for Wireless Networks antenna diversity coding power control tseng:59

60 definition: Diversity to send multiple copies of the same information signal through several channels goal: to combat fading in wireless channels example: time, frequency, antenna tseng:60

61 Antenna Diversity also known as space diversity method a set of array elements (also referred to as branches), spaced sufficiently apart from each other usually 2 elements can combat multipath fading because multipath fading is usually independent at distances in the order of channel s wavelength tseng:61

62 Example a 2-branch diversity system a number of algorithms have been proposed to reconstruct the original transmission ex: pick the strongest signal from one of the antennas tseng:62

63 Smart Antennas multi-antennas that change in order to adapt to the conditions of wireless channels can focus toward the receivers can focus to the transmitters also known as beamforming Already available for several years not widely used due to costs tseng:63

64 Coding Parity check Hamming code Cyclic redundancy check (CRC) tseng:64

65 Power Control properly tuning the transmission power to reduce coverage and interference tseng:65

66 Summary What have we discussed? Electromagnetic Spectrum Wireless Propagation Models Digital Modulation Techniques Multiple Access Performance Issues Cellular and Ad Hoc Concepts Link Budget Analysis Performance Improvement Techniques tseng:66

WLAN 2

WLAN 2 1 WLAN 2 IEEE 802.11 HomeRF GSM/ GPRS CDMA 3 ( ) (Infrared) (Laser) (Microwave) (DSSS) (FHSS) (HomeRF) (Bluetooth) 4 ( ) IrDA (Direct-Beam IR, DB/ IR) (Diffuse IR, DF/ IR) (Ominidirectional IR, Omini/