HY539: Spring 2005 Wireless networks and mobile computing

1. HY539: Spring 2005 Wireless networks and mobile computing Lecture2: Radio Channel Issues Prof. Maria Papadopouli Assistant Professor Department of Computer Science University of North Carolina at Chapel Hill

2. Review of Last Lecture • Heterogeneous networks of devices with different capabilities • Pervasive computing spaces • Mobile Computing Challenges • Battery capacity, Energy and Bandwidth constraints • Mobility • Intermittent connectivity • Delays • Packet losses • Wireless networks more vulnerable than the wired ones • Wireless infrastructures cannot support QoS for applications with real-time constraints

3. Support Intelligent Mobile Clients • Efficient position sensing mechanisms • Adaptive systems Monitor the environment and adapt based on their resources (battery, application requirements, channel capacity, throughput) in an energy-efficient manner • Intelligent and robust wireless infrastructures

4. Roadmap High-level introduction to • Mobile data access • Physical layer

5. Mobile Data Access • Infrastructure • Client-Server paradigm • Ad Hoc (without infrastructure) • Peer-to-Peer paradigm

6. Fundamentals • Wireless channel model • Antenna • Impairments • Radio Propagation • Digital modulation and detection techniques • Error control techniques

7. Baseband Modulation Carrier Radio Channel Data In Transmitter Digital Radio Communications Conversion of a stream of bits into signal Carrier Detection Decision Bit &Frame Sync Data Out Receiver Conversion of the signal to a stream of bits

8. Adds redundancy to protect the digital information from noise and interference Bits mapped to signal (analog signal waveform) e.g., GFSK e.g., TDMA, CDMA

9. Transmitter & Radio Channel Transmitter Receiver Multi-path Fading Receiver Transmitter + Noise

10. Antenna • Made of conducting material • Radio waves hitting an antenna cause electrons to flow in the conductor and create current • Likewise, applying a current to an antenna creates an electric field around the antenna. As the current of the antenna changes, so does the electric field. A changing electric field causes a magnetic field, and the wave is off …

11. Antenna (cont’d) • The gain is the extent to which it enhances the signal in its preferred direction • Measured in dBi, decibels relative to an isotropic radiator • Isotropic antenna: radiates power with unit gain uniformly in all directions

12. Assignment (for your log) • Different types of antennas • Multiple & directional antennas • State of art • Cost

13. Channel Coding • Often used to protect the digital information from noise and interference and reduce the number of bit errors • Accomplished by selectively introducing redundant bits into the transmitted information stream • These additional bits allow detection and correction of bit errors in the received data stream

14. Types of Impairments • Noise (thermal, human) • Radio frequency signal path loss • Fading at low rates • Inter-Symbol interference (ISI) • Shadow fading • Co-channel interference • Adjacent channel interference

15. Impairments • Impacts radio system design • Impact on indoor and outdoor communications • Difficult to control

16. Adjacent Channel Interference • Interference from signals adjacent in frequency to the desired signal • Results from imperfect receiver filters which allow nearby frequencies to leak into the passband • Prevented by keeping the frequency separation between each channel in a given cell as large as possible

17. Inter-Symbol Interference (ISI) • Overflowing symbols

18. ISI (cont’d) • Waves that take different paths from the transmitter to the receiver will travel different distances and be delayed with respect to each other • Waves are combined by superposition, but the effect is that the total waveform is garbled • Delay spread: time between the arrival of the first wavefront and the last multipath echo • Longer delay spreads require more conservative coding • 802.11b networks can handle delay spreads of up to 500 ns, but performance is much better when the delay spread is lower • When delay spread is large, many cards reduce transmission rate

19. Limits of wireless channel • How many bits of informationcan be transmitted without error per sec over a channel with a bandwidth B, when the average signal power is limited to P watt, and the signal is exposed to an additive, white (uncorrelated) noise of power N with Gaussian probability distribution Shannon (1916-2001) Norbert Wiener (1894-1964)

20. Shannon’s limit • For a channel without shadowing, fading, or ISI, the maximum possible data rate on a given channel of bandwidth B is R=Blog2(1+SNR) bps, where SNR is the received signal to noise ratio • Shannon’s is a theoretical limit that cannot be achieved in practice but design techniques improve data rates to approach this bound

21. Signal-to-noise ratio (SNR) • The ratio between the magnitude of background noise and the magnitude of un-distorted signal (meaningful information) on a channel • Higher SNR is better (i.e., cleaner) • It determines how much information each symbol can represent

22. Propagation models • One of the most difficult part of the radio channel design • Done in statistical fashion based on measurements made specifically for an intended communication system or spectrum allocation • Predicting the average signal strength at a given distance from the transmitter • Large-scale propagation model: signal strength over large T-R separation distances • Small-scale or fading model: rapid fluctuations of the received signal strength over very short travel distances or short time durations (order of seconds)

23. Our measurements at UNC

24. Free-space propagation model • Used to predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between them Examples: satellite, and microwave line-of-sight radio links Derived from first principles - power flux density computation • Any radiating structure produces electric and magnetic fields: its current flows through such antenna and launches electric and magnetic fields • The electrostatic and inductive fields decay much faster with distance than the radiation field • At regions far way from the transmitter, the electrostatic and inductive fields become negligible and only the radiated field components need be considered

25. Free Space Model Pr(d)=PtGtGrl2/[(4p)2d2L] Pt,Pr: transmitter/receiver power Gt, Gr: transmitter/receiver antenna gain G = 4pAe/l2 L: system loss factor (L=1 no loss) Ae: related to the physical size of the antenna l: wavelength in meters, f carrier frequency, c :speed of light • l = c/f

26. Path Loss • Difference (in dB) between the effective transmitted power and the received power

27. Radio wave propagation • Electromagnetic wave propagation mechanisms are diverse • Due to reflection, diffraction, scattering

28. Propagation Mechanisms (cont’d) • Reflection: when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave Reflections occur from the surface of the earth, buildings, and walls • Diffraction: when the radio path between transmitter and receiver is obstructed by a surface that has sharp irregularities (edges) Secondary wavelets into a shadowed region • Scattering: when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength and where the number of obstacles per unit volume is large (e.g., street signs, lamp posts) Reflected energy is spread out/diffused in all directions

29. Multipath Propagation Wall Scattering Receiver Transmitter Cabinet Diffraction (Shadow Fading) Reflection Wall

30. Mobile radio channel • A single direct path between the base station and the mobile is seldom the only physical means for propagation Hence, the free space propagation model is inaccurate when used alone • Two-ray ground reflection model considers both the direct path and a ground reflected propagation path between transmitter and receiver • Reasonably accurate for predicting the large-scale signal strength over distances of several km for mobile radio systems that use tall tower (heights which exceed 40m) or for line-of-sight micro-cell channels in urban environment

31. Two-ray ground reflection model T (transmitter) Pr(d)=PtGtGrhr2ht2/d4 R (receiver) ht hr d

32. Wave combination by superposition When multiple waves converge on a point, the total wave is simply the sum of any component waves

33. Modulation • The process of taking information from a message source (baseband) in a suitable manner for transmission • It involves translating the baseband signal onto a radio carrier at frequencies that are very high compared to the baseband frequency

34. Demodulation • The process of extracting the baseband from the carrier so that it may be processed and interpreted by the receiver (e.g., symbols detected and extracted)

35. Why not modulate the baseband • We must consider the fact that for effective signal radiation the length of the antenna must be proportional to the transmitted wave length • For example, voice range 300-3300Hz At 3kHz at 3kbps would imply an antenna of 100Km! By modulating the baseband on a 3GHz carrier the antenna would be 10cm • To ensure the orderly coexistence of multiple signals in a given spectral band • To help reduce interference among users • For regulatory reasons

36. Modulations Techniques • Carrier wave s : s(t)=A(t)*cos[(t)] A(t) time varying amplitude Time varying angle (t) (t)= + (t) phase (t), : radian frequency

37. Ideal Digital Modulation • Provides low bit error rates at low received signal-to-noise ratio • Performs well in multi-path and fading conditions • Occupies a minimum of bandwidth • Is easy and cost-effective to implement Existing modulation schemes do not simultaneously satisfy all of these requirements

38. Performance of Modulation Schemes • Tradeoff between fidelity and signal power To increase noise immunity, it is necessary to increase the signal power • Power efficiency: the amount by which the signal power should be increased to obtain a certain level of fidelity (ie acceptable bit error probability) depends on the particular type of modulation • Bandwidth efficiency: the ability to accommodate data within a limited bandwidth

39. Modulation Examples: Frequency Hopping Timing the hops accurately is the challenge Frequency slot 5 User A 4 User B 3 2 1 0 Time slot

40. Reading material on 802.11 • 802.11 Wireless Networks, The definitive guide. Matthew S. Gast, O'Reilly, 2002, ISBN 0-596-00183-5. http://www.csd.uoc.gr/~maria/802.11.book.pdf • Papers: http://sss-mag.com/pdf/802_11tut.pdf http://sss-mag.com/pdf/80211p.pdf • Theoretical paper on its performance: http://www.ece.utexas.edu/~jandrews/ee381k/EE381KTA/802.11_throughput.pdf