Introduction

1. Introduction Lecture # 2

2. Overview • Fundamentals of wireless communication technology • The electromagnetic spectrum • Radio propagation mechanisms • Characteristics of the wireless channel • Modulation techniques • Multiple access techniques • Error control • Computer networks • Computer network software • IEEE 802 networking standard • Wireless networks

3. Reference Material • Ad hoc Wireless Networks, - Ch #1 Architectures and Protocols By C. Siva Ram Murthy • Wireless Communication - Ch #5 and Networks By William Stallings

4. Multipath Propagation • Reflection - Occurs when signal encounters a surface that is large relative to the wavelength of the signal • Diffraction - Occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave • Scattering – Occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

5. Multipath Propagation

6. The Effects of Multipath Propagation • Multiple copies of a signal may arrive at different phases • If phases add destructively, the signal level relative to noise declines, making detection more difficult • Inter-symbol interference (ISI) • One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

7. Fading • Fluctuation in signal strength when received at the receiver • Four types • Fast fading Or small-scale fading • Slow fading Or large-scale fading • Flat fading • Selective fading • In fixed environment, fading is affected by changes in atmospheric (environmental) conditions, such as rainfall or electric noise • In a mobile environment, effect of fading is more complex • Effects • Add signals constructively or destructively at the receiver, causing a variation in the power level of the received signal • Introduce bit error rate and packet error rate

8. Fast Fading • Rapid fluctuations in the amplitude, phase, or multipath delays of the received signal • Reasons • Due to the interference between multiple version (copies) of the same transmitted signal arriving at the receiver at slightly different times • Delayed Spread • Time between the reception of the first version of the signal and the last echoed signal is called delayed spread • Reasons • Occurs because of the three mechanism, reflection, diffraction, and scattering • Error control coding, interleaving, frequency hopping and diversity is used to mitigate the effect of fading

9. Slow Fading or Shadow Fading • Cause the received signal power to vary, though the distance between the transmitter and receiver remains the same • Occurs when objects that partially absorb the transmission lie between the transmitter and receiver • It is called Slow Fading because the variations are much slower as compare to other fading phenomena • Also referred to as Shadow Fading since the objects that cause the fade, (buildings or other structures), block the direct transmission path from the transmitter to the receiver • A fade margin is added as an additional signal power to overcome the problem

10. Flat Fading • All frequency components of the received signal fluctuate in the same proportions simultaneously

11. Selective Fading • Affects unequally the different spectral components of a radio signal

12. Counter Measures Against Fading • Diversity • Principle - Independent paths between the same transmitter and receiver nodes experiences independent fading effects • Solution • Providing multiple logical channel between the transmitter and receiver, and sending parts of the signal over each channel, the error effects due to fading can be compensated • Independent path can be distinct in space, time, frequency

13. Diversity Types • Time diversity • Spreads the data over time so that the effects of burst errors are minimized • Frequency diversity • Spreads the transmissions over a wider frequency spectrum, or use multiple carriers for transmitting the information • Space diversity • Use of different physical transmission paths with the help of an antenna array • Each antenna element receives an independent fading signal, • Received signals are combined in some fashion such that the most likely transmitted signal could be reconstructed

14. Counter Measures Against Fading • Adaptive modulation techniques • Channel characteristics are estimated at the receiver and sent to the transmitter through a feedback channel • Transmitter adapts its transmission based on the received channel estimates in order to counter the errors that could occur due to the characteristics of the channel • More complex to implement

15. Interference • Adjacent channel interference • Reason • Signals in nearby frequencies have components outside their allocated ranges, these components may interferes with on-going transmissions in the adjacent frequencies • Avoidance • Introduce guard bands between the allocated frequency ranges

16. Interference • Co-channel interference/narrow band interference • Reason • Due to other nearby systems using the same transmission frequency • Avoidance • Can be minimized with the use of multiuser detection mechanisms, directional antennas, and dynamic channel allocation methods

17. Interference • Inter-symbol interference • Reason • Caused by the temporal spreading and the consequent overlapping of individual pulses in the signal • Individual pulses (delay spread) goes above a certain limit (symbol detection time), the receiver becomes unable to reliably distinguish between changes of state in the signal

18. Interference • Avoidance • Adaptive equalization is commonly used technique against inter symbol interference • Gathers the dispersed symbol energy into its original time interval • Estimates the channel pulse response to periodically transmitted well-known bit pattern, knowing as a training sequences • This enables a receiver to determine the time dispersion of the channel and compensate accordingly

19. Doppler Shift • Change/shift in the frequency of the received signal when the transmitter and receiver are mobile with respect to each other • If transmitter and receiver are moving towards each other, then the frequency of the received signal will be higher that that of the transmitted signal • If transmitter and receiver are moving from each other, the frequency of the signal at the receiver will be lower than that at the transmitter fd = v/λ fd – Doppler shift frequency v – relative velocity between the transmitter and receiver λ – wavelength of the signal

20. Transmission Rate Constraints • Two important constraints that determines the maximum rate of data transmission on a channel are • Nyquist’s theorem • Shannon’s Theorem • Signaling speed of a transmitted signal • Number of times per second the signal changes its value/voltage • Number of changes per second is measured in terms of baud • Baud and bit rate may or may not be same

21. Bit Rate & Baud Rate • Bit rate • Number of bits transmitted during one second • Baud rate • Number of signal units per second that are required to represent those bits • Baud rate is less than or equal to the bit rate • Baud rate is analogous to a car, and a bit is analogous to a passenger Baud rate = bit rate / number of bits per signal unit

22. Bit Rate & Baud Rate

23. Nyquist’s Theorem • Gives the maximum data rate possible on a channel C = 2 x B x log2 L bits/sec C – maximum channel capacity B – bandwidth of the channel in Hz L – number if discrete signal levels/voltage values used • Valid only for a noiseless channel

24. Shannon’s Theorem • Noise level is the channel is represented by the SNR (ratio of signal power (S) to noise power (N)) SNR = 10 log10 (S/N) decibels • Maximum data rate possible in a noisy channel is C = B x log2 (1 + (S/N)) bits/sec C – maximum data rate in noisy channel B – bandwidth of the channel in Hz S – signal power N – noise power

25. Modulation • Modulation is the process where a Radio Frequency or Light Wave's amplitude, frequency, or phase is changed in order to transmit intelligence • Radio communication superimposes this information bearing signal onto a carrier signal • These high frequency carrier signals can be transmitted over the air easily and are capable of traveling long distances • The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal • In the field of communication engineering, the information bearing signal is also known as the modulating signal • The modulating signal is a slowly varying signal - as opposed to the rapidly varying carrier frequency

26. Modulation Techniques • Data (analogue & digital) has to be converted into electromagnetic waves for transmission over a wireless channel • Analogue modulation • Digital Modulation • Modulation process alters certain properties of a radio wave, called carrier wave, whose frequency is the same as the frequency of the wireless channel being used for transmission

27. Analogue Modulation • Used for transmitting analogue data • Analogue data signal is superimposed on a carrier signal • Aimed at altering certain properties • Amplitude • Angle Modulation • Frequency • Phase

28. Amplitude Modulation • Carrier signal is modulated so that its amplitude varies with changing amplitude of the modulating signal • Simple • Creates additional unwanted signals (side bands) in the frequencies on either side of the of the carrier

29. Frequency Modulation • Frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal • Complex • More resistant to noise

30. Phase Modulation • Phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal

31. Digital Modulation • Used for transmitting digital signals (sequence of 0 and 1 bits) • Changes occurs in continuous manner in analogue modulation whereas, they occur at discrete time intervals in digital modulation • The number of such changes per second is known as baud rate • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • Phase shift keying (PSK)

32. Amplitude Shift Keying (ASK) • Strength of the carrier signal is varied to represent binary 1 or 0 • Peak amplitude of the signal during each bit duration is constant, and its value depends on the bit (0 or 1) • Highly susceptible to noise interference

33. Frequency Shift Keying (FSK) • Frequency of the carrier signal is varied to represent binary 1 or 0 • Frequency of the signal during each bit duration is constant, and its value depends on the bit (0 or 1) • Avoids most of the problem of noise

34. Phase Shift Keying (PSK) • Phase of the carrier is varied to represent binary 1 or 0 • Phase of the signal during each bit duration is constant, and its value depends on the bit (0 or 1)

35. Multiplexing • Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared • Multiplexing, is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link

36. Categories of Multiplexing

37. Multiple Access Techniques • Transmission medium in wireless networks is broadcast in nature, a node can not transmit on the channel whenever it wants to • Multiple access techniques are used to control access to the shared channel • Frequency division multiplexing (FDM) • Time division multiplexing (TDM) • Code division multiplexing (CDM) • Space division multiplexing (SDM) • Also called channelization

38. Frequency Division Multiplexing (FDM) • Available bandwidth is divided into multiple frequency channels/bands • A transmitter-receiver pair uses a single dedicated frequency channel for communication • Frequency bands are separated from each other by guard frequency bands in order to eliminate inter-channel interference • Results in underutilization of frequency spectrum

39. Frequency Division Multiplexing (FDM) & De-multiplexing

40. Frequency Division Duplex (FDD) • Two different frequencies are used for communication between a pair of stations • In cellular network, base station (BS) dynamically allocates a pair of different carrier frequency to each mobile station (MS) • One for the traffic from the MS to BS (uplink frequency) • Other for carrying traffic from BS to the MS (downlink frequency) • Downlink frequency is always higher than the uplink frequency • High frequency transmission suffers greater attenuation as compared to low frequency transmissions, high transmission power is required for high frequency channels for compensating the transmission losses • Power available at MS is limited, hence, in order to conserve energy at MS the uplink frequency is always lower than the downlink frequency

41. Orthogonal Frequency Division Multiplexing (OFDM) • Multi-carrier transmission mechanisms • Data signal is split into multiple smaller sub-signals that are then transmitted simultaneously at different carrier frequencies (sub-carriers) • Sub-carriers are made orthogonal to each other, though spectral overlapping among carriers occurs, but the orthogonality ensure error free reception at the receiver, thereby, providing better spectral efficiency • Reduces the signal distortion at the receiver caused due to multipath propagation of the transmitted signal • Also referred to as Discrete Multi-tone modulation (DMT) • Used in WLANs

42. Time Division Multiple Access (TDMA) • Each frequency band is divided into periodically repeating time slots (channels) • A set of such time slots is known as TDMA frame • Each node is assigned one or more time slots in each frame, and the node transmits only in those slots • Perfect synchronization is required between sender and the receiver • To prevent synchronization errors and inter-symbol interference due to signal propagation time differences, guard intervals are used between time slots • Result in significance overhead for the system • Widely used in 2G cellular systems such as GSM

43. Time Division Multiplexing (TDM) • Time division duplex – TDMA (TDD-TDMA) • For two-way communication, the uplink and downlink time slots, used for transmitting and receiving data, are on same frequency band (TDMA frame) • Frequency division duplex – TDMA (FDD-TDMA) • For two-way communication, the uplink and downlink time slots, used for transmitting and receiving data, are on different frequency band (TDMA frame)

44. Spread Spectrum • Multiplexing combines signals from several sources to achieve BW efficiency • SS also combines signals from different sources to fit into a larger BW but with a different perspective • Stations in wireless environment must be able to share medium (air) without interception or without being subject to jamming • To achieve these goals, SS adds redundancy • Spread the original spectrum needed for each station, making interception and jamming difficult • Resistance to multipath interference is also provided

45. Code Division Multiple Access (CDMA) • Does not divide frequency or time to among user • Instead ever channel uses the entire spectrum • Spreading is done independent of the original signal • Based on the coding theory • Each station is assigned a code, which is a sequence of numbers called chips • Frequency hopping spread spectrum (FHSS) • Direct sequence spread spectrum (DSSS)

46. Frequency Hopping Spread Spectrum (FHSS) • Transmission switches across multiple frequencies in a pseudo-random manner • Sequence of transmission frequencies in known both at transmitter and receiver only

47. Direct Sequence Spread Spectrum (DSSS) • Each bit send by the sender is replaced by a sequence of bits called a spreading code (chip code) • Each node transmits using these codes • At the receiver, the transmission is received and information is extracted using the transmitter’s code • For transmitting a binary 1, the sender transmits its code; for a binary 0, the one’s complement of the code is transmitted • Provides • Privacy, if the intruder does not know the code • Immunity against interference if each station uses a different code

48. Direct Sequence Spread Spectrum (DSSS)

49. Space Division Multiple Access (SDMA) • Entire circular region around transmitter is divided using directional transmitters/antennas • Thus different areas/regions can be served using the same frequency channel • Best suited to satellite systems

50. Error Control • Bit error rate (BER) is very high in wireless • Unreliable nature of the wireless channel makes error control even more important in the wireless network context • Several coding schemes are available, which try to provide resistance against errors by adding redundant bits to be transmitted • Channel coding • These redundant bits help the receiver to either • Detect errors and request for retransmission (error detection) • OR to identify and correct the faulty bits (error correction)