Wireless Radio Communications

1. IT351: Mobile & Wireless Computing Wireless Radio Communications Objectives: • To study the wireless radio communication medium, spectrum and signals. • To study antennas and their role in wireless communications. • To study the process of wireless signal propagation. • To introduce basic issues in signal processing and signal modulation. • To study signal modulation techniques including ASK, FSK and PSK. • To detail the concept of spread spectrum and study its techniques; FHSS, DSSS. • To study issues in radio resource management and detail the cellular concept of channel allocation.

2. Outline • The radio spectrum • Signals • Antennas • Signal propagation problems • Multiplexing • Modulation • Spread spectrum • Radio Management

3. Wireless communications • The physical media – Radio Spectrum • There is one finite range of frequencies over which radio waves can exist – this is the Radio Spectrum • Spectrum is divided into bands for use in different systems, so Wi-Fi uses a different band to GSM, etc. • Spectrum is (mostly) regulated to ensure fair access

4. Frequencies for communication • VLF = Very Low Frequency UHF = Ultra High Frequency (DAB, dig-TV, mobile phone, GSM) • LF = Low Frequency (submarine) SHF = Super High Frequency (satellite) • MF = Medium Frequency (radio AM) EHF = Extremely High Frequency (direct link) • HF = High Frequency (radio FM & SW) UV = Ultraviolet Light • VHF = Very High Frequency (analog TV broadcast) • Frequency and wave length •  = c/f • wave length , speed of light c  3x108m/s, frequency f twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV

5. Frequencies for mobile communication • VHF-/UHF-ranges for mobile radio • simple, small antenna for cars • deterministic propagation characteristics, reliable connections • SHF and higher for directed radio links, satellite communication • small antenna, beam forming • large bandwidth available • Wireless LANs use frequencies in UHF to SHF range • some systems planned up to EHF • limitations due to absorption by water and oxygen molecules (resonance frequencies) • weather dependent fading, signal loss caused by heavy rainfall etc.

6. Frequencies and regulations • ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

7. Wireless communications • Signals • Physical representation of data is the signal • Signals are function of time and location • In wireless sinewaves are used as the basic signal: • Amplitude: strength of the signal • Frequency: no of waves generated per second • Phase shift: where the wave starts and stops • These factors are transformed into the exactly required signal by Fourier transforms (complicated equations that parameterise the sine wave)

8. Signals • Sine wave representation • signal parameters: parameters representing the value of data • signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  • sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t) Amplitude Phase Shift t Frequency

9. Fourier representation of periodic signals • It is easy to isolate/ separate signals with different frequencies using filters 1 1 0 0 t t ideal periodic signal real composition (based on harmonics)

10. Antennas • Sending and receiving signals is performed via antennas • Role: Radiation and reception of electromagnetic waves, coupling of wires to space and vice versa for radio transmission • Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna z y z ideal isotropic radiator y x x

11. Antennas • Real antennas do not produce radiate signals in equal power in all directions. They always have directive effects (vertically and/or horizontally) • Radiation pattern: measurement of radiation around an antenna • Most basic antenna is the dipole • Two antennas both of length /4 (/2 in total) • Small gap between the two antennas • Produces an omni-directional signal in one plane of the three dimensions Source: Wikipedia

12. Antennas • Omni-Directional Antennas are wasteful in areas where obstacles occur (e.g. valleys) • Directional antennas reshape the signal to point towards a target, e.g. an open street • Placing directional antennas together can be used to form cellular reuse patterns • Antennas arrays can be used to increase reliability (strongest one will be received) • Smart antennas use signal processing software to adapt to conditions – e.g. following a moving receiver (known as beam forming), these are some way off commercially

13. Antennas /4 /2 y y z simple dipole x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) y y z directed antenna x z x side view (xy-plane) side view (yz-plane) top view (xz-plane)

14. Signal propagation sender transmission distance • In perfect conditions (a vacuum) wireless signals will weaken predictably • Transmission range: receivers can understand enough of the signal (i.e. low error) for data • Detection range: receivers hear the signal but cannot recover the data (i.e. high error) • Interference range: there is a signal but it is indistinguishable from other noise • Wireless is less predictable since it has to travel in unpredictable substances – air, dust, rain, bricks detection interference

15. Signal propagation: Path loss (attenuation) • In free space signals propagate as light in a straight line (independently of their frequency). • If a straight line exists between a sender and a receiver it is called line-of-sight (LOS) • Receiving power proportional to 1/d² in vacuum (free space loss) – much more in real environments(d = distance between sender and receiver) • Situation becomes worse if there is any matter between sender and receiver especially for long distances • Atmosphere heavily influences satellite transmission • Mobile phone systems are influenced by weather condition as heavy rain which can absorb much of the radiated energy

16. Signal propagation • Radio waves can penetrate objects depending on frequency. The lower the frequency, the better the penetration • Low frequencies perform better in denser materials • High frequencies can get blocked by, e.g. Trees • Radio waves can exhibit three fundamental propagation behaviours depending on their frequencies: • Ground wave (<2 MHz): follow the earth surface and can propagate long distances – submarine communication • Sky wave (2-30 MHz): These short waves are reflected at the ionosphere. Waves can bounce back and forth between the earth surface and the ionosphere, travelling around the world – International broadcast and amateur radio • Line-of-sight (>30 MHz): These waves follow a straight line of sight – mobile phone systems, satellite systems

17. Additional signal propagation effects • Receiving power additionally influenced by • fading (frequency dependent): signals can change as the receiver moves • Blocking/ Shadowing: large objects may block signals (building,..etc) • Reflection: waves can bounce off dense objects • Refraction: waves can bend through objects depending on the density of a medium • scattering : small objects may reflect multiple weaker signals • diffraction at edges refraction shadowing reflection scattering diffraction

18. Multipath propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction,… • Different signals use different length paths • The difference is called delay spread • Systems must compensate for the delay spread • Interference with “neighbor” symbols, Inter Symbol Interference (ISI) • Symbols may cancel each other out • Increasing frequencies suffer worse ISI multipath pulses LOS pulses signal at sender signal at receiver

19. Effects of mobility • Channel characteristics change over time and location • signal paths change • different delay variations of different signal parts • different phases of signal parts •  quick changes in the power received (short term fading) • Additional changes in • distance to sender • obstacles further away •  slow changes in the averagepower received (long term fading) long term fading power t short term fading

20. Multiplexing • Multiplexing describes how several users can share a medium with minimum or no interference • It is concerned with sharing the frequency range amongst the users • Bands are split into channels • Four main ways of assigning channels • Space Division Multiplexing (SDM) : allocate according to location • Time Division Multiplexing (TDM): allocate according to units of time • Frequency Division Multiplexing (FDM): allocate according to the frequencies • Code Division Multiplexing (CDM) : allocate according to access codes • Guard Space: gaps between allocations

21. Multiplexing channels ki k1 k2 k3 k4 k5 k6 • Multiplexing in 4 dimensions • space (si) • time (t) • frequency (f) • code (c) • Goal: multiple use of a shared medium • Important: guard spaces needed! c t c s1 t s2 f f c t s3 f

22. Space Division Multiplexing (SDM) channels ki k1 k2 k3 k4 k5 k6 • Space Division • This is the basis of frequency reuse • Each physical space is assigned channels • Spaces that don’t overlap can have the same channels assigned to them • Example: FM radio stations in different countries c t c s1 t s2 f f c t s3 f

23. Frequency Division Multiplexing (FDM) • Separation of the whole spectrum into smaller non overlapping frequency bands (guard spaces are needed) • A channel gets a certain band of the spectrum for the whole time – receiver has to tune to the sender frequency • Advantages • no dynamic coordination necessary • works also for analog signals • Disadvantages • waste of bandwidth if the traffic is distributed unevenly • inflexible k1 k2 k3 k4 k5 k6 c f t

24. Time Division Multiplexing (TDM) • A channel gets the whole spectrum for a certain amount of time • Guard spaces (time gaps) are needed • Advantages • only one carrier in themedium at any time • throughput high even for many users • Disadvantages • precise clocksynchronization necessary k1 k2 k3 k4 k5 k6 c f t

25. Time and frequency multiplexing • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example: GSM • Advantages • better protection against tapping • protection against frequency selective interference • but: precise coordinationrequired k1 k2 k3 k4 k5 k6 c f t

26. Code Division Multiplexing (CDM) • Code Division • Instead of splitting the channel, the receiver is told which channel to access according to a pseudo-random code that is synchronised with the sender • The code changes frequently • Security: unless you know the code it is (almost) impossible to lock onto the signals • Interference: reduced as the code space is huge • Complexity: very high

27. Code multiplexing k1 k2 k3 k4 k5 k6 • Each channel has a unique code • All channels use the same spectrum at the same time • Advantages • bandwidth efficient • no coordination and synchronizationnecessary • good protection against interferenceand tapping • Disadvantages • precise power control required • more complex signal regeneration • Implemented using spread spectrum technology c f t

28. Modulation • Definition: transforming the information to be transmitted into a format suitable for the used medium • The signals are transmitted as a sign wave which has three parameters: amplitude, frequency and phase shift. • These parameters can be varied in accordance with data or another modulating signal • Two types of modulation • Digital modulation: digital data (0, 1) is translated into an analog signal (baseband signal) • Analog modulation: the center frequency of the baseband signal generated by digital modulation is shifted up to the radio carrier

29. Why we need digital modulation? • Digital modulation is required if digital data has to be transmitted over a medium that only allows analog transmission (modems in wired networks). • Digital signals, i.e. 0/1, can be sent over wires using voltages • Wireless must use analogue sine waves • This translation is performed by digital modulation • digital data is translated into an analog signal (baseband) • Shift Keying is the translation process • Amplitude, Freq., Phase Shift Keying (ASK/FSK/PSK) • differences in: • spectral efficiency: how efficiently the modulation scheme utilizes the available frequency spectrum • power efficiency: how much power is needed to transfer bits • Robustness: how much protection against noise, interference and multi-path propagation

30. Why we need analogue modulation ? • Analogue modulation then moves the signal into the right part of the channel • Motivation • smaller antennas (e.g., /4) • Frequency Division Multiplexing • medium characteristics – path loss, penetration of objects, reflection,..etc • Basic schemes • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM)

31. Modulation and demodulation analog baseband signal digital data digital modulation analog modulation radio transmitter 101101001 radio carrier analog baseband signal digital data analog demodulation synchronization decision radio receiver 101101001 radio carrier

32. Digital Modulation - Amplitude Shift Keying (ASK) • Amplitude Shift Keying (ASK) • 0 and 1 represented by different amplitudes • i.e. a basic sine wave • Problem: susceptible to interference • Constant amplitude is hard to achieve • ASK is used for optical transmissions such as infra-red and fibre (simple + high performance) • In optical  light on = 1 light off = 0

33. Digital Modulation - Frequency Shift Keying (FSK) • Frequency Shift Keying (FSK) • 0 and 1 represented by different frequencies • Switch between two oscillators accordingly • Twice the bandwidth but more resilient to error

34. Digital Modulation - Phase Shift Keying (PSK) • Phase Shift Keying (PSK) • 0 and 1 represented by different (longer) phases • Flip the sine wave 180 to switch between 0/1 • Better still than FSK but more complex • Other modulation schemes are mostly complex variants of ASK, FSK, or PSK…

35. Digital modulation - summary 1 0 1 • Modulation of digital signals known as Shift Keying • Amplitude Shift Keying (ASK): • very simple • low bandwidth requirements • very susceptible to interference • Frequency Shift Keying (FSK): • needs larger bandwidth • more error resilience than AM • Phase Shift Keying (PSK): • more complex • robust against interference t 1 0 1 t 1 0 1 t

36. Analog modulation • Definition: Impress an information-bearing analog waveform onto a carrier waveform for transmission

37. Spread spectrum technology • Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference • Solution: spread the narrow band signal into a broad band signal using a special code • Advantage: protection against narrow band interference • Side effects: • coexistence of several signals without dynamic coordination • tap-proof signal power interference power spread signal spread interference detection at receiver f f

38. Spread spectrum • Basic idea • Spread the bandwidth needed to transmit data • Lower signal power, more bandwidth, same energy • Resistant to narrowband interference • Steps • Apply spreading (convert narrow band to broadband) • Send low power spread signal • Signal picks up interference • Receiver can de-spread signal • Signal is more powerful than remaining interference • Signal is therefore able to be interpreted

39. Effects of spreading and interference dP/df dP/df user signal broadband interference narrowband interference i) ii) f f sender dP/df dP/df dP/df iii) iv) v) f f f receiver

40. channelquality 2 2 2 2 2 1 frequency spreadspectrum Spreading and frequency selective fading channelquality 2 1 5 6 narrowband channels 3 4 frequency narrow bandsignal guard space spread spectrum channels

41. Spread spectrum problems • Spread spectrum problems: • Increased complexity of receivers • Raising background noise • Spread spectrum can be achieved in two different ways: • Direct Sequence • Frequency Hopping

42. Spread Spectrum – Direct Sequence Spread Spectrum (DSSS) • Each bit in original signal is represented by multiple bits in the transmitted signal • Spreading code spreads signal across a wider frequency band • XOR of the signal with pseudo-random number (chipping sequence) • many chips per bit (e.g., 128) result in higher bandwidth of the signal • Advantages • reduces frequency selective fading

43. DSSS • Chipping sequence appears like noise, to others • Spreading factor S = tb /tc • If the original signal needs a bandwidth w, the resulting signal needs s*w • The exact codes are optimised for wireless • E.g. for Wi-Fi 10110111000 (Barker code) • For civil application spreading code between 10 and 100 • For military application the spreading code is up to 10,000 tb user data 0 1 XOR tc chipping sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 = resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit period tc: chip period

44. DSSS • New modulation process: • Sender: Chipping  Digital Mod.  Analog Mod. • Receiver: Demod.  Chipping  Integrator  Decision? • At the receiver after the XOR operation (despreading), an integrator adds all these products, then a decision is taken for each bit period • Even if some of the chips of the spreading code are affected by noise, the receiver may recognize the sequence and take a correct decision regarding the received message bit.

45. DSSS (Direct Sequence Spread Spectrum) spread spectrum signal transmit signal user data X modulator chipping sequence radio carrier transmitter correlator lowpass filtered signal sampled sums products received signal data demodulator X integrator decision radio carrier chipping sequence receiver

46. Spread Spectrum – Frequency Hopping Spread Spectrum (FHSS) • Uses entire bandwidth for signals • Signal is broadcast over seemingly random series of radio frequencies • A number of channels allocated for the FH signal • Width of each channel corresponds to bandwidth of input signal • Signal hops from frequency to frequency at fixed intervals • Transmitter operates in one channel at a time • At each successive interval, a new carrier frequency is selected. Pattern of hopping is the hopping sequence • Time on each frequency is the dwell time • Fast hopping = many hops per bit • Slow hopping = many bits per hop • Fast hopping is more robust but more complex • FHSS is used in Bluetooth - 1600 hops/s, 79 channels

47. Frequency Hopping Spread Spectrum (FHSS) • Process 1 - Spreading code modulation • The frequency of the carrier is periodically modified (hopped) following a specific sequence of frequencies. • In FHSS systems, the spreading code is this list of frequencies to be used for the carrier signal, the “hopping sequence” • The amount of time spent on each hop is known as dwell time and is typically in the range of 100 ms. • Process 2 - Message modulation • The message modulates the (hopping) carrier, thus generating a narrow band signal for the duration of each dwell, but generating a wide band signal if the process is regarded over periods of time in the range of seconds.

48. FHSS • Discrete changes of carrier frequency • sequence of frequency changes determined via pseudo random number sequence • Two versions • Fast Hopping: several frequencies per user bit • Slow Hopping: several user bits per frequency • Advantages • frequency selective fading and interference limited to short period • simple implementation • uses only small portion of spectrum at any time • Disadvantages • not as robust as DSSS • simpler to detect

49. FHSS tb user data 0 1 0 1 1 t f td f3 slow hopping (3 bits/hop) f2 f1 t td f f3 fast hopping (3 hops/bit) f2 f1 t tb: bit period td: dwell time

50. FHSS (Frequency Hopping Spread Spectrum) narrowband signal spread transmit signal user data modulator modulator frequency synthesizer hopping sequence transmitter narrowband signal received signal data demodulator demodulator hopping sequence frequency synthesizer receiver