Chapter 2 Mobile Communication

Transcription

1 Page 77 Chapter 2 Mobile Communication 2.1 Characteristics of Mobile Computing 2.2 Wireless Communication Basics 2.3 Wireless Communication Technologies PANs (Bluetooth, ZigBee) Wireless LAN (IEEE ) Cellular Networks (GSM/UMTS) 2.4 Mobile Ad-hoc Networks (MANET)

2 Page 78 How Wireless Communication influences the Layers Application Layer Transport Layer service location new applications, multimedia adaptive applications congestion and flow control quality of service Network Layer Datalink Layer Physical Layer addressing, routing, device location hand-over authentication media access multiplexing media access control encryption modulation interference attenuation frequency

3 Page 79 Radio Waves Electromagnetic oscillation Frequency ƒ (Hz): Number of oscillations per second Speed of light: c 3 * 10 8 m/s (30 cm/ns) Wavelength: λ = c/ƒ Examples of wavelengths: 1 MHz == 300 m 100 MHz == 3 m 10 GHz == 3 cm

4 Page 80 Electromagnetic Spectrum [Image source: ]

5 Page 81 Signal Transmission Signal: physical representation of data The Signal is a function of time and data Classification of possible functions continuous time / discrete time continuous values / discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values Periodical signal: g(t) = A t sin(2πƒ t t + ϕ t ) A t : amplitude (amplitude modulation) ƒ t : frequency (frequency modulation) ϕ t : phase shift (phase modulation)

6 Page 82 Modulation Digital modulation digital data is translated into an analog signal (baseband) differences in spectral efficiency, power efficiency, robustness Basic schemes Amplitude Modulation (AM) / Amplitude Shift Keying (ASK) Frequency Modulation (FM) / Frequency Shift Keying (FSK) Phase Modulation (PM) / Phase Shift Keying (PSK) Animation of AM/FM at

7 Page 83 Digital modulation 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 t t Phase Shift Keying (PSK): robust against interference t 1 0 1

8 Page 84 Nyquist Theorem Nyquist Theorem (1924) A bandwidth limited signal with a maximum frequency of H can be restored if 2H sample values are known. Calculate the maximal data rate that can be reached on a bandwidth limited channel Max. Datarate =2H log2 V Bit V: number of discrete values per sample s Achievable bandwidth still unlimited Just code more bits per symbol/sample Interactive Demos: nykvist_eng\nykvist.exe [Nota bene: Compressed Sensing goes beyond the Nyquist Theorem]

9 Page 85 Signal Noise Ratio Nyquist theorem only for noise-free channels In reality: noise/interference distorts the signal Signal-Noise Ratio (SNR) S : Signal Strength N : Noise Level SNR = 10 log 10 S/N db S/N SNR db db db

10 Page 86 Shannon-Hartley Shannon-Hartley-Law (1948) Maximum data rate in a noisy channel is limited by Signal to Noise Ratio Max. Datarate = H log (1 S ) Bit 2 + N s Example: Telephone system Bandwidth 3000 Hz SNR 30 db S/N = 1000 max. Datarate ~ Bit/s Claude Elwood Shannon Ralph Hartley

11 Page 87 Bandwidth vs. Propagation Carrier: Radio Frequency Microwave Infrared light Visible Light / Laser Frequency Wavelength Tradeoff: Higher Frequency More available bandwidth but lower propagation Lower Frequency Less available bandwidth but higher propagation

12 Page 88 Signal propagation ranges Transmission range communication possible low error rate Detection range sender detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise transmission detection interference distance Notion of a Radio Cell

13 Page 89 Attenuation / Loss / Fading Attenuation A = 10 log 10 (P s /P r ) db P s = Power Sent; P r = Power Received Ideal situation: P r ~P s /r 2 In reality: signal energy gets absorbed by atmosphere, rain, walls etc. Higher Frequencies higher fading Differences in free space fading vs. fading in solid material Influence of gravity Ground-waves (< 2 MHz): follow surface Sky-wave (2-30 MHz): Ionospheric reflection Line of Sight (>30 MHz): like visual light

14 Page 90 Interfering Effects Blocking (Abschattung) by large objects Reflection (Reflektion) at large objects Refraction (Brechung/Refraktion) if density of medium changes Scattering (Streuung) at small objects Diffraction (Beugung) at edges

15 Page 91 Multipath propagation Signal can take many different paths between sender and receiver due to reflection, refraction, scattering, diffraction line of sight pulses multipath pulses signal at sender signal at receiver Time dispersion signal is dispersed over time interference with neighbor symbols, Inter Symbol Interference (ISI) Delay spread the signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts

16 Page 92 Short/Long Term Fading Moving Sender/Receiver Short Term Fading quick changes in the power received due to rapidly changing characteristics of transmission channel (caused e.g. by multipath propagation) Long Term Fading slow changes in the power received due to changing distance between sender and receiver may be compensated by adapting transmit power Other Problem at high speeds Doppler effect

17 Page 93 Signal Strength in real world examples

18 Page 94 Unregulated Radio Bands ISM = Industrial, Science and Medical use Can effectively be used by anybody (ITU-R) 900 MHz, 2.4 GHz and 5.8 GHz frequency bands Restrictions regarding max. transmit power, etc. Becomes increasingly crowded Bluetooth, WLAN, wireless home appliances and many more

19 Page 95 Multiplexing Problem: How should multiple senders operate in parallel? Solution: Multiplexing in Space (SDM) in Frequency (FDM) in Time (TDM) in Code (CDM) Important: Guard spaces separate the channels

20 Page 96 Space Division Multiplex Large distance between two senders using the same frequency Utilizes attenuation cf. cellular network Same Frequency

21 Page 97 Frequency Division Multiplex Different senders use different frequencies Advantages no dynamic coordination necessary works also for analog signals Disadvantages waste of bandwidth if the traffic is distributed unevenly inflexible c c 1 c 2 c 3 c 4 c 5 c 6 Guard space f t

22 Page 98 Time Division Multiplex Different senders all use the same frequency, but at different times Advantages only one carrier in the medium at any time throughput high even for many users Disadvantages precise synchronization necessary c c 1 c 2 c 3 c 4 c 5 c 6 f t Guard space

23 Page 99 Combination TDM/FDM Often combinations of TDM/FDM are being applied A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: but: better protection against tapping protection against frequency selective interference higher data rates compared to code multiplex Precise coordination required c c 1 c 2 c 3 c 4 c 5 c 6 Guard space f t Guard space

24 Page 100 Code Division Multiplex c Different senders all use the same frequency and time, but separate codes Codes should be orthogonal Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology t Guard space f c 1 c 2 c 3 c 4 c 5 c 6

25 Page 101 Multiple Access Methods based on the Multiplex Methods SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (e.g. Bluetooth) TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time (e.g. WLAN, GSM) CDMA (Code Division Multiple Access) assign orthogonal codes to different senders