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2. Wireless Transmission Frequencies for Communication Frequencies for Communication (1) twisted coax cable optical transmission pair 1 Mm 10 km 100 m 1 m 10 mm 100 µm 1 µm 300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz 2. Wireless Transmission VLF LF MF HF VHF UHF SHF EHF infraredvisible light UV VLF = Very Low Frequency UHF = Ultra High Frequency Frequencies and Signals LF = Low Frequency SHF = Super High Frequency Multiplexing MF = Medium Frequency EHF = Extra High Frequency Modulation and Spread Spectrum HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length: λ = c/f wave length λ, speed of light c ≅ 3x108m/s, frequency f © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 1 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 2 Frequencies for Mobile Communication (2) Frequencies and Regulations VHF-/UHF-ranges for mobile radio ITU-R holds auctions for new frequencies and manages – Simple, small antenna for cars frequency bands worldwide (WRC, World Radio Conferences) – Deterministic propagation characteristics, reliable connections Europe USA Japan Cellular GSM 450-457, 479- AMPS, TDMA, CDMA PDC Phones 486/460-467,489- 824-849, 810-826, 496, 890-915/935- 869-894 940-956, SHF and higher for directed radio links, satellite communication 960, TDMA, CDMA, GSM 1429-1465, 1710-1785/1805- 1850-1910, 1477-1513 – Small antenna, focusing 1880 1930-1990 – Large bandwidth available UMTS (FDD) 1920- 1980, 2110-2190 UMTS (TDD) 1900- 1920, 2020-2025 Wireless LANs use frequencies in UHF to SHF spectrum Cordless CT1+ 885-887, 930- PACS 1850-1910, 1930- PHS Phones 932 1990 1895-1918 – Some systems planned up to EHF CT2 PACS-UB 1910-1930 JCT 864-868 254-380 – Limitations due to absorption by water and oxygen molecules (resonance DECT 1880-1900 frequencies) Wireless IEEE 802.11 902-928 IEEE 802.11 LANs 2400-2483 IEEE 802.11 2471-2497 • Weather dependent fading, signal loss caused by heavy rainfall etc. HIPERLAN 2 2400-2483 5150-5250 5150-5350, 5470- 5150-5350, 5725-5825 5725 Others R F-C ontrol RF-Control RF-Control 27, 128, 418, 433, 315, 915 426, 868 868 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 3 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 4 Signals (1) Signals (2) Physical representation of data Different representations of signals: Function of time and location – Amplitude (amplitude domain) – Frequency spectrum (frequency domain) – Phase state diagram (amplitude M and phase ϕ in polar coordinates) Signal parameters: ϕ – Parameters representing the value of data A [V] A [V] Q = M sin Classification: – Continuous time/discrete time t[s] ϕ – Continuous values/discrete values I= M cos ϕ – Analog signal = continuous time and continuous values ϕ f [Hz] – Digital signal = discrete time and discrete values Composed signals transferred into frequency domain using Signal parameters of periodic signals: Fourier transformation – Period T, frequency f=1/T, amplitude A, and phase shift ϕ – Sine wave as special periodic signal for a carrier: Digital signals need: π ϕ – Infinite frequencies for perfect transmission s(t) = At sin(2 ft t + t) – Modulation with a carrier frequency for transmission (analog signal!) © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 5 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 6 1 Fourier Representation of Periodic Signals Antennas: Isotropic Radiator Radiation and reception of electromagnetic waves: – Coupling of wires to space for radio transmission ∞ ∞ = 1 + π + π Isotropic radiator: g(t) c ∑an sin(2 nft) ∑bn cos(2 nft) 2 n=1 n=1 – Equal radiation in all directions (three dimensional) – Only a theoretical reference antenna Real antennas always have directive effects: 1 1 – Vertically and/or horizontally Radiation pattern: – Measurement of radiation around an antenna 0 0 tt z y z Ideal periodic signal Real composition yxIdeal isotropic radiator (based on harmonics) x © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 7 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 8 Signal Propagation Ranges Signal Propagation Transmission range: Propagation in free space always like light (straight line) – Communication possible Receiving power proportional to 1/d² – Low error rate – d = distance between sender and receiver Receiving power additionally influenced by: Detection range: – Fading (frequency dependent) – Detection of the signal – Shadowing (Abschattung) possible Sender – Reflection (Reflexion) at large obstacles – No communication – Refraction (Brechung) depending on the density of a medium possible Transmission – Scattering (Streuung) at small obstacles Distance – Diffraction (Beugung) at edges Interference range: Detection – Signal may not be Interference detected – Signal adds to the background noise shadowing reflection refraction scattering diffraction © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 9 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 10 Real World Examples Multi-path Propagation Signal can take many different paths between sender and receiver due to: – Reflection, scattering, and diffraction LOS pulses Multi-path pulses Signal at sender Signal at receiver Time dispersion: signal is dispersed over time Î Interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted Î Distorted signal depending on the phases of the different parts © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 11 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 12 2 Effects of Mobility Multiplexing Channel characteristics change over time and location: Channels ki – Signal paths change permanently Multiplexing in 4 dimensions: k k k k k k – Different delay variations of different signal parts 1 2 3 4 5 6 –Space (si) – Different phases of signal parts –Time (t) c Î Quick changes in the power received (short term fading) – Frequency (f) t c – Code (c) t s Power Long term 1 f Additional changes in: fading Goal: s 2 f – Distance to sender – Multiple use of a shared medium c – Obstacles further away t Î Slow changes in the average power received (long term fading) t Important: guard spaces needed! Short term fading s 3 f © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 13 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 14 Frequency Multiplexing Time Multiplexing Separation of the entire spectrum into smaller frequency bands A channel gets the whole spectrum for a certain amount of time A channel gets a certain band of the spectrum for the full time Advantages: Advantages: – Only one carrier in the – No dynamic coordination k k k k k k medium at any time 1 2 3 4 5 6 k1 k2 k3 k4 k5 k6 necessary – Throughput high even c – Works also for analog signals for many users f c f Drawbacks: Drawbacks: – Waste of bandwidth – Precise if the traffic is synchronization distributed unevenly necessary – Common – Inflexible t –Guard spaces t clock © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 15 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 16 Time and Frequency Multiplexing Code Multiplexing Combination of both methods Each channel has a unique code k k k k k k A channel gets a certain frequency band for a certain 1 2 3 4 5 6 amount of time All channels use the same spectrum –Example: GSM at the same time c k1 k2 k3 k4 k5 k6 Advantages: Advantages: – Bandwidth efficient – Better protection against tapping c – No coordination and synchronization necessary – Protection against frequency f – Good protection against interference and selective interference tapping f – Higher data rates compared Drawbacks: to code multiplex – Lower user data rates – More complex signal regeneration But: t Implemented using spread spectrum t – Precise coordination required technology © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 17 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 18 3 Modulation Modulation and Demodulation Digital modulation: Analog – Digital data is translated into an analog signal (baseband) Base-band – ASK, FSK, PSK - main focus in this chapter Digital Data Signal – Differences in spectral efficiency, power efficiency, robustness Digital Analog Analog modulation: 101101001 Modulation Modulation Radio Transmitter – Shifts center frequency of baseband signal up to the radio carrier Radio Motivation: Carrier – Smaller antennas (e.g., λ/4) – Frequency Division Multiplexing Analog – Medium characteristics Base-band Digital Signal Data Basic schemes: Analog Synchronization – Amplitude Modulation (AM) Demodulation Decision 101101001 Radio Receiver – Frequency Modulation (FM) radio – Phase Modulation (PM) carrier © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 19 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M2 – 20 Digital Modulation Advanced Frequency Shift Keying Modulation of digital signals known Bandwidth needed for FSK depends on the distance between 101 as Shift Keying the carrier frequencies Special pre-computation avoids sudden phase shifts: Amplitude Shift Keying (ASK): t Î MSK (Minimum Shift Keying) – Very simple Bit-separated into even and odd bits, the duration of each bit is – Low bandwidth requirements 101 doubled – Very susceptible to interference Depending on the bit values (even, odd) the higher or lower t frequency, original or inverted is chosen Frequency Shift Keying (FSK): – Needs larger bandwidth The frequency of one carrier is twice the frequency of the other 101 Equivalent to offset QPSK Even higher bandwidth
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