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Communication Channel

Communication Channel. Outline. Information Transmission Attenuation: dB Equivalent Noise Temperature Communication Limits Broadband Channel BER . Frequency Response. All communication channels modify/ distort signals transmitted.

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Communication Channel

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  1. Communication Channel Property of R. Struzak

  2. Outline • Information Transmission • Attenuation: dB • Equivalent Noise Temperature • Communication Limits • BroadbandChannel • BER Property of R. Struzak

  3. Frequency Response • All communication channels modify/ distort signals transmitted. • A linear, time-invariant channel is characterized in frequency domain by its transfer function (frequency response or frequency characteristics):H() = Y() / X() • Valid for fixed (or moving slowly) systems (otherwise other effects have to be taken into account, e.g. Doppler frequency shift) Input signal, frequency domain (amplitude spectrum) Output signal, frequency domain (amplitude spectrum) Property of R. Struzak

  4. Frequency Response Measurement Signal Generator Transmission Channel Receiver/ Spectrum Analyzer Synchronized Property of R. Struzak

  5. Time Response • The time domain and frequency domain are uniquely linked by the Fourier transform Channel impulse response Output signal (time domain) An example of (analogy to) impulse response: a bell rings when hit by clapper Property of R. Struzak

  6. Time Response Measurement Impulse Generator Transmission Channel Oscilloscope Synchronized Property of R. Struzak

  7. T-Domain & F-Domain Property of R. Struzak

  8. Output power (dBm) Noise Floor Nonlinearity: BDR P1dB-out 1dB MDS = Minimum Detectable Signal (Output Noise Floor) P1dB-in Input power (dBm) MDS BDR (Blocking Dynamic Range) Property of R. Struzak

  9. Nonlinarity: SFDR Extrapolated Third-Order Distortion Extrapolated Linear Output OIP2 Output power (dBm) Extrapolated Second-Order Distortion OIP3 IIP3 = Third-Order Intercept Point IIP2 = Second-Order Intercept Point MDS = Minimum Detectable Signal (Output Noise Floor) SFDR = [(2/3)(IIP3 – MDS)] = Spurious-Free Dynamic Range Noise floor Input power (dBm) MDS IIP3 IIP2 SFDR OIP2 = Output Referred Second-Order Intercept Point OIP3 = Output Referred Third-Order Intercept Point Property of R. Struzak

  10. 2-Tone Test P  Signal power (dBm) 2f1-f2 f1 f2 2f2-f1 Frequency IIP3 = 1/2 + P f1 ~ f2 Property of R. Struzak

  11. Communication Channel Signal transmitted Signal received Input signal Output signal Information destination Information source Transmitter Propagation Channel Receiver Signal transformationsdue to natural phenomena;external noise/signals added Transmitter signal processing Receiver signal processing Property of R. Struzak

  12. Main Natural PhenomenaAffecting Communication • Attenuation • Noise/ interference • Additive (thermal noise) • Multiplicative (fading) Property of R. Struzak

  13. Loss & dB • Abbreviation fordecibel(s). One tenth of the common logarithm of the ratio of relative powers, or power ratios, equal to 0.1 B (bel). Property of R. Struzak

  14. Various dBs • dBi: In the expression of antenna gain, the number of decibels of gain of an antenna referenced to the zero dB gain of a free-space isotropic radiator. • dBm: dB referenced to one milliwatt. ‘dBm’ is often used in communication work as a measure of absolute power values. Zero dBm means one milliwatt. • dBV :dB referenced to 1 microvolt. Used often for receiver sensitivity measurement. • dBmV:dB referenced to one millivolt across 75 ohms. This is 1.33 × 10-5 milliwatts. • dBv:dB relative to 1 volt peak-to-peak. ‘dBv’ is often used for television video signal level measurements. • dBW:dB referenced to one watt. Zero dBW means one watt. • Note: There are also other ‘dBs’ in use! Source: Telecommunication Glossary 2000 Property of R. Struzak

  15. Radio Transmission Loss Components ITU-R Rec. Property of R. Struzak

  16. Sum of Two Signals (Deterministic, Linear System) Resultant signal Property of R. Struzak

  17. Uncertainty due to Noise Small uncertainty, Signals can easily be differentiated Large uncertainty, Signals cannot easily be differentiated Property of R. Struzak

  18. Thermal Noise N = kTB N – available noise power from resistor [W] k – Boltzmann’s constant (1.37 x 10-23 [J/o]) T – temperature [oK] B – frequency bandwidth [Hz] 1J=1Ws Thermal Noise = fundamental limiting factor Property of R. Struzak

  19. Equivalent Noise Temperature S+N Actual Receiver Internal Noise Identical Output Signal-to-Noise Ratio S+N Noise-less Receiver kTeB Property of R. Struzak

  20. Communication Channel (2) Original message m(t) m(t) = message (information, data) s(t) = signal carrying the message f = f(a,b,c,…, t) (carrier function) a,b,c, … = modulation parameters U, V, W = operators  = noise, interference, perturbations x(t) = perturbed signal at the receiver input y(t) = reproduced message Task: make y≈m (within an acceptable error) Transmitter s(t) = U(m, f) Transport medium x(t) = V(s, ) Receiver y(t) = W(x) Reproduced (received) message y = W{V[,U(m,f)]} Property of R. Struzak

  21. Shannon’s Law • The maximum rate of information transmission without errors through a communication channel equals the channel capacity • The channel capacity of a noisy channel is limited. It depends on the channel bandwidth B and signal-to-noise power ratio SNR: it is proportionaltoB, and increases with SNR Notes: (1) Isolated system. (2) AWGN (Additive White Gaussian Noise) only. (3) Noise-like signal using full bandwidth. (4) No signal-noise correlation. (5) Ideal coding, but Shannon says nothing how to implement such a code. Special coding required that may take very log time, but the signal latency is ignored. (6) Claude Shannon, 1948 Property of R. Struzak

  22. Communication Limits • Claude Shannon defined the limits for communication channels • C: channel capacity (max. data rate), bps • B: frequency band, Hz • S/N: received signal-to-noise power ratio Property of R. Struzak

  23. Transmission Time & Speed Property of R. Struzak

  24. Data Rate per Hz vs. SNR Property of R. Struzak

  25. Bit Rate & Boud Rate • The bit rate defines the rate at which information is passed • The boud (or signalling) rate (Bd) is a unit of modulation rate and defines the number of symbols per second. • Each symbol represents n bits, and has M signal states, where M = 2n. This is called M-ary signalling. Property of R. Struzak

  26. WidebandChannel C = Blog2{1 + [S/(NoB)]} Noise density, W/Hz (const) Received signal power, W Bandwidth, Hz Capacity (data rate), bit/s With signal power S and noise power density N0 constant, enlargement of the bandwidth increases also noise. For B  , (S/N0B)  0 and log2(1+S/N0B) = 1.44 loge(1+S/N0B)  1.44S/N0B, or R  1.44S/N0. With thermal noise only, C  1.44S/kT. R does not become greater with any further increase of B. In these conditions, S0.693kTR. Property of R. Struzak

  27. Wideband Channel 2 • With large bandwidth involved, the assumption of flat channel frequency response and/or white noise is likely not to be valid. In such a case, the following equation is frequently used: Delogne P, Bellanger M, The impact of Signal Processing on an Efficient Use of the Spectrum, Radio Science Bulletin No 289, june 1999, 23-28 Property of R. Struzak

  28. Data Rate vs. Bandwidth(Wideband Channel) C = B log [1+ S / kT] Thermal noise asymptote: C = 1.44 S / kT Property of R. Struzak

  29. BER vs. S/N • BER or bit error ratio: The number of erroneous bits divided by the total number of bits transmitted, received, or processed over some stipulated period. • It is usually expressed as a coefficient and a power of 10; e.g. 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 × 10-5. • Acceptable BER: 10-3 for a voice link, 10-9 for a data link • BER decreases with S/N to a degree that depends on the signal processing applied BER S/N Property of R. Struzak

  30. BER vs Input Signal BER Errors due to thermal noise, Quantization, Sampling jitter Errors due to self-induced spurious interference (overload) Input signal level Property of R. Struzak

  31. Countermeasures Against Errors • Repeating transmission/ Error control • Increase S/N (filtration/ frequency, time, direction selection) • Noise-resistant Modulation/ Demodulation / Encoding/ Decoding • Spreading/De-spreading signals Applied during signal generation, transmission, reception in digital/ analogue technology Property of R. Struzak

  32. Retransmission Schemes • Stop and Wait • Only one packet at a time can be transmitted. The tranbsmitter waits for an acknowledgment (ACK), positive or negative, from the receiver. If no ACK is received after a fixed amount of time (timeout) the packet is retransmitted • Go-Back-N • Extension of Stop and Wait. Transmitter sends up to N packets without reception of corresponding ACK. On reception of negative ACK or when the timeout expires, the packets are retransmitted. • Selective Repeat • Extension of Go-Back-N. Only the packet in error is retransmitted. Requires packet buffering and reordering at the receiver end. Property of R. Struzak

  33. Channel Summary • Information is carried by signals that are limited in time, frequency, and energy • Signal travel distance with limited speed – require time to travel at a distance • During transmission, signal suffer attenuation and is affected by noise, etc. • The channel capacity is limited Property of R. Struzak

  34. References • Many good books, e.g. • Pierce JR, An Introduction to Information Theory, Dover Publ. • Dunlop J, Smith DG, Telecommunications Engineering, Chapmann & Hall Property of R. Struzak

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