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ECE 4371, Fall, 2014 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 2 Aug. 27th, 2014 Overview Chapter 4.1-4.3, basics of amplitude modulation Other – GPS – Type of waves – Satellite communication basics Baseband and Carrier Communication Baseband: Describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency Voice: Telephone 0-3.5KHz; CD 0-22.05KHz Video: Analog TV 4.5MHz, TV channel is 0-6MHz. Digital, depending on the size, movement, frames per second, … Example: wire, coaxial cable, optical fiber, PCM phone Carrier Communication: Carrier: a waveform (usually sinusoidal) that is modulated to represent the information to be transmitted. This carrier wave is usually of much higher frequency than the modulating (baseband) signal. Modulation: is the process of varying a carrier signal in order to use that signal to convey information. Example on the board. Modulation Modulation A process that causes a shift in the range of frequencies of a signal. Gain advantages Antenna size: half of the antenna size. Thousands of miles for baseband Better usage of limited bandwidth: less side lopes Trade bandwidth for SNR: CDMA Robust to inter-symbol-interference (multipath delay) Robust to errors and distortions Types Analog: AM (DSB, SSB, VSB), FM, Delta modulation Digital: ASK, FSK, PSK, QAM, … Pulse modulation: PCM, PDM, … Fiber, phone Advanced: CDMA (3G), OFDM (WLAN, WMAN), …. Double Sideband Modulation: m(t)cos(wct) 0.5[M(w-wc)+M(w+wc)] Low/upper side band (LSB/USB), Double side band (DSB) DSB-SC: suppressed carrier, no carrier frequency Wc >= bandwidth of the signal to avoid aliasing. Demodulation: e(t)=m(t)(cos(wct))^2=0.5(m(t)+m(t)cos(2wct)) E(w)=0.5M(w)+0.25(M(w+2wc)+M(w-2wc)) Low pass filter to remove the higher frequency Coherent and non-coherent detection – Receiver can recover the frequency and phase of the transmitter by PLL. Error of timing can cause the performance error floor – Non-coherent receiver has 3dB worst performance than coherent. – Cheaper for Non-coherent receiver, Nextel. AM-DSB-SC Example 4.1 t t ( t ) ( t ) t m( t ) cos( c t ) M( ) F{cos( c t )} 0 0 () c 0 Lower sideband (LSB) c c Upper sideband (USB) Categories of Modulators Multiplier Modulators – Multiply m(t) by cos(wct) – Hard for linearity for high energy. Expensive. e.g. sound system Nonlinear Modulators – Example Switching Modulators – FFT transform to series of frequencies – Series-bridge diode modulator, shunt-bridge diode modulator – Ring Modulators Frequency Conversion Move the signals to other frequency Multiplying two sinusoids results in two frequencies which are the sum and difference of the frequencies of the sinusoids multiplied. (t ) m(t ) cos(C t ) [email protected] I cos(MIX t ) EXAMPLE : Let m(t) be as shown. m(t) cos(t) cos(t) 21 [cos(( )t) cos(( )t)] To change the carrier frequency c of a modulated signal to an intermediate frequency I we use an oscillator to generate a sinusoid of frequency MIX such that e1 (t) 21 m(t) cos(I t ) (t) t t 0 Example 4.2, 4.3 () c c 0 Then m(t)cos( c t ) cos(MIX t ) 21 m( t )[cos(( c MIX )t ) cos(( c MIX )t )] 21 m( t )[cos(( 2 c I )t ) cos(( I )t )] t M() SPECTRA I c MIX . e1(t) E1() I 0 I Amplitude Modulation Why DSB-SC not working: do not know the carrier frequency in receiver. The last impulse functions indicate that the carrier is not suppressed in this case. For some M() shown, the modulated signal spectrum is as shown. AM (t ) [ A m(t )] cos(ct ) ( ) 12 M ( c ) M ( c ) A ( c ) ( c ) M() 0 () c 0 c With this type of AM the demodulation can be performed with/without a local oscillator synchronized with the transmitter. AM Example • m(t) has a minimum value of about -0.4. Adding a dc offset of A=1 results in A+m(t) being always positive. Therefore the positive envelope of is just A+m(t). An envelope detector can be used to retrieve this. A=1 m(t) A+m(t) 0.7 0. -0.4 1. t t AM (t ) [ A m(t )] cos(c t ) t AM Example (cont.) The choice of dc offset should be such that A+m(t) should always be positive. Otherwise envelope detector cannot be used, but coherent still ok For example, the minimum value of m(t) = -0.4 . Therefore A > |min(m(t))| for successful envelope detection. What if A< |m(t) |. In the previous example let A=0.3. A+m(t) m(t) 0.7 0. -0.4 0 t AM (t ) [ A m(t )] cos(c t ) t t Modulation Index • Let mp be the absolute negative peak of m(t). EXAMPLE : Single-tone modulation. Let m(t)=2sin(20t) A mp A is the carrier amplitude. MODULATION INDEX : mp A Then we see that for A m p , 0 1 When 1 (or A m p ) the signal is overmodula ted, and envelope detection can not be used. (However, we can still use synchronou s demodulati on). mp 2 . i) 0.5 A A For dc offset of 1 2. mp 2; A 4 ii) 1 A 2 1 0.5 t m(t) t 2 t t Sideband and Carrier Power AM ( t ) A cos(c t ) m( t ) cos(c t ) The first term is the carrier and the second term is sidebands which contain the signal itself. The total AM signal power is the sum of carrier power and the sideband power. A2 Carrier power Pc 2 Sideband power Ps 21 Pm where Pm is the power of m(t). The sideband power is the useful power. Ps Pm useful power Efficiency : . Total power Pc Ps A 2 Pm For example , let m(t) Bcos(m t ) mp B, B or B A. A Pm B2 2 2 A 2 2 For 1, max 2 2 2 x100 % 1 x100 % 33 % 2 1 Example 4.4, 4.5 AM Generator + m(t) - BPF @ c + c cos(ct) - AM output Coherent detector for demodulating DSB-SC modulated wave. AM Decoder Rectifier Detector: synchronous Envelope Detector: asynchronous t t t + AM signal R C vc(t) - t RC Selection Assume that the capacitor is charged to voltage E (the envelope voltage at the instant)at the instant when the diode turns OFF. The capacitor begins to dischrage through the resistor according to v c ( t ) Ee t RC E(1 dv c ( t ) dt t ) RC for RC 1 c . E RC The slope of the capacitor discharge is - E RC . For the capacitor discharge to follow the envelope, the magnitude of the capacitor discharge slope must be greater than the envelpe slope. dv c ( t ) dt E RC E(t)=A(1+cos(wct)) dE . dt 1 1 2 RC wc GPS Orbits GPS Position By knowing how far one is from three satellites one can ideally find their 3D coordinates To correct for clock errors one needs to receive four satellites Differential GPS: local FM Type of waves Radio Frequency Bands Classification Band Initials Frequency Range Characteristics Extremely low ELF < 300 Hz Infra low ILF 300 Hz - 3 kHz Very low VLF 3 kHz - 30 kHz Low LF 30 kHz - 300 kHz Medium MF 300 kHz - 3 MHz Ground/Sky wave High HF 3 MHz - 30 MHz Sky wave Very high VHF 30 MHz - 300 MHz Ultra high UHF 300 MHz - 3 GHz Ground wave Space wave Super high SHF 3 GHz - 30 GHz Extremely high EHF 30 GHz - 300 GHz Tremendously high THF 300 GHz - 3000 GHz Satellite Communications Large communication area. Any two places within the coverage of radio transmission by satellite can communicate with each other. Seldom effected by land disaster ( high reliability) Circuit can be started upon establishing earth station (prompt circuit starting) Can be received at many places simultaneously, and realize broadcast, multi-access communication economically( feature of multi-access) Very flexible circuit installment , can disperse over-centralized traffic at any time. One channel can be used in different directions or areas (multiaccess connecting). Rain Attenuation