ECE 4371, Fall, 2017 Introduction to

37 Slides1.84 MB

ECE 4371, Fall, 2017 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 2 Sep. 6th, 2017

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 ) F{cos( ct )} M( ) 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. To change the carrier frequency c modulated signal cos(of t )acos( t ) 21 [cos(( to )t )an cos(( )t )] intermediate frequency I we use an oscillator to generate a sinusoid of frequency MIX such that ( t ) m( t ) cos( C t ) e 1 ( t ) 21 m( t ) cos( I t ) [email protected] I cos( MIX t ) EXAMPLE : Let m(t) be as shown. m(t) e1(t) (t) t t t M( ) SPECTRA 0 ( ) c I c MIX . Example 4.2, MIX 4.3 Then m(t)cos( c t ) cos( t ) 21 m( t )[cos(( c MIX )t ) cos(( c MIX )t )] 21 m( t )[cos(( 2 c I )t ) cos(( I )t )] c 0 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( c t ) ( ) 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 1. 0. -0.4 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 overmodulated, and envelope detection can not be used. (However, we can still use synchronous demodulation). mp m(t) 2 . i) 0.5 A 4 ii) 1 A 2 A A For dc offset of 1 2. mp 2; 1 0. 5 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 2 Pm B2 2A 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 Nocoherent 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 t ) E(1 RC dv c ( t ) dt for RC 1 . c E RC E . The slope of the capacitor discharge is - 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 dE . RC dt E(t) A(1 cos(wct)) 1 RC wc 1 2

QAM AM signal BANDWIDTH : AM signal bandwidth is twice the bandwidth of the modulating signal. A 5kHz signal requires 10kHz bandwidth for AM transmission. If the carrier frequency is 1000 kHz, the AM signal spectrum is in the frequency range of 995kHz to 1005 kHz. QUADRARTURE AMPLITUDE MODULATION is a scheme that allows two signals to be transmitted over the same frequency range. Coherent in frequency and phase. Expensive TV for analog Most modems

Single Sideband (SSB) Purpose : to reduce the bandwidth requirement of AM by one-half. This is achieved by transmitting only the upper sideband or the lower sidebband of the DSB AM signal.

SSB Frequency M( ) baseband 2 B 2 B 0 DSB c 0 SSB ( ) c c SSB (Upper sideband) SSB c 0

SSB Generator Selective Filtering using filters with sharp cutoff characteristics. Sharp cutoff filters are difficult to design. The audio signal spectrum has no dc component, therefore , the spectrum of the modulated audio signal has a null around the carrier frequency. This means a less than perfect filter can do a reasonably good job of filtering the DSB to produce SSB signals. Baseband signal must be bandpass Filter design challenges No low frequency components c 0 c

SSB Generator Phase shift method using Hilbert transformer Non-causal filter, approximations x m(t) cos c ) H( ) 1 Hilbert Transformer H( ) 2 2 2 X ssb ( t )

SSB Demodulation Synchronous, SSB-SC demodulation SSB ( t ) cos( c t ) m( t ) cos( c t ) jm h ( t ) sin( c t ) cos(n( c t ) 21 m( t )(1 cos( c t )) jm h ( t ) sin(2 c t ) A lowpass filter can be used to get 21 m( t ). SSB C, envelop detection SSB C ( t ) A cos( c t ) m( t ) cos( c t ) m h ( t ) sin( c t ) An envelope detector can be used to demodulate such SSB signals . What is the envelope of SSB C ( t ) ( A m( t )) cos( c t )) m h ( t ) sin( c t ) E( t ) cos( c t ) ? {Recall Acos( ) Bsin( ) A 2 B 2 1 2 cos( ), tan -1( B )) A E(t) (( A m( t )) 2 m h2 ( t )) (( A 2 m 2 ( t )) m h2 ( t ) 2Am( t )) 1 2 2 m (t) m (t) 2m( t ) A 1 A A A A m( t ) for A m(t) , A m h (t) . 2 h 2 2 The efficiency of this scheme is very low since A has to be large. 1 2

SSB vs. AM Since the carrier is not transmitted, there is a reduction by 67% of the transmitted power (-4.7dBm). --In AM @100% modulation: 2/3 of the power is comprised of the carrier; with the remaining (1/3) power in both sidebands. Because in SSB, only one sideband is transmitted, there is a further reduction by 50% in transmitted power Finally, because only one sideband is received, the receiver's needed bandwidth is reduced by one half--thus effectively reducing the required power by the transmitter another 50% (-4.7dBm ( ) -3dBm ( ) -3dBm -10.7dBm). Relative expensive receiver

Vestigial Sideband (VSB) VSB is a compromise between DSB and SSB. To produce SSB signal from DSB signal ideal filters should be used to split the spectrum in the middle so that the bandwidth of bandpass signal is reduced by one half. In VSB system one sideband and a vestige of other sideband are transmitted together. The resulting signal has a bandwidth the bandwidth of the modulating (baseband) signal but the DSB signal bandwidth. DSB c c c c 0 SSB ( ) SSB (Upper sideband) 0 VSB ( ) VSB Spectrum c c

Filtering scheme for the generation of VSB modulated wave.

VSB Transceiver VSB ( ) m(t) Hi( ) e(t) VSB ( ) M( ) LPF Ho( ) 2cos( c t ) 2cos( c t ) Transmitter Receiver M( ) is bandlimite d to 2 B rad/sec VSB ( ) [M( c ) M( c )]Hi ( ) E( ) [ VSB ( c ) VSB ( c )] [Hi ( c )M( 2 c ) Hi ( c )M( ) Hi ( c )M( ) Hi ( c )M( 2 c )] High freq. term High freq. term M( ) E( )H o ( ) Hi ( c ) Hi ( c ) M( )Ho ( ) [Hi ( c )M( 2 c ) Hi ( c )M( 2 c )]H o ( ) Lowpass filter removes this. Thus we should have OR H o ( ) Hi ( c ) Hi ( 1 Hi ( c ) Hi ( c ) c ) H o ( ) 1 for 2 B

Other Facts about VSB Envelope detection of VSB C Analog TV: DSB, SSB and VSB – DSB bandwidth too high – SSB: baseband has low frequency component, receiver cost – Relax the filter and baseband requirement with modest increase in bandwidth

(a) Idealized magnitude spectrum of a transmitted TV signal. (b) Magnitude response of VSB shaping filter in the receiver.


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 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 Super high SHF 3 GHz - 30 GHz Extremely high EHF 30 GHz - 300 GHz Tremendously high THF 300 GHz - 3000 GHz Characteristics Ground wave Space wave

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 multiaccess) Very flexible circuit installment , can disperse over-centralized traffic at any time. One channel can be used in different directions or areas (multi-access connecting).

Satellite Orbit Geostationary (GEO): 35,786 km Beidou (MEO): 27,878 km Hubble Space Telescope (LEO): 347mile International Space Station (LEO): 249mile

Rain Attenuation

Related Articles

Back to top button