1、1,Chapter 2: 無線傳輸之原理 Part I: Transmission Fundamentals,YC-TsengCS.NCTU,2,Electromagnetic Signal,is a function of time can also be expressed as a function of frequency Signal consists of components of different frequencies,3,Time-Domain Concepts,Analog signal - signal intensity varies in a smooth fas
2、hion over time No breaks or discontinuities in the signal Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level Periodic signal - analog or digital signal pattern that repeats over times(t +T ) = s(t ) - t + where T is the per
3、iod of the signal Aperiodic signal - analog or digital signal pattern that doesnt repeat over time,4,5,Time-Domain Concepts (cont.),Peak amplitude (A) maximum value or strength of the signal over time typically measured in volts. Frequency (f ) Rate, in cycles per second, or Hertz (Hz), at which the
4、 signal repeats.,6,Time-Domain Concepts (cont.),Period (T) amount of time it takes for one repetition of the signal T = 1/f Phase () - measure of the relative position in time within a single period of a signal Wavelength () - distance occupied by a single cycle of the signal Ex: Speed of light is v
5、 = 3x108 m/s. Then the wavelength is f = v (or = vT).,7,Sine Wave Parameters,General sine wave s(t ) = A sin(2ft + ) note: 2 radians = 360 = 1 period Figure 2.3 shows the effect of varying each of the three parameters (a) A = 1, f = 1 Hz, = 0; thus T = 1s (b) Reduced peak amplitude; A=0.5 (c) Increa
6、sed frequency; f = 2, thus T = (d) Phase shift; = /4 radians (45 degrees),Sine Wave Parameters,9,Frequency-Domain Concepts,An electromagnetic signal can be made up of many frequencies. Example: s(t) = (4/p)x(sin(2pft) + (1/3)xsin(2p(3f)t) Fig. 2.4(a) + Fig. 2.4(b) = Fig. 2.4(c) There are two compone
7、nt frequencies: f and 3f. Based on Fourier analysis, any signal is made up of components at various frequencies, in which each component is a sinusoid wave, at different amplitudes, frequencies, and phases.,10,11,Frequency-Domain (cont.),Spectrum - range of frequencies that a signal contains In Fig.
8、 2.4(c), spectrum extends from f to 3f. Absolute bandwidth - width of the spectrum of a signal In Fig. 2.4(c), it is 3f f = 2f. Effective bandwidth A signal may contain many frequencies. But most of the energy may concentrate in a narrow band of frequencies. These frequencies are effective bandwidth
9、.,12,Fundamental frequency when all frequency components of a signal are integer multiples of one frequency, its referred to as the fundamental frequency (earlier example) f and 3f fund. freq = f The period of the total signal is equal to the period of the fundamental frequency. refer to Fig. 2.4 ag
10、ain!,13,Data vs. Signal,Signals - electric or electromagnetic representations of data Data - entities that convey meanings or information Transmission - communication of data by the propagation and processing of signals,14,Approximating Square Wave by Signals,adding a frequency of 5f to Fig. 2.4(c)
11、Fig. 2.5(a) adding a frequency of 7f to Fig. 2.4(c) Fig. 2.5(b) adding all frequencies of 9f, 11f, 13f, . Fig. 2.5(c), a square wave This square wave has an infinite number of frequency components, and thus infinite bandwidth.,15,16,Data Rate vs. Bandwidth,Case I: (Fig. 2.5(a) Let f = 106 cycles/sec
12、 = 1 MHz frequency components: 1f, 3f, 5f absolute bandwidth = 5f 1f = 4f = 4 MHz data rate = 2 Mbps (1 bit per 0.5 us) Case II: (Fig. 2.5(a) Let f = 2x106 cycles/sec = 2 MHz frequency components: 1f, 3f, 5f absolute bandwidth = 10M 2M = 8 MHz data rate = 4 Mbps (1 bit per 1/4 us),17,Case III: (Fig.
13、 2.4(c) Let f = 2x106 cycles/sec = 2 MHz frequencies: 1f, 3f absolute bandwidth = 6M 2M = 4 MHz data rate = 4 Mbps (1 bit per 1/4 us),Analog and Digital Signal Conversion: Examples,19,Some Terms about Channel Capacity,Data rate - rate at which data can be communicated (bps) Bandwidth - the bandwidth
14、 of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) Noise Channel Capacity the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions Error rate - rate at which errors occur,20,Nyqui
15、st Bandwidth,Given a bandwidth of B, the highest signal transmission rate is 2B: C = 2B Ex: B=3100 Hz; C=6200 bps With multilevel signaling C = 2B log2 M, where M is the number of discrete signal or voltage levels,21,Signal-to-Noise Ratio,Signal-to-noise ratio (SNR) = power of signal/power of noise
16、typically measured at a receiver Signal-to-noise ratio (in db)A high SNR means a high-quality signal.,22,Shannon Capacity Formula,The max. channel capacity:note: SNR not in db. In practice, only much lower rates are achieved Formula assumes white noise (thermal noise) Impulse noise is not accounted
17、for Attenuation distortion or delay distortion not accounted for,23,Classifications of Transmission Media,Transmission Medium Physical path between transmitter and receiver Guided Media Waves are guided along a solid medium E.g., copper twisted pair, copper coaxial cable, optical fiber Unguided Medi
18、a Provides means of transmission but does not guide electromagnetic signals Usually referred to as wireless transmission E.g., atmosphere, outer space,24,General Frequency Ranges,Microwave frequency range 1 GHz to 40 GHz Directional beams possible Suitable for long-distance, point-to-point transmiss
19、ion Used for satellite communications Radio frequency range 30 MHz to 1 GHz Suitable for omnidirectional applications Infrared frequency range Roughly, 3x1011 to 2x1014 Hz Useful in local point-to-point multipoint applications within confined areas,25,Multiplexing Techniques,Time-division multiplexi
20、ng (TDM) Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signalFrequency-division multiplexing (FDM) Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal,26,Summary,signal analog vs. digital transmissions channel capacity transmission media TDM/FDM,
