无线通信基础(双语)_教学课件_6.ppt

1、Review of Last Class,Log-Term Fading,Review of Last Class,Radio Cell Coverage,Without shadowing With shadowing,Review of Last Class,Okumura Model,5、 Indoor/Outdoor Path Loss Model,Okumura Model Okumura-Hata Path Loss Model COST 231 Model Lees Path Loss Model,Empirical formulation to match Okumura mo

2、del Suitable for large cell,5、 Indoor/Outdoor Path Loss Model,Okumura-Hata Path Loss Model,The path loss is represented as a function of The carrier frequency, 150, 1500MHz Antenna heights of base station and mobile station, hb30, 200m, hm1, 10m The distance between the base station and mobile stati

3、on, 1, 20Km,5、 Indoor/Outdoor Path Loss Model,Okumura-Hata Path Loss Model,The path loss in dB is given by,where,5、 Indoor/Outdoor Path Loss Model,Okumura-Hata Path Loss Model,a(hm) is the correction factor for mobile antenna height, and is given by,For a small and medium city,For a large city,5、 In

4、door/Outdoor Path Loss Model,Okumura-Hata Path Loss Model,fc=900MHz hb=50m hm=3m,5、 Indoor/Outdoor Path Loss Model,Okumura-Hata Path Loss Model,The Hata model was extended by the European cooperative for scientific and technical research (EURO-COST) to 2GHz,Where a(hr) is the same correction factor

5、as before and CM is 0 dB for medium sized cities and suburbs and 3dB for metropolitan areas.,5、 Indoor/Outdoor Path Loss Model,COST 231 Model,The COST 231 model is restricted to the following range of parameters: The carrier frequency, 1.5, 2GHz Antenna heights of base station and mobile station, hb

6、30, 200m, hm1, 10m The distance between the base station and mobile station, 1, 20Km,5、 Indoor/Outdoor Path Loss Model,COST 231 Model,Lees model can be used to predict area-to-area path loss. The model consists of two parts: Path loss prediction for a specified set of conditions Adjustment factors f

7、or a set of conditions different from the specified one The model requires two parameters: The power at a 1.6km (1 mile) of interception P0 in dBm The path-loss exponent k.,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,The specified set of conditions is as follows: Carry frequency fc = 900M

8、Hz Base station antenna height = 30.48 m(100 ft) Base station power at the antenna = 10 W Base station antenna gain = 6 dB above dipole gain Mobile station antenna height = 3 m (10 ft) Mobile station antenna gain = 0 dB above dipole gain,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,5、 Indo

9、or/Outdoor Path Loss Model,Lees Path Loss Model,The received signal power in dBm is represented by:,Where d0=1.6km, d(d0) is the distance between the mobile station and the base station in km, and n is a constant between 2 and 3 dependent on the geographical locations and the operating frequency ran

10、ges. n=2 is recommended for a suburban or open area with f 450MHz.,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,The parameter a0(dB) is an adjustment factor for a different set of conditions:,Where,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,The parameter a0(dB) is an adjustment

11、 factor for a different set of conditions:,Where,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,The value v in a2 is obtained from empirical data and is given by: A 2dB signal gain is provided by an actual 4dB gain antenna at the mobile unit in a suburban area, and less than 1dB gain receive

12、d from the same antenna in an urban area for adjusting a5.,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,fc=900MHz hb=50m Antenna gain=6dB Transmitter power=10W hm=3m,5、 Indoor/Outdoor Path Loss Model,Lees Path Loss Model,Review of Sect

13、ion 2.4,Free-space propagation Two-ray ground reflection model Log-distance path loss model Log-normal path loss model Okumura Hata model, Lees model,Received power reduces with propagation distance and terrain characteristics models,Keep in mind when studying section 2.4: The channel model is indep

14、endent of signals, symbol rate, modulation, etc.,Channel characterized statistically by the median path loss and lognormal shadowing, varies relatively slowly with time. Small-scale fading: Due to the local environment, signal variability on a scale of , have been characterized statistically as inst

15、antaneously received signal level variations with the local average.,Shadowing or lognormal fading,Time delay spread Time-varying,In this section, we made the assumption that the channel was linear, according to this assumption, all distortions can be characterized by the attenuation or superpositio

16、n of different signals. In addition, we allow the possibility that the propagation channel may be time varying. As a consequence of these assumptions, the channel can be presented by a dual time time-varying impulse response, defined as Linear time-variant channel.,Chapter 2 - Lecture 5,Linear time-

17、variant channel model,2.2 Linear time-variant channel model,2.2.2 Time-Variant Transfer Function,2.2.4 Example on the Channel Functions,2.2.1 Channel Impulse Response,2.2.3 Doppler Spread Function and Delay-Doppler spread function,2.2 Linear time-variant channel model,1、 Channel Impulse Response,Bas

18、eband Equivalent Channel Model Linear Time-Variant (LTV) Channel,1、 Channel Impulse Response,Consider a multipath propagation environment with N distinct scatters. The path associated with the nth distinct scatter is characterized by: , represents the amplitude fluctuation by the scatter at time t ,

19、 associated propagation delay,Baseband Equivalent Channel Model,Consider a narrowband signal transmitted over the wireless channel at a carrier frequency fc, such that,And the received signal at the channel output is,Baseband Equivalent Channel Model,1、 Channel Impulse Response,1、 Channel Impulse Re

20、sponse,Baseband Equivalent Channel Model,1、 Channel Impulse Response,Baseband Equivalent Channel Model,The channel can be characterized equivalently by its impulse response at baseband.,1、 Channel Impulse Response,Baseband Equivalent Channel Model,Let us first review the impulse response of a linear

21、 time-invariant (LTI) channel,The time variable t in the impulse response actually represents the propagation delay of the channel,1、 Channel Impulse Response,Linear Time-Variant (LTV) Channel,1、 Channel Impulse Response,Linear Time-Variant (LTV) Channel,Time-varying Discrete Impulse Response Model

22、for multipath channel,1、 Channel Impulse Response,Linear Time-Variant (LTV) Channel,Definition 2.1 The impulse of an LTV channel, h(, t ), is the channel output at t in response to an impulse applied to the channel at t - .,In definition 2.1, the variable represents the propagation delay. Thus, the

23、channel output can be represented in terms of the impulse response and the channel input by,1、 Channel Impulse Response,Linear Time-Variant (LTV) Channel,The Channel impulse response for the channel with N distinct scatters is then,1、 Channel Impulse Response,Linear Time-Variant (LTV) Channel,Exampl

24、e 2.9,1、 Channel Impulse Response,Example 2.10,Same as 2.9, but receiver move at v=10m/s toward path #1 while away from path #2. What is the channel at time t=0.1 s? Carrier is 1 GHz.,1、 Channel Impulse Response,Review of Last Class,Free-space propagation Two-ray ground reflection model Log-distance

25、 path loss model Log-normal path loss model Okumura Hata model, Lees model,Review of Last Class,Linear Time-Variant (LTV) Channel,Channel impulse response,Fourier transform,Channel Transfer Function,h(t),H(f),h(,t),？,2.2 Linear time-variant channel model,2、 Time-Variant Transfer Function,Definition

26、2.2 The time-variant transfer function of an LTV channel is the Fourier transform of the impulse response, h(, t ), with respect to the delay variable .,Where the time variable t can be viewed as a parameter.,2、 Time-Variant Transfer Function,2、 Time-Variant Transfer Function,At any instant, say t =

27、 t0, the transfer function H(f, t0) characterizes the channel in the frequency domain. As the channel changes with t, the frequency domain representation also changes with t. Therefore, we have the channel time-varying transfer function.,2、 Time-Variant Transfer Function,In general, the output signa

28、l of an LTI system does not have frequency components different from those of the input signal On the other hand, both nonlinear and time-varying systems introduce new frequency components other than those existing in the input signal.,Linear Time-Variant Channel,Doppler effect Doppler shifts,2.2 Li

29、near time-variant channel model,3、 Doppler Spread Function and Delay-Doppler spread Function,Doppler Shift in frequency: where v is the moving speed, is the wavelength of carrier.,As a wireless channel can be characterized equivalently in both time and frequency domains, a channel being time varying

30、 in the time domain means a channel introducing Doppler shifts in the frequency domain.,3、 Doppler Spread Function and Delay-Doppler spread Function,Doppler Shift,Assume That the delay spread is negligible as compared with the symbol interval of the transmitted signal. That this mean delay does not

31、change with time The time-variant impulse response of the channel can be approximately described in the form:,Where,3、 Doppler Spread Function and Delay-Doppler spread Function,Spectral broadening,Given that the transmitted signal is x(t), the received signal is,3、 Doppler Spread Function and Delay-

32、Doppler spread Function,Spectral broadening,In frequency domain, the received signal is,Has a finite but nonzero pulse width in the frequency domain,3、 Doppler Spread Function and Delay-Doppler spread Function,Spectral broadening,This means that the channel indeed broadens the transmitted signal spe

33、ctrum by introducing new frequency components, a phenomenon referred to as frequency dispersion.,3、 Doppler Spread Function and Delay-Doppler spread Function,Spectral broadening,Definition: The Doppler spread function is defined by the following function:,where: v is a variable describing the Dopple

34、r shift introduced by the channel. H(f,v) is the channel gain associated with Doppler shift v to the input signal component at frequency f.,3、 Doppler Spread Function and Delay-Doppler spread Function,Doppler Spread Function,Since both the time-variant transfer function H(f,t) and the Doppler spread

35、 function H(f,v) can be used to describe the same channel, there exists a relation between the two channel functions. It can be sown that,Where the frequency variable f can be viewed as a parameter.,3、 Doppler Spread Function and Delay-Doppler spread Function,Doppler Spread Function,The preceding Fo

36、urier transform relation verifies that being time-variant in the time domain can be equivalently described by having Doppler shifts in the frequency domain.,3、 Doppler Spread Function and Delay-Doppler spread Function,Doppler Spread Function,Definition: Delay-Doppler spread function defined as the F

37、ourier transform of the channel impulse response with respect to t, as follows:,3、 Doppler Spread Function and Delay-Doppler spread Function,Delay-Doppler Spread Function,Given the channel input signal x(t), it can be shown that the channel output signal is,3、 Doppler Spread Function and Delay-Doppler spread Function,Delay-Doppler Spread Function,3、 Doppler Spread Function and Delay-Doppler spread Function,Relationships among the channel functions,2.2 Linear time-variant channel model,4、 Example on the Channel Functions,Example 2.11 LTV Channel Model,Consider an LTV cha