Simulation of Outdoor Radio Channel Peter Brída, Ján Dúha Department of Telecommunication, University of Žilina Univerzitná 8215/1, 010 26 Žilina Email:
[email protected],
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Abstract Wireless communication is one of the most demanding applications for the telecommunications equipment designer. The mobile radio channel defines fundamental limitations on the performance of wireless communications systems. In this paper is presented mobile radio channel situated in different environments – rural and urban. Some of the most important factors are multipath propagation and shadowing.
1. Introduction An understanding of radio propagation is essential to the complete wireless professional because the loss in signal caused by propagation limits the received signal strength, impacting on the quality of the received signal [1]. To simplify the analysis of radio propagation, we generally consider the received signal strength to be a composite of three discrete effects known as path loss (Large Scale Propagation), slow fading (Medium Scale Propagation), and fast fading (Small Scale Propagation). Although such characterization does not exactly reflect reality, it has proved to be sufficiently useful and accurate to model mobile radio systems and is in widespread use to date. Each of these
separate elements will be examined in detail.
2. The Mobile Radio Channel Radio channel is generally hostile in nature. It is very difficult to predict its behavior. Therefore, radio channel is modeled in a statistical way using real propagation measurement data. In general, the signal fading in a radio environment between a transmitter and receiver can be decomposed into a largescale path loss component together with a medium-scale slow-varying component (having a lognormal distribution) and a small-scale fast varying component (with Rician or Rayleigh distribution).
2.1. Large Scale Propagation Model Path loss is the simplest of all the propagation mechanisms to understand and reflects the fact that the signal drops as the distance from the transmitter increases. Well known models for path loss are Hata’s, Lee’s, COST231 model etc. Hata [2, 3] developed a useful model for path loss in macrocells on the experimental results of Okumura [2]. The model expresses the path loss as a function of BS height, MS height, carrier frequency and the type of environment (urban, suburban or rural). In Hata’s
model, the path loss is expressed as [in dB] L PL (Urban ) = 69 ,55 + 26 ,16 log( f ) − 13,82 log (hb ) − a (hm )
,
(1)
+ (44 ,9 − 6 ,55 log(hb )) log (d )
a(hm ) = (1,1log( f ) − 0,7)hm − 1,56 log( f ) + 0,8
,
Figure 1. A mobile radio signal fading representation [2].
LPL (Rural ) = LPL (Urban ) − 4,78(log( f )) + 18,33 log( f ) , (2) − 40,94 2
d is distance between BS and MS in km, hb is height of BS (transmitter) and hm is height of MS (receiver), both in m [2]. The range of parameters over Hata model is valid under the following conditions
150 ≤ f [MHz ] ≤ 1500 30 ≤ hb [m] ≤ 200 . 1 ≤ hm [m] ≤ 10
(3)
2.2. Medium and Small Scale Propagation Model Mobile radio envelope r(t), illustrated in Fig. 1, is composed of m(t) and r(t) s (t ) = m(t ).r (t ) .
(4)
The component m(t) is called long-term fading or lognormal fading and its variation is due to terrain contour between base station and mobile station. It represents a slow variation in the mean envelope over a distance. Medium-scale variations take on Gaussian statistics when the signal is expressed in dB. The Gaussian or normal density function takes the form ⎡ (a − μ )2 ⎤ 1 exp ⎢− (5) p(a ) = ⎥, 2σ 2 ⎦ 2πσ 2 ⎣ where a is signal level [dB], σ2 is variance of the signal distribution [dB], and μ is the mean signal level stated [dB]. Small-scale propagation models are characterized by the fast variation of received signal strength over a short distance on the order of a few wavelengths or over short time durations on the order of seconds. The second component represents short-term fading or multipath fading. Variation of this fading is due to wave reflection from the surrounding buildings and other structures [3], [4].
2.2.3. Rayleigh channel Channel with a large number of paths can be modeled as a Rayleigh channel. The signal amplitude is Rayleigh distribution in a case of large number of obstacles and there are dominant reflected waves (NLoS environment – Non Line of Sight).
This is a case of LoS (Line of Sight) propagation path. The LoS propagation model is given by the Rician distribution [4] ⎧ a 2 + s 2 ⎫ ⎛ a.s ⎞ a p(a ) = 2 exp⎨− ⎟ , (7) ⎬I 0 ⎜ σ 2.σ 2 ⎭ ⎝ σ 2 ⎠ ⎩ where I0(.) is modified Bessel function of the first kind and zeroth order, s is the peak value of the specular radio signal, variable a is signal amplitude and σ2 is variance of Rician distribution.
(
)
Figure 3. Propagation model.
Figure 2. LoS and NLoS examples propagation situation.
NLoS propagation always exists in cities or built-up environments. Rayleigh distribution is defined p (a ) =
⎧ a2 ⎫ , exp ⎨− 2 ⎬ σ2 ⎩ 2.σ ⎭ a
(6)
where a is signal amplitude and σ2 is variance of the signal distribution [dB2]. 2.2.3. Rician channel In a situation where the distance between the transmitter and the receiver is small and the environment is static, there is a fixed spatial pattern of maxima and minima. Mostly there is a dominant stationary (nonfanding) signal component.
Rx = Tx − (LLS + LMS + LSS ) ,
(8)
where Rx is received signal strength, Tx is transmitted signal strength. Following parameters influence signal attenuation: LLS is signal degradation, caused by large-scale propagation, LMS is signal degradation, caused by mediumscale propagation and finally LSS is signal degradation, caused by small-scale propagation. All of the parameters are in [dB].
3. Simulation results Simulations are done in two cases. In the first case is assumed urban environment. In the second case is assumed rural environment. These conditions are important for correct choosing environment properties.
Table 1. Mean and variance of signal particular parts for different environments. Environment Urban Rural μ σ μ σ Parameter [dB] [dB] [dB] [dB] Shadowing 0 6 0 1 Rayleigh -2 6 Rician -1 3
3.1. Urban Environment The signal can be decomposed to particular parts – long-term fading (path loss and shadowing) and short-term fading. We assume for urban environment following conditions, large-scale propagation (path loss) is according to equation (1), medium-scale is according to (5) with parameters, which is shown in table 1, and for small-scale propagation is assumed Rayleigh channel, which parameters is shown in table 1.
Figure 5. Shadowing in urban environment.
Figure 6. Short-term (Rayleigh) fading in urban environment.
Figure 4. Path Loss in urban environment.
Figure 7. Signal strength of mobile radio signal in urban environment.
3.2. Rural Environment In rural environment is possible decompose the signal to particular parts – long-term fading (path loss and
shadowing) and short-term fading as in the case of urban environment. The differences are only in way of definition of particular parts of signal i.e. given model for path loss, mean μ [dB] and standard deviation σ [dB]. We assume for rural environment following conditions, largescale propagation (path loss) is according to equation (2), medium-scale is according to (5) with parameters, which is shown in table 1, and for small-scale propagation is assumed Rician channel, which parameters is shown in table 1.
Figure 8. Path Loss in rural environment.
Figure 10. Short-term (Rician) fading in rural environment.
Figure 11. Signal strength of mobile radio signal in rural environment.
4. Conclusion
Figure 9. Shadowing in rural environment.
At the previous figures (4 - 11) are shown particular parts of radio signal. Comparison of signal attenuation characteristics in the urban and rural environment is very interesting (fig. 4. and fig. 8). Basic path loss difference between urban and rural environment is that in the urban environment is higher level of attenuation, which is caused by bigger number of obstructions. Another significant part of signal attenuation is shadowing. You can see characteristics for both kinds of observed environment at the fig. 5 and fig. 9. Level of shadowing in the urban environment is approximately
three times higher than in the rural environment. Fig. 6 and fig. 10 show the difference of short-term fading between urban and rural environment that were simulated with different models. In the urban environment is the channel in deep fading and it means that the signal amplitude is significantly attenuated. In the urban environment is higher level of fading, than in the rural environments and the fading is deeper. Fig. 7 and fig. 10 depicts comparison of the complete signals.
5. Acknowledgment Research described in the paper was supported by the state programme of Slovak republic, No. 2003 SP 51/028 09 00/028 09 10 “Communication Networks and Services of New Generations”. This paper was supported by the grant VEGA - 1/0140/03 “Effective radio resources management methods in next generations of mobile communication networks”.
References: [1]WEBB, W. The Complete Wireless Communications Professional: A Guide for Engineers and Manageres, Artech House, Norwood 1999, ISBN: 0-89006-338-9 [2] HATA, M. Empirical formula for propagation loss in land mobile radio services, IEEE Transaction on Vehicular Technology, vol. VT-29, no. 3,pp. 317-325, August 1980 [3] RAPPAPORT, T. S. Wireless Communications: Principles and Practise, Prentice Hall PTR, 1996 [4] SIWIAK, K. Radiowave Propagation and Antennas for Personal Communications 2nd ed. Artech House, 1998
