First, why would decline
Compared to a wired channel, a wireless channel is more severe: the mobile station can either be between city buildings, may also be in complex terrain area mountains, forests and oceans, due to fading and mobility of a mobile station, a radio wave characteristics is more complex, with a lot of randomness. In addition to the direct radio wave, each encounters an obstacle, will depending on the relative size of the obstacle and own wavelength is reflected thereby, diffraction, scattering, which will have a certain energy loss of radio waves, resulting in a fading signal is generated.
To put it simply, is the signal will encounter a lot of ups and downs (trees, buildings, mountains, etc.) in the actual channel transmission, each will be a little rough on the heart signal itself cause some damage.
Second, the decline classification
Fading classification is divided according to the distance of radio wave propagation and the magnitude relationship between wave length
The interval size of a received signal field intensity changes, the radio waves can be divided into large-scale fading, fading mesoscale and small-scale fading. Here the interval size, which means that the distance and signals we have observed the spread of wavelengths relationship itself,That is, to look at this interval contains a total number of wavelengths。
2.1 Large-scale fading
Large-scale fading is described transmitter and a receiver over long distances (several hundred or several thousand meters, is certainly a lot wavelength) signal strength or a change in a long time range.
2.2 scale fading
Scale fading often used to describe the transmitter and the receiver medium distance (hundreds of wavelength signal strength variation within) range.
2.3 small-scale fading
Small-scale fading is described small scale interval (several or several tens of wavelengths) signal strength changes rapidly or within a short time (in seconds).That is, look at the signal propagation distance is very short period of rapid fluctuations in the signal, this is very short distance and the wavelength of the signal must be is an order of magnitude.
In general, radio wave fading can also be divided into large-scale and small-scale fading fading, while the scale fading is considered to be a large-scale fading.
Then carefully tell you the large-scale fading and small scale fading.
Third, the large-scale fading
Including large-scale path loss and shadow fading are two types of fading.
3.1 path loss
Path loss means the loss introduced in an amount of propagation environment between the base station and the mobile station. It has a power law propagation characteristics, irrespective of some variation that is inversely proportional to the power level of the signal from the increased length, the receiving antenna directivity and other parameters,Only the relevant transmission path, the longer the path, the path loss is greater. The average mean path loss refers to loss in some distance in the path, the distance generally can be used for path loss model number denoted.
3.2 Shadow fading
Radio wave blocking obstacle encountered in rugged terrain, buildings and trees in the propagation path, behind an obstacle shadow zone formed waves. Shaded area signal field strength is weak, when the mobile station passes through the shaded area in the movement, will cause slowly varying received signal strength value field, usually called this shadow effect. Statistical laws have proven shadow fading approximate lognormal distribution.
Since the shadow fading is superimposed on the path loss, shadow fading and the lognormal distribution, therefore, the path loss of a point a certain distance from the transmitter, it is a normally distributed random variable.
Fourth, small-scale fading (very important !!!)
4.1 What caused small-scale fading
4.1.1 multipath propagation (multipath)
Since the radio wave propagation process will be a variety of reflection, diffraction and scattering of incident wave propagation from different directions with different propagation delays, which leads to signals arriving at the mobile station by radio waves of many synthetic route. The random distribution of amplitude, phase and incident angle of multipath components of the vector are combined by the receiver antenna into amplitude and phase of the signal changes sharply, so that the reception signal fading distortion, this fading due to multipath propagation is called a multi- by fading, it is a small-scale fading.
For an extreme example, if after two paths only a single frequency sinusoidal signal reaches the receiving end, and if the two paths of the received sinusoidal signals is just a half wavelength phase difference (i.e. a phase difference of 180 °), then the receiving end Synthesis , the two signals will be exactly offset, and directly to zero, and this is the result of multipath propagation caused. (Of course, this is just a very extreme case, the reality is not so terrible)
4.1.2 mobile station moving speed (Doppler effect)
Relative motion of the base station and the mobile station causes multipath components have different Doppler shift, causing random frequency modulation.
4.1.3 Environment object moving speed (Doppler effect)
If the wireless channel of the object is in motion, the Doppler shift change time can cause. If the environment is greater than the speed of the moving object at the mobile station, then the small-scale fading such movement would play a decisive role. Otherwise, the mobile station can be considered only affect the speed of movement, while ignoring the environmental movement speed of the object.
4.1.4 signal transmission bandwidth
If the transmission bandwidth of the wireless signal is greater than the bandwidth of the multipath channel, the received signal will be distorted, but the local receiver signal strength not decline much (i.e., small-scale fading is not dominant). With respect to the cable channel for, if a transmission signal is a narrowband signal, the amplitude of the signal changes rapidly, but the signal does not appear temporal distortion.
4.2 Small-scale fading characteristic parameters
4.2.1 delay spread and correlation bandwidth
Receiving signals by a plurality of independent multipath signals distinguishable composition, resulting in a received signal duration longer than the duration of the transmission signal (because some length of the path, then the time it arrives at the receiver will be longer) , resulting in a temporal dispersion in the time domain, with a delay spread measured; reflected in the frequency domain as a frequency selective fading, measured with the coherence bandwidth.
(1) delay spread : finally reaches the first signal receiver and the arrival time difference between the signal receiver.
(2) the coherence bandwidth : approximately equal to the reciprocal of the delay spread, but in the general case, to accurately determine the influence of multipath signals for a particular channel, it is necessary to use spectral analysis and simulation techniques. When the frequency interval of two frequency components less than the coherence bandwidth of the amplitude thereof has a strong correlation.
4.2.2 Doppler spread and coherence time
Time varying nature of the channel is determined by the relative movement between the mobile station and the base station caused by the channel or path of the object motion induced, its essence is superimposed a plurality of non-resolvable path fluctuation caused by time-domain signal. Determines the frequency characteristics of the time-varying dispersive channel, the Doppler spread and coherence time are described in these two characteristics (dispersion time varying characteristic and frequency) is an important parameter.
(1) Doppler spread : Doppler spread frequency broadening measurements, defined as a single frequency sine wave (non-modulated carrier) transmission spectrum bandwidth . Effect of the received signal is a transmission signal frequency Doppler spread , rather than the maximum of the Doppler shift . (As difference Doppler spread and Doppler shift, the current I also do not understand)
(2) coherence time : the channel coherence time is a measure of time, for varying characteristics of the time domain channel frequency dispersion described. Over the duration of the desired signal response of the channel it is substantially constant, i.e., during this interval, the two arriving signals has a strong correlation between the amplitude (we mentioned above very coherent bandwidth strong amplitude correlation).
4.3 Classification of small-scale fading
Small scale fading characteristics depending on the type of transmission signal (signal bandwidth and symbol period) and the channel characteristics (delay spread and Doppler spread). The relationship between the signal parameters and channel parameters determine the different transmitted signals will experience different fading characteristics. The delay spread of the channel, the channel can be divided into a flat fading channel and frequency selective fading channel; The Doppler spread channel, the channel can be divided into fast and slow fading channel fading channel.
4.3.1 Based fading effect of the multipath delay spread
Multipath time dispersion characteristics due to a number such that the receiver receives the signal pulses of different delays. Corresponds to the frequency domain, the channel transmission signal is filtered, the signal components of different frequencies is not the same magnitude of the fading, resulting in a transmission signal generation flat fading and frequency selective fading.
(1) flat fading : if the mobile radio channel bandwidth larger than that of the transmission signal, and a constant gain and linear phase within the bandwidth range , then the received signal will experience flat fading. In the case of flat fading, multipath channel structure so that the spectral characteristics of the transmitted signal at the receiver remains unchanged. However, due to multipath effects cause structural channel gain fluctuations, the strength of the received signal may vary over time. Typical flat fading channel may lead to deep fades, so a deep fade during a certain constant need to increase transmission power.
(2) frequency selective fading : if bandwidth channel having a constant gain and linear phase is less than the bandwidth of the transmission signal , the channel characteristics may cause the received signal to generate frequency selective fading. In the case of frequency selective fading, the channel impulse response having a plurality of through delay spread, which is greater than the value of the transmission signal period. At this point in the received signal subjected to multipath transmission signal wave attenuation and delay wave, thus distorted received signal, thereby causing inter-symbol interference (ISI).
4.3.2 based on Doppler spread fading effect
Since the relative movement between a mobile station and a base station, or be generated due to the channel path of a moving object, the Doppler spread, causing a channel change over time, resulting in time-varying characteristics of a channel (time selectivity). The comparison signal transmission speed and the degree of channel variation, the channel can be divided into fast and slow fading channel fading channel.
(1) Fast fading : when the coherence time of the channel is shorter than the period of the transmission signal, and the bandwidth of the baseband signal is less than the Doppler spread , the channel impulse response in symbol period changes rapidly, resulting in fast fading signal is generated. As can be seen from the frequency domain, exacerbated by the signal distortion caused by fast fading with increasing Doppler spread transmission signal bandwidth.
(2) slow fading : when the cycle is much greater than the coherence time of the channel of the transmission signal, and the bandwidth of the baseband signal is much greater than the Doppler spread , the channel impulse response variation obtained lower than the signal element to be transmitted cycle more, it may be considered that the channel is a slow fading channel, slow fading channel, it can be considered stable when the channel parameters within one or more signal symbol period.
When the channel is identified as a fast fading or slow fading channel, and accordingly is not recognized as a fading frequency flat fading or selective fading. Only fast fading associated with a channel variation caused by the movement. In the frequency selective fading and fast fading channels, of any amplitude, phase, and time change rate faster than the multipath components of the transmission rate signal. Indeed, fast fading occurs only at a data rate is very low.
4.4 Small-scale fading statistical characteristics of the channel envelope
A typical small-scale fading distribution function description has Rayleigh distribution and Rice distribution . Rayleigh distribution for the statistical properties of the envelope line of sight multipath component independently propagation (LOS) between the absence of description the transceiver; Rice distribution is based on the Rayleigh distribution, coupled with a direct path the impact of the decline caused by type.
Note: This article is a reference to most of the textbooks, then present, some specific knowledge I do not really understand the future needs to be further study, now to know something about the framework on the line.
Reference books are: Principles and mobile communication system (by the Beijing Jiaotong University Li Xu, Ai Bo, Zhang Zhong team eds., Science Publishing House)
