Delay spread is defined as the difference between the maximum transmission delay and the minimum transmission delay, that is, the difference between the arrival time of the last distinguishable delay signal and the first delay signal, which is actually the time of pulse broadening. Delay spread is an important index to measure the quality of multipath propagation channel. In digital transmission, due to delay spread, the waveform of one symbol in the received signal will spread to other symbol periods, resulting in inter-symbol crosstalk, so the code redundancy period is greater than the delay spread caused by multipath.

2. Reasons for delayed transmission:

The multipath fading of signals caused by the multipath environment of mobile channels is described from the perspective of time domain: different path lengths lead to different signal arrival times. When the base station sends a pulse signal, the received signal contains not only the signal, but also various delayed signals. This phenomenon that the pulse width of the received signal expands due to multipath effect is called delay spread. The limit of multipath delay of digital signal transmission is one digital signal period, otherwise, waveform broadening will cause intersymbol interference of digital signal.

3. The influence of delay spread on digital mobile communication system;

Delay spread has an effect on digital signal transmission. Generally speaking, delay spread will inevitably lead to the increase of symbol width, which will lead to inter-symbol crosstalk and distort the received waveform. In order to avoid inter-symbol crosstalk, the maximum delay spread Tm must be less than the duration Tb of one symbol, that is

Tm & ltTb or Rb < 1/Tm

Where: Rb= 1/Tb, which is the symbol rate.

However, when the relative delay between two multipath signals exceeds the chip width of the spreading code, the spread spectrum system can combine two separable multipath signals through diversity reception technology, thus improving the quality of the received signal. For example, in IS-95 or the third generation WCDMA mobile communication system, this is how to improve the reception performance.

4. Statistical significance:

Delay spread is a statistical variable, which is closely related to the radio wave propagation environment (time, region, user situation) and is a statistical description of the delay characteristics of multipath channels. The parameters describing the multipath delay phenomenon are: maximum delay spread Tm(xdB), average delay τm and mean square delay spread. Tm is defined as the signal delay when the direct (strongest) signal power drops by xdB (for example, 30dB); τm is the mathematical expectation of each delayed signal; Is the mean square value of each delayed signal. that is

Typical values of delay spread are shown in table 1.

Table 1 Typical value of delay spread