Coherent time and bandwidth (20141212) for wireless communication network learning, 20141212 for wireless communication

Coherent time and bandwidth (20141212) for wireless communication network learning, 20141212 for wireless communication

Before learning key LTE technologies, let's take a look at two concepts: coherent time and coherent bandwidth.

Coherent time: The channel maintains a constant maximum time difference. The same signal at the transmitting end reaches the receiving end within the coherent time. The weak signal has similar characteristics. The receiving end considers the same signal.

Time diversity requires that the time interval between two transmissions be greater than the coherent time. Otherwise, the signal will experience the same weakness, and the weak performance of diversity will not exist.

Coherent bandwidth:It describes latency expansion. It means that any two frequency components in a specific frequency range have a strong amplitude correlation, that is, within the coherent bandwidth range, multi-path channels have constant gain and linear phase.

Generally, coherent bandwidth is the reciprocal of multi-path latency. That is, if the maximum multi-path latency of the channel is extended to Tm, the coherent band Bc = 1/Tm of the channel;

From the frequency domain, if the coherent bandwidth is less than the bandwidth of the transmit channel, the channel feature will lead to weak frequency selectivity of the received signal waveform, that is, the increase of some frequency component signals can be enhanced, in addition, the signal increase of some frequency components will be weakened.

The above is a general definition. The following is my understanding (if there is any discrepancy, please click it ):

It can be imagined as two sine waves, assuming that the cycle is T. From the coherent time point of view, when the time interval is an integer multiple of T, the two sine waves are enhanced to the strongest in the amplitude after superposition; when the interval is an integer multiple of T/2, the amplitude is exactly offset.

Similarly, for coherent bandwidth, you can think of the time of the xuanbo as the Emission frequency f, and then refer to the time difference between the two sine waves. When the time difference Tm is an integer multiple of the reciprocal 1/f of the frequency, the amplitude gain is the maximum. So the coherent bandwidth is equal to 1/Tm.

Today, we would like to introduce the knowledge of OFDM. When we encounter coherent time and coherent bandwidth, we have never understood it. Simply put them out and explain them separately. Next, we will start learning key LTE technologies.