"Road" in wireless networks-wireless channels (1)

A wireless channel is a "network" in a wireless network ". That is, the path of transmission. If the roads are uneven, how can the wireless network be unobstructed? This article will give you a detailed description of what the wireless channels are in the wireless network.

Wireless Channel:

A channel is an image metaphor for the channel between the sending end and the receiving end in wireless communication. For radio waves, it is transmitted from the sending end to the receiving end, and there is no tangible connection between them, there may be more than one transmission path. However, to visually describe the work between the sender and the receiver, we imagine there is an invisible link between the two, call this connection channel a channel. The channel has a certain frequency bandwidth, just as the road has a certain width.

The propagation of electrical waves in wireless channels is not a single path, but the synthesis of reflection waves from many paths. Because the distance of the radio wave through each path is different, the arrival time of the reflected wave in each path is different, that is, the delay of each signal is different. When the sender sends a very narrow pulse signal, the signal received by the Mobile Station is composed of many pulses with different latencies, which are called latency extension.

At the same time, because the arrival time of the reflected wave in each path is different, the phase is also different. Multiple signals of different phases are superimposed on the receiving end, sometimes overlapping while enhancing in the same direction), sometimes overlapping and weakening in the opposite direction ). In this way, the amplitude of the received signal will change dramatically, that is, rapid decline. This kind of fading is caused by multiple paths, so it is called multi-path fading.

In addition, in addition to the instantaneous fading of the received signal, the average value of the field strength also changes slowly. It is mainly caused by changes in regional locations and meteorological conditions. As a result, the refraction propagation of the radio waves changes with time, and the delay of the signal that transmits the multi-path transmission to a fixed receiving point changes accordingly. This kind of signal change caused by shadow effect and meteorological reasons is called slow fading.

In addition, due to the mobility of mobile devices in mobile communications, as mentioned above, there will also be a Doppler Effect in wireless channels. In mobile communication, when the mobile station moves to the base station, the frequency increases, and when it is far from the base station, the frequency decreases. We must fully consider the Doppler effect in mobile communication ". Although, due to the limitation of our mobile speed in daily life, it is impossible to bring a very large frequency offset, this will inevitably affect mobile communication, in order to avoid this impact, we have to consider it technically. It also increases the complexity of mobile communication.

To sum up, wireless channels include the multi-path propagation, latency expansion, fading characteristics, and Doppler Effect of radio waves. in mobile communication, we must fully consider these features and solutions.

For wireless network installation, the management tool provided by wireless network devices is generally used to set the connection parameters. The most important configuration items for any wireless network include the network mode centralized or peer-to-peer Wireless Network), SSID, channel, transmission rate, but some wireless device drivers or settings software to simplify these steps, generally use the default settings that do not require any settings) it is easy to use wireless networks. However, many problems may arise due to the pursuit of convenience. As you know, the commonly used IEEE 802.11b/g works in 2.4 ~ 2.4835GHz frequencies, which are divided into 11 or 13 channels. When there are more than two aps in the Wireless AP coverage range, you need to set different frequency bands for each AP to avoid conflicts between the shared wireless channels. The default setting of wireless devices used by many users is Channel 1. Conflicts between two or more Wireless AP devices are inevitable.

As we all know, the mainstream wireless protocols are all developed by the IEEE American electrical and electrical Association. Among the three wireless standards recognized by IEEE, IEEE802.11b, IEEE802.11g, and IEEE802.11a, the number of channels is different.