In previous sections, some wireless communication problems were discussed. For example, effects appearing on the receiving side in multipath propagation (see 2. Radio signal propagation fundamentals). In this section, the list of wireless link possible problems will be expanded.
When the signal is reflected during propagation, its polarization may change and mismatch the receiving antenna orientation. In Figure 1 is shown the wireless link operating according to the MIMO scheme: during of propagation, signals of each polarizations are reflected from the buildings, and change it's polarization. There are two effects on the receiving side: since the reflected signals has same polarization, they interfere; since the orientation of receiver antennas mismatchs the received signal polarization, the RSSI will be lower than the calculated values.
Figure 1 The polarization shift during the signal propagation
An important link quality indicator is the received SNR value, if the transmitter power increases, the SNR on the receiving side increases too. However, this solution has several disadvantages: the transmitter power is regulated by the state, the transmitter operation at increased power reduces its service life, the out-of-band radiation level increases in accordance with transmitted power. The out-of-band radiation level is determined by the transmitting device spectral mask (see. 1. Radio waves main characteristics) and a radiation power increasement will affect the operation of systems in adjacent channels:
Figure 2 Co-channel interference: a - before increasing the adjacent channel signal power, b - after increasing the adjacent channel signal power
In figure 2(a) are shown spectra of two frequency-adjacent communication channels. The orange color indicates adjacent channel out-of-band emission, which interfere to the received signal. With increasing the signal power in the adjacent channel, the interference level will increase - Figure 2(b).
Hidden node problem
When using the multiple access method CSMA/CA (see 5. Multiple Access Methods) hidden node problem appears. In Figure 3 is shown a diagram of a wireless network operating on CSMA / CA, in which there is one sector with a circular radiation pattern and two subscribers. In accordance with the algorithm, each subscriber scans the air before transmission and, in case it is free, starts data transfer. So, subscriber 1, after preliminary listening to the air, starts the transfer. Due to significant distance subscriber 2 can't receive signals from subscriber 1. In this case, subscriber 2, after scanning the air and not detecting the subscriber 1 signal, starts the transfer. The sector will receive simultaneously signals from two subscriber devices, which will cause a collision.
Figure 3 Hidden node problem
The "bad" subscriber problem
In wireless communication systems with a large number of subscribers, a situation in which the quality indicators of all client devices will be the same is impossible: the signal levels, the signal-to-noise ratio and the number of retries will differ. These parameters will determine the modulation level for each subscriber, which indirectly affects the performance of the whole wireless communication system.
Fow example three subscribers are connected to the sector and one of them is "bad", connected in a high noise environment, so the subscriber works at low modulation and with a high retries percentage. In Figure 4 is shown the time diagram of the subscribers and sector interaction: subscribers 1 and 2 use high modulation, so they manage to transfer data quickly enough and occupy a small part of the sector total time, while the third subscriber needs more time to transfer their data, which reduces the overall system performance.
Figure 4 The "bad" subscriber problem in a polling system