Radio channel is a random time-specific channel, whose fading feature will reduce the performance of the communication system. There are several measures to avoid fading, such as channel coding/decoding technology, anti-fading receive technology or spread spectrum technology. Diversity reception technology is considered as an effective and economical anti-fading technology.
As we all know, the signal received in radio channel is an combination of multi-path components arriving at the receiver. If the signals of different paths obtained at the receive end at the same time can be combined into the whole receive signal properly, the effect of fading will be reduced. That is how diversity is designed. Literally, the meaning of diversity is to obtain compound signals separately and then combine them. The signals, if statistically independent, can be combined such that the performance of the system can be improved greatly.
Those received signals, completely or almost independent of each other, can be obtained by the following receiving means: different paths, different frequencies, different angles and different polarizations.
1) Space Diversity: set several antennas upon the receive end or the transmit end, and leave enough space (generally exceeding 10 signal wavelength) between each two antennas to ensure the signals sent/received in each antenna are mutually independent. Figure 4-7 is an example of dual-antenna transmit diversity to improve receive signals. Thanks to dual-antenna transmit diversity, the independent receive paths obtained by the receiver are increased and accordingly combination effect is gained:
The two antennas transmit different data: antenna 1 transmits the data in even location while antenna 2 transmits the data in odd location. Owing to the irrelevant transmit data, the data arriving at receiver antenna via different antenna paths have corresponding diversity, and accordingly the power of data transmission can be reduced. In the meantime, the reliability of data transmission is greatly improved due to lower bit rate of single antenna transmit data. Therefore, transmit diversity can increase the data transmission speed of the system.
2) Polarization Diversity: receive horizontal polarized wave and perpendicular polarized wave respectively.
Time Diversity: Another way of diversity is to combine irrelevant signals transmitted asynchronously. Frequency Diversity: The same information is transmitted with several different carrier frequencies. If frequency difference interval of each carrier frequency is so large that it exceeds channel relevant bandwidth, the signals transmitted with carrier frequencies are irrelevant of each other. Angle Diversity: Signals are not related because antennal beams point to different direction. For example, set several irradiators on the microwave antenna to generate little-related beams. The diversities can be combined in practice because they are not mutually repulsive.
We can adopt different ways to combine diversity signals:
1) Selective Combination (SC): Select the signal with the best S/N from several discrete ones as the receive signal.
2) Equal Gain Combination (EGC): Combine several discrete signals by the same branch gain and take the combined signal as the receive signal.
3) Maximum Ratio Combination (MRC): Control each combined branch gain to make them in proportion to the S/N of the existing branch, and then combine them to get receive signal.
The above ways are different between the diversity gains in improving the combined S/N. Generally, diversity reception is effective to improve the effect of radio channel reception.
As the diversity number K increases, the improvement of SC is not ideal, while that of EGC and MRC is better, whose difference is only about 1 dB.