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.
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