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Chapter 7: Digital Bandpass Transmission System
What are the methods of binary keying? What are the corresponding modulation and demodulation methods? What is its bandwidth and noise immunity (bit error rate)? Pros and cons of each?
There are four types of binary keying: 2ASK, 2FSK, 2PSK and 2DPSK, among which 2DPSK is an improvement of 2PSK.
For 2ASK, it belongs to amplitude keying, and the modulation methods include analog multiplication and keying. The analog multiplication method refers to treating the binary as a special analog signal for modulation, and the keying method is to control the amplitude change of the carrier wave according to the binary 1 or 0.
Demodulation methods include envelope detection and coherent demodulation. The former is through an envelope detector (full-wave rectification (diode) + lowpass filter), and the latter is through a multiplier (coherent carrier multiplication) + lowpass filter.
Its bandwidth is twice that of the baseband transmission bandwidth. The advantage of 2ASK is that the equipment is simple and the utilization rate of the frequency band is high. The disadvantage is that the anti-noise performance is poor.
For 2FSK, it belongs to frequency modulation, and the modulation methods include direct frequency modulation method and key switch method. As the name implies, the frequency modulation method refers to modulating a frequency modulator according to a baseband signal, which refers to outputting symbols of two different frequencies. The keying rule is to control the output of two independent carriers with different frequencies. The main difference is that the phase of the frequency modulation method is continuous, but the keying method is not necessarily. The direct frequency modulation method is to directly pass through a frequency tuner, which will not be shown here.
Demodulation methods include non-coherent demodulation (envelope detection and zero-crossing detection) and coherent demodulation. In fact, the coherent demodulation method is to distinguish the carriers of different frequency bands through a band-pass filter, and then perform multiplication and filtering respectively, and the final sampling is two-way comparative sampling.
The envelope detection method in the non-coherent demodulation method is similar to the above, first pass through the band-pass filter, then pass the envelope detection, and finally compare and sample in pairs. The zero-crossing detection method is not described.
Its transmission bandwidth is the difference between the two carrier frequencies + 2 times the baseband bandwidth. The advantage is that it has strong anti-noise performance, but the disadvantage is that it occupies a wide frequency band and the utilization rate of the frequency band is low.
For 2PSK, it belongs to phase modulation, and the modulation methods include multiplication method and keying method. The multiplication method is to convert the baseband signal into a bipolar non-return-to-zero code and multiply it directly with the carrier, so that there will be a phase difference of π to realize the phase modulation; the keying method is to divide the carrier signal into two channels, one channel Add a phase π to the phase, and then use the baseband signal as a switch to control the output.
The demodulation method of 2PSK is a coherent demodulation method, multiplied by the local carrier and passed through a low-pass filter, and finally sampled and judged. Since there are only two phases, phase ambiguity is very likely to occur, causing the final judgment to be reversed with 1 and 0, and the final result is completely reversed, so there is 2DPSK.
The idea of 2DPSK is that the phase difference of two symbols before and after modulation is based on an initial phase, and the difference is made before and after to obtain the relative phase for modulation. The modulation first converts the absolute code into a relative code, and then performs 2PSK modulation;
The demodulation method is composed of phase comparison method and coherent demodulation method. The phase comparison method is to divide the received signal into two channels, one of which is delayed by one symbol time, in order to compare the difference between the current symbol and the previous symbol. Road multiplication, and finally through the low-pass filter and sampling decision. The coherent demodulation method is the same as 2PSK, but an inverse code transformation is added at the end to restore the absolute code.
2PSK and 2DPSK have the same bandwidth as 2ASK, and their anti-noise performance is better than 2ASK and 2FSK, so they are more widely used. Of course DPSK actually uses more.
The following is the bit error rate formula of the binary digital modulation system. Generally speaking, the bit error rate: 2PSK>2FSK>2ASK.
What about multiple system?
The multi-ary system is based on the original binary digital modulation, and the binary is changed into a multi-ary system. The core idea is not much different, but the modulation and demodulation method is more complicated. The multi-ary system also greatly improves the transmission efficiency.