Digital communication technology and instrument measurement technology

　　With the advent of the digital wireless communication era, some of the communication equipment used in daily life have gradually begun to be digitized.

　　Digital Communication Technology and Applications


　　Nowadays, various communication systems can basically be distinguished and classified by "modulation mode":


　　1. MSK/GMSK: used by European cellular mobile phones GSM and mobile data transmission CDPD of AMPS.


　　2. BPSK: used by Cable Modem.


　　3. QPSK/DQPSK: suitable for satellite communication, narrowband CDMA in the United States, North American digital cellular system NADC, PDC and PHS in Japan.


　　4. OQPSK: used by narrowband CDMA and satellite communication in the United States.


　　5. FSK/GFSK: used by European digital cordless phones DECT, digital paging systems POCSAG/FLEX, ERMES.


　　6. 8/16 VSB: used by North American digital television (ATV), broadcasting and digital cable television.


　　7. 16QAM: used by microwave digital communication, modems, DVB-C, and DVB-T.


　　Generally speaking, "communication" is a mature technology, but this usually refers to analog communication, because the revolution of digital communication is just in the ascendant. First of all, the difference between analog communication and digital communication is:


· Analog communication does not make any changes to the audio and voice from beginning to end, while digital communication digitizes the analog audio and voice signals from the beginning. Therefore, digital modulation can not only transmit audio and voice signals, but also transmit data communication;


· Secondly, there are only three modulation technologies for analog communication, namely amplitude modulation, frequency modulation and phase modulation, while in digital modulation, there is only I-Q modulation.


· Especially in the technology of radio frequency transmission, since analog signals are inseparable, but digital signals are a very simple thing, the technology for analog communication to process radio frequency transmission is only frequency division multiple access (FDMA)/frequency division duplex (FDD); in digital communication, there are many technologies to choose from, such as frequency division multiple access, time division multiple access or code division multiple access, and in terms of transceiver duplex, in addition to FDD, TDD duplex technology can also be used.



Instrument measurement technology and application


　　From the above analysis and comparison, we can find that analog communication and digital communication are compared:


· In terms of baseband, it is a sine wave to a digital bit;


· In terms of modulation, it is AM/FM/PM to IQ modulation;


· In terms of RF transmission, it is a continuous wave to a time-varying signal.


Therefore, in analog communication, the sensitivity is measured, but in digital communication, it becomes a bit error rate BER or a frame error rate FER; the modulation parameters in analog communication are modulation coefficient, modulation frequency deviation, etc., while in digital modulation, it is EVM, I-Q, I-Q deviation, etc.; in analog communication, the RF measurement measures carrier frequency, transmission power, bandwidth, etc., while in digital communication, some RF measurement parameters are the same as those in analog communication, and there are more time-varying parameters, such as carrier ON/OFF ratio, adjacent channel power, etc.


　　Therefore, in terms of baseband and modulation measurement parameters, the difference between analog modulation communication and digital modulation communication is very large, so the measurement instruments used can be said to be completely different. However, instruments that can usually demodulate digital signals can also cope with the needs of analog demodulation. As for the measurement parameters of RF, some of them overlap, so the RF parameter measurement instruments used for analog communication can usually be extended to the RF measurement work of digital communication. The most typical one is the spectrum analyzer, which can basically cover all the functions. If it is properly "changed", it can also measure the relevant RF parameters of digital communication. With different specific software, a spectrum analyzer can become a test instrument for GSM, DECT, NADC, CDMA, etc. It can not only measure RF tests, but also demodulate I-Q signals.


　　Usually, spectrum analyzers are loaded with appropriate software and are more suitable for use in test stations on production lines or QA-related departments. As for research and development, higher-performance measuring instruments should be selected. Digital communication measuring instruments suitable for research and development can be divided into two parts: transmitting and receiving. Whether it is an electronic instrument with transmitting or receiving analysis and demodulation, it is best to have the ability to expand functions. For example, if the GSM system is currently being developed, the measuring instruments purchased should not only consider GSM measurements, but also analyzers and signal generators that can cope with narrowband CDMA, even broadband CDMA, and digital video in the future.


　　Usually, digital communication instruments in the R&D stage can be divided into five parts, including baseband signal processing and modulation, RF transmission, RF reception, demodulation and analysis, and communication protocols. This corresponds to an electronic instrument that has a baseband modulation signal generator with multiple modulation modes, plus a RF signal generator with high-performance I-Q modulation and up to 3 to 4 GHz; in terms of reception, an instrument usually has the demodulation function of various digital modulation signals, the ability to trigger signals, the analysis capabilities of time domain, frequency domain and modulation domain, RF reception, demodulation analysis and other functions.