synonymAmplitude keying(Amplitude keying) generally refers to ask modulation
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ASK is also known as "amplitude shift keying" or "amplitude keying", in which 2ASK is also known as "on-off keying" (on-off keying), so it is also recorded as OOK signal.ASK is a relatively simple modulation mode.Amplitude shift keying (ASK) is equivalent to amplitude modulation in analog signals, but it is only a binary number multiplied by the carrier frequency signal.Amplitude shift takes frequency and phase as constants and amplitude as variables. Information bits are transmitted through carrier amplitude.
11Mbps DSSSphysical layeruseComplementKeying (CCK) modulation mode.CCK has the same channel scheme as the existing IEEE802.11 DSSS, and the channel scheme is 2.4GHzISM bandThere are three independent channels that do not interfere with each other, and each channel accounts for about 25MHz.Therefore, CCK has multi-channel operating characteristics.
Quadrature amplitude modulation（QAM）: High modulation efficiency, requiring high signal-to-noise ratio of transmission path, suitable for cable TV transmission.
KeyingPhase-shifting modulation（QPSK）: High modulation efficiency, requiring transmission pathSignal-to-noise ratioLow, suitable for satellite broadcasting.
Coded orthogonal frequency division modulation（COFDM）: Anti multipath propagation effect andCo frequency interferenceGood, suitable for terrestrial broadcasting and co channel network broadcasting.
TDM QPSK modulation system is adopted for the modulation technology of the downlink carrier of the World Wide Digital Satellite Broadcasting System.It is better than codingOrthogonal frequency division multiplexing(COFDM) modulation technology is more suitable for large area coverage of satellites.
The ultimate purpose of communication is to transmit information within a certain distance.althoughbasebandDigital signals can be transmitted directly when the transmission distance is relatively short, but ifLong distance transmissionEspecially in wireless orFibre ChannelWhen uploading, the signal spectrum must be modulated to move to the high frequency before transmission in the channel.In order to transmit the digital signal in the high frequency channel with limited bandwidth, it is necessary to carry out carrier modulation on the digital signal.As with analog signals, there are three basic modulation modes for digital signals: amplitude shift keying (ASK)Frequency shift keying(FSK）AndPhase shift keying(PSK)。They correspond to the amplitudefrequencyAnd phaseDigital baseband signal, which can be regarded as simulationLinear modulationandAngular modulationSpecial circumstances.
theoretically,digital modulation In essence, they are not different from analog modulation. They are all sine wave modulation.However, digital modulation isModulated signalIt is digital sine wave modulation, while analog modulation is continuous sine wave modulation.
In the three modulation modes of digital communication (ASK, FSK, PSK)bandwidth efficiency And anti noise performance (or power utilization). Generally speakingPSKThe system is optimal.Therefore, PSK is widely used in high-speed data transmission.
ASK
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The carrier amplitude varies with the modulation signal.The simplest form is that the carrier isBinaryModulated signalThis mode can also be called on-off keying or switch keying (OOK).
L Modulation method: realized by multiplierModulator。
L Modulation type: 2ASK, MASK.
L Demodulation method: coherent method, non coherent method.
MASK，Also called multi basedigital modulation Law.In binary digital modulation, each symbol can only represent 0 and 1 (+1 or - 1).But in many practicalDigital transmission systemHowever, M-ary digital modulation mode is often used in.Compared with binary digital modulation system, the M-ary digital modulation system has the following two characteristics: First, in the same channel code source modulation, each symbol can carry log2M bit information, so when the channel frequency band is limited, the information transmission rate can be increasedbandwidth efficiency 。But the cost is increasing the signal power and complexity of implementation.Second, at the same information rate, due to theChannel transmission rateIt can be lower than the binary one, so the duration of the M-ary signal code source is wider than the binary one.Widening the symbol width will increase the energy of signal symbols and reduce the impact of inter symbol interference caused by channel characteristics.
Binary 2ASK andQuaternaryComparison of MASK modulation performance:
At the same output power andChannel noiseUnder the condition, the demodulation performance of MASKSignal-to-noise ratioThe rate of deterioration is much faster than OOK.This indicates that MASK applicationSNRThe requirement of OOK is higher than that of ordinary OOK.In the sameChannel transmission rateLower M-level modulation and two-level modulation have the sameSignal bandwidth。That is, when the symbol rate is the same, they have the samePower spectrum。
Although multi-level MASK modulation is an efficient transmission mode, it is generally suitable for constant parameter channels only because of its poor anti noise ability, especially its weak anti fading ability.
FSK is an earlier modulation mode used in information transmission. Its main advantages are that it is easy to implement and has good anti noise and anti attenuation performance.It has been widely used in medium and low speed data transmission.The so-called FSK is to modulate the carrier frequency with digital signals.
L Modulation method: 2FSK can be regarded as two differentcarrier frequency The sum of ASK modulated signals of.
L Demodulation methods: coherent method and non coherent method.
In addition to the above three basic modulation methods, with the development of high-capacity and long-distance digital communication technology, some new problems have emerged, mainly the bandwidth limitation of the channel and the impact of nonlinearity on the transmission signal.In this case, the traditional digital modulation method can no longer meet the needs of applications, and new digital modulation methods need to be adopted to reduce the impact of the channel on the transmitted signal, so as to obtain higher transmission rate under the condition of limited bandwidth resources.The research of these technologies is mainly around the full spectrum saving and efficient use of frequency bands.M-ary modulation is to improveSpectrum utilizationConstant envelope technology can adapt to the nonlinearity of the channel and maintain a small spectrum occupancy.
