Comparison of several common modulation methods for wireless modules
Date:2019/11/28 16:23:24   Browse:0

At present, there are many types of common wireless module on the market. The differences between them are often reflectedin the supported frequency, maximum rate, modulation method, power consumption,etc. This article compares several modulation methods commonly used in wireless module.

ASK definition

ASK is short for Amplitude Shift Keyingmodulation. For example, binary, and the binary symbols 0 and 1 are representedby different amplitudes respectively, which is ASK.

"Amplitude Shift Keying", alsoknown as "Amplitude Shift Keying", is called ASK, and is a commonmethod of modulation technology. If the possible state of the digitalmodulation signal corresponds to the binary information symbol or its correspondingbaseband signal state one-to-one, the modulated signal is called a binarydigital modulation signal. Keying with binary information symbols is calledbinary amplitude keying and is represented by 2ASK.


Shift Keying Schematic

In the amplitude shift keying mode, when 1 appears, thecarrier with amplitude A is turned on, and when 0 appears, thecarrier is turned off. This is equivalent to moving the original basebandsignal (pulse train) spectrum to side.

Amplitude shift keying (ASK) is equivalent to amplitude modulationin an analog signal, except that it is a binary number multiplied by a carrierfrequency signal. Amplitude shifting uses frequency and phase as constants andamplitude as variables. Information bits are transmitted through the amplitudeof the carrier. Binary amplitude keying (2ASK), because the modulation signalhas only two levels of 0 or 1, the result of the multiplication is equivalentto turning the carrier frequency off or on. The practical significance is thatwhen the modulated digital signal is "1 ", The carrier istransmitted; when the modulated digital signal is" 0 ", the carrieris not transmitted. The principle is shown in Figure 1, where s (t) is abaseband rectangular pulse. Cosine signals are generally used for carriersignals, and modulated signals are used to convert digital sequences intounipolar baseband rectangular pulse sequences, and the role of this on-offkeying is to multiply this output by the carrier to shift the spectrum to thecarrier frequency. Nearby, realize 2ASK. The realized 2ASK waveform is shown inFigure 2.


        2ASK waveform

    


The simplest and most commonly used form of amplitude modulationkeying for this modulation technique is switching. The presence of a carrier isrepresented by a "1" and the absence of a carrier is represented by a"0". This type of modulation is called on-off keying (OOK), and it isthe most energy-efficient modulation method because it radiates energy onlywhen a "1" is sent. Amplitude shift keying requires a high signal-to-noiseratio to demodulate the signal, because according to its characteristics, mostsignals are transmitted at very low power. The advantages of the ASK modulatedradio frequency system are that the structure of the transmitting and receivingequipment is simple, and the power consumption is relatively low.Unfortunately, the bandwidth occupied by the ASK / OOK modulation system isless than 500kHz or the peak density will not fall into the range required bythe "digital modulation system". This shows that the transmissionpower of the ASK / OOK modulation system is limited to 50mV / m, or some FHSStechnology must be adopted to meet the requirements of FCC clause 15.247. 

OOK

On-Off Keying

OOK is a special case of ASK modulation. Ifone amplitude is 0 and the other is non-zero, it is OOK. Binary on-off keying(OOK: On-Off Keying), also known as binary amplitude keying (2ASK), is aunipolar non-return-to-zero code sequence to control the opening and closing ofa sine carrier. This modulation method appeared earlier than the analogmodulation method, and the Morse code radio transmission uses this modulationmethod. OOK's anti-noise performance is not as good as other modulationmethods, so this modulation method is not used in current satellite communicationand digital microwave communication. However, because the modulation method issimple to implement, in fiber-optic communication systems, amplitude keyingmethod widely used. The analysis method of this modulation method is basic, soyou can study the basic theory of digital modulation from the entry of OOKmodulation method.

 

FSK (frequency shift keying)

An angular modulation in which thefrequency of a sinusoidal oscillation changes between a set of discrete values,where each discrete value represents a characteristic state of a time-dispersedmodulated signal.

 

Applied disciplines: communicationtechnology (first-level discipline); communication principles and basictechnologies (secondary discipline)


Frequency shift keying uses two oscillators with differentfrequencies F1 and F2 to represent signals 1 and 0. Use 1 and 0 of the digitalsignal to control two independent oscillation sources to output alternately.For binary frequency shift keying modulation, its effective bandwidth is B =2xF + 2Fb, where xF is the bandwidth of the binary baseband signal and themaximum frequency offset of the FSK signal. Since the bandwidth of the digitalsignal is the Fb value, the binary frequency shift key The signal bandwidth ofthe control is relatively large, and the frequency band utilization is small.

 

GFSK

Gaussian frequency shift keying GFSK-Gaussfrequency Shift Keying, is a Gaussian low-pass filter before modulation tolimit the signal's spectral width.

 

Modulation principle

GFSK Gaussian frequency shift keyingmodulation is a digital modulation method in which input data is pre-modulatedand filtered by a Gaussian low-pass filter, and then FSK modulated. Whilemaintaining a constant amplitude, it can control the spectrum of the modulatedsignal by changing the 3dB bandwidth of the Gaussian low-pass filter. It hasthe characteristics desired by wireless communication systems such as constantamplitude envelope, concentrated power spectrum, and narrow spectrum.Therefore, GFSK modulation and demodulation technology is widely used in manyfields such as mobile communications, aviation and marine communications.

GFSK modulation can be divided into directmodulation and quadrature modulation.

Direct modulation

Direct modulation is to directly simulatethe frequency modulation of the RF carrier after the digital signal is filteredby Gaussian low-pass. When the modulation index of the frequency modulator isequal to 0.5, it is the well-known GMSK (Gaussian Minimum Frequency ShiftKeying) modulation, so GMSK modulation can be regarded as a special case ofGFSK modulation. In some literatures, the GFSK modulation method with differentBT product and modulation index is called GMSK / FM, which actually notices thefact that this method cannot be called GMSK when the modulation index is notequal to 0.5.

Although the direct modulation method issimple, because the modulation signal is usually added to the VCO of the PLLfrequency synthesizer, its inherent loop high-pass characteristic will causethe low-frequency component of the modulation signal to be lost. Therefore, inorder to obtain the ideal GFSK modulation characteristics, a direct frequencymodulation technique called two-point modulation is proposed. In thistechnique, the modulation signal is divided into two parts, one part is addedto the VCO end of the PLL according to the conventional frequency modulationmethod, and the other part is added to the main oscillator end of the PLL.Because the main oscillator is not in the control feedback loop, it can be modulatedby the low-frequency component of the signal. In this way, the generatedcomposite GFSK signal has spectral characteristics that can be extended to DC,and the modulation sensitivity is basically a constant, which is not affectedby the loop bandwidth. However, two-point modulation increases the difficultyof GFSK modulation index control.

Quadrature modulation

Orthogonal modulation is an indirectmodulation method. In this method, a digital signal is subjected to Gaussianlow-pass filtering and an appropriate phase integration operation, and thendivided into two parts: in-phase and quadrature. The in-phase and quadraturecomponents of the carrier are multiplied respectively, and then the GFSK signalis synthesized. Relatively speaking, the physical concept of this method isclear, and it also avoids the damage of the signal spectral characteristicsduring direct modulation. On the other hand, GFSK parameter control can beimplemented in a Gaussian filter with a calibration factor, and is not affectedby the subsequent frequency modulation circuit, so the parameter control issimpler. Because of this, the baseband signal processing of the GFSK quadraturemodem is particularly suitable for digital implementation.

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