RF Demodulation is the process of extracting a signal containing information from an RF carrier wave. A demodulator generally is an electronic circuit that extracts the signal from the carrier wave. There are many types of demodulators and methods of demodulating a signal from a carrier wave. For example, phase-shift keying is a digital demodulation method that changes the phase of a constant frequency reference signal (i.e., the carrier signal). Some examples of processes that use phase-shift keying include wireless local area networks (LAN), radiofrequency identification (RFID) and Bluetooth communication.
Features and advantages of examples of the present disclosure will be apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, but in some instances, not identical, components. Reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.
Conventional phase shift keyed (PSK) RF receivers demodulate a signal by using analog circuitry to multiply the received signal by a sine and cosine function oscillating at the carrier frequency, recovering (after going through a low pass filter) the In Phase “I” (sine product), and recovering the Quadrature Phase “Q” (cosine product) of the IQ plane phasor defining the “symbol” being transmitted. In an example, a “constellation” of symbols may be a binary 0 or 1 having constant amplitude and a phase difference of 180° to arrays of I and Q values with varying amplitudes and phases to circular patterns of symbols. A large separation of symbols in the IQ plane provide a larger tolerance to amplitude and phase noise. However, the signal to noise ratio (SNR) is proportional to the received amplitude, and therefore symbols with a lower amplitude are more vulnerable to error due to noise. In other examples, an RF receiver that demodulates a signal using circular constellations that use a constant amplitude avoid having a symbol dependent error vulnerability. However, RF receivers with circular constellations require more extensive calculations (the arc-tangent function of I and Q) to determine the symbol value. In addition, current methods of demodulation favor array based symbol arrays because the orthogonal I and Q vectors are the output.
Mathematically, I and Q of a RF signal S is defined by:
The conventional method of extracting the IQ phasor from an RF signal is to determine I(t) and Q(t) by multiplying the signal S(t) by 2·sin(2πft+δ) and 2·sin
respectively and filtering out the resulting high frequency 4πft components (note: δ is the phase difference (t−t)·2πf). The use of analog RF mixers (for signal multiplication) and frequency filters add and remove frequency components to the resulting I(t) and Q(t).
The method herein produces the IQ phasor phase angle, IQ amplitude, or a combination of the IQ phasor phase angle and IQ amplitude as the output, which favors circular symbol constellations. The method herein does not require analog local oscillators, mixers, or frequency filters. The IQ phasor phase angle and IQ amplitude can be calculated digitally. The method uses a digital sampling rate of the received signal at 3 times the carrier frequency f (i.e., 3 samples per carrier frequency period). When sampling at 3 times per carrier frequency period, two of any three adjacent sample values will add up to be the negative of the third sampled value. Since each sample is part of 3 sets of 3 samples, random noise can also be removed from the signal by averaging each sample value with each negative sum of the other two sampled values from each set. This feature not only provides a means of removing noise from the signal, but also provides a means of calculating the IQ phasor phase angle and IQ amplitude without the use of trigonometric functions. In addition, the method herein can determine which of the 6 phase regions the IQ phasor phase angle resides in without the use of multiplication or division.
The method herein includes demodulating an RF signal to polar in-phase and quadrature (IQ) components that includes converting an RF signal with an analog-to-digital converter and calculating the polar in-phase and quadrature (IQ) components of the RF signal as an IQ phasor phase angle and an IQ amplitude using a digital processor. The analog-to-digital converter uses a sampling rate, where, when the sampling rate used has sampling rates other than 3 times an RF carrier frequency of the RF signal, a digital logic circuit splines data to the sampling rate of 3 times the RF carrier frequency of the RF signal. The digital processor calculates the polar in-phase and quadrature (IQ) components of the RF signal as an IQ phasor phase angle and an IQ amplitude using addition, subtraction, multiplication, division, and absolute value.
Referring now to
Digital logic circuits can perform Boolean logic resulting in addition, subtraction, absolute value, as well as multiplication, division. For example, the digital logic circuit calculations define A1, A2, and A3 as the 3 adjacent signals, Sn, Sn+1, and Sn+2 where S is the RF signal and n is a positive integer provided that 120° is subtracted from the IQ phasor phase angle when n−1 is a multiple of 3 or 120° is added to the IQ phasor phase angle when n+1 is a multiple of 3.
Referring back to
First, the calculation of the IQ phasor phase angle is described using the following thirteen equations. The calculation of the Normalization factor, N, at approximately 1.78 times the carrier signal amplitude, is shown below as equation (I):
N=(abs(A1)+abs(A2)+abs(A3)+(abs(A1−A2)+abs(A1−A3)+abs(A2−A3))/2)/2 (I)
First, in equation (I), the absolute value of every possible set of 3 adjacent samples 120° apart are taken. An example of a plot of the absolute value of 3 samples 120° apart is shown in
Next, the equations (II)-(XIII) are shown below:
E1=abs(A1)+A1−abs(A2)−A2; (II)
E2=abs(A2)−A2−abs(A3)+A3; (III)
E3=abs(A3)+A3−abs(A1)−A1; (IV)
E4=abs(A1)−A1−abs(A2)+A2; (V)
E5=abs(A2)+A2−abs(A3)−A3; (VI)
E6=abs(A3)−A3−abs(A1)+A1; (VII)
F1=abs(A1)−A1+abs(A2)−A2; (VIII)
F2=abs(A2)+A2+abs(A3)+A3; (IX)
F3=abs(A3)−A3+abs(A1)−A1; (X)
F4=abs(A1)+A1+abs(A2)+A2; (XI)
F5=abs(A2)−A2+abs(A3)−A3; (XII)
F6=abs(A3)+A3+abs(A1)+A1; (XIII)
In equations II-VII, E1-E6 are the difference of the sums and the difference of the absolute value and value of every possible set of 3 adjacent samples 120° apart. In equations VIII-XIII, F1-F6 are the sums of the sums and the difference of the absolute value and value of every possible set of 3 adjacent samples 120° apart. An example of a plot of the difference of the sums and the difference of the absolute value and value of every possible set of 3 adjacent samples 120° apart is shown in
IQ Phasor Phase Angle=(1−sign(F1))*(30+30*E1/N)+(1−sign(F2))*(90+30*E2/N)+(1−sign(F3))*(150+30*E3/N)+(1−sign(F4))*(210+30*E4/N)+(1−sign(F5))*(270+30*E5/N)+(1−sign(F6))*(330+30*E6/N); (XIV)
The calculation of the IQ phasor phase angle for every possible carrier phase from equation (XIV), along with the error from the actual IQ phasor phase angle, is shown in
Referring back to calculating the polar in-phase and quadrature (IQ) components of the RF signal as an IQ phasor phase angle and an IQ amplitude using a digital processor 104, the IQ amplitude is also calculated before, after, or simultaneously with the IQ phasor phase angle. The IQ amplitude is calculated using the following equation (XV):
IQ Amplitude=N/1.78 (XV)
The IQ amplitude, along with the IQ phasor phase angle, can be used to calculate the polar in-phase component and quadrature component of the RF signal.
In addition, in some examples, the frequency modulation may be determined by the digital processor. The frequency modulation is a difference between a targeted frequency and the actual frequency of the RF signal. The difference between a targeted frequency and the actual frequency of the RF signal, ΔF (i.e., the frequency modulation), is determined by equation (XVI):
where ϕn is the IQ phasor phase angle and Fs is the sampling rate.
Z1(0°-60°)=1−sign(F1); (XVII)
Z2(60°-120°)=1−sign(F2); (XVIII)
Z3(120°-180°)=1−sign(F3); (XXIX)
Z4(180°-240°)=1−sign(F4); (XXX)
Z5(240°-300°)=1−sign(F5); (XXI)
Z6(300°-360°)=1−sign(F6); (XXII)
In equations (XVII-XXII), the RF signal is determined to be in the zone (Z1-Z6) when the calculation of one of the zones is equal to 1. This is also shown in
The method 100 herein is continuously preformed as long as the RF signal is being received. The IQ phasor phase angle, IQ amplitude, or any additional data being calculated by the digital processor is also continuously forwarded to a computer storage device or a computer for further processing.
Referring now to
Referring back to
Referring back to
S′(n)=[S(n)−[S(n+1)+S(n+2)]−[S(n−1)+S(n+1)]−[S(n−1)+S(n−2)]]/4; (XXXIII)
where S(n) is the RF signal and n is a positive integer. Multiple iterations of the filter can be realized by redefining the RF signal S(n) as S′(n).
Referring back to
In addition to the methods 100, 900, a system for demodulating an RF signal to in-phase and quadrature (IQ) components is disclosed herein. In one example, the system includes an analog-to-digital converter and a digital processer that performs the same functions and calculations as previously disclosed herein. In another example, the system includes an analog-to-digital converter, a digital processer, and a shaped bandpass filter that performs the same functions and calculations as previously disclosed herein.
As used herein, the term “about” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “a little above” or “a little below” the endpoint. The degree of flexibility of this term can be dictated by the particular variable and would be within the knowledge of those skilled in the art to determine based on experience and the associated description herein.
As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of a list should be construed as a de facto equivalent of any other member of the same list merely based on their presentation in a common group without indications to the contrary.
Unless otherwise stated, any feature described herein can be combined with any aspect or any other feature described herein.
Reference throughout the specification to “one example”, “another example”, “an example”, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise.
In describing and claiming the examples disclosed herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.