The emergence of new wireless technologies forces the use of multi-standard multi-band radios, so software defined radio (SDR) will play a key role in future radio architectures.SDR only uses a hardware front-end, but it can change its operating frequency, occupied bandwidth and different wireless standards by invoking different software algorithms.This approach enables inexpensive and efficient interoperability between existing standards and frequency bands.The SDR front end consists of the standard subsystems used in most receiver transmitters: modulator and demodulator, frequency converter, power amplifier (PA), and low noise amplifier (LNA).However, the modulation and coding and operating frequency are controlled by software.Such radios typically rely on digital signal processors (DSPs) for their flexibility.The SDR can self-regulate according to the transmission conditions, thereby minimizing interference from other signals present at the air interface.Implementation of such a system requires the ability to scan the spectrum from low to high frequencies in software.This concept has motivated many researchers to study the concept of Cognitive radio-CR (Cognitive radio-CR) proposed by Mitola in [2], in which the radio conducts its own Adjusts to suit the air interface conditions it is in to minimize interference and keep communications open for a given condition.Figure 2, (a) A superheterodyne receiver structure in which an RF signal is received, filtered, amplified, downconverted to an IF frequency, and then filtered and amplified again.The signal is then converted to baseband by a quadrature demodulator, filtered on each path (I and Q), amplified, and then converted to the digital domain.(b) A zero-IF structure in which the RF signal is filtered, amplified, and converted directly to baseband by a quadrature demodulator.Subsequently, the signal is filtered, amplified and digitized.(c) A bandpass sampling receiver, in which the signal is filtered, amplified, and sampled by a sample-and-hold circuit, which is usually part of an analog-to-digital converter.The signal is mixed down to the first Nyquist zone, digitized by an analog-to-digital converter, and processed in the digital domain.ADC: Analog-to-Digital Converter, BPF: Band Pass Filter, FIR: Finite Impulse Response Filter, I: In-phase Component, LNA: Low Noise Amplifier, LO: Local Oscillator, LPF: Low Pass Filter, Q: Quadrature Component; VGA: Variable Gain Amplifier.where fc is the carrier frequency, fs is the sampling frequency, fix(a) is the value obtained by truncating the fractional part of parameter a and parameter b, and rem(a, b) is the remainder of dividing a by b.The first structure [Fig. 3(a)] is a general-purpose superheterodyne transmitter, which is the dual system of the superheterodyne receiver shown in Fig. 2(b).The signal is generated in the digital domain and then converted to the analog domain by a simple sampling digital-to-analog converter (DAC).The signal is modulated at an intermediate frequency, where it is amplified and filtered to remove harmonics generated during the modulation process.Finally, a local oscillator source (LO2) is used to up-convert the signal to an RF signal, filter to remove the undesired image sidebands, amplify by the RF amplifier and feed it to the transmit antenna.I/Q modulation is performed at IF, which means that the design of hardware components is easier than with RF modulation.Finally, the overall gain is controlled at mid-frequency, where it is relatively easy to make a high-quality variable-gain amplifier.However, like the receiver, there are many problems with such a structure.Therefore, this structure is mainly used for microwave point-to-point wireless links, eg for backhaul communication [6], [7], and of course in the field of radio transmitters mentioned above.The number of circuits and low level of integration, along with the linearity required for power amplifiers, combined with difficult multi-mode operation often hinder the use of superheterodyne transmitters in SDR.Figure 3(b) shows the block diagram of a direct conversion transmitter [20], [21], which is a simplified superheterodyne front-end.Like the last example, it uses two digital-to-analog converters to convert the baseband digitized I and Q signals to the analog domain.A subsequent low-pass filter removes the Nyquist image signal, thereby improving the noise floor (background noise).These signals are modulated directly at the radio frequency using a high performance I/Q modulator.The signal is then filtered by a bandpass filter centered at the desired output frequency and amplified by a power amplifier.Figure 3. (a) A superheterodyne transmitter structure in which the I/Q digital signal is converted to the analog domain, low-pass filtered, and modulated at an intermediate frequency.The signal is then amplified, filtered, and up-converted to RF frequencies, which are then filtered and amplified prior to transmission.(b) A direct-conversion architecture in which the I/Q digital signal is passed through a digital-to-analog converter to the analog domain, filtered, and then modulated directly at the desired RF frequency.After this, the RF signal is filtered and amplified by a power amplifier.BPF bandpass filter, DAC: digital-to-analog converter, DPA: driving power amplifier, I: in-phase component, LO: local oscillator source, LPF: low-pass filter, PA: power amplifier, Q: quadrature component;In the previous configurations, the RF power amplifiers (power amplifier modules) used are class A, AB or B, which exhibit the highest efficiency when operating in the compression region, and when operating in switch mode, the Class D, Class E or Class F are used [23].The latter high-efficiency power amplifier operates in a strongly nonlinear mode.Therefore, they can only amplify constant envelope modulated signals, as used in the Global System for Mobile Communications (GSM) access format.The Quadrature Amplitude Modulation (QAM) type used in newer access modes such as Wideband Code Division Multiple Access (W-CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) has a high peak-to-average power ratio (PAPR) .The standard practice to prevent amplifiers from going into compression is to operate in Back-off mode, which reduces input power until the power amplifier is no longer driven into compression.Unfortunately, this greatly reduces efficiency, especially for high PAPR signals.Several linearization techniques such as feedback, feedforward, or digital predistortion have been proposed and evaluated [23], [24], but these techniques have not been widely used in fully integrated power amplifiers.If we consider the circuit of Figure 6, the class S amplifier simply amplifies the envelope of the input signal (detected in the digital domain by the digital signal processor DSP).In this case, the class S amplifier is only used to change the bias voltage of the RF high power amplifier, Vdd(t).On the phase path, the constant envelope phase modulated signal is generated in the DSP, then upconverted to RF frequency and fed into the RF power amplifier.This RF power amplifier is always saturated, resulting in high efficiency.Nonetheless, the main concern of this design is the time alignment of the baseband envelope path and the RF path.This can be compensated in the digital domain by using the use of DSP.Other proposed structures include amplifiers based on Doherty [32], [33] and out-of-phase techniques [34].The Doherty structure consists of a combination of two power amplifiers of equal capacity (a carrier power amplifier biased in class B and a peaking power amplifier biased in class C) via quarter wavelength line segments or networks.In a modern implementation, a DSP can be used to improve the performance of the Doherty amplifier by controlling the drive and bias applied to the two power amplifiers.For an ideal Class B amplifier, the average efficiency can be as high as 70% for signals with high PAPR values.The instrumentation industry [35], [37] has developed various instruments suitable for SDR characterization, such as mixed-signal oscilloscopes that can operate in both the analog and digital domains.This allows time synchronization of analog and digital signals on the same instrument.However, mixed-signal oscilloscopes only offer asynchronous sampling.This means that, like a traditional sampling oscilloscope, a mixed-signal oscilloscope uses its built-in clock to sample the data.As discussed in [38] and [39], when testing SDR devices (including analog-to-digital converters), accurate estimation of the phase and magnitude of the transfer function is required between the input, output and clock signals. Perform correlation sampling.If these signals were sampled asynchronously, there would be enough spectral leakage to completely degrade any amplitude and phase information from the SDR.Spectral leakage occurs because when the necessary Fourier transform (DFT or FFT) is performed, the two signals do not share the same time-domain grid and, therefore, are uncorrelated with each other.The instrumentation industry has also proposed other methods of combining several instruments, including logic analyzers, oscilloscopes, vector signal analyzers or real-time signal analyzers [40]-[42].To test an SDR transmitter configuration, these instruments can be used in a configuration similar to that shown in Figure 7.Using reference signals, trigger signals, and markers, one can make simultaneous measurements between the digital and analog domains, as well as the time and frequency domains.Typical tests performed with these systems can be used to evaluate the transmit and receive chains in SDR, including error vector magnitude (EVM) and adjacent channel power ratio (ACPR) in the signal chain.A metric commonly used in transmitter testing quantifies spectrum regeneration in adjacent channels.Adjacent Channel Power Ratio [ACPR, sometimes called Adjacent Channel Level Ratio (ACLR)] is specified using out of band masks, while out-of-band specifications define the maximum transmit power allowed in adjacent channels .ACPR usually results from spectral regeneration caused by nonlinear distortion.As with testing of many radio structures, for SDR testing, the stimulus signals used in the test can affect the measurement performance of the radio system.The influence of the test signal on radio performance is usually analyzed by means of the statistical properties inherent in the excitation, either using probability density (PDF) or complementary cumulative distribution function (CCDF).The PAPR value (peak/average power ratio) of the signal is also often used as a figure of merit [44]-[48].In probability theory, a probability density function (PDF) is a function that expresses the probability that a random variable X has a value less than x.Usually, the PDF is determined on the basis of a lot of measurements, it determines the probability of all possible values ​​of x, which is a non-negative function with unit areaThe complementary cumulative distribution function (CCDF) curve is closely related to PDF because it is obtained by CCDF=1-PDF.CDF is the cumulative distribution function that can be derived directly from PDF statisticsPeak to average power ratio (PAPR) is the ratio of the maximum peak power to the average power of a given signal and is the most interesting measurement in wireless communications.The evaluation of the impact of PAPR on the communication system is mainly obtained through the analysis of the CCDF curve, we can define a specific percentage in the CCDF curve to obtain the value of PAPRAdjacent channel power ratio (ACPR) is a measure of the amount of distortion produced by a wireless system in the adjacent channel relative to the main channel.It is usually defined as the ratio of the average power of the adjacent frequency channel (offset channel) to the average power of the transmit frequency channelAmong them, F1 and F2 represent the frequency spectrum interval, S(W) is the fundamental frequency signal, and U1 and U2 are the frequency spectrum interval of the upper adjacent channel.The bit error ratio (BER) is the ratio of the number of erroneous bits in the received message to the total number of bits of data transmitted.BER is usually expressed as a percentage, where 0% means no erroneous bits were detected at the receiverError vector magnitude (error vector magnitude-EVM) is a parameter used to test modulation and demodulation accuracy, as well as the degree of channel impairment.It can be used to quantify the performance of a digital radio transmitter or receiver.The signal transmitted by the transmitter or received by the receiver is subject to all the different imperfections in the implementation of hardware and software that can cause the K modulated signal constellation points Zc(k) to deviate from their ideal positions, S(k ).In everyday use, the EVM is a measure of how far the points deviate from their ideal positions, where, for N transmitted symbols, we can getFor transmitters, EER technology and its modified versions are promising options for SDR applications because their efficiency is largely independent of PAPR.Therefore, they can be easily applied to multi-standard and multi-band operation [50].Such SDR and CR transmitter architectures require not only high-efficiency amplifiers, but also broadband amplifiers [51].The SDR field is moving from analog to digital in terms of signal transmission, so the requirement to increase the switching speed of RF amplifiers is becoming more pronounced and more stringent, leading to class S transmitters in the future.[1] J. 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