# Design and implementation of AD9361-based software radio receiver

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## Abstract

Existing software radio platforms constructed by discrete devices have many disadvantages, such as high power consumption, high cost, and poor portability. In this study, an AD9361-based software radio communication system was designed on the basis of the zero-IF bandpass sampling software radio structure to solve the poor universality and expansibility problem of traditional software radio receivers. In the AD9361-based software radio communication system, the influences of channels on received signals and the inter-symbol interference caused by the multipath configuration are offset and eliminated by the minimum mean-squared error (MMSE) equalization algorithm. The simulation analyses on the core functions, including group detection, frame synchronization, channel estimation, and frequency-domain equilibrium, of the designed receiver was performed by ModelSim. The receiving functions of the software radio were realized by the core radio frequency (RF) board of AD9361 and the digital baseband development board of ZC706. The signal frequency spectra received and sent by the designed receiver overlap on the basis of the joint debugging and testing of the RF and digital baseband modules. Test results demonstrate that the designed software radio receiver has a reasonable structural design and can meet the design requirements in terms of overall performance. Additionally, the repeated development process of traditional software radio receivers is simplified, and the integration level and expansibility of the system can be improved. The results can provide valuable references for the development of universal software radio receivers.

## Keywords

Software radio Receiver AD9361 Frequency-domain equalization Channel estimation## Abbreviations

- FFT
Fast Fourier transform algorithm

- FMC
FPGA Mezzanine Card

- FPGA
Field programmable gate array

- IF
Intermediate frequency

- JTAG
Joint Test Action Group

- MMSE
Minimum mean-squared error

- MSE
Mean square error

- QAM
Quadrature amplitude modulation

- RF
Radio frequency

- SC-FDE
Single-carrier frequency domain equalization

- SNR
Signal-to-noise ratio

- SOC
System on chip

- SPI
Serial Peripheral Interface

- TDMA
Time-division multiple address

- UW
Unique words

## 1 Introduction

Software radio has overcome the disadvantage that previous communication platforms with different communication functions and frequency bands cannot communicate mutually. Existing software radio platforms are constructed by discrete devices, which are constrained by high power consumption and high cost. This situation not only requires technicians to be experienced in hardware design and radio frequency (RF) signal processing, but it also poses high access requirements for software. Given the coexistence of 3G and 4G communication standards, and even that of 5G, there are multiple frequency bands around the world. Traditional software radio designs require different hardware platforms to support varied communication protocols and frequency bands, and they require long development periods and high design costs.

Andrews et al. [1] suggested the use of a mixer as the first-level structure of a software radio to reduce noise at the cost of power with the supply voltage of the baseband low-noise amplifier increased. Murphy et al. (2012) put forward the use of a noise-canceling technology as the software radio structure. Useful signals were enhanced by increasing the auxiliary channels and offset noises at the output terminal, thus improving the noise reduction capability and the overall performance of the software radio system [2]. Zhang et al. propounded a software radio technique that can eliminate the spatial incident angle interference and frequency interference in signal filtering [3]. Loubser and Swart [4] encoded two existing CR-specific media access control protocols by using a CR-specific simulator. Kamaleldin and Ahmed put forward that hardware platforms of software radio system which support many wireless standards could be realized by dynamical program reconfiguration [5]. Marwanto et al. [6] proposed ARDUINO UNO and X-Bee technologies for software radio systems to reduce the costs of spectrum exchange information based on OFDM. Sahoo et al. propounded a multichannel finite impulse response filter for software radio, which can reduce power consumption effectively by the launcher umbilical tower, and can be applied to software radio systems with multichannel filter efficiently [7].

Tsinghua University (2014) completed a chip for software radio receivers in the working frequency band of 0.1–5 GHz. Qin and Wang et al. constructed a radio communication system platform in Matlab and a universal software radio peripheral to increase the utilization of spectrum resources. Spectrum sensing and available spectral bandwidth estimation of signals of master users were obtained by performing an energy detection method. Thus, spectrum detection was realized, and a set of judgment criteria was provided for the spectral access of secondary users [8]. Xu and Yu designed and completed a fault prediction software platform for an airborne software radio system by analyzing its structure [9]. Cui [10] designed a communication terminal for the time-hopping spread spectrum of the TDMA system based on the software radio concept, realized the single-channel launching and multichannel reception of RFs, and accomplished the design of RF modules and their link terminals. Zhang et al. [11] designed a monitoring system over interferences and multipath in the signal bands and adjacent frequencies of current four navigation systems based on software-defined radio concept. Yin and Cheng [12] built a new hardware design program for the software radio processing platform with high-performance and low power consumption based on the requirements of special radio communication systems with low power consumption.

Extant studies have reported that software radio systems are generally limited by their structures. Studies based on AD9361 RF modules and processing modules of SOC digital basebands remain underexplored. Covering most bands with the charter and license-free bands, the working frequency range of AD9361 is from 70 MHz to 6.0 GHz. The supported channel bandwidth is from less than 200 kHz to 56 MHz. With the RF front end and the baseband of the flexible mixed signal integrated, AD9361 provides a configurable digital interface for the processor and integrates frequency synthesizer, thus simplifying the import of the design, which can achieve lower noise and higher accuracy of modulation with the high programmability. In this study, an AD9361-based software radio structure was constructed by using the broadband zero-IF bandpass sampling software radio structure. The corresponding software radio receiver was designed, which achieved many core functions, such as group detection, frame synchronization, channel estimation, and frequency-domain equilibrium. The results can provide new universal platforms and methods for software radio receivers.

## 2 Methods

### 2.1 Structure of software radio system

### 2.2 Implementation of software radio receiving terminal

### 2.3 The structure of AD9361

### 2.4 Group detection

*C*

_{n}is:

*r*

_{n}is the received signal, and

*C*

_{n}is the mutual correlation between the currently received

*L*data and the

*L*data received before

*D*.

*P*

_{n}is:

*m*

_{n}of the delay correlation algorithm is:

The value of *m*_{n} for group detection can be determined by the leading structure when the signal-to-noise ratio (SNR) is 15 dB. A value of *C*_{n} approaching 0 indicates that effective data transmission has not been achieved and only noise exists. Parameter *m*_{n} begins to increase with the occurrence of the secondary short-training symbol and begins to decrease when the ninth period is reached.

DataInRe and DataInIm are the real part and imaginary part of the current data, respectively. SumMagnitude is the sum of relevant window energies, and SumDelayCorrelation is the sum of correlation coefficients of relevant windows. BufferForDetection represents the initial judgment samples of 32 continuous groups, and BufferForDetection represents the judgment samples at the end of 48 continuous groups. As shown in Figs. 4 and 5, SumMagnitude and SumDelayCorrelation are immediately calculated after a system reset and when the effective signal of the grouping detection elevates. When 32 samples are detected continuously, data grouping of judgment arrives. When the SumMagnitude of the 48 continuous samples is smaller than the threshold, the data grouping is completed.

### 2.5 Frame synchronization

*C*

_{k}can be expressed as:

^{∗}is a conjugation, and

*D*is the length of the cross-correlation coefficient, which is determined to be 16. The positions of the short-training symbols are judged according to the value of ∣

*C*

_{k}∣. The moment of the last peak of ∣

*C*

_{k}∣ is designated as the end point of the short-training symbols.

DataInRe and DataInIm are the real part and imaginary part of input data, respectively. DataInEnable is the enable signal of input data, and PCouter is the number of detected peaks. First, quantization is implemented when the data to be synchronized arrives. Then, the correlation is calculated on the basis of the 16 local short-training symbols, and the moment at the ninth peak is viewed as the end point of the short-training symbols. Finally, long-training symbols and data symbols are designated with serial output according to the output format with the cyclic prefix eliminated at the same time. As shown in Fig. 7, DataOutEn is the effective time for outputting one symbol denoted as 1 and 2 successively which calculates from the long-sequence, with the data symbols started from 3. This scheme is viewed as one cycle of output data.

### 2.6 Channel estimation

*h*(

*t*) is the impulse response,

*r*(

*t*) represents the received signals,

*s*(

*t*) denotes the theoretically received signals, and

*n*(

*t*) is the signal noise. The estimated value of the input signal \( \widehat{s}(t) \) is produced by the convolution of inverse channel system \( \widehat{h}(t) \) that is composed of

*r*(

*t*) and

*h*(

*t*), where \( h(t)\otimes \widehat{h}(t)=\delta (t) \).

*H*(

*jω*) is estimated by using

*r*(

*t*), and the inverse channel system \( \widehat{H}\left( j\omega \right) \) is constructed by using

*H*(

*jω*). Therefore,

*x*

_{m}} with a length of

*P*, then the channel frequency response \( {\widehat{H}}_k \) can be estimated by FFT from the transmitting sequences {

*x*

_{m}} and {

*y*

_{m}} to {

*x*

_{m}} and {

*Y*

_{m}}.

The corresponding time-domain discrete signal {*h*_{m}} can be initially obtained from the IFFT operation of \( {\widehat{H}}_k \) at the point *P*, where *P* denotes the length. Then, the zero-padding operation of {*h*_{m}} is performed, thus obtaining the {*h*_{m}} of the point *M*. Finally, the frequency response value \( {\widehat{H}}_k \) is acquired from the FFT operation of {*h*_{m}} of the point *M*.

*Y*=

*XH*+

*V*matrix as follows:

*X*= diag[

*X*(0),

*X*(1), … ,

*X*(

*N*

_{p − 1})],

*Y*= [

*Y*(0),

*Y*(1), … ,

*Y*(

*N*

_{p − 1})]

^{T}, and

*H*= [

*H*(0),

*H*(1), … ,

*H*(

*N*

_{p − 1})]

^{T}.

*N*

_{p}is the number of UW. The first-order derivative and the second-order derivative of

*J*

_{LS}for

*H*are calculated as follows:

*n*is the estimation error, and

*n*=

*X*

^{−1}

*V*. The simulation results of channel estimation based on the ModelSim platform are shown in Fig. 8.

DataInRe and DataInIm are the real part and imaginary part of input data, respectively. DataInEnable is the enable signal of input data, and ChannelcoeEnable is the enable signal of output data. ChannelcoeIm and ChannelcoeRe are the real part and imaginary part of output channel estimation, respectively.

### 2.7 Frequency-domain equilibrium module

Frequency-domain equilibrium is performed to offset the effects of channels on the received signals. Here, the minimum mean-squared error (MMSE) equilibrium algorithm is applied as the frequency-domain equilibrium [17].

*s*= [

*s*

_{0},

*s*

_{1}, … ,

*s*

_{N − 1}]

^{T}),

*N*is the number of FFT points (

*h*= [

*h*

_{0}, … ,

*h*

_{L − 1}.0, … , 0]

^{T}), and

*L*is the length of impulse response. Then, the received signal vector is

*r*= [

*r*

_{0},

*r*

_{1}, … ,

*r*

_{N − 1}]

^{T}. Accordingly,

*v*= [

*v*

_{0},

*v*

_{1}, … ,

*v*

_{N − 1}]

^{T}is the channel noise. On the basis of the FFT of Eq. (14),

*R*

_{k},

*S*

_{k},

*H*

_{k}, and

*V*

_{k}are the frequency domain values of received signals, transmitting signals and channel impulse response function, and additive white Gaussian noise.

*W*

_{k}, then the frequency-domain output after equilibrium is:

*z*

_{1}= arg

*z*

_{2}, |

*z*

_{1}−

*z*

_{2}|≥||

*z*

_{1}| − |

*z*

_{2}| |(

*z*

_{1},

*z*

_{2}∈

*W*) is true, then:

*F*

_{k}uses the lower limit value based on the condition of arg

*W*

_{k}

*H*

_{k}= arg 1 = 0.

*F*

_{k}is

*y*, then the minimum

*y*should be calculated, such that:

*y*:

Given that \( \frac{\sigma_S^2}{\sigma_N^2}=\mathrm{SNR} \), the MMSE equilibrium coefficient can be expressed as \( {W}_k=\frac{H_k^{\ast }}{{\left|{H}_k\right|}^2+1/\mathrm{SNR}} \). In Eq. (24), 0 ≤ *k* ≤ *N* − 1 and SNR denotes the signal-to-noise ratio of the transmitting terminal.

The simulation of signals based on the MMSE algorithm is conducted in Matlab. The following parameters are included:

Multipath channel: the corresponding power of the SUI.3 channel model is [0, − 5, − 10 dB];

Modulation mode: 16QAM;

SC-FDE system parameters: UW uses the Chu sequence, and the length is *N* = 64;

*M* = 256, and the MMSE equilibrium algorithm is used.

SC-FDE symbols in the time domain are read from “buffer of RX frame sample” by the channel equalization module and sent to the FFT module to calculate the frequency domain values of SC-FDE symbols.

Frequency domain values are read from “buffer of CSI” by the CSI_ACQ module, which can complete the integration of corresponding samples meanwhile.

With the complex multiplication of the corresponding sample points completed by the FDE_CORE module, the frequency domain equalization is achieved.

Meanwhile, with the subsequent IFFT_256 module controlled by the FDE_CORE module, the sample points in the frequency domain after the equilibration are restored to the time domain and stored in the symbol buffer of SIG domain and the time buffer of DATA domain, respectively.

DataInRe and DataInIm are the real part and imaginary part of the input data, respectively. DataInEnable is the enable signal of input data. DataOutRe and DataOutIm are the real part and imaginary part of output data, respectively.

### 2.8 RS decoding

- 1.
The adjoint polynomial

*s*(*x*) of RS codes is calculated from the receiving codes. - 2.
The error position polynomial

*a*(*x*) and error value polynomial*δ*(*x*) are solved by an adjoint polynomial. - 3.
The error position can be acquired by using the Chien searching method to calculate the roots of error location polynomials.

- 4.
The error magnitude corresponding to each error location can be obtained from the error value polynomial by using the Fomey algorithm, namely

*C*(*x*) =*R*(*x*) −*E*(*x*). - 5.
After decoding, the adjoint formula of the codeword is calculated again, and the adjoint formula is determined by detecting whether the adjoint formula is zero or not.

According to the above procedures, RS decoder should include four parts: the adjoint polynomial calculation module, the key equation solving module, the money search module, and the Fomey algorithm module [18].

Specific procedures of RS decoding design are as follows:

1. Solving the adjoint polynomial of RS decoding. The parameters of RS (255,191) are as follows:

Encoding length: *n* = 255

Information bit length: *k* = 191

Parity bit length: 64

Error correcting capability: *t* = 32

*a*,

*a*

^{2},

*a*

^{3}, … ,

*a*

^{32}, in which

*R*(

*x*) =

*r*

_{0}+

*r*

_{1}

*x*+

*r*

_{2}

*x*

^{2}+ …

*r*

_{n − 1}

*a*

^{(n − 1)}. The 32 adjoint expressions of RS (255,191) codes are acquired, namely

*s*

_{1},

*s*

_{2},

*s*

_{3}, … ,

*s*

_{32}.

*δ*(

*x*) is obtained, then the error location polynomial and the error value polynomial are obtained. The error location polynomial

*δ*(

*x*) can be defined as:

*θ*

_{1}. …

*θ*

_{t}. The right part of the expansion equation is simplified as follows:

3. Solving the error position. The error location of receiving polynomial *R*(*x*) = *r*_{0} + *r*_{1}*x* + Λ + *r*_{n − 2}*x*^{n − 2} + *r*_{n − 1}*x*^{n − 1} is acquired according to the root of *δ*_{1}*x*.

*E*(

*x*) and the polynomial of

*c*(

*x*). The error value polynomial is defined as follows:

*ω*(

*x*) =

*S*(

*x*)

*δ*(

*x*), which is simplified as follows:

The error value polynomial *ω*(*x*) = *ω*_{1}*x* + *ω*_{2}*x*^{2} + *ω*_{t}*x*^{t} can be obtained if *ω*_{1} = *s*_{1}, *ω*_{2} = *s*_{2} + *δ*_{1}*s*_{1}, Λ, *ω*_{t} = *s*_{t} + *δ*_{1}*s*_{t − 1} + Λ*δ*_{t − 1}*s*_{1}. The error pattern \( E(x)=\sum \limits_{i=1}^t{Y}_i{x}_i^{ti} \) can be obtained by \( {Y}_j=\frac{-{x}_j\omega \left({x}_j^{-1}\right)}{\delta \left({x}_j^{-1}\right)} \). Here, *x*_{j} is the root of the Chien searching method. The final actual code *C*(*x*) is obtained with *E*(*x*) and the receiving code *R*(*x*) superposed.

5. Calculating the adjoint formula of the codeword again. The decoding result is determined by detecting whether the adjoint formula is zero or not.

As is shown in Figs. 11 and 12, the output of RS encoding is used as the input of RS decoding data. The input is as follows: (1, 2, 3, …, 190, 191, 204, 5, 85, 10, 239, 109, 76, 117, 180, 235, 220, 44, 210, 158, 235, 68, 138, 211, 46, 185, 196, 249, 194, 92, 219, 237, 254, 229, 151, 239, 246, 19, 26, 219, 66, 100, 210, 157, 6, 208, 187, 169, 68, 168, 78, 28, 34, 163, 42, 134, 149, 43, 0, 88, 70, 90, 93, 129, 173, 131, 235, 192, 66, 34). If the output data of RS decoding is the input of RS encoding, the output is correct. Namely, the output is (1, 2, 3, …, 190, 191). As is shown in Fig. 13, the RS decoding decodes the encoded data. The RS encoding is correct.

### 2.9 16QAM demodulation module

RS decoding is followed by the 16QAM demodulation. With the orthogonal coherent demodulation method applied, the signal is judged, detected, and converted in series and parallel, and the final output is generated.

*I*branch and

*Q*branch are shown in Eq. (33).

With \( \frac{1}{2}{X}_k\cos 2 at+\frac{1}{2}{Y}_k\sin 2 at \), \( \frac{1}{2}{Y}_k\cos 2 at+\frac{1}{2}{X}_k\sin 2 at \), \( \frac{1}{2}{X}_k \), and \( \frac{1}{2}{Y}_k \) filtered by the low pass filter, the output of 16QAM demodulation is obtained. The expression is as follows:

*d*= (

*I*+

*jQ*) ×

*K*

_{MOD}, where \( {K}_{\mathrm{MOD}}=1/\sqrt{10} \). The constellation of the 16QAM modulation is shown in Fig. 14.

The *I*-way component and *Q*-way component correspond to *b*_{0}*˴b*_{1} and *b*_{2}*˴b*_{3} in the code elements *b*_{0}*˴b*_{1}*˴b*_{2}*˴b*_{3}, respectively. With the decision thresholds set as −2 × *K*_{MOD}, 0 and 2 × *K*_{MOD}, respectively, the *I* and *Q* can be demodulated now.

## 3 Experiment results and discussion

As shown in Fig. 16, the transmitting frequency spectrum is located in the upper position, while the receiver frequency spectrum is located in the lower position. The overlapping of the frequency spectra of the transmitting signals and receiver indicates the consistency of parameters between the transmitting and receiving signals. Therefore, transmitting signals are received accurately.

In this study, a joint experiment of RF module and digital baseband processing module is carried out. The experiment is carried out by two modules combined with a signal source and a spectrum analyzer. But there is no video display part and signal compression part, which can be added to the system to promote the applications in our future research.

## 4 Conclusions

- 1.
The single-carrier frequency domain equilibrium is applied on the digital baseband physical layer, which is optimized on the basis of the SC-FDE communication protocol. Then, the receiving function on the basis of the software radio receiver is realized by using the optimized communication protocol. The system can effectively resist the frequency-selective fading of channels, thus achieving high-rate and large-capacity communication transmission.

- 2.
Peak to average power ratio and the sensitivity to the phase noise are decreased by the MMSE equilibrium algorithm, thus enhancing the resistance against multipath interference.

The AD9361-based software radio receiver presented in this study has a reasonable structure, and its performance indexes are satisfactory. The proposed system can effectively increase the communication speed and capacity, remarkably ameliorate the reduction of signal quality caused by multipath fading, and excellently offset the poor universality and expansibility of traditional radio receivers. The results of this study can serve as a useful reference in the development of next-generation universal software radio receivers.

## Notes

### Acknowledgements

This work was supported by the Project of Science and Technology of Shaanxi (No.2018GY-151).

### Funding

This work was supported by the Project of Science and Technology of Shaanxi (No.2018GY-151).

### Availability of data and materials

The authors declare that all the data and materials in this manuscript are available.

### Authors’ contributions

The contributions of all authors are equal in this manuscript, and all authors read and approved the final manuscript.

### Authors’ information

Feng Tian is currently an associate professor of College of Communication and Information Engineering, Xi’an University of Science and Technology.

Hanqing Li is currently pursuing an MS degree at the College of Communication and Information Engineering, Xi’an University of Science and Technology.

Liangchen Yuan is currently pursuing an MS degree at the College of Communication and Information Engineering, Xi’an University of Science and Technology.

### Competing interests

The authors declare that they have no competing interests.

### Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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