Array signal recovery algorithm for a singleRFchannel DBF array
 Duo Zhang^{1}Email authorView ORCID ID profile,
 Wen Wu^{1} and
 Da Gang Fang^{1}
https://doi.org/10.1186/s1363401603916
© The Author(s) 2016
Received: 1 May 2016
Accepted: 25 August 2016
Published: 14 September 2016
Abstract
An array signal recovery algorithm based on sparse signal reconstruction theory is proposed for a singleRFchannel digital beamforming (DBF) array. A singleRFchannel antenna array is a lowcost antenna array in which signals are obtained from all antenna elements by only one microwave digital receiver. The spatially parallel array signals are converted into timesequence signals, which are then sampled by the system. The proposed algorithm uses these timesequence samples to recover the original parallel array signals by exploiting the secondorder sparse structure of the array signals. Additionally, an optimization method based on the artificial bee colony (ABC) algorithm is proposed to improve the reconstruction performance. Using the proposed algorithm, the motion compensation problem for the singleRFchannel DBF array can be solved effectively, and the angle and Doppler information for the target can be simultaneously estimated. The effectiveness of the proposed algorithms is demonstrated by the results of numerical simulations.
Keywords
1 Introduction
Digital beamforming (DBF) technology offers a significant improvement in performance over analog beamforming technology [1]. Many applications, including modern wireless communications, radar systems, surveillance, radio astronomy, and sonar, take advantage of this technology to gain benefits in terms of beam steering, improved signaltointerference ratio (SIR), and interference rejection [2]. Recently, multiinput multioutput (MIMO) technology has attracted the attention of many researchers because it offers the ability to transmit multiple probe signals via transmit antennas, thereby providing additional diversity [3]. At the receiving end, DBF technology has been applied to sample combinations of reflected array signals from each element of an array to achieve different functions [4, 5]. Another research topic of interest is a type of flexible array called a frequency diverse array (FDA), which was proposed in [6, 7]. A small frequency increment is applied across all array elements to obtain a rangeangledependent beam pattern. DBF technology has thus been used to enhance beamforming performance [8–10].
The DBF technique is based on the signals received at each antenna element. The set of such signals collected at a given instant of time constitutes a snapshot. The traditional way to obtain such snapshots is to connect each array element to an independent radio frequency (RF) receiver. Therefore, the number of receivers must be equal to the number of antenna elements, which usually causes the overall system to be complicated, bulky, and costly [11]. This problem becomes more serious in the case of largescale arrays. Moreover, the channel uniformity, which significantly affects the performance of the system, should be carefully designed [12].
Various efforts have been made to reduce the hardware cost and design complexity of such systems. One feasible solution is to use a singleRFchannel structure in the antenna array. There are two main kinds of singleRFchannel structures, which use only one receiver channel and therefore are cost effective. The first is the switched antenna array (SAA), in which the connections of the antennas to the single receiving channel are periodically switched to sample the array signals. This structure was proposed in [13], and the corresponding array signal processing method has been discussed in [14–16]. However, although the number of lownoise amplifiers (LNAs) and mixers is reduced, the numbers of lowpass filters and analogtodigital converters (ADCs) still must be proportional to the number of elements [17]. The second type of structure is the time sequence phase weighting (TSPW) antenna array [18, 19], in which the combined phase weighted array signals are sequentially sampled by a single RFreceiving channel. Each antenna element is followed by a 1bit digitally controlled 0/ π phase shifter. The weighted array signals are combined to form a singlechannel output, and only one ADC is required to sample this output. In previous research, the baseband signal has typically been assumed to be timeinvariant. This assumption is acceptable for most communication scenarios in which the sampling cycle is equal to the period of each data symbol [20, 21]. However, when the target has a relative radial velocity, the echo signal will contain a Doppler frequency, which causes the received signal to be timevarying. Therefore, a TSPW array cannot obtain the correct array signals in the case of movement. This is a key problem that must be addressed before the use of TSPW antenna arrays can become widespread.
A Doppler frequency compensation algorithm was proposed in [22] to solve this problem. By observing the targets for a certain length of time, the number of targets and the Doppler frequency of each target are estimated beforehand. This prior information is used to compensate the received singlechannelsampled signals to obtain the correct original signals. However, this algorithm requires preobservation to estimate the number of targets and the Doppler frequencies, which obviously increases the total processing time. In [23], a timereversal motion compensation algorithm was proposed to overcome this drawback. Without requiring prior information, this method allows the correct original array signals to be obtained through interpolation. However, this method requires the sampling frequency to be N times higher than that in a typical multichannel system. This requirement limits the application of this method for largescale arrays.
In this paper, a sparsereconstructionbased signal processing algorithm is proposed. It uses analog mixers and standard (highrate) shift registers [24] to transform the spatially parallel array signals into timesequence signals. This signal processing algorithm is quite different from that used in a TSPW array. A random sampling matrix is adopted in place of the WalshHadamard matrix, which is used in TSPW arrays, to achieve the reduceddimension transformation. The original spatially parallel array signals are recovered from the timesequence sampled signals by exploiting the secondorder sparse property of the array signals. No prior observation is required to obtain the number and Doppler information of the targets. The required sampling frequency is equal to that of a typical multichannel system. With the aid of the proposed algorithm, the direction of arrival (DOA) and the Doppler frequency of the target can be estimated simultaneously. Additionally, this paper also proposes a hardwareachievable measurement matrix optimization method based on the artificial bee colony (ABC) algorithm to improve the reconstruction performance in practical applications.
The remainder of this paper is organized as follows. In Section 2, the array structure and signal model are discussed. In Section 3, the sparsereconstructionbased signal processing algorithm is proposed. To improve the reconstruction performance, a method for optimizing the measurement matrix, which is based on the artificial bee colony (ABC) algorithm, is proposed in Section 4. The simulation results are provided in Section 5 to demonstrate the effectiveness of the proposed algorithm. Conclusions are given in Section 6.
2 The system model
2.1 SingleRFchannel DBF array
2.2 Signal model
To simplify the analysis, this paper adopts the following assumptions. (1) The echo signal is assumed to be the farfield narrowband signal. (2) The target is moving with a constant relative velocity within one transformation period T _{ single }, where T _{ single } = 1/PRF. In other words, the Doppler frequency f _{ d } exhibits no significant change during one T _{ single }. 3). The direction of arrival (DOA) of the target does not change within one T _{ single }.
where G _{ n } is the gain of the nth low noise amplifier (LNA) and ε _{ n }(t) is the receiver noise of the nth element.
After M pulses, the signal S _{ B }(m T _{ single }) has been multiplied by H _{ n }(m T _{ single }), m=1,…,M. It contains all of the information of the target and can be used to recover the original signals. The signal at point C represents the recovered spatially parallel signal for the ith singlechannelsampling period.
It can be seen that the signal is disturbed by both the Doppler phase difference \(\Delta {\tilde \varphi _{{f_{d}}}}\) and the spatial phase difference \({\Delta }{\tilde \phi _{n}}\). If the matrix inversion algorithm proposed in [19] was still to be used to obtain the original signals, then the recovered signal S _{ C,n }(i T _{ single }) would contain incorrect target information and could not be used for further signal processing.
3 Sparsereconstructionbased signal processing
3.1 Timesequence sampling model
where G=[G _{1},⋯,G _{ N }]^{ T }; Φ(m)=[H _{1}(m),⋯,H _{ N }(m)]^{ T }; x(m)=[x _{1}(m),⋯,x _{ N }(m)]^{ T }; \(\bigodot \) and <,> denote the Hadamard product and the inner product, respectively; and σ(m) is the receiver noise. The weighting vector Φ(m) is produced by the analog mixers and shift registers shown in Fig. 1, and the value of each element of Φ(m) is 1 or −1. Thus, the vector Φ(m) can be designed as a Bernoulli random vector, which also consists of values of ±1 and has a low coherence with any fixed basis.
where \(\boldsymbol {\bar {\Phi }}\) is the equivalent measurement matrix and 0 is a 1×N zero vector.
The (NM)dimensional vector signal X=[x ^{ T }(1),⋯,x ^{ T }(M)]^{ T } contains the same information as that contained in the multisnapshot array signal \(\tilde {\mathbf {X}} = [\textbf {x}\left (1 \right), \cdots,\textbf {x}\left (m \right)]\), which is the N×Mdimensional matrix obtained by a normal multichannel DBF array. However, there are certain differences between these two signals. The signal \(\tilde {\mathbf {X}}\) has a parallel structure and is obtained by directly sampling the signals from each array element with multiple receivers. It is known immediately after sampling. By contrast, the signal X has a sequential structure and cannot be obtained directly after sampling. It is wrapped in the singlechannelsampled vector Y _{1:M } which consists of the real signals sampled in the singlechannel DBF system.
3.2 The firstorder sparse expression for the array signal
Because Eq. (11) transforms the array signal only in spatial domain, it can be regarded as the firstorder sparse expression of the array signal. However, the signal X contains M snapshots, and the time information has been added into the signal. The sparse projection vector \({\boldsymbol {\tilde \xi } = {[\!{\boldsymbol {\xi }}^{T}}\left (1 \right), \cdots,{{\boldsymbol {\xi }}^{T}}\left (M \right)]^{T}}\) will have K×M nonzero entries, which causes (11) to be an imperfect sparse expression. The signal X should be written in a sparser form to ensure a better performance before applying sparse reconstruction theory to recover it.
3.3 The secondorder sparse expression for the array signal
3.4 Sparse reconstruction
where C _{0} is a positive constant. Notably, increasing M can improve the Doppler frequency resolution at the cost of significantly extending the processing time. There is therefore a tradeoff between the performance and the processing time cost in practical applications.
where η>0 is the regularization parameter, which corresponds to the noise power, and ∥·∥_{1} denotes the l _{1}norm, which is equal to the sum of the absolute values of the vector entries. The parameter η controls the tradeoff between the sparsity approximation and the least square error of the recovered signal [34]. The parameter selection method has been comprehensively studied in [35], and we therefore omit such a discussion in this paper.
where u n v e c _{ M,P }(·) is the inverse vectorization function such that \(unvec_{M,P}({\boldsymbol {\bar \chi }})\) is an M×P matrix that enables the conversion from the M P×1 vector \(\boldsymbol {\bar \chi }\).
4 Optimization
4.1 Optimization of the measurement matrix
Because analog mixers and shift registers can easily be realized in practical applications, they are used in the singlechannel DBF array to measure the array signals. The shift registers provide only two phaseweighting states. Therefore, the measurement matrix Φ consists of values of ±1.
The measurement matrix can be optimized to achieve better reconstruction performance [36–39]. However, the existing algorithms generally ignore the practical problem that arises when the values of the optimal measurement matrix elements are too precise to be achieved by the hardware. In other words, the hardwarebased measurement matrix generally cannot provide the measurement values that are produced by such optimization algorithms. For example, the digital micromirror device (DMD) in a singlepixel camera [40] cannot provide reflection directions other than ±10°. The recovery algorithm based on the singleRFchannel DBF array also faces the same problem. Thus, to improve the reconstruction performance, an optimization algorithm is proposed that can produce hardwarerealizable measurement values.
where \(\textbf {L} = {\tilde {\mathbf {D}}^{H}}\tilde {\mathbf {D}}\) is the socalled Gram matrix and t r i u(·) is a function such that triu(L) extracts the upper triangular entries from the offdiagonal part of the matrix L. To achieve highreconstruction performance, the value of (18) should be designed to be as small as possible [41]. From section 3.4, we know that \(\boldsymbol {\bar \Psi }\) is a fixed matrix associated with the detection requirements; therefore, we can optimize only \(\boldsymbol {\bar \Phi }\) to reduce μ(D).
Based on the fact that the hardware can provide only two weighting values of ±1, all possible observations form the feasible solution set Ω. The optimal observation matrix Φ _{ opt } that minimizes μ(D) must be included in this set. For a singleRFchannel system with N elements and M observations, the number of feasible solutions in Ω is 2^{ N M }. Directly finding an optimal solution from within this enormous feasible set is certainly an NPhard problem.

The steps of the ABC algorithm for optimizing the measurement matrix for the recovery algorithm are as follows.

The control parameters are initialized, and the initial food sources are prepared. Each food source is a feasible matrix from the collection Ω. The Bernoulli random matrix can be used for initialization.

The employed bees search for new food sources within the neighborhood of the current food source. The strategy for determining neighboring food sources is described as follows: (a) A zero entry of the matrix \(\bar{\boldsymbol{\Phi}}_{m}  \bar{\boldsymbol{\Phi}}_{k}\) is randomly selected, where \(\bar{\boldsymbol{\Phi}}_{m}\) is the current food source and \(\bar{\boldsymbol{\Phi}}_{k}\) is the randomly selected neighboring food source. (b) The element value of \(\bar{\boldsymbol{\Phi}}_{m}\) is reversed at the position selected in a). (c) The new food source is used to calculate the fitness value by applying the function described in [42]

The unemployed bees then work to find the best solutions.

If the maximum number of cycles has been reached, then the algorithm terminates and outputs the optimal solution. Otherwise, the process is repeated from step 2.
4.2 Optimization of the computational complexity
The numbers of angle grids P and singlechannel samplings M influence the angular and velocity resolutions, respectively, of the recovery algorithm. The values of these two parameters can be increased to improve the recovery performance. However, this will significantly increase the computational complexity. To reduce the computational complexity for practical applications, a coarse grid can be used first to find the approximate source information. Then, the neighboring regions of the targets can be searched again, using a finer grid. The accurate sparse projection vector is thus obtained by using a fine grid in a few small regions.
In practice, when searching in these small regions, the processing time can be shortened by using multiple Digital Signal Processors (DSPs) with a parallel structure. The search region can be separated into several segments according to the angle and Doppler information shown in Fig. 2. Based on the detection results obtained using the coarse grid, the corresponding segments can be extracted and assigned to multiple processors to search for more accurate results with finer grids.
The CPU processing time
W  P  Angle resolution (degree)  Velocity resolution (m/s)  CPU time (s)  CPU time of parallel structure (s) 

100  30  3  30  2.24  0.96 
100  90  1  30  4.19  1.18 
200  180  0.5  15  66.47  11.87 
300  180  0.5  10  147.71  24.80 
The processing time of the proposed algorithm includes two primary components. The first is the time consumption of the singlechannel sampling procedure. This procedure has the same complexity as that for a typical multichannel DBF array, which is O(M). The second component is the time consumption of the sparse reconstruction procedure. The computational complexity of this procedure primarily depends on the chosen recovery algorithm. For example, the computational complexity of the recovery algorithm used in this paper is O((M P)^{3}) [43]. However, this recovery algorithm could be replaced with a faster one. For example, the Orthogonal Matching Pursuit (OMP) algorithm could be adopted, in which case the computational complexity would be O(K M ^{2} P).
5 Simulation results
In this section, several numerical simulations are presented to evaluate the proposed algorithms. First, the original spatially parallel signals are recovered using the proposed algorithm, and the results are compared with those that are obtained directly by a typical multichannel DBF array in a moving scenario. Second, the ability of the proposed algorithm to simultaneously estimate the angle and Doppler information of the targets is evaluated by means of a classical parameter estimation problem. Finally, a simulation is presented to demonstrate the performance improvement achieved using the proposed measurement matrix optimization method.
A linear Xband antenna array with N=64 elements is used. The spacing of the adjacent elements is 0.5 λ, and the center frequency is 10 GHz. The PRF is 200 kHz, the number of singlechannel samples is M=200, and the number of angle grids is P=180. The receiver noise is assumed to be zeromean Gaussian white noise.
5.1 Evaluation of the signal reconstruction
The performance of the algorithm proposed in [22] is associated with the frequency estimation errors. When the frequencies of two targets are close to each other, it becomes difficult to distinguish them with the normal methods. This may cause a serious decline in recovery performance. The performance of the algorithm proposed in [23] is associated with the number of samples. More samples will obtain a higher reconstruction accuracy. However, it will spend much more time in sampling and computing. In this simulation, the number of samples is 1024, which is four times as much as that used in the proposed method. It can be seen from Fig. 5 that the recovery error of the proposed method is less than −10 dB when the SNR is greater than 0 dB. However, in the lowSNR scenario, the algorithm fails to function properly.
5.2 Parameter estimation
It can be seen that the angle and Doppler information of the targets are estimated correctly. Moreover, it can be seen from the estimated results for targets A and B that the sources can be resolved even when the angle of separation between them is less than the classical Rayleigh resolution limit. This finding indicates that the algorithm has the capability of angular superresolution.
5.3 Optimization of the measurement matrix
In this subsection, the measurement matrix optimization method is evaluated. For these calculations, the parameters of the ABC algorithm were set as follows: the number of employed and unemployed bees is 40, the number of food sources is 20, and the number of cycles for foraging is 3000.
5.4 Simulations of the weighting circuit error
The weighting circuit that is used in a practical application is subject to gain and phase shift errors. Because these errors generate a nonorthogonal WalshHadamard measurement matrix, these errors can result in completely incorrect recovered array signals.
6 Conclusions
Array signal processing algorithms based on sparse signal reconstruction are proposed for a lowcost singlechannel DBF array. The secondorder sparse property of the array signals is used to decouple the angle and velocity information and to obtain the correct original parallel array signals from the weighted timesequence signals. The DOA and Doppler information of the targets can be simultaneously estimated using this lowcost scheme, which uses a lowcost, highefficiency receiver array in place of the traditional design. The proposed approach can be adopted in several expensive and complicated array systems, such as FDAs and MIMO systems, to reduce their total cost.
Declarations
Competing interests
The authors declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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