 Research
 Open Access
Superimposed signaling inspired channel estimation in fullduplex systems
 Abbas Koohian^{1}Email authorView ORCID ID profile,
 Hani Mehrpouyan^{2},
 Ali A. Nasir^{3},
 Salman Durrani^{1} and
 Steven D. Blostein^{4}
https://doi.org/10.1186/s1363401805299
© The Author(s) 2018
 Received: 3 July 2017
 Accepted: 11 January 2018
 Published: 24 January 2018
Abstract
Residual selfinterference (SI) cancellation in the digital baseband is an important problem in fullduplex (FD) communication systems. In this paper, we propose a new technique for estimating the SI and communication channels in a FD communication system, which is inspired from superimposed signaling. In our proposed technique, we add a constant real number to each constellation point of a conventional modulation constellation to yield asymmetric shifted modulation constellations with respect to the origin. We show mathematically that such constellations can be used for bandwidth efficient channel estimation without ambiguity. We propose an expectation maximization (EM) estimator for use with the asymmetric shifted modulation constellations. We derive a closedform lower bound for the mean square error (MSE) of the channel estimation error, which allows us to find the minimum shift energy needed for accurate channel estimation in a given FD communication system. The simulation results show that the proposed technique outperforms the dataaided channel estimation method, under the condition that the pilots use the same extra energy as the shift, both in terms of MSE of channel estimation error and bit error rate. The proposed technique is also robust to an increasing power of the SI signal.
Keywords
 Fullduplex systems
 Channel estimation
 Selfinterference cancellation
 Superimposed signaling
1 Introduction
1.1 Background
Fullduplex (FD) communication, allowing devices to transmit and receive over the same temporal and spectral resources, is a promising mechanism to potentially double the spectral efficiency of future wireless communication systems [1]. The main challenge in implementing a FD communication system is the cancellation of the strong selfinterference (SI) signal, which is caused by transmission and reception in a single frequency band [2, 3]. This strong selfinterference signal has to be suppressed to the receiver noise floor in order to ensure that it does not degrade the system performance. For instance, for smallcells in Long Term Evolution (LTE), the maximum transmit power is typically 23 dBm (200 mW) and the typical noise floor is − 90 dBm [4]. Ideally, this requires a total of 113 dB SI cancellation for realizing the full potential of FD systems [4].
Recently, there has been a lot of interest in SI cancellation techniques for FD systems [5–11]. The SI cancellation techniques in the literature can be divided into two main categories [5]: (i) passive suppression in which the SI signal is suppressed by suitably isolating the transmit and receive antennas [5, 6] and (ii) active cancellation which uses knowledge of the SI signal to cancel the interference in either the analog domain (i.e., before the signal passes through the analogtodigital converter) [5, 7] and/or the digital domain [8–10]. Depending upon the design, passive suppression and analog cancellation can provide about 40–60 dB cancellation in total [11]. Hence, in practice, the SI is cancelled in multiple stages, beginning with passive suppression and followed by cancellation in the analog and digital domains. In this paper, we focus on the SI after the passive suppression and analog cancellation, termed residual SI.
1.2 Motivation and related work
The residual SI can still be relatively strong in the baseband digital signal, e.g., for the LTE smallcell example, it can be as high as 50 dB assuming stateoftheart passive suppression and analog cancellation provide 60 dB of the total required SI cancellation of 113 dB. Thus, accurate digital SI cancellation is required to bring the SI as close to the noise floor as possible. The effectiveness of any digital interference cancellation technique depends strongly on the quality of the available channel estimates for both the SI and desired communication channels [12–14]. Typically, the baseband channels are estimated by using dataaided channel estimation techniques, where a portion of the data frame is allocated for known training sequences or pilot symbols [13, 15–17]. In this regard, a maximumlikelihood (ML) approach was proposed in [13] to jointly estimate the residual SI and communication channels by exploiting the known transmitted symbols and both the known pilot and unknown data symbols from the other intended transceiver. Another approach was proposed in [18] where a subspacebased algorithm was developed to jointly estimate the residual SI and communication channels.
Compared to halfduplex (HD) systems, dataaided channel estimation in FD systems can be bandwidth inefficient. This is because, firstly, two channels need to be estimated and, secondly, accurate channel estimation requires a larger number of pilots [19, 20]. A bandwidth efficient channel estimation technique in HD systems is superimposed training, where no explicit time slots are allocated for channel estimation. Instead, a periodic low power training sequence is superimposed with the data symbols at the transmitter before modulation and transmission [21, 22]. The downside of this approach is that some power is consumed in superimposed training which could have otherwise been allocated to the data transmission. This lowers the effective signaltonoise ratio (SNR) for the data symbols and affects the bit error rate (BER) at the receiver. In contrast to dataaided and superimposed trainingbased channel estimation techniques, blind techniques avoid the use of pilots altogether by exploiting statistical and other properties of the transmitted signal [23–27]. However, blind estimators can only estimate the channel up to a scaling factor and cannot recover the channel phases [23]. The necessary and sufficient conditions for ambiguityfree blind estimation can be determined using identifiability analysis, which determines whether a parameter can be uniquely estimated without any ambiguity [19, 23, 28–30]. To the best of our knowledge, bandwidth efficient and accurate channel estimation methods for FD systems are still an important open area of research.
1.3 Paper contributions

We derive the condition for identifiability of channel parameters in a FD system (cf. Theorem 1) and show that symmetric modulation constellations with respect to the origin cannot be used for ambiguityfree channel estimation in a FD system. Based on Theorem 1, our proposed technique is able to resolve the inherent ambiguity of blind channel estimation in FD communication via shifting the modulation constellation away from origin.

Using the proposed technique, we derive a computationally efficient expectation maximization (EM) estimator for simultaneous estimation of both SI and communication channels. We derive a lower bound for the channel estimation error, which depends on the energy used for shifting the modulation constellations, and use it to find the minimum signal energy needed for accurate channel estimation in a given FD communication system.

We use simulations to compare the performance of the proposed technique against that of the dataaided channel estimation method, under the condition that the pilots use the same extra power as the shift. Our results show that the proposed technique performs better than the dataaided channel estimation method both in terms of the mean square error (MSE) of channel estimation and BER. In addition, the proposed technique is robust to an increasing SI power.
1.4 Notation and paper organization
The following notation is used in this paper. Capital letters are used for random variables, and lower case letters are used for their realizations. f_{ Y }(y) denotes the probability density function (PDF) of random variable Y. \(\mathbb {E}_{Y}[\!\cdot ]\) denotes the expectation with respect to the random variable Y. p_{ X }(x) denotes the probability mass function (PMF) of a discrete random variable X, and P(X=a) is the probability of the discrete random variable X taking the value a. \(\mathcal {CN}\left (\mu,\sigma ^{2}\right)\) denotes a complex Gaussian distribution with mean μ and variance σ^{2}. Bold face capital letters, e.g., Y, are used for random vectors, and bold face lower case letters, e.g., y, are used for their realizations. Capital letters in upright Roman font, e.g., G, are used for matrices. Lower case letters in upright Roman font, e.g., g, are used for functions. I_{ N } represents the N×N identity matrix. [·]^{ T } denotes vector and matrix transpose. \(\mathrm {j}\triangleq \sqrt {1}\) and the real and imaginary parts of a complex quantity are represented by ℜ{·} and I{·}, respectively. z^{∗} and z indicate scalar complex conjugate and the absolute value of complex number z, respectively. Finally, det(·) is the determinant operator.
This paper is organized as follows. The system model is presented in Section 2. The channel estimation problem and the proposed technique are formulated in Section 3. The EM estimator and the lower bound on the channel estimator error are derived in Section 4. The performance of the proposed technique is assessed in Section 5. Finally, conclusions are presented in Section 6.
2 System model
where \(\mathbf {x}_{\mathbf {a}}\triangleq \left [x_{a_{1}},\cdots, x_{a_{N}}\right ]^{T}\) and \(\mathbf {x}_{\mathbf {b}}\triangleq \left [x_{b_{1}},\cdots, x_{b_{N}}\right ]^{T}\) are the N×1 vectors of transmitted symbols from nodes a and b, respectively, \(\mathbf {y}_{\mathbf {a}}\triangleq \left [y_{a_{1}},\cdots, y_{a_{N}}\right ]^{T} \) is the N×1 vector of observations, w_{ a } is the noise vector, which is modeled by N independent Gaussian random variables, i.e., \(f_{\mathbf {W}_{\mathbf {a}}}(\mathbf {w}_{\mathbf {a}}) = \mathcal {CN}\left (\mathbf {0},\sigma ^{2} \mathrm {I}_{N}\right)\), and h_{ aa } and h_{ ba } are the residual SI and communication channel gains, respectively. Furthermore, we model h_{ aa } and h_{ ba } as independent random variables that are constant over one frame of data and change independently from frame to frame [12].
Remark 1
Including all the hardware impairments and unknown parameters in mathematical modeling of parameter estimation problem in FD communication results in a highly nonlinear system model, which may not have a tractable solution. The current approach is to separate the estimation of the linear and nonlinear parameters [13,18]. In this paper, we focus on the estimation of linear parameters, while the estimation of nonlinear parameters can be the topic of future works.
2.1 Modulation assumptions and definitions
In this paper, we assume that the transmitted symbols are all equiprobable and call the set \(\mathcal {A}\triangleq \{x_{1},x_{2},...,x_{M}\}\), which contains an alphabet of M constellation points, a modulation set. Let \(\mathcal {K} \triangleq \{1,\cdots,M\}\) denote set of indices of the constellation points.
where \(x_{k} \in \mathcal {A}\). Note that the average symbol energy can be related to the average bit energy as \(E_{b} \triangleq E/\log _{2}(M)\).
3 Channel estimation for FD systems
In this section, we first formulate the blind channel estimation problem for the FD system considered in Section 2. Based on this formulation, we present a theorem which provides the necessary and sufficient condition for ambiguityfree channel estimation. Finally, we discuss the proposed technique to resolve the ambiguity problem.
3.1 Problem formulation
Without loss of generality, we consider the problem of baseband channel estimation at node a only (similar results apply at node b). In formulating the problem, we make the following assumptions: (i) the transmitter is aware of its own signal, i.e., x_{ a } is known at node a, which is a commonly adopted assumption in the literature [3,5], (ii) the interference channel h_{ aa } and the communication channel h_{ ba } are unknown deterministic parameters, (iii) the transmit symbol from node b is modeled using a discrete random distribution, and (iv) we observe N independent received symbols.
where \(i \in \mathcal {I}\triangleq \{1,\cdots,N\}\), \(y_{a_{i}}\) is the i^{ t h } received symbol, and \(x_{a_{i}}\) and \(x_{b_{i}}\) are the i^{ t h } transmitted symbols from nodes a and b, respectively.
where we substitute the value of \(f_{Y_{a_{i}}}(y_{a_{i}};h_{aa},h_{ba})\) from (4).
Using (5), we can state the channel estimation problem as shown in the proposition below.
Proposition 1
where \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};h_{aa},h_{ba})\) is given by (5).
In the next subsection, we show that (6) does not have a unique solution if modulation sets which are symmetric around the origin are used.
3.2 Identifiability analysis
where θ(1) and θ(2) represent the first and second elements of θ.
We start the identifiability analysis by presenting the following definition and remark:
Definition 1
([28], Definition 5.2) If Y is a random vector distributed according to f_{ Y }(y;θ), then θ is said to be unidentifiable on the basis of y, if ∀y there exists θ^{′}≠θ for which f_{ Y }(y;θ)=f_{ Y }(y;θ^{′}).
Remark 2
Definition 1 states that θ and θ^{′} (θ≠θ^{′}) cannot be distinguished from a given set of observations if they both result in the same probability density function for the observations. This implies that if θ is unidentifiable, then it is impossible for any estimator to uniquely determine the value of θ.
In order to present the main result in this subsection, we first give the definitions of a symmetric modulation constellation [31] and a bijective function [32].
Definition 2
We mathematically define modulation constellation as the graph of the function f(x_{ k })=x_{ k }, where \(x_{k} \in \mathcal {A} \forall k \in \mathcal {K}\). Then a modulation constellation is symmetric with respect to the origin if and only if \(\mathrm {f}(x_{k})=\mathrm {f}(x_{k}) \forall x_{k} \in \mathcal {A}\)[31].
Definition 3
Let \(\mathcal {C}\) and \(\mathcal {D}\) be two sets. A function from \(\mathcal {C}\) to \(\mathcal {D}\) denoted \(\mathrm {t}: \mathcal {C} \rightarrow \mathcal {D}\) is a bijective function if and only if it is both onetoone and onto.
The above definition states that a bijective function is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other set and there are no unpaired elements. Note that a bijective function from a set to itself is also called a permutation [32].
In this work, we define and use the bijective function g: \(\mathcal {K} \rightarrow \mathcal {K}\), i.e., g is a onetoone and onto function on \(\mathcal {K}\rightarrow \mathcal {K}\). Using this bijective function, we present the main result as below.
Theorem 1
There exists a θ^{′}≠θ for which the joint probability density \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta })\) given by (7) is equal to \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }') \; \forall \mathbf {y}_{a}\), if and only if there exists a bijective function g: \(\mathcal {K} \rightarrow \mathcal {K}\), such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \; \forall k \in \mathcal {K}\), where c≠1 is a constant and c=1, i.e., the modulation constellation is symmetric about the origin.
Proof
We prove the result in Theorem 1 in three steps. First, we assume θ^{′}≠θ for which \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta })=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }') \forall \mathbf {y}_{\mathbf {a}}\) exists and show that it leads to a bijective function g satisfying \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k \in \mathcal {K}\). Then, we assume that a bijective function g satisfying \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k \in \mathcal {K}\) exists and show that there exists a θ^{′}≠θ for which \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta })=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }') \forall \mathbf {y}_{\mathbf {a}}\). Finally, using Definition 2, we show that the condition \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k \in \mathcal {K}\) is equivalent to the modulation constellation being symmetric with respect to the origin. The details are in Appendix 1. □
Remark 3
From Theorem 1, we can see that since the modulation constellations, such as Mary quadrature amplitude modulation (MQAM), satisfy the definition of symmetric modulation constellations in Definition 2, the blind channel estimation problem in (6) does not have a unique solution and suffers from an ambiguity problem.
3.3 Proposed technique
In this subsection, we present our proposed technique to resolve the ambiguity problem in (6).
The rationale behind the proposed technique comes from the fact that Theorem 1 shows that symmetry of the modulation constellation with respect to the origin is the cause of the ambiguity. A simple way to achieve constellation asymmetry^{2} is to add a constant s to each element of \(\mathcal {A}\). The resultant asymmetric shifted modulation constellation is formally defined as follows:
Definition 4
where \(\mathbb {R}^{+}\) is the set of positive real numbers.
In the rest of the paper, we also use \(\bar {x}_{k}= x_{k}+s\) to denote the kth element of \(\mathcal {\bar {A}}\).
Remark 4
The addition of the DC component lowers power efficiency similar to the use of superimposed training [22]^{3}. However, the proposed shifted modulation technique has the offsetting advantages that (i) bandwidth efficiency is not reduced and (ii) the DC offset can be used to reduce the peaktoaverage power of the signal envelope during transmissions resulting in lowered cost/complexity power amplifiers. Moreover, the proposed scheme is wellsuited to MQAM as investigated here. Since, this is a spectrally efficient modulation scheme used where power efficiency is not critical
For the sake of numerically investigating the problem of smallest possible shift energy, we define β as the portion of the average energy per symbol that is allocated to the shift and use the real constant \(s\triangleq \sqrt {\beta E}\), where 0<β<1 to shift the symmetric modulation constellation. In this case, the problem of smallest shift energy corresponds to the problem of finding the minimum value of β. The minimum value of β is an indication of how much extra energy is needed compared to the perfect channel knowledge scenario.
In Section 4.1, we derive a lower bound on the estimation error, which allows us to numerically find the minimum value of β.
4 EMbased estimator
In this section, we derive an EM estimator to obtain channel estimates in a FD system with asymmetric shifted modulation constellation defined in Definition 4. We derive a lower bound on the estimation error of the estimator. Finally, we investigate the complexity of the proposed estimator.
where \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}} (\mathbf {y}_{\mathbf {a}} ; \boldsymbol \phi)\) is given by (7) and \(\ln f_{\mathbf {Y}_{\mathbf {a}}} (\mathbf {y}_{\mathbf {a}} ; \boldsymbol \phi)\) is known as the loglikelihood function.
 1.
Expectation step: In the Estep, the expectation of the loglikelihood is taken over all the values of the hidden variable, conditioned on the vector of observations, and the nth estimate of ϕ (ϕ^{(n)}). In (1), the hidden variable is \(\bar {\mathbf {x}}_{\mathbf {b}}\) and consequently, we need to evaluate \(Q(\boldsymbol {\phi }\boldsymbol {\phi }^{(n)})\triangleq \mathbb {E}_{\bar {\mathbf {X}}_{\mathbf {b}}\mathbf {y}_{\mathbf {a}},\boldsymbol {\phi }^{(n)}}[\ln {f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}},\bar {\mathbf {x}}_{\mathbf {b}}\boldsymbol {\phi })}]\).
 2.
Maximization step: In the Mstep, the function Q(ϕϕ^{(n)}) obtained from the Estep is maximized with respect to ϕ.
 3.
Iterations: We iterate between the E and Msteps until convergence is achieved.
The equations needed for the E and Msteps are summarized in the propositions below.
Proposition 2
where \(k \in \mathcal {K}\), \(i \in \mathcal {I}\triangleq [1,2,\cdots,N]\), \(\bar {x}_{k} \in \mathcal {\bar {A}}\), \(\bar {x}_{a_{i}} \in \mathcal {\bar {A}}\) and \(\bar {x}_{\bar {k}} \in \mathcal {\bar {A}}\).
Proof
See Appendix 2. □
Proposition 3
Proof
See Appendix 2. □
Remark 5
It is wellknown that the EM algorithm may be very sensitive to initialization [34]. Although different methods exist for EM initialization, generally they are not computationally efficient [34, 35]. For the given channel assumptions in Section 5, our empirical results showed that initializing the EM algorithm by \(\boldsymbol {\phi }^{(0)} \triangleq [0,0,0,0]\) resulted in the lowest estimation error. Hence, this initialization is used in this work.
4.1 Lower bound on the estimation error
In this section, we derive a closedform lower bound on the estimation error of the proposed estimator. The derived lower bound directly links the channel estimation error to the parameter β, defined in Section 3.3.
where ϕ_{ l } is the lth element of the parameter vector ϕ, \(\hat {\phi }_{l}\) is an estimate of ϕ_{ l }, for l∈{1,2,3,4}, [ ·]_{l,l} is the lth diagonal element of a square matrix, and \(\mathrm {I}^{1}\left [f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{a};\boldsymbol {\phi })\right ]\) is the inverse of FIM. Since the inverse of FIM in (15) cannot be found in closedform [24, 37], we derive a lower bound on the MSE of the proposed estimator, which is in closedfrom. The result is presented in the proposition below.
Proposition 4
where l∈{1,⋯,4}, N is the number of observations, E is the average symbol energy of the modulation constellation before the shift, and β is the portion of E that is allocated to the shift.
Proof
See Appendix 3. □
Remark 6
The result in (16) links the closedform lower bound of the estimation error to the average energy of the modulation constellation before the shift and the portion of this average energy allocated to the shift. This is important because in Section 5.1, we will use (16) to find the minimum shift energy needed for the proposed technique.
4.2 Complexity analysis
To evaluate the feasibility in implementing the proposed estimator, we investigate the computational complexity of the estimator in terms of required floating point multiplications and additions (flops) [38].
It is clear from Table 1 that the complexity of EM estimator per iteration is proportional to NM^{2}. This analysis shows that the EM algorithm is computationally very efficient since, for a given modulation constellation with size M, the computational complexity of the EM estimator only grows linearly with the number of observations, N.
Remark 7
In dataaided approaches to channel estimation, both x_{ a } and x_{ b } in (1) are assumed to be known. Consequently, linear channel estimation can be performed for linear Gaussian models (LGMs) as explained in [36]. It is a wellknown fact that linear estimator complexity estimators for LGM is independent of the modulation size M, and only grows linearly with the number of observations [36]. The extra complexity of the proposed algorithm compared to linear estimators is expected. This is because as opposed to linear estimators, the proposed estimator requires no dataaided piloting, and hence, it allows for efficient use of the bandwidth for channel estimation.
5 Simulation results
In this section, we present numerical and simulation results to investigate the performance of the proposed estimator with asymmetric shifted modulation constellation. We consider a FD communication system as illustrated in Fig. 1. The analysis in Section 4.1 shows an identical lower bound for the estimation error of both h_{ aa } and h_{ ba }. Hence, in this section, we only present the results for the communication channel h_{ ba } since identical results are obtained for the SI channel h_{ aa }.
where ζ is uniformly distributed angle of arrival of the LOS component of the SI channel [39].
where the signaltointerference ratio \(\text {SIR}=\frac {\sigma ^{2}_{h_{ba}}}{\sigma ^{2}_{h_{aa}}}\) assuming both nodes use constellations with the same average energy, the desired signaltonoise ratio \(\text {SNR}=\frac {\sigma ^{2}_{h_{ba}}\log _{2}{(M)}E_{b}}{N_{0}}\), E_{ b } is the average bit energy which is defined below (2) and N_{0} is the noise power spectral density.
As discussed in Section 1, even with stateoftheart passive suppression and analog cancellation, the SIR can still be around − 5 dB [5,40]. Hence, we adopt this value of the SIR in the simulations while assuming that the communication channel has average energy of unity, i.e., \(\mathbb {E}\left [h_{ba}^{2}\right ]=\sigma ^{2}_{h_{ba}}=1\). Furthermore, in order to investigate the performance of the proposed estimator over a range of SINR, we fix N_{0}=1 and run the simulations for different values of E_{ b }/N_{0} (in dB). The figures of merit used are the average mean square error (MSE) and the BER, which are obtained by averaging over 5000 Monte Carlo simulation runs.
5.1 Minimum energy needed for channel estimation
In this subsection, we are interested in finding the minimum value of β, for a given E_{ b }/N_{0} and N. As discussed in Section 3.3, we use \(s\triangleq \sqrt {\beta E}\), where 0<β<1, to shift the symmetric modulation constellation. Hence, a lower value of β is desirable since it means less energy is used to shift the modulation constellation.
In order to find a minimum value of β suitable for a practical range of E_{ b }/N_{0} and N, we use the average MSE lower bound in (16) to observe the behavior of the proposed estimator as a function of β at low N and low E_{ b }/N_{0}. This is motivated by the fact that the minimum value of β found for low N and low E_{ b }/N_{0} will ensure that the desired estimation error will also be achieved for high E_{ b }/N_{0} and/or when the number of observations N is large. This intuition is confirmed from (16), which indicates that higher values of β are needed at low E_{ b }/N_{0} to reach a given estimation error. Furthermore, since the lower bound on the estimation error also decreases with N, the minimum value of β found for smaller N can also serve for larger N. Since the experimental results of [5,41] show that the FD communication channel is normally constant for more than N>128 symbols, we propose to find the minimum β at N=128 and E_{ b }/N_{0}=0 dB.
In the following sections, we set β=0.2 and N=128 to study the performance of the FD communication system.
5.2 Comparison with dataaided channel estimation
In this section, we compare the MSE and BER performance of the proposed estimator against a dataaided channel estimator for the case that the average energy per transmitted frame is the same for both methods^{4}. For the proposed technique, we assume that (i) all the transmitted symbols are data symbols and (ii) shifting the modulation constellation increases the average energy by 20% compared to the ideal scenario when no channel estimation is needed (corresponds to β=0.2). For the dataaided channel estimation, we assume that (i) 64 pilot symbols are used in a frame of 128 symbols and (ii) these pilots also require an extra 20% energy.
5.2.1 MSE performance
It has been shown in [9] that the effect of quantization error in FD communication system can be modeled as an additive Gaussian noise. This means the system model given by (1) implicitly includes the effect of quantization noise as well as the effect of thermal noise in the Gaussian noise term w_{ a }. Consequently, the effect of quantization noise on the performance of the proposed estimator can be studied by observing the MSE results of the proposed estimator in the low SNR region as shown by Fig. 5. The results of Fig. 5 show that at low SNR region, a noticeable MSE gain is not obtained by using the proposed estimator instead of the dataaided technique that uses 128 pilots. However, the proposed technique is still more attractive compared to the dataaided technique because of the bandwidth efficiency.
5.2.2 BER performance
As shown in this subsection, in comparison to the dataaided algorithms, the proposed algorithm is more bandwidth efficient. Also compared to the existing blind algorithms, which suffer from phase ambiguity [23], the proposed algorithm can estimate the channel with no phase ambiguity. These advantages have been obtained by an increase in complexity as explained in Remark 7. However, the superior performance of the proposed algorithm as shown in Figs. 5 and 6 produces attractive tradeoffs compared to the existing dataaided and blind algorithms.
5.3 Effect of power of SI signal
In the results so far, we have set the SIR to − 50 dB. In this section, we assess the impact of the SI power level on the performance of the proposed technique.
6 Conclusions
In this paper, we have proposed a new technique to estimate the SI and communication channels in a FD communication systems for residual SI cancellation. In the proposed technique, we add a real constant number to each constellation point of a modulation constellation to yield asymmetric shifted modulation constellations with respect to the origin. Using identifiability analysis, we show mathematically that such a modulation constellation can be used for ambiguityfree channel estimation in FD communication systems. We proposed a computationally efficient EMbased estimator to estimate the SI and communication channels simultaneously using the proposed technique. We also derived a lower bound for the estimation error of the proposed estimator. The results showed that the proposed technique is robust to the level of SI power.
7 Appendix 1
7.1 Proof of Theorem 1
The proof consists of three main steps.
Step 1: We show that if θ^{′}≠θ exists such that \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }')=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }) \forall \mathbf {y}_{a}\), then a bijective function \(\mathrm {g}: \mathcal {K}\rightarrow \mathcal {K}\) exists, such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k\in \mathcal {K}\), where c≠1 is a constant and c=1. This is done as follows.
If \(f_{Y_{a_{i}}}(y_{a_{i}};\boldsymbol {\theta }')\) and \(f_{Y_{a_{i}}}(y_{a_{i}};\boldsymbol {\theta })\) are equal \(\forall y_{a_{i}}\), they should also be equal for \(y_{a_{i}}=\boldsymbol {\theta }(1)x_{a_{i}}+\boldsymbol {\theta }(2)x_{1}\). In this case, we have
The LHS of (25) does not depend on k; consequently, the RHS of (25) should also be independent of k and should be a constant. Hence, for bijective function g, \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c\), where c≠1 is a constant. We note that if c=1 then θ(2)=θ^{′}(2) and hence θ=θ^{′}, which violates the assumption that \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }')=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta })\) for θ≠θ^{′}.
The sequence (x_{ k },Π(x_{ k }),Π(Π(x_{ k })),⋯,x_{ k }) forms an orbit of the permutation Π [32]. If \(x_{k}=c x_{\mathrm {g(k)}} \forall k \in \mathcal {K}\), then from the definition of the orbit it is clear that \(x_{k}=c^{m}x_{k} \forall k \in \mathcal {K}\), where m is the length of the orbit sequence and c≠1 is a constant. Since, \(x_{k}=c^{m}x_{k} \forall k \in \mathcal {K}\), we can conclude that c^{ m }=1 and c=1.
Step 2: We show that if the bijective function g: \(\mathcal {K} \rightarrow \mathcal {K}\) exists, such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k \in \mathcal {K}\), then there exists a θ^{′}≠θ for which \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }')=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }) \forall \mathbf {y}_{a}\). This is done as follows.
Comparing (30) with (28) reveals that for θ^{′}≠θ, \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }')=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }) \forall \mathbf {y}_{a}\). Consequently, if bijective function g: \(\mathcal {K} \rightarrow \mathcal {K}\) exits, such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k \in \mathcal {K}\), then there exists a θ^{′}≠θ for which \(\phantom {\dot {i}\!}f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }')=f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}};\boldsymbol {\theta }) \forall \mathbf {y}_{a}\).
 (i)
Firstly, we need to show that if a bijective function \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) exists such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c\), then the modulation constellation is symmetric with respect to origin. Equivalently, we can show that if a bijective function \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) does not exist such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c\), then the modulation constellation is not symmetric with respect to origin. To prove this equivalent statement, we use proof by contradiction. We assume \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) does not exist such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c\), but the modulation constellation is symmetric with respect to origin. If the modulation is symmetric with respect to the origin, then it satisfies the condition of Definition 2 and hence,
$$\begin{array}{*{20}l} \mathrm{f}(x_{k})=\mathrm{f}(x_{k}). \end{array} $$(31)However, since the function f(x_{ k }) is defined on set \(\mathcal {A}\), (31) holds if and only if both x_{ k } and −x_{ k } are in the set \(\mathcal {A}\). Consequently, set \(\mathcal {A}\) can be represented by
$$\begin{array}{*{20}l} \mathcal{A}=\{x_{1},x_{2},\cdots,x_{\frac{M}{2}},x_{1},x_{2},\cdots,x_{\frac{M}{2}}\}. \end{array} $$(32)Now, if the bijective function g is defined as \(\mathrm {g}(k)=k+\frac {M}{2}\), then \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=1\). However, this contradicts the assumption that bijective function \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) does not exist, such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c\forall k \in \mathcal {K}\). Hence, if \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) does not exist such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c\), then the modulation constellation cannot be symmetric with respect to origin.
 (ii)
Secondly, we need to show that if the modulation is symmetric then a bijective function \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) exists such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=c \forall k \in \mathcal {K}\). This easily follows from the proof of previous step, where we showed that if the modulation constellation is symmetric then \(\mathcal {A}\) can be represented by (32). Consequently, a bijective function \(\mathrm {g}:\mathcal {K}\rightarrow \mathcal {K}\) exists such that \(\frac {x_{k}}{x_{\mathrm {g}(k)}}=1 \forall k \in \mathcal {K}\), i.e., \(\mathrm {g}(k)=k+\frac {M}{2} \forall k \in \mathcal {K}\).
Combining the proofs of the three steps, Theorem 1 is proved.
8 Appendix 2
8.1 Proof of Propositions 2 and 3
Propositions 2 and 3 correspond to the E and M steps of the EM algorithm. We assume that both transmitters at nodes a and b use the asymmetric shifted modulation constellation \(\mathcal {\bar {A}}\) defined in Definition 4, i.e., \(\bar {x}_{a_{i}},\bar {x}_{b_{i}} \in \mathcal {\bar {A}}\) and assume a uniform discrete distribution for the transmitted symbols.
8.1.1 Proof of Estep
To calculate (33), we require
Finally, substituting (39) into (37), Q(ϕϕ^{(n)}) can be found as in (33). This concludes the proof of Proposition 2.
8.1.2 Proof of Mstep
where the elements of S and v are given by (14b)–(14d).
It can easily be shown that \(\det (\mathrm {S})=\left (s_{1}s_{4}s_{2}^{2}s_{3}^{2}\right)^{2}\) and is always positive. According to (14b), s_{1} is always positive, and it is clear that the second determinant is always positive. However, the positivity of the third determinant, i.e., \(s_{1}s_{4}s_{2}^{2}s_{3}^{2}\), directly depends on the initialization. This is evident from definitions in (14b)–(14d), which link s_{1}, s_{2}, s_{3}, and s_{4} to the function \(T_{k,i}^{(n)}\) and the derivation of function \(T_{k,i}^{(n)}\) in (39), which is a function of \(\hat {h}^{(n)}_{aa}\) and \(\hat {h}^{(n)}_{ba}\), i.e., the estimates from the nth iteration. Our numerical investigation shows that for the initialization vector \(\boldsymbol {\phi }^{(0)}\triangleq [0,0,0,0]\), the Hessian matrix H is always positive and hence the critical point ϕ^{(n+1)} is indeed the minimum of function r(ϕ). Consequently, the EM algorithm with initialization vector \(\boldsymbol {\phi }^{(0)}\triangleq [0,0,0,0]\) converges to the maximum of the likelihood function.
This concludes the proof of Proposition 3.
9 Appendix 3
9.1 Proof of Proposition 4
where \(\mathrm {I}_{avg}=\mathbb {E}_{\bar {\mathbf {X}}_{\mathbf {b}}, \bar {\mathbf {X}}_{\mathbf {a}}}\left [\mathrm {I}[f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}}\bar {\mathbf {x}}_{\mathbf {b}};\boldsymbol {\phi })]\right ]\). The value of \(\mathrm {I}[f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}}\bar {\mathbf {x}}_{\mathbf {b}};\boldsymbol {\phi })]\), needed to evaluate I_{ avg }, is presented in the Lemma below.
Lemma 1
Proof
where m,n∈{1,2,3,4}, ϕ_{ m } and ϕ_{ n } are the mth and nth elements of ϕ=[ℜ{h_{ aa }},I{h_{ aa }},ℜ{h_{ ba }},I{h_{ ba }}]. By evaluating (49), using the joint PDF given by (48), the nonzero elements of \(\mathrm {I}[f_{\mathbf {Y}_{\mathbf {a}}}(\mathbf {y}_{\mathbf {a}}\bar {\mathbf {x}}_{\mathbf {b}};h_{aa},h_{ba})]\) can be found and are given by (47b)–(47c). □
Using (46) and considering the diagonal elements of (51), we arrive at the result in (16).
Note that it may be possible to achieve constellation asymmetry through other means, such as design of irregular modulation constellations. The optimum design of such modulation constellations is outside the scope of this work.
Note that the existing hardware implementations for superimposed training [42] can also be used here for shifting the modulation constellation.
Note that the simulation results have been obtained by normalizing the shifted modulation so the extra power needed to shift the modulation does not push the power amplifiers into saturation and hence, power amplifiers do not experience any nonlinearities.
Declarations
Acknowledgements
The authors would like to thank Mohammad Azarbad for insightful discussions and comments.
Funding
This work was supported by the Australian Research Councils Discovery Project Funding Scheme (Project number DP140101133). The work of Abbas Koohian was supported by an Australian Government Research Training Program (RTP) Scholarship. The work of Hani Mehrpouyan was partially funded by the NSF ERAS grand award number 1642865. The work of Steven Blostein was partially funded by the Natural Sciences and Engineering Research Council of Canada 050612014.
Competing interests
The authors declare that they have no competing interests.
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