Linearly Time-Varying Channel Estimation and Symbol Detection for OFDMA Uplink Using Superimposed Training
© Han Zhang et al. 2009
Received: 30 July 2008
Accepted: 27 January 2009
Published: 4 March 2009
We address the problem of superimposed trainings- (STs-) based linearly time-varying (LTV) channel estimation and symbol detection for orthogonal frequency-division multiplexing access (OFDMA) systems at the uplink receiver. The LTV channel coefficients are modeled by truncated discrete Fourier bases (DFBs). By judiciously designing the superimposed pilot symbols, we estimate the LTV channel transfer functions over the whole frequency band by using a weighted average procedure, thereby providing validity for adaptive resource allocation. We also present a performance analysis of the channel estimation approach to derive a closed-form expression for the channel estimation variances. In addition, an iterative symbol detector is presented to mitigate the superimposed training effects on information sequence recovery. By the iterative mitigation procedure, the demodulator achieves a considerable gain in signal-interference ratio and exhibits a nearly indistinguishable symbol error rate (SER) performance from that of frequency-division multiplexed trainings. Compared to existing frequency-division multiplexed training schemes, the proposed algorithm does not entail any additional bandwidth while with the advantage for system adaptive resource allocation.
Orthogonal Frequency-Division Multiplexing Access (OFDMA) is a promising technique for future high-speed broadband wireless communication systems, and it has recently been proposed or adopted in many industry standards (e.g., IEEE 802.16e , 3 GPP Long Term Evolution (LTE) ). In OFDMA, subcarriers are grouped into sets, each of which is assigned to a different user. Interleaved, random, or clustered assignment schemes can be used for this purpose. Such a system, however, relies on the knowledge of propagating channel state information (CSI). Explicitly, in many mobile wireless communication systems, transmission is impaired by both delay and Doppler spreads [3–10], resulting in inside- and out-of-band interferences.
Channel estimation in OFDMA uplinks is challenging, however, since different channel responses for the individual user need to be tracked simultaneously at the base station (BS). OFDMA systems with adaptive resource allocation are even more critical since the uplink channels have to be estimated over the whole frequency band. In conventional pilot-aided approaches wherein the pilot symbols are frequency-division multiplexed (FDM) with the data symbols [3–8, 10–15]; however, channel estimation can only be performed within each subband of individual user separately since each user is only assigned a subset of the whole frequency band. This may be a great disadvantage for OFDMA systems with adaptive resource allocation. In addition, extra bandwidth is required for transmitting known pilot symbols. In recent years, an alternative and promising approach, referred to as superimposed training (ST), has been widely studied in [9, 16–24]. In the idea of ST, additional periodic training sequences are arithmetically added to information sequence in time or frequency domain, and the channel transfer function can thus be estimated by using the first-order statistics. The advantage of the scheme is that there is no loss in information rate and thus enables higher bandwidth efficiency. In this scheme, however, the information sequences are viewed as interference to channel estimation since pilot symbols are superimposed at a low power to the information sequences at the transmitter. To circumvent the problem, it was recommended in [16–22, 24] that a periodic impulse train of the period larger than the channel order is superimposed in time-domain, and the channel is thus estimated by averaging the estimations of multiple training periods to reduce the information sequence interference. For a multicarrier systems, that is, SISO/OFDM system,  suggested a similar scheme that superimposes the periodic impulse training sequences on time-domain modulated signals, while for single-carrier systems, a novel block transmission method is proposed in frequency domain in , where an information sequence dependent component is added to the superimposed training so as to remove the effect of the information sequence on the channel estimation at receiver. In , an iterative approach is provided where the information sequence is exploited to enhance the channel estimation performance. These above-mentioned schemes, however, are restricted to the case that the channel is linearly time-invariant (LTI), and cannot be extended to the linearly time-varying (LTV) channel since the variation of channel coefficients may degrade the simple average-based solution extensively. A combined approach is developed in [9, 11] to solve the problem of channel estimation of LTV channels. However, it is only suitable for single-carrier transmission. In addition, some useful power is wasted in ST which could have otherwise been allocated to the information sequence. This lowers the effective signal-to-noise ratio (SNR) for information sequence and affects the symbol error rate (SER) at receiver. This may be a great disadvantage to wireless communication systems with a limited transmission power. On the other hand, the interference to information sequence recovery due to the embedded training sequences may degrade the SER performance severely at receiver. Previous papers merely focus on the information sequence interference suppression; whereas few researches are contributed to the superimposed training effect cancellation for information sequence recovery.
In this paper, we propose a new ST-based channel estimator that can overcome the aforementioned shortcomings in estimating LTV channel for OFDMA uplink systems. In contrast to the previous works, the main contributions of this paper are twofold. First, we extend conventional LTI-based ST schemes [16–24] to the case where the channel coefficient is linearly time-varying. By resorting to the truncated Fourier bases (DFBs) to model the LTV channel, we adopt a two-step approach to estimate the time-varying channel coefficients over multiple OFDMA symbols. Unlike conventional FDM training strategy [12–15] where channel estimation can only be performed within each subband of individual user separately, the LTV uplink channel transfer functions over the whole frequency band can be estimated directly by using specifically designed superimposed training. Furthermore, we present a performance analysis of the channel estimator. We demonstrate by simulation that the estimation variance, unlike that of conventional ST-based schemes of LTI channel [16–22, 24], approaches to a fixed lower bound as the training length increases. Second, an iterative symbol detection algorithm is adopted to mitigate the superimposed training effects on information sequences recovery. In simulations presented in this paper, we compare the results of our approaches with that of the FDM training approaches [12–15] as latter serves as a "benchmark" in related works. It is shown that the proposed algorithm outperforms FDM trainings, and the demodulator exhibits a nearly indistinguishable SER performance from that of .
The rest of the paper is organized as follows. Section 2 presents the channel and system models. In Section 3, we estimate the LTV channel coefficients by using the proposed channel estimator. In Section 4, we present the closed-form expression of the channel estimation variances of Section 3. An iterative symbol detector is provided in Section 5. Section 6 reports on some simulation experiments carried out in order to test the validity of theoretic results, and we conclude the paper with Section 7.
The letter represents the time-domain variable, and is the frequency-domain variable. Bold letters denote the matrices and column-vectors, and the superscripts and represent the transpose and conjugate transpose operations, respectively. denotes the identity matrix of size , and denotes the element of the specified matrix.
2. Channel and System Model
where is the impulse response vector of the propagating channel with the channel coefficients being the functions of time variable The notation represents the cyclic convolution, and is the additive noise with variance
As mentioned in , the coefficients of the time- and frequency-selective channel can be modeled as Fourier basis expansions. Thereafter, this model was intensively investigated and applied in block transmission, channel estimation, and equalization (e.g., [4–8]). In this paper, we extend the block-by-block process [4–8] to the case where multiple OFDMA symbols are utilized. Consider a time interval or segment the channel coefficients in (3) can be approximated by truncated discrete Fourier bases (DFBs) within the segment as
where is a constant coefficient, is the multipath delay, represents the basis expansion order that is generally defined as [3–8], is the segment length, and is the segment index. Unlike [4–8], the approximation frame covers multiple OFDM symbols, denoted by where and .
where represents the FFT vector of the specified function with a length , and is the frequency-domain noise. Note that the vectors in (6) should be computed corresponding to the variations of the propagating channel during an OFDM symbol time interval. Specifically, the variation of LTV channel is associated with the OFDM symbol-size as well as the Doppler frequency or mobile velocity.
where is the mid-sample of the th OFDMA symbol. In (7), the LTV channel coefficients are in fact approximated by the mid-values of the LTV channel model (4) at the th symbol. Since the proposed channel estimation will be performed within one single frame , we omit the frame index and thus have for simplification.
In conventional FDM training schemes [12–14] where each user is only assigned a subset of the whole subcarriers, the channel estimation, however, cannot be performed over the whole frequency band. This may be a great disadvantage for OFDMA systems with adaptive resource allocation.
3. Superimposed Training-Based Solution
In this section, we propose an ST-based two-step approach to estimate the channel transfer functions over the whole frequency band and, meanwhile, overcome the above-mentioned shortcoming of conventional ST-based schemes in estimating LTV channels.
3.1. Channel Estimation over One OFDMA Symbol
Note that the pilot symbols in (10) are complex exponential functions superimposed over the whole subcarriers, the corresponding time-domain signals of various users are in fact a -sequence as that follows a disjoint set with an interval Therefore, using the specifically designed training sequence (10), the training signals of various users are decoupled. The sequence (10), however, possibly leads to high signal peaks at the instant samples , One of the simple ways to suppress the above undesired signal peaks may refer to the scrambling procedure  (details will not be addressed here since it is beyond the scope of this paper).
Substituting the specifically designed pilot sequence (10) into (8), we have
3.2. Channel Estimation over Multiple OFDMA Symbols
From (14), we note that the information sequence interference vector (the second entry of (14) can hardly be neglected unless using a large pilot power The conventional ST trainings stated in [16–22, 24] employ averaging the channel estimates over multiple OFDM symbols (or training periods) to suppress the information sequence interference in the case that the channel is linearly time-invariant during the record length. This arithmetical average operation in [16–22, 24], however, is no longer feasible to the channel assumed in this paper wherein the channel coefficients are time-varying over multiple OFDMA symbols.
In this section, we develop a weighted average approach to suppress the abovementioned information sequence interference over multiple OFDMA symbols, and thus overcoming the shortcoming of conventional ST-based schemes for linearly time-varying channel estimation.
where is the complex exponential coefficients modeling the LTV channel, and is a matrix with Thus, when the matrix is of full column rank, and the basis exponential model coefficients can be estimated by
By (16)-(17), the modeling coefficients are estimated over the whole frame OFDMA symbols and can be rewritten by
In fact, (18) is estimated over multiple OFDMA symbols with a weighted average function of . Similar to the average procedure of LTI case [16–22, 24], it is thus anticipated that the weighted average estimation may also exhibit a considerable performance improvement for the time-varying channels over a long frame .
Compared with the conventional STs that are generally limited to the case of LTI channels [16–22, 24], the proposed weighted average approach can be performed to estimate the LTV channels of OFDMA uplink systems. In fact, the proposed channel estimation is composed of two steps: first, with specially designed training signals in (10), we estimate the channel coefficients during each OFDMA symbol as temporal results. Second, the temporal channel estimates are further enhanced over multiple OFDMA symbols by using a weighted average procedure. That is, not only the target symbol, but also the OFDMA symbols over the whole frame are invoked for channel estimation.
On the other hand, the proposed ST-based approach can be utilized to estimate the uplink channel over the whole frequency band, thus overcome the shortcoming of FDM training methods [12–14] where channel estimation can only be performed within each subband of individual user, separately.
4. Channel Estimation Analysis
In this section, we analyze the performance of the proposed channel estimator in Section 3 and derive a closed-form expression of the channel estimation variance which can be, in turn, used for superimposed training power allocation. Before going further, we make the following assumptions.
Since (22) depends upon the channel transfer functions (equivalently, the channel impulse response), we define the normalized variance as
From (24), we can find that the estimation variance due to the information interference is directly proportional to the information-to-pilot power ratio thereby resulting in an inaccurate solution for the general case that
By (H1)-(H2), the MSE of the weighted average estimator is given by
Note that the column vectors of the matrix in (15) are in fact the FFT vectors of a matrix, we thus have and Substituting (21)-(22) into (26), we then obtain the variance of the weighted average estimation associated with , as
Combining the variances in (27) and (28), we have the weighted average estimation variances
In (29), the last term is due to the additive noise. In general, since the LTV channel model satisfies the additive noise is greatly suppressed by the weighted average procedure. On the other hand, estimation variance due to the information sequence interference (the first term in (29) may be the dominant component of the channel estimation error, especially for high SNR. Similar to (23), we derive the normalized variance of information sequence interference by removing the channel gain by
From (31), the normalized variance is directly proportional to the information-pilot power ratio and the ratio of the unknown parameter number over the frame length In particular, with the specifically designed training sequence (10), the closed-form estimation variance (31) may provide a guideline for signal power allocation at transmitter, for example, for a given threshold of the estimation variance (channel gain has been normalized), the minimum training power should at least satisfy the approximated constraint as .
Compared with the variances of channel estimation over one OFDMA symbol as in (22)–(24), the estimation variances (29)–(31) of the weighted average estimator (15)–(18) are significantly reduced owing to the fact that Theoretically, the weighted average operation can be considered as an effective approach in estimating LTV channel, where the information sequence interference can be effectively suppressed over multiple OFDMA symbols. As stated in the conventional ST-based schemes [16–22, 24], channel estimation performance can be improved along with the increment of the recorded frame length , that is, the estimation variance approaches to zero as This can be easily comprehended that larger frame length means more observation samples, and hence lowers the MSE level. From the LTV channel model (4), however, we note that as the frame length is increased, the corresponding truncated DFB requires a larger order to model the LTV channel (maintain a tight channel model), and the least order should be satisfied where and are the Doppler frequency and sampling rate, respectively [1–8]. Consequently, as the frame length increases, the LTV channel estimation variance (31) approaches to only a fixed lower-bound associate with the system Doppler frequency as well as the information-pilot power ratio. This is quite different from the ST trainings in estimating LTI channels [16–22, 24].
5. Iterative Symbol Detector
Unlike the FDM trainings [10, 12–15, 25], the pilot sequences in (10) are superimposed on the information sequences and thus produce interferences on the information sequences recovery. The existing ST approaches [9, 11, 16–22, 24] merely focus on the information sequence interference suppression; whereas few researches are contributed to the ST effect cancellation for information sequence recovery. In this section, we provide a new iterative symbol detector to cancel the residual training effects on symbol recovery.
As in the symbol detection of conventional ST-based approach, the contribution of the training sequences is firstly removed at OFDMA uplink receiver before recovering the data symbols
where is an matrix with the diagonal elements being the estimated channel frequency-domain transfer function, that is, (with ) and the remaining entries being zeros. is the residual error of the superimposed pilots.
Note that is distributed over the whole frequency tone; whereas owing to the specifically designed training signals in (10), the time-domain received signals affected by the residual error are concentrated only during a sequence of sample periods , , In order to mitigate the residual error, a natural idea is to reconstruct the above time-domain signals of , , In our proposed iterative method, we carry out the following steps.
Using the updated signals in (36), we detect the information symbols by (32)–(36) in the forthcoming iteration.
Repeat the Steps 1–4 until the increment changes of the improved SER performance over successive iterations are below a given threshold.
When the SER of the initial hard detector in (34) is lower than a certain threshold, the reconstructed signals in the current iteration should approach to the original signals more than that of the previous iteration, that is, where is the pure IFFT modulated information signals of and are the reconstructed signals by (36) in the current and previous iterations, respectively. Additionally, the iteration index depends crucially on the size of the reconstructed signals over one OFDMA symbol period, that is, Base on experiment studies, the proposed iterative method should satisfy the constraint of Commonly, such constraint for practical implementation can be satisfied freely by simply adjusting the total frequency bandwidth and the number of active users.
Obviously, the SER performance degradation owing to the residual effect of superimposed training is guaranteed with the proposed iterative approach. Compared with conventional ST methods [9, 11, 16–22, 24], the iterative scheme offers an alternative to enhance the channel estimation performance by using a large training power while without sacrificing SER performance degradation.
6. Simulation Results and Discussion
In this section, we present the numerical examples to validate our analytical results. We assume the OFDMA uplink system with and all subcarriers are equally divided into subband that assigned to four users. The transmitted data symbol is QPSK signals with symbol rate /second. The channel is assumed with , and the coefficients are generated as low-pass, Gaussian, and zero-mean random processes and correlated in time with the correlation functions according to Jakes' mode where is the Doppler frequency associated with the th user. CP length is chosen to be 15 to avoid intersymbol interferences. The additive noise is a Gaussian and white random process with a zero mean.
We run simulations with the Doppler frequency Hz that corresponds to the maximum mobility speed of 162 km/h as the users operate at carrier frequency of 2 GHz. In order to model the LTV channel, the frame is designed as that is, each frame consists of 256 OFDMA symbols. During the frame, the channel variation is Notice that the channel variation during an OFDM symbol is and thus can be neglected. Over the total frame we utilize the truncated DFB of order to model the LTV channel coefficients. The LTV channel modeled by the truncated DFB, however, exhibits modeling errors at the outmost samples. A possible explanation is that as the Fourier basis expansions are truncated in (4), an effect similar to the Gibbs phenomenon, together with spectral leakages, may lead to modeling inaccuracy at the beginning and the end of the frame [3, 5, 7–9]. To circumvent the problem, the frames are designed to be partially overlap, for example, , where is a positive integer. By the frame-overlap, the LTV channel at the beginning and the end of the frame can be modeled and estimated accurately from the neighboring frames.
6.1. Channel Estimation
6.2. Symbol Detection
As aforementioned, symbol detection in demodulator of ST-based schemes [9, 11, 16–22, 24] is affected by the residual contribution of embedded pilot symbols. Herein, we carry out simulation experiments to assess the effectiveness of the proposed iterative symbol detector.
6.3. Complexity Analysis
The description of the proposed channel estimation method in Section 3 shows that the overall complexity comes from the complex matrix pseudoinverse operation in (16). Note that (16) can be deduced into a weighted average process in (18). Thus, compared to the ST-based estimator within one OFDMA symbol (13), only additional complex multiplication and complex additions are required to obtain the accurate time-domain CSI of uplink OFDMA systems.
In this paper, we have developed a new method for estimating the LTV channels of uplink OFDMA systems by using superimposed training. We extend conventional LTI-based ST schemes to the case where the channel coefficient is linearly time-varying. By resorting to the truncated Fourier bases (DFBs) to model the LTV channel, we adopt a two-step approach to estimate the time-varying channel coefficients over multiple OFDMA symbols. We also present a performance analysis of the channel estimation approach and derive a closed-form expression for the channel estimation variances. It is shown that the estimation variances, unlike conventional superimposed training, approach to a fixed lower-bound that can only be reduced by increasing the pilot power. In addition, an iterative symbol detector was presented to mitigate the superimposed training effects on information sequence recovery, thereby offering an alternative to enhance the channel estimation performance by using a large training power while without sacrificing SER performance degradation. Compared with the existing FDM training schemes, the new estimator can estimate the channel transfer function over the whole frequency band without a loss of rate, and thus enables a higher efficiency with the advantage for system adaptive resource allocation.
The authors would like to thank the editor and the reviewers for their helpful comments. This work is supported by the National Natural Science Foundation of China (NSFC), Grant 60772132, Key Project of Natural Science Foundation of Guangdong Province, Grant 8251027501000011, Science & Technology Project of Guangdong Province, Grant 2007B010200055, Industry-Universities-Research Cooperation Project of Guangdong Province and Ministry of Education of China, Grant 2007A090302116, and also supported in part by joint foundation of NSFC and Guangdong Province U0635003.
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