Optimized Paraunitary Filter Banks for Time-Frequency Channel Diagonalization
© Ziyang Ju et al. 2010
Received: 17 June 2009
Accepted: 31 December 2009
Published: 18 January 2010
We adopt the concept of channel diagonalization to time-frequency signal expansions obtained by DFT filter banks. As a generalization of the frequency domain channel representation used by conventional orthogonal frequency-division multiplexing receivers, the time-frequency domain channel diagonalization can be applied to time-variant channels and aperiodic signals. An inherent error in the case of doubly dispersive channels can be limited by choosing adequate windows underlying the filter banks. We derive a formula for the mean-squared sample error in the case of wide-sense stationary uncorrelated scattering (WSSUS) channels, which serves as objective function in the window optimization. Furthermore, an enhanced scheme for the parameterization of tight Gabor frames enables us to constrain the window in order to define paraunitary filter banks. We show that the design of windows optimized for WSSUS channels with known statistical properties can be formulated as a convex optimization problem. The performance of the resulting windows is investigated under different channel conditions, for different oversampling factors, and compared against the performance of alternative windows. Finally, a generic matched filter receiver incorporating the proposed channel diagonalization is discussed which may be essential for future reconfigurable radio systems.
Motivated by the heterogeneity of today's world of wireless communications—which includes cellular mobile radio systems of the second and third generations and beyond, wireless local and personal area networks, broadband wireless access systems, digital audio and video broadcast, emerging peer-to-peer radio, and so forth—particular attention is given to reconfigurable radio architectures. Essential in this context are radio resource management solutions on the higher layers and the ability to comply with a range of different air interfaces on the physical layer. Devices comprising the logic for handling multiple air interfaces in the form of parallel implementations are widely available. However, in view of the still increasing number of standards, monolithic transceiver architectures are desirable which enable a uniform processing of different signals by means of reconfigurable multipurpose signal processing units.
A major challenge in the design of a universal baseband receiver architecture is posed by the dispersive radio channel. For dealing with signal dispersion, fundamentally different approaches are followed in traditional radios depending on the type of modulation. Receivers for single-carrier signals typically model the channel as a tapped delay line. For known coefficients of the delay line, the information in the transmitted signal can be recovered by means of a matched filtering followed by a sequence detector or using instead an equalizer followed by a simple detector. The complexity of the coefficient estimation and detection schemes increases with the delay dispersion and thus with the number of taps. Orthogonal frequency-division multiplexing (OFDM) can evade the need for complex equalizers in high data rate systems. The cyclic extensions in OFDM signals facilitate a frequency domain representation of the multipath channel in the form of parallel single-tap lines. On the basis of the frequency domain signal description resulting from the block-wise Discrete Fourier Transform (DFT), the signal mapping by multipath channels can be represented as diagonal matrices. This channel diagonalization enables straightforward demodulation and coefficient estimation and has, along with the availability of Fast Fourier Transform (FFT) algorithms, led to the popularity of OFDM.
The aforementioned approach for a simple channel inversion based on a frequency domain description is not limited to OFDM receivers. Single-carrier modulation with frequency domain equalization (FDE) can achieve similar performance as OFDM if a proper cyclic prefix is appended to each block of signals . In  the computational complexities of time and frequency domain equalizers are compared and it is shown that FDE is simpler when the length of the stationary channel impulse response exceeds the sample time by a factor of 5 or more. Processing signals without cyclic prefix result in errors at the block boundaries. These errors have a limited impact at sufficiently large block sizes, which makes FDE an interesting alternative for code-division multiple access receivers [3, 4].
The limitations of OFDM receivers and FDE to time-invariant channels and certain signal formats can be overcome by resorting to alternative signal representations. A natural choice for the signal transform is the discrete-time Gabor expansion  based on a system of time-frequency (TF) shifted versions of a certain window function. Even though a TF domain channel diagonalization based on such a Gabor expansion is approximative in the general case of time-variant channels and aperiodic signals, for the typical underspread channels encountered in mobile radio scenarios the inherent model error can be limited to a usually acceptable level by choosing an adequate window underlying the signal transform .
The transform of discrete-time signals into the TF domain can be accomplished by DFT filter banks, for which similarly efficient FFT-based implementations are available as for plain DFTs . There is plenty of literature on filter bank design in the context of generalized multicarrier/multitone modulation in wireless/wired communications. Replacing the block-wise inverse DFT and DFT in the transmitter and receiver, respectively, by more general filter banks is a way to get rid of the rigid framework of rectangular windows and cyclic prefixes in OFDM systems. Interference between adjacent sub-bands or multicarrier symbols can be avoided, or at least limited, by choosing appropriate transmit pulses. Filter banks for transmission over dispersive channels with limited interchannel and intersymbol interference are designed in [8–14].
The optimization of filter banks for specific objective functions and constraints can sometimes be formulated as a convex optimization (CO) problem . In , CO methods are employed for the design of a two-channel multirate filter bank, in  for the design of pulse shapes which minimize intercarrier interference due to frequency offsets in OFDM systems, in  for finding optimized prototype filters for filtered multitone modulation used in digital subscriber line systems, and in  for the design of filter banks for sub-band signal processing under minimal aliasing and induced distortion. Semidefinite programming (SDP), a branch of CO for which efficient numerical solution methods are available, was employed in  for the design of a linear phase prototype filter with high stopband attenuation for cosine-modulated filter banks. In  two-channel filter banks are optimized under similar criteria by SDP.
In this paper we are not concerned with the design of transmit pulses. Rather, we optimize filter banks in the context of channel diagonalization. We are interested exclusively in paraunitary filter banks, which are related to the concept of tight Gabor frames . The signal transform associated with discrete-time tight Gabor frames fulfills Parseval's identity. This property is crucial for flexible receivers as it lets the correlation between two time domain signals be computed based on the respective TF signal representations. A main concern of this paper is the design of tight Gabor frames facilitating TF domain channel diagonalization with minimal model error for given channel conditions. More specifically, we minimize the mean-squared error (MSE) resulting from the diagonalization of random channels with known second-order statistical properties, complying with the wide-sense stationary uncorrelated scattering (WSSUS) model, with respect to the TF window function. As we showed in , window functions minimizing the MSE appearing in the TF domain can be computed by SDP. In this paper we directly focus on the more relevant MSE in the time domain signal. We show that for weak assumptions on the channel statistics, the optimization problem can likewise be turned into a tractable form through semidefinite relaxation. In order to be able to constrain the windows to constitute tight frames, we extend the parameterization of tight Gabor frames presented in . Optimized windows can then be computed off-line for different channel conditions encountered by reconfigurable receivers, such as the generic matched filter-based inner receiver discussed in this paper.
1.1. Outline of This Paper
In Section 2, the mathematical concepts for TF representation and processing of signals are introduced. A parameterization of tight Gabor frames, needed for the constrained optimization in Section 5 is presented in Section 3. In Section 4, TF domain channel diagonalization is discussed, resulting in a certain error in the case of doubly dispersive channels. As shown in Section 5, semidefinite relaxation lets the window design problem be formulated as a CO problem. Numerical results are shown in Section 6 for different channel conditions. In Section 7, a generic matched filter architecture incorporating the channel diagonalization is presented. Finally, conclusions are drawn in Section 8.
We enclose the arguments of functions defined on a discrete domain in square brackets in order to distinguish them from functions defined on . The Hilbert space of the square summable functions is denoted as , and the associated inner product and -norm are given by and , respectively, where the asterisk in the superscript denotes complex conjugation. Furthermore, we use to denote convolution, and for the one-by-one multiplication of two compatible functions and , that is, corresponds to for all . Vectors and matrices are denoted by boldface characters. The transpose and Hermitian transpose of a matrix are denoted as and , respectively, stands for the paraconjugate of a polynomial matrix ( is obtained from by transposing it, conjugating all of the coefficients of the rational functions in , and replacing by .), for the trace, and denotes the identity matrix of size . The th element of the th row of a matrix is represented as . Also, denotes the expected value, and represent the real and imaginary parts, respectively, of complex arguments, the modulo operation, , and .
2. DFT Filter Banks and Discrete-Time Gabor Frames
In this section, we introduce signal representation concepts needed subsequently. Some important properties of discrete-time Gabor frames are recapitulated with an emphasis on tight frames and the relationship to DFT filter banks. For more insight into Gabor analysis and filter bank theory the reader is referred to the rich literature, for instance [7, 23–26].
is referred to as a Gabor system in . The elements of the Gabor system can be associated with the grid points of a lattice overlaying the TF plane . If there exist two positive constants and such that
For an arbitrary signal the inner products of with every element of the system (1) form a linear TF representation. In the following, the corresponding transform onto is represented by the analysis operator
The mapping (4) can be implemented by a -channel DFT (analysis) filter bank with a prototype filter with impulse response followed by a down-sampling by a factor . Conversely, a synthesis operator can be defined based on (1) which maps an arbitrary TF representation onto an element of according to
Henceforth we assume that (1) represents a tight Gabor frame. We note that the range of the operator is a subspace of , and the mapping is an isometry. If the operator represents the orthogonal projection from onto . As a direct consequence,
Tight Gabor frames are associated with paraunitary DFT filter banks. To enable the design of windows with favorable properties, for instance in regard to TF concentration, it is often necessary to indeed choose , resulting in oversampled filter banks. Besides of available efficient implementations of paraunitary DFT filter banks, the properties (6) and (7) are of prime interest for reconfigurable baseband receivers since they allow operations for the signal demodulation, such as signal energy computations and crosscorrelations with reference waveforms, to be performed directly in the TF domain.
3. Parameterization of Tight Gabor Frames
The conditions under which (1) represents a tight Gabor frame can be formulated via the polyphase representation. Let denote the least common multiple of and , and define and such that and . The -component polyphase representation of the -transform of the window reads
Furthermore, the polyphase matrix of size associated with the DFT filter bank implementing (4) can be expressed as 
Here, is the diagonal matrix with diagonal elements . The Gabor system (1) represents a tight frame in if and only if the polyphase matrix is paraunitary with . Or, equivalently, if and only if the polynomial matrix is paraunitary with , since .
We observe that if , where . Consequently, is paraunitary if and only if the matrices of size , which comprise the possibly nonzero elements of according to , are all paraunitary. As follows from (12) the elements of the matrices are given as
Note that if the sequences were identical for all column indices except for differing offsets, then the factor could be omitted in (13) without affecting the condition . Replacing some by the equivalent is a way to align the sequences. Having this in mind, we define matrices of size according to
with the index map
4. Time-Frequency Channel Diagonalization
where denotes the time-variant impulse response. We consider random channels where represents a two-dimensional zero-mean random process complying with the WSSUS model. The second-order statistics of are determined by the time correlation function and the delay power spectrum according to
Of interest in the context of TF signal processing is the time-variant transfer function
For compatibility with the TF signal representations introduced in Section 2, the sampling intervals and are chosen in line with those for the Gabor system (1). The time-variant transfer function represents the complex-valued channel gain over time and frequency. Hence, given the TF representation of a signal at the channel input, it is straightforward to approximate the signal at the channel output as
As a result of the sampling of the model (22) is usually only approximative, and is an approximation of the channel output. The accuracy of depends on the channel characteristics and the underlying Gabor frame. We may expect the model error to be limited if every elementary function is concentrated around in the TF plane such that is essentially constant within the sphere of . Window functions fulfilling this can be designed for the typical underspread channels encountered in mobile radio scenarios by CO, as shown in Section 5.
The error from the channel diagonalization is given by
a white random signal at the channel input,
The error signal sample energy relative to the unit average sample energy of the desired signal, in the following termed relative mean-squared sample error (RMSSE), can be expressed as
Making use of the above assumptions, the RMSSE can be written as
as shown in the appendix. Having formulated both conditions for the window to define a tight Gabor frame (in Section 3) and the error resulting from the channel diagonalization based on , we can now turn to window optimization.
5. Window Design
Next, we need to incorporate the constraints under which will be minimized. In order to formulate the constraints (16) on the window in the time domain, it is helpful to permute the samples in . Let us introduce a window of length defined as
( ) Case . From each of the above-defined matrices , unique block diagonal matrices of dimension are reproduced which contain the original matrix as one of the diagonal blocks of dimension . Hence, there are constraints in total. The constraint matrices are mutually orthogonal in the sense that for .
We can now formulate the optimization problem in the form
where are the matrices resulting from by permuting the rows and columns in accordance with (29), and . This problem is difficult to tackle for large . Let us thus introduce and reformulate the optimization problem as
In order to reduce the rank to , we resort to the matrix composed by the dominant eigenvector of , since is the matrix nearest to in terms of the Frobenius norm .
We finally obtain by the algorithm , which yields a window defining a tight frame and at the same time minimizes the distance to a given window (i.e., ) in terms of the -norm.
Employing steepest descent methods for solving (35) may result in very slow convergence, whereas alternative methods may not be applicable when the number of dimensions is large. Neglecting the quadratic terms in the objective function leads to the simplified optimization problem
As shown in , the linear objective function reflects the mean-squared deviation of from , that is, the model error in the TF domain. Problems of the form (36) are dealt with by SDP, a subfield of CO. For the efficient solution of these optimization problems a number of sophisticated software packages are widely available. However, because generally the windows resulting from solving (36) do not minimize the time domain error signal, the magnitude of which determines the performance of the channel diagonalization.
6. Numerical Results
We consider a WSSUS channel with an exponentially decaying delay power spectrum, the sampled version of which reads
with denoting the unit step function and the root mean-squared (RMS) delay spread . As for the Doppler power spectrum, a two-sided exponentially decaying shape is assumed, which results in the time correlation function
Obviously, solving (35) leads to better windows than solving (36). The considerable offset of the RMSSEs from the lower bound for smaller indicates that here the rank reduction has a significant impact on the windows. We observe that rank reduction generally has a limited effect when the delay and Doppler spreads are of similar extent, that is, when in the TF plane the delay spread relative to the sampling interval in time (i.e., ) is of the same order of magnitude as the Doppler spread relative to the sampling interval in frequency (i.e., ).
7. Generic Matched Filter Receiver
The considered TF channel diagonalization does not rely on a particular signal format, making it suitable for application in multimode receivers . The burst structures defined in the various standards for wireless communications differ substantially. However, commonly the bursts incorporate preamble signals for the channel estimation along with information-bearing signals which are usually subject to a linear modulation scheme. The transmitted baseband signals generally follow the form with representing elementary waveforms, possibly complex exponentials such as in the case of OFDM, or pseudo-noise sequences as in the case of direct-sequence spread-spectrum systems. For performing channel estimation and information recovery the receiver needs to estimate the signals on the basis of the known waveforms . To this end, the inner receiver correlates the received signal with the elementary signals as appearing at the channel output, resulting in
For example, in the case of signal decoding in the presence of additive white Gaussian noise, represent a sufficient statistic. In other situations, such as for the channel parameter estimation (Efficient parameter estimators which are applicable in the context of filter bank-based multicarrier transmission are presented in .), is unknown.
where for obtaining expression (41) we exploit that while represents the orthogonal projection from onto . The error signal is in line with the error signal definition (25). Under the assumptions that the relation between and found in Section 4 carries over to the relation between and , and that represents a random signal with and , the RMSSE determines the signal-to-noise ratio at the matched filter output. Since the pulse is typically a component of , the aforementioned assumptions, however, do not hold in general. Nevertheless, may in practice serve as a rough characterization of the performance of the matched filter in Figure 5. The performance of the TF domain matched filtering in a reconfigurable receiver architecture configured to the reception of direct-sequence spread-spectrum signals is studied in .
We have derived paraunitary filter banks facilitating diagonalization of doubly dispersive channels at limited inherent MSE. Making use of a suitable parameterization of tight frames, we have shown that the optimization of paraunitary DFT filter banks for given channel statistics and oversampling factors can be formulated as a CO problem. An investigation of the MSE performance achieved by the optimized windows shows that the windows obtained by CO are more favorable than conventional windows with an RRC spectrum. However, in certain configurations the necessary rank reduction following the CO has a significant impact on the window shapes. The induced potential degradation of the MSE performance may be evaded by choosing appropriate lattice constants and , specifying the down-sampling factor and the number of sub-bands, respectively, or by alternative rank reduction procedures which are yet to be devised. In general, the MSE performance can be improved at the cost of a higher complexity in terms of numbers of coefficients by increasing the oversampling factor.
In this paper our main concern was mathematical techniques for designing optimized filter banks in the context of channel diagonalization. Reconfigurable radios are clearly a prospective field of application. Since tight frames are natural generalizations of orthonormal bases used for the signal transform in OFDM receivers, the efficient handling of dispersive channels in OFDM can be inherited by receivers not limited to signals with cyclic extensions. Flexible radio architectures which incorporate the channel diagonalization considered in this paper have been investigated within the IST project URANUS (Universal RAdio-link platform for effieNt User-centric accesS) . In this project the performance of such flexible receiver architectures has been studied in the context of different air interfaces and on different levels, from the inner receiver performance with perfect and imperfect channel estimation to the link level performance. While channel diagonalization by means of properly designed filter banks has been shown to have a great potential, there are a number of related issues that need to be addressed on the way to practical solutions, such as adequate channel estimation methods, synchronization, radio resource management, and others. A comparison of the performance of flexible receivers taking advantage of the channel diagonalization as compared to conventional receiver architectures has therefore been out of the scope of this paper.
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