Browsing by Author "Phoong, SM"

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A frequency-domain SIR maximizing time-domain equalizer for VDSL systems
(2005)
Lin, YP
;
Liang, LH
;
Chung, PJ
;
Phoong, SM
;
電控工程研究所
In this paper we propose a semi-blind time domain equalizer (TEQ) design method that maximize SIR (signal-to-interference ratio) in frequency domain for VDSL systems. The proposed method exploits the training symbols in VDSL initialization using an eigen approach. Unlike earlier eigen based TEQ designs, the proposed method does not require the channel impulse response. The TEQ can be obtained by computing an eigenvector of a positive definite matrix that depends only on the averaged received VDSL symbols. Examples will be given to demonstrate that the proposed TEQ design method can effectively shorten the channel impulse response, and achieve very good bit rates with only a small number of training symbols.
A new class of optimal biorthogonal subband coder
(1999-01-01)
Phoong, SM
;
Lin, YP
;
電控工程研究所
;
Institute of Electrical and Control Engineering
In this letter, we introduce a novel two-channel biorthogonal subband coder. The new coder employs the ladder structure (or so-called wavelet with lifting scheme). The coding gain of the biorthogonal coder is always greater than or equal to unity, and it can be expressed in closed form. Moreover the proposed coder has a very low complexity and the perfect reconstruction property is structurally preserved. Given any filter order, the optimal solution can be found by the well-known Levinson fast algorithm. For both AR(1) process and MA(1) process, the proposed biorthogonal coder with 2 taps has a higher coding than the optimal orthonormal coder with infinite number of taps.
An semi-blind eigen approach to time-domain equalizer design for VDSL systems
(2005)
Lin, YP
;
Liang, LH
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
In this paper we propose a semi-blind time domain equalizer (TEQ) design method for VDSL system. In the VDSL system frequency division duplex is used to separate upstream and downstream signals. In downstream or upstream transmission, only half the tones carry data and the other half are null tones. The proposed semi-blind method exploits the null tones of VDSL training symbols. It does not require the channel impulse response nor the channel noise statistics, like all eigen approaches proposed earlier. Example will be given to demonstrate that proposed TEQ design method can effectively shorten the channel impulse responses for VDSL application.
Analog representation and digital implementation of OFDM systems
(2003)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
Many existing results on the analysis of OFDM systems are based on an analog representation. The actual implementation of OFDM transmitters typically consists of a discrete DFT matrix and a digital-to-analog (DAC) converter. In this paper, we show that the analog representation admits the implementation of a DFT matrix followed by a DAC converter only in special cases. Necessary and sufficient conditions for such cases will be given. Analyses based on the analog representation, e.g., spectral roll-off of transmitter outputs and carrier frequency offset, may not be appropriate when digital implementation is employed.
Asymptotical optimality of DFT based DMT transceivers
(1999)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
The DMT (discrete multitone modulation) technique has been widely applied to data transmission over fading channels of twisted pairs. It has been shown that the DMT system with ideal filters can achieve within 8 to 9 dB of the channel capacity of ADSL. The DFT based DMT system is proposed as a practical DMT implementation but its optimality is never asserted. In this paper we will show that the DFT based DMT systems are asymptotically optimal although they are not optimal for finite number of channels. The DFT based DMT system and the DMT system with ideal filters achieve the same bound. However, for a modest number of channels the optimal transceiver can provide substantial gain over the DFT based system as will be demonstrated by examples.
BER minimized OFDM systems with channel independent precoders
(2003-09-01)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
We consider the minimization of uncoded bit error rate (BER) for the orthogonal frequency division multiplexing (OFDM) system with an orthogonal precoder. We analyze the BER performance of precoded OFDM systems with zero forcing and minimum mean squared error (MMSE) receivers. In the case of MMSE receivers, we show that for quadrature phase shift keying (QPSK), there exists a class of optimal precoders that are channel independent. Examples of this class include the discrete Fourier transform (DFT) matrix and the Hadamard matrix. When the precoder is the DFT matrix, the resulting optimal transceiver becomes the single carrier system with cyclic prefix (SC-CP) system. We also show that the worst solution corresponds to the conventional OFDM system; the conventional OFDM system has the largest BER. In the case of zero forcing receivers, the design of optimal transceiver depends on signal-to-noise ratio (SNR). For higher SNR, solutions of optimal precoders are the same as those of MMSE receivers.
BER optimized channel independent precoder for OFDM system
(2002)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
We consider the minimization of bit error rate (BER) for OFDM systems with orthogonal precoders. For low SNR, we show that the conventional OFDM system is the optimal solution and the optimal precoder is the identity matrix. For high SNR, corresponding to a relatively practical range of BER, there are channel independent solutions for optimal precoders. One such solution is the DFT matrix and the resulting optimal transceiver becomes the single carrier transmission system. Furthermore, in this case the conventional OFDM system has the largest BER among all OFDM systems with orthogonal precoders.
Block based DMT systems with reduced redundancy
(2001)
Lin, YP
;
Weng, WL
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
There has been great interest in the design of DMT (discrete multitone) transceivers. An M-band DMT transceiver: is called block based if the transmitter and the receiver consist of constant matrices. The commonly used DMT systems are mostly block based, e.g., the DFT based system used in transmission over digital subscriber lines. For an FIR channel of order L, it is known that redundancy of length L enables the receiver to cancel ISI completely. Such a scheme allow us to trade bandwidth for ISI cancellation. In block based DMT (BDMT) systems, the redundancy K is typically chosen to be the same as the order of the channel L. In this paper we will consider BDMT transceiver with redundancy K less than or equal to L. With the reduced redundancy better bandwidth efficiency can be obtained as will be demonstrated by examples. Furthermore minimum redundancy for BDMT systems will be derived and the transceivers will be parameterized whenever ISI solutions exist.
Closed-form expression of smith forms for pseudo-circulants
(2002)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
The pseudo-circulant matrices arise naturally in the block filtering application of multirate systems. More recently, there has been considerable interests in pseudo-circulants for the applications of precoding and discrete multitone communications systems. In these systems, a scalar FIR channel P(z) is cast as a pseudo-circulant channel matrix. Many important channel properties have been derived from the Smith form and the decomposition of pseudo-circulants. In this paper, we will show that the Smith form of an FIR pseudo-circulant matrix can be given in terms of the zeros of the underlying scalar filter P(z). Once the zeros of P(z) are known, the Smith form of the corresponding pseudo-circulant matrix can be obtained in closed form.
DFT based transceivers with-windowing
(2004)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
In this paper we consider window designs for DFT (discrete Fourier transform) based multicarrier transceivers without using extra cyclic prefix. As in previous works of window designs for DFT based transceiver, a post processing matrix, generally channel dependent, is needed to have a zero-forcing receiver. We show that post processing is channel independent if and only if the window itself has the cyclic-prefixed property. We design optimal windows with minimum spectral leakage subject to the cyclic-prefixed condition. Moreover, we analyze how post processing affects SNR (signal to noise ratio) at the receiver, an aspect that is not considered in most of the earlier works. The resulting SNR can be given in a closed form. Join optimization of spectral leakage and SNR are also considered. Furthermore, examples demonstrate that we can have a significant reduction in spectral leakage at the cost of a small SNR penalty.
Discrete multitone modulation with principal component filter banks
(2002-10-01)
Vaidyanathan, PP
;
Lin, YP
;
Akkarakaran, S
;
Phoong, SM
;
電控工程研究所
Discrete multitone (DMT) modulation is an attractive method for communication over a nonflat channel with possibly colored noise. The uniform discrete Fourier transforn (DFT) filter bank and cosine modulated filter bank have in the past been used in this system because of low complexity. We show in this paper that principal component filter banks (PCFB) which are known to be optimal for data compression and denoising applications, are also optimal for a number of criteria in DMT modulation communication. For example, the PCFB of the effective channel noise power spectrum (noise psd weighted by the inverse of the channel gain) is optimal for DMT modulation in the sense of maximizing bit rate for fixed power and error probabilities. We also establish an optimality property of the PCFB when scalar prefilters and postfilters are used around the channel. The difference between the PCFB and a traditional filter bank such as the brickwall filter bank or DFT filter bank is significant for effective power spectra which depart considerably from monotonicity. The twisted pair channel with its bridged taps, next and fext noises, and AM interference, therefore appears to be a good candidate for the application of a PCFB. This will be demonstrated with the help of numerical results for the case of the ADSL channel.
ISI-free FIR filterbank transceivers for frequency-selective channels
(2001-11-01)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
Discrete multitone modulation transceivers (DMTs) have been shown to be very useful for data transmission over frequency selective channels. The DMT scheme is realized by a transceiver that divides the channel into subbands. The efficiency of the scheme depends on the frequency selectivity of the transmitting and receiving filters. The receiving filters with good stopband attenuation are also desired for combating narrowband noise. The filterbank transceiver or discrete wavelet multitone (DWMT) system has been proposed as an implementation of the DMT transceiver that has better frequency band separation, but usually, inrtersymbol interference (IST) cannot be completely canceled in these filterbank transceivers, and additional equalization is required. In this paper, we show how to use over interpolated filterbanks to design IST-free FIR transceivers. A finite impulse response (FIR) transceiver with good frequency selectivity can be designed, as will be demonstrated by design examples.
Lapped unimodular transform and its factorization
(2002-11-01)
Phoong, SM
;
Lin, YP
;
電控工程研究所
;
Institute of Electrical and Control Engineering
Two types of lapped transforms have been studied in detail in the literature, namely, the lapped orthogonal transform (LOT) and its extension, the biorthogonal lapped transform (BOLT). In this paper, we will study the lapped unimodular transform (LUT). All three transforms are first-order matrices with finite impulse response (FIR) inverses. We will show that like LOT and BOLT, all LUTs can be factorized into degree-one unimodular matrices. The factorization is both minimal and complete. We will also show that all first-order systems with FIR inverses can be minimally factorized as a cascade of degree-one LOT, BOLT, and LUT building blocks. Two examples will be given to demonstrate that despite having a very small system delay, the LUTs have a satisfactory performance in comparison with LOT and BOLT.
Minimum redundancy for ISI free FIR filterbank transceivers
(2002-04-01)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
There has been great interest in the design of filter-bank transceivers. Usually, with proper time domain equalization, the channel is modeled as an FIR filter. It is known that for FIR channels, the introduction of certain redundancy allows the receiver to cancel intersymbol interference (ISI) completely, and channel equalization is performed implicitly using FIR transceivers. This scheme allows us to trade bandwidth for ISI cancellation. In this paper, we will derive the minimum redundancy required for the existence of FIR transceivers for a given channel. We will see that the minimum redundancy is directly related to the zeros of the channel and to the Smith form of an appropriately defined channel matrix.
Minimum-redundancy ISI-free FIR filterbank transceivers
(2000)
Lin, YP
;
Phoong, SM
;
電機資訊學士班
;
Undergraduate Honors Program of Electrical Engineering and Computer Science
The discrete multitone modulation system (DMT) has been demonstrated to be a very useful technique for high speed transmission over frequency selective channels such as the digital subscriber loops. The DR IT system can be realized using a filterbank transceiver, the synthesis bank as the transmitter and the analysis bank as the receiver. With proper time domain equalization, the channel call usually be modeled as an FIR filter with order L. It is known that if a redundancy of length L is introduced, FIR filterbank transceivers with zero ISI (Inter-Symbol Interference) property can be achieved. For example, in DFT based DMT system, redundancy is introduced by adding a cyclic prefix of length L. In this paper, we will derive the minimum length of redundancy required for FIR filterbank transceivers with ISI Free property. For a give channel, we will show that the minimum length is directly related to the Smith form of an appropriately defined channel matrix.
MINLAB: Minimum noise structure for ladder-based biorthogonal filter banks
(2000-02-01)
Phoong, SM
;
Lin, YP
;
電控工程研究所
;
Institute of Electrical and Control Engineering
In this paper, we introduce a minimum noise structure for ladder-based biorthogonal (MINLAB) filter bank. The minimum noise structure ensures that the quantization error has a unity noise gain, even though the filter bank is biorthogonal, The coder has a very low design and implementation cost. Perfect reconstruction property is structurally preserved, Optimal bit allocation and coding gain formulas are derived, We show that the coding gain of the optimal MINLAB coder is always greater than or equal to unity, For both AR(1) process and MA(1) process, the MINLAB coder with two taps has a higher coding gain than the optimal orthonormal coder with an infinite number of taps, In addition to its superior decorrelation ability, it has many other desired features that make it a potentially valuable and attractive alternative to the orthonormal coder, especially for the high-fidelity compression.
MMSE OFDM and prefixed single carrier systems: BER analysis
(2003)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
The single carrier system with cyclic prefix (SC-CP) has been demonstrated to outperform the OFDM system through simulations. In this paper, we consider minimum mean squared error (MMSE) receivers for the SC-CP system. We show analytically that, for uncoded SC-CP symbols, the SC-CP system with an MMSE receiver has a smaller bit error rate (BER) than the OFDM system for all SNR.
OFDM transmitters: Analog representation and DFT-based implementation
(2003-09-01)
Lin, YP
;
Phoong, SM
;
電控工程研究所
;
Institute of Electrical and Control Engineering
The implementation of OFDM transmitters typically consists of a discrete DFT matrix and a digital-to-analog (DAC) converter. Many existing results on the analysis of OFDM systems, e.g., spectral roll-off, are based on a convenient analog representation. In this paper, we show that the analog representation and the DFT-based OFDM transmitters are equivalent only in special cases. Using the analog system to analyze the DFT-based OFDM system may not be valid if there is no equivalent analog representation.
On the duality of optimal DMT systems and biorthogonal subband coders
(2000)
Lin, YP
;
Vaidyanathan, PP
;
Akkarakaran, S
;
Phoong, SM
;
電控工程研究所
The discrete multitone modulation (DMT) systems have been widely used in various applications. The DMT system can be considered as a dual of a subband coder, obtained by using the synthesis bank as the transmitter and analysis bank as the receiver. In designing optimal subband coders, the objective is to minimize output quantization noise, whereas in the problem of designing optimal DMT system, the objective function to be minimized is the transmitted power. In this paper we will show that the design of optimal DMT systems can be formulated as a hypothetical design problem of optimal subband coders. The solution of optimal DMT system can be obtained using existing design methods for optimal biorthogonal subband coders.
Optimal biorthogonal transform for colored noise suppression with subband Wiener filtering
(2002-05-01)
Phoong, SM
;
Lin, YP
;
電控工程研究所
;
Institute of Electrical and Control Engineering
It is well-known that the Karhunen-Loeve transform (KLT) is optimal for white noise suppression. Recently, Akkarakaran and Vaidyanathan showed that for the case of colored noise, if both the noise and signal have a common KLT, then the KLT is optimal for subband noise suppression. In this paper, we will derive the optimal transform (not restricting to the class of unitary transforms) for the noise suppression problem when the signal and noise have arbitrary spectrum.