E.K.S. Au

University of British Columbia - Vancouver, Vancouver, British Columbia, Canada

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Publications (42)37.62 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper proposes a statistical-eigenmode space-division multiple-access (SE-SDMA) transmission approach for a two-user downlink system where two transmit antennas are deployed at the base station and each mobile user has only one receive antenna, assuming that only statistical channel state information (SCSI) is available at the transmitter. By maximizing a lower bound of the ergodic signal-to-leakage-and-noise ratio (SLNR), the proposed SE-SDMA approach selects two users with orthogonal principal statistical eigen-directions and then transmits to each user along the corresponding eigenmode. For this approach, we derive an exact expression of the achievable ergodic rate, and compare it with the zero-forcing beamforming (ZFBF) system exploiting instantaneous CSI. We show that SE-SDMA can achieve the maximum ergodic sum-rate of the two selected users, and provide significant user selection gains. For comparison with ZFBF, we show that the rate gap between SE-SDMA and ZFBF with perfect CSI at the transmitter (CSIT) tends to zero in highly correlated channels. Further, when ZFBF is implemented with inaccurate CSIT, the operating regions of SE-SDMA and ZFBF are investigated for different ranges of signal-to-noise ratio (SNR), instantaneous CSIT accuracy levels, and spatial correlation levels. Numerical results confirm the analytically derived conclusions, and physical insights are provided.
    IEEE Transactions on Signal Processing 10/2012; 60(10):5371-5383. · 2.81 Impact Factor
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    ABSTRACT: This paper proposes a statistical-eigenmode spacedivision multiple-access (SE-SDMA) transmission for a two-user downlink system where two transmit antennas are equipped at the base station and each mobile user has one receive antenna, assuming that only statistical channel state information (CSI) is available at the transmitter. By maximizing a lower bound of the ergodic signal-to-leakage-and-noise ratio, the proposed SE-SDMA approach selects two users with orthogonal principal statistical eigen-directions and transmits to each user along the corresponding eigenmode. We derive an exact expression of the ergodic achievable rate, and compare it with the zero-forcing beamforming (ZFBF) system exploiting instantaneous CSI. It is shown that SE-SDMA can achieve the maximum ergodic sum-rate of the two selected users, and provide significant user selection gain. Analytical and simulation results show that the rate gap between SE-SDMA and ZFBF with perfect CSI at the transmitter can tend to zero in highly correlated channels, which indicates that statistical precoding can be used instead of instantaneous precoding in certain environments.
    Communications (ICC), 2012 IEEE International Conference on; 01/2012
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    ABSTRACT: We perform a minimum distance analysis of a class of two-dimensional intersymbol interference (ISI) channels applicable to multitrack magnetic recording and orthogonal frequency division multiplex transmission systems. Exact minimum distance for a wide class of ISI responses is derived. The fundamental analytical technique is to transform the channel into an equivalent minimum phase channel. The results improve upon the prior work of Soljanin and Georghiades.
    IEEE Transactions on Information Theory 01/2012; 58(2):878-887. · 2.62 Impact Factor
  • Xinping Yi, E. Au
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    ABSTRACT: In practical downlink multiuser MIMO channels, users' channels are non-orthogonal, which results in inter-user interference and in turn degrades system performance. In this paper, a new subspace-domain linear transmit preprocessing technique is proposed in enhancing users' spatial separation and suppressing inter-user interference. We show by simulations that sum rate is significantly improved and its saturation is avoided.
    Communications (ICC), 2012 IEEE International Conference on; 01/2012
  • Source
    Xinping Yi, E.K.S. Au
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    ABSTRACT: In downlink multiuser multiple-input-multiple-output (MU-MIMO) systems, users are practically heterogeneous. However, many existing user-scheduling algorithms are designed with an implicit assumption that users are homogeneous. In this paper, we revisit the problem by exploring the characteristics of heterogeneous users from a subspace viewpoint. With an objective of minimizing interference nonorthogonality among users, three new angular-based user-scheduling criteria are proposed. While the first criterion is heuristically determined by identifying the incapability of the largest principal angle to characterize subspace correlation and, hence, interference nonorthogonality between users, the second and third criteria are derived by using the sum rate capacity bounds with block diagonalization and the change in capacity by adding a new user into an existing user subset, respectively. Aiming to capture fairness among heterogeneous users while maintaining multiuser diversity gain, two new hybrid user-scheduling algorithms, whose computational complexities are only linearly proportional to the number of users, are proposed. We show by simulations the effectiveness of our proposed user-scheduling criteria and algorithms with respect to those commonly used in a homogeneous environment.
    IEEE Transactions on Vehicular Technology 11/2011; · 2.06 Impact Factor
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    ABSTRACT: Single-carrier frequency division multiple access (SC-FDMA) has appeared to be a promising technique for high data rate uplink communications. Aimed at SC-FDMA applications, a cyclic prefixed version of the offset quadrature amplitude modulation based OFDM (OQAM-OFDM) is first proposed in this paper. We show that cyclic prefixed OQAM-OFDM (CP-OQAM-OFDM) can be realized within the framework of the standard OFDM system, and perfect recovery condition in the ideal channel is derived. We then apply CP-OQAM-OFDM to SC-FDMA transmission in frequency selective fading channels. Signal model and joint widely linear minimum mean square error (WLMMSE) equalization using a prior information with low complexity are developed. Compared with the existing DFTS-OFDM based SC-FDMA, the proposed SC-FDMA can significantly reduce envelope fluctuation (EF) of the transmitted signal while maintaining the bandwidth efficiency. The inherent structure of CP-OQAM-OFDM enables low-complexity joint equalization in the frequency domain to combat both the multiple access interference and the intersymbol interference. The joint WLMMSE equalization using a prior information guarantees optimal MMSE performance and supports Turbo receiver for improved bit error rate (BER) perform BER) performance. Simulation results confirm the effectiveness of the proposed SC-FDMA in terms of EF (including peak-to-average power ratio, instantaneous-to-average power ratio and cubic metric) and BER performances.
    IEEE Transactions on Communications 06/2011; · 1.75 Impact Factor
  • IEEE Transactions on Communications. 01/2011; 59:1467-1480.
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    ABSTRACT: In this paper, we consider the multiple base station cooperative transmission, a new technology being developed to lift the interference limits on wireless network. In practice, the biggest challenge facing multiple base station collaboration is the stringent requirement of the high-speed backbone to enable information exchange, especially for large deployment. We address this backbone issue and propose to define a collaborative zone within each cell. Only those users within the collaborative zone will be served by multiple coordinated base stations. Whereas users outside the collaborative zone will be served by a single base station as in a conventional non-collaborative scheme. The criterion defining the collaborative zone is proposed and designed in such a way that the users outside the collaborative zone sacrifice only marginal potential collaboration gain in terms of achievable rates. Since the amount of information to be exchanged will be a square function of the number of users served by the collaborative base stations, the introduction of the collaborative zone can reduce the backhaul overhead significantly.
    01/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we consider the multiple base station cooperative transmission, a new technology being developed to lift the interference limits on wireless network. In practice, the biggest challenge facing multiple base station collaboration is the stringent requirement of the high-speed backbone to enable information exchange, especially for large deployment. We address this backbone issue and propose to define a collaborative zone within each cell. Only those users within the collaborative zone will be served by multiple coordinated base stations. Whereas users outside the collaborative zone will be served by a single base station as in a conventional non-collaborative scheme. The criterion defining the collaborative zone is proposed and designed in such a way that the users outside the collaborative zone sacrifice only marginal potential collaboration gain in terms of achievable rates. Since the amount of information to be exchanged will be a square function of the number of users served by the collaborative base stations, the introduction of the collaborative zone can reduce the backhaul overhead significantly.
    GLOBECOM Workshops (GC Wkshps), 2010 IEEE; 01/2010
  • Liang Sun, E.K.S. Au, M.R. McKay
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    ABSTRACT: We propose a novel low-complexity adaptive transmission scheme for multiple-input-multiple-output (MIMO) multichannel beamforming systems in the presence of cochannel interference. By measuring and exploiting the characteristics of the cochannel interference, the proposed adaptive algorithm jointly selects the active eigenmodes, the power allocation, and the modulation formats by utilizing a set of eigenvalue-based thresholds to maximize the spectral efficiency while meeting a target bit-error-rate (BER) constraint. We analyze the performance of the proposed scheme by deriving tractable expressions for the suspension probability and the error-free rate and demonstrate significant performance gains with respect to prior adaptive systems.
    IEEE Transactions on Vehicular Technology 11/2009; · 2.06 Impact Factor
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    ABSTRACT: Precise time-of-arrival (TOA) estimation is challenging in dense multipath environments because the leading-edge path of a received signal may not be the strongest path. In this paper, we propose a threshold-based coherent TOA estimation scheme for impulse radio-based ultrawideband (UWB) systems. The scheme uses a dedicated ranging sequence with a good cross-correlation property and a search-back process that considers the count of threshold crossings to find the leading edge of the received signal. We derive an approximate closed-form expression for the optimal threshold that restrains the mean absolute error. We show by simulations that high-precision short-distance ranging is achieved without introducing too much latency.
    IEEE Transactions on Vehicular Technology 11/2009; · 2.06 Impact Factor
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    ABSTRACT: Zero-IF architecture is a competitive alternative to the conventional super-heterodyne architecture for RF-to-baseband conversion in OFDM systems. Despite its advantages of low cost and high integratability, it suffers from the I/Q imbalance that contributes amplitude and phase mismatches between in-phase and quadrature branches, resulting in signal distortion and performance degradation. In this paper, we revisit the problem in a theoretical viewpoint through a minimum distance analysis. In particular, we aim at proving that when the subcarriers are paired symmetrically to a center frequency and the maximum likelihood decoding is employed, the transmitter I/Q imbalance can provide an additional degree of diversity in frequency selective fading channels.
    Proceedings of the Global Communications Conference, 2009. GLOBECOM 2009, Honolulu, Hawaii, USA, 30 November - 4 December 2009; 01/2009
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    Lu Zhang, E.K.S. Au, V.K.N. Lau
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    ABSTRACT: We propose a three-stage cross correlation-based timing synchronization algorithm for multiband-OFDM UWB systems. By utilizing the time-domain base sequence of the packet/frame synchronization preamble, the proposed algorithm first performs a coarse estimation on the reference symbol boundary, followed by a fine estimation on locating the exact boundary of the FFT window for the symbol, and a final check if the exact boundary is within an intersymbol interference-free zone. Numerical results reveal the effectiveness of the proposed algorithm in terms of mean square error and synchronization probability with respect to prior works.
    Communication Systems, 2008. ICCS 2008. 11th IEEE Singapore International Conference on; 12/2008
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    Rong Zhang, E.K.S. Au, R.S. Cheng
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    ABSTRACT: OFDM systems are sensitive to front-end imperfections including CFO, IQ imbalance and phase noise. In practice, these imperfections are inevitable and would lead to significant performance degradation. In this paper, a general closed-form expression on average signal-to-interference-and-noise ratio (SINR) over multipath fading channels is derived for performance evaluation. When additive white Gaussian noise channels and flat fading channels are considered, the expression reduces to an exact expression and a tight upper bound, respectively. Our results show that the average SINR is inversely proportional to these front-end non-idealities.
    Communication Systems, 2008. ICCS 2008. 11th IEEE Singapore International Conference on; 12/2008
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    ABSTRACT: In this paper, a novel time-of-arrival estimation algorithm is proposed for impulse radio-based UWB systems in dense multipath environment. In particular, a dedicated ranging sequence with good cross-correlation property is proposed and a threshold-based search-back unit is introduced, with the purpose of supporting high-precision ranging even at low signal-to-noise ratio. An approximate closed-form expression on the optimal threshold is derived based on the Rayleigh statistical analysis, and its accuracy and tightness are verified by simulations. Results show that the proposed ranging scheme achieves an excellent mean absolute error performance in the order of nanoseconds without introducing too much latency.
    Personal, Indoor and Mobile Radio Communications, 2008. PIMRC 2008. IEEE 19th International Symposium on; 10/2008
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    ABSTRACT: In this paper we perform a minimum distance analysis of a class of two dimensional intersymbol interference channels. In particular, some important cases of multitrack multihead magnetic recording systems fall into the studied class. Previously, Soljanin and Georghiades have studied the same problem as we do. The results derived in this paper are more conclusive and they improve upon theirs. The fundamental proof technique that we will use is to transform the channel into an equivalent minimum phase channel.
    Information Theory, 2008. ISIT 2008. IEEE International Symposium on; 08/2008
  • J.W. Huang, E.K.S. Au, V.K.N. Lau
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    ABSTRACT: We address the problem of designing a linear precoder for space-time coded multiple-input multiple-output (MIMO) system with imperfect knowledge of channel state information at the transmitter (CSIT), subject to a total transmit power constraint. Assuming an uncorrelated flat-fading channel and using a maximum likelihood decoder at the receiver, we analytically reveal that the derived precoder is a function of the noise variance, the eigenvalues of the estimated channel matrix and the eigenvalues of the codeword distance matrix. Furthermore, the power allocation on the eigenvalues of the precoder is shown to follow the water-pouring policy and hence the proposed precoder allocates more power to the stronger channels.
    IEEE Transactions on Wireless Communications 07/2008; · 2.42 Impact Factor
  • E.K.S. Au, J.W. Huang, V.K.N. Lau
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    ABSTRACT: We propose a joint set of linear precoder designs for single cell uplink multiuser space-time block coded multiple- input multiple-output systems with multi-packet reception by exploiting outdated channel state information. By deriving the pairwise error probability with respect to both minimum and average codeword distance design metrics, we formulate the design as an optimization problem subject to transmit power constraint for each user. Due to the non-convex nature of the optimization problem, we devise an iterative algorithm to solve for linear precoding structure for general space-time block code. For orthogonal space-time block code, we also propose a simplified distributed algorithm to solve for a closed-form solution. Asymptotic analysis on the effect of quality of the outdated channel state information (CSI) on the precoder structure is also presented. Simulation results are provided to demonstrate the effectiveness of the proposed designs for different space-time block codes at various CSI qualities.
    IEEE Transactions on Wireless Communications 07/2008; · 2.42 Impact Factor
  • E.K.S. Au, Wai Ho Mow
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    ABSTRACT: The input-output weight enumerator of a convolutional code characterizes the distance spectrum and allows error probability bounds to be conveniently evaluated. To efficiently compute the weight enumerator, Pimentel recently introduced the so-called state reduction algorithm which has a convenient implementation using existing symbolic mathematical software. In this paper, we propose a dynamic state elimination ordering heuristic to further accelerate the algorithm. As demonstrated by our empirical results, the accelerated state reduction algorithm can achieve impressive complexity savings relative to the original algorithm when applied to compute the weight enumerators and its various truncated versions of convolutional codes with moderate-to-large constraint lengths.
    IEEE Transactions on Wireless Communications 07/2008; · 2.42 Impact Factor
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    ABSTRACT: This paper analyzes bit error rate (BER) and outage probability of singular value decomposition-based multiple-input multiple-output systems with channel estimation error and feedback delay over uncorrelated Ricean fading channels. By utilizing marginal unordered and ordered eigenvalue distributions of complex noncentral Wishart matrices, we derive exact closed-form expressions on the average system performance and high signal- to-interference-plus-noise ratio (SINR) approximations on the individual eigen-subchannels, respectively, under the assumption of equal power allocation. Our expressions apply for various modulation formats and arbitrary numbers of transmit and receive antennas. Our results show that in low-to-moderate SINR regimes, both the BER and the outage probability increase with channel estimation error, feedback delay and the Ricean K-factor at a polynomial rate that is inversely proportional to the difference between the numbers of transmit and receive antennas. We also show that, with channel estimation error and feedback delay, the diversity orders of the BER and outage probability are zero and an irreducible error floor exists at high SINR.
    IEEE Transactions on Wireless Communications 05/2008; · 2.42 Impact Factor

Publication Stats

234 Citations
37.62 Total Impact Points

Institutions

  • 2008
    • University of British Columbia - Vancouver
      • Department of Electrical and Computer Engineering
      Vancouver, British Columbia, Canada
    • Lund University
      • Department of Electrical and Information Technology
      Lund, Skane, Sweden
  • 2004–2008
    • The Hong Kong University of Science and Technology
      • Department of Electronic and Computer Engineering
      Kowloon, Hong Kong