Yu-Sun Liu

A new universal random coding bound for the multiple-access channel

The minimum average error probability achievable by block codes on the two-user multiple-access channel is investigated. A new exponential upper bound is found which can be achieved universally for all discrete memoryless multiple-access channels with given input and output alphabets. It is shown that the exponent of this bound is greater than or equal to those of previously known bounds. Moreover, examples are given where the new exponent is strictly larger.

Performance analysis of frequency-hop packet radio networks with generalized retransmission backoff

A fully connected radio network is considered in which packets are sent using Reed-Solomon error-control coding, slow frequency-hop (FH) modulation, and generalized retransmission backoff policy. An easy-to-use analytical method for finite population FH networks is developed which offers insight into how system dynamics is affected by the number of users and the input rate. The performance of several retransmission backoff policies is examined. It is shown that the performance of exponential backoff policy with small minimum retransmission probability is not too far away from the optimum, when the code-block length is small and the number of users is not unreasonably large. It is also shown that packet rejection after a certain number of unsuccessful transmissions can stabilize some networks as one would expect, but may destabilize some others contrarily. Finally, the accuracy of the method is verified by extensive simulation results.

Distributed Dynamic Spectrum Management for Digital Subscriber Lines

This paper investigates the dynamic spectrum management problem for digital subscriber lines. Two new distributed dynamic spectrum management algorithms, which improve upon the existing iterative water-filling algorithm, are proposed. Unlike the iterative water-filling algorithm, in which crosstalk interference is reduced by using adaptive power backoff, the new algorithms employ full power and mitigate crosstalk interference by shifting one users spectrum away from the others. Simulation results show that the new algorithms achieve significant performance gains over the iterative water-filling algorithm in mixed central office/remote terminal (CO/RT) deployment asymmetric digital subscriber line (ADSL) and upstream very-high bit-rate digital subscriber line (VDSL).

Polling an image for circles by random lines

A new random sampling strategy, designed for retrieving subsets consisting of two edge pixels from an input image, is proposed as the sampling process for RANSAC circle detection using coaxal transform. The proposed strategy is shown to have the following advantages over the conventional random sampling strategy. First, a poll size can be planned in a principled manner. Second, once a poll size is set, the probability that a circle is missed by the sampling process is kept relatively constant regardless of noise. Third, the actual number of subsets taken is automatically adjusted for different image complexities. Experimental results in agreement with the claimed advantages are presented.

Diversity-Combining and Error-Correction Coding for FFH/MFSK Systems over Rayleigh Fading Channels under Multitone Jamming

The combined application of diversity and error-correction coding for combating multitone jamming (MTJ) in fast frequency-hopping/M-ary frequency-shift-keying (FFH/MFSK) systems over Rayleigh fading channels is investigated. A log-likelihood-combining metric which requires only easy-to-obtain system parameters for its computation is proposed. In addition, four soft-decision coding schemes which employ the proposed diversity-combining metric as decoding metric are considered. An upper bound on the bit error rate of the proposed metric is derived. Simulation results show that the proposed metric outperforms robust metrics. Moreover, the improvement is most substantial for low signal-to-jamming ratio (SJR). Performances of convolutional codes and turbo codes together with either bit-interleaving or symbol-interleaving are examined. Upper bounds are derived for the bit error rates of the convolutional coding schemes, and simulations are performed for all coding schemes. The results show that significant performance improvement can be achieved in difficult transmission conditions by changing the interleaving method from bit-interleaving to symbol-interleaving, but without changing the code. In particular, the symbol-interleaved turbo coding scheme is the best among the four for maintaining system performance at an acceptable level for most situations.

Cubic convolution interpolation function with variable coefficients and its application to channel estimation for IEEE 802.16 initial downlink

In this study, the authors propose to use the cubic convolution interpolation function with a free variable to generate a class of constant-coefficient interpolation functions. The interpolations functions can be applied to an orthogonal frequency-division multiplexing-based receiver for channel estimation. For a given channel, different interpolation functions from the class have different bit error rates (BERs). Therefore by using a search algorithm, a better interpolation function is chosen with the aid of a performance indicator, such as the estimated mean-squared error (EMSE). Although the uneven distribution of pilots in the IEEE 802.16e system prohibits direct utilisation of the cubic interpolation function, a simple approach has been proposed to solve this problem. The proposed method is validated in the IEEE 802.16e downlink system by Monte Carlo simulation. The simulation results also confirm that channels with different statistical properties should use different interpolation functions. In addition to the Monte Carlo simulation, we also analyse the BER of the proposed approach. We find that the BERs based on the analytical evaluation and on the simulation closely match each other. Finally, it is shown that the EMSE is an acceptable performance indicator for medium-to-high signal-to-noise ratio.

MAC Protocols for Multi-Channel WLANs

Wireless local area networks, in which every station can transmit via any one of the available operating channels, but only one channel at a time, are investigated. Two distributed random access protocols are proposed for these WLANs. The CSMA/CA protocol is similar to the IEEE 802.11 standard but with slight modifications for multiple operating channels. The fuzzy logic controlled protocol employs a simple fuzzy logic controller to tune the size of backoff window. Extensive simulations are provided to evaluate the channel utilization, fairness, and responsiveness of these two protocols. Furthermore, the effects of employing RTS/CTS mechanism with both protocols are considered. Finally, performances of these two protocols are also investigated under conditions of burst traffic and noisy channels. Simulation results show that the fuzzy logic controlled protocol is a great improvement of the CSMA/CA protocol.

Trellis lengths and truncation depths for TBCC decoders in mobile WiMAX

The circular decoding algorithm is a simple and effective decoding method for tail-biting convolutional codes, which are the mandatory error-correcting codes in the mobile WiMAX standard. The performance of the circular decoding algorithm is evaluated for mobile WiMAX systems under different choices of truncation depth and decoding trellis length. A rule of thumb for the choice of the trellis length is also obtained.

Soft-Decision Decoding in Asynchronous FH/SSMA Networks Using MFSK Modulation

Soft-decision decoding techniques are applied to asynchronous frequency-hop/spread-spectrum multiple-access (FH/SSMA) networks, where M-ary frequency shift keying (MFSK) is employed to transmit one modulated symbol per hop. Coding schemes using soft-decision decoded binary convolutional codes or turbo codes are considered, both with or without bit-interleaving. Performances of several soft metrics are examined for each coding scheme. It is shown that when multiple access interference is the main source of errors, the product metric offers the best performance among the soft metrics considered for all coding schemes. Furthermore, the application of soft-decision decoded convolutional codes or turbo codes without bit-interleaving is shown to allow for a much larger number of simultaneously transmitting users than hard-decision decoded Reed-Solomon codes

Iterative channel estimation method for long-term evolution downlink transmission

Long-term evolution (LTE) standard employs orthogonal frequency division multiplexing (OFDM) for downlink (DL) data transmission. As pilot subcarriers are non-uniformly and sparsely spaced in the OFDM DL transmission, more complicated channel estimation methods may have to be adopted in the mobile devices to ensure adequate performance. An iterative approach is proposed for the channel estimation task, in which an inverse fast Fourier transform and fast Fourier transform pair is iteratively used to smooth channel estimates provided by decision feedback. The performance of this approach is compared with two two-dimensional interpolation methods and one discrete Fourier transform-based estimation method that have been proposed for the LTE DL transmission. Simulation results show that the performance of the proposed method surpasses all the other methods except at high signal-to-noise ratio, and moreover, not a single estimation method consistently performs better than the proposed method for all the channel models considered. Thus, the proposed method is the best among the considered methods for LTE DL transmission.