Chi-chao Chao

Two Low-Complexity Reliability-Based Message-Passing Algorithms for Decoding Non-Binary LDPC Codes

This paper presents two low-complexity reliability-based message-passing algorithms for decoding LDPC codes over non-binary finite fields. These two decoding algorithms require only finite field and integer operations and they provide effective trade-off between error performance and decoding complexity compared to the non-binary sum product algorithm. They are particularly effective for decoding LDPC codes constructed based on finite geometries and finite fields.

Estimation and Compensation of I/Q Imbalance in OFDM Direct-Conversion Receivers

In this paper, we consider estimation and compensation of the I/Q imbalance generated by the direct-conversion receiver in orthogonal frequency division multiplexing (OFDM) systems. Two methods for I/Q imbalance compensation are proposed: the maximum-likelihood estimation (MLE)-based method and the least-square estimation (LSE)-based method. Computer simulations are conducted for comparisons of the performance. Effects of timing and frequency offsets on performance are also analyzed. The MLE-based method generally performs well even at low signal-to-noise ratio (SNR) and is robust against timing and frequency offsets. The LSE-based method outperforms the MLE-based method at higher SNR. Both methods are robust against different gain and phase imbalances.

Analysis of LMS-adaptive MLSE equalization on multipath fading channels

We consider a practical maximum-likelihood sequence estimation (MLSE) equalizer on multipath fading channels in conjunction with an adaptive channel estimator consisting of a least mean square (LMS) estimator and a linear channel predictor, instead of assuming perfect channel estimates. A new LMS estimator model is proposed which can accurately characterize the statistical behavior of the LMS estimator over multipath fading channels. Based on this model, a new upper-bound on block error rate is derived under the consideration of imperfect channel estimates. Computer simulations verify that our analytical results can correctly predict the real system performance and are applicable over a wide range of the step size parameter of the LMS estimator.

Signal-to-Interference-Plus-Noise Ratio Analysis for Direct-Sequence Ultra-Wideband Systems in Generalized Saleh–Valenzuela Channels

In this paper, exact signal-to-interference-plus-noise ratio (SINR) analysis of direct-sequence ultra-wideband (UWB) systems with Rake receiving in the presence of inter-symbol interference and multiple-access interference over a generalized Saleh-Valenzuela (GSV) channel with a generic pulse shaping function is provided. The SINR expression, for synchronized multiple-access, is first obtained without assuming random spreading. The GSV channel structure under consideration is a generalization of the Saleh-Valenzuela channel structure with generalized path-gain and path-arrival models, examples of which can include all the IEEE 802.15.3a UWB channel models and some of the IEEE 802.15.4a models. Then, by the novel treatment of renewal processes, the exact average SINR over the GSV channel statistics is obtained. Our analytical results well match computer simulations and can readily be applied to evaluate and improve the performance of UWB systems over realistic channel and interference models.

Path-compatible pruned convolutional (PCPC) codes

Path pruning, a new coding concept to achieve free distance enlargement for convolutional codes, is proposed. Through path pruning, every convolutional code can be used for unequal error protection (UEP), no matter whether it is originally a UEP code. To avoid undesired path discontinuity and reduce possible path distance loss, a cascaded implementation together with a path-compatible criterion is proposed, under which path-compatible pruned convolutional (PCPC) codes are constructed. Necessary and sufficient conditions are also derived for a subclass of PCPC codes whose decoding can be done by a single decoder for the parent code. Finally, some PCPC codes with good UEP capabilities found by computer search are given.

Capacity of synchronous coded DS SFH and FFH spread-spectrum multiple-access for wireless local communications

The performance of synchronous spread-spectrum multiple-access (SSMA) communications based on direct-sequence (DS), slow frequency-hopped (SFH), and fast frequency-hopped (FFH) systems for wireless local communications of micro-cellular personal communications is analyzed. Using an indoor multipath fading channel model with clusters of arriving rays, we investigate multiuser DS systems with RAKE and diversity reception by selection combining (SC), multiuser SFH systems with equal-gain (EG) diversity reception, and multiuser FFH systems with correlated EG and self-normalization (SN) combining techniques. Reed-Solomon codes are considered to further improve the system performance. Given a fixed available bandwidth with narrow band interference (NBI), capacities and packet error rates are determined under various system configurations. Total capacities of hybrid frequency-division multiple-access (FDMA)/SSMA (DS and SFH) systems are compared with those of wide-band SSMA systems. For high data rate communications, wide-band DS-SSMA systems have larger capacities than hybrid FDMA/DS-SSMA systems. For low data rate communications, a capacity comparison between wide-band DS-SSMA and hybrid FDMA/DS-SSMA systems depends on fading statistics. Hybrid FDMA/SFH-SSMA systems have larger capacities than wide-band DS-SSMA systems, FFH-SSMA systems could not provide satisfactory performance due to correlation among hopping bands.

Complete complementary codes and generalized reed-muller codes

Due to ideal autocorrelation and cross-correlation properties, complete complementary codes (CCCs) can be employed in CDMA systems to eliminate the multiple-access interference. In this letter, we propose a direct general construction of CCCs from cosets of the first-order Reed-Muller codes, which includes previous results as a special case. The larger number of CCCs constructed by our method can provide advantages in applications to cellular CDMA systems.

On generalized Hamming weights of binary primitive BCH codes with minimum distance one less than a power of two

The generalized Hamming weights introduced by Wei (1991) have been shown to be fundamental descriptive parameters of a linear block code. They have been found to be useful in certain cryptographic applications and in the studies of minimal trellis diagrams of linear block codes. In this correspondence, we determine the first few and the last few generalized Hamming weights of binary primitive BCH codes with minimum distance one less than a power of two, of their extensions, and of the duals of both.

Resource Allocation and Partner Selection for Cooperative Multicarrier Systems

Subcarrier allocation, power allocation, and partner selection (PS) algorithms are examined for amplify-and-forward (AF) cooperative multicarrier systems. Consider a network that consists of multiple cooperative pairs that employ a two-phase cooperation scheme, where the users first transmit their own messages on each of their subcarriers in phase I and either retransmit their own messages or relay the messages of their partners in phase II. In general, subcarrier allocation and partner selection problems are known to be intractable for systems with large number of users and subcarriers. In this paper, we propose a suboptimal but mathematically tractable approach to address these issues and show its effectiveness compared with existing algorithms. This approach is done by considering the power allocation over a relaxed problem formulation, where each subcarrier is allowed to retransmit or relay for all other subcarriers in phase II over virtually orthogonal channels. The power allocation in this case is a convex optimization problem and, thus, can efficiently be solved. Based on the results of the relaxed power allocation problem, an efficient subcarrier allocation algorithm for the original problem can then be derived by allowing subcarriers to forward the data that are allocated the most power. The results also yield a necessary and sufficient condition on when cooperation is helpful among two users. A partner selection algorithm is then devised based on the criterion given in the condition. With the proposed subcarrier allocation scheme, an iterative procedure is then adopted for resource allocation in the two phases. The complexity of the proposed algorithm is discussed, and its efficacy is demonstrated through numerical simulations.

Performance of binary block codes at low signal-to-noise ratios

The performance of general binary block codes on an unquantized additive white Gaussian noise (AWGN) channel at low signal-to-noise ratios is considered. Expressions are derived for both the block error and the bit error probabilities near the point where the bit signal-to-noise ratio is zero. These expressions depend on the global geometric structure of the code, although the minimum distance still seems to play a crucial role. Examples of codes such as orthogonal codes, biorthogonal codes, the (24,12) extended Golay code, and the (15,6) expurgated BCH code are discussed. The asymptotic coding gain at low signal-to-noise ratios is also studied. >