Francois P. S. Chin

Peak-to-average power reduction using partial transmit sequences: a suboptimal approach based on dual layered phase sequencing

A high peak-to-average power ratio (PAPR) is a major shortcoming in multicarrier systems, as it causes nonlinearity in the transmitter, degrading the performance of the system significantly. Partial transmit sequences (PTS) is one of the best methods in reducing PAPR, in which the information-bearing subcarriers are divided into M disjoint subblocks, each controlled by a phase rotation factor which brings PAPR down. Though PAPR reduction by PTS is more effective with more subblocks, there is a corresponding exponential increase in complexity. In this paper, a novel implementation of PTS is presented, in which a dual-layered approach is employed to reduce the complexity.

Added pilot semi-blind channel estimation scheme for OFDM in fading channels

An added pilot semi-blind (APSB) scheme used for channel identification of OFDM systems, which have been adopted for DAB and DVB-T, is proposed. This scheme uses pilot symbols that are added directly to the data symbols and hence does not require any additional bandwidth for its implementation. To remove the pilots, a two dimensional Wiener filter is used for the purpose of channel interpolation by using successive iterations. Although its complexity increases with additional iterations, the APSB scheme is found to compare favorably to existing blind schemes proposed for the OFDM system in terms of bit error rate and convergence.

Capacity enhancement of a multi-user OFDM system using dynamic frequency allocation

A dynamic subcarrier allocation algorithm is developed and studied in order to improve the capacity of a multi-user OFDM system in the downlink environment. The proposed algorithm uses a decentralized approach and considers the instantaneous channel response of each user in parallel. In order to reduce the complexity of the system, the available subcarriers are divided into a number of partitions and the algorithm attempts to allocate the partition with the highest average channel gain to each user. However, situations may arise whereby two or more users attempt to select the same partition. As such, an important aspect of the algorithm is to resolve such conflicts. Based on the above allocation plan, adaptive modulation is employed for each user on the allocated partitions. The results obtained show that the proposed dynamic allocation scheme outperforms a static allocation scheme in terms of a lower BER and a higher system capacity for the same SNR. In addition, it is possible to obtain performance improvements at the expense of an increase in the system complexity by dividing the subcarriers into a larger number of partitions.

Combined beamforming with space-time block coding for wireless downlink transmission

The paper proposes a downlink transmission scheme achieving both transmit diversity gain and beamforming gain via combining space-time block coding (STBC) with adaptive beamforming techniques. The beamforming weights of the combined system are optimized in such a way that the virtual channel coefficients corresponding to STBC-encoded data streams, seen at the mobile receiver, are guaranteed to be uncorrelated. Therefore, the diversity order promised achievable by a conventional system with STBC can be obtained completely. At the same time, an additional beamforming gain is achieved by maximizing the received signal-to-noise ratio (SNR) at the mobile receiver. The superiority of the proposed scheme is confirmed by numerical simulation results.

Enhanced architecture for residue number system-based CDMA for high-rate data transmission

This paper presents an advanced architecture for residue number system (RNS)-based code-division multiple-access (CDMA) system for high-rate data transmission by combining RNS representation, phase shift keying/quadrature amplitude modulation (PSK/QAM) and orthogonal modulation. The residues obtained from a fixed number of bits are again divided into spread code index and data symbol for modulation. The modulated data symbol is spread using the indexed orthogonal codes and transmitted through a communication channel. The proposed system uses a lower number of orthogonal codes than conventional RNS-based CDMA and the performance is comparable. The computational complexity of the proposed system is compared against alternative schemes such as M-ary CDMA and conventional RNS-based CDMA. The modified system is simulated extensively for different channel conditions and the results are discussed.

Characterization of ultra-wideband channels for outdoor office environment

We present a statistical model for the characterization of ultra-wideband (UWB) channels for the outdoor office environment. The bandwidth is from 3-6 GHz and measurements are done for the line-of-sight (LOS) and non-line-of-sight (NLOS) cases. Small scale effects are modeled by the Saleh-Valenzuela (S-V) model with modifications on the ray arrival times and amplitude statistics to fit the empirical data. The ray arrival times are described by a mixture of Poisson processes while the amplitude statistics are follow a Nakagami distribution with the m parameter following a lognormal distribution. The parameters for the cluster and ray power-decay time constants, cluster and ray arrival times, mean excess delay and RMS excess delay are also extracted to define the channel model completely.

ML-based frequency estimation and synchronization of frequency hopping signals

A maximum likelihood (ML)-based algorithm for frequency estimation and synchronization of frequency hopping signals is proposed in this paper. By using a two-hop signal model that incorporates the unknown hop transition time, the likelihood function of the received frequency hopping signal is formulated. A new iterative method is then derived to estimate the hopping frequencies and hop transition time. Without using any pilot signal or sync bit, the new algorithm is able to implement synchronization and frequency estimation at the same time. Unlike the time-frequency distribution (TFD) and the wavelet-based algorithms in papers by Barbarossa and Scaglione (1997) and by Khalil and Hippenstiel (1996), the new ML algorithm does not require the selection of a kernel or mother wavelet function. In addition, compared with the TFD-based algorithm, it has a better performance with a lower implementation complexity.

Entropy-Based Time Window for Artifact Removal in UWB Imaging of Breast Cancer Detection

In this letter, we propose an entropy-based time window function for artifact removal in ultra-wideband (UWB) imaging of breast cancer detection. The artifact is due to the incident and skin-breast backscattered signal in such an imaging system. Observing that artifacts at different antennas are very similar, we define an entropy function in the antenna domain to measure the similarity of different antenna signals and design a rectangular window function to eliminate the artifacts. This window function is applied to the computed finite-difference time-domain (FDTD) data of the breast model with a tumor embedded. Compared with the Wiener filtering algorithm, this entropy-based algorithm is computationally simple. In addition, it requires no prior knowledge about the breast and the tumor and brings no distortion to the tumor reflection

Quasi-Orthogonal Space-Time Block Codes for Two Transmit Antennas and Three Time Slots

In this paper, a class of quasi-orthogonal space-time block codes (Q-STBC) is proposed for systems with two transmit antennas and three time slots, where the Alamouti code is not applicable due to the odd time slots. The proposed Q-STBC codes achieve rate one and full diversity with low complexity maximum likelihood decoding. The Q-STBC design also shows excellent properties in other practical aspects, such as the compatibility with the single antenna transmission mode, low power fluctuation, and low receiver decoding and transmitter encoding complexity.

Joint downlink beamforming, power control, and data rate allocation for DS-CDMA mobile radio with multimedia services

Power control and rate allocation are two fundamental issues for improving the spectrum efficiency of downlink transmission for wireless multimedia communications. Downlink beamforming, on the other hand, can be used to suppress stronger interference induced by high rate users, thereby improving the system performance. We present a joint downlink beam-forming, power control and rate allocation technique suitable for DS-CDMA systems with multimedia services. To simplify the computational complexity a practical rate allocation algorithm as also proposed. Computer simulation results are given to evaluate downlink capacity of DS-CDMA systems using base station antenna and the new algorithm proposed.