Khiam-Boon Png

Kronecker Modelling for Correlated Shadowing in UWB MIMO Channels

The shadowing in ultra wideband (UWB) channels may degrade the system performance significantly. Although UWB multipath channels exhibit high resolution in space and time. The large scale shadow fading were moderately correlated based on UWB indoor channel measurement. In this paper, a new modelling method for correlated shadowing with log-normal distribution in UWB multiple-input multiple-output (MIMO) channels is proposed based on the Kronecker model and IEEE 802.15.3a channel model recommendation. Moreover, antenna selection technique with low hardware complexity is proposed for a UWB system with multiple receive or transmit antennas to mitigate its channel shadowing effect. The BER performance for orthogonal frequency division multiplexing (OFDM) UWB system with power based antenna selection is evaluated. Antenna selection technique is shown to be effective in mitigating the correlated shadow fading.

Joint carrier and sampling frequency offset estimation for MB-OFDM UWB system

In this paper, a novel joint carrier and sampling frequency offset estimation scheme for multi-band orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) system is introduced. We investigate the relationship between carrier and sampling frequency offset when they have a common error source and derive the joint estimation scheme. The proposed scheme consists of two stages. In the first stage, a robust estimation of the frequency offset is carried out using preamble symbols. In the second stage, pilot subcarriers within OFDM symbols is used to track the phase distortion caused by the presence of estimation error, if any, in the first stage. Extensive simulation is carried out and the results show that when coupled with the appropriate compensation algorithms, the proposed scheme is effective in limiting the MB-OFDM system performance degradation due to frequency offset to within 0.5 dB.

Block Spread IFDMA: An Improved Uplink Transmission Scheme

In this paper, a block-spread interleaved frequency division multiple access (BS-IFDMA) scheme is proposed. It is capable of allocating users in block-level code-spreading process for multiple access on top of the IFDMA multiple access scheme for better user allocation. Orthogonality among users in block spreading is achieved when the block spreading, carried out in the time domain, is over a few IFDMA block symbols with negligible channel variation (i.e. transmission over quasi time-invariant channel). Given the same number of supported users and same system bandwidth, by allocating more users in the block level code spreading domain, more frequency diversity can be achieved for the proposed scheme since more subcarriers can be allocated to individual user in the frequency domain compared to the conventional IFDMA scheme.

Two-Dimensional Iterative Sampling Frequency Offset Estimation for MB-OFDM System

In this paper, we introduce a novel sampling offset estimation scheme based on iterative two-dimensional estimation principle for multi-band orthogonal frequency division multiplexing (MB-OFDM) system. The proposed scheme estimates the sampling frequency offset through the phase distortion of pilot sub-carriers in each OFDM symbol and improves the accuracy by averaging the estimate from each pilot sub-carrier across both the sub-carrier dimension and the symbol dimension. Through analysis and simulation, we show that the proposed scheme is superior to existing schemes in minimizing the performance degradation due to sampling frequency offset in MB-OFDM system effectively, especially in long packet transmission

A low-complexity adaptive channel estimation scheme for MB-OFDM system

In this paper, we proposed an adaptive channel estimation scheme with low implementation complexity suitable for small portable MB-OFDM UWB devices. Through the introduction of a simple adaptive filter in the frequency domain, the mean squared error (MSE) of the channel estimates and system performance improves substantially. Moreover, the proposed scheme can adapt to the different operating channel environments considered in indoor wireless personal area networks (WPANs) and provide a robust performance.

Two-Layer Spreading CDMA: An Improved Method for Broadband Uplink Transmission

This paper proposes and investigates a novel code-division multiple-access (CDMA) system. This two-layer spreading CDMA (TLS-CDMA) system can combat multiple-access interference (MAI) and multipath interference (MPI) simultaneously and effectively in a multiuser scenario over frequency-selective fading channels. Moreover, a two-layer cell-specific scrambling code is proposed for the TLS-CDMA system in the uplink transmission to efficiently suppress other-cell interference (OCI) in a multicell environment. The proposed TLS-CDMA system allows the two-layer spreading factors to be adapted to the cell structure, the channel conditions, and the number of active users to support variable data rate transmission among multiple users. The superior performance of the TLS-CDMA system over other uplink transmission systems, such as cyclic prefix CDMA (CP-CDMA) and multicarrier CDMA (MC-CDMA), is also illustrated using performance analysis and simulation results.

Iterative Sampling Frequency Offset Estimation for MB-OFDM UWB Systems With Long Transmission Packet

A multiband orthogonal frequency-division multiplexing (MB-OFDM) system, which is one of the effective modulation techniques that have been adopted for high-speed ultrawideband (UWB) systems, is very sensitive to sampling frequency offset (SFO) due to the mismatch between local oscillators at the transmitter and the receiver. In this paper, we propose an iterative SFO estimation method for a high-data-rate MB-OFDM UWB system to improve its SFO estimation accuracy in the case of a long transmission packet. The proposed method is an iterative process of 2-D SFO estimation across pilot subcarriers and consecutive OFDM symbols, together with joint channel estimation. Furthermore, we derive the Cramer-Rao lower bound (CRLB) for the proposed SFO estimation method. This CRLB can be used as a guide for algorithm design and to explore the theoretical limit. Our performance analysis and simulation results show that the proposed iterative SFO estimation method is both practical and effective.

Design and performance of cyclic delay diversity in UWB-OFDM systems

This paper addresses cyclic delay diversity (CDD) in an ultra-wideband communication system based on orthogonal frequency division multiplexing (OFDM) technique. Symbol error rate and outage probability have been derived. It is shown that with only two transmit antennas, CDD effectively improves SER performance and reduces outage probability significantly especially when the channel delay spread is short. Both simulation and analytical results agree well in all considered cases. The selection of delay times for CDD is also addressed for some special cases.

Mobility-Based Interference Cancellation Scheme for BS-IFDMA Systems With Optimum Code Assignment

In this paper, we propose a mobility-based multiple-access interference (MAI) cancellation scheme for block spread-interleaved frequency-division multiple-access (BS-IFDMA) systems. We optimize the assignment of spreading codes to users to maximize the performance of the cancellation scheme. The optimization is formulated as an NP-hard quadratic assignment problem, which is solved using the branch-and-bound approach. We propose an efficient search algorithm that significantly reduces the number of branches to be searched and hence speeds up the computation by a factor equal to the number of users in the system. Through analysis and simulations, we demonstrate that the proposed cancellation scheme is able to mitigate the MAI in BS-IFDMA systems due to the presence of high-mobility users. We show that without the proposed scheme, a single high-mobility user in the system will cause significant system performance degradation in terms of high bit error floor for most users in the system. In comparison, the proposed cancellation scheme with proper code assignment allows half of the users in BS-IFDMA systems to have high mobility without any significant performance degradation.

Soft QRD-M decoding for coded IFDMA systems

In this paper, a low complexity a posterior probability (APP) detector based on breadth-first tree search algorithm is purposed for coded interleaved frequency division multiple access (IFDMA) system. The proposed detector combines the principles of the M-algorithm with QR decomposition (QRD-M) to generate soft decision statistics in place of the full APP detector which has a high computational cost. We first demonstrate that an IFDMA system can be viewed as a MIMO system at the receiver and then derive the proposed detector. Simulated results show that the bit error rate (BER) performance of a system using the proposed QRD-M detector performs better than a system using frequency domain equalizer, especially for channels with greater diversity. Moreover, the simulated BER performance exhibits similar diversity order as the maximum channel diversity order.