Bit-interleaved time-frequency coded modulation for OFDM systems over time-varying channels
Orthogonal frequency-division multiplexing (OFDM) is a promising technology in broadband wireless communications, with its ability to transform a frequency-selective fading channel into multiple flat-fading channels. However, the time-varying characteristics of wireless channels induce the loss of orthogonality among OFDM subcarriers, which was generally considered harmful to system performance. In this paper, we propose a bit-interleaved time-frequency coded modulation (BITFCM) scheme for OFDM to achieve both the time and frequency diversity inherent in broadband time-varying channels. We will show that the time-varying characteristics of the channel are beneficial to system performance. Using the BITFCM scheme, and for relatively low maximum normalized Doppler frequency, a reduced-complexity maximum-likelihood decoding approach is proposed to achieve good performance with low complexity. For high maximum normalized Doppler frequency, the intercarrier interference (ICI) can be large, and an error floor will be induced. To solve this problem, we propose two ICI-mitigation schemes by taking advantage of the second-order channel statistics and the complete channel information, respectively. It will be shown that both schemes can reduce the ICI significantly.

title: An interference-cancellation scheme for carrier frequency offsets correction in OFDMA systems
abstract: Recently, orthogonal frequency-division multiplexing (OFDM), with clusters of subcarriers allocated to different subscribers (often referred to as OFDMA), has gained much attention for its ability in enabling multiple-access wireless multimedia communications. In such systems, carrier frequency offsets (CFOs) can destroy the orthogonality among subcarriers. As a result, multiuser interference (MUI) along with significant performance degradation can be induced. In this paper, we present a scheme to compensate for the CFOs at the base station of an OFDMA system. In the proposed scheme, circular convolutions are employed to generate the interference after the discrete Fourier transform processing, which is then removed from the original received signal to increase the signal-to-interference power ratio (SIR). Both SIR analysis and simulation results will show that the proposed scheme can significantly improve system performance.

title: Source routing based pairwise key establishment protocol for sensor networks
abstract: Sensor networks are composed of a large number of low power sensor devices. For secure communication among sensors, secret keys must be established between them. The establishment of secret keys after deployment of sensors requires wireless communication. Because of the energy constraints, an efficient key establishment scheme cannot be designed without considering the power consumption factor in wireless communication. In order to reduce the communication overhead, we propose a source routing based pairwise key establishment protocol for large-scale sensor networks. We then use a probability model proposed in [D. Huang et al., (2004)] to analyze the communication overhead, and security strength of the scheme.

title: A state-transition-based incremental protocol specification

title: A key-chain-based keying scheme for many-to-many secure group communication
abstract: We propose a novel secure group keying scheme using hash chain for many-to-many secure group communication. This scheme requires a key predistribution center to generate multiple hash chains and allocates exactly one hash value from each chain to a group member. A group member can use its allocated hash values (secrets) to generate group and subgroup keys. Key distribution can be offline or online via the key distribution protocol. Once keys are distributed, this scheme enables a group member to communicate with any possible subgroups without the help of the key distribution center, and without having to leave the overall group, thus avoiding any setup delay. Our scheme is suitable for applications where the population of a system is stable, group size is moderate, subgroup formation is frequent, and the application is delay sensitive. Through analysis, we present effectiveness of our approach.