Chunjie Duan

A Non-Coherent 802.15.4a UWB Impulse Radio

The new IEEE 802.15.4a standard for Wireless Personal Area Networks (WPAN) extends the capabilities of its popular predecessor IEEE 802.15.4, and enables accurate range/distance estimation between communicating devices. In addition to its ranging capabilities, the new standard also defines several new data rates ranging from 100 kb/s to 27 Mb/s and a variety of options that give IEEE 802.15.4a compliant devices a high degree of flexibility and scalability. In this article we describe our implementation of an UWB impulse radio device, which is compliant with the 802.15.4a standard. Our implementation uses non-coherent energy-detection based receivers both to demodulate data and to detect ranges. We will present our prototype system and discuss some implementation details (energy detection, sampling rates, acquisition/synchronization) specific to non-coherent receivers. The performance of our prototype system is discussed as well. The reported range estimation error is less than 30cm, which agrees well with theoretical predictions.

Capacity, MSE and Secrecy Analysis of Linear Block Precoding for Distributed Antenna Systems in Multi-User Frequency-Selective Fading Channels

Block transmission with cyclic prefix is a promising technique to realize high-speed data rates in frequency-selective fading channels. Many popular linear precoding schemes, including orthogonal frequency-division multiplexing (OFDM), single-carrier (SC) block transmission, and time-reversal (TR), can be interpreted as such a block transmission. This paper presents a unified performance analysis that shows how the optimal precoding strategy depends on the optimization criterion such as capacity, mean-square error, and secrecy. We analyze three variants of TR methods (based on maximum-ratio combining, equal-gain combining and selective combining) and two-types of pre-equalization methods (zero-forcing and minimum mean-square error). As one application of our framework, we derive optimal precoding (i.e., OFDM with optimal power and phase control) in the presence of interference limitation for distributed antenna systems; we find that without power/phase control, OFDM does not have any capacity advantage over SC transmissions. When comparing SC and TR, we verify that for single-antenna systems in the high SNR regimes, SC has a capacity advantage; however, TR performs better in the low SNR regime. For distributed multiple-antenna systems, TR always provides higher capacity, and the capacity of TR can approach that of optimal precoders with a large number of distributed antennas. Furthermore, we make an analysis of secrecy capacity which shows how high-rate messages can be transmitted towards an intended user without being decoded by the other users from the viewpoint of information-theoretic security. We demonstrate that TR precoding can be the best candidate among the non-optimal precoders for achieving high secrecy capacity, while the optimal precoder offers a significant gain over those non-optimal precoders.

A Low-Complexity Synchronization Design for MB-OFDM Ultra-Wideband Systems

In this paper, we investigate the low-complexity synchronization design for multi-band orthogonal-frequency-division-multiplexing (MB-OFDM) ultra-wideband (UWB) systems. We propose a unified synchronizer design based on auto-correlation-function. The key component in the proposed synchronizer is a parallel signal detector structure in which multiple auto-correlation units are instantiated and their outputs are shared by other functional units in the synchronizer, including time-frequency pattern detection, symbol timing, carrier frequency offset estimation and correction and frame synchronization. We show that, via analysis and simulations, such a design achieves not only a low computation cost which makes it attractive in implementation, but also equal or better performance compared to the cross-correlation based designs.

A Synchronization Design for UWB-Based Wireless Multimedia Systems

Multi-band orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) technology offers large throughput, low latency and has been adopted in wireless audio/video (AV) network products. The complexity and power consumption, however, are still major hurdles for the technology to be widely adopted. In this paper, we propose a unified synchronizer design targeted for MB-OFDM transceiver that achieves high performance with low implementation complexity. The key component of the proposed synchronizer is a parallel auto-correlator structure in which multiple ACF units are instantiated and their outputs are shared by functional blocks in the synchronizer, including preamble signal detection, time-frequency code identification, symbol timing, carrier frequency offset estimation and frame synchronization. This common structure not only reduces the hardware cost but also minimizes the number of operations in the functional blocks in the synchronizer as the results of a large portion of computation can be shared among different functional blocks. To mitigate the effect of narrowband interference (NBI) on UWB systems, we also propose a low-complexity ACF-based frequency detector to facilitate the design of (adaptive) notch filter in analog/digital domain. The theoretical analysis and simulation show that the performance of the proposed design is close to optimal, while the complexity is significantly reduced compared to existing work.

High-order statistical equalizer for nonlinearity compensation in dispersion-managed coherent optical communications

Fiber nonlinearity has become a major limiting factor to realize ultra-high-speed optical communications. We propose a fractionally-spaced equalizer which exploits a trained high-order statistics to deal with data-pattern dependent nonlinear impairments in fiber-optic communications. The computer simulation reveals that the proposed 3-tap equalizer improves Q-factor by more than 2 dB for long-haul transmissions of 5,230 km distance and 40 Gbps data rate. We also demonstrate that the joint use of a digital backpropagation (DBP) and the proposed equalizer offers an additional 1-2 dB performance improvement due to the channel shortening gain. A performance in high-speed transmissions of 100 Gbps and beyond is evaluated as well.

On probabilistic data association for achieving near-exponential diversity over fading channels

Machine-to-Machine (M2M) wireless communication requires the transmission of short blocks of data with high reliability over fading channels. We discuss the use of the probabilistic data association (PDA) detector in conjunction with precoding to design high-performance systems for these links. First, the performance of the traditional PDA algorithm with precoding over ideal Rayleigh fading links is analyzed, which provides insight into its performance, and evidence of an error floor at high SNRs. Then, a novel ordering mechanism is proposed that takes advantage of the precoder characteristics. It is shown by simulation that the proposed modified algorithm can achieve near-ML performance for block sizes as small as 32 symbols.