Watcharapan Suwansantisuk

On the performance of wide-bandwidth signal acquisition in dense multipath channels

This paper investigates important properties of acquisition receivers that employ commonly used serial-search strategies. In particular, we focus on the properties of the mean acquisition time (MAT) for wide bandwidth signals in dense multipath channels. We show that a lower bound of the MAT over all possible search strategies is the solution to an integer programming problem with a convex objective function. We also give an upper bound expression for the MAT over all possible search strategies. We demonstrate that the MAT of the fixed-step serial search (FSSS) does not depend on the timing delay of the first resolvable path, thereby simplifying the evaluation of the MAT of the FSSS. The results in this paper can be applied to design and analysis of fast acquisition systems in various wideband scenarios.

Multipath Aided Rapid Acquisition: Optimal Search Strategies

In this paper, we propose a search technique that takes advantage of multipath, which has long been considered deleterious for efficient communication, to aid the sequence acquisition in dense multipath channels. We consider a class of serial-search strategies and use optimization and convexity theories to determine fundamental limits of achievable mean acquisition times (MATs). In particular, we derive closed-form expressions for both the minimum and maximum MATs and the conditions for achieving these limits. We prove that a fixed-step serial search, a form of nonconsecutive serial search, achieves a near-optimal MAT. We also prove that the conventional serial search, in which consecutive cells are tested serially, should be avoided as it results in the maximum MAT. Our results are valid for all signal-to-noise ratio (SNR) values, regardless of the specifics of the detection layer and the fading distributions

Outage behavior of selective relaying schemes

Cooperative diversity techniques can improve the transmission rate and reliability of wireless networks. For systems employing such diversity techniques in slow-fading channels, outage probability and outage capacity are important performance measures. Existing studies have derived approximate expressions for these performance measures in different scenarios. In this paper, we derive the exact expressions for outage probabilities and outage capacities of three proactive cooperative diversity schemes that select a best relay from a set of relays to forward the information. The derived expressions are valid for arbitrary network topology and operating signal-to-noise ratio, and serve as a useful tool for network design.

Frame Synchronization for Variable-Length Packets

A cognitive radio can sense its environment and adapt some of its features, such as carrier frequency, transmission bandwidth, transmission power, and modulation, thus allowing dynamic reuse of the available spectrum. Due to their high degree of adaptability to environmental variations, cognitive radios are expected to utilize packet-based transmission with variable-length frames. Packet-based transmission requires the receiver to perform frame synchronization, an important enabling step that allows adaptation in cognitive radios. However, proper metrics to characterize the performance of frame synchronization for transmission of variable-length frames are currently unavailable. To address this issue, we put forth two performance metrics, namely the expected duration to complete frame synchronization and the probability of correct acquisition within a given duration. We then develop analytical expressions for these important metrics. This paper advances our understanding of frame synchronization for the continuous transmission of variable-length frames and for bursty transmission.

Outage Behavior of Cooperative Diversity with Relay Selection

Cooperative diversity is a useful technique to increase reliability and throughput of wireless networks. To analyze the performance gain from cooperative diversity, outage capacity is an important figure of merit that captures the inherent diversity-multiplexing trade-off in cooperative diversity schemes. In this paper we derive the outage capacity for several cooperative diversity schemes in decode-and-forward relay networks with a finite number of relay nodes. The obtained expressions are simple and applicable to arbitrary network topologies and signal-to-noise ratios in Rayleigh fading channels. The analysis shows that there exists a signal-to-noise ratio threshold, below which some cooperative diversity schemes are better than direct communication. We propose a new diversity scheme, which, compared to the conventional counterpart, offers improved performance and requires protocol overhead.

Multipath Aided Rapid Acquisition Techniques for Spread Spectrum Signals

Spread-spectrum systems with large transmission bandwidth present significant challenges from the standpoint of achieving acquisition before the communication commences. This paper investigates a rapid acquisition procedure that exploits multipath to aid the synchronization. In particular, we consider a randomized search strategy and derive the corresponding mean acquisition time (MAT). We show that the MAT of the randomized search is at most twice the MAT of the optimal serial search. Using our results, we compare various search techniques in a multipath fading channel

Ultrawide bandwidth serial-search multi-dwell acquisition-Part I: Asymptotic theory

The use of ultrawide bandwidth (UWB) signals requires highly accurate timing, and, hence, presents significant challenges from the standpoint of acquisition receiver design. Improving the acquisition performance is important, especially for time-critical applications that cannot tolerate long processing delay. This paper deals with the design and analysis of UWB rapid acquisition receivers. In particular, we consider serial-search strategies that employ a multi-dwell detector with optimal thresholds. Specific contributions include derivation of lower and upper bounds on the asymptotic mean acquisition times for both the conventional and the optimal serial search strategies. The bounds serve as ultimate performance limits of acquisition receivers that operate in high and low signal-to-noise ratio regimes. We propose a search strategy that is near-optimal in these asymptotic regimes. The results in this paper are valid for environments with arbitrary power dispersion profiles, provided that some mild assumptions about the channel and the receiver design are satisfied.

Ultrawide bandwidth serial-search multi-dwell acquisition-Part II: System considerations

This paper quantifies the performance of ultrawide bandwidth (UWB) serial-search multi-dwell receivers operating in Nakagami-m fading channels. In particular, the receiver employs a bank of correlators with optimal detection thresholds. The mean acquisition time is given as a function of the signal-to-noise ratio (SNR) for different search strategies and numbers of correlators. The numerical results show that the improvement in acquisition performance from using intelligent serial search strategies is consistent with the analytical bounds derived in the companion paper for arbitrary fading channels.

On the Asymptotic Performance of Multi-Dwell Signal Acquisition in Dense Multipath Channels

This paper investigates the mean acquisition time (MAT) of an acquisition system that employs an optimal multi-dwell detector at the detection layer. We derive lower bound and upper bound expressions for the cases when the search layer employs the optimal search procedure and the conventional serial search. The bounds depend only on a few parameters at the detection layer and can be used for the design and analysis of a broad class of multi-dwell detectors. The results indicate that in a high signal-to-noise ratio (SNR) regime, an intelligent search procedure improves the MAT of the conventional serial search by approximately Nhit times, where Nhit is proportional to the number of propagation paths. On the other hand, for a low SNR regime, only moderate improvement can be obtained. The results in this paper are valid for any fading statistic

First passage time problems with applications to synchronization

First passage time problems arise in many fields, but few of them are explicitly solved. Motivated by an application to synchronization of clocks, this paper obtains the probability distribution for the first passage time for a Brownian motion with quadratic drift to exit from two constant barriers that are not necessarily symmetric. The expression for the distribution is explicit, consisting of elementary functions and functions that are characterized by Laplace transforms. Applications of first passage times are demonstrated in the context of synchronization.