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Combinational cooperative detection (CCD) is a method where signal energies from multiple secondary users (SUs) are gathered in different combinations in order to reach a unique decision for each secondary user regarding the activity of the primary user. In earlier implementations of the CCD method, the locations of the SUs have been assumed to be fixed and selected near the border points of the communication area, and the detection performance has been studied for different parameters. In this paper, the performance of the CCD method has been studied under realistic conditions. Initially, the effect of random locations of SUs on the detection performance is considered and compared with the performance for fixed locations. Then, the effect of distance variations between the master and the user is investigated. Finally, the effect of different propagation environments on the detection performance is studied. The results presented are important to assess the performance of the CCD method under realistic conditions.

We investigate reliable data packet delivery employing random linear packet coding for a half-duplex underwater acoustic link. Packet coding is performed on a group of M information-bearing packets so as to form N ≥ M coded packets, where N is chosen such that a pre-specified probability of successful decoding is maintained at the receiver. We regard a group of N coded packets as one super-packet, on which we apply an ARQ technique. Specifically, we group L super-packets and apply a group stop-and-wait acknowledgment procedure to the so-obtained unit (a super-group) to achieve full reliability. We also explore adaptive power and rate control combined with the proposed technique in order to improve the performance on fading channels. Using the channel gain information obtained via feedback from the receiver, the transmitter adjusts its power and coding rate such that the average energy per bit is minimized. In doing so, two constraints are observed, one on the maximum transmit power and another on the size N of the super-packet. Under these constraints, we develop the adaptation policy and provide analytical results for the throughput efficiency. Finally, we compare the performance of the proposed technique to that of conventional stop-and-wait, as well as a full-duplex benchmark, showing that packet coding technique on a half-duplex link can achieve a throughput efficiency that is very close to that of a full-duplex link.

Random linear packet coding combined with adaptive power and rate control is being considered as an alternative to traditional ARQ techniques for the underwater acoustic channel. We first analyze a scheme where the number of coded packets to transmit is determined so as to achieve a pre-determined reliability at the receiver, and hence eliminate the need for explicit feedback. We next investigate joint power and rate control with constrained resources. Using the channel gain information which is obtained via feedback from the receiver, the transmitter adjusts its transmit power and the number of coded packets such that the average energy per bit is minimized. We impose two main constraints for the optimization: (a) the transmit power should be within a maximum available power, and (b) the number of coded packets should not exceed a maximum value. Under these constraints, we define adaptation policies to minimize the average energy per bit and show analytical results for average energy savings available from the adaptation policies. We also show experimental results obtained using the measured channel gains from the SPACE-08 experiment, an at-sea experiment conducted off the coast of Martha's Vineyard in Massachusetts.

Random linear packet coding is considered for channels that experience fading and have long propagation delay, such as the underwater acoustic channels. Previously, we employed power control (adjusting the transmission power according to the channel gain) and rate control (adjusting the number of coded packets according to the channel gain) to counteract the effects of fading. For such policies, it was shown that there exists an optimal number of coded packets (when employing power control) or optimal transmission power (when employing rate control) for which the energy required per bit of information transmitted is minimized. In this paper, we present a strategy to jointly optimize the transmission power and the number of coded packets. We present two optimization criteria, i.e., (a) minimizing the average energy per bit for transmission and (b) maximizing the throughput. Given the statistics of the channel, we compute the average energy per bit and the average throughput under each criterion, to make a choice based on the desired tradeoff. In applications with limited power, minimizing the average energy per bit will be the guiding principle, while in time-critical applications maximizing the average throughput will dictate the choice.

Random linear packet coding is considered for efficient broadcasting in networks with large propagation delays, such as underwater acoustic networks. To additionally overcome the effects of fading, we combine packet coding with adaptive power control, whereby the transmitter adjusts its power upon receiving feedback from the receiver on the current state of the channel (locally-averaged, large-scale gain). We investigate two power adjustment rules: the worst link rule and the average link rule. In the first case, the transmit power is adjusted in accordance with the link that has the lowest channel gain, while in the second case, the power is adjusted in accordance with the average of the gains on all links. System performance is evaluated based on the average energy per bit of successfully transmitted bit of information, using (i) simulated channels, (ii) experimentally recorded gain values from the MISSION 2012 experiment, and (iii) actual network deployment from the MISSION 2013 experiment conducted off the coast of Singapore. Results indicate energy savings on the order of several dB compared to systems that do not use power control.

This work focuses on random linear packet coding as an efficient method for reliable high-speed underwater communications over acoustic links with long delay. The transmitter sends coded packets with a pre-determined redundancy over a simplex link with no feedback. The receiver checks each received packet, discarding packets with errors, until the necessary degrees of freedom for successful decoding have been achieved. Each coded packet is modulated using Orthogonal Frequency Division Multiplexing(OFDM) for transmission over an acoustic channel. Differentially coherent detection is employed at the receiver for symbol detection. We focus on an experimental analysis using an in-air testbed and verifying the results with signals transmitted during an experiment conducted off the coast of Kauai, HI, in July 2011.