Yougan Chen

OFDM-Modulated Dynamic Coded Cooperation in Underwater Acoustic Channels

Dynamic coded cooperation (DCC) allows relay diversity without altering the transmission procedure from the source to the destination. In this paper, we propose a practical orthogonal frequency-division multiplexing (OFDM)-modulated DCC scheme for underwater relay networks, where OFDM modulation accommodates multipath fading channels with large delay spread. Two cooperation strategies are studied, where the relay transmits either identical or different OFDM blocks as the source during the cooperation phase. The block-level synchronization between the OFDM blocks from the source and the relay is achieved by a delay control mechanism at the relay, by knowing the distances among the source, relay, and destination. Two OFDM-DCC design examples are presented, one based on nonbinary rate-compatible low-density parity-check (LDPC) codes applied across multiple OFDM blocks, and the other using layered interblock erasure correction and intrablock error-correction coding. In addition to simulation studies, one particular design has been implemented on practical OFDM modems, and tested in a swimming pool and in a recent sea experiment. The proposed OFDM-DCC scheme is particularly appealing to underwater acoustic (UWA) networks where a relay node with abundant resources (e.g., a surface buoy) can enhance communications among underwater nodes without changing their transmission procedure.

HFM spread spectrum modulation scheme in shallow water acoustic channels

Owing to the large delay spread caused by multipath propagation and the severe Doppler Effect of the shallow water acoustic channel, a spread spectrum modulation scheme using hyperbolic frequency modulated (HFM) signal named as “HFM spread-spectrum modulation” (HFM-SS) was exploited to substantially improve the performance of communications over such channels. Apart from some advantages such as reducing influence of narrow-band noise, HFM signals provide Doppler-invariability and low power cost. The effects of multipath and Doppler shift on the proposed scheme were investigated by establishing a model of a shallow water acoustic communication system. The emulated results demonstrate that it has a good prospect for underwater acoustic communication system, especially for moving platforms. High transmission stability of this approach was validated in lake experiments carried out in the summer 2011.

Joint carrier frequency offset and impulse noise estimation for underwater acoustic OFDM with null subcarriers

Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) has been actively studied for underwater acoustic communications. In this paper, we investigate the OFDM performance in the presence of impulse-like noises. We present one method that estimates the carrier frequency offset (CFO) and the impulse noise separately, based on existing approaches, and one method that estimates them jointly. Simulation results in both time-invariant and time-varying channels have verified the receiver performance.

Dynamic Node Cooperation in an Underwater Data Collection Network

In this paper, we consider a practical underwater data collection network, where one destination needs to collect data from multiple underwater nodes. With the conventional automatic-repeat-request (ARQ) protocol, the destination requests retransmission from each node individually without any node cooperation. We propose two protocols, selective relay cooperation and dynamic network coded cooperation, utilizing the fact that underwater nodes can overhear the transmission of the others. In the selective relay cooperation, one node can be selected as a relay to transmit the data from another undecoded node in the retransmission phase. In the network coded cooperation, the selected relay nodes transmit network coded packets to the destination. The relay nodes participating the cooperation are selected by the destination based on the channel quality, as measured by the effective signal-to-noise ratio. In addition to simulation results, we have carried out several lake tests based on a full protocol implementation. Simulation and field testing results demonstrate that the proposed schemes can gain significant performance improvement compared with the conventional ARQ scheme.

Dynamic network coded cooperative OFDM for underwater data collection

In this paper, we consider an underwater data collection problem, where multiple underwater nodes need to send data to one common destination. With the conventional ARQ protocol, the destination requests retransmission from each node individually without any node cooperation. We propose two protocols, selective relay cooperation and dynamic network coded cooperation, utilizing the fact that underwater nodes can overhear the transmission of others. In the selective relay cooperation, one node can be selected as a relay to transmit the data from another undecoded node in the retransmission round. In the network coded cooperation, the selected relay nodes transmit network coded packets, combining the packets from several undecoded nodes to the destination. The relay nodes participating the cooperation are selected by the destination based on the measured pilot signal to noise ratio of the underlying OFDM modulation. Simulation results based on one specific underwater topology show that the proposed schemes can achieve significant gains compared with the conventional ARQ scheme.

Performance Analysis of LDPC Codes over Shallow Water Acoustic Channels

The major obstacle to shallow water acoustic communication is the interference of multipath signal results from surface and bottom reflections. Channel coding is indispensable in practical system to accommodate these adverse channel transmission characteristics due to the coding gain. In this paper, a channel model including time-varying fading, multipath and additive noise for the shallow water acoustic channels is built. Based on the channel model, the Probability Density Function (PDF) of initial decoding messages with Belief Propagation (BP) algorithm of Low-Density Parity-Check (LDPC) codes is deduced. The performance of the LDPC codes with BP algorithm is simulated, and the effects of multipath, fading velocity and channel interleaver on the decoding performance are studied. As the result, the following conclusions can be given: The performance of LDPC codes is excellent (Bit Error Rate (BER) is less than 10-4) over three-multipath shallow water acoustic channels, with 3-5 iterative decoding times and about 1000 bits code lengths; The BER of LDPC codes degrades with the increasing of the number of multipath; The BER of LDPC codes degrades slowly with the decreasing of the fading velocity, which shows the proposed channel code is not sensitive to the fading velocity of the channel.

Energy-efficient mobile data collection adopting node cooperation in an underwater acoustic sensor network

This paper considers an underwater acoustic sensor network with one mobile surface node to collect data from multiple underwater nodes, where the mobile destination requests retransmission from each underwater node individually employing traditional automatic-repeat-request (ARQ) protocol. We propose a practical node cooperation (NC) protocol to enhance the collection efficiency, utilizing the fact that underwater nodes can overhear the transmission of others. To reduce the source level of underwater nodes, the underwater data collection area is divided into several sub-zones, and in each sub-zone, the mobile surface node adopting the NC protocol could switch adaptively between selective relay cooperation (SRC) and dynamic network coded cooperation (DNC). The difference of SRC and DNC lies in whether or not the selected relay node combines the local data and the data overheard from undecoded node(s) to form network coded packets in the retransmission phase. The NC protocol could also be applied across the sub-zones due to the wiretap property. In addition, we investigate the effects of different mobile collection paths, collection area division and cooperative zone design for energy saving. The numerical results show that the proposed NC protocol can effectively save energy compared with the traditional ARQ scheme.

Dynamic coded cooperative ARQ for multi-hop underwater acoustic networks

Dynamic coded cooperation (DCC) does not need extra transmission time scheduled for the relay, which is appealing to the bandwidth-limited high-delay underwater acoustic (UWA) environment. In this paper, we propose dynamic coded cooperative automatic repeat request (DCC-ARQ) protocol for multi-hop UWA networks. A transmission packet with multiple blocks is taken as a one-shot unit, where an erasure-correction code is used for inter-block encoding. Adopting the DCC scheme in each hop UWA transmission, the half-duplex cooperative node switches to cooperation phase immediately after it decodes the cooperative message, which provides a more reliable cooperative path for the specific three-node network. Further, if the relay (or destination in the last hop) node sends a negative acknowledgement (NACK) to the upstream cooperative node, the cooperative node only needs to retransmit parts of the packet under DCC-ARQ mechanism, hence a reduced end-to-end transmission latency can be achieved. Simulation results show that for a one-shot transmission, the proposed protocol achieves good balance between the reduced end-to-end delay and decent outage performance, relative to existing protocols.

The analysis of hops for multi-hop cooperation in Underwater Acoustic Sensor Networks

Underwater Acoustic Sensor Networks (UASNs) have recently attracted significant attention for exploration and monitoring in underwater environments. The sensor nodes used in UASNs are powered by batteries that are difficult to recharge or replace. It is imperative that the number of hops deployment in multi-hop networks should be very energy efficient whilst at the same time satisfying necessary delay constraints. In this paper, we firstly analyze the relationship of energy consumption, end-to-end delay and number of hops in a multi-hop UASN, and then we investigate how to optimize the number of hops with a fixed distance in term of trade-offs between energy consumption and end-to-end delay. Then we define a cost function to research the minimum number of hops. Simulation results show that, taking into consideration the energy consumption and end-to-end delay, we can obtain an appropriate number of hops for the multi-hop UASN by adopting co-operation over a given distance. The model obtained in this paper is used to determine the number of hops in an energy efficient UASN for an underwater time-critical mission.

Mobile data collection paths for node cooperative underwater acoustic sensor networks

Given a middle-scale underwater acoustic network, to collect data from the multiple underwater nodes, the surface destination node need to be working on mobile data collection mechanism. In this paper, compared to traditional automatic-repeat-request (ARQ) protocol, we proposed node cooperation (NC) to increase the data collection efficiency for the surface node, utilizing the fact that underwater nodes can overhear the transmission of the others. However, different data collection paths can lead to different data collection efficiency for the middle-scale network when employing NC protocol. For one specific node cooperative underwater acoustic network, the underwater nodes are distributed at random within a circle planar region. As the restriction of underwater transmission power, the planar region is divided into several sub-zones according to different mobile collection paths. And then we present several different data collection path examples and show their data collection efficiency. The numerical results show that the optimal mobile collection path should consider both the cooperative zone and the distribution of the underwater nodes.