Glauber Brante

Spatial Diversity Using Analog Joint Source Channel Coding in Wireless Channels

We consider the use of spatial diversity to improve the performance of analog joint source-channel coding in wireless fading channels. The communication system analyzed in this paper consists of discrete-time all-analog-processing joint source-channel coding where Maximum Likelihood (ML) and Minimum Mean Square Error (MMSE) detection are employed. By assuming a fast-fading Rayleigh channel, we show that MMSE performs much better than ML at high Channel Signal-to-Noise Ratios (CSNR) in single-antenna wireless systems. However, such performance gap can be significantly reduced by using multiple receive antennas, thus making low complexity ML decoding very attractive in the case of receive diversity. Moreover, we show that the analog scheme can be considerably robust to imperfect channel estimation. In addition, as an alternative to multiple antennas, we also consider spatial diversity through cooperative communications, and show that the application of the Amplify-and-Forward (AF) protocol with single antenna nodes leads to similar results than when two antennas are available at the receiver and Maximal Ratio Combining (MRC) is applied. Finally, we show that the MMSE implementation of the analog scheme performs very close to the unconstrained capacity of digital schemes using scalar quantization, while its complexity is much lower than that of capacity-approaching digital systems.

Outage Probability and Energy Efficiency of Cooperative MIMO with Antenna Selection

This paper compares the energy efficiency of some cooperative MIMO schemes in wireless networks. By energy efficiency we denote the spectral efficiency seen at the receiver normalized by the total energy consumption, which includes the circuitry, the efficiency of the power amplifier, and the transmission rate. We focus on transmit antenna selection (TAS) and switch and stay combining (SSC) at the receiver. The performance of TAS+SSC is compared to that of TAS and maximal ratio combining (MRC), and to that of transmit/receive beamforming using a singular value decomposition (SVD) technique. We derive closed-form outage probability expressions, and analyze the effect of selecting the antenna from the source to optimize the communication with the relay or with the destination. Our results show that selecting the antennas with respect to the destination is in general a better option when the energy consumption is accounted. Moreover, some power allocation strategies are described and the comparison of the schemes in terms of energy efficiency reveals a considerable improvement with the use of TAS+SSC for low to moderate spectral efficiency, while beamforming outperforms the other schemes for high spectral efficiency when the number of antennas at each node is small.

Short Channel Hopping Sequence Approach to Rendezvous for Cognitive Networks

The rendezvous process allows cognitive users to find an available channel and establish a communication link. The implementation of rendezvous strategies to minimize the time to rendezvous (TTR) is a major challenge in this context. In this paper we present a comprehensive analysis of the benefits of periodic channel hopping sequences in the rendezvous process. We also introduce a novel short-sequence-based (SSB) rendezvous algorithm which works under symmetric and asymmetric channel models. Theoretical analysis and simulations show that the proposed SSB outperforms other recently developed rendezvous protocols in terms of expected TTR and maximum TTR.

Analog joint source-channel coding in Rayleigh fading channels

We consider discrete-time all-analog-processing joint source-channel coding, using non-linear spiral-like curves. We assume a Rayleigh channel, where the receiver may employ or not multiple antennas. Maximum Likelihood (ML) and Minimum Mean Square Error (MMSE) detection are considered. Our results show that MMSE performs much better than ML in high CSNR in single-antenna wireless systems, while diversity combining is able to significantly reduce such performance gap, therefore making the low complexity ML decoding very attractive in the case of multiple receive antennas.

On the Performance of Secure Full-Duplex Relaying under Composite Fading Channels

We assume a full-duplex (FD) cooperative network subject to hostile attacks and undergoing composite fading channels. We focus on two scenarios: a) the transmitter has full CSI, for which we derive closed-form expressions for the average secrecy rate; and b) the transmitter only knows the CSI of the legitimate nodes, for which we obtain closed-form expressions for the secrecy outage probability. We show that secure FD relaying is feasible, even under strong self-interference and in the presence of sophisticated multiple antenna eavesdropper.

Energy Efficiency of Transmit Diversity Systems Under a Realistic Power Consumption Model

We compare the downlink energy efficiency of spatial diversity multiple transmit antenna schemes. We determine the minimum required transmit power for a given outage probability. Our analysis shows that antenna selection is in general the most energy efficient option as it requires a single radio-frequency chain. We also investigate the limiting distances up to which the antenna selection technique outperforms the transmit beamforming scheme for different numbers of transmit antennas.

Comparing the energy efficiency of single-hop, multi-hop and incremental decode-and-forward in multi-relay wireless sensor networks

In this paper we analyze the energy efficiency of single-hop, multi-hop and incremental decode-and-forward in wireless sensor networks composed of multiple nodes. The energy efficiency analysis is constrained by a target outage probability and an end-to-end throughput, while the Nakagami-m distribution is used to model the small scale fading. Our results show that in most scenarios the cooperative incremental decode-and-forward scheme is more energy efficient than the other methods.

On the performance of full-duplex relaying under phy security constraints

In this paper we investigate the performance of a cooperative network in the presence of an eavesdropper (Eve). Alice and Bob communicate with the help of a relay, which can operate either in half-duplex (HD) or full-duplex (FD) mode. We account for the self interference at the relay when operating under FD mode. Our analysis focus in the case that the CSI of Eve is not available at Alice. Thus, we derive closed-form expressions for secrecy outage probability. Our results allow us to compare the performance of FD and HD cooperative scenarios under secrecy constraints and, despite the additional interference at the relay, show the advantages of FD relaying over HD. In addition, we also show that a cooperative network is more vulnerable if Eve is closer to Alice than to the relay.

Energy Efficient Relay Placement in Dual Hop 802.15.4 Networks

IEEE 802.15.4 has emerged as a popular standard for short range wireless sensor networks used in industrial, military, health, and environmental sectors. The limited lifetime of such networks is one of the critical design challenges. This paper examines the how relaying through intermediate sensor nodes can enhance the lifetime of an 802.15.4 network. In particular, novel energy consumption models for both AF and DF relays have been developed. Different relay gain scaling mechanisms and forwarding strategies under each of the relay categories were also considered and their energy efficiencies were compared. For every relaying protocol, it was found that there exists an optimum location where energy saving is maximum and this location is not necessarily different for different modes. In summary, it has been observed that the optimum location for AF relays is the equidistant point from source and destination. In contrast, the optimal location for DF relays is closer to source. The effect of different PHY level (outage probability, path loss) and MAC level parameters (frame length) on the energy efficiency are also studied.

Optimizing the Code Rate of Energy-Constrained Wireless Communications With HARQ

Retransmissions due to decoding errors have a big impact on the energy budget of low-power wireless communication devices, which can be reduced by using hybrid automatic repeat request (HARQ) techniques. Nevertheless, this reduction comes at the cost of extra energy consumption introduced by the added computational load. No complete analysis of the tradeoff between retransmissions reduction and baseband consumption of low-power communications over fading channels has been reported so far. In this paper, we study the energy efficiency achievable by HARQ schemes when the code rate of the error-correcting code is optimized. For this purpose, we develop an energy consumption model that focuses on simple HARQ (S-HARQ) and Chase combining (HARQ-CC) transmissions, which are studied under fast-fading and block-fading scenarios with Nakagami-m fading. The retransmission statistics are analyzed, and expressions for the expected number of transmission trials are derived. Using this framework, it is shown that transmission schemes with high diversity gain are the most efficient choice for long range transmissions, which in our case correspond to HARQ-CC and codes with low code rate. On the other hand, schemes with good multiplexing capabilities are optimal for short link distances, which in our analysis correspond to S-HARQ and high code rates. It is also shown that HARQ-CC can effectively extend the transmission range of a low-power communication device.