C. Ibars

Cooperative distributed MIMO channels in wireless sensor networks

The large number of network nodes and the energy constraints make Wireless Sensor Networks (WSN) one of the most important application fields for Cooperative Diversity. Node cooperation increases the spatial diversity of wireless channels and, thus, reduces the transmitted power. In this paper, we propose a multi-hop WSN with nodes grouped in cooperative clusters that exploits transmit and receive cooperation among cluster nodes. Multi-hop transmission is carried out by concatenating single cluster-to-cluster hops, where every cluster-to-cluster link is defined as a cooperative distributed multiple-input-multiple-output (MIMO) channel. Transmit diversity is exploited through a time-division, decoder-and-forward, relaying scheme based upon two time slots: the Intracluster Slot, used for data sharing within the cluster, and the Intercluster Slot, used for transmission between clusters. At the receiver side, a distributed reception protocol is devised based upon a Selection Diversity algorithm. The proposed multi-hop cooperative WSN is optimally designed for minimum end-to-end outage probability by deriving the optimum time and power allocated on the intracluster and intercluster slots of every single hop, given a per-link energy constraint. A simplified suboptimum resource allocation is also proposed, which performs close to the optimal policy. Results show that the proposed scheme achieves diversity equal to the equivalent MIMO system and significantly reduces energy consumption with respect to. the non-cooperative channel

Virtual MIMO Channels in Cooperative Multi-hop Wireless Sensor Networks

In this paper we analyze the performance of virtual multiple-input-multiple-output (MIMO) channels for multi-hop transmission in wireless sensor networks. First, we propose a clustered network topology with sensor nodes grouped in collaborative sets attending to proximity. Then, we propose that all nodes belonging to any given cluster cooperatively transmit and receive data from other clusters, exploiting the diversity advantages of cluster-to-cluster virtual MIMO channels. In order to construct the cooperative transmission, we arrange each hop into two consecutive time slots: the intracluster slot, that accounts for data sharing within the cluster, and the intercluster slot for transmission between clusters; and we devise a cooperative reception protocol within the clusters based upon a simplified selection diversity algorithm. Optimum time assignment and power allocation for both slots are derived taking the cluster-to-cluster probability of outage as the metric. Results for one hop networks and for multi-hop networks are obtained, showing substantial diversity gains and energy savings. Furthermore, results show that the performance of the proposed virtual MIMO channels is equal to that of real MIMO channels but for a small SNR loss.

Comparing the performance of coded multiuser OFDM and coded MC-CDMA over fading channels

The bit error rate of a coded Multiuser OFDM (M-OFDM) system and coded multicarrier CDMA (MC-CDMA) system with multiuser detection is analyzed. The paper focuses on downlink wireless applications, where the channel suffers from frequency-selective fading. We investigate both systems when long time interleaving is possible, though introducing large delays, and when time interleaving is limited. Bit error rate lower bounds and simulation results show that, when the system load is high, M-OFDM performs similar to MC-CDMA while its implementation is less complex as it does not require multiuser detection.

MIMO applicability to satellite networks

This paper evaluates the applicability of MIMO techniques to satellite networks in order to achieve diversity and multiplexing gain through dual polarized antennas. In single satellite scenarios the proposed STTC and OSTBC techniques proposed offer better BER than plain stream multiplexing along each polarizations and SISO transmissions. By adding a satellite with dual polarization antennas and performing a joint distributed OSTBC, the spectral efficiency increases as the satellites transmit in the same frequency band. Finally the hybrid satellite-terrestrial network has been considered for MIMO transmission. In this case, the spectral efficiency can be multiplied by 4 if a joint encoding of satellite and terrestrial signals is performed. In each scenario the broadcast signal follows the DVB-SH standard.

Achievable rates for the AWGN channel with multiple parallel relays

Relaying is a key technology to increase capacity in wireless networks. In this paper, the point-to-point AWGN channel with N parallel relays and time-invariant, frequency-flat fading is studied. For it, we derive achievable rates with four coding techniques, namely: decode-and-forward, partial decode-and-forward, compress-and-forward and linear relaying. The first two are based on signal regeneration at the relay nodes and aim at mimicking a transmit antenna array. We study their scaling law for N rarr infin and Rayleigh fading, and show that is lower than log2 log (N) due to the source-relays broadcast limitation. In turn, compress-and-forward aims at mimicking a receive antenna array and consists of relay nodes distributedly compressing their signals and transmitting them to destination. We provide its achievable rate considering distributed Wyner-Ziv compression at the relays, and show that it also scales as log2 log (N); in this case, due to the relays-destination MAC limitation. Finally, linear relaying is the extension of amplify-and-forward to full-duplex operation. For it we derive the optimum transmitted signal at the source and propose suboptimum linear relaying functions at the relays. All techniques are compared with the max-flow-min-cut upper bound, evaluated for AWGN channels.

Partial Decoding for Synchronous and Asynchronous Gaussian Multiple Relay Channels

Relaying diversity is a key technology to increase capacity in wireless networks. In this paper, the capacity of a single source-destination wireless channel, aided by a set of N potential relays, is studied. For such a link (analyzed under full- duplex and half-duplex constraint), we derive a capacity upper bound based upon the max-flow-min-cut theorem stated by Cover in [1]. Furthermore, a lower bound is proposed considering the achievable with partial decoding at the relay nodes. To maximize the later, optimum relay selection is carried out. Both bounds are then applied to Gaussian channels with channel knowledge at both transmitter and receiver sides, and two synchronization modes are addressed: the synchronous mode, where relays perform coherent beamforming, and the asynchronous mode. Results show that, surprisingly, partial decoding is more spectrally efficient for the half-duplex scheme, becoming the capacity achieving technique for low number of relays.

Outage Capacity of Two-Phase Space-Time Coded Cooperative Multicasting

Multi-path fading poses severe limitations to the reliable transmission rate of multicast in wireless networks. To improve robustness and throughput, cooperation among the nodes of the network can be used to crate spatial diversity and thus overcome fading impairments. This paper proposes a two-phase, space-time coded, cooperative multicast protocol, and analyzes its capacity performance in finite size networks. We consider two different channel state information (CSI) scenarios: (1) no transmit CSI at the base station and receive CSI at the multicast nodes; (2) channel knowledge at the base station of base-station-to-users channels but not of intra-user channels, and receive CSI at the multicast nodes. Results show that cooperation is worthwhile for sufficiently large networks and for the low SNR regime.

Inter-Carrier Interference Cancellation for OFDM Systems with Macrodiversity and Multiple Frequency Offsets

Macrodiversity provides improved coverage in wireless communications. It isused in single frequency networks (SFN), and in some cellular systems, whena mobile is in a soft handoff. The combination of OFDM and macrodiversity isvery attractive for the ability of OFDM to cope with delay spread resultingfrom macrodiversity. However, such system is very sensitive to poor frequencysynchronization among transmitters, which results in multiple frequencyoffsets. These cause inter-carrier interference (ICI), which severely degradesperformance. In this paper we quantify the degradation, providing expressionsfor the receiver SINR, and find the optimum receiver frequencysynchronization. Furthermore, we propose to mitigate ICI degradation usinglinear and decision feedback interference cancellation. In each case, weprovide a solution using both the decorrelating and the MMSE criteria.Simulation results show that, for macrodiversity OFDM, our approach resultsin significantly improved performance and robustness to frequency offsets.

Achievable Rate for Gaussian Multiple Relay Channels with Linear Relaying Functions

The achievable rate of a Gaussian multiple relay channel with linear relaying functions is derived here. With linear relaying, relays transmit (on every channel use) a causal linear combination of their past received inputs. In this paper, the optimum temporal covariance of the signal transmitted by the source is derived and linear relaying functions proposed. Results show that linear relaying outperforms all known relaying techniques for relay channels and generates the tightest lower bound on the capacity of the multiple relay channel.

Bounds on ergodic capacity of multirelay cooperative links with channel state information

Cooperation among users of wireless networks has been lately presented as a suitable technique to obtain reliable communications and capacity gains over fading environments. This paper studies the impact that mutual relaying has over the ergodic capacity of wireless networks when full channel state information (CSI) is available at both receiver and transmitter side, and at the relay nodes. Upper and lower bounds are derived for the ergodic capacity of multiple relay, half-duplex, decode-and-forward Gaussian channels with CSI, assuming Rayleigh flat fading and random relay positions, and a short-term power constraint. Both bounds show spectral efficiency gains growing with the logarithm of the total number of network users, and better performance of cooperative links in the low SNR regime. Likewise, it is shown that the proposed bounds approach the ergodic capacity as SNR diminishes