A. de Baynast

Low density parity check codes for the relay channel

We propose Low Density Parity Check (LDPC) code designs for the half-duplex relay channel. Our designs are based on the information theoretic random coding scheme for decode-and-forward relaying. The source transmission is decoded with the help of side information in the form of additional parity bits from the relay. We derive the exact relationships that the component LDPC code profiles in the relay coding scheme must satisfy. These relationships act as constraints for the density evolution algorithm which is used to search for good relay code profiles. To speed up optimization, we outline a Gaussian approximation of density evolution for the relay channel. The asymptotic noise thresholds of the discovered relay code profiles are a fraction of a decibel away from the achievable lower bound for decode-and-forward relaying. With random component LDPC codes, the overall relay coding scheme performs within 1.2 dB of the theoretical limit.

Performance of dynamic spectrum access based on spectrum occupancy statistics

At present multiband wireless devices are able to select their working frequency only to a limited extent because of the strict, current spectrum regulation. Dynamic spectrum access is a promising approach that might solve this inefficiency. The authors focus on spectrum sensing, one of the main tasks involved. First, three strategies are compared to efficiently sense the current spectrum based on the spectrum occupancy information statistics. In contrast to the simulation-based studies, the authors evaluate the performance of those strategies on real spectrum occupancy data gathered during an extensive measurement campaign. The authors show that the usage of historical information considerably improves the spectrum sensing process and also that the modelling of the periodic behaviour of the licensed signals leads to negligible performance enhancements because only very few periods shorter than several minutes can be found within 20 MHz–6 GHz. Secondly, the authors unveil the fundamental tradeoff between the required bandwidth for the transmission and the total bandwidth that has to be sensed in order to guarantee that the required bandwidth is available. All the results are provided in terms of outage probability that can be viewed as an approximation of the packet loss rate.

Exploiting Historical Spectrum Occupancy Information for Adaptive Spectrum Sensing

At present wireless devices are able to select their working frequency only to a limited extend although several measurements have shown that the current spectrum regulations are inefficient. Dynamic spectrum access is seen as a promising approach that might solve this inefficiency. Spectrum sensing is one of the main tasks involved. We compare in this paper four methods to efficiently sense the current spectrum based on the spectrum occupancy information statistics. The parameters of all methods are extracted from spectrum occupancy data gathered during an extensive measurement campaign. We show that the usage of historical information considerably improves the spectrum sensing process. We also show that the modelling of the periodic behaviour of the licensed signals leads to negligible performance enhancements because only very few periods shorter than several minutes can be found within 20 MHz-6 GHz.

Half-Duplex Estimate-and-Forward Relaying: Bounds and Code Design

We propose a practical coding scheme for half-duplex estimate-and-forward relaying. The proposed construction is guided by the information theoretic coding scheme for the estimate-and-forward relay protocol. Our construction incorporates several design features to reduce receiver complexity without compromising performance. Observing that the relaying gain is significant only at low SNRs, we use binary LDPC codes in the source broadcast phase of relaying. Estimation is performed by entropy constrained scalar quantization of the received signal at the relay. Finally, a procedure similar to maximal ratio combining is used to aggregate direct and relayed signals at the destination. An important practical advantage of our scheme is that it does not require source-relay symbol synchronization. The codes outperform direct and two-hop channel capacities, as well as decode-and-forward relaying when the relay-destination link is strong

Blind PARAFAC receivers for multiple access-multiple antenna systems

We present a new blind receiver for a multiple access channel with multiple transmit antennas per user and multiple receive antennas (MIMO channel). After being multiplied by a spreading sequence, each users data is split into N/sub t/ streams that are simultaneously transmitted using N/sub t/ transmit antennas. The received signal at each receive antenna is a linear superposition of the N/sub t/ transmitted signals of the N/sub u/ users perturbed by noise. We propose a new blind detection/identification algorithm under the assumption that the fading is slow and frequency non-selective. This algorithm relies on a generalization of parallel factor (PARAFAC) analysis (Kruskal, J., Linear Algebra Applications, vol.18, p.95-138, 1977; Sidiropoulos, N. et al., IEEE Trans. Sig. Process., vol.48, no.3, p.810-23, 2000). We show that a generalized canonical decomposition of the 3D data tensor is unique under mild assumptions without noise. Neither algebraic orthogonality nor independence between sources is needed for uniqueness of the decomposition. By performing this decomposition in rank (N/sub t/, N/sub t/, 1) terms, we are able to retrieve the three sets of parameters: the symbols; the channel fading coefficients (including the antenna gains); the spreading sequences. In a noisy context, we propose a simple algorithm of the alternating least squares (ALS) type, which yields a performance close to the linear minimum mean square error (LMMSE) receiver which requires knowledge of the channel and spreading sequences.

Cooperative Strategies and Achievable Rate for Tree Networks With Optimal Spatial Reuse

In this paper, a low-complexity cooperative protocol that significantly increases the average throughput of multihop upstream transmissions for wireless tree networks is developed and analyzed. A system in which transmissions are assigned to nodes in a collision free, spatial time division fashion is considered. The suggested protocol exploits the broadcast nature of wireless networks where the communication channel is shared between multiple adjacent nodes within interference range. For any upstream end-to-end flow in the tree, each intermediate node receives information from both one-hop and two-hop neighbors and transmits only sufficient information such that the next upstream one-hop neighbor will be able to decode the packet. This approach can be viewed as the generalization of the classical three node relay channel for end-to-end flows in which each intermediate node becomes successively source, relay and destination. The achievable rate for any regular tree network is derived and an optimal schedule that realizes this rate in most cases is proposed. Our protocol is shown to dramatically outperform the conventional scheme where intermediate nodes simply forward the packets hop by hop. At high signal-to-noise ratio (SNR), it yields approximately 66% throughput gain for practical scenarios.

Chip-level LMMSE equalization for downlink MIMO CDMA in fast fading environments

In this paper, we consider linear MMSE equalization for wireless downlink transmission with multiple transmit and receive antennas in fast fading environments. We propose a new algorithm based on the conjugate-gradient algorithm with enhanced channel estimation. In order to be robust to the channel variations, the channel coefficients are estimated by using a weighted sliding window. Two methods to determine optimal weights with respect to the Doppler frequency are proposed. The algorithm has been simulated in a fast fading environment (vehicular A with a velocity for the mobile station of 120 km/h). We show by simulations that good performance is obtained in a correlated fast fading environment with reasonable complexity. Moreover, this approach outperforms methods based on basic sliding window or forgetting factor and the LMS algorithm.

Cognitive Radio Testbed: Exploiting Limited Feedback in Tomorrow's Wireless Communication Networks

The next generation of wireless communication devices should support advanced features such as high spectral efficiency, broad bandwidth, diverse quality of service (QoS) requirements, and adaptivity. The cognitive radio (CR) is a new paradigm which has a high potential to become a basis for the future wireless systems. This paper is a first step towards the implementation of such a system. Our CR testbed is based on a GNU Radio platform which enables flexibility and reconfigurability of transmission parameters. As machine learning component, we invoke genetic algorithm (GA) to optimize the transmission parameters such as transmission power, modulation order and frequency channel based on the current spectrum conditions. Unlike other CR implementations, our approach requires very limited feedback information at the transmitter (ap 8 bits/packet duration). No transmission model nor additional network state information (NSI)is needed at the transmitter side. Experimentations show that our CR is capable to find free channels within 4-5 iterations even in a highly occupied spectrum scenario. It also offers the optimal trade-off between throughput, reliability, and power consumption depending on the users QoS requirements.

LDPC code design for OFDM channel: graph connectivity and information bits positioning

We present an optimized channel coding scheme for OFDM transmission. In the future wireless standards such IEEE 802.11n, frame error rates (FER) as low as 0.0001 are targeting. Irregular LDPC codes have excellent performance at moderate computational complexity: they are strong candidates for channel coding in such systems. In the context of an OFDM transmission, Mannoni et al. (2002) proposed to optimize the graph connectivity of the irregular LDPC code accordingly to the channel spectrum. However, their codes did not have good performance for a FER range of 0.001-0.0001. In this paper, we propose to optimize the LDPC code in two steps: first, we optimize the graph connectivity in order to get a minimum operational average SNR and therefore excellent FER; then we optimize the information bits placement accordingly to the channel. By simulation, we show that our approach provides substantial performance gain in term of FER over the existing methods (1 dB or more for FER equal to 10/sup -4/).

Cooperative Strategies and Optimal Scheduling for Tree Networks

In this paper, we develop and analyze a low-complexity cooperative protocol that significantly increases the average throughput of multi-hop upstream transmissions for wireless tree networks. We consider a system in which transmissions are assigned to nodes in a collision free, spatial time division fashion. This protocol exploits the broadcast nature of wireless networks where the communication channel is shared between multiple adjacent nodes within interference range. For any upstream end-to-end flow in the tree, each intermediate node receives information from both one-hop and two-hop neighbors and transmits only sufficient information such that the next upstream one-hop neighbor will be able to decode the packet. This approach can be viewed as the generalization of the classical three node relay channel for end-to-end flows in which each intermediate node becomes successively source, relay and destination. We derive the achievable rate and propose an optimal schedule that realizes this rate for any regular tree network. We show that our protocol dramatically outperforms the conventional scheme where intermediate nodes simply forward the packets hop by hop. At high signal-to-noise ratio, it yields approximatively 80% throughput gain.