Ezio Biglieri

Low-Density Parity-Check Codes for Nonergodic Block-Fading Channels

We design powerful low-density parity-check (LDPC) codes with iterative decoding for the block-fading channel. We first study the case of maximum-likelihood decoding, and show that the design criterion is rather straightforward. Since optimal constructions for maximum-likelihood decoding do not perform well under iterative decoding, we introduce a new family of full-diversity LDPC codes that exhibit near-outage-limit performance under iterative decoding for all block-lengths. This family competes favorably with multiplexed parallel turbo codes for nonergodic channels.

Neighbor Discovery in Wireless Networks: A Multiuser-Detection Approach

We examine the problem of determining which nodes are neighbors of a given one in a wireless network. We consider an unsupervised network operating on a frequency-flat Gaussian channel, where K + 1 nodes associate their identities to nonorthogonal signatures, transmitted at random times, synchronously, and independently. A number of neighbor-discovery algorithms, based on different optimization criteria, are introduced and analyzed. Numerical results show how reduced-complexity algorithms can achieve a satisfactory performance.

Design and Analysis of Low-Density Parity-Check Codes for Block-Fading Channels

We solve the problem of designing powerful low-density parity-check (LDPC) codes with iterative decoding for the block-fading channel. We present a new family of full-diversity LDPC codes that exhibit near outage limit performance. This family competes with multiplexed parallel turbo codes suitable for non-ergodic channels and recently reported in the literature.

MIMO Link Adaptation in Mobile WiMAX Systems

Mobile WiMAX systems are based on the IEEE 802.16e specifications, which include two mandatory MIMO profiles for the downlink. One of these is Alamoutis space-time code (STC) for transmit diversity, and the other is a 2times2 spatial multiplexing MIMO scheme. In this paper, we compare the two schemes assuming that the latter employs maximum-likelihood detection. The analysis shows that at the same spectral efficiency, Alamoutis STC combined with maximum-ratio combining (MRC) at the receiver significantly outperforms the 2times2 spatial multiplexing scheme at high values of the signal-to-noise ratio (SNR). Next, selection of a MIMO option is included in link adaptation to maximize network capacity, and operating SNR regions are determined for different modulation, coding and MIMO combinations.

A Simple Algorithm for Neighbor Discovery in Wireless Networks

We consider the neighbor-discovery problem in a fixed wireless network where each node is identified by its signature, and the signatures are chosen so as to limit collisions. Limited-complexity constraints lead to a simple algorithm based on incoherent detection of nodes. The performance of this algorithm is evaluated by computing the probability of a false alarm and of a missed detection of a single node.

An overview of Cognitive Radio for satellite communications

The present scarcity of frequency spectrum allocated to radio communications has motivated the search for technologies able to alleviate it by adapting the transmission to changing environmental and network-usage conditions. Cognitive Radio is one of the most promising among these technologies. Three paradigms for its application have emerged. With “interweaving,” the secondary (unlicensed) users are able to occupy the portions of the spectrum left temporarily free by the primary (licensed) users. In the “underlay” paradigm, the secondary transmitter overlaps in frequency with the primary user, after making sure that the interference level it causes is below a given threshold. The “overlay” paradigm uses the secondary users knowledge of the primary users transmission scheme and of the channel to choose a transmission scheme that causes an acceptable amount of interference. In this paper we briefly review the basic concepts underlying Cognitive Radio, and examine their applications to satellite communications.

Effect of uncertainties in modeling interferences in coherent and energy detectors for spectrum sensing

We examine an extreme case of modeling uncertainties in spectrum sensing, whereby the interference which, in addition to additive noise, affects a sensor has an unknown probability distribution, and only its maximum power is known. We deal with a situation in which the sensor disregards the presence of the interference, and we study its performance. Application of moment-bound theory allows the derivation of sharp (i.e., achievable) upper and lower bounds to the receiver operating characteristics of the sensor. Coherent and energy detectors are analyzed. In particular, it is proved that, even in the large signal-to-noise ratio regime, reliable sensing becomes impossible if the maximum interference power exceeds a certain threshold.

A Poltyrev outage limit for lattices

Nonergodic fading is a useful model for various wireless communication channels in both indoor and outdoor environments. With this model, a codeword is divided into multiple blocks such that fading is constant within a block and independent across blocks. Building on Poltyrevs work on infinite lattice constellations for the Gaussian channel, we derive a Poltyrev outage limit for lattice constellations transmitted over a block-faded channel. We prove that the diversity order of this Poltyrev outage limit is equal to the number of degrees of freedom in the channel. An important application is in decoding low-density lattice codes. With block fading, the presence of an outage may dramatically increase the runtime of both sphere and iterative decoders. Using our newly defined Poltyrev outage limit, decoding is not performed whenever an outage is declared, which drastically reduces the overall decoding time.

Linear-quadratic detectors for spectrum sensing

Spectrum sensing for cognitive-radio applications may use a matched-filter detector (in the presence of full knowledge of the signal that may be transmitted by the primary user) or an energy detector (when that knowledge is missing). An intermediate situation occurs when the primary signal is imperfectly known, in which case we advocate the use of a linear-quadratic detector. We show how this detector can be designed by maximizing its deflection, and, using moment-bound theory, we examine its robustness to the variations of the actual probability distribution of the inaccurately known primary signal.

A systolic array for linear MIMO detection based on an all-swap lattice reduction algorithm

A systolic array to implement lattice-reduction-aided linear detection is proposed for a MIMO receiver. The lattice reduction algorithm and the ensuing linear detections are operated in the same array, which can be hardware-efficient. All-swap lattice reduction algorithm (ASLR) is considered for the systolic design. ASLR is a variant of the LLL algorithm, which processes all lattice basis vectors within one iteration. Lattice-reduction-aided linear detection based on ASLR and LLL algorithms have very similar bit-error-rate performance, while ASLR is more time efficient in the systolic array, especially for systems with a large number of antennas.