1,721,239 research outputs found
Quasi-cyclic Generalized LDPC codes with low error floors
No full textIn this paper, a novel methodology for designing structured generalized LDPC (G-LDPC) codes is presented. The proposed design results in quasi-cyclic G-LDPC codes for which efficient encoding is feasible through shift-register-based circuits. The structure imposed on the bipartite graphs, together with the choice of simple component codes, leads to a class of codes suitable for fast iterative decoding. A pragmatic approach to the construction of G-LDPC codes is proposed. The approach is based on the substitution of check nodes in the protograph of a low-density parity-check code with stronger nodes based, for instance, on Hamming codes. Such a design approach, which we call LDPC code doping, leads to low-rate quasi-cyclic G-LDPC codes with excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel
Outage and spectrum efficiency analysis in microcellular systems
No full textOutage probability due to interference and spectrum efficiency for a microcellular mobile radio system, are considered. A new general expression to evaluate outage probability in the presence of a Rician channel for the useful signal and an Rayleigh channel for the interferers, is presented. Traffic distribution and power control techniques are taken into account by means of a Monte Carlo simulation methods. The protection ratio for coherent and non-coherent demodulation of continuous phase-modulated signals and its impact on overall performance are estimated and discussed
A new approach to time-of-arrival estimation based on information theoretic criteria
No full textExisting energy detector (ED)-based methods for
time-of-arrival (ToA) estimation in ultrawide band (UWB) im
pulse radios (IRs) use a threshold to discriminate the time bins
containing noise from those containing signal plus noise. For
these threshold crossing (TC) methods the key problem is the
design of the threshold level to obtain the best possible ranging
accuracy. T hreshold design for TC methods is a difficult problem,
and requires receiver noise power knowledge. We here propose a
completely different approach, where there is no threshold and
where the noise power knowledge is not required. Our blind and
robust ToA estimation technique is based on information theoretic
criteria (ITC). T he proposed ToA algorithm is universal, in that
it does not require any information about the channel or the
noise power level, and it eliminates the need to set a predefined
threshold. T he performance of the proposed universal ToA
estimator is evaluated in realistic multi path channels, revealing
excellent performance and robustness
A Decoding Algorithm for LDPC Codes Over Erasure Channels with Sporadic Errors
No full textAn efficient decoding algorithm for low-density parity-check (LDPC) codes on erasure channels with sporadic errors (i.e., binary error-and-erasure channels with error probability much smaller than the erasure probability) is proposed and its performance analyzed. A general single-error multiple-erasure (SEME) decoding algorithm is first described, which may be in principle used with any binary linear block code. The algorithm is optimum whenever the non-erased part of the received word is affected by at most one error, and is capable of performing error detection of multiple errors. An upper bound on the average block error probability under SEME decoding is derived for the linear random code ensemble. The bound is tight and easy to implement. The algorithm is then adapted to LDPC codes, resulting in a simple modification to a previously proposed efficient maximum likelihood LDPC erasure decoder which exploits the parity-check matrix sparseness. Numerical results reveal that LDPC codes under efficient SEME decoding can closely approach the average performance of random codes
High-Throughput Random Access via Codes on Graphs
No full textRecently, contention resolution diversity slotted ALOHA (CRDSA) has
been introduced as a simple but effective improvement to slotted ALOHA. It relies on MAC burst repetitions and on interference cancellation to increase the normalized throughput of a classic slotted ALOHA access scheme. CRDSA allows achieving a larger throughput than slotted ALOHA, at the price of an increased average transmitted power. A way to trade-off the increment of the average transmitted power and the improvement of the throughput is presented in this paper. Specifically, it is proposed to divide each MAC burst in k sub-bursts, and to encode them via a (n, k) erasure correcting code. The n encoded sub-bursts are transmitted over the MAC channel, according to specific time/frequency-hopping patterns. Whenever n − e ≥ k sub-bursts (of the same burst) are received without collisions, erasure decoding allows recovering the remaining e sub-bursts (which were lost due to collisions). An interference cancellation process can then take place, removing in e slots the interference caused by the e recovered sub-bursts, possibly allowing the correct decoding of sub-bursts related to other bursts. The process is thus iterated as for the CRDSA case
Impact of cochannel interference on vehicle-to-vehicle communications at millimeter waves
No full textIn order to assess a vehicle-to-vehicle millimeter wave mobile radio network the evaluation of co-channel interference effects is of great importance. In fact the definition of a coverage strategy requires suitable frequency reuse thus producing the above-mentioned interference effects. As regards vehicle-to-vehicle communication, a scenario oriented to highways is considered; in this situation, a first step in estimating the transmission performance has to be done when the link between two communicating vehicles is affected by a single co-channel interferer. This analysis is carried out by considering anomalous propagation conditions both for the useful and the interference signals: to describe the system performance the authors not only consider their power levels, but evaluate the interfered system outage probability. Moreover, in order to counteract the effects of anomalous propagation some kind of diversity techniques are investigated. Finally, rain effects will be evaluated at different carrier frequencie
Can we trust in HDLC?
No full textIn spite of the theoretical error detecting capability of its CRC, the actual
degree of data integrity provided by HDLC is quite low .
The paper gives a quantitative assessment of HDLC's deficiencies and of their reasons, paying particular attention to actual applications of the HDLC family of standards.
Finally, we look at some strategies that have been suggested to allow the use of HDLC in the most demanding applications
Adaptive Modulation Systems Subject to Interference
No full textAdaptive modulation techniques enable wireless systems to achieve high spectral efficiency (SE) while maintaining a target quality of service and are rising a great interest for high data rate communications in fading channels. In this paper, we analyze slow adaptive M-ary quadrature amplitude modulation systems with diversity subject to interference. We derive the performance in terms of mean SE and bit error outage for systems experiencing co-channel interference with optimum combining. The analysis leads to a new definition of in-service regions, to be used for the optimum constellation size selection in place of signal-to-noise ratio thresholds
Short Low-Rate Non-Binary Turbo Codes
No full textA serial concatenation of an outer non-binary turbo
code with different inner binary codes is introduced and analyzed.
The turbo code is based on memory-1 time-variant
recursive convolutional codes over high order fields. The resulting
codes possess low rates and capacity-approaching performance,
thus representing an appealing solution for spread spectrum
communications. The performance of the scheme is investigated
on the additive white Gaussian noise channel with coherent and
noncoherent detection via density evolution analysis. The proposed
codes compare favorably w.r.t. other low rate constructions
in terms of complexity/performance trade-off. Low error floors
and performances close to the sphere packing bound are achieved
down to small block sizes (k = 192 information bits)
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