9 research outputs found
High-Throughput Random Access via Codes on Graphs
Recently, 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.Comment: Presented at the Future Network and MobileSummit 2010 Conference,
Florence (Italy), June 201
Spatially-Coupled Random Access on Graphs
In this paper we investigate the effect of spatial coupling applied to the
recently-proposed coded slotted ALOHA (CSA) random access protocol. Thanks to
the bridge between the graphical model describing the iterative interference
cancelation process of CSA over the random access frame and the erasure
recovery process of low-density parity-check (LDPC) codes over the binary
erasure channel (BEC), we propose an access protocol which is inspired by the
convolutional LDPC code construction. The proposed protocol exploits the
terminations of its graphical model to achieve the spatial coupling effect,
attaining performance close to the theoretical limits of CSA. As for the
convolutional LDPC code case, large iterative decoding thresholds are obtained
by simply increasing the density of the graph. We show that the threshold
saturation effect takes place by defining a suitable counterpart of the
maximum-a-posteriori decoding threshold of spatially-coupled LDPC code
ensembles. In the asymptotic setting, the proposed scheme allows sustaining a
traffic close to 1 [packets/slot].Comment: To be presented at IEEE ISIT 2012, Bosto
ALOHA Random Access that Operates as a Rateless Code
Various applications of wireless Machine-to-Machine (M2M) communications have
rekindled the research interest in random access protocols, suitable to support
a large number of connected devices. Slotted ALOHA and its derivatives
represent a simple solution for distributed random access in wireless networks.
Recently, a framed version of slotted ALOHA gained renewed interest due to the
incorporation of successive interference cancellation (SIC) in the scheme,
which resulted in substantially higher throughputs. Based on similar principles
and inspired by the rateless coding paradigm, a frameless approach for
distributed random access in slotted ALOHA framework is described in this
paper. The proposed approach shares an operational analogy with rateless
coding, expressed both through the user access strategy and the adaptive length
of the contention period, with the objective to end the contention when the
instantaneous throughput is maximized. The paper presents the related analysis,
providing heuristic criteria for terminating the contention period and showing
that very high throughputs can be achieved, even for a low number for
contending users. The demonstrated results potentially have more direct
practical implications compared to the approaches for coded random access that
lead to high throughputs only asymptotically.Comment: Revised version submitted to IEEE Transactions on Communication
Check-hybrid GLDPC Codes: Systematic Elimination of Trapping Sets and Guaranteed Error Correction Capability
In this paper, we propose a new approach to construct a class of check-hybrid
generalized low-density parity-check (CH-GLDPC) codes which are free of small
trapping sets. The approach is based on converting some selected check nodes
involving a trapping set into super checks corresponding to a 2-error
correcting component code. Specifically, we follow two main purposes to
construct the check-hybrid codes; first, based on the knowledge of the trapping
sets of the global LDPC code, single parity checks are replaced by super checks
to disable the trapping sets. We show that by converting specified single check
nodes, denoted as critical checks, to super checks in a trapping set, the
parallel bit flipping (PBF) decoder corrects the errors on a trapping set and
hence eliminates the trapping set. The second purpose is to minimize the rate
loss caused by replacing the super checks through finding the minimum number of
such critical checks. We also present an algorithm to find critical checks in a
trapping set of column-weight 3 LDPC code and then provide upper bounds on the
minimum number of such critical checks such that the decoder corrects all error
patterns on elementary trapping sets. Moreover, we provide a fixed set for a
class of constructed check-hybrid codes. The guaranteed error correction
capability of the CH-GLDPC codes is also studied. We show that a CH-GLDPC code
in which each variable node is connected to 2 super checks corresponding to a
2-error correcting component code corrects up to 5 errors. The results are also
extended to column-weight 4 LDPC codes. Finally, we investigate the eliminating
of trapping sets of a column-weight 3 LDPC code using the Gallager B decoding
algorithm and generalize the results obtained for the PBF for the Gallager B
decoding algorithm
Doubly-Generalized LDPC Codes: Stability Bound over the BEC
The iterative decoding threshold of low-density parity-check (LDPC) codes
over the binary erasure channel (BEC) fulfills an upper bound depending only on
the variable and check nodes with minimum distance 2. This bound is a
consequence of the stability condition, and is here referred to as stability
bound. In this paper, a stability bound over the BEC is developed for
doubly-generalized LDPC codes, where the variable and the check nodes can be
generic linear block codes, assuming maximum a posteriori erasure correction at
each node. It is proved that in this generalized context as well the bound
depends only on the variable and check component codes with minimum distance 2.
A condition is also developed, namely the derivative matching condition, under
which the bound is achieved with equality.Comment: Submitted to IEEE Trans. on Inform. Theor
Coded Slotted ALOHA: A Graph-Based Method for Uncoordinated Multiple Access
In this paper, a random access scheme is introduced which relies on the
combination of packet erasure correcting codes and successive interference
cancellation (SIC). The scheme is named coded slotted ALOHA. A bipartite graph
representation of the SIC process, resembling iterative decoding of generalized
low-density parity-check codes over the erasure channel, is exploited to
optimize the selection probabilities of the component erasure correcting codes
via density evolution analysis. The capacity (in packets per slot) of the
scheme is then analyzed in the context of the collision channel without
feedback. Moreover, a capacity bound is developed and component code
distributions tightly approaching the bound are derived.Comment: The final version to appear in IEEE Trans. Inf. Theory. 18 pages, 10
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