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.
The rise of machine-to-machine communications has rekindled the interest in random access protocols as a support for a massive number of uncoordinatedly transmitting devices. The legacy ALOHA approach is developed under a collision model, where slots containing collided packets are considered as waste. However, if the common receiver (e.g., base station) is capable to store the collision slots and use them in a transmission recovery process based on successive interference cancellation, the design space for access protocols is radically expanded. We present the paradigm of coded random access, in which the structure of the access protocol can be mapped to a structure of an erasure-correcting code defined on graph. This opens the possibility to use coding theory and tools for designing efficient random access protocols, offering markedly better performance than ALOHA. Several instances of coded random access protocols are described, as well as a case study on how to upgrade a legacy ALOHA system using the ideas of coded random access.
In this paper, coded slotted ALOHA (CSA) is introduced as a powerful random
access scheme to the MAC frame. In CSA, the burst a generic user wishes to
transmit in the MAC frame is first split into segments, and these segments are
then encoded through a local a packet-oriented code prior to transmission. On
the receiver side, iterative interference cancellation combined with decoding
of the local code is performed to recover from collisions. The new scheme
generalizes the previously proposed irregular repetition slotted ALOHA (IRSA)
technique, based on a simple repetition of the users' bursts. An interpretation
of the CSA interference cancellation process as an iterative erasure decoding
process over a sparse bipartite graph is identified, and the corresponding
density evolution equations derived. Based on these equations, asymptotically
optimal CSA schemes are designed for several rates and their performance for a
finite number of users investigated through simulation and compared to IRSA
competitors. Throughputs as high as 0.8 are demonstrated. The new scheme turns
out to be a good candidate in contexts where power efficiency is required.Comment: 6 pages, 2 figures. To be presented at IEEE ICC 201
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