Spreading modulation for multi-level non-volatile memories

The aggressive scaling of NAND flash memories has caused significant degradation in their reliability and endurance. One of the dominant factors in this degradation is the intercell-interference, by which the programming of a cell can affect nearby neighboring cells corrupting the information that they store. This paper proposes a new data representation scheme, which increases endurance and significantly reduces the probability of error caused by inter-cell-interference. The method is based on using an orthogonal code to spread each bit across multiple cells, resulting in lower variance for the voltages being programmed in the cells. This new data representation method is also shown to present many of the advantages that spreading sequences bring to wireless communications. For example, multiple information sequences can be written on the same cells at different times without interfering with each other. It also allows storing additional information on an already programmed memory in such a way that the new information is hidden by the noise

Optimizing HARQ and relay strategies in limited feedback communication systems

One of the key challenges for future communication systems is to deal with fast changing channels due to the mobility of users. Having a robust protocol capable of handling transmission failures in unfavorable channel conditions is crucial, but the feedback capacity may be greatly limited due to strict latency requirements. This paper studies the hybrid automatic repeat request (HARQ) techniques involved in re-transmissions when decoding failures occur at the receiver and proposes a scheme that relies on codeword bundling and adaptive incremental redundancy (IR) to maximize the overall throughput in a limited feedback system. In addition to the traditional codeword extension IR bits, this paper introduces a new type of IR, bundle parity bits, obtained from an erasure code across all the codewords in a bundle. The type and number of IR bits to be sent as a response to a decoding failure is optimized through a Markov Decision Process. In addition to the single link analysis, the paper studies how the same techniques generalize to relay and multi-user broadcast systems. Simulation results show that the proposed schemes can provide a significant increase in throughput over traditional HARQ techniques

Increasing Throughput in Wireless Communications by Grouping Similar Bits

This letter proposes and studies a technique for grouping the bits transmitted through a wireless channel into codewords according to their quality (SNR). It proves that splitting the bits into multiple codewords of different rates provides a higher throughput than mixing heterogeneous quality bits into fixed-rate codewords. The letter first analyzes the pros and cons of different mappings of bits and codewords to the available time, frequency, and modulation resources. Then it describes the proposed scheme for a 16-QAM modulation and illustrates its benefits through simulations. Finally, it provides a mathematical proof of its superiority in a binary-input parallel AWGN channel with finite length error correcting codes. The proposed scheme can be applied to any communications channel using error correcting codes (ECC), but it is of particular interest for millimeter-wave (mmWave) wireless communications, where the channel quality is closely monitored and high order modulations are used over wide bandwidths. The simulations suggest that modest gains in throughput can be obtained with negligible additional complexity

Coded Caching for Distributed Storage

Content delivery networks store information distributed across multiple servers, so as to balance the load and avoid unrecoverable losses in case of node or disk failures. Coded caching has been shown to be a useful technique which can reduce peak traffic rates by pre-fetching popular content at the end users and encoding transmissions so that different users can extract different information from the same packet. On one hand, distributed storage limits the capability of combining content from different servers into a single message, causing performance losses in coded caching schemes. But, on the other hand, the inherent redundancy existing in distributed storage systems can be used to improve the performance of those schemes through parallelism. This paper designs coded caching and delivery schemes tailored towards systems where the library is distributed across multiple servers, possibly with some redundancy in the form of maximum distance separable (MDS) erasure codes. Different schemes are proposed based on the capacity of the users' caches, as well as the number of parity servers. The main focus is on scenarios with one (RAID-4) or two (RAID-6) parity servers, but the paper also includes simple extensions for cases with more than two or no parity servers at all. The proposed schemes are shown to reduce the worst case latency, or equivalently the peak transmission rate from any server, below that of state-of-the-art algorithms