We investigate how the maximum-distance separable (MDS) coding can be incorporated into probabilistic caching to utilize the limited storage space efficiently. In a user-centric clustered wireless network, each caching helper node probabilistically caches a segment of MDS coded sequence of each file. The segment size is optimized to maximize the cache hit probability or successful file retrieval probability. We reveal that the best way of storing files is determined by the condition whether the average amount of MDS coded information stored for the requested file within a user's cluster exceeds the amount required for file retrieval or not. In terms of the cache hit probability maximization, if the condition is not fulfilled, it is proved that storing the complete file with a low probability is optimal. Otherwise, storing either a segment as small as possible with a high probability or a complete file with a low probability, according to a given environment, is shown to be desirable. We also analyze the successful retrieval probability, which accounts for both a cache hit event and successful transmissions from multiple caching helper nodes. Since the successful retrieval probability is in an intractable form, to find the desirable segment size, the theoretically driven algorithms with low search complexity are developed.

In this paper, we consider a cellular network, where a macro BS serves a single user with the aid of helper nodes, and find the optimal way to save a file in the helper nodes. Storing a file into distributed storage affects both hitting probability and successful recovery of the data at a user. With MDS coding, cache equipped helper nodes can store small fragments to improve the hitting probability. When a file is relatively important, we show that the optimal way to save a file is to save either the whole file or the smallest MDS fraction constrained on the amount of fragments. For less important file, we show that the conventional replica storing is optimal in terms of hitting probability.

In wireless cloud storage systems, the recovery failure probability depends on not only wireless channel conditions but also storage size of each distributed storage node. For an efficient utilization of limited storage capacity and the performance characterization of allocation strategies, we asymptotically analyze the recovery failure probability of a wireless cloud storage system with a sum storage capacity constraint for both high signal-to-noise ratio (SNR) regime and low SNR regime. Then, we find the optimal storage allocation strategy across distributed storage nodes in terms of the asymptotic recovery failure probability. Our analysis reveals that the maximal symmetric allocation is optimal for high SNR regime and the minimal allocation (with ⌊T⌋ complete storage nodes and an incomplete storage node) is optimal for low SNR regime, where T is the sum storage capacity. Based on the numerical investigation, we also show that in intermediate SNR regime, a balance allocation between the minimal allocation and the maximal symmetric allocation would not be required if we select one between them according to SNR.

This paper studies diversity-multiplexing tradeoff (DMT) of decode-and-forward (DF)-based half-duplex relay networks without a direct path from source to destination. We propose a new multiple relay coded cooperation (MRCC) protocol to overcome the half-duplex loss in DF relay networks and show that the proposed MRCC protocol achieves the MISO/SIMO DMT upper bound as the number of assisting relays increases. The loss of multiplexing gain from the half-duplex constraint is recovered by coded cooperation and successive cancellation. Despite its notable gain in terms of DMT, the proposed MRCC protocol requires only limited feedback information/signaling for relay and operation mode selection. We also derive the achievable DMT of the proposed MRCC protocol when each node has multiple antennas. It is shown that the proposed MRCC protocol achieves K times diversity gain of the point-to-point N × N MIMO case. Compared with existing protocols, the MRCC protocol shows much higher diversity gain for almost all multiplexing gains. Furthermore, our numerical results verify that the proposed MRCC protocol outperforms existing protocols in terms of outage probability for the environments of practical interest.

In distributed wireless storage systems, failed recovery probability depends on not only wireless channel conditions but also storage size of each distributed storage node. For efficient utilization of limited storage capacity, we asymptotically analyze the failed recovery probability of a distributed wireless storage system with a sum storage capacity constraint when signal-to-noise ratio goes to infinity, and find the optimal storage allocation strategy across distributed storage nodes in terms of the asymptotic failed recovery probability. It is also shown that when the number of storage nodes is sufficiently large the storage size required at each node is not so large for high exponential order of the failed recovery probability.

In this paper, we devise and comprehensively analyze an efficient transmission protocol for multiple half-duplex relay communication without a direct link but with inter-relay interference. The proposed protocol achieves high diversity gain with the help of Hybrid Automatic Retransmission reQuest (H-ARQ) even when multiplexing gain is quite high. As the maximum allowable number of retransmission rounds (L) increases, an additional time diversity gain is obtained in short-term static channels. Compared to pre-existing protocols which adopt H-ARQ in a similar manner, its diversity gain at high multiplexing gain is much improved. As L increases, the dominating region become wider. In addition, in long-term static channels, we show that more than two retransmissions (i.e., L > 2) are unnecessary for improvement of the diversity gain.

We investigate the cooperative diversity gain in underlay cognitive radio (CR) systems where secondary users access the spectrum licensed to primary users under an interference power constraint at primary receivers. Our analysis shows that the cooperative diversity gain has different characteristics from that in non-CR systems since the transmit power of a secondary transmitter is limited by the interference power constraint. If a fixed interference power constraint is imposed, the cooperative diversity gain is lost regardless of the number of relays even if peak transmit power grows to infinity. We analyze the critical value of peak transmit power beyond which increasing peak transmit power does not help in improving the outage probability. However, if a proportional interference power constraint is imposed, a full diversity gain is shown to be attained even without instantaneous interference channel information at the secondary transmitters.