Cognitive radio (CR) has been proposed as an efficient method to reuse the licensed spectrum. It recognizes the primary (licensed) users' signals and adapts its own to minimize the interference it generates. When perfect channel state information is known at the transmitters, the capacity of CR system can be achieved by utilizing the dirty paper coding (DPC). In this paper, we consider the performance of CR systems under both fast and slow fading channels with only channel statistics known at the transmitters. Due to the limited channel state information, the original DPC fails and is replaced by the so-called linear-assignment Gel'fand-Pinsker coding. By carefully designing the parameters based on this precoding, we show that significant rate gains over naively treating primary users' signals as interference can be obtained for both fast and slow fading scenarios. A nested-lattice based coding/decoding scheme is also proposed to implement this precoding in practice, and it validates our theoretical claims.
Relay-aided multiuser communications are crucial for future 5G systems. In this paper, we consider the twouser multiple access relay channel (MARC), in which two users transmit messages to a common destination with the assistance of a half-duplex relay. The decode-and-forward (DF) based lattice coding was shown to be effective for the MARC in our previous work [1]. However when the links from the users to the relay are weak, DF protocol may fail to decode all users at the relay. Aiming to solve this problem, we propose a new lattice coding where the relay only needs to decode an integer-weightedsum of users' lattice codewords, re-maps it with a modulobased mapper and then forwards the corresponding codeword. Although the decoding at the relay is akin to the orthogonal compute-and-forward protocol, we relax the restriction imposed by previous works that the users have to be silent when the relay is transmitting to avoid interference. The key ingredient is the joint multi-user lattice decoding performed at the destination. This jointly decoding strategy not only complicates the corresponding code design but also the error analysis. To find the proper integerweighted-sum at the relay for the destination's joint decoder, we also solve a non-convex integer problem by carefully transforming and relaxing it to a convex one. Simulation results show that the proposed non-orthogonal lattice coding can outperform existing schemes in a variety of channel settings.
The dynamic decode-and-forward (DDF) protocol for the multiple access relay channel (MARC) with quasi static fading is evaluated using the Zheng-Tse diversity-multiplexing tradeoff (DMT). We assume that there are two users, one half-duplex relay, and a common destination, each equipped with single antenna. For the Rayleigh fading channel, the DDF protocol is well known and has been analyzed in terms of the DMT with the infinite block length assumption by Azarian et al. However, to make the protocol feasible, the practical constraint of finite block length must be enforced, which may result in a loss in the DMT. Another practical effect not considered in the infinite block length DDF protocol is the possible decoding error at the relay. By carefully dealing with these practical issues due to finite block length, we characterize the finite block length DMT of the DDF protocol. We further consider the situation where the destination does not have a priori knowledge of the relay decision time at which the relay switches from listening to transmitting, and show that the optimal DMT is still achievable as if there is no decoding error at the relay. Therefore, the assumption of error-free decoding at the relay and additional protocol overhead to communicate the decision time are not needed for the DDF to achieve the optimal DMT.
This paper considers the multi-antenna multiple access relay channel (MARC), in which multiple users transmit messages to a common destination with the assistance of a relay. In a variety of MARC settings, the dynamic decode and forward (DDF) protocol is very useful due to its outstanding rate performance. However, the lack of good structured codebooks so far hinders practical applications of DDF for MARC. In this work, two classes of structured MARC codes are proposed: 1) one-to-one relay-mapper aided multiuser lattice coding (O-MLC), and 2) modulo-sum relay-mapper aided multiuser lattice coding (MS-MLC). The former enjoys better rate performance, while the latter provides more flexibility to tradeoff between the complexity of the relay mapper and the rate performance. It is shown that, in order to approach the rate performance achievable by an unstructured codebook with maximum-likelihood decoding, it is crucial to use a new K-stage coset decoder for structured O-MLC, instead of the one-stage decoder proposed in previous works. However, if O-MLC is decoded with the one-stage decoder only, it can still achieve the optimal DDF diversity-multiplexing gain tradeoff in the high signal-to-noise ratio regime. As for MS-MLC, its rate performance can approach that of the O-MLC by increasing the complexity of the modulo-sum relay-mapper. Finally, for practical implementations of both O-MLC and MS-MLC, practical short length lattice codes with linear mappers are designed, which facilitate efficient lattice decoding. Simulation results show that the proposed coding schemes outperform existing schemes in terms of outage probabilities in a variety of channel settings.
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