Lattice Codes for Multiple-access Relay Channel: Decode-and-Forward and Compute-and-Forward
Date Issued
2014
Date
2014
Author(s)
Lee, Chung-Pi
Abstract
For the uplink relay-aided transmission in a cellular system, the spectral efficiency of mobile radio networks can be improved by introducing relays to assist the transmissions of mobile stations. Further performance improvement can be expected if each relay aids not just a single mobile station, but many simultaneously. In this dissertation, we study the multiple-access relay channel (MARC), in which multiple users transmit messages to a common destination with the assistance of a relay. There are two protocols to be considered: 1) dynamic decode-and-forward (DDF) protocol and 2) compute-and-forward (CMF)protocol. In a variety of MARC settings, 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 the MARC. For the DDF protocol, two classes of structured codes for theMARC are proposed: 1) one-to-one relay-mapper aided multiuser lattice coding (OMLC), 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 MSMLC, 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.
When the channel links from the users to the relay are weak, DF-based protocol may fail to decode all users at the relay. Aiming to solve this problem, we propose a new lattice coding based on the CMF protocol, where the relay only needs to decode an integerweighted-sum of users’ lattice codewords, re-maps it with a modulo-basedmapper and then
forwards the corresponding codeword. Although the decoding at the relay is akin to the orthogonal CMF 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 multiuser lattice decoding performed at the destination. This jointly decoding strategy not only complicates the corresponding code design but also the error analysis.
Simulation results show that the proposed coding schemes outperform existing schemes in terms of outage probabilities and the achievable rates in a variety of channel settings.
Subjects
多重存取中繼通道
解調後傳輸
計算後傳輸
晶格編碼
共 集合解碼器
SDGs
Type
thesis
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