On The Study Of Diversity Theory And Key Technologies In Multiuser Cooperative Communication Systems

Diversity is used as a promising solution to combat the channel impair?ment which is caused by random fading in wireless communication systems.Cooperative communication is considered as one of the most effective means to provide spatial diversity in a distributed manner.It attracts much attention and it is a very important technology in future mobile communication systems.In a cooperative communication system with multiple relays,opportunistic re-laying is a relatively low complexity cooperative scheme.It selects the "best“relay to assist transmission,and it is able to achieve full cooperative diversity with high spectrum efficiency.On the other hand,there exists another kind of diversity which is called multiuser diversity in future mobile communication systems.Because the users experience independent random fading,the prob-ability that the "best" user having a "strong" channel is very high.Therefore,multiuser diversity can be obtained to improve the outage probability and/or capacity performance by allowing only the user with the highest instantaneous signal-to-noise ratio(SNR)to transmit.To conclude,both multiuser diversity and cooperative diversity exist in multiuser cooperative communication sys-tems.However,current research studies focusing on the use of diversity in multiuser cooperative communication systems mainly extend the scheduling schemes from the non-cooperative systems to the cooperative systems without much considering of the cooperative characteristic,and might cause perfor-mance loss.This dissertation studies the theory and technologies of diversi-ty in multiuser cooperative communication systems under a new framework and then proposes cooperative schemes with higher spectrum efficiency and/or lower complexity.The major contributions of this dissertation are summarized below.Firstly,this dissertation proposes a dual network coding based retrans-mission scheme to improve the transmission efficiency for wireless broadcast systems.The transmission efficiency of the proposed scheme and its counter-parts is analyzed.This example reveals the key idea of this dissertation:the researcher should design diversity schemes according to the characteristic of the practical system to achieve higher spectrum efficiency and/or lower com-plexity.Secondly,this dissertation studies how to achieve full cooperative diver-sity with round-robin scheduling,and proposes a novel round-robin relaying based cooperative scheme.Round-robin scheduling is a fair scheduling in which all the users transmit in turn.The existing opportunistic relaying work-s under the traditional time-slot allocation framework in which one broadcast time slot and one relay time slot compose one user's transmission.In the re-lay time slot,optimal relay selection is exploited to provide full cooperative diversity.However,the proposed scheme is based on a new time-slot alloca-tion framework where the entire transmission of all the users is divided into two phases:the broadcast phase and the relay phase.In the proposed scheme,all the users transmit one by one in the broadcast phase.Then in the relay phase,all the relays transmit in a round-robin manner,thus no relay selec-tion is needed.By introducing user selection,the proposed scheme achieves the "relay-sharing”effect,which means each relay's transmission assists al-1 the users in the system.Compared with opportunistic relaying,two merits are obtained.One is that in future mobile communication systems where the users are more than the relays,the proposed scheme achieves better diversity-multiplexing tradeoff.The other one is that the proposed scheme reduces the implementation complexity because user selection is much easier to carry out than the relay selection.Moreover,based on the diversity-multiplexing trade-off analysis,an adaptive relay activation scheme which is capable of achieving higher diversity-multiplexing tradeoff by dynamically selecting the number of relays to be activated in the entire network is proposed.Thirdly,based on the previously proposed time-slot allocation frame-work,a new cooperative scheme is proposed,which achieves multiuser di-versity and cooperative diversity simultaneously with the greedy scheduler.Compared with existing schemes which are based on traditional time-slot al-location framework,the proposed scheme achieves higher spectral efficiency while obtaining multiuser diversity and cooperative diversity with proper de-sign.Furthermore,the proposed scheme has asymptotical optimality in terms of diversity-multiplexing tradeoff.Moreover,the proposed scheme does not need to perform relay selection in each relay time slot,and it results in simpler implementation.Fourth,this dissertation tries to answer the question that whether round-robin scheduling conflicts with multiuser diversity in cooperative communica-tion systems.A round-robin scheduling based scheme is proposed to achieve both kinds of diversity.Theoretical analysis and simulation results verify that the proposed scheme achieves multiuser diversity and cooperative diversity si-multaneously under round-robin scheduling.Therefore,the proposed scheme provides better tradeoff between performance and fairness.Finally,this dissertation proposes two cooperative schemes in multi-input-multi-output(MIMO)relay systems with linear processing.The proposed schemes can achieve the same diversity order when compared with traditional protocol while using much fewer total transmission time slots,thus leads to significant throughput improvement.