Research On Node Selection And Scheduling Mechanism For Wireless Cooperative Networks

Wireless mobile communication is one of the most active areas of research. With the increasing requirements of high transmission rate and high quality of service, there is continuous evolution in wireless network architecture and advanced transmission technique. As an effective method to improve spectrum utilization efficiency, multi-input multi-output (MIMO) has unique technique advantage and draws much attention. It is adopted in many systems. By employing multiple antennas at both transmitter and receiver, spatial diversity can be achieved, system capacity and spectrum efficiency can be improved greatly. On the other hand, it is difficult to equip multiple antennas considering the size, implementation complexity, Power constraint and cost of mobile terminals. Recently, a new technique called cooperative communication is proposed to solve this problem, the basic idea of cooperative communication is that every terminal has one partners or more and terminals share antennas of each other, which can be form a virtual antenna array to get the same diversity gain as conventional MIMO technique does. It can also reduce the transmit power and extend the radio cover range. This transmission technique has attracted a lot of research attention from industry and academic circles. Cooperative communication is not only a transmission method, it can also be regarded as a kind of thought, it can inosculate with other techniques and be broadly applied in many areas such as wireless sensor network (WSN), ad-hoc network, wireless mesh network and cellular network. It will be the one of the most important key techniques in the next generation heterogeneous and convergent network.This thesis surveys recent advances related to cooperative communication and lists the efficiency of the recent references. The first disadvantage is the improper of system model. The initial model is based on three nodes, then the model develops to multiple relays. But in practical communication systems, the model is more heterogeneous and there maybe multiple source or multiple relay in the system. Because of the complexity of channel fading, in many practical scenarios, different links in relay networks can experience asymmetric fading conditions. However, the current research mostly focuses on the Rayleigh fading in both two hops. In fact, Nakagami fading and Rician fading can represent the actual scenario more accurately in despite of complexity of two channels. The other disadvantage of the current research is that the node selection in multiple node networks. The traditional node selection means relay selection. It is waste of resources that all the relay nodes participate in the cooperation, so the best relay node can be selected from relay cluster to take part in the communication, This best relay selection outperforms traditional cooperative communication (all relay nodes forward the signal) and can be implemented in a distributed fashion with limited feedback. There are many reports about this relay selection. However, for multiple source or multiple user networks, node selection also includes source selection or user selection. There are two node clusters in multi-source networks or multiuser networks:source (user) node cluster and relay node cluster. In this case, node selection scheme is two nodes selection in two clusters synchronously. This scheme is brand-new research field and there are few reports about this scheme.Based on the aforementioned issues of cooperative networks, we concentrate our efforts on the research of new cooperative model and node selection schemes for wireless networks. The main contribution and innovation can be summarized as follows:1) If all the channels undergo deep and serious fading in multi-relay networks, even the best relay is used, the system performance can not achieve the goal. In this case, more then one relay or all the relays are needed to forward the signal. Based on this scenario, a new iterative generalized selection combining (IGSC) based on error detection is proposed. First, all the relays are rearranged in decreasing order of magnitude of SNR of the indirect link, then destination node combines the links using maximal rate combining (MRC) from the direct link according to the descend order of magnitude of the SNR, every time the new signal comes, the error detection is carried on. If no error is detected, the communication is over, if error is detected, the destination node continues combining and checks again. This process will keep on until no error is detected or the destination node gathers diversity paths and performs MRC with the signals from all the relay nodes and the source node. In this scheme, an error detection check is performed at every time frame to reduce the excessive burden of MRC with all diversity paths, save system resource and shorten the communication process. Although this scheme increases the computational complexity, it overcomes the disadvantage of the traditional best relay selection and fully strengthens the role of the second best relay or even all the relays.2) In cooperative communication of two hops, channel fading can experience asymmetric conditions because of variety of fading, the first hop and the second hop can experience different fading. So in this thesis, we concentrate on model of the asymmetric fading, the research object contains not only the common Rayleigh fading, but also Nakagaimi fading and Rician fading which is more complex. In this system model, we also consider that the best relay might be unavailable due to some scheduling or load balancing conditions. The decision may be made to use the second best relay or more generally the Nth best relay. This setting makes the system model confirm with practical scenario more appropriately.3) Based on the multiple source node in actual communication scenario, we propose a system model with multi-source multi-relay and two new source-relay selection schemes:two-step selection scheme and two-step selection scheme based on incremental relaying. In the first scheme, one source node is selected first and then one relay node is selected. Although this scheme has a slight loss in performance compared with the optimal joint source-relay selection scheme, it reduces the complexity greatly and has strong practicability. In the second scheme, a new channel state evaluation of the direct link is added to the first scheme. If the SNR of the direct best link is sufficiently high, then cooperation is omitted and the communication is carried on using the direct link, if the SNR of this best link is not sufficiently high for successful direct transmission, relay selection is carried on and it is only based on the CSI of the first hop, rather than the traditional relay selection which takes two hops into account. This kind of relay selection only needs to know few CSI and can prolong the network lifetime. A combination of incremental scheme and partial relay selection can save the system resources greatly.4) In multiuser cooperative networks, when user selection is carried out in a practical system, the CSI required for user selection may be different from their actual values during data transmission, due to several factors (e.g., channel changes with time, feedback delay). In other words, the user is chosen based on outdated CSI. In this thesis, the performances analysis of a multiuser Amplify-and-Forward (AF) cooperative communication networks is presented based on the outdated CSI. Asymptotical expressions of outage probability, channel capacity and Symbol Error Rate (SER) are derived. Exact expression of capacity is also derived. Analysis is applicable to independent identically distributed (i.i.d.) or independent none-identically distributed (i.n.d.) fading channels.5) We propose a new multi-relay multiuser system and based on this architecture, we proposed a two-step relay-user selection scheme. The proposed scheme can significantly reduce the amount of channel estimations and link quality comparisons compared to the optimal relay-user joint selection scheme. Futher more, we assume that two kinds of relay are implemented in this system:fixed gain relay and CSI-assisted relay. When fixed gain relay is used, asymptotical expression for channel capacity is derived, and when CSI-assisted relay is used, exact expression for channel capacity is derived.Finally, problems solved in this thesis and future research topics are summarized.