Study On Sub-carrier Selection And Power Allocation Algorithm For Multi-relay-assisted Cooperative OFDM Systems

Multiple input multiple output (MIMO) technology is an important way toachieve high data transmission rates and improves system capacity in wireless fadingenvironments for future mobile communication systems. However, the restrictions onthe size of nodes and the transmission power make it very difficult to achieve.Cooperative communication technology can make full use of the broadcastcharacteristics of radio waves. The wireless nodes form virtual antenna array throughmutual cooperation to obtain spatial diversity gain of traditional MIMO technology.Therefore, cooperative communication has been concerned more and more as a newmode of communication.Relay node selection and power allocation has always been a research hot spot inthe cooperative communication system. When some relay nodes with bad channelstate participate in the cooperation, these nodes take up valuable system resources, butalmost contribute nothing to improve the performance of the system. Relay nodeselection is the process to choose the most appropriate relay to participate incooperative communication from the potential relay node in order to maximize systemcooperative diversity gain. Since the cooperative communication system link state ofeach channel are different, if the average power is allocated to each link, some linkwith poor channel quality consume the power of the system but hardly contribute tothe system performance. Therefore, the control algorithm for limited power allocatedto each sub-carrier has important practical significance.Most of current resource allocation algorithms in multi-relay-assistedcooperative OFDM system has considered as a joint optimization problem ofsub-carrier selection and power allocation. Such joint optimization methods canprovide an optimal solution for power allocation with the expense of higher systemcomplexity. And the existing sub-optimal algorithms execute subcarrier allocationbefore the power allocation, without considering the influence of power to sub-carrierallocation. In terms of relay selection, the relay nodes are selected only based onchannel condition, these algorithms do not take relay nodes position and the numberof sub-carriers relay nodes forwarded into consideration.Rather than providing a less feasible joint optimization strategy, we present amore feasible and effective solution for source allocation in multi-relay-assistedsystem. In order to simplify the analysis, the optimization problem is decomposed totwo sub-problems: sub-carrier selection and power allocation, these two sub-problemsare all solved through an optimal solution. First, selective sub-carrier relaying schemeis realized according to whether it can bring the gain, and it is compared with all sub-carrier relaying scheme in the single-relay system. Then the relay nodes areselected by the channel conditions and the forwarding gain in multi-relay system. Forpower allocation, when the target of optimization is to maximize the system capacity,the optimal power allocation scheme is obtained by KKT conditions under thecondition of the individual power constraints.Considering the location of the relay nodes will influence the performance of thesystem, this paper puts forward an improved algorithm. When relay nodes are close tothe source node, higher system gain is achieved, otherwise, the system will get lowergain. Under the condition of each relay node has individual power constraint, themore subcarriers relay nodes forward, the less power each subcarrier is allocated.Greater power gain can be acquired when some of the subcarriers are forwarded byfree relay node instead of the previous busy node. Therefore, when relay nodes areclose to the source node, we can make full use of the power of the free relay node toincrease the system capacity. When relay nodes are far from the source node, we candecrease the relay node number to reduce the system complexity.Simulation results show that the algorithm proposed in this paper can effectivelysolve the problem of sub-carrier selection and owner allocation formulti-relay-assisted cooperative OFDM systems. The selective sub-carrier relayingstrategy can obtain greater system capacity than all sub-carrier relaying strategy. Theimproved algorithm can adjust the sub-carrier selection scheme according to theposition of the relay nodes.When the relay nodes are close to the source node, theproposed algorithm can obtain higher system capacity. When the relay nodes are farfrom the source node, the proposed algorithm can reduce the system complexity andmake it be implemented more easily, while maintaining the system capacityapproximately unchanged.