The Research On Signal Space Alignment In Two-way Relay Channel

Two-way relaying (TWR) is a promising spectral efficient transmission protocol for wireless networks with half-duplex terminals. By means of network coding, TWR channel is able to overcome the limitation of the half-duplex mode to clearly improve the spectral efficient. Motivated by two elegent schemes:interference alignment (IA) and network coding (NC), signal space alignment (SSA) is proposed to use wireless resource more efficient. The performance of two-way relaying networks (TWRNs) can be further improved by integrating SSA.However, the study on the degree of freedom (DoF), which is used to characterize the TWR model capacity, is in its infancy; the horizontal comparisons between SSA and multi-user multiple-input and multiple-output (MU-MIMO) are rare; the problems which will encounter in the realistic application of SSA are hardly considered. Based on the situation mentioned above, this paper investigates SSA in the TWR model, and the contents can be gengeralized as four parts:First of all, the original MIMO Y channel is extended into a more generalized circumstance, called generalized MIMO Y channel with general signal demands. The degree of freedom (DoF) is used to characterize the channel capacity. SSA and zero-forcing beamforming are used to prove the achievement of the DoF. Then, it is found that SSA places new antenna restrictions at users. Then a method is introduced to compare SSA and MU-MIMO, mainly from the perspective of the total number of antennas used. The results show that if each node must be able to exchange signals with all others, the model with SSA will definitely use more antennas to supply the same DoF, when K>5. If it is not necessary for every node to exchange signals with all others, the comparison can still be made by using the method introduced here. As a result, the conditions under which SSA-NC is needed are presented.In addition, a method to find the constellation condensing (CC) design at the relay is proposed for signal space alignment (SSA). When SSA is employed, the relay receives a sum signal composed by an M order modulation signal and an N (M≤N) order modulation signal. Follow the three proposed steps; the constellation of the sum signal can be condensed to an N order modulation. The CC operation for the sum signal at the relay is capable of reducing the demodulation complexity and improving BER performance at the users.At last, this paper proposes an asymmetric transceiving scheme for SSA. Utilizing the proposed scheme, two paired users with different data rates could employ SA to communicate with each other without any control signalling. The relay and the receiving user can analyze the received signal to obtain the necessary information to demodulate it. The expression of the bit error rate (BER) at the receiving user is derived.