Design And Analysis Of The Cooperative Strategy In High Altitude Platform Communication System

Comparing with the traditional communication system, High Altitude Platform System（HAPS） has many advantages, such as low cost of system construction, flexible maintenance, easy update of the platform and the load, and environment protection. However, the link between HAPS and the subaerial station often suffer from fading in low elevation coverage area due to the barrier of the buildings, trees, and other objects.Diversity technology can overcome the effect of the fading and improve the system performance. In recent years, cooperative communication draws a lot of research interest. It can improve the reliability of the system by exploiting the transmit diversity obtained with cooperation between different wireless equipment.This paper aims at improving the performance of cooperative HAPS system in fading channel, and studies the cooperative strategy in uplink and link adaptation technology in cooperative communication. The main works and contributions of this paper are summarized as follows.Firstly, the system model of HAPS is introduced, and the cooperative communication model in HAPS is given. Then,the performance of two basic cooperative protocols, i.e., Amplify-and-Forward（AF） and Decode-and-Forward（DF）, is analyzed. Two link adaptation techniques of HARQ and AMC are introduced.Secondly, a cooperative decode-and forward（DF） clustered protocol with multiple relays is designed. The clustered system model for HAPS is introduced, and the relationships between the system BER performance and the transmit power, the receive SNR threshold of the relay node, and the number of the relay nodes are discussed. Research results show that the BER performance for different number of cooperative relays tends to become consistent when the SNR threshold of each relay is fixed and that the declination rate of the BER performance becomes flatter with the increasing transmit power. This performance tendency begins at a higher transmit power and the consistent BER value becomes lower with the increasing SNR threshold value for the relay node. In practical application, when the transmit power is low, more relay nodes will be selected to cooperate with the source node to ensure the reliability. However, a higher SNR threshold at the relay nodes will limit the available number of relay nodes and thus decrease the reliability, so different SNR threshold values can be adopted according to the transmit power.Finally, according to the variety of the HAPS channel, a cross-layer design, which combining adaptive modulation and coding（AMC） at physical layer with automatic repeat request（ARQ） at data link layer, is proposed for cooperative wireless networks that use opportunistic relaying scheme. The proposed scheme can maximize the system throughput by employing a proper broadcast rate and a more effective retransmission rate according to the adaptive cooperative diversity. TCARQ-MR scheme perform much better than TCARQ-FR and TARQ-MR. Compared with TARQ-MR scheme, TCARQ-MR scheme can achieve better throughput performance by exploiting the cooperative diversity gain to decrease the PER or improve the TM order of retransmission. However, when the average SNR is high enough, the source can transmit the packet to the destination correctly with the highest order TM and few retransmission is needed, thus, the throughput of two schemes become close with each other and converge to the same bound with average SNR converge to infinity. as the relay number N increases, the throughput of TCARQ-MR improves in a wide SNR range. For the retransmission numbermax reN, research results show that increasing Q is a more effective way to improve throughput of TCARQ-MR and TCARQ-FR compared with increasing N_re^max.