Vol.23 No.4 JOURNAL OF ELECTRONICS (CHINA) July 2006 RELAY ASSISTED TRANSMISSSION WITH BIT-INTERLEAVED CODED MODULATION 1 Meng Qingmin You Xiaohu John Boyer * (National Mobile Communications Research Lab, Southeast University, Nanjing 210096, China) * (Department of Systems and Computer Engineering, Carleton University, Ottawa, ON KIS 5B6, Canada) Abstract We investigate an adaptive cooperative protocol in a Two-Hop-Relay (THR) wireless system that combines the following: (1) adaptive relaying based on repetition coding; (2) single or two transmit antennas and one receive antenna configurations for all nodes, each using high order constellation; (3) Bit-Interleaved Coded Modulation (BICM). We focus on a simple decoded relaying (i.e. no error correcting at a relay node) and simple signal quality thresholds for relaying. Then the impact of the two simple thresholds on the system performance is studied. Our results suggest that compared with the traditional scheme for direct transmission, the proposed scheme can increase average throughput in high spectral efficiency region with low implementa- tion-cost at the relay. Key words Cooperative diversity; Multihop diversity; relay channel; iterative decoding; Alamouti coding I. Introduction The use of diversity to mitigate the detrimental effects of path loss and fading in radio environments has been well documented. Similar to transmit di- versity [1,2] , relayed transmission also provide the means of effective diversity to combat channel fad- ing. Recently, cooperative diversity techniques [3,4] , have attracted significant attention in wireless net- works. Emerging dual-mode mobile terminals used as intermediate regenerative relays can enhance the coverage extension and capacity of next-generation wireless systems [5] . The Two-Hop-Relay (THR) ar- chitecture (single decoded relay-based) is a form of decoded relaying in which a Relay node (R) decodes the signal from a Source node (S) or an Access Point (AP) in the downlink and then re-encodes and for- wards the decoded symbols to a Destination node (D). This architecture can benefit from path loss savings and multihop diversity. Studies in Refs.[6−8] evaluated the performance in specific two-frame coded cooperative schemes where channel decoder is used to perform data recovering (i.e. error correct and so on) and then interleaving and re-encoding at R. An incremental codeword from R to D will be transmitted if R correctly decodes the signal from S, otherwise S has to transmit the corresponding codeword to D in the consecutive frame, and thus a 1 Manuscript received date: October 18, 2005; revised date: January 16, 2006. Supported by the National Natural Science Foundation of China (No.60496311) and partially supported by the B3G project (2003AA123310). Communication author: Meng Qingmin, born in 1965, male, Ph.D. candidate. National Mobile Communica- tions Research Lab, Southeast University, Nanjing 210096, China. qm_meng@seu.edu.cn. feedback path between S and R is required to control signaling of S in such coded communication system with low transmit rate. However, when high order constellation is considered to support high transmit rate in the THR system, a powerful error correct code will be required to obtain a good signal quality in S-R link under a variety of network geometries, which leads to high implementation complexity and substantial power cost at R. Hence, one has to care- fully study an adaptive cooperative protocol to ex- ploit the benefits of the simple coded cooperation under the low-cost constraint at R when high order constellation is used in the THR system. In this paper, motivated by the adaptive relaying for its good performance [4] over fixed relaying, we propose an adaptive cooperative protocol based on two different signal quality thresholds, i.e., instan- taneously received Signal-to-Noise Ratio (SNR) (related to threshold 1) and magnitude square of channel gain (related to threshold 2), in flat block fading relay channel. With diversity combining in- stead of coded combining that is used in Refs.[6−8], the proposed scheme with repetition coding and simple feedback mechanism among all nodes has its low cost advantage in terms of date recovering at R since no error correct processing is required except for simple SNR comparison. Previous studies [4,9] discussing regenerative or decoded relaying have noted that although there is less danger of noise propagation than non-regenerative relays, the detec- tion error at R is usually dominant in the relay channel since D has diversity from two independent branches. In a practical coded cooperation scheme, when the signal quality that is modeled by instanta- neous received SNR value or magnitude square