On the Design of Cooperative Transmission Schemes

In this paper a new cooperative transmission scheme is proposed that enjoys some important advantages over similar protocols First it achieves cooperative diversity without relying on orthogonal subspaces e g di erent spreading codes or frequency bands allowing full rate transmission Second its performance degrades gracefully as the path loss between cooperating partners increases never falling below that of non cooperative protocols Introduction Multipath fading is the predominant factor limiting the speed and reliability of wireless communication systems Spatial diversity a powerful means to mitigate fading is unfor tunately not available when communicating between a pair of single antenna terminals In ad hoc network applications however nearby single antenna sources can cooperatively transmit to a far away destination thereby leveraging the spatial diversity inherent to a virtual multi antenna source Several cooperative schemes have been proposed Sendonaris et al proposed a scheme in which sources utilize di erent spreading codes to communicate with the destination as well as among cooperative partners Laneman et al designed a cooperative scheme in which sources communicate with their partners and the destination using non overlapping time slots Both of these schemes rely on orthogonal subspaces for cooperation essentially trading rate for diversity In this paper we present a scheme which does not rely on orthogonal subspaces allowing a more e cient use of resources The Proposed Scheme For simplicity we consider the case of two source nodes and one destination node each with a single antenna The channels between the sources are assumed to be quasi static at Rayleigh fading and the noises at each receiving antenna are additive white and Gaussian We assume that the inter source channel gain is known by both sources as well as the destination though the source destination channel gains are known only by the destination i e not the sources In the proposed scheme sources transmit once per frame where a frame is de ned by two consecutive time slots Each source transmits a linear combination of its current symbol and the noisy signal received from its partner during the last time slot For source j and frame k we denote the broadcast and repetition gains by aj and bj respec tively the information symbol by xj k and the transmitted signal by tj k At startup the transmitted signals will take the form t a x t a x b ht w t a x b ht w t a x b ht w where h denotes the inter source channel gain and jwj k the noise observed by the j th source during the k frame We assume that wj k has unit variance The corresponding signals received by the destination node are y g t v y g t v y g t v y g t v where gj is the gain of the channel connecting source j to the destination and vj k the destination noise of variance v Note that in this scheme sources are either transmit ting or receiving but not both The broadcast and repetition gains faj bjg are chosen experimentally for now to minimize outage probability at the destination As a conse quence of symmetry a and a as well as b and b will have the same optimal value Thus we assume that broadcast and repetition gains are the same at each source and omit the subscripts yielding fa bg System Model In order to develop an e cient system model for the proposed scheme we de ne the following frame based quantities x k x k t x k x k t xk x k x k t w k w k t w k w k t tk t k t k t r k t k r k t r k r k t k t yk y k y k t