Abstract-The capacity bounds are studied for the multipleantenna real full-duplex Gaussian relay channel with t1 transmitting antennas at the sender, r2 receiving and t2 transmitting antennas at the relay, and r3 receiving antennas at the receiver. It is shown that compress-forward and partial decode-forward achieve within (1/2)(min(t1 + t2, r3) + r2) bits and (1/2) min(t1, r2) bits, respectively, from the cutset bound. Unlike the single-antenna case, partial decode-forward can be arbitrarily better than optimal selection between decode-forward and direct transmission. Similar gap results for half-duplex models are briefly discussed.
I. INTRODUCTIONThe relay channel, whereby point-to-point communication between a sender and a receiver is aided by a relay, is an important building block for cooperative wireless communication. Introduced by van der Meulen [1], this channel model has been studied extensively in the literature, including the now classical paper by Cover and El Gamal [2]. Nonetheless, even for the most basic Gaussian relay channel, the problem of characterizing the capacity in a computable form remains open under any nondegenerate channel gain and power constraint. Consequently, a large body of the literature has been devoted to the study of bounds on the capacity. Reminiscent of the maxflow min-cut theorem [3], the cutset bound was developed by Cover and El Gamal [2] that sets an upper bound on the capacity. There are myriads of coding schemes [4] and corresponding lower bounds on the capacity.The main focus of this paper is on two canonical coding schemes, compress-forward [2, Th. 6] and partial decodeforward [2, Th. 7] for Gaussian relay channels; see also [5, Ch. 16] for the detailed descriptions of the coding schemes. For the single-antenna full-duplex real Gaussian relay channel, compress-forward and partial decode-forward, respectively, achieve within half a bit from the cutset bound [6], [7], providing a half-bit approximation of the capacity. Moreover, partial decode-forward, which is superposition of decodeforward and direct transmission, reduces to the better of the two [8].Paralleling these results for the single-antenna model, we study the performance of compress-forward and partial decode-forward for multiple-antenna (also known as multipleinput multiple-output or MIMO) Gaussian relay channels. Capacity bounds for MIMO relay channels have been studied in numerous papers. For example, by convex programming techniques, Wang, Zhang, and Høst-Madsen [9] derived upper and lower bounds based on looser versions of cutset bound