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The unicasting mode is a natural choice in multi-relay systems to utilize the spatial potential of source and destination nodes. In the unicasting phase, the source node transmits different data streams to several relay nodes with the downlink multi-user precoding; in the multi-accessing phase, relay nodes transmit the received data streams to the destination node simultaneously. In this paper, the received signal-to-noise-ratio (SNR) at the destination node of the unicasting mode is analyzed and derived, which can be calculated with two SNRs related to the two phases as an approximate harmonic mean function. Then, an approximate method for calculating the system ergodic capacity is presented. The inner products between singular vectors are approximately replaced by their expectations and distribution functions of singular values are derived. The numerical and simulation results demonstrate the efficiency of the analysis and the approximate method. Copyright © 2010 John Wiley & Sons, Ltd. The received signal-to-noise-ratio (SNR) at the destination node of the unicasting mode in multi-relay systems is analyzed and derived, which can be calculated with two SNRs related to the two phases as an approximate harmonic mean function. Then, an approximate method for calculating the system ergodic capacity is presented. The inner products between singular vectors are approximately replaced by their expectations and distribution functions of singular values are derived. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]