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In this paper, we propose a closed-form (i.e., without the need of any iterative procedure) joint optimization framework of the source precoder, the amplify-and-forward relaying matrices, and the destination equalizer for a cooperative multiple-input multiple-output (MIMO) wireless network. We study in depth such a design and carry out its performance analysis in terms of average symbol error probability (ASEP), which allows one to calculate the diversity order of the cooperative system. The ASEP is also evaluated via Monte Carlo simulations and compared with recent competitive alternatives. Results show that the proposed design performs better than or comparably to recently proposed iterative approaches, with lower computational requirements.

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This paper deals with performance analysis of a cooperative multiple-input multiple-output (MIMO) network with spatial multiplexing, wherein multiple half-duplex relays perform noncoherent (i.e., without channel state information) amplify-and-forward (AF) relaying and the destination employs a linear zero-forcing (ZF) equalizer. The correlation among the noise samples at the destination, the non-Gaussian nature of the dual-hop channel, and the fact that the relays are located in different positions significantly complicate the performance analysis of the system. In the high signal-to-noise ratio regime, we derive a simple and accurate approximation of the symbol error probability at the destination. In the special case of a relaying cluster, such a result allows to discuss the best placement of the relays and the performance gain of cooperation over the direct (i.e., without relaying) transmission. The theoretical analysis is validated by comparison with semi-analytical Monte Carlo simulations.Index Terms-Amplify-and-forward (AF) relaying, linear zero-forcing (ZF) equalization, multiple-input multiple-output (MIMO) systems, spatial multiplexing. 0018-9545 (c)

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In this paper, we propose a cognitive radio scheme that allows a secondary user (SU) to transmit over the same time-frequency slot of a primary user (PU), even when the PU is active. In our scheme, the SU amplifies and forwards the signal of the PU, by using as scaling factor the value of its information symbol to be transmitted towards the secondary receiver. The information-theoretic limits of the proposed protocol are investigated in terms of ergodic channel capacities of both the PU and SU links. It is shown that: (i) under certain operating conditions, the SU can superimpose its information symbols on the PU signal, without violating the cognitive radio principle of protecting the PU transmission; (ii) when the primary link is busy, the SU offers the PU its own transmitting power in exchange for a low-capacity communication channel, which improves the packet delay performance of the SU. In this barter, the tempting incentive for the PU consists of a noticeable improvement of its achievable rate at the price of a slight increase in the computational complexity of the primary receiver.Index Terms-Amplify-and-forward relaying, cognitive radio, ergodic channel capacity, wireless communications.

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