Abstract-This paper investigates the linear precoder design for K-user interference channels of multiple-input multipleoutput (MIMO) transceivers under finite alphabet inputs. We first obtain general explicit expressions of the achievable rate for users in the MIMO interference channel systems. We study optimal transmission strategies in both low and high signal-to-noise ratio (SNR) regions. Given finite alphabet inputs, we show that a simple power allocation design achieves optimal performance at high SNR whereas the well-known interference alignment technique for Gaussian inputs only utilizes a partial interferencefree signal space for transmission and leads to a constant rate loss when applied naively to finite-alphabet inputs. Moreover, we establish necessary conditions for the linear precoder design to achieve weighted sum-rate maximization. We also present an efficient iterative algorithm for determining precoding matrices of all the users. Our numerical results demonstrate that the proposed iterative algorithm achieves considerably higher sumrate under practical QAM inputs than other known methods.

In this paper, we consider a heterogeneous ad-hoc network where primary users may cooperate with cognitive radio (CR) users for the transmission of their data. We propose a new cooperation protocol that allows CR users to relay primary user signals in exchange for some spectrum. The spectrum released by primary users is used by CR users for their own data transmission. The proposed protocol maximizes the primary user power savings and the CR users' own data transmission rate. In addition, it provides more robust (potentially continuous) service for CR users, compared to the conventional practice in cognitive networks where cognitive users transmit in the spectrum holes of primary users (i.e., their service is interrupted when primary users need to transmit and no spectrum holes are available). More specifically, we propose a CR user power allocation scheme that maximizes the rate of transmission of CR user own data, for any given CR user power budget and a given bandwidth released from the primary user. Furthermore, we determine a range of possible transmission power levels that can be used by the primary user during cooperation without sacrificing its target transmission rate, and we derive a necessary condition on the quality of the channel between the primary user and the CR user that enables cooperation. Extensive numerical and simulation studies illustrate our theoretical developments and show that cooperation between a primary and CR user may lead, for example, to up to 80% savings of primary user power when compared to a noncooperation scheme at the same transmission power level.Index Terms-Cooperative communications, amplify-and-forward relaying, cognitive radio, heterogeneous ad-hoc networks.

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Abstract-Throughput maximization is a key challenge in cognitive radio ad hoc networks, where the availability of local spectrum resources may change from time to time and hopby-hop. To achieve this objective, cooperative transmission is a promising technique to increase the capacity of relay links by exploiting spatial diversity without multiple antennas at each node. This idea is particularly attractive in wireless environments due to the diverse channel quality and the limited energy and bandwidth resources. In this paper, decentralized and localized algorithms for joint dynamic routing, relay assignment, and spectrum allocation in a distributed and dynamic environment are proposed and studied. A cross-layer protocol to implement the joint routing, relay selection, and dynamic spectrum allocation algorithm is also introduced, and its performance is evaluated through simulation. Performance evaluation results show that the proposed protocol achieves much higher throughput than solutions that do not rely on cooperation.