Hans H. Brunner

Mitigation of intercell interference without base station cooperation

We consider the downlink of a cellular system based on the spatial channel model of the 3GPP MIMO urban macro cell with multiple antenna base stations and single antenna mobile devices. We analyze the impact of path-loss and intercell interference and classify mobile devices according to their location in a cell. We discuss non-cooperative approaches for maximizing the network sum-rate and propose an algorithm that combines altruistic avoidance of intercell interference and egoistic sum-rate maximization. The sum-rate reached with this algorithm is still limited by intercell interference but outperforms existing non-cooperative methods without introducing a large overhead.

Two-Phase Scheduling and Leakage-Based Precoding in Wireless Cellular Networks

We consider the downlink of a wireless cellular network where the base stations are equipped with multiple antennas and operate in the same frequency band. Since scheduling changes the spatial transmit signal processing with each time slot, information from neighboring base stations is required for data encoding. This can, in theory, be accomplished by a high-capacity backhaul network through which the base stations exchange channel state information (CSI) and other control signals. In reality, however, the temporal granularity of the scheduler does not allow for timely distribution of CSI among base stations. We propose a two-phase scheduler which optimizes the precoding in the first phase and allows the users to feed back their instantaneous interference power in the second phase. For the single-user case, we present a practical scheme that combines two-phase scheduling with precoders that maximize the signal-to-leakage-plus-noise ratio. If the users feed back the interference power together with a supported rate, communication between base stations can be limited to integers. By comparing the performance to multi-user two-phase scheduling with dirty paper coding and to algorithms that share CSI among base stations we show that two-phase scheduling is a technically and practically feasible solution to deal with non-stationary intercell interference.

How much training is needed for interference coordination in cellular networks

Cooperative techniques for cellular networks promise very high data rates, but require additional and precise knowledge of the serving and interference channels. We show, how the pilot symbols required for achieving this information affect the possible data rates. The measurements of the channels are suffering from pilot contamination, due to the measurements in adjacent cells. On the one hand, with a too short pilot length, cooperation is not possible and the channels are learned too poorly, degrading the possible data rates. On the other hand, a too long pilot length reduces the efficiency of the system, leaving no resources for the data transmission. In addition, the channel measurements are outdated before they can be applied. With an upper bound to the sum rate of a system with interference coordination and a sub-optimal pilot allocation strategy, we discuss the pilot length trade-off.

Handling unknown interference in cellular networks with interference coordination

We consider the downlink of a cellular network with cooperating multiple antenna base stations. On the one hand, each base stations tries to serve its associated mobile devices optimally, on the other hand, they try to minimize the interference they cause. Both goals could be achieved, if the interference channels would be zero. An adaptive beamforming based interference mitigation can only be performed for measured interference channels. To find an unachievable, but relatively tight upper bound we set the measured interference channels to zero. For such a limited cooperation, we show that the rates can be improved by predicting the interference over the unknown channels.

Point-to-point MIMO MMSE vector precoding and thp achieving capacity

Non-linear precoding for point-to-point (P2P) multiple-input multiple-output (MIMO) systems is considered. First, the minimum mean square error (MMSE) optimal vector precoding (VP) is presented for different receiver structures, viz., weighted identity matrix, diagonal matrix, weighted unitary matrix, and matrix without particular structure. Whereas the former two structures can also be applied to the vector broadcast channel, the latter two are only realizable for cooperative receivers. Second, VP is derived that minimizes the MSE but is restricted to maximize the mutual information of the MIMO channel. Third, the corresponding Tomlinson-Harashima precoding (THP) is found by applying the nearest-plane approximation to the computation of the perturbation signal. The resulting maximum mutual information THP clearly outperforms the state-of-the-art P2P-MIMO THP based on the generalized triangular decomposition (GTD) with respect to MSE and BER.

Weighted sum rate maximization with multiple linear conic constraints

In the downlink (DL) of a multi-user multiple-input and multiple-output (MU-MIMO) system, the maximization of the weighted sum rate with dirty paper precoding (DPC) is treated under multiple linear and linear conic constraints. By network duality, the problem is transformed to a minimax uplink (UL) problem. In the UL, the minimization of the utility with respect to the noise covariance and the maximization of the utility with respect to the transmit covariances is solved either jointly or alternately with the gradient-projection algorithm. The proposed algorithms do not only allow to find the maximum weighted sum rate with respect to conic constraints, they are also efficient implementations with respect to multiple linear constraints.

PRECODING FOR SYSTEMS WITH SOFT COMBINING TO COUNTERACT INSTATIONARY INTERCELL INTERFERENCE

We consider the downlink of a cellular network with multiple antenna base stations and single antenna user terminals. In cellular networks with and without cooperation, the problem of instationary intercell interference arises. Some base stations change their beamforming unpredictably and the signal to interference plus noise ratios of the served user terminals are unknown at the base station. Consequently, the precoding and link rate adaption are outdated and the transmission might fail. Hybrid automatic repeat request can be used to mitigate the risk of such a fail. We propose to optimize the precoders at the base stations based on the expectation of the rate, where we include the effects of soft combining in the optimization.

Fun examples for teaching linear and nonlinear circuits

Circuit theory is essentially a physically based theory of dynamical systems. To demonstrate basic phenomena example circuits have to be selected and designed. These circuits consist of a few essential elements, which makes it easier to explain the mathematics. But, the abstraction of these simplified circuits can be a hurdle for students. To give students with a strong interest in the application an access to the theory and to add more verve to the lectures, we gathered some circuit examples, which are easy to explain in detail and have an entertaining effect. In this paper, we focus on circuits with at least one reactive element.