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Sun L.,NEC Laboratories | Li P.,Hong Kong University of Science and Technology | McKay M.R.,Hong Kong University of Science and Technology | Murch R.D.,Hong Kong University of Science and Technology
IEEE Transactions on Signal Processing | Year: 2012

This paper deals with the fundamental limit of capacity for MIMO wireless systems with antenna mutual coupling. Motivated by practical considerations, the capacity-achieving precoder is found by maximizing the capacity subject to two separate power constraints; namely, a constraint on the available power at the transmitter which is imposed on a per-amplifier basis, and a constraint on the allowable radiated power. These two separate constraints occur because MIMO antenna systems are usually unable to use a perfect conjugate multiport impedance match in practice and therefore MIMO systems with mutual coupling usually do not radiate all of the power input or available to them. Assuming that perfect channel state information is available to both the transmitter and receiver, we first derive sufficient conditions on the power constraints, in terms of the mutual coupling, under which the dual-constraint problem reduces to effectively a single-constraint problem. This greatly simplifies the optimization problem in some cases. For the more general case where both constraints are relevant, we present an efficient suboptimal algorithm by first relaxing the per-amplifier power constraint to a sum power constraint and then solving the dual-constraint problem based on dynamic water-filling and mode-dropping. Finally, we investigate the optimal precoder design at high signal to noise ratios, and provide stronger theoretical results in this regime. Numerical results are provided to demonstrate the effectiveness of our technique when dual power constraints are important to consider. © 2011 IEEE. Source


Yuan F.,Beihang University | Yang C.,Beihang University | Wang G.,NEC Laboratories | Lei M.,NEC Laboratories
IEEE Transactions on Wireless Communications | Year: 2013

In this paper, a channel direction information (CDI) feedback strategy is proposed to assist coordinated multi-point beamforming (CoMP-CB) between a macro base station (MBS) and multiple pico BSs (PBSs) in heterogeneous networks, where the numbers of bits for feedback are adaptive to the average channel gains of multiple users. To maximize the average sum rate achieved by the users with given feedback resource, we optimize the overall number of feedback bits for each user and the codebook sizes for quantizing the per-cell CDIs of each user. To provide feasible feedback scheme for practice use, a low complexity two-step solution is developed from deriving the lower bounds of the per-user average rate. In the first step, we optimize the number of overall feedback bits for each user, which provides a tradeoff between the accuracy of CDI quantization and the reliability of feedback transmission. In the second step, we optimize the bit allocation for quantizing the per-cell CDIs, which accommodates the difference in transmit powers and antennas of the MBS and PBSs and in the channels of macro and pico cells. Two adaptive feedback schemes are developed from two lower bounds. By characterizing the degree of freedom (DoF) achieved by each user, we show that the systems using the proposed adaptive feedback schemes are no longer interference-limited. The maximum per-user DoFs achieved by the proposed schemes depend on the per-antenna feedback resource constraint for each user. Simulation results show significant performance gain of the proposed scheme over NonCoMP and CoMP-CB with fixed codebook size. © 2002-2012 IEEE. Source


Tang X.,Link A Media Devices | Wang S.,NEC Laboratories
IEEE Transactions on Computers | Year: 2010

A self-diagnosis circuit that can be used for built-in self-repair is proposed. The circuit under diagnosis is assumed to be composed of a large number of field repairable units (FRUs), which can be replaced with spares when they are found to be defective. Since the proposed self-diagnosis circuit is implemented on the chip, responses that are scanned out of scan chains are compressed by the group compactor, the space compression circuit, and finally, the time compression circuit to reduce the volume of test response data. Both the space and time compression circuits implement a Reed-Solomon code. Unlike prior work, in the proposed technique, responses of all FRUs are observed at the same time to reduce diagnosis time. The proposed diagnosis circuit can locate up to l defective FRUs. We propose a novel space compression circuit that reduces hardware overhead by exploiting the frequency difference of the scan shift clock and the system clock and by combining scan cells into groups of size r. When the size of constituent multiple-input signature register (MISR) is m, the total number of signatures to be stored for the fault-free signature is 2lmB bits, where 1\le B \le m. The experimental results show that the proposed diagnosis circuit that can locate up to four defective FRUs in the same test session can be implemented with less than one percent of hardware overhead for a large industrial design. Hardware overhead for the diagnosis circuit is lower for large CUDs. © 2006 IEEE. Source


Jiang L.,NEC Laboratories | Lei M.,NEC Laboratories
IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC | Year: 2012

This paper proposes a resource allocation scheme for enhanced inter-cell interference coordination (eICIC) in heterogeneous networks. In the eICIC system, the macro eNBs will stop data transmission in almost blank subframes (ABSs), where the UEs in small cells can be protected. The proposed scheme takes into account both the number of the ABSs configured at the macro eNBs and the UE partition scheme in small cells to achieve the performance balance between the macro cells and the small cells. Simulation results show that the proposed scheme outperforms the existing methods regardless of the configured ABS pattern and the UE partition scheme. The performance gain in the cell spectrum efficiency is achieved by slightly sacrificing the medium UEs with medium values of signal to interference plus noise ratio (SINRs) to further improve the best UEs with high SINRs without degradation of the edge UEs. © 2012 IEEE. Source


Yi S.,NEC Laboratories | Lei M.,NEC Laboratories
IEEE Wireless Communications and Networking Conference, WCNC | Year: 2012

In 3GPP LTE-Advanced, relay is considered as an important means for extending cell coverage and improving capacity. Backhaul resource allocation needs to be carefully designed to achieve the performance advantage of relay. In this paper, we propose a two-step backhaul resource allocation scheme for an inband relaying system based on relay buffer level. The resource assignment to relays meets the actual traffic demand, takes into account the link quality of the backhaul link and the access link, and the resource partitioning between relays and macro UEs is then optimized. Our evaluations show that the proposed framework can improve the cell average and cell-edge user performance, for different deployment scenarios. This approach has very low complexity in implementation and the revision to current standard is very minimal. © 2012 IEEE. Source

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