On Energy Efficient Design for Dynamic CoMP Transmission in K-Tier Heterogeneous Cellular Networks

Motivated by the objective of pursuing revenue, improvement in coverage and reduction in energy cost for wireless communication networks have been of great significance for mobile operators. Therefore, heterogeneous cellular networks(HCNs) and Coordinated Multipoint(Co MP) transmission are considered as promising solutions to enhance the performances of wireless communication systems. This paper analyzed the K-tier HCNs with a dynamic downlink Co MP scheme, in which the flexible clusters of cooperative stations are determined by a connecting threshold θ. Using stochastic geometry, the coverage probability(CP) and energy efficiency(EE) of a K-tier HCN operating under this scheme are derived, based on which the trade-off between CP and EE is discovered and discussed. Simulation results show the validity of our derivations. The proposed schememay significantly reduce energy consumption sacrificing a small amount of CP, and outperforms the fixed scheme as well. The CP-EE trade-off are also revealed, whichsuggests suitable trade-off points between CP and EE that will deliver the maximum economic profitability. Tendencies discovered in this paper may provide the operators with opportunities for further optimization in pursuit of economic profitability.

Interference Coordination for Secrecy Enhancement in Random Wireless Networks

The co-channel interference modeling is vital for evaluating the secrecy performance in random wireless networks,where the legitimate nodes and eavesdroppers are randomly distributed.In this paper,a new interference model is proposed from the userdominant perspective.The model can provide a better analytical assessment of secrecy performance with interference coordination for the presence of eavesdroppers.The typical legitimate is assumed to be located at the origin,and chooses the closest base station(BS) as its serving BS.The field of interferers is obtained by excluding the desired BSs(including the serving BS and its cooperative BS(s)).In contract with the exiting interference model,it is assumed that desired BSs and interferers belong to the same Poisson Point Process(PPP),and eavesdroppers are distributed according to another independent PPP.Based on this model,the average secrecy transmission capacity is derived in simply analytical forms with interference coordination.Analysis and simulation results show that the secrecy performance can be significantly enhanced by exploiting interference coordination.Furthermore,the average secrecy transmission capacity increases with increasing number of cooperative BSs.

Design of hybrid automatic repeat request (HARQ) for TDD-OFDM system

Reliability-based hybrid automatic repeat request （HARQ） （RB-HARQ） is a recently developed form of incremental-redundancy ARQ. It achieves good performance whereas large retransmission request packets should be fed back. In this paper, in order to reduce the number of the fed back bits, we propose a HARQ scheme applied in time duplex division orthogonal frequency division multiplexing （TDD- OFI）M） system over the slow fading channel which is named channel-based HARQ （CB-HARQ）. Because one bit which meets deep fading is always with small value of log likelihood ratio （LLR） during the process of decoding of LDPC code, the bits transmitted on the carrier with deep fading are retransmitted. At the receiver, the decoder will compute the locations of retransmission bits according to the channel fading values which are gotten by utilizing the feature of channel symmetry in TDD mode. So the indices of retransmission bits are avoided to be transmitted. Simulation results show that this method achieves better BER performance and requires much smaller request packets in feedback link.

User selection strategies for multiuser MIMO systems with block diagonalization

Block diagonalization （BD） is an efficient precoding technique that eliminates inter-user interference in downlink multiple-input multiple-output （MIMO） systems. User selection strategies applied to multiuser MIMO systems with BD are investigated in this article. To enhance the capacity of multiuser MIMO systems, an equivalent capacity maximum （ECM） user selection strategy is proposed with low computational complexity. Considering both the factors of channel correlations and channel conditions, the proposed strategy can select a group of users to serve for maximizing the total throughput. Simulation results indicate that, for various channel conditions, proposed ECM strategy gains a better performance compared with traditional user selection strategies, and achieves a near optimal throughput as the exhaustive search.

A new access scheme in OFDMA systems

This article presents a dynamic random access scheme for orthogonal frequency division multiple access （OFDMA） systems. The key features of the proposed scheme are it is a combination of both the distributed and the centralized schemes, it can accommodate several delay sensitivity classes, and it can adjust the number of random access channels in a media access control （MAC） frame and the access probability according to the outcome of Mobile Terminals access attempts in previous MAC frames. For floating populated packet-based networks, the proposed scheme possibly leads to high average user satisfaction.