in this paper, based on the analysis of the UCA beamforming algorithm, an efficient SDMA scheme using the uniform circular array (UCA) with 8 elements is proposed. As the scheme devised, the cell is sectored by 2 crossed beam groups formed by UCA and the large elevation area of the cell is covered by UCA omni-directional pattern. With SDMA technique applied in the sectors, the cellular capacity can be increased nearly 5 times with low co-channel interference. The simulation results verify that, the scheme is of good feasibility in implementation and has a practical worth to be applied in hot spots.

Internet real-time multimedia communication brings a further challenge to Quality of Service (QoS). A higher QoS in communication is required increasingly. As a new framework for providing QoS services, Differentiated Services (DiffServ) is undergoing a speedily standardization process at the IETF. DiffServ not only can offer classified level of services, but also can provide guaranteed QoS in a certain extent. In order to provide QoS, DiffServ must be properly configured. The traditional DiffServ mechanism provides classifier for edge router to mark the different traffic streams, and then the core router uses different Drop Packet Mechanisms to drop packets or transmit data packets according to these classified markers. When multiple edge routers or other core routers transmit data packets high speedily to a single core router, the core router will emerge bottleneck bandwidth. The most valid solution to this problem is that the edge router adopts drop packet mechanism. This paper proposes an Modified Edge Router Mechanism that let the edge router achieve marking, dropping and transmitting packets of hybrid traffic streams based on DiffServ in a given bandwidth, the core router will only transmits packets but won’t drop packets. By the simulation of ns2, the modified mechanism ensure the QoS of high priority traffics and simplify the core router, it is a valid method to solve the congestion of the core router.

Recent research reveals that the fluid-based model can describe the dynamic behavior for bottleneck networks. The stability of the fluid-based model can inflect whether there exists a congestion of the network. This paper develops a linearization processing for fluid-based model at equilibrium points for the stability analysis of the networks with Active Queue Management (AQM). The bottleneck networks are described by uncertain linear time-delay systems, then 2-D (two-dimensional) Laplace-z transform has been applied in the stability test of the network. Simulations verify the stability analysis for AQM network to be valid, the AQM network approaches to full utilization, while the buffer size on the order of bandwidth-delay product is necessary for the stability of the fluid model with full utilization.

The real application of differentiated services (DiffServ) network requires the DiffServ routers to support different data rates and time delays for the subscribers with different priorities. However, classical routers and AQM algorithms for the routers can not meet the requirements. To solve the problems, this paper develops the schemes of DiffServ edge router (DER) and DiffServ core router (DCR) with multiqueues, classifier and scheduler and a DiffServ AQM algorithm. The proposed DiffServ AQM algorithm based on DER can process the heterogeneous and classified traffic flows. The DiffServ AQM algorithm is derived from two theorems about the congestion conditions of DiffServ network in different links, it can prevent the possible congestion and ensure the data rates and time delays for the subscribers with higher priorities. The system simulations validate the proposed DiffServ routers and DiffServ AQM algorithm.

Based on recent results for 2-D continuous-discrete systems, this paper develops 2-D Laplace-z transform, which can be used to analyze 2-D continuous-discrete signals and system in Laplace-z hybrid domain. Current 1-D Laplace transformation and z transform can be combined into the new 2-D s-z transform. However, 2-D s-z transformation is not a simple extension of 1-D transform, in 2-D case, we need consider the 2-D boundary conditions which don’t occur in 1-D case. The hybrid 2-D definitions and theorems are given in the paper. To verify the results of this paper, we also derived a numerical inverse 2-D Laplace-z transform, applying it to show the 2-D pulse response of a stable 2-D continuous-discrete system.

A new stability test algorithm for two-dimensional (2-D) digital filters is proposed, which uses the inner term polynomials of the corresponding 2-D test polynomial to construct a test table in order to simplify the stability test procedure. Different from other well-known table test algorithms, the new test algorithm can directly use the onedimensional (1-D) Schur procedure to test the zeros' distribution of a 2-D complex polynomial in the unit bidisk.

Active Queue Management (AQM) based on nonlinear difference equations has been proposed to solve the end-to-end TCP network congestion problem recently. The proposed AQM scheme can guarantee the stability of the multiple bottleneck network by nonlinear control for dropping probability of the routers. Nonlinear control often relies on some heuristics and network traffic controllers that appear to be highly correlated with the multiple bottleneck network state. Based on the proposed nonlinear difference equations for TCP flows control across the network, this paper reveals the reasons of congestion of multiple bottleneck AQM, and provide three theorems for avoiding network congestion. Moreover, we give simulations to verify the results for nonlinear control of the multiple bottleneck network congestion.

This paper develop a low complexity composite CDMA system based on MIMO (Multiple-Input-Multiple-Output) processing and LDPC(low-density parity-check) codec based CDMA systems, involved the complicated case of multipath, multiusers and short length LDPC codes. To explore the practical application, the mobile user in the composite CDMA systems is with only two antennas and adopts short length irregular LDPC codec, each user's data has been divided two LDPC encoded substreams being sent to two transmitter antennas at base station. Since the LDPC encoded substreams of reaching the mobile user are orthogonal each other in space and time, the CDMA system performances (BER and SINR) can be improved much, but the multipath may ruin the orthogonal property. To solve the problems, the paper provides the algorithms of main function modules of transmitter and receivers, gives a simple method to test the girth of LDPC codes, and analyzes the performance of MIMO-LDPC CDMA systems theoretically and experimentally. The simulation results show that the hybrid CDMA systems can be of better performance than the conventional CDMA systems based on single transmitted antenna at a base station.

This letter presents a necessary and sufficient condition for a class of quasi-cyclic low-density parity-check (QC LDPC) codes without girth four. Girth-four property of a class of QC LDPC codes is investigated. Good QC LDPC codes without girth four can be constructed by selecting proper shifting factors according to the proposed theorems. Examples are provided to verify the theorems. The simulation results show that the QC LDPC codes without girth four achieve a better BER performance compared with that of randomly constructed LDPC codes.

The necessary and sufficient conditions of Schur stability of polytopes of bivariate polynomials have been established. Based on a simplification, the two-dimensional (2-D) analysis for stability of polytopes of 2-D polynomials is turned into that of polytopes of one-dimensional (1-D) polynomials with complex variable coefficients. We reveal that the uncertain coefficients of the 2-D polytopes are of linear affine property, then we showthat the stability of a polytope of bivariate polynomials can be guaranteed by that of finite edge polynomials of the polytope. An algorithm about the stability test of edge polynomials is provided.