Spectral Efficiency of Dynamic Time Division Duplex Fixed Wireless Cellular System for Asymmetric and Dynamic Multimedia Traffic

Recently, wireless multimedia services have been growing because of the spread of various wireless applications. Hence, time division duplex (TDD) systems and their crossed-slot interference problems in a cellular environment have been attracting growing interests. Considering large signaling overhead between cells, decentralized time slot allocation (TSA) strategy is suitable for practical implementation. Thus, Fixed-TSA strategy which fixes the same ratio of uplink and downlink time slots in all cells is adopted in commercial WiBro systems (IEEE802.16e Mobile WiMax) to mitigate the crossed-slot interference. However, the Fixed-TSA strategy reduces the flexibility in time slot allocation due to a strong constraint set by a predefined boundary. Moreover, the mathematical derivation of the optimal value for the predefined boundary has not been presented yet. In this paper, we propose two decentralized TSA strategies. The first one is Enhanced Fixed-TSA strategy, which dynamically adapts the predefined boundary of the conventional Fixed-TSA strategy according to traffic conditions. The second one is region-based Decentralized Time Slot Allocation (RED-TSA) strategy, which utilizes partial location information of MSs to reduce crossedslot interference. We fully analyze the proposed TSA strategies in terms of new call blocking probability, average bit error probability, and the overall system throughput. Numerical results show that the proposed RED-TSA strategy provides the highest system throughput by compromising both new call blocking performance and average bit error performance, whereas it requires additional location information and complicated computation. On the contrary, the proposed Enhanced Fixed-TSA strategy requires reasonable computational complexity while it provides almost the same system throughput with that of the proposed RED-TSA strategy. In a nutshell, the proposed Enhanced Fixed- TSA strategy is more appropriate for the practical systems. Moreover, it can be directly applied to the current commercial WiMax/WiBro systems with minimum changes.

Dynamic heterogeneous networks (HetNets) in which small cells operate in dynamic time division duplex (D-TDD) systems enable the instantaneous traffic load to be handled with greater spectral efficiency than conventional HetNets. However, combining HetNets and D-TDD systems causes cross-link interference (CLI) both within the same tier and across tiers, negatively impacting system performance. In this paper, we point out that small cells are underprivileged in terms of interference mitigation under the conventional spectrum partitioning usage. Accordingly, we propose employing a novel spectrum partitioning scheme that brings up interference scenarios where we can take advantage of multiple-input multiple-output (MIMO) compatibility from macro base stations. Simulation results show that the proposed spectrum usage in combination with general MIMO schemes achieves significant performance enhancement in terms of small cell capacity at the cost of a relatively small degradation in macro cell capacity.

Dynamic time-division duplex (D-TDD) aided mobile communication systems bear the potential to achieve significantly higher spectral efficiency than traditional static TDD based systems. However, strong cross-link interference (CLI) may be caused by different transmission directions between adjacent cells in D-TDD systems, thus degrading the performance. Most existing CLI mitigation schemes require sharing certain information among base stations (BSs) via backhaul links. This strategy is usually expensive and suffers high latency. Alternatively, we propose a pilot information sharing scheme based on over-the-air forwarding of the downlink pilot of the interfering BS to the interfered BS via a wireless terminal, along with a dedicated CLI channel estimation method. Simulation results demonstrate that thanks to the proposed pilot information sharing scheme the classic interference rejection combining (IRC) receiver achieves a signal detection performance highly comparable to that of the IRC detector with perfect pilot information, necessitating no information sharing among BSs via backhaul links. Furthermore, the proposed CLI channel estimation scheme reduces the impact of errors introduced by pilot forwarding, thereby improving the performance of both CLI channel estimation and signal detection.

The code division multiple access system with time division duplex mode (CDMA/TDD) where the same frequency allocation is made for both uplink and downlink is a good solution when the traffic asymmetry between uplink and downlink exists. However, when the asymmetry of each cell may be significantly different from each other, the traffic may experience severe interference from each other when each cell's own slot allocation is made according to the level of traffic asymmetry in each cell. The interference in the W-CDMA/TDD system when the slot allocations are made according to traffic patterns of each cell is analyzed, and a time slot allocation scheme considering the traffic asymmetry is proposed. In this paper, system performances are simulated under the proposed time slot allocation strategy and its optimal operation of the strategy is investigated.

Dynamic time division duplexing (TDD) is envisioned as a vital transmission technology of the 5G new radio, due to its reciprocal propagation characteristics. However, the potential cross-link interference (CLI) imposes a fundamental limitation against the feasibility of the ultra-reliable and low latency communications (URLLC) in dynamic-TDD systems. In this work, we propose a near-optimal and complexity-efficient CLI suppression scheme using orthogonal spatial projection, while the signaling overhead is limited to B-bit, over the back-haul links. Compared to the state-of-the-art dynamic-TDD studies, proposed solution offers a significant improvement of the URLLC outage latency, e.g., $\sim -199$% reduction, while boosting the achievable capacity per the URLLC packet by $\sim +156$%.

The Third Generation Partnership Project (3GPP) has been studying dynamic allocation of sub-frames to uplink (UL) or downlink (DL) in Time Division Duplex (TDD), called ‘Dynamic TDD,’ since the Long Term Evolution (LTE) Rel. 11 timeframe. At the same time, 3GPP is also standardizing Enhanced Local Area (eLA) small-cell heterogeneous architectures for inclusion in LTE-B (LTE Rel. 12) as a solution offering high data rate to user terminals (UEs) along with high system capacity through spatial reuse of spectrum. In this paper, we focus on a particular eLA architecture proposed by DOCOMO, called the Phantom Cell architecture, that has the option to support dynamic TDD. For an arbitrarily-located UE in an eLA cell network, we apply results from stochastic geometry to derive expressions for the distribution of DL signal to interference plus noise ratio (SINR) at an arbitrary UE and the distribution of UL SINR at its serving eLA base station (BS). We use these results to study aspects of eLA cell system design, and the sensitivity of SINR to the extent of coordination across eLA cells employing dynamic TDD.

Under dynamic time division duplexing (D-TDD) architecture in OFDM cellular systems, the inter-BS and inter-MS interference is inevitable during the cross time slot (CTS) period, and this interference seriously degrades the wireless access system performance. To mitigate such interference, we propose a region and time partitioning D-TDD architecture for OFDM systems. Each time slot in the CTS period is split into several minislots, and then each cell is divided into as many regions as the number of minislots per time slot. We then assign each minislot only to the users in its corresponding region. On top of such architecture, which inherently separates interfering entities farther from each other, we design a robust time slot allocation scheme considering the channel status of each mobile station. By computer simulation, it has been verified that the proposed scheme outperforms conventional time slot allocation methods in both the outage probability and the bandwidth efficiency.

We consider a dynamic time division duplex (DTDD) enabled cell-free massive multiple-input multiple-output (CF-mMIMO) system, where each half-duplex (HD) access point (AP) is scheduled to operate in the uplink (UL) or downlink (DL) mode based on the data demands of the user equipments (UEs), with the goal of maximizing the sum UL-DL spectral efficiency (SE). We develop a new, low complexity, greedy algorithm for the combinatorial AP scheduling problem, with an optimality guarantee theoretically established via showing that a lower bound of the sum UL-DL SE is sub-modular. We also consider pilot sequence reuse among the UEs to limit the channel estimation overhead. In CF systems, all the APs estimate the channel from every UE, making pilot allocation problem different from the cellular case. We develop a novel algorithm that iteratively minimizes the maximum pilot contamination across the UEs. We compare the performance of our solutions, both theoretically and via simulations, against a full duplex (FD) multi-cell mMIMO system. Our results show that, due to the joint processing of the signals at the central processing unit, CF-mMIMO with dynamic HD AP-scheduling significantly outperforms cellular FD-mMIMO in terms of the sum SE and 90% likely SE. Thus, DTDD enabled HD CF-mMIMO is a promising alternative to cellular FD-mMIMO, without the cost of hardware for self-interference suppression.

Average transmission time delay is an important parameter to measure UAV ad hoc network performance. In TDMA system, time slot allocation scheme leads to structure time delay (STD) which is an important factor to influence network transmission time delay. In order to optimize structure time delay, this paper firstly studies the structure of TDMA transmission frame which is universally adopted by UAV ad hoc network, and then takes chain topology as basic study object, and analyzes the average STD of chain topology after optimization. Using genetic algorithm, this paper proposes a method to solve time delay problems caused by time slots structure for general networks. Simulation results show that, compared with traditional time slots allocation schemes, the algorithm put up by this paper could reduce the STD up to about 80%. The research results could provide a valuable reference for the design of UAV ad hoc network.KeywordsUAV Ad hoc networkTime slot allocationStructure time delayGenetic algorithm

This paper presents the design of a compact, short range, conferencing direct sequence spread spectrum radio for applications in both specialized and commercial communications. The design aims to be simple to achieve low cost. Conferencing is achieved through packetisation of digitised speech which allows time division multiple access. There is no time slot allocation scheme but collision avoidance of the packets is achieved through sensing of incoming packets. Direct sequence spread spectrum is used to enhance system robustness and to cater for future implementation of CDMA.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In this paper, we consider a satellite communication system which consists of a number of source low earth orbit (SLEO) satellites, a number of relay low earth orbit (RLEO) satellites and a destination ground station (GS). Under the assumption that data flows can be split while transmitted through the system, a joint route selection and time-slot allocation algorithm is proposed. Considering the constraints on the transmission requirements of data flows, system resource allocation and the amount of data flows, the joint route selection and time-slot allocation problem is formulated as an optimization problem which minimizes total system energy consumption. As the original optimization problem is NP-hard, which cannot be solved conveniently, the optimization problem is equivalently converted into candidate link selection subproblem and route selection subproblem. The two optimization problems are solved by means of the K-shortest path algorithm and the knapsack algorithm, respectively, and the optimal strategy of joint route selection and time-slot allocation is obtained. Simulation results demonstrate the effectiveness of the proposed scheme.

The Internet of Things (IoT) aims to interconnect and converge the physical world and cyberspace. It represents the future of networking, and leads the wave of the third industrial revolution. As IoT applications evolve, more and more regions lacking infrastructures are also experiencing an urgent need to deploy and connect IoT devices into the network. Low earth orbit satellites, due to its wide coverage and relatively low latency, can provide network access for IoT system. In this article, we propose a low-latency heterogeneous network MAC approach for space-earth integrated IoT network, which can significantly reduce network latency, improve system scalability and balance total system load. Specifically, we design a geographic priority time slot allocation strategy which ensures optimal load and latency, besides, we also design a continuous forward time slot allocation strategy, which guarantees data timeliness while reducing the utilization of satellite resources. Then we design three algorithms based on above strategies, which can effectively reduce network latency and balance the satellite resources. The simulation experimental results are consistent with the theoretical results, which embodies the effectiveness of the algorithm.

IoT device are usually built as edge networks in smaller areas, but with the development of technology, more and more areas without infrastructure need to be covered. LEO satellites, due to its full coverage of the Earth and relatively low latency, can provide network access for IoT system. In this paper, we propose a hybrid TDMA MAC protocol LST-MAC for IoT network based on LEO satellite, which can reduce network delay and improve the system scalability. Specifically, we design a geographic priority time slot allocation strategy which can ensure optimal load and delay, besides, we also design a continuous forward time slot allocation strategy, which guarantees real-time data while reducing the utilization of satellite resources. Then we design two algorithms based on both strategies, which can effectively reduce network delay and balance the satellite resources. The simulation experimental results are consistent with the theoretical results, which embodies the effectiveness of the algorithm.

Currently, most Landsat satellites are deployed in the low earth orbit (LEO) to obtain high resolution data of the Earth surface and atmosphere. However, the return channels of LEO satellites are unstable and discontinuous intrinsically resulting from the high orbital velocity, long revisit interval and limited ranges of ground-based radar receivers. Space-based information networks (SBIN), in which data can be delivered by the cooperative transmission of relay satellites, can greatly expand the spatial transport connection ranges of LEO satellites. While different types of these relay satellites deployed in orbits of different altitudes represent distinctive performances when they are participating in forwarding. In this chapter, we consider the cooperative mechanism of relay satellites deployed in the geosynchronous orbit (GEO) and LEO according to their different transport performances and orbital characteristics. To take full advantage of the transmission resource of different kinds of cooperative relays, we propose a multiple access and bandwidth resource allocation strategy for GEO relay, in which the relay can receive and transmit simultaneously according to channel characteristics of space-based systems. Moreover, a time slot allocation strategy which is based on the slotted time division multiple access is introduced for the system with LEO relays. Based on the queueing theoretic formulation, the stability of the proposed systems and protocols is analyzed and the maximum stable throughput region is derived as well, which provide the guidance for the design of the system optimal control. Simulation results exhibit multiple factors that affect the stable throughput and verify the theoretical analysis.KeywordsSpace-based Information NetworksCooperative CommunicationResource AllocationQueueing TheoryStability Analysis

In the Time Division Multiple Access (TDMA) Ad Hoc network system, time slot allocation is one of the most important factors affecting the performance of the network. A good time slot allocation algorithm can effectively reduce the average delay and delay jitter of the network, thus improve the network efficiency. This paper proposes an uniform time slot allocation scheme for TDMA-based Ad Hoc network based on genetic algorithms. The scheme aims at searching out the optimal one in the all feasible solutions for the time slot allocation by the genetic algorithms. The simulation results show that the proposed algorithm is correct and effective.