Jimmi Grönkvist

Comparison between graph-based and interference-based STDMA scheduling

Spatial reuse TDMA is a fixed assignment access scheme for multi-hop radio networks. The idea is to increase network capacity by letting several radio terminals use the same time slot when the interferences caused are not too severe. We consider two methods of generating traffic controlled reuse schedules. One method uses full knolwedge of the interference environment to generate schedules. The other method uses a graph representation of the network, assuming limited knowledge of the interferences. By simulations, we evaluate the proposed methods in terms of average delay and throughput. The simulation results indicate that the network performance of the graph-based scheduling may suffer compared to the interference-based scheduling, depending on how the graph is created. In a stationary situation, or temporary stationary situation, where knowledge of the full interference environment can be assumed, interference based scheduling can improve the network capacity by up to one thrid, thereby being worth its increased complexity

On the performance of antenna arrays in spatial reuse TDMA ad hoc networks

We have investigated how, the capacity and average delay can be improved by using antenna arrays in an ad hoc network with Spatial Time Division Multiple Access (STDMA). The study is based on different antenna combinations consisting of a single isotropic antenna element, beam steering and adaptive beamforming. We have also studied how the number of antenna elements and the terrain affects the performance. The study shows that the maximum capacity is improved dramatically for the examined networks, with up to 980%. In addition, the average delays are decreased substantially when using antenna arrays. Depending on the beamforming combination used, the capacity gain and the average delay reduction will differ. The highest capacity gain is achieved when combining beam steering (transmitting node) and adaptive beamforming (receiving node). For broadcast traffic omni-directional transmission is most feasible (no transmit beamforming); however,,ever, beamforming is still possible in the receiving nodes. For this beamforming combination the capacity gain is significant, about 560%. The study also indicates that the benefit from antenna arrays is higher in aflat terrain than in a rough terrain.

Novel assignment strategies for spatial reuse TDMA in wireless ad hoc networks

Spatial reuse TDMA has been proposed as an access scheme for multi-hop radio networks where real-time service guarantees are important. The idea is to increase capacity by letting several radio terminals use the same time slot when possible. A time slot can be shared when the radio units are geographically separated such that small interference is obtained. In reuse scheduling, there are several alternative assignment methods. Traditionally, transmission rights are given to nodes or to links, i.e., transmitter/receiver pairs. We present a comparison of these two approaches and show that both have undesirable properties in certain cases, e.g. link assignment gives a higher delay for low traffic loads but can achieve much higher throughput than node assignment. Furthermore, we propose a novel assignment strategy, achieving the advantages of both methods. Simulation results show that the proposed method can achieve the throughput of link assignment for high traffic loads as well as the lower delay characteristics of node assignment for low traffic loads.

Distributed scheduling for mobile ad hoc networks - a novel approach

Spatial reuse TDMA (STDMA) is a collision-free access scheme for ad hoc networks. The idea is to let spatially separated radio terminals reuse the same time slot when the resulting interferences are not too severe. In this paper we first describe the properties a distributed STDMA algorithm must have in order to be efficient. No existing algorithm fulfills all of these properties. Second we focus on how to efficiently use distributed information and describe an algorithm that can handle different amount of information. Furthermore, we evaluate this algorithm for different information and show that it can give the same capacity as a centralized reference algorithm.Spatial reuse TDMA (STDMA) is a collision-free access scheme for ad hoc networks. The idea is to let spatially separated radio terminals reuse the same time slot when the resulting interferences are not too severe. In this paper we first describe the properties a distributed STDMA algorithm must have in order to be efficient. No existing algorithm fulfills all of these properties. Second we focus on how to efficiently use distributed information and describe an algorithm that can handle different amount of information. Furthermore, we evaluate this algorithm for different information and show that it can give the same capacity as a centralized reference algorithm.

A comparison of access methods for multi-hop ad hoc radio networks

For mobile radio networks without guaranteed connections to fixed infrastructure, simple non-centralized network control is desirable in order to reduce the administrative traffic in the network. In this paper, we analyze and compare two asynchronous non-centralized access schemes in the case of multi-hop ad hoc networks. The first is a new protocol based on time-hopping code division multiple access (TH-CDMA), and the second is based on carrier-sense multiple access (CSMA). The new ultra-wideband (UWB) impulse radio technique makes the first scheme an interesting option. A third scheme, a synchronous spatial time-division multiple access (STDMA) scheme is also included in the comparison as a reference.

Joint Node and Link Assignment in an STDMA Network

Spatial re-use TDMA (STDMA) is a medium access control (MAC) protocol which allows multiple transmitter-receiver pairs to communicate simultaneously, as long as they do not interfere too much with each other. Node and link assignment strategies for spatial re-use TDMA networks have previously been studied independently. The metric chosen in these studies is the uniform capacity, the minimum rate at which any node can communicate with any other node in the network. It has been shown that for large frame lengths, node assignment leads to lower spatial re-use than link assignment, resulting in lower uniform capacity than link assignment. In this paper, we first explore the behaviour of node and link assignment at small frame lengths. We show that node assignment achieves non-zero throughput values at lower frame lengths than link assignment, and that it outperforms link assignment at small frame lengths. This motivates the need for an assignment strategy that performs well for all values of the frame length. We propose a joint node-link assignment strategy and show that it performs as well as node assignment at small frame lengths and as well as link assignment at large frame lengths, while exceeding the performance of both node and link assignment at intermediate frame lengths.

Architectural issues for integrating tactical radio access networks in civilian infrastructure

We investigate architectures where a tactical radio subsystem can integrate in civilian systems. The idea is that civilian communication infrastructure should be used wherever it is available. One such possibility may be found in the proposed next generation wireless communication system UMTS, where the definition of a radio network subsystem (RNS), encapsulating all the features of wireless mobile communication has been proposed. We adopt this design paradigm to see if it is possible to find a low cost military network (by maximal use of civilian network technology and communication applications). The idea is to within the framework of an RNS, encapsulate also features specific to tactical wireless communications, a tactical radio access network (TRAN). Based on UMTS we describe two system concepts for designing a TRAN and analyze the architectures.

Comparing the capacity of cooperative broadcast to spatial reuse TDMA in ad hoc networks

Cooperative broadcasting is a promising technique for efficient as well as robust broadcast of information in ad hoc networks. The paper compares the capacity of scheduled cooperative broadcasting with the capacity of Spatial Reuse TDMA (STDMA) for broadcast traffic in ad hoc networks. Three different routing methods for STDMA are considered: a tree-based solution; default MPR-flooding; and a robust version of MPR-flooding obtained by increasing the MPR coverage parameter. The results show that scheduled cooperative broadcasting, besides being robust, also has a higher capacity than STDMA unless the networks are dense, i.e., many neighbors and short average hop lengths. In that case STDMA is efficient as a packet can be broadcasted to the whole network with just a few retransmissions.

Performance Analysis of Reuse Distance in Cooperative Broadcasting

Cooperative broadcasting is a promising technique for robust broadcast with low overhead and delay in mobile ad hoc networks. The technique is attractive for mission-oriented mobile communication, where a majority of the traffic is of broadcast nature. In cooperative broadcasting, all nodes simultaneously retransmit packets. The receiver utilizes cooperative diversity in the simultaneously received signals. The retransmissions continue until all nodes are reached. After the packet has traveled a specific number of hops out from the source, denoted as reuse distance, the source node transmits a new broadcast packet in the time slot used for the previous broadcast packet. If the reuse distance is too small, interference causes packet loss in intermediate nodes. In the literature, a reuse distance of three is common. With an analysis based on a realistic interference model and real terrain data, we show that a reuse distance of at least four is necessary to avoid packet loss in sparsely connected networks, especially for high spectral efficiencies. For frequency hopping, widely used in military systems, we propose a novel method. This method almost eliminates interference for a reuse distance of three, increasing the throughput by 33% compared to systems with a reuse distance of four.

Measurement-Based Analysis of Two-Hop Cooperative Relaying

For tactical military communications, ad hoc networks are an attractive choice because they can provide good area coverage without the vulnerabilities of any central node or base station. Their performance is, however, strongly dependent on the ability to maintain link quality information, and to distribute the changing routing tables, in a timely and efficient manner in highly mobile scenarios. A relatively new concept of cooperative relaying, in which several relay nodes simultaneously retransmits identical copies of the message on the same channel, is promising as it eliminates the need of routing. In this paper, we analyze the performance of an OFDM based cooperative simultaneous relaying scheme for two-hop networks. The networks are simulated based on measured channels for the individual links. The results are compared with the performance for the traditional single relay technique with optimal and suboptimal route information. The results show that the performance of the cooperative relaying scheme in the investigated scenario is equal to, or better than, the performance for the best route for the single relay scheme in a large majority of the simulated networks. However, some fraction of the networks shows a worse result for the cooperative scheme. This fraction decreases as the bandwidth increases. Furthermore, the performance for the cooperative relay scheme show significant increase in performance when compared to the single relay scheme for the next-best route. Moreover, we observe that the performance of the cooperative relay scheme, in general, is non-reciprocal with respect to the direction of transmission for a given pair of terminal nodes.