Terence D. Todd

Dynamic slot allocation (DSA) in indoor SDMA/TDMA using a smart antenna basestation

We introduce and study the use of dynamic slot allocation (DSA) in packet-switched space-division-multiple-access (SDMA) systems. In conventional SDMA, a smart antenna is used at the basestation to simultaneously communicate with multiple stations on the same frequency channel. When dynamic slot allocation is added, the basestation uses uplink channel measurements to intelligently construct future SDMA/TDMA frames. It is shown that under a simple minimum signal-to-interference-plus-noise ratio (SINR) constraint, the problem of performing optimal dynamic slot allocation is NP-complete. Heuristic slot allocation algorithms are introduced which are capable of greatly increasing SDMA/TDMA frame capacity compared with a random allocation of stations. The paper uses both theoretical results and measured data from an experimental testbed to characterize the performance of dynamic slot allocation. The experimental system operates at a carrier frequency of 1.86 GHz and uses an eight-element circular antenna array. It is demonstrated that significant increases in system capacity are possible using DSA in the indoor situations that were tested. Dynamic slot allocation requires the channel to be essentially constant from the time that channel measurements are made until the SDMA/TDMA frame is transmitted. We also present channel measurements which show the effects of channel time coherence in the presence of indoor pedestrian movement. This and other results we have taken suggest that dynamic slot allocation is possible at the frequency considered, provided turnaround times are in the low-to-mid tens of milliseconds.

Ad hoc networks with smart antennas using IEEE 802.11-based protocols

Smart antennas have been studied extensively for use in cellular radio base station applications. Recently however, low cost array technologies have suggested that adaptive antennas may soon be cost effective for mobile ad hoc networks. In this paper we consider the potential use of adaptive antenna arrays in networks using protocols based on the IEEE 802.11 distributed coordination function (DCF). In the system under study, omnidirectional RTS/CTS exchanges are used to initiate array-mode data packet transmissions. Several variations on the basic protocol are considered. When two stations communicate using their antenna arrays, the ensuing gain across the link can be very large. In many cases the transmit power can be significantly reduced while still maintaining a sufficient link margin. We show that this reduction in power is a key factor in improving the capacity of an ad hoc network. Results are presented for various parameters which show how the capacity of the system scales with the size of the system. Significant capacity improvements are possible compared with a network using conventional IEEE 802.11 protocols. In our simulations a relatively inexpensive circular antenna array configuration is used with a fairly modest number of elements.

Handoff trigger nodes for hybrid IEEE 802.11 WLAN/cellular networks

Future mobile handsets will often be multi-mode, containing both wireless LAN (WLAN) and cellular air interfaces. Vertical handoffs will commonly be used to pass voice calls to a cellular network when the user roams outside of WLAN radio coverage. Unfortunately, the transition from WLAN hotspot to cellular coverage is often very abrupt and leads to unacceptable call dropping rates. In this paper we propose and investigate the use of explicit WLAN/cellular handoff triggering. A simple Wi-Fi handoff trigger node (HTN) can be installed in the WLAN/cellular transition region, and generates link layer triggers which cause the initiation of the vertical handoff process. A key function provided by the HTN is to significantly reduce the call dropping rate even when there is very little collaboration between the cellular and WLAN hotspot providers. Results are presented which show that the call dropping probability can be dramatically reduced by the use of a handoff trigger node.

The need for access point power saving in solar powered WLAN mesh networks

Wireless LAN mesh networks are now being used to deploy Wi-Fi coverage in a wide variety of outdoor applications. In these types of networks, conventional WLAN mesh nodes must be operated using continuous electrical power connections. This requirement may often be very expensive, especially when the network includes expansive outdoor wireless coverage areas. An alternative is to operate some of the WLAN mesh nodes using an energy sustainable source such as solar or wind power. This eliminates the need for a fixed power connection, making the node truly tetherless and allowing more flexibility in node positioning. In this article we first review the background and recent activities in the area of energy sustainable WLAN mesh networks. These types of networks are provisioned geographically, in that the assigned resources are a function of the geographic region where the network is to be deployed. The theory behind this is briefly described using some sample North American locations. We then discuss the current shortcomings of IEEE 802.1 1 when used in these types of networks. IEEE 802.11 requires that the access point be continuously powered, and this requirement is a major barrier to deploying cost-effective sustainable energy networks in certain applications. Recent work is then reviewed that has begun to address the changes that would be required to the standard to better support these types of networks.

Resource Allocation and Outage Control for Solar-Powered WLAN Mesh Networks

In this paper, resource allocation and outage control are considered for solar-powered WLAN mesh networks. Solar-powered nodes are a very cost effective option in WLAN mesh deployments where continuous power sources are not practical. In such nodes, the cost of the solar panel and battery can be a significant fraction of the total and, therefore, reducing AP power consumption is very important. A solar panel/battery configuration methodology is introduced based on a proposed AP power-aware version of IEEE 802.11. Public meteorological data is used to provision each node based on an averaged offered capacity profile. Since a node is configured statistically, it is possible that future loading may result in nonzero outage even when negligible outage is the design target. Control algorithms are introduced which can improve node outage performance by sometimes introducing an access point capacity deficit. Results are presented which show the value of the proposed configuration methodology and show that the control algorithms can prevent outage even at high levels of excess loading.

Power saving access points for IEEE 802-11 wireless network infrastructure

In the past decade, there has been a huge proliferation of wireless local area networks (WLANs) based on the IEEE 802.11 WLAN standard. As 802.11 connectivity becomes more ubiquitous, multihop communications will be increasingly used for access point range extension and coverage enhancement. In this paper, we present a design for an IEEE 802. 11 -based power saving access point (PSAP), intended for use in multihop battery and solar/battery powered applications. These types of APs have many practical applications and can be deployed very quickly and inexpensively to provide coverage enhancement in situations such as campuses, building complexes, and fast deployment scenarios. Unlike conventional wired access points, in this type of system, power saving on the AP itself is an important objective. A key design constraint is that the proposed PSAP be backward compatible to a wide range of IEEE 802.11 functionality and existing wired access points. In this paper, we introduce the protocols required to achieve this compatibility, show the constraints imposed by this restriction, and present performance results for the proposed system.

Indoor SDMA capacity using a smart antenna base station

In this paper we consider the capacity of a set of portable stations sharing a single indoor radio channel. The stations communicate with a base station which is equipped with a smart antenna operating in multibeam SDMA/FDMA mode, Both theoretical models and measured data from an experimental testbed are presented. The experimental system operates at 1.86 GHz and uses an 8 element circular antenna array. This system was built at the Communications Research Laboratory at McMaster University. The paper focuses on the static TDMA network capacity of this system. In particular, we explore the value of performing dynamic slot assignment when constructing the SDMA/TDMA frames. Slot assignment algorithms are introduced which are capable of increasing static system capacity under non-ideal propagation situations. In all cases, optimal SINR beamforming is used to determine the performance of the system. The results presented give clear insights into the network capacity possible in such systems and indicate the value of dynamic slot assignment under time division duplex operation. The results can also be used to motivate the design of media access protocols for these types of networks.

Cellular CDMA capacity with out-of-band multihop relaying

In this paper, we consider the capacity of cellular code division multiple access (CDMA) when there is out-of-band ad hoc traffic relaying. The mobile stations (MSs) are dual-mode, having both ad hoc and cellular CDMA radios. An active MS is free to choose any available relay station (RS) within its ad hoc radio coverage area for dual-hop communication with the CDMA base station (BS). Communications between the RSs and the MSs use bandwidth which is available to the ad hoc radio and does not consume the CDMA capacity. Using this mechanism, CDIVIA interference can be reduced by dynamically selecting RSs which have more favorable CDMA link characteristics. Several relay station selection criteria are considered, namely, ad hoc relaying with low relative interference (ARRI), with best link gain (ARLG), and with shortest distance (ARSD). The relay station selection protocols are compatible with existing wireless local area network (WLAN) standards such as IEEE 802.11. An analytic model is used to compute the effects on uplink and downlink CDMA capacities when out-of-band relaying is added. The results show that very significant capacity improvements are possible by using these criteria compared with conventional CDMA with hard or soft handoff. Ad hoc relaying which dynamically tracks CDMA link quality can achieve greater capacity improvements than that using a distance-based relay station selection. Relaying, which considers both signal and interference conditions, achieves better capacity than that based on signal link quality alone.

A selective CSMA protocol with cooperative nulling for ad hoc networks with smart antennas

In this paper a media access control protocol is proposed for ad hoc network stations with adaptive antenna arrays. The protocol is based on the IEEE 802.11 distributed coordination function (UCF) and uses omnidirectional RTS/CTS exchanges to initiate data packet transmissions. Unlike previous directional antenna MAC protocols, the proposed protocol accommodates the active nulling of co-channel interferers that may arise during the course of ongoing transmissions. This is done using a three-way handshake where neighboring stations cooperate during link activations thus allowing the active receiver to dynamically null potential future interfering packet transmissions. The protocol uses omnidirectional transmission combined with beamformed reception, and is referred to as selective CSMA with cooperative nulling (SCSMA/CN). Packet type identification is used to enable/disable carrier sensing based upon whether or not existing transmissions are protected by antenna beamforming. Simulation results are presented which show that the proposed protocol can achieve higher capacity than previous comparable MAC protocols. The results also include the effects of transmit power control on system performance.

Multi-constraint QoS routing using a new single mixed metric

Multi-constraint quality-of-service routing becomes increasingly important as the Internet evolves to support real-time services. It is well known however, that optimum multi-constraint QoS routing is computationally complex, and for this reason various heuristics have been proposed for routing in practical situations. Among these methods, those that use a single mixed metric are the most popular. Although mixed metric routing discards potentially useful information, this is compensated for by the significantly reduced complexity. Exploiting this tradeoff is becoming increasingly important where low complexity designs are desired, such as in battery operated wireless applications. In this paper, a novel single mixed metric multi-constraint routing algorithm is introduced. The proposed technique has similar complexity compared with existing low complexity methods. Simulation results are presented which show that it can obtain better performance than comparable techniques in terms of generating feasible multi-constraint QoS routes.