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High Capacity Hotspots Based on Bluetooth Technology

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High Capacity Hotspots Based on Bluetooth Technology John Dunlop and Nathan Amanquah Mobile Communications Group University of Strathclyde Glasgow G1 1XW, Scotland Abstract Bluetooth is a short range wireless interface that offers data transmission rates of the order of 721 kb/s which is comparable with ADSL data rates, but which could be regarded as being too low for deployment as a limited range wireless access point. The capacity of a wireless access point based on Bluetooth technology can be
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  High Capacity Hotspots Based on Bluetooth Technology John Dunlop and Nathan AmanquahMobile Communications GroupUniversity of StrathclydeGlasgow G1 1XW, Scotland Abstract Bluetooth is a short range wireless interface that offers data transmission rates of the order of 721 kb/s which is comparable with ADSL data rates, but which could be regarded as beingtoo low for deployment as a limited range wireless access point. The capacity of a wirelessaccess point based on Bluetooth technology can be increased significantly by co-locating anumber of Bluetooth transducers in a hotspot scenario. This paper describes a techniquewhich reduces mutual interference in co-located Bluetooth transducers by coordinating thehopping frequencies of the individual devices. Such a system can then provide attractive datatransmission rates when deployed in wireless access point applications. 1 Introduction The concept of service delivery over heterogeneous wireless networks is shown in Figure 1.In this case services may be provided via a 3G cellular link, via wireless hotspots, includinginfostation networks, or via a digital broadcast network (DAB and/or DVB). In fact servicesmay be provided over a single network or over a combination of different (heterogeneous)networks including personal area networks (PAN), which is also indicated in this figure. Toaccommodate such service delivery options a concept known as the Personal DistributedEnvironment (PDE) has been introduced [1] in which the traditional user terminal is replacedby a distributed set of devices which may be local or remote to the user. Such a collection of devices will have a variety of radio interfaces available for connection to the core network including low power Bluetooth and its derivatives.Consequently there is growing interest in deploying short range wireless access pointsbecause they offer much higher throughput at reasonable cost than cellular counterparts.Wireless LAN access points based on 802.11 currently dominate this sector, but severalscenarios can be envisaged where alternative access can be acquired via low range low powerdevices such as Bluetooth. In a Personal Distributed Environment, for example, a user’sdevices are likely to be connected to the core network by means of several heterogeneousaccess technologies and access networks. This has the prospect of reducing service disruptionand also facilitates cooperation between these networks for service delivery.Bluetooth is therefore a potential access point technology that could be combined with othertechnologies to provide a heterogeneous component for service delivery. However there are anumber of problems associated with deployment of Bluetooth access points which must beaddressed. The Bluetooth interface can support up to 7 clients per access point, which islimited compared to the number of potential clients that may be co-located, for examplepassengers in a railway carriage. Furthermore, an increasing number of concurrent users hasa negative impact on the attainable throughput as the available capacity is shared between thenumber of connected clients. Capacity may be increased by co-locating access points, but asthe Bluetooth radio interface is based on frequency hopping it is evident that interference willbe present whenever two or more co-located Bluetooth devices choose the same hopping- 1 -  frequency. This problem and a method of minimising its effect are the topics considered inthis paper. 2 The Bluetooth Radio Interface The basic unit of a Bluetooth system is a piconet, which consists of a master node and up toseven active slave nodes within a distance of approximately 10 metres. Figure 2 shows ascatternet consisting of two piconets, and an example of a slave device shared between twodifferent piconets in the scatternet..The node that initiates a connection, by sending either a PAGE or INQUIRY message,becomes the master of the piconet that is formed. Bluetooth devices operate in the ISM(Industrial, Scientific, and Medical) band at 2.4 GHz. Most of Europe and the USA haveallocated the space between 2400 and 2483.5 MHz for this band and Bluetooth devices use 79channels within this band, each occupying 1 MHz. Bluetooth uses a frequency hopping, timedivision duplexing (TDD) scheme for each channel, where the master device determines thefrequency hopping scheme used and also sets the piconet clock. The frequency-hoppingscheme is determined by a cyclic code of length 2 27 - 1, hopping at a nominal rate of 1600hops per second. Transmissions are performed in 625 µ  s slots, with a single packet beingtransmitted per slot; the frequency used for transmission does not change during thetransmission of a packet. However, packets are allowed to be transmitted over multiple slots,allowing asymmetric connections. When a packet occupies multiple slots, the transmittingfrequency stays the same, dropping the hopping rate below 1600 hops per second. The TDDmechanism is implemented by alternating the master and slave transmission slots, with themaster transmitting in even-numbered slots, and slave(s) transmitting in odd numbered slots.In order to maintain this scheme, packets can last one, three, or five slots. A single Bluetoothunit may send/receive at a maximum data rate of 721 kb/s or a maximum of 3 dedicatedvoice channels of 64 kb/s each.Bluetooth has two link types, Synchronous Connection-Oriented (SCO) and AsynchronousConnection-Less (ACL) links. A SCO link is a symmetric, dedicated link between twodevices. In effect, this is a circuit-switched connection, although the actual transmissions arebased on a packet format. The ACL link is an asynchronous link that uses those slots in apiconet that are not dedicated to a SCO link, this is effectively a packet-switched connection.In ACL links, a slave is limited to transmitting to the master only in the slot directly after theslot where the master addressed this particular slave. The master controlled methodology,while it seems restrictive, can lead to an efficient use of the spectrum occupied by the piconet,since a single entity controls the usage of the channel and the probability of collisionsbetween members of the piconet is zero. Also the dynamic nature of the master/slavearchitecture allows devices to establish themselves as masters in a piconet very easily. Thisdynamic behaviour provides devices that need to transmit information with the ability toestablish a link and transmit its information when the application requires it. Broadcastmessages to the whole piconet are possible using an ACL link. Another benefit of ACL linksis that if the master has no information to send, and no polling is taking place, then thechannel can be idle.In principle two SCO connections to an access point take up all available capacity. Thisindicates the need to deploy multiple access points in a single location if, for example, morethan two audio sessions and other data sessions are to be supported. This paper considers the- 2 -  transmissions in terms of packet length and thus the results presented are applicable to bothSCO and ACL links. 3 The Coordinated Co-located Access Point Scheme The co-located devices forming a Bluetooth access point are essentially the masters of thepiconets which they form. The objective is to increase throughput in a hotspot whilstminimising interference between the co-located devices which form the access point. Thismay be achieved by synchronising the frequency hopping of the co-located devices whichform the access point. This arrangement, is referred to as a Coordinated Co-located AccessPoint (CCAP) scheme. The basic principle is to establish a condition where no two devicesuse the same frequency at the same time, in a given location, and this can be achieved if themaster nodes hop in tandem using the same hopping sequence, but separated in time by aconstant offset, as shown in Figure 1. This is equivalent to applying a frequency offsetbetween co-located devices which means that mutual interference can, in principle, be totallyeliminated.An assessment of co-channel and adjacent channel interference in Bluetooth systems has beengiven by Sousissi [2], but this does not consider methods for interference reduction orelimination. Coexistence issues between Bluetooth and other ISM band devices has beenconsidered by several authors [3,4,5], but this work concentrates on the potential for reducinginterference within Bluetooth systems with co-located devices operating in realistic scenarios. 4 Coordination of Hop Frequency Selection The frequency hopping sequence is unique for a Bluetooth piconet and is determined by twoinputs to the Hop Frequency Selection Kernel (FHKernel) embedded in each device. Theseare the Bluetooth device address (BD_ADDR) of the master, and an input derived from theBluetooth clock of the master which determines the position in the hopping sequence. Theclock, which has a period of 312.5 µ s (0.5 × slot duration), is applied to a 28 stage counter,which consequently has a cycle time of about 23.30 hrs. The input to the FHKernel is the 27most significant bits or all 28 bits of the counter (depending on the hopping sequence or sub-state of the Bluetooth device), and least significant 28 bits of the 48 bit Bluetooth address.The requirement is to produce hopping sequences that will not mutually interfere, asillustrated in Figure 3. It would not be possible to generate the desired non interfering sets of hop sequences by supplying as input values to the FHKernel arbitrary sets of {address, clock-counter}. Two methods which have been considered to produce the appropriate hoppingsequences for co-located devices. One is to use a single pair of values for {address, clock-counter}and then add multiples of a fixed frequency offset to each output. The other is tochange the {address, clock-counter} input values by adding offsets to the clock counter value. 4.1 Use of a fixed offset In this case frequency offsets are added to the identical hop frequencies generated by{address, clock-counter}. It gives the distinct advantage that adjacent interference can becontrolled, because the offsets can be added such that the frequencies chosen are fairly widelyspaced. Another advantage is the ability to use an arbitrary Bluetooth device address as theLead Master BD_ADDR (this is the BD_ADDR shared by all participants). The disadvantageof this method is that the resulting implementation will be incompatible with the largeexisting installed base of the Bluetooth wireless interface. In particular a new slave device- 3 -  would not know what the specific offset is and additional signalling would be required to passthe value to the client 4.2 Change in the Frequency Hop Kernel Input Values In this case, the inputs to the FHKernel are unrelated (but not arbitrary) sets of {address,clock-counter} values. A common {address, clock-counter} value is applied, with clock offsets added to the clock-counter  value. Thus input sets such as {addr1, clock-counter + δ 1 },{addr1, clock-counter + δ 2 }, {addr1, clock-counter + δ 3 }…etc., are applied to the differentparticipating master nodes. The conceptual model for the modified FHKernel is given inFigure 4. This method preserves compatibility with existing nodes. When a client terminal(slave node) establishes a connection with a master, it will correctly evaluate the next hopfrequency because the master passes to the slave the {addr1, clock-counter} value it has itself used, as dictated by the Bluetooth standard. However it should be noted that arbitrary {addr1,clock-counter} values cannot be used and that specific offsets for the clock counter valueshave to be determined beforehand. It should be further noted that only the nodes used asaccess points (or used in coordinated piconets) need to be modified in this way. All clientnodes can maintain their existing Bluetooth implementation. The preferred method of determining hop frequencies is thus based on adding offsets to the clock counter value. Byusing a coordinated hop frequency selection scheme, guarantees can be provided that mutualinterference will not occur, since no two frequencies will be the same at any point in time.A universally available BD_ADDR of 0x00000 was selected as the address to be used insuch high capacity hotspots. Using this parameter, a set of clock offsets was determined, fromcomputer based iterations, to ensure that no co-channel interference would be experiencedbetween any two co-located piconets using identical packet lengths. 5 Evaluation of the Coordinated Co-located Access Point Scheme  A simulator was constructed to evaluate the CCAP concept in terms of the probability of occurrence of interference, rather than in terms of modelling the Bluetooth radio channelitself, the latter having been evaluated in other studies [6]. In the simulations conducted eachpiconet consisted of a one master and one slave. It should be noted that a single slave issufficient in this case as all master frequency hops will be generated (for both master-to-slaveand slave-to-master transmissions), thus covering all possible frequencies that can be used inany piconet, irrespective of the number of slave nodes. Results for the CCAP scheme arecompared to both asynchronous and synchronised uncoordinated  scenarios. 5.1 Scenarios Simulated The effectiveness of the CCAP has been evaluated by considering possible scenarios thatcould occur in a practical Bluetooth access point deployment, in particular scenarios whichwould involve the use of different packet lengths in the co-located piconets. The scenariossimulated in the evaluation were: ã All nodes use the same packet lengths ã All master nodes use one packet length and all slave nodes use a different packet length ã An increasing proportion of master-slave pairs (piconets) are introduced using a packetassignment different from the other pairs.- 4 -

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