Mobility Management in the Satellite Access Network to UMTS A El-Hoiydi, R J Finean, F da Costa, M Dinis, A Saïdi, B Vazvan EPFL1, BT2, IT, IT, Alcatel CRC, Nokia Abstract: The Universal Mobile Telecommunication System (UMTS) will appeal to the existing cordless, paging and cellular markets and to the emerging satellite personal communications market by allowing standard delivery of a diverse range of communication services to people, no matter where they are. Satellite communications will feature as the preferred mode of access to rural and remote regions as well as being a means to rapidly deploy UMTS service at the initial commercial roll-out of UMTS networks. Race Monet, in conjunction with Race Saint, have developed a mobile network architecture capable of providing UMTS services through a wide variety of satellite networks in the same way as it provides service to a wide variety of terrestrial radio environments (public, business, domestic). This paper presents the features of the UMTS system specification relative to the mobility management in the satellite access network. We give a definition of the location area that allows similar routing and location updating procedures as in the terrestrial cellular case. Finally, several intelligent paging options are presented, together with an analytical comparison of their performances.
Introduction Location update is a function performed in cellular mobile networks to allow the network to page a mobile terminal for an incoming call. Classically, a location area is the smallest area unit used to locate a terminal when it is in idle mode (i.e. not engaged in a call). Within the terrestrial segment, a location area is usually defined as the area (cluster of adjacent cells) wherein the mobile terminal can roam without having to perform a location update. Ultimately, the mobile terminal is known to be reachable through one of those terrestrial cells. The cell site antennas are static and location area reselection can be performed as new cells are selected based on received signal strength at the terminal. For the conception of the mobility management for the space segment, the satellite engineers’ approach was to make minimal changes to the terrestrial derived model and to keep the network model generic, to allow participation in UMTS of a number of different satellite networks. For satellite use, network design needs to account for the possibility that the satellites (acting as cell site antennas) will be moving overhead very fast relative to the mobile terminals (MTs) and to the Fixed Earth Stations (FESs) which act as the point of connection from the mobiles to the core UMTS network. Therefore, it is not possible to permanently associate an area of the Earth’s surface with the radio coverage of a specific spot footprint. This prevents us from using the same location management techniques as the terrestrial cellular component. Another consideration is that satellite bandwidth is expensive and that its use for signalling needs to be minimised. Location Area and its Update A location area is formed by an FES’s instantaneous coverage. An assumption is that each FES will be designed to cover a particular geographic area. Because of the dynamics of a satellite network, the shape of an FES’s total coverage (the surface covered by at least one of all the visible satellites) is changing as the satellites move relative to the Earth. However, for each FES it is possible to define a guaranteed coverage area (GCA) which is the geographic area over which the FES is designed to 1
Swiss Federal Institute of Technology Lausanne, Telecom. Lab., E-mail:
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BT Laboratories, MLB4/67 Martlesham Heath, Ipswich IP5 7RE, UK. E-mail:
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provide service 100% of the time. To maintain coverage of this area whilst allowing for satellite guaranteed coverage area orbital motion an FES may need to use a number of different satellites and it may need to share satellites instantaneous with other FESs at certain times. The FES will be coverage area programmed to always cover the GCA but minimise the transmission of its location area identity outside footprint the GCA. The spot beams covering the edges of the spot-beam GCA will inevitably transmit the location area identity in a small region around the GCA. The location area is where the location area identity is received, i.e. it is Figure 1 the instantaneous coverage area shown in figure 1. Using the same scheme as for terrestrial cellular networks, an MT will location update only if it has lost the FES’s location area broadcast channel. From the network viewpoint, the location of the MT is “somewhere within reach of the FES”. The FES (either on its own or with the help of the MT) may provide a way of intelligently reducing the area over which it pages in the event of an incoming call. In a satellite system designed to provide coverage throughout a region, these FES GCAs will overlap in places and combine to cover the region with no gaps. If an MT is in overlapping FES coverage and location updates to an FES only intermittently covering its location (because the MT is not in the FES’s GCA), it will loose that FES’s location area broadcast after a while and be forced to location update to another FES which covers its location properly. Examples of Guaranteed Coverage Areas In order that the GCA concept can be applied, it must be possible to define a GCA that is big enough so that we will not need too many Odyssey Globalstar 90 90 FES to cover the Earth's surface. We show here the maximum size of 75 75 GCAs for some constellations. A 60 60 GCA can be characterised by γMT , 45 45 the minimum satellite elevation angle that is tolerated by an MT and 30 30 by the type of desired coverage 15 15 (single or multiple satellites). The maximum size of the GCA depends 0 0 -45 -30 -15 0 15 30 45 -45 -30 -15 0 15 30 45 of the minimum size of the overlap between the coverage of different Figures 2 satellites. Odyssey provides generous multiple coverage at all latitudes (and hence large overlaps), as does Globalstar at temperate latitudes. The figures 2 show the largest possible GCA for each, from an FES at 45°N, 0°E using all satellites at elevations above 5°. The outer contour is the GCA with at Globalstar 90 least a single satellite at γMT ≥ 20°. The inner contour is the GCA for at least two satellites visible, both with γMT ≥ 10°. The smallest diameter 75 of the single and diverse GCAs are for Globalstar 3430 km and 60 2020 km, respectively, and for Odyssey 6060 km and 3390 km. Closer to the equator, Globalstar satellites are more sparse and the 45 requirements for a GCA must be reduced for these latitudes to 30 guarantee only single satellite coverage with γMT ≥ 10°. The figure 3 15 shows such a GCA for an FES at 15°N, 0°E. Iridium provides single satellite coverage with very little overlap at the equator but its use of -45 0 -30 -15 0 15 30 45 inter-satellite links allows an FES to guarantee coverage of an area as Figure 3 large as desired.
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Paging Implementation Options As in the terrestrial case, any incoming call will be routed to the FES which can guarantee that the MT, if it is working, is within the area the FES is covering with its broadcast channels. With the FES at its simplest, the FES would then transmit a paging message for the mobile through every spot beam which it is using to cover its GCA. If incoming calls occur infrequently, this may be acceptable but otherwise paging through this many spot beams is considered a waste of power and spectral resources. Satellite network designers could use a number of techniques to reduce the number of spot beams paged, some examples of which are presented below. Using Multiple Location Areas per FES As shown above, the GCA of an FES can be very big, so it may be convenient for the FES operator to split the area into two or more location areas, each with a distinct broadcast location area. As with the GCA, these location areas would be geographically fixed. For example, an FES covering southern Europe, the Mediterranean and the Middle East might split the location area along a border which is seldom crossed, say the middle of the Mediterranean sea. This then reduces the maximum area over which paging is necessary whilst increasing the location update signalling traffic only marginally. Paging Areas Smaller than the Location Area - Intelligent Paging Other approaches use information about the MT’s position to avoid paging throughout a location area and only page through spot beams where it is likely that the MT is. This is “intelligent paging” [1]. In each of the following cases the FES, possibly with help from the MT, uses additional position information in order to page the MT in a paging area that is smaller than the location area. The network beyond the FES is not affected by this extra intelligence. We will call position update the refreshing of this additional position information. A) Using Location Update Spot Beam Position A first approach to intelligent paging could be the FES identifying and recording the instantaneous size, shape and location (latitude, longitude) of the spot beam in which the MT last made contact (for location update, call set-up or any other reason), with a time-stamp. In the event of an incoming call, the FES would page only those spot beams required to completely cover the recorded area in the first instance. In case the mobile does not respond the paging is repeated over a wider area, depending on the age of the time-stamp and on the MT mobility profile. If the mobile still doesn't respond (or directly in second instance), it will be paged over the whole GCA. B) Using a Terminal Position Fix If an MT is capable of making the necessary measurements and calculations to fix its own position then the FES could record the measured position of the MT, which would be more accurate than a spot beam area. The location update message from the MT to the FES would be modified to include the latitude and longitude of the MT and an uncertainty radius that determines the circular area where the terminal can be found at any time. The MT would continuously monitor its own position and if it moves outside of its uncertainty area it would perform a position update. If the MT is still receiving the location area broadcasts of the FES then position update could remain local to the MT and FES, since from the network point of view the MT is still contactable through the same FES as before. However, if at any time the MT looses the location area broadcast of the FES then it would search for and location update to a new FES, invoking the full location update procedure defined by Race Monet.
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On an incoming call, the FES only needs to page the spot beams that cover the MT’s declared uncertainty area (or at least the part of it within the FES’s location area). Depending on the mobility of the MT and its users’ incoming and outgoing call rates, the MT might vary the uncertainty area radius to minimise either the paging area or the number of position updates in an attempt to minimise the total spectrum and power resource consumed by this signalling. One can show with the mobility model presented below, that the paging signalling requirement rises as ρU2 as the uncertainty radius ρU increases whilst the position update signalling decreases as 1/ρU. Disadvantages of this approach are the extra complexity in the MTs and the additional spectrum and power resources needed to implement position fixing. A terminal capable of position fixing will be more expensive to produce than one that is not but position fixing might be useful as a UMTS service 3 for other purposes . Position fixing signals could either be supplied by a third party (e.g. the US Navy’s GPS system) or extra bandwidth on UMTS satellite broadcast channels could be reserved for the timing signal and satellite almanac details. C) Using Dual Satellite Coverage Position Fix If an MT is covered by two or more satellites (both in use by the FES) when it makes its location update then the FES could perform position fixing ranging measurements for itself. Based on the relative measured delay and Doppler frequency shift via the different satellites, the terminal position could be calculated and stored along with an uncertainty area and time-stamp. As in approach A, this position would not be updated unless a call was set-up or the MT lost the FES location area broadcast. On an incoming call, the FES would page an area covering the uncertainty area or a wider area, depending on the age of the position time-stamp. Unlike approach B, this FES intelligence does not require any extra position fixing features in the MT. A periodical position update could be implemented, either MT or FES initiated, to reduce uncertainty of old location updates. Comparison of the Intelligent Paging Options In all mobile networks location update signalling is a trade-off against signalling required to page an imprecisely located mobile terminal for incoming calls. In satellite networks, where an antenna's radio coverage is much wider than in terrestrial networks, the optimum trade-off between precision location update and wide-area paging is different to that in terrestrial cellular networks. As the frequency spectrum is reused in different spot beams, the signalling information transmission has to be minimised for one spot beam. The measure for the total signalling load will be the sum of the location update and the paging rates (per spot beam and per hour). For this comparison, we developed a simple mobility model: A proportion of the population do never move, the other part move all the time in any constant direction with a constant speed v. Although this model is very simple, it could be close to the reality: As all main transport means (highways, trains) will be covered by terrestrial cellular systems, the satellite is only responsible for the rest of the mobile users that travel by other sparser transport means. From this model, one can derive the probability that a mobile user, starting anywhere in a circle and covering a certain distance, will arrive out of the circle. This probability will allow to derive the location update and paging rates. As traffic model, we simply use the incoming call rate Rin (per user and per hour), which determine the amount of necessary paging and the outgoing call rate, which, together with Rin , determine the frequency of "contacts", i.e. of position updates in option A. In option A, the surface to page is in first instance of the size of a spot beam and in second instance, the whole GCA (we do not take into account possible steps in-between). The location area is the
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Note that the accuracy required for position fixing as a UMTS service might be different to that required for location management. The accuracy of a positioning system like GPS would be expensive to match.
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Total signalling load
Total signalling load
GCA. In option B, the surface to page is the uncertainty area. The uncertainty area acts also as a (local) location area because the MT has to position update when it leaves it. From the network point of view, the location area remains the GCA. It is possible, for given input parameters (spot beam size, users mobility, call rates) to find a value for the size of the GCA for option A and for the uncertainty radius for option B that minimise the total signalling load. Option C is a mix of the two others. The position is known more accurately than in option A, but the location area keeps the size of the GCA. It could be seen as an improvement of option A. As this scheme presents many unknown parameters (positioning precision, time between periodical location updates, ...) we will only compare the two other options. Option A demands very little location update signalling because its location area can be very large (diameters between 2000 and 6000 km). But as the size of the paging area is given by the spot beam size, which can be relatively large (diameters between 400 and 1000 km), the paging signalling load can be heavy. Figure 4 shows the total signalling load for the two options in function of the movement speed v of the users and of the incoming call rate Rin . The conclusion is that approach A is the best 6 9 x 10 10 4.5 when v is large and Rin low (high mobility and few 4 8 10 incoming calls), i.e. when 3.5 the location update 7 10 3 signalling load is higher 2.5 than the paging signalling 6 10 load. The advantage of 2 5 option B is to be able to 10 1.5 0.001 0.01 0.1 1 10 0 20 40 60 80 100 locate the MT more Rin [per user and per hour] v [km/h] precisely, as the Figure 4: Comparison of option A (plain line) and option B (dashed line). uncertainty area can be smaller that a spot beam. If the incoming call rate is high and the user mobility low, this ability makes option B preferable (see figure 4). At the time of writing, it is difficult to predict whether the UMTS market will favour one approach or the other. Conclusion With the revisions described in this paper incorporated, the Monet network functional model will enable the design of UMTS networks just as capable of supporting mobility in satellite cells as it is in terrestrial macro-cells and micro-cells. From the core UMTS network viewpoint, the location management functional entities operate in the same way in terrestrial cellular and satellite networks. Acknowledgements The authors wish to thank all the other partners in Race Monet work package RAS2 (satellite aspects) and in Race Saint work package A4100 and the Race Monet project management team. Reference [1] “Location Areas, Paging Areas and the Network Architecture”. Deliverable No 1 of Race 2066, Monet.
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