Impact of Satellites on UMTS Network

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 communications 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. This paper presents the features of the UMTS system specification that were added specifically to support satellite access networks. These include enhancements to the functional entities which handle location update, domain update and handover. A description of the assumptions that have been made and an indication of the performance of the system specification when satellite access is used are presented. Approach to Defining Network Functions for Satellites The Race Monet project had been working on network models for three years before satellite aspects were considered in detail. 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) might 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. Another consideration is that satellite bandwidth is expensive and that its use for signalling needs to be minimised. Few changes to the terrestrial model have been made, all of which have been confined to location update and to handover. Location Area and its Update A location area is formed by an FES’s instantaneous coverage. Using the same functional model used for terrestrial cellular networks, an MT will location update only if it looses 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. An assumption is that each FES will be designed to cover a particular geographic area. To maintain coverage of this area whilst allowing for satellite orbital motion an FES may need to use a number of different satellites and it may need to share satellites with other FESs at certain times. Because of the dynamics of a satellite network, the edges of an FES’s coverage are intermittently covered by this FES. 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 provide service 100% of the time. The FES will be programmed to always cover the GCA but minimise the transmission of its location area identity outside the GCA. In a satellite system designed to provide coverage throughout a region, these FES 1 BT Laboratories, MLB4/67 Martlesham Heath, Ipswich IP5 7RE, UK. E-mail: bob@garden.bt.co.uk 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 A GCA can be characterised by γMT , the minimum satellite elevation angle that is tolerated by an MT and by the type of desired coverage (single or multiple satellites). Inmarsat P21 provides generous multiple coverage at all latitudes, as does Globalstar at temperate latitudes. The figures on the left 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 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 of the single and diverse GCAs are for Globalstar 3430 km and 2020 km, respectively, and for Inmarsat 6060 km and 3390 km. Closer to the equator, Globalstar satellites are more sparse and the requirements for a GCA must be reduced for these latitudes to guarantee only single satellite coverage with γMT ≥ 10°. The figure on the right 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 inter-satellite links allows an FES to guarantee coverage of an area as large as desired. Paging Implementation Options As in the terrestrial segment, 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. -45 -30 -15 0 15 30 45 90