Broadband systems

Development of MSS architecture beyond the IMT 2000 era will be influenced by trends in telecommunications growth, technology, spectrum demands and regulatory issues. Recent trends in the mobile industry illustrate that developments in the mobile environment closely follow those in the fixed network, as users expect fixed service facilities to be available in the mobile environment. The most notable technical development influencing telecommunications has been in the field of information technology, and in particular the internet, resulting in an exponentially increasing demand for large throughputs as individuals access remote databases and download large files. Furthermore, a variety of multimedia services, such as video retrieval, image transfer, etc., are on offer through the fixed network and asymmetric direct broadcast satellite system, which indicates the potential of personal wideband systems. Fixed satellite systems are already catering to the internet demand at the ISP level, whereas DBS systems offer the facility from the user's satellite dish.

Assuming that users will demand in the mobile environment the services on offer in the fixed band, it can be anticipated that there will be a demand for broadband services in the mobile environment. Thus it is possible to postulate a basic set of requirements of a future mobile personal communication service:

wideband;
bandwidth on demand;
protocols to support a mix of services;
circuit and packet mode services;
hand held or small portable sets;
propagation delay compliant with multimedia requirements.

A major problem in the provision of wideband service in the L and S band is spectrum scarcity. Studies conducted as part of S UMTS development demonstrate a shortfall of up to 17 and 61 MHz in global hot spots for the years 2005 and 2010, respectively (Barani et at., 1999). Throughput can be increased by improving the modulation efficiency, restricting mobility, employing more efficient accessing techniques, narrowing the size of spot beams, and migrating to less crowded parts of the spectrum. While all techniques are being refined and improved, migration of services to less problematic bands appears promising in a longer perspective, and therefore there is considerable interest in utilizing bands above 20 GHz for wideband MSS.

Most proposed commercial wideband systems are in FSS parts of the Ka band and therefore do not formally belong to a mobile service, but the terminals are small enough to be portable and thus there is a fuzziness between applications of these services. A variety of orbits and architectures have been proposed, including low, geostationary orbits or hybrid orbits, intersatellite links, onboard signal processing, etc. In the USA alone, the FCC has awarded licences to over a dozen applicants for K,, band FSS systems for a variety of broadband solutions, such as interactive multimedia personal communications. By 1997, the ITU had received filing for over a thousand Ka band satellites. The main features of a sample of US systems are summarized in Table 11.5 (Farserotu and Prasad, 2000). The list is representative but not exhaustive and is included to demonstrate trends; note that a number of broadband systems have also been proposed in Europe and elsewhere.

Consider, as an example, the K,, band broadband technology demonstrator system proposed under the SECOMS/ABATE (Satellite EHF Communications for Multimedia Mobile Services) project under the European Union's Fourth Framework Advanced Communications Technologies and Services (ACTS) programme, which aims to develop a broadband Ka (20/30 GHz) and EHF (40/50 GHz) system to cover Europe and the North Atlantic with data rates in the range 4 kbps to 2 Mbps, the proposed system is a multi spot, K,,/EHF band, multi satellite, geostationary orbit, private satellite network to provide 4 kbps 2 Mbps mobile multimedia service for Europe and the North Atlantic (Losquadro et al., 1998). Implementation is envisaged in two phases; in phase one, a Ka band payload will be introduced, followed by the introduction of an EHF payload in phase two; both systems will be interconnected through intersatellite links to ensure compatibility.

The network architecture is illustrated in Figure 11. 11 (Losquadro et at., 1998).

The main components are: four types of user terminals; K. and EHF band regenerative transponders; gateway and service provider Earth stations; and a master control station interfaced to a network operator and a satellite operation control centre. The SECOM network is considered as a private network which interfaces with terrestrial fixed networks through a user network interface at the SECAM gateway stations. There are three rates of user terminals for the Ka band SaT A, SaT B and SaT C capable of delivering data rates of 160, 512 and 2048 kbps, and a single terminal type, SaT D, at 64 kbps for the EHF band. The main categories of users are individuals and groups, with a further differentiation on mode of operation, i.e. portable, fixed and mobile. Provided that data rates are compatible, users can be interconnected. Gateways, service provider centres and the MCS operate up link and downlink transmissions at 32 Mbps in the Ka band sub network, and at 1,024 Mbps with the capability to operate more than one link in the EHF sub net. Protocol adapters at fixed and mobile stations allow interworking with a host of terrestrial systems, such as ISDN, BISDN, IP, etc. The satellite operation control centre is used for satellite operations, and the network operator manages administration and billing of external public terrestrial networks and user equipment. The satellite payload concept is illustrated in Figure 11.12(a) and its traffic routing processor (Losquadro et al., 1998).