GSM Solutions over Satellite

As cellular networks evolve and expand, there is an increasing demand from
users to have ubiquitous coverage. To satisfy these needs, service providers
must deploy equipment into ever more challenging areas. Difficulties facing the
operators can include;

a) Remote/Rural Areas. To service remote areas, it is often economically unfeasible to provide backhaul facilities (BTS to BSC) via terrestrial lines (fiber/microwave).
b) Time to deploy. Terrestrial build-outs can take years to plan and implement.
c) Areas of ‘minor’ interest. These can include small isolated centers such as tourist resorts, islands, mines, oil exploration sites, hydro-electric facilities, etc.
d) Temporary Coverage. Special events, even in urban areas, can overload the existing infrastructure.

This paper presents the advantages of using satellite-based solutions to augment traditional terrestrial infrastructure in supporting the build-out of cellular networks.

GSM Overview

The following diagram depicts the major components and subsystems in a typical GSM cellular network.

Figure 1: GSM Network Architecture

There are two areas where it can be advantageous to use satellite-based communications links: the Abis interface (interconnecting the BSC with multiple BTSs) and the A interface (interconnecting the MSC with multiple BSCs). The Abis interface is the key air interface in the Access portion of the network. The A interface is the primary interface within the Core portion of the network. These two interfaces have different characteristics and requirements which necessitate different solutions to optimize their performance over satellite. PolarSat has a cost-effective solution for both Access and the Core.

Access Network Using FlexiDAMA

Figure 2 shows the GSM network of Figure 1, with the Abis interface circled for clarity. A typical Base Station Controller (BSC) will have links to multiple Base Transceiver Stations (BTS). The Abis interface consists of voice and data traffic, signaling information and provides synchronization information from the BSC to the BTS.

Figure 2: The Abis Interface within the GSM Network

The simple but inefficient way to support these interfaces used to be to put in SCPC modems. However, these provide fixed rate, static links with little or no M&C capabilities. Due to dynamic traffic loads within BTS cell sites, there is an opportunity to share the satellite bandwidth, thus dramatically reducing the overall cost of providing the Abis links over satellite. Network solutions with Demand-Assigned Multiple Access (DAMA) capabilities are a must and ones that can efficiently provide single satellite hop connectivity are highly desirable. PolarSat’s FlexiDAMA product is ideally suited to providing such an optimal solution.

A FlexiDAMA network consists of a centralized Hub and various classes and sizes of remotes. The remote terminals are either multi-channel mesh/star FlexiDAMA terminals, or single carrier star-only SkyIP terminals. The FlexiDAMA/SkyIP terminals provide a powerful and efficient satellite link, taking advantage of DAMA and true BOD algorithms to match the occupied satellite bandwidth with the current load on the various Abis interfaces. To condition the Abis signals into IP format, FlexiDAMA/SkyIP equipment is integrated with backhaul optimizing equipment from such industry leading vendors as Memotec, Cisco, or others.

Advanced Features of FlexiDAMA for GSM Access Networks

• Direct packet data interface (IP on 10BaseT Ethernet).
• DAMA – inactive voice circuits will cause a minimization of assigned satellite bandwidth to that particular BTS station.
• BOD – as additional voice circuits become active at a BTS, more bandwidth will be assigned to the link, thus matching the satellite resource to the actual demand.
• Multiple mini-star topology. Each FlexiDAMA terminal at a BSC site acts as a mini-Hub for a group of SkyIP terminals at the associated BTS sites. Overall network management, timing and control exist at the Satellite Hub which may be at a different location.

Core Network Using VSATPlus II

Figure 3 shows the same generic GSM network architecture diagram, but this time the A interface is circled. This is the link between BSCs and the MSC (or possibly between BSCs). This link is typically a non-compressed full E1 span and the signaling scheme invoked on this link is vendor specific.

Figure 3: The A Interface within the GSM Network

Connectivity at the A interface may be star, mesh or a hybrid of the two. User traffic between BSCs does not have to route through an MSC. If these A links are being carried over a satellite communication network, it is highly desirable for direct BSC to BSC connectivity, thus requiring the satellite network to satisfy full mesh connectivity.

PolarSat’s VSATPlus II product line provides a full mesh network solution and a direct E1 interface. The E1 interface may be connected directly to the BSC/MSC equipment or pass through a signaling converter, depending upon the interface. Key features of the VSATPlus II solution include:

• Integrated voice encoding (16/24/32 kbps) and G.165 echo cancellation.
• Full mesh, single hop connectivity.
• DAMA operation, thus sharing satellite bandwidth amongst a group of sites.
• Partial/Fractional/E1 operation for thick-route trunks between major sites.
• Embedded MFC/R2 and DTMF signaling, providing a standard interface to a variety of switch vendors.
• Transparent C7/SS7 operation, whereby the signaling channel is sent in TS16 and any number of telephony trunks are linked between two sites.
  

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