Wireless

June 5, 2004
Cellular backhaul

John Stanton

The Yankee group projects that about 40 percent of global wireless infrastructure spending is taking place in Asia, particularly in Japan and China. Pyramid Research says the region is likely to double its subscriber base from 2003 to 2005 and reach over a billion total subscribers by 2008.

UNITED STATES -- India will rapidly increase from 2 percent to 9 percent by 2008. This growth in subscribers will in some part due to increased competition because of market liberalization, limited access to telephony services and pre-pay options that provide access to lower income subscribers. The need to extend access is important to the advancement of certain markets. The technological developments such as cellular backhaul give providers a greater range of effective solutions that can overcome traditional issues, such as cost of deployment and terrain impediments by using satellite-based backhaul networks that provide greater flexibility.

Backhaul optimisation
The mobile industry has realised significant achievements in managing radio frequency (RF) challenges. Solutions have been found for geographic, technical and legal issues, thereby improving coverage, capacity and quality. Looking at the base station and improving the cell concept made most of these optimisation approaches. While optimisation at the cell level has attracted interest, the infrastructure network, called the backbone or backhaul network, deserves some exploration.

Primarily leased lines from telecom companies and microwaves create cellular backhaul networks. A full set of economics and technical parameters, which may vary significantly, depending on variables including the country and population density, rules the use of fiber versus microwave. Overall, data from microwave equipment providers and consultants indicate that microwaves are used twice as often as fiber on a per route basis. The design of the backbone network of a mobile operator is close to that of a fixed line operator in that they offer fixed point-to-point service. Capacity on a per route basis is based on traffic patterns that match the classic peak and off-peak use of a fixed telephony network.

Because the mobility of the user is a significant component of the variation of traffic on a per cell basis, the peak and off-peak characteristics of a mobile operator include more variables than those of a fixed line operator. In addition to the connection, the mobility element is another factor in the design of the core network. However, because of the point-to-point approach taken in the design of the core network, this mobility factor can generate strong variation of traffic at a cell level. Together, connectivity and mobility patterns define the peak traffic for a cell. Determining the backbone link capacity for that cell may require a significant allocation of bandwidth, of which a significant portion may be largely unused. Usually, the fixed backbone bandwidth matches the maximum number of channels that the RF equipment on the cell site allows.

This sizing approach of the backbone routes is repeated as per the number of sites, and a significant number of equivalent E1s/T1s is set up. In fact, the number of E1s/T1s usually becomes relatively large when compared to the number of subscribers on a network. At any moment, a significant amount of backbone capacity on a mobile network goes unused; this has a strong negative impact on financial results.

For example, a network containing 500,000 subscribers at a grade of service of 2 percent theoretically needs about 10,000 voice channels of capacity. About 45 equivalent E1s or 62 equivalent T1s provide the 10,000 voice channels. Usually, a mobile network backbone sized for a half million subscribers will present hundreds of E1s/T1s of capacity. Anecdotal data shows that, at least 10 times the theoretical needed capacity is implemented to provide backhaul functionality.

Satellite mobility
The fixed aspect of the core network design does not leave a lot of room to improve the unused capacity factor. With a fixed point-to-point design approach of a network, demands for high variations of traffic are stressful to the mobile or fixed network and, consequently, the cost of serving customers is increased. An alternative way of configuring the backbone network of a cellular operator is to look at the mobility factor. By using fixed routes sizing approach, it is difficult to apply a mobility factor at the design stage. The transfer of the user mobility factor on the network design can be successfully achieved by increasing the flexibility of the backhaul network, but this is difficult to achieve with a fixed infrastructure.

A cellular backhaul network that would allow users to re-allocate core bandwidth capacity would answer the variations in bandwidth demand on a per route basis. Unfortunately, it is not possible to do this as quickly as would be necessary when using in-ground, fixed-line assets such as leased lines or microwaves. With satellite, however, it can be accomplished.

The re-allocation of bandwidth can be done between earth stations, and when managed appropriately, it is done in a matter of minutes or hours rather than the weeks and months that this kind of activity can take with microwaves and leased lines. By matching the capacity requirement and the connection requirement on a per channel basis, a flexible satellite network can avoid the provisioning of approximately 10 times the backbone capacity required to run a mobile network.

In the case of cellular backhaul via satellite, the issue is no longer defined by the capacity size of the different routes of a point-to-point network, but simply by determining how a customer can be given access to an available channel.

Hybrid backhaul networks
A flexible, satellite-based backhaul network should complement the traditional fixed transmission lines for a cellular network. When fixed-line assets and flexible satellite bandwidth are used to build the routes, the result provides more efficiency. In addition to the flexibility aspect for standard usage, the availability of satellite-based capacity is also a powerful tool for emergencies and planned outages, as network redundancy needs can be adequately resolved.

Satellite operators such as Intelsat have overcome quality issues involving satellite-based backhaul networks, which are helping over 30 mobile operators to deploy flexible backhaul networks. As discussed, a fixed point-to-point-based infrastructure needs to pre-provision about 10 times the equivalent of capacity than will be actually used. The main cost component of a satellite-based network is for used capacity while the cost component of the terrestrial capacity is most significant for the unused capacity.

A mix of terrestrial and satellite-based backhaul capacity is, therefore, likely to provide the best level of efficiency technically and economically. This is of particular importance in areas like the Asia-Pacific and India, with telecom networks serving an increasingly important role as a driver of economic development.

Cellular wireless already has proved itself as an efficient way of increasing capacity and extending access to areas ranging from remote rural communities and islands, to high-density urban areas such as Hong Kong. The rapid growth of mobile users in Asia gives FSS providers an opportunity to display the geo-synchronous satellite advantage when it comes to factors including flexibility, speed of implementation, efficiency and reach.

(The author is president, data, carrier and Internet, Intelsat.)

Contact:
Intelsat
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