The wireless industry has become such a large user of satellite bandwidth and Earth stations that the cellular backhaul market consistently is rated as one of the best growth markets for satellite. While the business continues to evolve, it remains focused on the basic act of moving traffic from one cell site to another, but the technology behind an operator’s system can have an impact on the operations and the revenue the network produces.

Although the central theme of cellular backhaul is the transmittal of voice traffic, traffic patterns and volumes differ greatly from one cell site to another, which begs the question: Which satellite technology is best suited for the cellular backhaul market? 

Architectural Overview of GSM Network

Wireless service providers around the globe primarily use GSM and CDMA architectures to deliver wireless services to subscribers. The majority of wireless carriers in the United States use CDMA technology, and their combined networks represent almost the entire global market share for this wireless technology. GSM, on the other hand, enjoys a far larger market penetration with approximately 85 percent global market share. Since there is ready access to fiber and microwave connectivity in the United States for backhauling traffic from cell sites, the majority of which are CDMA, the largest demand for satellite connectivity is from GSM carriers

The architecture of a GSM wireless system is hierarchal in nature, with the lowest level being a base station subsystem, the part of the network responsible for handling traffic and signaling between a mobile phone and the network switching subsystem that covers a geographic area known as a cell. Each base station subsystem has one or more base station controllers which communicates to one or more base transceiver stations (BTS) that typically control six to 10 transceivers, the hardware that controls the radio links between the subscribers handsets and the radio towers. The base station controllers aggregates the traffic for each cell and then communicates with the mobile switching center. In addition to routing calls to other wireless handsets and to the PSTN, the mobile service switching center maintains several databases, which keep track of subscriber’s phone numbers and which cell each handset in the network is communicating through.

An Abis interface is the connection between a BTS and a base station controller and is designed around the T1/E1 interface. One of the challenges to this interface is that it must transport an entire E1 frame (2.048 Mbps) from the BTS to the base station controller, even if the system supports only a small handful of voice channels. IP optimizers converts the E1 Abis interface to IP and then optimizes traffic by removing silence and unused channels from the traffic that must be sent, thereby reducing the amount of satellite bandwidth that is needed to support an individual BTS. 

Market Drivers

The demand for satellite backhaul in GSM networks around the world is growing as wireless carriers continue to expand their networks. Jose Del Rosario, senior analyst for NSR, says the global revenues in 2009 for cellular backhaul applications totaled $316.6 million. Of that total, 93 percent represented bandwidth, while hardware purchases made up 7 percent. “This market is showing steady growth, and we are forecasting global revenues to increase to $583.4 million by 2019,” says Del Rosario. “When you total the escalating revenues over the 10-year period, we expect the combined revenues to total $4.8 billion.
For satellite operators, this represents a good revenue stream. Of course, it is good for hardware manufacturers, too. You won’t see double digit growth rates, but the steady growth you have seen will continue.”

If the mobile operator has a thick route, with 500-plus Kbps, 1 Mbps or more, and the traffic is constant, SCPC is definitely the best option, but TDMA is better in some medium-route and most thin-route applications.
— Rehbehn, Hughes

Del Rosario also notes several important trends which will have an affect on the wireless industry. The first is the shift in the wireless industry to IP. “All network devices are moving in this direction. This will have a positive impact on wireless hardware, bringing down the hardware pricing and increasing bandwidth efficiencies. This is where all of the future development will be.” Del Rosario also says more countries are beginning to leverage universal service obligations to put pressure on wireless carriers to increase their wireless footprints. “In essence, the governments are telling the wireless service providers that if they want to serve the urban areas, they have to serve rural areas,” he said. In order to grow their business, wireless carriers have just two options: increase their ARPU (average revenue per user) or increase their number of subscribers. Both generally bode well for the satellite industry. 

In developed countries, wireless operators tempt subscribers with advanced features and handsets, so the operators continually can ratchet up monthly rates. Text messaging and intelligent phones have both been used effectively to increase ARPU. While successful in affluent countries, the technology envy game enjoys little success if a country’s population has limited means to pay. In these areas, expanding the number of subscribers is the key to growing the wireless operator’s revenue base. As wireless operators push farther into rural areas, satellite quickly becomes the technology of choice for backhauling traffic, but as cell sites get smaller, the number of voice channels drops. 

TDMA Versus SCPC

When the term “cellular backhaul” was coined, the principal applications involved the transport of voice traffic from one mobile switching center to another, to a long distance gateway for a country, or to a central office interconnected to the PSTN. As the market has matured and wireless service providers expanded their networks outside urban settings, the need to support base station controllers and even RTXs became necessary. While the traffic was still a mix voice and signaling, traffic patterns began to vary. “All technologies can achieve results, but you need to look at the solution from the wireless operator’s perspective,” says Daniel Enns, senior vice president, marketing and business development, Comtech EF Data. “The total cost of ownership involves working to optimize the capex and opex of any system deployed. In some cases, such as a large voice trunk between two points with constant traffic in both directions, the solution should be a point-to-point SCPC backhaul solution. In other cases, such as a wireless operator wanting to support multiple BTSs from a given central [base station controller] and the individual BTS traffic is lower, a point-to-multi-point shared SCPC solution brings down both the capex and opex by using a TDM outbound shared carrier,” he says.

Comtech EF Data’s Memotec subsidiary supplies cellular GSM and telephony backhaul network optimization solutions and has hundreds of sites operational across satellite and terrestrial transport networks in Latin America, the Middle East, Africa, Asia and the Pacific Rim. “When it comes to choosing the right backhaul technology, we like to look at it from the mobile operator’s perspective. The basic question is: ‘How can the mobile operator get more subscribers on line?’ There are many steps to this complex puzzle. The mobile operator must then often must look at satellite technology that achieves the lowest cost of ownership. The next billion wireless customers will, at first, not be reached by fiber or microwave. A correctly deployed satellite backhaul solution will have an important role to their success. The role that IP optimizers have played in the growth of the satellite backhaul market can’t be overlooked. By suppressing silence time and idle channels, the amount of satellite bandwidth between a BTS and [a base station controller] is dramatically reduced, plus legacy protocols are converted to IP.”

Hughes classifies satellite links in a GSM network as either thick route, medium route or thin route, says Dave Rehbehn, senior director, marketing, International Division at Hughes. “If the mobile operator has a thick route, with 500-plus Kbps, 1 Mbps or more, and the traffic is constant, SCPC is definitely the best option, but TDMA is better in some medium-route and most thin-route applications. With SCPC, you need to nail up satellite bandwidth between the [base station controller] and BTS. If traffic is variable and drops below 50 percent, you may waste a lot of bandwidth with an SCPC solution. A TDMA system can dynamically assign capacity but it has more overhead than does an SCPC circuit. TDMA terminals have a price advantage over SCPC terminals, but you must factor in the capital cost of the hub. If the mobile operator needs to deploy more than 20 to 30 remotes, a TDMA solution will generally lower the overall capex requirement of the network.”

iDirect’s Richard Deasington, a wireless industry veteran and the company’s director of verticals, says: “If you have a continuous wall of traffic, SCPC works well. If there is no variability of traffic, such as trunks between major nodes, you don’t need clever systems to reassign bandwidth. But traffic levels across an entire network aren’t constant and this is where TDMA works well.” iDirect’s TDMA systems allows mobile operators to create a pool of bandwidth that is shared between all of the nodes, says Deasington. “There is a misconception that voice traffic is deterministic. It isn’t. If you aren’t talking on a wireless call, it only takes 2 kbps to keep the call connected, but the traffic immediately jumps to 10 kbps when you begin speaking. This causes traffic levels to vary. Peaks come at different times and for different reasons. For instance, road construction can cause traffic in a geographic region to spike. When you examine the traffic of the entire network, it is common for a TDMA network to achieve bandwidth reductions in the 30 percent to 80 percent range, which creates large monthly savings in bandwidth,” he says.

While TDMA networks allow operators to fine tune their networks, all TDMA networks are equal when it comes to the speed in which they can reallocate bandwidth. “iDirect can change configurations up to eight times per second, allowing the network to contract or expand instantly. Without this type of responsiveness and the ability to rapidly adapt, the voice calls being transported will experience clipping,” says Deasington.

iDirect’s modems also will be able to support SCPC mode in the future should a mobile operator outgrow a TDMA link at a particular location. “A wireless operator needs a crystal ball to estimate their future traffic needs. Most apply some sort of fudge factor, guessing that traffic may increase 200 percent. Frankly, it is hard to predict. Our new software can issue a software command and immediately begin operating a link in SCPC mode. This frees wireless operators from having to make rigid decisions in advance,” he says.

Gilat installs and operates both TDMA and SCPC networks, and there is room for both technologies in the GSM market, says Doron Elinav, Gilat’s vice president of marketing and business development. “TDMA offers advantages for connecting relatively small base stations with six to nine TRXs, while SCPC is usually a better solution for higher capacity base stations. Elinav also points to the growing amount of data being carried by today’s wireless networks. “More data means greater variability in network traffic. Mobile operators craft very stringent service level agreements for voice traffic but they aren’t quite so rigid on data traffic. As advanced versions of LTE are deployed, base stations will be able to communicate directly between themselves, which will create more demand for mesh network,” he says.

LinkSat president Marc Nadon says while TDMA systems allow bandwidth pooling, the buffering that is done to share time slots can create significant delay, which would then cause problems with signaling between BTSs and the base station controller. If too much delay is created by the buffering scheme, the BTS will lose its link. Nadon also notes that TDMA systems add a significant amount of overhead, up to 40 percent in some cases, which causes mobile operators to barter inefficiency for flexibility. “LinkSat’s IP pouter is an overlay on top of SCPC networks which adds multi-destination routing, allowing mobile operators to become more bandwidth efficient and get better performance. Satellite modems are typically the biggest single expense in a satellite network. Having to do a fork lift upgrade on all of the modems in a wireless operator’s network to increase efficiency is a bitter pill to swallow.” 

Future Trends

The wireless industry is deploying new generations of wireless architectures, with the foreseeable goal being LTE. As the progression moves forward, the transportation of data traffic over the wireless infrastructure may surpass voice traffic. Wireless manufacturers, such as Ericsson and Motorola, have recognized the mobile operator’s need to grow the number of subscribers by expanding into areas with low subscriber density. This has resulted in a new class of IP-enabled base station with a small number of TRXs.

Xavier Verrans, product line manager for Newtec, says, “What we will see in the future are SCPC/TDMA hybrid networks. There aren’t very many today. The reason is that the industry is lacking a single network management system which can effectively manage both types of networks, but that is coming in the near future.”

The future for the cellular backhaul market appears bright with plenty of opportunities for both SCPC and TDMA vendors.