Estimates indicate there are nearly three billion mobile phones in the world. As much as 90 percent of the world’s population will have mobile phone coverage by 2010. The approaching saturation of mobile phone ownership comes with the expectation that mobile network access is equally ubiquitous. The advances in backhauling technology, particularly the improvements in picocell capabilities, mean that GSM and UMTS networks can literally be established anywhere—temporary networks in remote areas on land, as well as mobile networks on planes, ships and trains.
TECHNOLOGY INNOVATION
Satellite communication in itself is not new or particularly innovative and has been an expensive ‘fix’ when compared to terrestrial wireless or fixed–line technology. As a result, satellite–based usage has been, by and large, limited to critical communications in remote, inaccessible areas, frequently as a result of a disaster or when secure transmission is a high priority. Until now, satellite usage certainly has not been seen as a mainstream contender for network access.
Traditional satellite technology has been relatively crude. A GSM network is created by simply transporting a standard GSM interface over a dedicated link protocol, and a Base Transmitter Station (BTS) is connected to a base station controller (BSC) using the industry–standard Abis protocol, which is directly transported over satellite.
However, the disadvantages have been significant. In addition to high installation and running costs, the solutions have typically been large and expensive and certainly not spectrum–efficient, in that they usually require a large capacity link of around 2 Mbit/s to be up and running, whether there is GSM traffic or not.
But the fortune for satellite–based network provision is about to change—all thanks to the picocell. Picocells, which act similarly to a base station, are simple to install, compatible with existing GSM handsets, and use existing IP infrastructure for cost–effective backhaul. Picocells have traditionally been used to extend coverage to indoor areas where physical building restrictions mean poor signal reception. Or, they have been used to add network capacity in areas with very dense phone usage, such as train stations. However, they can now be used as the BST and can be backhauled to provide coverage anywhere on land in the air or at sea.
The new picocell–based technology can offer the one thing that has eluded previous satellite communications—effective communications management. System efficiency is the key enabler to the technology behind the new GSM, GPRS, EDGE and UMTS networks based on satellite transport. Much of the pioneering development work that has gone into enabling picocell backhauling has been based around optimization, including compression of payload, IP Header and signalling.
System architectures
Before addressing compression techniques, an understanding of the principle backhauling architectures is required.
There are two principle architectures for satellite–based communications. The first is the adaptation of current 2G GSM networks, offering comparable voice, GPRS data and SMS messaging services, as with terrestrial networks. The architecture of these systems connects different network elements of the GSM network at the remote area to the operator, via a satellite link. For this to occur, the functionality of the BSC is broken up into a BSC at the remote site, which acts as the terminal for the BTS, and a ground BSC, which performs data transcoding and routing to the core network—see Figure 1.
The second implementation is for 3G—UMTS and 3G GSM networks—offering all of the additional advantages of 3G, including broadband wireless data and video conferencing. The architecture is based on the same approach as with 2G, except that the elements follow the 3G terminology, as shown in Figure 2. One of the key elements in the design is that it is scaleable by the addition of further BTSs/Node Bs, meaning the network can be expanded to meet increased capacity needs.
The first practical architecture was devised as part of the EU–funded WirelessCabin project. That paved the way for the development of picocell backhauling on commercial aircraft. The architecture, which is significantly more complex than the previous examples, consists of three segments: the cabin segment at the remote site— in this case the aircraft—the satellite transport segment and the ground segment at provider site. See Figure 3. The key elements work as follow:
- The SS7 Gateway in the Service Provider Domain is used to convert SS7 signalling from 2G/3G networks into sessions initiation protocol (SIP) and/or remote authentication dial in user service messages
- The WirelessCabin Location Register in the Service Provider Domain has Authentication, Authorization and Accounting server functionalities. The register checks that the user has an international roaming agreement with their mobile telecoms provider
- The SIP server and MGW in the SPD allow the system to interconnect with the 2G/3G domain. In particular, the SIP server, together with the one located in the SID, converge as the session is established using SIP
Optimization
IP networks provide the versatility required for backhauling. However, the protocol headers involved can more than double the bandwidth required. Development focus has been on compressing the IP, real–time transport (RTP) and user datagram protocol (UDP) headers to maximize optimization.
The reason the compression of RTP, UDP and IP headers is so important is because they add a total of 40 Bytes to each voice packet, regardless of the size of the packet. This means that a voice packet of 12 Bytes adaptive multi–rate (ARM), codecs, used by GSM and UMTS picocells, is increased by more than four times by the headers.
The choice of an appropriate header compression protocol depends on the configuration of the modem and the satellite gateway. If they both support the compression technique, then the complete IP header can be removed. If they do not, the IP header is required for routing, and the RTP/UDP headers can be compressed between the BTS/RNC pairs, or the cabin and ground service provider domain—their equivalent in the WirelessCabin architecture.
In addition, there is the potential of using other compression techniques. IETF RFC 2507 can be spanned between the modem and the satellite gateway and it can compress the IP and UDP headers, but not RTP headers. It can reduce the IP/UDP header from 20 Byte to only 2 Bytes, leaving the 12 Byte RTP header untouched. Robust Header Compression (RoHC) is an extremely efficient way of reducing total overheads. However, RoHC uses the redundancy of the different packets and does not retransmit redundant information, instead storing it as context information at the compressor and decompressor. When combined with the technique of bundling several voice packets into a single IP datagram, very significant compression can be achieved, with overheads being reduced to only 5 Bytes.
APPLICATIONS
Picocell–based satellite technology can be used to set up a GSM or UMTS network anywhere in the world. The picocell can pick up signals within a 700–meter radius, and can handle up to 14 calls at any one time. There is an almost endless range of applications, some of which have already been developed into commercial applications.
Transport
Aircraft Connectivity
Passengers have been able to communicate from planes for many years, using seat–back phones. What picocell backhauling enables them to do now is to use their own mobile phones and Blackberrys and other smartphones during flights, just as they would on the ground. In addition to being able to access all their normal phone functions, users will be billed as per normal, with call details and costs, based on international roaming rates, appearing on their normal monthly billing statement.
It also provides new crew applications. Transferring of non–flight–critical information in real–time is one example. Plus this can be used for pilots’ electronic flight bag updates, catering updates, as well as real–time charging of credit cards for duty free sales.
A plane is a harsh environment: equipment is subject to significant forces and to dramatic changes in temperature and atmospheric pressure. It is also subject to the stringent safety requirements of one of the most tightly regulated industries. Aviation’s safety record is a very good and no one—regulators, airframe manufacturers, airlines or passengers—want that to change. It was essential to ensure the equipment cannot jeopardize the safety of the plane, its crew and passengers.
In addition, the equipment has to be small and light. Margins are very tight in the air transport industry, particularly with oil prices rising, so airlines do not want to carry heavy equipment that will increase fuel consumption. Nor do they want to use limited space that could otherwise be filled with passengers or cargo. Figure 4 shows how the equipment fits into an aircraft.
The first commercial operation of a GSM network on a commercial aircraft will start in 2008; by the end of the year, the technology will be installed on as many as 100 aircraft in Europe. Installations on business aircraft, typically less constrained by cost issues, are expected to follow quickly.
Maritime Connectivity
Ships, be they leisure or merchant, once in open seas need a cost–effective mobile connection solution for crew and passengers. In addition, there is increasing demand for the real–time monitoring of containers while at sea. The first commercial trial of the technology is already up and running on a German cruise ship. It is anticipated that once approved, the service will quickly come into service.
Another good opportunity for growth is the tracking of containers. An RFID door seal attached to the container is connected to a SIM card, which sends messages via satellite to the control centers both on the ship and on the ground. These can include the shipping company’s headquarters, the ship’s homeport operator, or government security services. If the seal is compromised in any way, a panic ‘alarm’ signal is transmitted immediately, as is information about changes in temperature, humidity, vibration, light and volumetric pressure, as well as levels of oxygen, carbon monoxide and carbon dioxide.
The two main benefits of the system are that it monitors the security of the container, so can be used to demonstrate to government security services that there has been no breach of security, which can dramatically reduce the time it takes to enter a country, thereby cutting costs. It also means that valuable cargos can be monitored at all times, which can lead to a reduction in insurance premiums.
A solution for container tracking was tested in the South China Sea early in 2007. Figure 5 shows a map of where the ship, the Kota Gemar, travelled during the test.
Train Connectivity
On most trains, passengers can connect to the standard terrestrial network. On high–speed trains, the handover process from one base station to the next often does not work effectively, meaning calls are dropped or, in some cases, connections are impossible.
A robust satellite radio link is being developed to react to the impairments caused by obstacles located along the railway (i.e., power lines and their metallic infrastructures). Today, Ku–band satellite terminals are available in industry, but also a prototype of a train terminal is being developed that will use a Ka–band two–way satellite to provide multimedia services to train passengers with smaller antenna sizes and higher bit rates.
LAND
The equipment needed to set up a backhauled picocell is extremely portable. For use on land, equipment does not have to be built to withstand the same pressures that it does for transport, though it does have to be sufficiently rugged and sufficiently compact to be carried. It does require a power source, even as small as a 100 W generator that could be included as part of the equipment. The hardware has been adapted to make it transportable by air, wheeled or tracked vehicle, or even by rucksack.
Military Applications
Maintaining lines of communication during military operations is of paramount importance. The standard technology used is radio. The ability to establish a 2 or 3G network anywhere, at any time, significantly increases the options available.
The encryption links indicate the network can be used for secure communication, with end–to–end privacy, quite appropriate for the military environment. It can be programmed to accept connections only from registered phones, and therefore reject unknown devices. It can also be used to jam, monitor GSM and GPRS content, and to redirect certain calls for intelligence purposes.
Because it can handle up to 14 simultaneous calls, the network can be used for operational and personal communications. While personal communications are good for morale purposes, operational communications are a “must have” priority. To that end, the network can be set up to ensure the most important communications always have preference for the use of the network.
Finally, the technology allows for point–to–point calls within the 700m radius of the picocell. These calls are free, as they are not routed via the satellite.
Emergency Preparation
As with military operations, effective and efficient communications are essential when dealing with large–scale emergencies, either man–made or natural, such as terrorist activity, flooding or widespread fires.
Emergency services use dedicated emergency networks where they exist, but often rely on public land–based and mobile networks for their communications. All terrestrial mobile networks are subject to failure should power supplies be interrupted. Back–up power supplies are generally in place but are not designed to last more than one or two hours, in many cases. The satellite–based technology using a picocell (with its own power supply) represents an excellent back–up solution.
Disaster Recovery
When disaster strikes, coordinating a response is often hampered by a lack of information and the ability to communicate to emergency services, government and aid organizations. For example, when the earthquake hit Pakistan, the international community responded quickly. Unfortunately, much of the effort was misdirected as there was little information about what and where the equipment was needed. Also difficult was communicating what little information was available to the relevant people. That meant that some villages were visited by emergency services several times a day, while others received no help at all.
In the first few hours of a disaster response, the opportunity to set up a robust communications network is critical to direct aid to the most badly effected areas to save more lives.
Other Applications
The number and range of potential applications for the backhauling of picocells is endless. It is suitable for any remote site, where the laying of cables is prohibitively expensive or impossible. It is suitable whenever a communications network needs to be established very quickly and securely. And the backhauling of picocells is highly suited for anyone who wishes to be in control of their communications—at all times.
Dr. Axel Jahn is Managing Director of TriaGnoSys, a leading provider of mobility satellite communications solutions for remote mobile air, sea and land communications from anywhere to anywhere via satellite.
Before founding TriaGnoSys, Axel worked at DLR, the German Space Agency, and he was the project manager on WirelessCabin. He has been at the forefront of the development of picocell–based satellite backhauling. TriaGnoSys solutions are used for a range of commercial applications, including the provision of GSM/GPRS services on commercial aircraft, cruise ships, container tracking and for military use.
TriaGnoSys is also involved in a wide range of research projects, focusing on focus on a broad range of mobile satellite communication areas in conjunction with leading academic, government and industry researchers to advance the state of the art in such areas as mobile end–to–end solutions, next generation satcom and aircom, and combined navigation/communications applications and technologies.