Mobile Communications Data Transfer Rates
UMTS and More
The mobile communications systems of tomorrow will offer much more than just telephony and text messaging. Experts predict that mobile multimedia services will play a major role in our daily lives. Ushering in the new era will be radio standards such as UMTS backed up by local mobile networks and digital radio services.
In late February, the Palais des Festivals in the French resort of Cannes played host to a show that was reminiscent of the town's more famous film festival: Long carpets and flowing canopies adorned the entrances, jugglers and hostesses were on hand to welcome the crowds, and luxury yachts bobbed at anchor in the bay. And yet it was not the stars of the silver screen that tens of thousands of visitors from around the world had come to see. Instead, they were here to catch up with the crème de la crème of a completely different sort: cell phone and computer companies plus the software industry and network operators. As you might expect, talk at the event focused less on Hollywood than on the latest terminals and applications, platforms and standards, customer requirements and cooperative ventures, business models and refinancing needs. In short, there was one central topic that dominated the 3GSM World Congressthe future of mobile communications.
Although there was no lack of heated discussion on specific details, the general mood was unequivocal: "The industry's currently going through a period of upheavalwe're moving away from pure voice and text communications toward a much richer form of multimedia mobile communications," was the view of many experts. It's a position shared by Hiroshi Nakamura, president of DoCoMo Europe, a subsidiary of NTT DoCoMothe company that already offers integrated data, audio and video services for cell phone users in Japan (see article Japan's Pioneers). Nakamura is convinced that we are undergoing a revolution in our communications culture. "Mobile multimedia services are much more than a means of communication; they're set to become a major part of everyone's lifecutting right across the generations, from teenagers to senior citizens," he said.
At the same time, nobody has any illusions concerning the amount of work required to realize this vision. For while there's a clearly delineated path leading from voice communications and the bare text messages of SMS all the way up to multimedia messagingjust as there is one from GSM and GPRS to the forthcoming UMTS and other third-generation (3G) standards (see box)it's also clear that there are a number of issues that still need to be resolved. The most important of these are:
- How to ensure that users can easily access the information they want from wherever they happen to bee.g. making sure the data for a multimedia presentation can be transmitted rapidly enough?
- How to display multimedia data on a small mobile terminal? (see article Maxi Displays and Mini Projectors)
- How to meet the new terminals' power requirements? (see articleFilling Stations for Cell Phones)
- And, perhaps the crucial question for the success of mobile communications companies: Which applications and services are attractive and useful enough to persuade customers to spend money on them (see articles Plugging Customers into the Multimedia Picture, UMTS on the Isle of Man and Siemens Mobile Acceleration)
"Other challenges include ease of operability using voice or gesture commands and ensuring secure data transfer, particularly for e-business applications," explains Martin Gebler, who is one of the Siemens representatives working on Vision Wireless World 2010 in the international Wireless World Research Forum. "Another major aspect is the likely division of the terminals market into at least three segments: terminals that, like today's cell phones, are essentially used only for voice communication; those that serve as a mobile office and mainly process data; and those that are essentially for entertainment applications such as games or videos." Finally, as Gebler points out, there are also the key issues of transparent online billing procedures for the customer, intelligent business and cooperation models for the operators, and standardized solutions that make it possible to send a multimedia message from any terminal, via any network, to any other terminal.
Technologies and Transfer Rates: An Overview
The relationship between data transfer rate and speed/range
kbps: kilobits per second,
Mbps: megabits per second,
MHz: megahertz,
GHz: gigahertz
GSM: Global System for Mobile Communication (used in over 160 countries).
HSCSD: High Speed Circuit Switched Data. Combines (pools) several time slots of GSM. Circuit-switched; establishes an exclusive transmission link for each connection.
GPRS: General Packet Radio Service. Operates in packet-switched manner similar to the technology used for Internet Protocoli.e. transmits all information as a stream of small data packets that the receiver then reassembles into the right order. With GPRS, cell phones can always be online, as subscribers are billed not for the transfer time interval but rather the volume of data or the type of service. Depending on the coding procedure and number of channels pooled, GPRS can achieve approx. 40 to 100 kbps.
EDGE: Enhanced Data Rates for Global Evolution. With new modulation, channel pooling and coding max. 384 kbps.
UMTS: Universal Mobile Telecommunications System. Standard of third generation of mobile communications (3G). Operates in specially licensed frequency bands in the 2 GHz range. Both packet-switched and circuit-switched. Theoretical maximum data transfer rate of 2 Mbps. There are two UMTS modes: W-CDMA (Wideband Code Division Multiple Access, also known as FDDFrequency Division Duplex) and TD-CDMA (Time Division CDMAalso called TDD, Time Division Duplex). Both work with two separate linksa downlink from transmitter to receiver and an uplink in the other direction. FDD is the more common procedure (in the current implementation, downlink up to 384 kbps, uplink 64 kbps). TDD has special advantages, including a very high spectral efficiency in the case of asymmetric connections. These achieve very high data transfer rates in downlinksideal conditions for downloading large amounts of data from the network.
DECT: European standard for cordless telephones.
Bluetooth: Ideal for wireless communication between devices separated by only a few meters. Operates in the 2.4 GHz band with data transfer rates of up to 721 kbps.
W-LAN: Wireless Local Area Network. Operates on license-free frequencies such as 2.4 GHz (IEEE 802.11b, also Wi-FiWireless Fidelitywith up to 11 Mbps) or 5 GHz (IEEE 802.11a and HiperLAN/2 with max. 54 Mbps). Transmission range 10 to 50 m.
HiperAccess: Fixed local radio network, e.g. for buildings (operates at 40.5 to 43.5 GHz).
Codes, Time Slots and the Party Effect In a GSM network, each call takes place on a fixed frequency channel, several hundred of which fit within the reserved frequency bands around 900 and 1,800/1,900 MHz. To use these frequencies more efficiently, each channel is also split into eight time slots that alternate with one another so quickly that the switchover is imperceptible to callers. By contrast, W-CDMA/UMTS does not use different frequencies and time slots. Instead, each connection in a particular radio cell uses the same carrier bandwidth of 5 MHz. This bandwidth is 25 times larger than that of GSMhence the name Wideband CDMA. The only thing that distinguishes one cell phone signal from another is that the signals are encrypted with different codes. Using a special algorithm, the cell phone is able to decode only the signal addressed to it from among many superimposed signals. The effect is like entering a party where people are talking in different languages, but immediately being able to pick out your own.
In other words, there was more than enough to talk about at Cannes. It was ten years ago that the launch of the digital GSM standard ushered in the second generation of mobile communications. Since then, experts have been talking of little else other than the standards for the third generation. Siemens has been involved in this process from the very beginning and today employs 4,000 researchers and developers in this field at facilities in Munich, Berlin, Ulm (Germany), Beijing (China) and Roke Manor (England). Ten years ago, it was already clear that the data transfer rate with GSM (see box at left) was adequate for voice communications but way too low for multimedia services. The initial question therefore was how to achieve higher rates. One option includes techniques such as HSCSD and EDGE (see box at left) with which several GSM channels can be combined. Another is to use GPRS, a data transfer technology for mobile radio applications that functions in a manner similar to the one used for the Internet. Yet none of these solves the fundamental problem in this field.
The secret to attaining higher data rates lies in the following rule of thumb: "Use a larger portion of the spectrum for data transfer and use it more efficiently." That's the technological explanation of why new frequency bands had to be defined for UMTS and why there was an expensive auction for the corresponding licenses. With UMTS, the spectrum (carrier bandwidth) used for a radio link is 25 times broader than with GSM. What's more, the spectral efficiencyi.e. the data rate attained per hertz of bandwidthis also two to three times better. That means that UMTS will transfer a correspondingly larger amount of data per second than a GMS network.
Telecommunications engineers also use another formula to express the maximum achievable data transfer rate: "The crucial factor is the relation between the received usable signal and the interference," explains mobile communications expert Dr. Werner Mohr, who represents Siemens on international committees and EU projects. According to this rule, the data transfer rate falls as the radio interference power risesas a result of the interference generated by transmitters in neighboring cells, for example. By contrast, network operators can boost data transfer rates through the use of so-called smart antennas. Instead of transmitting in all directions, these focus the radio beam as precisely as possible at the receiver. But data transfer rates fall when the receiver moves quicklyfor example, in a car or train (see illustration). This is mainly due to the need to quickly transmit the signals from one radio cell to another, which is complex and time-consumingthe smaller the radio cells are, the more time is needed.
With UMTS (or, to be more exact, W-CDMA, FDDsee box), it's possible to achieve data transfer rates of up to 384 kbps, which must then be shared by all the people using a radio cell at any one time. As the world's first UMTS networks showon the Isle of Man, for example, or in Monaco and Japanthis is certainly adequate for a range of interesting applications and will even support simple video links. There are also, however, locations where many people will all want to use demanding multimedia services at the same timeat airports, for instance, or trade fairs, conferences, hotels, company sites and university campuses. In recent years, a mobile equivalent of the wired Ethernet has been developed for such "indoor" applications. The so-called W-LANs (Wireless Local Area Networks) merely require access pointstransmitter and receiver units about the size of a book, which provide access to the fixed networksand a special plug-in card with a small antenna for each notebook, pocket PC or other mobile terminal.
W-LAN systems offer the ideal radio-based solution for anyone wanting to surf the Internet or log on to a company network via a high-speed connection from "hot spots" such as hotels or airports. Here, a Siemens installation, including wall-mounted i-Gate access point, in a hotel in Munich, Germany
The frequencies for W-LANs are license-free and, depending on the standard used, provide very high data transfer rates of up to 11 Mbps (IEEE 802.11b) or even 54 Mbps (HiperLAN/2 and IEEE 802.11a) to a range of some tens of meters from the transmitter. Siemens has also developed W-LAN devices and systems, including the Access Point I-Gate and the i250-Access-Gateway, that can handle all the security and billing functions for a network, whether wireless or fixed. Today, W-LANs already operate at a number of hotels, airports and offices worldwide. And in a recent field test in Norway, Ericsson and network operator Telenor Mobile showed that it is feasible to combine a W-LAN with a GPRS or UMTS network. Wherever the limited range of the W-LAN comes to an end, communications can then be provided by mobile radio network.
For industry experts, the road to mobile communications is perfectly clear. "For indoor applications, a W-LAN represents the logical extension to a UMTS mobile radio network," said Ingelin Drepping, Vice President for Strategy at Telenor Mobile, at the 3GSM World Congress in Cannes. Experts are unanimous in their opinion that W-LAN or similar broadband networks would be much too expensive to provide comprehensive coverage for a city, as too many access points are needed, which all have to be connected to the fixed network. "In coming years, UMTS will be used to cover large areas," says Drepping. "W-LAN and UMTS won't compete with each other, since they are complementary. They are partner systems that allow us to provide customers with optimum transmission rates." Other extensions might also make sense. "These could include digital broadcast services such as Digital Video Broadcasting (DVB), transmission via satellite and terrestrial antennas," Mohr explains. "That would be useful for the kind of applications that many people use simultaneouslyTV programs, video films or news items for mobile terminals."
Ulrich Eberl