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LTE Standards - Jean-Gabriel Rémy
Introduction
Long Term Evolution (LTE) is commonly marketed as fourth generation (4G). LTE and LTE Advanced have been recognized by International Telecommunications Union – Radiocommunications (ITU-R) and International Telecommunications Union – Telecommunications (ITU-T) as the principal solution for the future mobile communication networks standards. Thus, they are the framework of what marketing calls 4G and maybe also fifth generation (5G). They have registered logos:
Figure I.1. LTE and LTE Advanced logo
It seems interesting to look at the evolution of mobile communication systems from their appearance upto LTE. This move has obviously been driven by commercial motivations as well as by the extraordinary improvement of microelectronics, especially from the 1960s to the present day. Functionalities, computing power and miniaturization have drastically progressed, while cost has constantly decreased.
I.1. Mobile communication systems: 0G, 1G, 2G, 3G, 4G and 5G
In this short introduction, many mobile communication systems will be omitted:
– military communications and public utilities communications;
– maritime and aviation communications;
– trunk systems and more generally all kinds of professional dedicated radio systems.
It does not mean that LTE will not have specific adaptations in order to fit the special requirements of such systems, especially for its radio interface, avoiding expensive developments being invested for a limited population of users.
Only public land mobile network (PLMN) will be considered: the so-called 4G
belongs to this category as long as LTE is used for public communication.
Also, the impressive list of various systems, which did not reach a high level of success, especially outside their country of origin, has been avoided.
The classification of mobile systems into generations is not strictly related to any given metrics or parameters. It corresponds to marketing considerations. Therefore, it is commonly agreed upon, both by industry and by academia, and hence conceived to be an unwritten standard.
I.1.1. Rationale
Mobile communications have always been a wish for most of the people. Of course, at the beginning, the mobile networks have been invested for precise applications, such as military communications or professional management. The introduction of PLMN came later. But the requirements for mobile services are most common for public systems and more specific networks.
For a network addressing all citizens, the investment is very high, especially in research and development – millions of coded instructions have to be written and validated. Also, the precise areas where the service will be necessary have to be determined. Therefore, it is necessary to analyze what the customers are ready to pay for to avoid vain efforts and investments. Excluding applications that are just using the mobile network as a support, mobile services can be classified into three categories:
– Mobile telephony: the mobile subscriber wants to discuss in realtime with distant interlocutors, who are connected with either a fixed telephone or a mobile set. Telephony offers the possibility to get immediate up-to-date information as well as the means to discuss any difficult item. Up until now it has been the most money making
application.
– Paging: by some means of collection of the information, the network offers the capacity to alert the mobile subscriber that something of interest is happening. The paging can be limited to a very simple binary signal – some tone or light – and the customer has to call an information center to get the message. It can also be accompanied by a short message, either written or vocal, giving the main details of the message. This paging is very popular and is now offered by the short message service (SMS) of Global System for Mobile communications (GSM) and further technologies. The SMS service is a teleservice
, which means that the operator must carry it to destination. The multimedia messaging service (MMS) delivers much richer information, but it is not as reliable, because the delivery of messages is not guaranteed by the network operator; it is supported by a bearer service
, the quality of service (QoS) is limited to the operator’s commitment.
– The Internet, fax or any written dialog: in the latter case, the mobile network offers the possibility to carry the office environment of its customer anywhere. Like MMS, the Internet and Internet-like services are generally bearer services, which are sold with a certain grade of QoS.
For these services, the mobile network can provide two kinds of access:
– nomadic access: the service is available anywhere inside the coverage of the network, but the customer must be static or is allowed to move very little;
– full mobile access: the service is available when the customer is moving, eventually at any speed, again within the limits of the geographical coverage service.
The paradigm of mobile communications is simple to summarize:
– be able to be connected to and receive information from any calling party;
– be able to be connected to any called party;
– full bidirectional access and real-time exchange of information;
– be accessed anywhere, outdoor, indoor, in urban and rural environment;
– full bidirectional access at anytime.
Going into detail shows a lot of issues:
– size of the mobile device: devices such as smartphones or tablets such have limited space to support the broadband module; these days, the terminal can also be some communication part of a machine for machine to machine (M2M) communications;
– nature and content of information to be transmitted, i.e. full telephony, television or data transmission, bilateral or unilateral.
I.1.2. Short history of mobile communications, milestones
I.I.2.I. 0G
The systems that allow customers to communicate on the move depend on electronics and microelectronics technology. Therefore, before the mass production of semiconductors, only experimental services have been deployed. The first network appeared in the United States in 1940, with mobiles using electronic tubes for car mounted terminals. Connection to the called party was made by human operators, in a way similar to that ensured for maritime communications.
Between 1960 and 1980, quite a few mobile communication systems were designed and deployed for either telephony or paging. Most of the advanced countries installed a home-made network. These systems offered automatic dialing with a good communication quality, obtained with a frequency/phase modulation radio access network. The radio path consisted of narrow frequency channels – 30 kHz in Northern America and 25 kHz everywhere else in the world. With the advent of transistors, a few handheld mobiles were available, especially for paging.
Of course, the service was only operated by incumbent fixed telecommunication operators, which found a new service for wealthy customers.
These systems will be called 0G.
1.1.2.2. 1G
During the 70s, some important innovations have brought a kind of revolution in the mobile communication world:
– computer driven frequency tuning (frequency synthesis) allowing us to reach with precision a given radio frequency channel among many with only one quartz oscillator. This technology opened the way to high-capacity systems in so-called analog technology – where each individual communication is allocated one (time division multiplex (TDM) or simplex) or two (frequency division duplex (FDD) or duplex) precise narrow band frequency channels – managing hundreds of radio frequencies instead of a few tens in the previous systems. With such a number of channels, the radio communication system becomes able to cope with a large number of customers. Also, frequency synthetization opened a way for massive production of handheld terminals:
– standardization and generalization of Signaling System No. 7 (SS7) designed for telephony, mainly the international version of ISDN;
– availability of microcomputers and computing chips offering greater speed and power for real-time processing, thus allowing us to implement sophisticated encoding, error correction and new transmission standards.
All these innovations were applied to new designs including some important breakthroughs:
– localization of the mobile terminal, which could be done manually, and automatically realized, in order to have the ability to route incoming calls;
– detection of the need for changing the communication in progress from one radio base station (one cell
) to another due to degradation of the radio link quality, and execution of the handover
(US: hand off) to the other base station/cell, which is selected to provide a good quality communication.
With all these new developments, the cost of R&D skyrocketed and only a few systems could be studied and deployed with a worldwide impact. Among them two standards will dominate the market:
– First, the advanced mobile phone system (AMPS), designed by the Bell Labs with a prototype rollout installed in Chicago in 1978, serving more than one thousand customers. AMPS has been the first system to offer real-time seamless handover. This network probably shows the best possible design for a system where each individual communication carried by an individual duplex frequency modulation (FM) (or phase modulation (PM)) channel, each channel being given a narrow frequency bandwidth. The main features were standardized by the American National Standard Institute (ANSI). This AMPS system has the particularity of being able to modify channel spacing and FM excursion very simply, which allowed us to adapt it to various frequency configurations (channel spacing of 30 kHz in the USA and 25 kHz in Europe and Japan). This is achieved simply by modifying the clock frequency driving the network. In North America, it was the genuine AMPS (initially, A stood for American).
In Europe and Japan, it was a modified version with a 25 kHz channel spacing, called Total Access Communication System (TACS), Europe TACS (ETACS) and Japan TACS (JTACS)). Due to some specific US political process aiming at introducing competition, AMPS and TACS massive deployment was delayed to 1985.
– However, the Scandinavian countries joined their strengths and developed the Nordic Mobile Telephone (NMT) system. This standard is by far simpler than the AMPS/TACS in all aspects of the technology. The spread of NMT is somehow due to the above-mentioned American political process, which delayed the mass deployment of AMPS. NMT became available around 1982 and was immediately rolled out in all Scandinavian countries.
Nevertheless, due to its transnational origin, NMT introduced a very interesting feature: automatic international roaming.
Another cellular system of the first generation was designed and deployed in Germany (C-Netz) and France (Radiocom, 2000) and counted a few hundred thousand subscribers. There was also a Japanese home-made cellular
system.
These systems and their unlucky competitors are considered to be 1G.
I.1.2.3. 2G
In the 1980s, with the spectacular increase of the computing power of integrated circuits, technology continued to progress with many breakthroughs:
– Development of vocoders. In concordance with the design of very powerful processors. Instead of needing a bitrate of 64 kbps to correctly digitalize narrow band voice telephony as calculated from the ordinary Shannon sampling, a telephony 4 kHz analog signal can be coded with a very good quality with 12 kbps, and even 6 kbps (GSM). For professional systems, vocoders provide a clear voice communication with a few hundred kilobits per second.
– Vocoders are the key to switch from analog FM (or PM) radio to full digital transmission for telephony. The compression of the voice signal is a question of processing power. Today, a very high quality sound can be coded with less than 10 kbps; and correct voice communications are now available for professional and military communications with a bitrate of less than 1 kbps.
– Development of identity chips. The 1G German C-Netz had introduced a device to dissociate the subscription from the mobile terminal hardware. Such chips make it possible to encrypt communications and protect customers’ privacy. AMPS or NMT were identifying the mobile terminal by a number which was included inside it and was very easy to copy or modify; so, customers were often suffering from pirated use of their identity. Concerning the privacy of communications, 1G networks did not provide protection against eavesdropping.
In the meantime, continental European countries have been conscious of their technological backwardness compared with AMPS. In 1982 the GSM
was created (at the beginning it was a special mobile group
led by German FTZ and French Centre national d’études des télécommunications (CNET)), which was commissioned to study a revolutionary mobile system based on a fully digital radio access subsystem, since it was considered difficult to surpass AMPS as an analog system. This new system, also called GSM, passed through a lot of studies until 1991. Code division multiple access (CDMA), which was in the 1980s a spread spectrum technique in use for military purposes, was experienced in 1985. At that time, CDMA showed need for too much computing power, far over the performance of the available chips, thus a simpler process, time division multiple access (TDMA), was chosen.
In 1987, all countries of the European Union signed a Memorandum of Understanding (MoU), which was accepted afterward by all GSM operators, always labeled as MoU. In this MoU, these countries decided:
– to roll out a GSM coverage from 1991 onward using the common frequency bands which had been decided in 1979 for a common mobile system;
– to authorize without restriction automatic international roaming for GSM mobiles, all expenses being paid by the home country of the subscription.
GSM takes up the C-Netz innovation of selling the mobile terminal and the operator subscription separately, the latter being materialized by a SIM card, which is inserted into the mobile set. The chip of the SIM card controls all the telecommunication functions of the mobile and masters the encryption of the radio path for the calls.
GSM introduces a kind of paging with the SMS
, which became a very important part of the communications.
As a response to the introduction of GSM, the AMPS industry designed the D-AMPS (IS-136 standard), where AMPS channels are used in TDMA mode in order to increase the overall network capacity.
Beside the TDMA systems, the American society Qualcomm introduced its proprietary design based on a CDMA encoding, later called CDMA 2000, which was standardized as IS-95 by ANSI. This standard was adopted by South Korea, which had to solve a lot of difficulties.
And again, Japanese NTT developed and rolled out a TDMA system, called PDC. They also rolled out a simpler system called PHS, which is probably the first implementation of a multiple input multiple output (MIMO) antenna system.
All these systems can be considered to be the 2G mobile standards.
I.1.2.4. 3G, the need for fast data transmission
Of course, as time passed, the technology of chips continued to improve drastically. During the 1990s it finally delivered processors having a sufficient computing power to cope with the Qualcomm CDMA mobile system.
In the 1990s, while GSM was being implemented all over the world including Northen America, the operators of fixed communications introduced the Internet services. At the beginning the available bitrate was limited to 50 kbps. Later it was increased to 10 Mbps downlink particularly with an Asymmetric Digital Subscriber Line (ADSL), provided the customer’s home is located a few hundred meters from the central office. The industry of mobile communications decided to adopt the internet service in their strategy, even when the response from the subscribers’ base surveys showed very little interest in telephony and SMS. GSM developed a wart
, called General Packet Radio Service (GPRS), supporting data transmission upto 50 kbps. In response, CDMA 2000 introduced data transmission upto 144 kbps. As an answer, GSM standardized Enhanced Data Rates for GSM Evolution (EDGE), providing upto 240 kbps, which was rolled out massively by ATT Wireless in the USA, where it was facing the competition of Verizon Wireless, the CDMA 2000 champion.
The way Qualcomm system manages data transmission makes it easy to reach good performances since the data flow and the telephony are transmitted by different networks, at least in the Evolution Data Optimized (EVDO) version. This conception answers the difficult challenge of mobility:
– telephony is a real-time communication, but accepts very short cuts, e.g. 300 ms; this is managed by a smooth handover process;
– data transmission in Transmission Control Protocol-Internet Protocol (TCP-IP) shows very poor performance if the flow is cut, as is the case when the mobile travels from one cell to another. In that case, a reselection is necessary and the usable bitrate is very poor
Considering that in a town like Paris the mobile terminals process an average of four handovers for a 2 min call, the network operator has to make a critical choice concerning the parameters of its network:
– either the parameter set favors telephony with a change of cell achieved as soon as possible to give the customer a very good telephony quality;
– or the parameters are stiffened and the mobile will drag its radio channel as far as possible in order to avoid reselection. It results in damaging the frequency planning, as well as creating poor quality telephone calls.
Of course, most of the GSM operators chose to favor telephone calls.
To examine what could be the future of mobile communications after the worldwide success of GSM, the European Union launched a consultation on the possible technologies which could be developed. Scandinavia pushed a variant of Qualcomm CDMA technology called wide band CDMA (WCDMA) very hard, which won the competition. This WCDMA technology immediately faced the issue of patents, since CEO of Qualcomm, who was a highly respected former professor of signal theory at MIT, had patented all possible implementation of CDMA. It also faced plenty of issues with the management of power, with the mobile needing too much energy, far more than GSM.
Nevertheless, the industry worked very hard and some 10 years later, beginning of the 2000s, the WCDMA, renamed High Speed Packet Access (HSPA) and HSPA+, could service data users correctly.
In the meantime, ATT had pushed in the 3rd Generation Partnership Project (3GPP) standard body, a variant of GSM, called EDGE, which had been rolled out by all GSM operators. The advantage of EDGE