Difference between revisions of "Mobile Communications/Technical Innovations of LTE"

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== Systemarchitektur==
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== System Architecture==
 
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Die LTE&ndash;Architektur ermöglicht ein vollständig auf dem IP&ndash;Protokoll basierendes Übertragungssystem. Um dieses Ziel zu erreichen, musste die für UMTS spezifizierte Systemarchitektur nicht nur im Detail verändert, sondern teilweise komplett neu konzipiert werden. Dabei wurden auch andere IP&ndash;basierte Technologien wie&nbsp; ''mobiles WiMAX''&nbsp; oder&nbsp; ''WLAN''&nbsp; integriert, um in diese Netze problemlos wechseln zu können.<br>
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The LTE&ndash;architecture enables a transmission system based entirely on the IP&ndash;protocol. In order to achieve this goal, the system architecture specified for UMTS not only had to be changed in detail, but in some cases completely redesigned. In the process, other IP-based technologies such as&nbsp; ''mobile WiMAX''&nbsp; or&nbsp; ''WLAN''&nbsp; were also integrated in order to be able to switch to these networks without any problems.<br>
  
In UMTS&ndash;Netzen (linke Grafik) ist zwischen einer Basisstation (NodeB) und dem Kernnetz noch der&nbsp; <i>Radio Network Controller</i>&nbsp; (RNC) zwischengeschaltet, der für den Wechsel zwischen verschiedenen Zellen hauptverantwortlich ist und der zu Latenzzeiten von bis zu 100 Millisekunden führen kann.<br>
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In UMTS&ndash;networks (left graphic), the&nbsp; <i>Radio Network Controller</i>&nbsp; (RNC) is inserted between a base station (NodeB) and the core network, which is mainly responsible for switching between different cells and which can lead to latency times of up to 100 milliseconds.<br>
  
[[File:EN_Mob_T_4_2_S6.png|center|frame|Systemarchitektur bei UMTS (UTRAN) und LTE (EUTRAN)|class=fit]]
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[[File:EN_Mob_T_4_2_S6.png|center|frame|System Architecture for UMTS (UTRAN) and LTE (EUTRAN)|class=fit]]
  
Die Neukonzipierung der Basisstationen (&bdquo;eNodeB&rdquo; anstelle von &bdquo;NodeB&rdquo;) und die Schnittstelle &bdquo;X2&rdquo; sind die entscheidenden Weiterentwicklungen von UMTS hin zu LTE. Die rechte Grafik illustriert insbesondere die mit der neuen Technologie einhergegangene Reduzierung der Komplexität gegenüber UMTS (linke Grafik).  
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The redesign of the base stations ("eNodeB" instead of "NodeB") and the interface "X2" are the decisive further developments from UMTS towards LTE. The graphic on the right illustrates in particular the reduction in complexity compared to UMTS that goes hand in hand with the new technology (left graphic).  
  
Die&nbsp; '''LTE&ndash;Systemarchitektur'''&nbsp; lässt sich in zwei große Bereiche einteilen:
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The&nbsp; '''LTE&ndash;system architecture'' &nbsp; can be divided into two major areas:
*das LTE&ndash;Kernnetz&nbsp; <i>Evolved Packet Core</i>&nbsp; (EPC),<br>
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*the LTE&ndash;core network&nbsp; <i>Evolved Packet Core</i>&nbsp; (EPC),<br>
  
*die Luftschnittstelle&nbsp; <i>Evolved UMTS Terrestrial Radio Access Network</i>&nbsp; (EUTRAN) &ndash; eine Weiterentwicklung von&nbsp; [[Examples_of_Communication_Systems/UMTS%E2%80%93Netzarchitektur#Architektur_der_Zugangsebene|<i>UMTS Terrestrial Radio Access Network</i>]]&nbsp; (UTRAN).<br><br>
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*the air interface&nbsp; <i>Evolved UMTS Terrestrial Radio Access Network</i>&nbsp; (EUTRAN), a further development of&nbsp; [[Examples_of_Communication_Systems/UMTS Network Architecture#Access Level_Architecture_of_the_Access Level|<i>UMTS Terrestrial Radio Access Network</i>]]&nbsp; (UTRAN).<br><br>
  
EUTRAN überträgt die Daten zwischen dem Endgerät und der LTE&ndash;Basisstation (&bdquo;eNodeB&rdquo;) über die sogenannte S1&ndash;Schnittstelle mit zwei Verbindungen, eine für die Übertragung von Nutzdaten und eine zweite für die Übertragung von SignalisierungsdatenAus obiger Grafik erkennt man:
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EUTRAN transmits the data between the terminal and the LTE&ndash;base station ("eNodeB") via the so-called S1&ndash;interface with two connections, one for the transmission of user data and a second for the transmission of signalling dataYou can see from the above graphic:
*Die Basisstationen sind außer mit dem EPC auch mit den benachbarten Basisstationen verbunden. Diese Verbindungen (X2&ndash;Schnittstellen) bewirken, dass  möglichst wenige Pakete verloren gehen, wenn sich das Endgerät aus dem Umkreis einer Basisstation in Richtung einer anderen bewegt.<br>
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*The base stations are connected not only to the EPC but also to the neighboring base stations. These connections (X2&ndash;interfaces) have the effect that as few packets as possible are lost when the terminal device moves from the vicinity of one base station towards another.<br>
  
*Dazu kann die Basisstation, deren Versorgungsgebiet der Nutzer gerade verlässt, eventuell noch zwischengespeicherte Daten direkt und schnell an die &bdquo;neue&rdquo; Basisstation weitergeben. Damit ist eine (weitgehend) durchgehende Übertragung sichergestellt.<br>
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*For this purpose, the base station whose service area the user is just leaving can pass on any cached data directly and quickly to the "new" base station. This ensures (largely) continuous transmission.<br>
  
*Die Funktionalität des RNC geht zum Teil in die Basisstation, zum anderen in die&nbsp; <i>Mobility Management Entity</i>&nbsp; (MME) im Kernnetz über. Diese Reduktion der Schnittstellen verkürzt die Signaldurchlaufzeit im Netzwerk und das Handover signifikant auf 20 Millisekunden.<br>
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*The functionality of the RNC is partly transferred to the base station and partly to the&nbsp; <i>Mobility Management Entity</i>&nbsp; (MME) in the core network. This reduction of the interfaces significantly shortens the signal throughput time in the network and the handover to 20 milliseconds.<br>
  
*Die LTE&ndash;Systemarchitektur ist zudem so ausgelegt, dass sich zukünftig&nbsp; <i>Inter&ndash;NodeB&ndash;Verfahren</i>&nbsp; (wie&nbsp; <i>Soft&ndash;Handover</i>&nbsp; oder&nbsp; <i>Cooperative Interference Cancellation</i>) einfach integrieren lassen.<br>
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*The LTE&ndash;system architecture is also designed so that future&nbsp; <i>Inter&ndash;NodeB&ndash;procedures</i>&nbsp; (such as&nbsp; <i>Soft&ndash;Handover</i>&nbsp; or&nbsp; <i>Cooperative Interference Cancellation</i>) can be easily integrated.<br>
  
== LTE&ndash;Kernnetz: Backbone und Backhaul ==
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== LTE&ndash;Core network: Backbone and Backhaul ==
 
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Das LTE&ndash;Kernnetz&nbsp; <i>Evolved Packet Core</i>&nbsp; (EPC) eines Netzbetreibers &ndash; in der Fachsprache&nbsp; <i>Backbone</i>&nbsp; &ndash; besteht aus verschiedenen Netzwerkkomponenten. Das EPC ist mit den Basisstationen über das&nbsp; <i>Backhaul</i>&nbsp; (englische Bezeichnung für <i>Rücktransport</i>) verbunden. Darunter versteht man die Anbindung eines vorgelagerten, meist hierarchisch untergeordneten Netzknotens an einen zentralen Netzknoten.<br>
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The LTE&ndash;core network&nbsp; <i>Evolved Packet Core</i>&nbsp; (EPC) of a network operator, in the technical language&nbsp; <i>Backbone</i, consists of various network components. The EPC is connected to the base stations via the&nbsp; <i>Backhaul</i>. This means the connection of an upstream, usually hierarchically subordinated network node to a central network node.<br>
  
Momentan besteht das&nbsp; <i>Backhaul</i>&nbsp; zum Großteil aus Richtfunk und sogenannten E1&ndash;Leitungen. Diese sind Kupferleitungen und erlauben einen Durchsatz von ca. 2 Mbit/s. Für GSM&ndash; und UMTS&ndash;Netzwerke waren diese Verbindungen noch ausreichend, aber bereits für großflächig konzipertes&nbsp; [[Examples_of_Communication_Systems/Weiterentwicklungen_von_UMTS#High.E2.80.93Speed_Downlink_Packet_Access| HSDPA]]&nbsp; reichen solche Datenraten nicht mehr. Für LTE ist ein solches&nbsp; <i>Backhaul</i>&nbsp; komplett unbrauchbar:
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Currently, the&nbsp; <i>Backhaul</i>&nbsp; consists mainly of directional radio and so-called E1&ndash;lines. These are copper lines and allow a throughput of about 2 Mbit/s. For GSM&ndash; and UMTS&ndash;networks these connections were still sufficient, however, for the large-scale conceived&nbsp; [[Examples_of_Communication_Systems/Further Developments_of_UMTS#High.E2.80.93Speed_Downlink_Packet_Access| HSDPA]]&nbsp; such data rates are no longer adequate. For LTE such a&nbsp; <i>Backhaul</i>&nbsp; is completely unusable:
*Das langsame Kabelnetzwerk würde die schnellen Funkverbindungen ausbremsen; insgesamt wäre kein Geschwindigkeitszuwachs festzustellen.<br>
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*The slow cable network would slow down the fast wireless connections; overall, there would be no increase in speed.<br>
  
*Aufgrund der geringen Kapazitäten der Leitungen mit E1&ndash;Standard wäre auch ein Ausbau mit weiteren baugleichen Leitungen nicht wirtschaftlich.<br><br>
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*Due to the low capacities of the lines with E1&ndash;standard, an expansion with further lines of the same construction would not be economical.
  
Im Zuge der LTE&ndash;Einführung musste also das&nbsp; <i>Backhaul</i>&nbsp; neu entworfen werden. Dabei war es wichtig, Zukunftssicherheit im Auge zu behalten, stand doch die nächste Generation&nbsp; ''LTE&ndash;Advanced''&nbsp; bereits vor der Einführung. Schenkt man dem von Experten propagierten&nbsp; <i>Moore's Law</i>&nbsp; für Mobilfunkbandbreiten Glauben, so ist die teure Neuverlegung besserer Kabel  der wichtigste Faktor für die Zukunftssicherheit.<br>
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In the course of the introduction of LTE, the&nbsp; <i>Backhaul</i>&nbsp; had to be redesigned. It was important to keep an eye on future security, since the next generation&nbsp; ''LTE&ndash;Advanced''&nbsp; was already in place before the introduction. If one believes the experts' propaganda&nbsp; ''Moore's Law'' for mobile phone bandwidths, the most important factor for future security is the expensive new installation of better cables.<br>
  
Aufgrund der rein paketorientierten Übertragungstechnik bietet sich für das LTE&ndash;Backhaul der ebenfalls IP&ndash;basierte Ethernet&ndash;Standard an, der mit Hilfe von Lichtwellenleitern realisiert wird. Die Firma Fujitsu stellte 2009 in der Studie&nbsp; [Fuj09]<ref name='Fuj09'>Fujitsu Network Communications Inc.: ''4G Impacts to Mobile Backhaul.'' [http://www.fujitsu.com/downloads/TEL/fnc/whitepapers/4Gimpacts.pdf PDF–Internetdokument].</ref>&nbsp; zudem die These auf, dass die momentane Infrastruktur noch für die nächsten zehn bis fünfzehn Jahre eine wichtige Rolle für das LTE&ndash;Backhaul spielen wird.<br>
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Due to the purely packet-oriented transmission technology, the Ethernet&ndash;standard, which is also IP&ndash;based, is suitable for the LTE&ndash;Backhaul, which is realized with the help of optical fibers. In 2009, the company Fujitsu presented in the study&nbsp; [Fuj09]<ref name='Fuj09'>Fujitsu Network Communications Inc.: ''4G Impacts to Mobile Backhaul.'' [http://www.fujitsu.com/downloads/TEL/fnc/whitepapers/4Gimpacts.pdf PDF Internet document].</ref>,&nbsp; also the thesis that the current infrastructure will continue to play an important role for LTE&ndash;Backhaul for the next ten to fifteen years.<br>
  
Für den Generationenwechsel hin zu einem Ethernet&ndash;basierten&nbsp; <i>Backhaul</i>&nbsp; gibt es zwei Ansätze:
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There are two approaches for the generation change to an Ethernet&ndash;based&nbsp; <i>Backhaul</i>&nbsp;:
*der parallele Betrieb der Leitungen mit E1 und Ethernet&ndash;Standard,<br>
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*the parallel operation of the lines with E1 and Ethernet&ndash;standard,<br>
  
*die sofortige Migration zu einem auf Ethernet basierenden&nbsp; <i>Backhaul</i>.<br><br>
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*the immediate migration to an Ethernet-based&nbsp; <i>Backhaul</i>.<br><br>
  
Ersteres hätte den Vorteil, dass die Netzbetreiber den Sprachverkehr weiterhin über die alten Leitungen laufen lassen könnten und ausschließlich den bandbreitenintensiven Datenverkehr über die leistungsfähigeren Leitungen abwickeln müssten.  
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The former would have the advantage that the network operators could continue to run voice traffic over the old lines and would only have to handle bandwidth-intensive data traffic over the more powerful lines.  
  
Die zweite Möglichkeit wirft einige technische Probleme auf:
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The second option raises some technical problems:
*Die vorher durch die langsamen E1-Standard&ndash;Leitungen transportierten Dienste müssten sofort auf ein paketbasiertes Verfahren umgestellt werden.<br>
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*The services previously transported through the slow E1-standard lines would have to be switched immediately to a packet-based procedure.
  
*Ethernet bietet (anders als der jetzige Standard) bisher keine&nbsp; <i>End&ndash;to&ndash;End&ndash;Synchronisierung</i>, was beim Funkzellenwechsel zu starken Verzögerungen bis hin zu Dienstunterbrechungen führen kann &ndash; also eine gewaltige Einbuße der Servicequalität.  
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*Ethernet does not offer (unlike the current standard) any&nbsp; <i>End&ndash;to&ndash;End&ndash;Synchronization</i>, which can lead to severe delays or even service interruptions when changing radio cells, thus a huge loss of service quality.  
*Im Konzept&nbsp; [https://en.wikipedia.org/wiki/Synchronous_Ethernet Synchronous Ethernet]&nbsp; (SyncE) wurden jedoch von der Fa. Cisco bereits Vorschläge unterbreitet, wie die Synchronisation realisiert werden könnte.<br>
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* However, in the concept&nbsp; [https://en.wikipedia.org/wiki/Synchronous_Ethernet Synchronous Ethernet]&nbsp; (SyncE), the Cisco company has already made suggestions as to how synchronization could be realized.<br>
  
  
Für Ballungsgebiete wäre eine direkte Umstellung des Backhauls sicher lohnenswert, da für eine vergleichsweise hohe Zahl an neuen Nutzern nur relativ wenige neue Kabel verlegt werden müssten.  
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For conurbations, a direct conversion of the backhaul would certainly be worthwhile, as relatively few new cables would have to be laid for a comparatively high number of new users.  
  
Im ländlichen Raum ergäben sich aber durch größere Grabungsarbeiten schnell hohe Kosten. Dies ist aber genau der Bereich, der laut der&nbsp; [[Mobile_Communications/Allgemeines_zum_Mobilfunkstandard_LTE#LTE.E2.80.93Frequenzbandaufteilung|getroffenen  Vereinbarung]]&nbsp; zwischen der Bundesregierung  und den (deutschen) Mobilfunkbetreibern als erstes abgedeckt werden muss. Hier müsste (und wird wohl) der meist vorhandene Richtfunk auf hohe Datenraten erweitert werden.<br>
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In rural areas, however, major excavation work would quickly result in high costs. However, this is exactly the area which must be covered first, according to the&nbsp; [[Mobile_Communications/General Information on the LTE Mobile Communications Standard#LTE frequency band splitting|agreement reached]]&nbsp; between the federal government and the (German) mobile phone operators. Here, the mostly existing microwave radio link would have to be (and probably will be) extended to high data rates.<br>
  
==Aufgaben zum Kapitel==
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==Exercises for Chapter==
 
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<br>
[[Aufgaben:4.2 FDD, TDD und Halb–Duplex|Aufgabe 4.2: FDD, TDD und Halb–Duplex]]
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[[Aufgaben:Exercise 4.2: FDD, TDD and Half-Duplex]]
  
[[Aufgaben:Aufgabe_4.2Z:_MIMO–Anwendungen_bei_LTE|Aufgabe 4.2Z: MIMO–Anwendungen bei LTE]]
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[[Aufgaben:Exercise 4.2Z: MIMO Applications in LTE]]
  
==Quellenverzeichnis==
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==List of Sources==
  
 
<references/>
 
<references/>
  
 
{{Display}}
 
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Revision as of 18:57, 18 October 2020

For voice transmission with LTE


Unlike previous mobile phone standards, LTE only supports packet-oriented transmission. For voice transmission, however, a connection-oriented transmission with fixed reservation of resources would be better, since a "fragmented transmission", as is the case with the packet-oriented method, is relatively complicated.

The problem of integrating voice transmission methods was one of the major challenges in the development of LTE, as voice transmission remains the largest source of revenue for network operators. There were a number of approaches, as it can be seen in the internet article   [Gut10][1]

(1)   A very simple and obvious method is  Circuit Switched Fallback  (CSFB). Here a wire-bound transmission is used for the voice transmission. The principle is:

  • The terminal device logs on to the LTE–network and in parallel also to a GSM– or UMTS–network. When an incoming call is received, the terminal device receives a message from the  Mobile Management Entity  (MME, control node in the LTE–network for user–authentication), whereupon a wire-bound transmission via the GSM– or the UMTS–network is established.
  • A disadvantage of this solution (actually it is a "problem concealment") is the greatly delayed connection establishment. In addition, CSFB prevents the complete conversion of the network to LTE.


(2)   Another possibility for the integration of voice in a packet-oriented transmission system is offered by  Voice over LTE via GAN  (VoLGA), which is based on the from  3GPP  developed by  [Generic Access Network) . In brief, the principle can be described as follows:

  • GAN enables line-related services via a packet-oriented network (IP–network), for example WLAN  (Wireless Local Area Network). With compatible end devices one can register oneself in the GSM–network over a WLAN–connection and use line-based services. VoLGA uses this functionality by replacing WLAN with LTE.
  • The fast implementation of VoLGA is advantageous, as no lengthy new development or changes to the core network are necessary. However, a so-called  VoLGA Access Network Controller  (VANC) must be added to the network as hardware. This takes care of the communication between the end device and the  Mobile Management Entity  or the core network.

Even though VoLGA does not need to use a GSM– or UMTS–network for voice connections like CSFB, it was considered by the majority of the mobile community as an (unsatisfactory) bridge technology due to its user-friendliness. T–Mobile has long been a proponent of the VoLGA–technology, but also stopped further development in February 2011.

In the following we describe a better solution proposal. Keywords are  IP Multimedia Subsystem  (IMS) and  Voice over LTE  (VoLTE). The operators in Germany switched to this technology relatively late:   Vodafone and O2 Telefonica at the beginning of 2015, Telekom at the beginning of 2016.

This is also the reason why the switch to LTE in Germany (and in Europe in general) was slower than in the USA. Many customers did not want to pay the higher prices for LTE as long as there was no well-functioning solution for integrating voice transmission.


VoLTE - Voice over LTE


From today's point of view (2016), the most promising approach to integrating voice services into the LTE–network, some of which are already established, is  Voice over LTE  – in short: VoLTE. This standard, officially adopted by the  GSMA,  the worldwide industry association of more than 800 mobile network operators and over 200 manufacturers of cell phones and network infrastructure, is exclusively IP–packet-oriented and is based on the  IP Multimedia Subsystem  ('IMS), which was already defined in the UMTS–Release 9 in 2010. The technical facts about IMS are:

  • The IMS–basic protocol is the one from  Voice over IP  known  Session Initiation Protocol  (SIP). This is a network protocol that can be used to establish and control connections between two users.
  • This protocol enables the development of a completely (for data and voice) IP-based network and is therefore future-proof.


The reason why the introduction of VoLTE has been delayed by four years compared to LTE–establishment in data traffic is due to the difficult interaction of "4G" with the older predecessor standards  GSM  ("2G") and  UMTS  ("3G"). Here is an example:

  • If a mobile phone user leaves his LTE–cell and switches to an area without 4G–coverage, an immediate switch to the next best standard (3G) must be made.
  • Language is transmitted here technically completely differently, no longer by many small data packets   ⇒   "packet-switched" but sequentially in the logical and physical channels reserved especially for the user   ⇒  "circuit-switched".
  • This implementation must be so fast and smooth that the end customer does not notice anything. And this implementation must work for all mobile phone standards and technologies.

According to all the experts, VoLTE will have a positive impact on mobile telephony in the same way that LTE has driven the mobile Internet forward since 2011. Key benefits for users are:

  • higher voice quality, as VoLTE  AMR–Wideband Codecs  with 12.65 or 23.85 kbit/s. Furthermore, the VoLTE–data packets are prioritized for lowest possible latencies.
  • An enormously  accelerated connection setup within one or two seconds, whereas with  Circuit Switched Fallback (CSFB) it takes an unpleasantly long time to establish a connection.
  • low battery consumption, significantly lower than "2G" and "3G", associated with a longer battery life. Also in comparison to the usual VoIP–services the power consumption is up to 40% lower.

From the provider's point of view, the following advantages result:

  • better spectral efficiency:   Twice as many calls are possible in the same frequency band than with "3G". In other words:   More capacity is available for data services for the same number of calls.
  • An easy implementation of  Rich Media Services  (RCS), for example for video telephony or future applications that can be used to attract new customers.
  • better acceptance  of the higher provisioning costs by LTE–customers if you don't need to outsource to a "low-value" network like "2G" or "3G" for telephony.


Bandwidth flexibility


LTE can be adapted to frequency bands of different widths with relatively little effort by using  OFDM  ("Orthogonal Frequency Division Multiplex"). This fact is an important feature for various reasons, see  [Mey10][2], especially for network operators:

  • The frequency bands for LTE may vary in size depending on the legal requirements in different countries. The outcome of the state-specific auctions of LTE–frequencies (separated into FDD and TDD) has also influenced the width of the spectrum.
  • Often LTE is operated in the "frequency–neighborhood" established radio transmission systems, which are expected to be switched off soon. If the demand increases, LTE can be gradually expanded to the frequency range that is becoming available.
  • For example, the migration of television channels after digitalization:   A part of the LTE–network will be located in the VHF–frequency range around 800 MHz, which has now been freed up, see  Frequency_Band_Splitting Graphic.
  • Actually the bandwidths could be selected with a degree of fineness of up to 15 kHz (corresponding to an OFDMA–subcarrier). However, since this would unnecessarily produce overhead, a duration of  one millisecond  and a bandwidth of  180 kHz  has been specified as the smallest addressable LTE–resource. Such a block corresponds to twelve subcarriers (180 kHz divided by 15 kHz).

In order to keep the complexity and effort of hardware standardization as low as possible, a whole range of permissible bandwidths between 1.4 MHz and 20 MHz has been agreed upon. The following list – taken from  [Ges08][3]  – specifies the standardized bandwidths, the number of available blocks and the "overhead":

  • 6 available blocks in the bandwidth 1.4 MHz   ⇒   relative overhead approx. 22.8%,
  • 15 available blocks in the bandwidth 3 MHz   ⇒   relative overhead about 10%,
  • 25 available blocks in the bandwidth 5 MHz   ⇒   relative overhead approx. 10%,
  • 50 available blocks in the bandwidth 10 MHz   ⇒   relative overhead approx. 10%,
  • 75 available blocks in the bandwidth 15 MHz   ⇒   relative overhead about 10%,
  • 100 available blocks in the bandwidth 20 MHz   ⇒   relative overhead about 10%.

Since otherwise some LTE–specific functions would not work, at least six blocks must be provided.

  • The relative overhead is comparatively high at small channel bandwidth (1.4 MHz):   (1.4 – 6 · 0.18)/1.4 ≈ 22.8%.
  • From a bandwidth of 3 MHz the relative overhead is constant 10%.
  • It also applies that all end devices must also support the maximum bandwidth of 20 MHz   [Ges08][3]

FDD, TDD and half duplex method


transmission scheme for FDD (top) or TDD (bottom)

Another important innovation of LTE is the half–duplex–procedure, which is a mixture of the two already known from UMTS  Duplex Procedure :

  • Frequency Division Duplex  (FDD), and
  • Time Division Duplex  (TDD) .

Such duplexing is necessary to ensure that uplink and downlink are clearly separated from each other and that transmission runs smoothly. The diagram illustrates the difference between FDD– and TDD–based transmission.
.

Using the FDD and TDD methods, LTE can be operated in paired and unpaired frequency ranges.
The two methods are present opposing requirements:

  • FDD requires a paired spectrum, i.e. one frequency band for transmission from the base station to the terminal (downlink) and one for transmission in the opposite direction (uplink). Downlink and uplink can be used at the same time.
  • TDD was designed for unpaired spectra. Now only one band is needed for uplink and downlink. However, transmitter and receiver must now alternate during transmission. The main problem of TDD is the required synchronicity of the networks.

In the graphic above the differences between FDD and TDD can be seen. In TDD a Guard Period has to be inserted when changing from downlink to uplink (or vice versa) to avoid an overlapping of the signals.

Although FDD is likely to be used more in practice (and FDD–frequencies were also much more expensive for the providers), there are several reasons for TDD:

  • Frequencies are a rare and expensive commodity, as the 2010 auction has shown. But TDD needs only half of the frequency bandwidth.
  • The TDD technique allows different modes, which determine how much time should be used for downlink or uplink and can be adjusted to individual requirements.

For the actual innovation, the Half–Duplex–Method, you need a paired spectrum as with FDD (see second graphic):

Transmission scheme for half-duplex
  • Base station transmitter and receiver still alternate like TDD. Each terminal device can either transmit or receive at a given time.
  • Through a second connection to another end device with swapped downlink/uplink–raster, the entire available bandwidth can still be fully used.
  • The main advantage of the half–duplex–process is that the use of the TDD–concept reduces the demands on the end devices and thus allows them to be produced at a lower cost.

The fact that this aspect was of great importance in the standardization can also be seen in the use of OFDMA in the downlink and of SC–FDMA in the uplink:

Multiple Antenna Systems


If a radio system uses several transmitting and receiving antennas, one speaks of  Multiple Input Multiple Output  (MIMO). This is not an LTE–specific development. WLAN, for example, also uses this technology.

$\text{Example 1:}$  The principle of multi-antenna systems is illustrated in the following figure using the example of 2×2–MIMO (two transmitting and two receiving antennas).

The difference between SISO and MIMO

The new thing about LTE is not the actual use of  Multiple Input Multiple Output, but the particularly intensive one, namely 2×2–MIMO in the uplink and maximum 4×4–MIMO in the downlink.

In the successor  LTE–Advanced  the use of MIMO is even more pronounced, namely "4×4" in the uplink and "8×8" in the opposite direction.


A MIMO–system has advantages compared to  Single Input Single Output  (SISO, only one transmitting and one receiving antenna). A distinction is made between several gains depending on the channel:

  • power gain  according to the number of receiving antennas:  
    If the radio signals arriving via several antennas are combined in a suitable way  (Maximal-ratio Combining), the reception power is increased and the radio connection is improved. By doubling the antennas, a power gain of maximum 3 dB.
    is achieved.
  • Diversity gain through spatial diversity (  Spatial Diversity): If several spatially separated receiving antennas are used in an environment with strong multipath propagation, the fading at the individual antennas is mostly independent from each other and the probability that all antennas are affected by fading at the same time is very low.
  • Data rate gain:  
    This increases the efficiency of MIMO, especially in an environment with increased multipath propagation, especially when transmitter and receiver do not have a direct line of sight and the transmission is done via reflections. Tripling the number of antennas for the transmitter and receiver results in approximately twice the data rate.

However, it is not possible for all advantages to occur simultaneously. Depending on the nature of the channel, it can also happen that one does not even have the choice of which advantage one wants to use.

In addition to the MIMO systems there are also the following intermediate stages:

  • MISO–Systems  (only one receiving antenna, therefore no power gain is possible), and
  • SIMO–Systems  (only one transmitting antenna, only small diversity gain).


$\text{Example 2:}$  The term "MIMO" summarizes multi-antenna techniques with different properties, each of which can be useful in certain situations. The following description is based on the four diagrams shown here.

Four multi-antenna procedures with different properties
  • If the mostly independent channels of a MIMO–system are assigned to a single user (top left diagram), one speaks of  'Single–User MIMO. With 2×2–MIMO, the data rate is doubled compared to SISO–operation and with four transmit&ndash each; and receiving antennas, the data rate can be doubled again under good channel conditions.
LTE allows maximum 4×4–MIMO but only in the downlink. Due to the complexity of multi-antenna systems, only laptops with LTE–modems can be used as receivers (end devices) for 4×4–MIMO. For a cell phone, the use of 2×2–MIMO is generally limited to 2×2–MIMO.
  • Contrary to Single–User MIMO, the goal with the  Multi–User MIMO  is not the maximum data rate for a receiver, but the maximization of the number of end devices that can use the network simultaneously (top right diagram). This involves transmitting different data streams to different users. This is particularly useful in places with high demand, such as airports or soccer stadiums.
  • Multi-antenna operation is not only used to maximize the number of users or data rate, but in the event of poor transmission conditions, multiple antennas can also combine their power to transmit data to a single user to improve the quality of reception. One then speaks of  Beamforming   (diagram below left), which also increases the range of a transmitting station.
  • The fourth possibility is  antenna diversity   (diagram below right). This increases the redundancy (regarding system design) and makes the transmission more robust against interferences. A simple example:   There are four channels that all transmit the same data. If one channel fails, there are still three channels for information transport.


System Architecture


The LTE–architecture enables a transmission system based entirely on the IP–protocol. In order to achieve this goal, the system architecture specified for UMTS not only had to be changed in detail, but in some cases completely redesigned. In the process, other IP-based technologies such as  mobile WiMAX  or  WLAN  were also integrated in order to be able to switch to these networks without any problems.

In UMTS–networks (left graphic), the  Radio Network Controller  (RNC) is inserted between a base station (NodeB) and the core network, which is mainly responsible for switching between different cells and which can lead to latency times of up to 100 milliseconds.

System Architecture for UMTS (UTRAN) and LTE (EUTRAN)

The redesign of the base stations ("eNodeB" instead of "NodeB") and the interface "X2" are the decisive further developments from UMTS towards LTE. The graphic on the right illustrates in particular the reduction in complexity compared to UMTS that goes hand in hand with the new technology (left graphic).

The  'LTE–system architecture   can be divided into two major areas:

  • the LTE–core network  Evolved Packet Core  (EPC),

EUTRAN transmits the data between the terminal and the LTE–base station ("eNodeB") via the so-called S1–interface with two connections, one for the transmission of user data and a second for the transmission of signalling data. You can see from the above graphic:

  • The base stations are connected not only to the EPC but also to the neighboring base stations. These connections (X2–interfaces) have the effect that as few packets as possible are lost when the terminal device moves from the vicinity of one base station towards another.
  • For this purpose, the base station whose service area the user is just leaving can pass on any cached data directly and quickly to the "new" base station. This ensures (largely) continuous transmission.
  • The functionality of the RNC is partly transferred to the base station and partly to the  Mobility Management Entity  (MME) in the core network. This reduction of the interfaces significantly shortens the signal throughput time in the network and the handover to 20 milliseconds.
  • The LTE–system architecture is also designed so that future  Inter–NodeB–procedures  (such as  Soft–Handover  or  Cooperative Interference Cancellation) can be easily integrated.

LTE–Core network: Backbone and Backhaul


The LTE–core network  Evolved Packet Core  (EPC) of a network operator, in the technical language  Backbone</i, consists of various network components. The EPC is connected to the base stations via the  Backhaul. This means the connection of an upstream, usually hierarchically subordinated network node to a central network node.

Currently, the  Backhaul  consists mainly of directional radio and so-called E1–lines. These are copper lines and allow a throughput of about 2 Mbit/s. For GSM– and UMTS–networks these connections were still sufficient, however, for the large-scale conceived  HSDPA  such data rates are no longer adequate. For LTE such a  Backhaul  is completely unusable:

  • The slow cable network would slow down the fast wireless connections; overall, there would be no increase in speed.
  • Due to the low capacities of the lines with E1–standard, an expansion with further lines of the same construction would not be economical.

In the course of the introduction of LTE, the  Backhaul  had to be redesigned. It was important to keep an eye on future security, since the next generation  LTE–Advanced  was already in place before the introduction. If one believes the experts' propaganda  Moore's Law for mobile phone bandwidths, the most important factor for future security is the expensive new installation of better cables.

Due to the purely packet-oriented transmission technology, the Ethernet–standard, which is also IP–based, is suitable for the LTE–Backhaul, which is realized with the help of optical fibers. In 2009, the company Fujitsu presented in the study  [Fuj09][4],  also the thesis that the current infrastructure will continue to play an important role for LTE–Backhaul for the next ten to fifteen years.

There are two approaches for the generation change to an Ethernet–based  Backhaul :

  • the parallel operation of the lines with E1 and Ethernet–standard,
  • the immediate migration to an Ethernet-based  Backhaul.

The former would have the advantage that the network operators could continue to run voice traffic over the old lines and would only have to handle bandwidth-intensive data traffic over the more powerful lines.

The second option raises some technical problems:

  • The services previously transported through the slow E1-standard lines would have to be switched immediately to a packet-based procedure.
  • Ethernet does not offer (unlike the current standard) any  End–to–End–Synchronization, which can lead to severe delays or even service interruptions when changing radio cells, thus a huge loss of service quality.
  • However, in the concept  Synchronous Ethernet  (SyncE), the Cisco company has already made suggestions as to how synchronization could be realized.


For conurbations, a direct conversion of the backhaul would certainly be worthwhile, as relatively few new cables would have to be laid for a comparatively high number of new users.

In rural areas, however, major excavation work would quickly result in high costs. However, this is exactly the area which must be covered first, according to the  agreement reached  between the federal government and the (German) mobile phone operators. Here, the mostly existing microwave radio link would have to be (and probably will be) extended to high data rates.

Exercises for Chapter


Exercise 4.2: FDD, TDD and Half-Duplex

Exercise 4.2Z: MIMO Applications in LTE

List of Sources

  1. Gutt, E.: LTE - a new dimension of mobile broadband use. PDF document on the Internet, 2010.
  2. Meyer, M.: Siebenmeilenfunk. c't 2010, issue 25, 2010.
  3. 3.0 3.1 Gessner, C.: UMTS Long Term Evolution (LTE): Technology Introduction. Rohde&Schwarz, 2008.
  4. Fujitsu Network Communications Inc.: 4G Impacts to Mobile Backhaul. PDF Internet document.