Mobile Communications/General Information on the LTE Mobile Communications Standard

1. Mobile Communications
2. LTE – Long Term Evolution
3. General Information on the LTE Mobile Communications Standard

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# OVERVIEW OF THE FOURTH MAIN CHAPTER #

This chapter provides an overview of  »Long Term Evolution«  $\rm (LTE)$.  From today's perspective  $(2011)$,  LTE is a new mobile communications standard that should replace UMTS and will probably continue to shape the next few years of mobile voice and data transmission.

In the following,  a rough overview of the motivation,  functionality and characteristics of LTE is given.  This is followed by a more detailed system description of the technical processes involved in LTE.  This chapter will deal with this in detail:

1. The  »motivation for LTE«  and the   »frequency band allocation«,
2. the  »development of mobile communications standards«  towards LTE,
3. some technical details about  »voice and data transmission«, 
4. the transmission method  »SC–FDMA«  used in the uplink and its  »differences to  OFDMA« ,
5. the  »description and function of the different logical channels«  in the bit transmission layer,
6. an outlook on the successor system  »LTE–Advanced«.

Addendum:   The LTE chapter was written in 2011, i.e. at the time when LTE had just been introduced.  During the last editorial revision in autumn 2017, some earlier statements were revised which no longer corresponded to the facts after six years of intensive use by many customers.  However, most of the chapter remained unchanged compared to 2011, as the LTE principle has not changed in the meantime.

Development of mobile users until 2010

Since the turn of the millennium, the number of mobile connections has increased dramatically.

Absolute and relative number of mobile devices in the years 2004 - 2010

• The graph shows for the years 2004 to 2010 an increase from 1.8 to approx. 5 billion mobile devices worldwide in absolute numbers  (red bars, left scale).
• The blue bars (left scale) show the development of the world population in the same period.

• The (percentage) number of cell phones  (green curve, right scale)  in relation to the world population increased from just under 30% to over 70% between 2004 and 2010.

• The statistics include users with more than one cell phone.  2010 possessed thus by no means 70% of the world population a mobile telephone.
  

• The use of mobile data services has sharply increased, especially since the introduction of flatrate tarifs.

The following statements refer to the year 2010:

1. Global mobile data traffic grew by 159% in 2010, a much stronger increase than expected.  Since then, mobile data transmission has caused more network load than voice transmission in the mobile network.
   
2. Mobile data traffic alone was three times as large in 2010 as the entire traffic volume in 2000  (at that time mainly voice traffic).
   
3. Although smartphones accounted for only 13% of all mobile devices in 2010, they were responsible for 78% of data and voice transmission.
   
4. To this development also 94 million laptop users contributed, who used the Internet on the way over UMTS modems.
5. Such a laptop user causes thereby on the average 22 times the data quantity of an average smartphone user.

Essential properties of LTE

The abbreviation  $\rm LTE$  stands for  "Long Term Evolution"  and refers to the mobile communications standard that follows UMTS.  The new conceptual development of LTE was intended to satisfy the ever-increasing demand for bandwidth and higher speeds over the long time  ("Long Term").

⇒   The LTE standard was first defined in 2008 as  "UMTS Release 8"  by the  $\rm 3GPP$  (Third Generation Partnership Project), a conglomerate of various international telecommunication associations, and has since been continuously developed further by so-called "Releases".  The commitment of the world's largest mobile communications providers has made LTE the first (largely) uniform standard for mobile communications technology.

According to  "UMTS Release 8", LTE is also called  $\rm 3.9G$  because it initially did not fully meet the conditions specified by the ITU  (International Telecommunication Union)  for fourth generation  $\rm (4G)$  mobile communications.

In contrast, the subsequent Release 10  (dated July 2011)  complies with the  $\rm 4G$ standard.  The chapter  "LTE Advanced"  lists the features of this LTE enhancement.  This technology is also referred to as  $\rm LTE–A$.

⇒   Here is a summary of the important system features of LTE from the page  "ITWissen" :

• LTE is based on the multiple access methods  $\rm OFDMA$  ("Orthogonal Frequency Division Multiple Access")  in the downlink and  $\rm SC–FDMA$  ("Single Carrier Frequency Division Multiple Access") in the uplink.  The detailed description of OFDMA and especially its differences to  $\rm OFDM$  can be found in chapter  "The application of OFDMA and SC–FDMA in LTE".

• The use of this modulation method enables orthogonality between individual users, resulting in an increased network capacity  [HT09].  In conjunction with "Multiple Input Multiple Output" $\rm (MIMO)$, this technology currently (2011) enables peak data rates of 100 Mbit/s in the downlink.

• In addition to the significantly higher data rate compared to the  $\rm 3G$  system UMTS, LTE technology makes more efficient use of the available bandwidth.  By combining the latest state-of-the-art technology with the existing experience of GSM and UMTS, the new standard is not only much faster, but also simpler and more flexible.

Motivation and goals of LTE

In 2010, the American telecommunications company  "Cisco Systems"  published a  "White Paper"  which assumes that in 2015

Graphic from the Ericsson Mobility Report 2015

• the use of mobile data will be twenty-six times higher than in 2010,
  

• this usage is increasing by a further 92% per year, and

• the gigantic amount of 6.3 Exabyte   ⇒   $\rm6.3 \cdot 10^{18}$  byte per month is reached.
  
  

It has also been predicted that five billion people will be connected to the Internet in 2015  [HT09].  In addition, other wireless transmission technologies are being developed at the same time, which promise equally high data transmission rates.  All these factors called for further development of the 3GPP mobile communications standard "UMTS".

The  "Ericsson Mobility Report"  of 2015 shows that the 2010 forecast has been exceeded.  In 2014 there were already 7.1 billion mobile users with Internet access, in 2020 there should be 9.2 billion.

The 3GPP consortium started early to define the LTE targets to keep up with the rapid development of line-based connections.  The exact targets were then set out in the  "LTE Release 6"  compared to  $\rm HSPA$  technology  ("High–Speed Packet Access")  at the end of 2004.

The main goals were mentioned:

1. A purely packet-oriented transmission and a high degree of mobility and security,
   
2. reduced complexity, cost reduction and optimized battery life of the end devices,
   
3. bandwidth flexibility between 1.5 MHz and 20 MHz,
   
4. a spectral efficiency  (data rate per one Hertz bandwidth)  as high as possible,
   
5. maximum possible data rates of 100 Mbit/s in downlink and 50 Mbit/s in uplink,
   
6. signal processing times less than 10 milliseconds.
   
   

Compared to HSPA, this means an increase in spectral efficiency by a factor of  $2 \ \text{...}\ 4$  and a reduction in latency by half and a tenfold increase in the maximum data rate.  The individual points, which represent a large part of the LTE specific technical characteristics, are described in more detail in the chapter  "Technical Innovations of LTE".

Development of the UMTS mobile phone standards towards LTE

The development of third generation mobile communications standards was already discussed in detail in the third chapter of this book.  For this reason, only the more recent developments are discussed in detail here.

First of all, a brief overview of UMTS releases before LTE from  [Hin08]:

• »Release 99«   (December 1999):   UMTS 3G FDD and TDD;   3.84 Mchip/s;   CDMA air interface.
  

• »Release 4«   (July 2001):   Lower chip rate (1.28 Mchip/s) for TDD;   some fixes and minor improvements.
  

• »Release 5«   (March 2002):   "IP Multimedia Subsystem"  $\rm (IMS)$;   "High-Speed Downlink Packet Access"  $\rm (HSDPA)$.
  

• »Release 6«   (March 2005):   "High-Speed Uplink Packet Access"  $\rm (HSUPA)$;   Multimedia Broadcast & Multicast Services  $\rm (MBMS)$;  
      Cooperation with Wireless LAN;   Push–to–Talk;   "Generic Access Network"  $\rm (GAN)$.
  

• »Release 7«   (December 2007):   Reduced latency;   improved Quality of Service  $\rm (QoS)$;   MIMO for UMTS;   TDD option  $7.68$  Mchip/s,   real-time applications,  e.g. 
  • "Voice over IP"  $\rm (VoIP)$, 
  • "Enhanced Data Rates for GSM Evolution EDGE Evolution"  $\rm (EDGE$  evolution$)$.
    
    

The  »Release 8«  of December 2008 was synonymous with the introduction of  "Long Term Evolution"  $\rm (LTE)$ and the basis for the first generation of LTE capable terminals.  The main innovations and characteristics of Release 8, summarized by  $\text{3gpp}$  (Third Generation Partnership Project), were:

• High spectral efficiency and very short latency,
  

• the support of different bandwidths,
  

• a simple protocol and system architecture,
  

• backwards compatibility and compatibility to other systems like  "cdma2000",
  

• $\rm FDD$  (Frequency Division Duplex) and  $\rm TDD$  (Time Division Duplex) are optionally usable,
  

• self-organizing networks  $\rm (SON)$  support.
  
  

These features (and some others more) are discussed in detail in the section  "Technical innovations of LTE" .

• The  »Release 9«, on the other hand, contains only minor improvements and will not be discussed in detail here.

• The  »Release 10«  from July 2011 describes the further development of LTE   ⇒   "LTE Advanced"  $\rm (LTE–A)$.
  

LTE frequency band splitting

LTE frequencies around  $\text{800 MHz}$  and  $\text{2.6 GHz}$

New frequencies were needed for LTE. In Germany, there was an auction of two frequency ranges in 2010, in which all German mobile network operators took part.

The chart illustrates the results of this auction of frequencies in

• $\text{Range around 800 MHz}$  $\text{(791 ... 862 MHz)}$:
  Here only paired spectra were assigned for FDD:   Telekom, O2 and Vodafone; got two times 10 MHz each
  

• $\text{Range around 2.6 GHz}$  $\text{(2.5 ... 2.69 GHz)}$:
  Here, paired spectra for FDD (140 MHz in total) and unpaired spectra for TDD (50 MHz) were assigned.

More about the difference between FDD and TDD can be found in section  "Motivation for xDSL"  in the book "Examples of communication systems".

The two auctioned frequency ranges have different system characteristics, which make them interesting for different applications.

• The lower frequency range (around 800 MHz) is also called  "digital dividend"  because it was freed up by the conversion of (terrestrial) TV transmission from  $\rm PAL$  to  $\rm DVB–T$  ("Digitization").

• By agreement of the Federal Government with the (German) network operators this range must be used to help poorly supplied regions to get "Fast Internet".  Four levels were defined for the level of broadband Internet coverage in a region.  Only when 90% of one level has been covered throughout Germany the developement of the next level may begin.

• The choice for this project was the comparatively low frequency range around 800 MHz with better propagation characteristics than 2600 MHz, which is both sensible and necessary for cost-effective coverage of rural areas.  A LTE–800 base station reaches a maximum transmission radius of about 10 km.  However, the ratio of users per area is lower than with LTE–2600, which means that LTE–800 is more suitable for sparsely populated regions.
  

• The frequency range from 821 MHz to 832 MHz is kept free to avoid interference between the uplink and the downlink.  One speaks of the  "Duplex Gap".  In addition, this frequency range can be used for event technology, since the frequency range around 800 MHz was already common for radio microphones before the introduction of LTE.  In areas where LTE is available everywhere, radio microphones must be able to use the duplex gap in the future.
  

The difference in importance of the frequency ranges from the operator's perspective is clearly illustrated by the results of the 2010 frequency auction:

• The 60 MHz around 800 MHz generated almost EUR 3.6 billion  $\text{(60 €/Hz)}$,  the 190 MHz around 2.6 GHz only EUR 344 million  $\text{ (1.80 €/Hz)}$.
• For comparison:   The UMTS auction in 2000 resulted in the astronomical sum of 50 billion euros for 60 MHz   ⇒   $\text{833 €/Hz}$.
  
  

3GPP - Third Generation Partnership Project

In the last sections the  "Third Generation Partnership Project"  $($or short $\rm 3GPP)$  has been mentioned several times.  Here is a short overview of the self-image of this group, its structure and its activities.  The information is taken directly from the  "3GPP Website".

3GPP is a group of various international standardization organizations that have joined forces for the purpose of standardizing mobile radio systems.  It was founded in December 1998 by five partners:

• »ARIB«   $($Association of Radio Industries and Businesses, Japan$)$
  
• »ETSI«   $($European Telecommunication Standards Institute$)$
  
• »ATIS«   $($Alliance for Telecommunications Industry Solutions, USA$)$
  
• »TTA«   $($Telecommunications Technology Association, Korea$)$
  
• »TTC«   $($Telecommunications Technology Committee, Japan$)$
  
  

The 3GPP develops, accepts and maintains a globally applicable standard in mobile communications.  The conferences, which are held regularly and frequently, are the most important bodies in the updating of the standardization of the technical specifications of LTE.

• Change requests go through a fixed standardization process with three stages, which enables high quality and good structuring of 3GPP's work.

• When a release has reached the last stage and is completed, it is passed on to the market by the telecommunications companies united in the partner organizations.
  

In  [Gut10]  the following assessment can be found:

"The goal of 3GPP standardization is the creation of technical specifications  $\rm (TS)$  that describe all technical details of a mobile communications technology in detail.  The specifications for LTE are extremely comprehensive.  The level of detail is chosen so high that mobile devices from different manufacturers can function without problems in all networks".

Exercise for the chapter

Exercise 4.1: General Questions about LTE