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3GPP Long Term Evolution
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3GPP LTE (Long Term Evolution) is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future technology evolutions. Goals include improving spectral efficiency, lowering costs, improving services, making use of new spectrum and refarmed spectrum opportunities, and better integration with other open standards. The LTE air interface will be added to the specification in Release 8 and can be found in the 36-series [1] of the 3GPP specifications. Although an evolution of UMTS, the LTE air interface is a completely new systems based on OFDMA in the downlink and SC-FDMA (DFTS-FDMA) in the uplink that efficiently supports multi-antenna techologies (MIMO). The architecture that will result from this work is called EPS (Evolved Packet System) and comprises E-UTRAN (Evolved UTRAN) on the access side and EPC (Evolved Packet Core) on the core side.

Contents


[edit] Current State

While 3GPP Release 8 has yet to be ratified as a standard, much of the standard will be oriented around upgrading UMTS to a so-called fourth generation mobile communications technology, essentially a wireless broadband Internet system with voice and other services built on top.
The standard includes:
  • Peak download rates of 326.4 Mbit/s for 4x4 antennas, 172.8 Mbit/s for 2x2 antennas for every 20 MHz of spectrum. [2]
  • Peak upload rates of 86.4 Mbit/s for every 20 MHz of spectrum.[2]
  • 5 different terminal classes have been defined from a voice centric class up to a high end terminal that supports the peak data rates. All terminal will be able to process 20 MHz bandwidth.
  • At least 200 active users in every 5 MHz cell. (i.e., 200 active data clients)
  • Sub-5ms latency for small IP packets
  • Increased spectrum flexibility, with spectrum slices as small as 1.5 MHz (and as large as 20 MHz) supported (W-CDMA requires 5 MHz slices, leading to some problems with roll-outs of the technology in countries where 5 MHz is a commonly allocated amount of spectrum, and is frequently already in use with legacy standards such as 2G GSM and cdmaOne.) Limiting sizes to 5 MHz also limited the amount of bandwidth per handset
  • Optimal cell size of 5 km, 30 km sizes with reasonable performance, and up to 100 km cell sizes supported with acceptable performance
  • Co-existence with legacy standards (users can transparently start a call or transfer of data in an area using an LTE standard, and, should coverage be unavailable, continue the operation without any action on their part using GSM/GPRS or W-CDMA-based UMTS or even 3GPP2 networks such as CDMA or EV-DO)
  • Supports MBSFN (Multicast Broadcast Single Frequency Network). This feature can deliver services such as Mobile TV using the LTE infrastructure, and is a competitor for DVB-H-based TV broadcast.
A large amount of the work is aimed at simplifying the architecture of the system, as it transits from the existing UMTS circuit + packet switching combined network, to an all-IP flat architecture system.
Preliminary requirements have been released for LTE Advanced, expected to be part of 3GPP Release 10[3]. If possible LTE Advanced will be a software upgrade for LTE networks and enable peak download rates over 1Gbit/s that fully supports the 4G requirements as defined by the ITU-R. It also targets faster switching between power states and improved performance at the cell edge. A first set of requirements has been approved in June 2008. [4]



Timetable

The LTE standard reached the functional freeze milestone in March 2008. Stage 2 Freeze is scheduled for mid 2008 and official ratification in December 2008. The standard has been complete enough that hardware designers have been designing chipsets, test equipment and base stations for some time. LTE test equipment has been shipping from several vendors since early 2008 & Motorola demonstrated a LTE RAN standard compliant eNodeB and LTE chipset at Mobile World Congress 2008.



An "All IP Network" (AIPN)

A characteristic of so-called "4G" networks such as LTE is that they are fundamentally based upon TCP/IP, the core protocol of the Internet, with higher level services such as voice, video, and messaging, built on top of this. In 2004, the 3GPP proposed this as the future of UMTS and began feasibility studies into the so-called All IP Network (AIPN.) These proposals, which included recommendations in 2005 for 3GPP Release 7[5] (though some aspects were in releases as early as 4[6]), form the basis of the effort to build the higher level protocols of evolved UMTS. The LTE part of this effort is called the 3GPP System Architecture Evolution.
At a glance, the UMTS back-end becomes accessible via a variety of means, such as GSM's/UMTS's own radio network (GERAN, UTRAN, and E-UTRAN), WiFi, and even competing legacy systems such as CDMA2000 and WiMAX. Users of non-UMTS radio networks would be provided with an entry-point into the IP network, with different levels of security depending on the trustworthiness of the network being used to make the connection. Users of GSM/UMTS networks would use an integrated system where all authentication at every level of the system is covered by a single system, while users accessing the UMTS network via WiMAX and other similar technologies would handle the WiMAX connection one way (for example, authenticating themselves via a MAC or ESN address) and the UMTS link-up another way.



E-UTRA Air Interface

Release 8's air interface, E-UTRA (Evolved UTRA, the E- prefix being common to the evolved equivalents of older UMTS components) would be used by UMTS operators deploying their own wireless networks. It's important to note that Release 8 is intended for use over any IP network, including WiMAX and WiFi, and even wired networks.[7]
The proposed E-UTRA system uses OFDMA for the downlink (tower to handset) and Single Carrier FDMA (SC-FDMA) for the uplink and employs MIMO with up to four antennas per station. The channel coding scheme for transport blocks is turbo coding and a contention-free quadratic permutation polynomial (QPP) turbo code internal interleaver.[8]
The use of OFDM, a system where the available spectrum is divided into thousands of very thin carriers, each on a different frequency, each carrying a part of the signal, enables E-UTRA to be much more flexible in its use of spectrum than the older CDMA based systems that dominated 3G. CDMA networks require large blocks of spectrum to be allocated to each carrier, to maintain high chip rates, and thus maximize efficiency. Building radios capable of coping with different chip rates (and spectrum bandwidths) is more complex than creating radios that only send and receive one size of carrier, so generally CDMA based systems standardize both. Standardizing on a fixed spectrum slice has consequences for the operators deploying the system: too narrow a spectrum slice would mean the efficiency and maximum bandwidth per handset suffers; too wide a spectrum slice, and there are deployment issues for operators short on spectrum. This became a major issue with the US roll-out of UMTS over W-CDMA, where W-CDMA's 5 MHz requirement often left no room in some markets for operators to co-deploy it with existing GSM standards.
OFDM has a Link spectral efficiency greater than CDMA, and when combined with modulation formats such as 64QAM, and techniques as MIMO, E-UTRA has proven to be considerably more efficient than W-CDMA with HSDPA and HSUPA.

Downlink

The subcarrier spacing in the OFDM downlink is 15 kHz and there is a maximum of 1200 subcarriers available. Number of subcarriers is dependent on the used bandwidth (1.4MHz and up to 20Mhz),subcarriers don't occupy 100% of the used bandwidth as Cyclic Prefixes (Guards) occupies a part of it.The Mobile devices must be capable of receiving all subcarriers but a base station need only support transmitting 72 subcarriers. The transmission is divided in time into time slots of duration 0.5 ms and subframes of duration 1.0 ms. A radio frame is 10 ms long.
Supported modulation formats on the downlink data channels are QPSK, 16QAM and 64QAM.
For MIMO operation, a distinction is made between single user MIMO, for enhancing one users data throughput, and multi user MIMO for enhancing the cell throughput.

Uplink

The currently proposed uplink uses SC-FDMA multiplexing, and QPSK or 16QAM (64QAM optional) modulation. SC-FDMA is used because it has a low Peak-to-Average Power Ratio (PAPR). Each mobile device has at least one transmitter. If virtual MIMO / Spatial division multiple access (SDMA) is introduced the data rate in the uplink direction can be increased depending on the number of antennas at the base station. With this technology more than one mobile can reuse the same resources.[9]

Technology Demos

  • In September 2006, Siemens Networks (today Nokia Siemens Networks) showed in collaboration with Nomor Research the first live emulation of a LTE network to the media and investors. As live applications two users streaming an HD-TV video in the downlink and playing an interactive game in the uplink have been demonstrated.[10]
  • The first presentation of an LTE demonstrator with HDTV streaming (>30 Mbit/s), video supervision and Mobile IP-based handover between the LTE radio demonstrator and the commercially available HSDPA radio system was shown during the ITU trade fair in Hong Kong in December 2006 by Siemens Communication Department.
  • In September 2007, NTT Docomo demonstrated LTE data rates of 200 Mbit/s with power consumption below 100mW during the test.[11]
Motorola demonstrated how LTE can accelerate the delivery of personal media experience with HD video demo streaming, HD video blogging, Online gaming and VoIP over LTE running a RAN standard compliant LTE network & LTE chipset. [1]
Ericsson demonstrated a portable LTE terminal showing streaming video. [12]
Freescale Semiconductor demonstrated streaming HD video with peak data rates of 96 Mbit/s downlink and 86 Mbit/s uplink . [13]
NXP Semiconductors demonstrated a multi-mode LTE modem as the basis for a software-defined radio system for use in cellphones. [14]
picoChip and Mimoon demonstrated a base station reference design. This runs on a common hardware platform (multi-mode / software defined radio) with their WiMAX architecture. [15]
  • In April 2008, Motorola demonstrated the first EV-DO to LTE hand-off - handing over a streaming video from LTE to a commercial EV-DO network and back to LTE. [2]
  • In April 2008, LG and Nortel demonstrated LTE data rates of 50 Mbit/s while travelling at 110 km/h. [16]

Carrier adoption

  • Most carriers supporting GSM or HSPA networks can be expected to upgrade their networks to LTE at some stage:
  • However, several networks that don't use these standards are also upgrading to LTE:
    • Verizon Wireless, Bell Mobility, the newly formed China Telecom/Unicom and Japan's KDDI have announced they have chosen LTE as their 4G network technology. This is significant, because these are CDMA carriers and are switching networking technologies to match what will likely be the 4G standard worldwide. They have chosen to take the natural GSM evolution path as opposed to the 3GPP2 CDMA2000 evolution path Ultra Mobile Broadband (UMB). Verizon Wireless plans to begin LTE trials in 2008. Bell Mobility plans to start LTE deployment in 2009-2010.
    • Telus Mobility has announced that it will adopt LTE as its 4G wireless standard.

Conformance testing

It has been suggested that TTCN-3 test specification language will be used for the purposes of LTE conformance testing. As of March 2008, TTCN-3 test suite development has been underway at ETSI.
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