3gpp LTE

Long Term Evolution (LTE) describes the latest standardization work by 3rd Generation Partnership Project (3GPP) in the mobile network technology tree previously realized the GSM/EDGE and UMTS/HSxPA network technologies that now account for over 85% of all mobile subscribers.^[1] In this latest standardization work which started in late 2004, the 3GPP (set out in December, 1998) defines a set of high level requirements (new high-speed Radio Access method) for mobile communications systems to compete with other latest cellular broadband technologies, particularly WiMAX.

In preparation for further increasing user demands and tougher competition from new radio access technologies, LTE is enhanced with a new radio access technique called Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).^[1] Via this technology LTE is expected to improve end-user throughput, increase sector capacity, reduce user plane latency, and consequently offer superior user experience with full mobility.

Unlike other latest deployed technologies such as HSPA, LTE is accommodated within a new Packet Core architecture called Enhanced Packet Core (EPC) network architecture. Technically, 3GPP specifies the EPC to support the E-UTRAN. EPC is designed to deploy TCP/IP protocols thus enabling LTE to support all IP-based services including voice, video, rich media and messaging with end-to-end Quality of Service (QoS). The EPC network architecture also enables improved connections and hand-over to other fixed-line and wireless access technologies while giving an operator the ability to deliver a seamless mobility experience.^[2]

To achieve all the targets mentioned herein, LTE Physical Layer (PHY) employs advanced technologies that are new to cellular applications. These include Orthogonal Frequency Division Multiple Access (OFDMA) and multiple-input and multiple-output (MIMO) data transmission. Smart Antennas are also deployed to accomplish those targets. Furthermore, the LTE PHY deploys the OFDMA for the Downlink (DU) - that is from the Base Station (BS) to the User Equipment (UE) and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the Uplink (UL). These technologies will further minimize the LTE system and UE complexities while allowing flexible spectrum deployment in existing or new frequency spectrum.^[2]

LTE enjoys the support from a collaborative group of international standards organizations and mobile-technology companies that form the 3GPP. However, a strong support comes from Ericsson and Qualcomm who decided not to support WiMAX, the competitor of LTE ^[3]. Alcatel-Lucent is also a key contributor to LTE standards, having held the rapporteur position on the MIMO working group since 2002. Motorola which also supports WiMAX claims to be the leading contributor in LTE standards such as Radio Access Network (RAN) 1 & 2 and a top three contributor to EPC 1 & 2 standards.^[1] The standardization work on LTE is continuing, and Motorola claims to introduce LTE in ‘Q4 2009 ^[4]. However, LTE is assumed to dominate world’s mobile infrastructure markets after 2011.^[5]

Standardization Path

The standardization of LTE started in November 2004, when the RAN Evolution Workshop in Toronto, Canada, accepted contributions from more than 40 operators, vendors and research institutes that included 3GPP members and nonmembers organizations. Contributions were merely a range of views and proposals on the UTRAN. Following those contributions, 3GPP started a feasibility study, in December 2004, so as to develop a new framework for evolution of the 3GPP Radio Access technology towards:

• Increased data rates
• Reduced cost per bit
• Increased service provisioning - that is more services with better user experience
• Flexibility in usage of both new and existing frequency bands
• High-data-rate
• Low-latency
• Simple architecture, open interface and packet-optimized RAN technology

Put simply, the study maps out specifications for RAN that are capable to support the wireless broadband internet scenario which is already enjoyed in today’s cable networks – adding full mobility to enable exciting new service possibilities.^[5]

Currently LTE specifications are described in 3GPP Release 8. The 3GPP Release 8 is the latest set of standards that describes the technical evolution of 3GPP mobile network systems. It is the successor of 3GPP Release 7 that includes a set of specifications for HSPA+, the ‘missing bridge’ between HSPA and LTE. Actually HSPA+ is described in both, the 3GPP Release 7 and 8, allowing the designing of simpler ‘flat’, all-IP based network architecture and bypassing many of the legacy equipments required for UMTS/HSPA.^[5]

The specifications of the 3GPP Release 8 standard are assumed to complete at the end of 2008. Obviously the finalization of the 3GPP Release 8 will further progress the market interest in commercial deployment of LTE. The 3GPP Release 8 will compile the completion of 3GPP Release 7 HSPA+ features, Voice over HSPA and EPC specification and Common IP Multimedia Subsystem (IMS) ^[6].

LTE Key Features

As it has been discussed earlier, technically speaking, a fundamental objective of the 3GPP LTE project is to offer higher data speeds for both DL and UL transmissions. In addition to that, it is obviously LTE to be characterized by reduced packet latency while promising a superior experience in online gaming, Voice over IP (VoIP) videoconferencing and other real-time professional services. Now that based on the feasibility study under 3GPP, the following are the important features of LTE:

OFDMA on the DL and SC-FDMA on the UL

3GPP Release 8 specifies an all-new RAN that combines OFDMA-based modulation and multiple access schemes for the downlink, as well as SC-FDMA for uplink. These new technologies (OFDM schemes) are deliberately deployed to split available spectrum into thousands of extremely narrowband carriers, such that each carrier is capable of carrying a part of signal. This is what is known as multiple carrier transmission.^[2]

To enhance the OFDM schemes, LTE also employs other higher order modulation schemes such as 64QAM and sophisticated Forward Error Correction (FEC) schemes such as tail biting, convolutional coding and turbo coding. Furthermore, complementary radio techniques such as MIMO and Beam Forming with up to four antennas per station are also deliberately deployed for further enhancement of innate spectral efficiency of OFDM schemes.^[5]

The results of these radio interface features are obvious, enabling LTE to have improved radio performance. As such they yield the spectral efficiency up to 3 to 4 times that of HSDPA Release 6 in the LTE DL and up to 2 to 3 times that of HSUPA Release 6 in UL.^[2][7] Consequently, theoretically, the DL peak data rates extend up to 300Mbit/s per 20MHz of spectrum. Similarly, theoretical UL peak data rates can reach 75Mbit/s per 20MHz of spectrum as well as supporting at least 200 active users per cell in 5MHz.^[5]

All-IP Packet Optimized Network Architecture

LTE has a ‘flat’, all-IP based core network with a simplified architecture, open interface and fewer system nodes. Indeed, the all-IP based network architecture together with the new RAN reduces network latency, improved system performance and provide interoperability with existing 3GPP and non-3GPP technologies. Within 3GPP, all-IP based core network architecture is now known as Evolved Packet Core (EPC). EPC is the result of standardization work within 3GPP which targeted to convert the existing System Architecture Evolution (SAE) to an all-IP system.^[5]

Advanced Antenna Techniques

LTE is enhanced with MIMO, Spatial-Division Multiple Access (SDMA) and Beam Forming.^[8] These are advanced radio antenna techniques which are complementary to each other. These techniques are deployed for better air interface via enhancing the innate spectral efficiency of OFDM schemes. Furthermore, these techniques can be used to trade-off between higher sector capacity, higher user data rates, or higher cell-edge rates, and thus enable mobile operators to have finer control over the end-user experience ^[9].

 

System Architecture

Evolved Radio Access Network (RAN)

The evolved RAN consists of the LTE base station (eNode B) that interfaces with the UE. The eNode B contains the PHY, Media Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers. Therefore the eNode B performs some tasks such as resource management, admission control, scheduling and enforcement of negotiated UL QoS.

Serving Gateway (SGW)

The SGW guides and forwards user data packets. Furthermore, during inter-eNode B handover SGW acts as the mobility anchor for the user plane. It can also act as an anchor for mobility between LTE technology and other 3GPP technologies. When the UE is in idle state, the SGW terminates the DL data path of the UE and triggers paging when DL data arrives for the UE.

Mobility Management Entity (MME)

MME handles Control Signaling for mobility. When the UE is in idle mode, the MME is responsible for UE tracking and paging procedure that includes retransmissions. MME is also involved in the bearer activation/deactivation process. In addition MME can choose the SGW for a UE at the initial attach and at time of intra-LTE handover involving core Network (CN) node relocation. MME can interact with the Home Subscriber Server (HSS) so as to authenticate the user.

Packet Data Network Gateway (PDN GW)

PDN GW is a point of exit and entry of traffic for the UE. PDN GW performs packet filtering and acts as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2.^[1]

Networks Upgrading to LTE

LTE is deemed to become a next generation mobile communications standard or 4G mobile communications standard that started with today’s 2G and 3G networks. Technically, the design of LTE is based on today’s 3GPP family of cellular networks that dominated by Global System for Mobile communication (GSM), General Packet Radio Service (GPRS) and Enhanced Data rate for GSM Evolution (EDGE) as well as Wideband Code Division Multiple Access (WCDMA) and High Speed Packet Access (HSPA). Therefore LTE ensures a smooth evolutionary path to higher speeds and reduced latency to these existing networks.

Contrary to today’s networks that deploy hybrid packet/circuit switched networks, LTE uses the advanced new radio interface. As such to harness the full potential of LTE it requires an evolution from the existing network architecture to a simplified, all-IP environment architecture. This evolution has advantages to operator’s point of view. These advantages include reduced costs for variety of services, blended applications combining voice, video and data services plus interworking with other fixed and wireless networks.

Furthermore since the design of LTE is based on today’s UMTS/HSPA family of standards, it will obvious enhance the capabilities of the existing cellular network technologies to delivery broadband services which were accustomed to fixed broadband networks. In other words, LTE will unify the voice-oriented environment of today’s mobile networks with the data-centric service possibilities of the fixed Internet. To the operator’s point of view, the smooth upgrading of the existing networks to LTE will allow the introduction of LTE’s all-IP concept progressively. As such operator will be able to retain the value of its existing voice-based service platforms at the same get the benefit of high performance in data services delivered by LTE network

[edit] Carrier adoption

• Most carriers supporting GSM or HSPA networks can be expected to upgrade their networks to LTE at some stage:
  • AT&T Mobility has stated that they intend on upgrading to LTE as their 4G technology, but will introduce HSUPA and HSPA+ as bridge standards. ^[10]
  • T-Mobile, Vodafone, France Télécom, Telia Sonera and Telecom Italia Mobile have also announced or talked publicly about their commitment to LTE.
• However, several networks that don't use these standards are also upgrading to LTE:
  • Alltel, Verizon Wireless, 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. ^[11] 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.^[12]
  • Telus Mobility and Bell Mobility have announced that they will adopt LTE as their 4G wireless standard.^[13]