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Technical Briefing
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LTE and SAE – A Clearer Picture Stabilization of Standards The development of UMTS continues, and Release 8 of the 3GPP standards sees the introduction of documents relating to LTE (Long Term Evolution) and SAE (System Architecture Evolution). Most LTE aspects are covered by the new 36 series documents and are now reasonably stable. SAE represents the general evolution of the core network and is applicable to LTE, UMTS and even to GSM. As such, SAE aspects are distributed throughout the existing core network standards. More work remains to be done on detailed SAE aspects.
Some Terminology LTE is the term used to describe collectively the evolution of the radio access network into Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the radio access technology into Evolved Universal Terrestrial Radio Access (E-UTRA). LTE
SAE is the term used to describe the evolution of the core network into the Evolved Packet Core (EPC). There is also a collective term, Evolved Packet System (EPS), which refers to the combined E-UTRAN and EPC.
SAE EPS
UE E-UTRA E-UTRAN
EPC
LTE Radio Access Network The LTE radio access network is known as the E-UTRAN. Compared to its 3G UTRAN predecessor it employs a much flatter and simpler architecture. Chiefly this involves the removal of the RNC node and the extension of IP/UDP/GTP transport out to the base stations; now known as eNode Bs (eNBs). These changes, coupled with a similar, much flatter core network architecture, should ensure reduced latency throughout the network. All layers of the air interface protocol stack, including Radio Resource Control (RRC), Radio Link Control (RLC) and Medium Access Control (MAC) have now been moved to the base station, and the eNB will anchor the main backhaul link to the EPC. The eNB supports a flexible association between access and core, permitting load sharing between core network nodes.
eNB S1 UE RRC RLC MAC PL
July 2008
RRC RLC MAC PL
X2
IP/UDP/GTP
EPC
S1
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With the removal of the RNC from the access network architecture, inter-eNB handovers will be negotiated and managed directly between eNBs using the X2 interface. Although not yet fully defined, it is assumed that the final protocol functions over the X2 interface will mimic those currently employed in the 3G UTRAN on the Iur interface of R99/4 networks.
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LTE Air Interface Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation are defined in the standards, but of these FDD operation is most significant in the shorter term. This is based on two different radio technologies. Orthogonal Frequency Division Multiple Access (OFDMA) will be employed in the downlink, while the uplink will employ Single Carrier Frequency Division Multiple Access (SC_FDMA). Even in its most basic form LTE has channel configurations with theoretical maximum bit rates approaching 100 Mbit/s. Initially, however, the maximum bit rates are likely to be between 20 and 30 Mbit/s. Nevertheless, with increased spectrum availability and the use of advanced antenna techniques, bit rates in excess of 300 Mbit/s may eventually be achievable. Low latency (10 ms round-trip delay), improved system capacity and coverage as well as reduced operating costs remain important additional benefits.
OFDM in LTE OFDM, although not a new technology in itself, is still relatively new in the context of mobile multiple access systems. The OFDM principle is at the core of the downlink OFDMA and uplink SC-FDMA physical layers for LTE. Traditional digital radio technologies transmit a data stream modulated onto a single radio carrier. However, to overcome the problems associated with transmitting high data rates over a single carrier – including Inter Symbol Interference (ISI) and narrowband interference – OFDM utilises a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Each subcarrier can then be modulated with a conventional modulation scheme such as QPSK, 16QAM or even 64QAM. As each subcarrier carries a relatively low data rate in comparison to high data rates modulated onto a single carrier, it will suffer far less from ISI resulting in far fewer errors. Also, as subcarriers are orthogonal, a spectral efficiency gain approaching 50% over traditional single-carrier methods can be achieved.
20 MHz/1200 15 MHz/900 10 MHz/600 5 MHz/300 3 MHz/180 1.4 MHz/72
LTE OFDMA and SC-FDMA are designed to work in a variety of bandwidths ranging initially from 1.4 MHz with 72 subcarriers up to 20 MHz with 1200 subcarriers. This will allow LTE to be backward compatible with current spectrum and flexible enough to fit into any potential future spectrum allocation. The abbreviation SC-FDMA is rather misleading as, like OFDMA, it also uses many subcarriers on the air interface. However, SC-FDMA adds an additional processing step that spreads the frequency components of each data symbol over each subcarrier used, hence the term ‘Single Carrier’. This improves power efficiency and consequently increases battery life for the mobile.
July 2008
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Briefing
Technical Briefing
Technical Briefing
Technical
SAE Evolved Packet Core Network The reduced complexity in the radio access network is mirrored by a similar simplification and flattening in the core network, with SAE architecture consisting of only five main nodes. A key difference from current networks is that the EPC is defined to support packet-switched traffic only. Interfaces are based on IP protocols. This means that all services will be delivered through packet connections, including voice.
EPC MME
PRCF
HSS S6a
Rx S1-MME
UE E-UTRAN
S1-U
IMS
Gx
S11
SGi
S5 Serving SAE Gateway
SGi
External PDNs
PDN SAE Gateway
The SAE gateway performs switching and routing services for user plane traffic. However, unlike R99/4 networks, bearer control has been removed from the gateway and now resides within the Mobility Management Entity (MME). The Policy and Charging Rules Function (PCRF) handles Quality of Service (QoS) management and also controls rating and charging. Subscriber management and security is the responsibility of the Home Subscriber Server (HSS). It is assumed that voice services will be implemented through the use of an IP Multimedia Subsystem (IMS). It should also be noted that Wireless LAN or WiMAX radio access could also be supported into the EPC via an S2 interface.
Need more? Delivered on your site or as a regularly scheduled 1-day public course, Wray Castle offers the follow-on comprehensive training:
LTE Evolved UTRAN Engineering Overview Evolved Packet Core (EPC) Engineering Overview
Please call us today or access our website for full course details and public course discounts European Office: Telephone +44 (0) 1539 742 742 E-mail
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July 2008
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