Initiated in 2004, the Long Term Evolution (LTE) project in 3GPP standardization focused on enhancing the Universal Terrestrial Radio Access (UTRA) and optimizing 3GPP’s radio access architecture. In 2007, LTE progressed from the feasibility study stage to the first issue of approved technical specifications. End 2008 the specifications were sufficiently stable for commercial implementation and the first commercial LTE network was launched in Sweden and Norway in December 2009. 35 commercial networks were launched by end October 2011.
From experience on GSM and UMTS it can be seen that the initial specification release after creation of a new technology is usually a minor one. This means this first follow up release includes leftovers, that were not completed in the initial release or some smaller features are added. As an example UMTS Release 4, the first release after creation of the UMTS technology in early 2000, included the additional UMTS TDD mode not finalized in Release 99 and just small optimizations of the FDD mode.
More significant enhancements like the HSDPA feature took another release to be included in the specifications. The same holds true for LTE Release 9. The Release includes a set of features that either were not completed in release 8 or which provide some smaller optimizations or improvements. These are namely:
• Multimedia Broadcast Multicast Services (MBMS) for LTE,
• LTE MIMO: dual-layer beamforming,
• LTE positioning
• PWS (Public Warning System)
• RF requirements for multi-carrier and multi-RAT base stations,
• Home eNodeB specification (femto-cell),
• Self-Organizing Networks (SON).
• History of MBMS
As the term evolved implies, Multimedia Broadcast Multicast Services (MBMS), is not fundamentally new to 3GPP and not defined as a LTE-only feature. In fact MBMS has first time been specified with 3GPP Release 6. It has than continuously been enhanced in the following versions of the specification. Initially defined for UTRAN/WCDMA (3G) it is also supported by GERAN/GSM (2G).
The goal with MBMS is to provide network operators with the possibility to broadcast over their cellular network. Some advantages are considered over traditional mobile broadcast technologies, such as DVB-H, DMB-H or former MediaFLO. These are:
• The same infrastructure is used
• No need for additional spectrum
• Interaction with user is possible due to available uplink.
An important aspect while defining MBMS was to keep the impact to the existing network architecture as minimal as possible. Therefore only one new network element was introduced, where for the existing nodes only new tasks have been added. The MBMS network architecture and the task of the different network elements is shown in Figure 1.
Fig 1: MBMS network architecture for 3G networks and related tasks.
At the terminal side an enhancement to the existing channel architecture was required to enable the delivery of broadcast and multicast services. Three new logical channels where introduced, providing information on the configuration of active MBMS services, scheduling information and the broadcast data itself. Only one new physical channel is required to inform terminals about the availability of MBMS services in that particular radio cell. The enhanced channel model is shown in Figure 2 illustrating minimal impact on terminal as well as on network side.
Fig 2: New 3G logical channels and physical channel due to MBMS.
MBMS in LTE (3GPP Release 8 and 9)
MBMS in UMTS/WCDMA offers 6 mobile TV channels at a data rate of 128 kbps in a 5 MHz channel. Also the FDMA/TDMA- and CDMA-based access technologies for 2G (GSM) and 3G (WCDMA) are not well-suited for broadcast. All classical broadcast technologies (e.g. Digital Video Broadcast (DVB), Digital Audio Broadcast (DAB)) utilizing Orthogonal Frequency Division Multiplex (OFDM) as the underlying transmission scheme and so does LTE.
In addition two aspects are impacting the success for a MBMS service from a technical perspective. First, good coverage, even at the cell edge, and second low powerconsumption. Evolved MBMS (eMBMS) as defined in 3GPP Release 8 and 9 aims to tackle these aspects. The goal is to increase spectral efficiency at the cell edge up to 1bps/Hz while realizing a Single Frequency Network (SFN). In addition MBMS capabilities are increased compared to 3G, by offering 20 TV channels at a data rate of 256 kbps in a 5 MHz channel. However, eMBMS has not been identified being critical for commercial LTE deployment. Therefore the specification has been segmented by adding physical layer aspects already to 3GPP Release 8 and have higher layer and network related aspects completed within 3GPP Release 9.
LTE defines also a simpler and flatter network architecture than 3G, thus there is an impact offering MBMS over LTE. Figure 3 shows the involved network elements.
Fig 3: LTE network architecture for MBMS.
The BM-SC (Broadcast/Multicast Service Center) has been already introduced with 3GPP Release 6. Its tasks are authentication, authorizing content provider, charging and the overall configuration of the data flow through the core network. The MBMS Gateway (MBMS GW) is the logical node handling the multi-cast of IP packets from the BM-SC to all LTE base station (enhanced Node B, eNodeB or eNB). It further handles session control via the MME.
Mobile Management Entity (MME) is not a MBMS-only related network element. In fact it is part of the 3GPP Release 8 network architecture. The MME handles all tasks, that are non-related to the air interface. That means all Non-Access Stratum (NAS) protocols are terminated in the MME.
Key element for MBMS in LTE is the MCE, the Multi-cell/Multicast Coordination Entity. It coordinates the use of the same resources and transmission parameters across all radio cells that belong to a MBSFN area1. There are two ways of integrating the MCE to the network. Directly to the LTE base station. This is very cost effective as it is in most cases only a simple software upgrade to the existing hardware. The drawback is of course, that only cells that belong to that particular base station can form a MBSFN area. To avoid this limitation the MCE can be added as separate network element to the architecture.
The full white paper continues to cover Positioning methods in LTE, LTE MIMO: dual-layer beamforming, a Public Warning System (PWS), a Commercial Mobile Alert System (CMAS), RF requirements for multi-carrier and multi-RAT base stations, Home eNodeB specification (femto-cell), New RAN functionalities and Self Organizing Networks. It concludes with am extensive list associated literature.
To download the white paper click here. (Registration to EE Times may be required.)
ArticleCourtesy: Communications DesignLine