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Sunday, March 13, 2011

Advocating for Single Frequency Networks (SFN) in LTE: Challenges for Multicast Delivery in Heterogeneous Networks


Voice communication and download data services such as web browsing are based on point-to-point (PTP) communication. On the other hand, multicast and broadcast services are based on point-to-multipoint (PTM) communication, where data packets are simultaneously transmitted from a single source to multiple destinations. The service delivery using point-to-multipoint (PTM) communication is generally more efficient when a large number of users is interested in receiving the same content such as a mobile TV channel. This results in efficient transmission not only over the wireless link but also in the core and access networks. This is because a single multicast broadcast packet travels in the core and access networks and is copied and forwarded to multiple Node-Bs in the multicast broadcast area.
In this article I provide an insight into the limitations of multicast delivery in heterogeneous networks hence hence justifying the use of SFNs in LTE.





A major challenge in a heterogeneous network environment is the lack of delivery coordination owing to the purely receiver-driven service model of IP multicast. In order to establish a multicast session, a mobile receiver enables the reception of data from a particular multicast group on one of its interfaces and sends a join request to the attached access network. This limitation is explained through an example illustrated below;


All receivers have different network access interfaces available, but they are assumed to be at the same location and expected to receive the same flow of session data. Receivers 1 and 2 join the service on the UMTS network, receivers 3 and 4 choose to initiate the session via WLAN and receiver 5 requests the multicast service on DVB. Consequently, the same service content is delivered via three different data paths to the same location. Knowing that all the receivers have a UMTS network access interface, the receivers could instead be instructed to join the UMTS network for receiving the traffic. Another challenge is the inherent heterogeneity of mobile receivers, which often makes it impossible to satisfy all interested receivers with a single service flow within a multicast session or with a common access network for the delivery. The provision of appropriate service flows to satisfy all interested receivers
in a multicast session and the selection of suitable delivery networks require knowledge of the existing heterogeneity of the receiver population and available networks. Advanced resource and session management mechanisms would allow for multicast service delivery with higher coordination. To over these challenges, an interworking architecture has been proposed that allows different operators to offer cooperative services to users across a heterogeneous wireless network environment, both securely and seamlessly. This new proposed architecture enables a shift in the paradigm from a purely receiver-driven to a network-controlled provision of multicast services. Cooperating network operators deploy so-called Interworking Gateways (IGWs) within their networks. An IGW implements various essential interworking functions and defines a logical link among them, which enables signalling message exchanges for interworking purposes. The IGW ensures that operators may interwork efficiently for service delivery, without the need to give up their own network autonomy and to disclose any sensitive network-related information. The figure below provides an overview of the interworking architecture.

The main parts of the proposed solution for coordinated delivery of multicast services are distributed resource management functionality and context-aware group management support. The proposal highlighted above is part of the multicast and broadcast capabilities that have already been introduced in the existing 3G wireless systems. The multimedia broadcast multicast service (MBMS) is specified in UMTS release 6 standard. The broadcast services need to be delivered in a cost effective way in order for these services to be popular among consumers. This demands, among other factors, extremely high spectral efficiency for these services due to scarcity of the radio spectrum. Therefore, techniques that provide increased spectral efficiency for broadcast services such as single frequency network (SFN) operation have recently been introduced in the LTE and in other cellular standards.

In SFN a broadcast or multicast service is delivered in a broadcast multicast service area which refers to the coverage area in which a specific broadcast service is available. A broadcast area
is defined on a per-broadcast-service basis which means that different broadcast multicast services may have different broadcast multicast coverage areas. A broadcast service area may represent the coverage area of the entire network or part of the network. As implified diagram of a cellular broadcast multicast system with broadcast multicast area consisting of six cells is shown in the figure below;

A content provider can be a cellular service provider or a third party and acts as the source of multicast broadcast content. The evolved broadcast multicast service center (eBM-SC) acts as an entry point for content-delivery services. The eBM-SC forwards the broadcast multicast packets to the eMBMS gateway from where the packets are distributed to eNode-Bs in the broadcast multicast area. A key point in broadcast multicast transmission over the air is that the same information is transmitted simultaneously from Node-Bs in the broadcast multicast area. This allows UEs to receive broadcast multicast signals from multiple cells.








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