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`
`
`Figure 12.5: System configuration: (a) users close to eNodeBs;(b)users at the cell edge.
`
`(b)
`
`serving cell, a frequency reuse factor of 1 can be adopted. For the outer part, scheduling
`restrictions are applied: when the cell schedules a user in a given part of band, the system
`capacity is optimized if the neighbouring cells do not transmit atall; alternatively, they may
`transmit only at low power(probably to users in the inner parts of the neighbouring cells)
`to avoid creating strong interference to the scheduled userin the first cell. This effectively
`results in a higher frequency reuse factor at the cell-edge;it is often referred to as ‘partial
`frequency reuse’or ‘soft frequency reuse’, andisillustrated in Figure 12.6.
`In order to coordinate the scheduling in different cells in such a way, communication
`between neighbouring cells is required. If the neighbouring cells are managed by the
`
`gEF
`
`Frequency
`
`Figure 12.6: Partial frequency reuse.
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`same eNodeB, a coordinated scheduling strategy can be followed without the need for
`standardized signalling. However, where neighbouring cells are controlled by different
`eNodeBs, standardized signalling is important, especially in multivendor networks. The
`main mechanism for ICIC in LTE Releases 8 and 9 is normally assumed to be frequency-
`domain-based, at least for the data channels, and the Release 8/9 inter-eNodeB ICIC
`signalling explained in the following two sections is designed to support this. In Release 10,
`additional time-domain mechanisms are introduced, aiming particularly to support ICIC for
`the PDCCH and for heterogeneous networks comprising both macrocells and small cells;
`these mechanisms are explained in Section 31.2.
`
`12.5.1 Inter-eNodeB Signalling to Support Downlink
`Frequency-Domain ICIC in LTE
`In relation to the downlink transmissions, a bitmap termed the Relative Narrowband Transmit
`Power (RNTP6) indicator can be exchanged between eNodeBs over the X2 interface. Each
`bit of the RNTP indicator corresponds to one RB in the frequency domain and is used to
`inform the neighbouring eNodeBs if a cell is planning to keep the transmit power for the RB
`below a certain upper limit or not. The value of this upper limit, and the period for which
`the indicator is valid into the future, are configurable. This enables the neighbouring cells to
`take into account the expected level of interference in each RB when scheduling UEs in their
`own cells. The reaction of the eNodeB in case of receiving an indication of high transmit
`power in an RB in a neighbouring cell is not standardized (thus allowing some freedom of
`implementation for the scheduling algorithm); however, a typical response could be to avoid
`scheduling cell-edge UEs in such RBs. In the definition of the RNTP indicator, the transmit
`power per antenna port is normalized by the maximum output power of a base station or cell.
`The reason for this is that a cell with a smaller maximum output power, corresponding to
`smaller cell size, can create as much interference as a cell with a larger maximum output
`power corresponding to a larger cell size.
`
`12.5.2 Inter-eNodeB Signalling to Support Uplink Frequency-Domain
`ICIC in LTE
`For the uplink transmissions, two messages may be exchanged between eNodeBs to facilitate
`some coordination of their transmit powers and scheduling of users:
`A reactive indicator, known as the ‘Overload Indicator’ (OI), can be exchanged over the
`X2 interface to indicate physical layer measurements of the average uplink interference plus
`thermal noise for each RB. The OI can take three values, expressing low, medium, and high
`levels of interference plus noise. In order to avoid excessive signalling load, it cannot be
`updated more often than every 20 ms.
`A proactive indicator, known as the ‘High Interference Indicator’ (HII), can also be sent
`by an eNodeB to its neighbouring eNodeBs to inform them that it will, in the near future,
`schedule uplink transmissions by one or more cell-edge UEs in certain parts of the bandwidth,
`and therefore that high interference might occur in those frequency regions. Neighbouring
`cells may then take this information into consideration in scheduling their own users to limit
`the interference impact. This can be achieved either by deciding not to schedule their own
`
`6RNTP is defined in [18, Section 5.2.1].
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`cell-edge UEs in that part of the bandwidth and only considering the allocation of those
`resources for cell-centre users requiring less transmission power, or by not scheduling any
`user at all in the relevant RBs. The HII is comprised of a bitmap with one bit per RB and,
`like the OI, is not sent more often than every 20 ms. The HII bitmap is addressed to specific
`neighbour eNodeBs.
`In addition to frequency-domain scheduling in the uplink, the eNodeB also controls the
`degree to which each UE compensates for the path-loss when setting its uplink transmission
`power. This enables the eNodeB to trade off fairness for cell-edge UEs against inter-cell
`interference generated towards other cells, and can also be used to maximize system capacity.
`This is discussed in more detail in Section 18.3.2.
`
`12.5.3 Static versus Semi-Static ICIC
`
`In general, ICIC may be static or semi-static, with different levels of associated communica-
`tion required between eNodeBs.
`For static interference coordination, the coordination is associated with cell planning, and
`reconfigurations are rare. This largely avoids signalling on the X2 interface, but it may result
`in some performance limitation since it cannot adaptively take into account variations in cell
`loading and user distributions.
`Semi-static interference coordination typically refers to reconfigurations carried out on a
`time-scale of the order of seconds or longer. The inter-eNodeB communication methods over
`the X2 interface can be used as discussed above. Other types of information such as traffic
`load information may also be used, as discussed in Section 2.6.4. Semi-static interference
`coordination may be more beneficial in cases of non-uniform load distributions in eNodeBs
`and varying cell sizes across the network.
`
`12.6 Summary
`In summary, it can be observed that a variety of resource scheduling algorithms may be
`applied by the eNodeB depending on the optimization criteria required. The prioritization
`of data will typically take into account the corresponding traffic classes, especially in regard
`to balancing throughput maximization for delay-tolerant applications against QoS for delay-
`limited applications in a fair way.
`It can be seen that multi-user diversity is an important factor in all cases, and especially
`if the user density is high, in which case the multi-user diversity gain enables the scheduler
`to achieve a high capacity even with tight delay constraints. Finally, it is important to note
`that individual cells, and even individual eNodeBs, cannot be considered in isolation. System
`optimization requires some degree of coordination between cells and eNodeBs, in order to
`avoid inter-cell interference becoming the limiting factor. Considering the system as a whole,
`the best results are in many cases realized by simple ‘on-off’ allocation of resource blocks,
`whereby some eNodeBs avoid scheduling transmissions in certain resource blocks which are
`used by neighbouring eNodeBs for cell-edge users.
`LTE Releases 8 and 9 support standardized signalling between eNodeBs to facilitate
`frequency-domain data-channel ICIC algorithms for both downlink and uplink. Additional
`time-domain ICIC mechanisms of particular relevance to the control signalling and to
`heterogeneous networks are introduced in Release 10 and explained in Section 31.2.
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`292
`References7
`[1] K. Knopp and P. A. Humblet, ‘Information Capacity and Power Control in Single-cell Multiuser
`Communications’, in Proc. IEEE International Conference on Communications, Seattle, WA,
`1995.
`[2] R. Knopp and P. A. Humblet, ‘Multiple-Accessing over Frequency-selective Fading Channels’,
`in Proc. IEEE International Symposium on Personal, Indoor and Mobile Radio Communications,
`September 1995.
`[3] D. N. C. Tse and S. D. Hanly, ‘Multiaccess Fading Channels. I: Polymatroid Structure, Optimal
`Resource Allocation and Throughput Capacities’. IEEE Trans. on Information Theory, Vol. 44,
`pp. 2796–2815, November 1998.
`[4] R. Jain, W. Hawe and D. Chiu, ‘A quantitative measure of fairness and discrimination for resource
`allocation in Shared Computer Systems’, Digital Equipment Corporation Technical Report 301,
`September 1984.
`[5] T. M. Cover and J. Thomas. Elements of Information Theory. Wiley, New York, 1991.
`[6] R. Knopp and P. A. Hamblet, ‘Multiuser Diversity’, Technical Report, Eurecom Institute, Sophia
`Antipolis, France, 2002.
`[7] G. Caire, R. Müller and R. Knopp, ‘Hard Fairness Versus Proportional Fairness in Wireless
`Communications: The Single-cell Case’. IEEE Trans. on Information Theory, Vol. 53, pp. 1366–
`1385, April 2007.
`[8] T. E. Kolding, ‘Link and System Performance Aspects of Proportional Fair Scheduling in
`WCDMA/HSDPA’, in Proc. IEEE Vehicular Technology Conference, Orlando, FL, October 2003.
`[9] B. Wang, K. I. Pedersen, P. E. Kolding and T. E. Mogensen, ‘Performance of VoIP on HSDPA’,
`in Proc. IEEE Vehicular Technology Conference, Stockholm, May 2005.
`[10] V. Vukadinovic and G. Karlsson, ‘Video Streaming in 3.5G: On Throughput-Delay Performance
`of Proportional Fair Scheduling’, in IEEE International Symposium on Modeling, Analysis, and
`Simulation of Computer and Telecommunication Systems, Monterey, CA, September 2006.
`[11] S. D. Hanly and D. N. C. Tse, ‘Multiaccess Fading Channels. II: Delay-limited Capacities’. IEEE
`Trans. on Information Theory, Vol. 44, pp. 2816–2831, November 1998.
`[12] I. Toufik, ‘Wideband Resource Allocation for Future Cellular Networks’. Ph.D. Thesis, Eurecom
`Institute, 2006.
`[13] I. Toufik and R. Knopp, ‘Channel Allocation Algorithms for Multi-carrier Systems’, in Proc.
`IEEE Vehicular Technology Conference, Los Angeles, CA, September 2004.
`[14] M. J. Neely, E. Modiano and C. E. Rohrs, ‘Power Allocation and Routing in Multi-beam Satellites
`with Time Varying Channels’. IEEE Trans. on Networking, Vol. 11, pp. 138–152, February 2003.
`[15] M. Realp, A. I. Pérez-Neira and R. Knopp, ‘Delay Bounds for Resource Allocation in Wideband
`Wireless Systems’, in Proc. IEEE International Conference on Communications, Istanbul, June
`2006.
`[16] M. Realp, R. Knopp and A. I. Pérez-Neira, ‘Resource Allocation in Wideband Wireless Systems’
`in Proc. IEEE International Symposium on Personal, Indoor and Mobile Radio Communications,
`Berlin, September 2005.
`[17] A. Gjendemsjoe, D. Gesbert, G. Oien and S. Kiani, ‘Binary Power Control for Sum Rate
`Maximization over Multiple Interfering Links’. IEEE Trans. Wireless Communications, August
`2008.
`[18] 3GPP Technical Specification 36.213, ‘Evolved Universal Terrestrial Radio Access (E-UTRA);
`Physical Layer Procedures’, www.3gpp.org.
`
`7All web sites confirmed 1st March 2011.
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`13
`
`Broadcast Operation
`
`Himke van der Velde, Olivier Hus and Matthew Baker
`
`13.1 Introduction
`The Multimedia Broadcast/Multicast Service (MBMS) aims to provide an efficient mode
`of delivery for both broadcast and multicast services over the core network. MBMS was
`introduced in the second release of the LTE specifications (Release 9), although the initial
`Release 8 physical layer specifications were already designed to support MBMS by including
`essential components to ensure forward-compatibility.
`The LTE MBMS feature is largely based on that which was already available in UTRAN1
`(from Release 6) and GERAN2 with both simplifications and enhancements.
`This chapter first describes general aspects including the MBMS reference architecture,
`before reviewing the MBMS features supported by LTE in more detail. In particular, we
`describe how LTE is able to benefit from the new OFDM downlink radio interface to achieve
`radically improved transmission efficiency and coverage by means of multicell ‘single
`frequency network’ operation. The control signalling and user plane content synchronization
`mechanisms are also explained.
`
`13.2 Broadcast Modes
`In the most general sense, broadcasting is the distribution of content to an audience of
`multiple users; in the case of mobile multimedia services an efficient transmission system
`for the simultaneous delivery of content to large groups of mobile users. Typical broadcast
`content can include newscasts, weather forecasts or live mobile television.
`Figure 13.1 illustrates the reception of a video clip showing a highlight of a sporting event.
`There are three possible types of transmission to reach multiple users:
`1UMTS Terrestrial Radio Access Network.
`2GSM Edge Radio Access Network.
`
`LTE – The UMTS Long Term Evolution: From Theory to Practice, Second Edition.
`Stefania Sesia, Issam Toufik and Matthew Baker.
`© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.
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`Figure 13.1: Receiving football game highlights as a mobile broadcast service. Adapted
`from image provided courtesy of Philips ‘Living Memory’ project.
`
`• Unicast: A bidirectional point-to-point transmission between the network and each of
`the multiple users; the network provides a dedicated connection to each terminal, and
`the same content is transmitted multiple times – i.e. separately to each individual user
`receiving the service.
`• Broadcast: A downlink-only point-to-multipoint connection from the network to
`multiple terminals; the content is transmitted once to all terminals in a geographical
`area, and users are free to choose whether or not to receive it.
`• Multicast: A downlink-only point-to-multipoint connection from the network to a
`managed group of terminals; the content is transmitted once to the whole group, and
`only users belonging to the managed user group can receive it.
`
`Point-to-multipoint transmission typically becomes more efficient than point-to-point
`when there are more than around three to five users per cell, as it supports feedback which
`can improve the radio link efficiency; the exact switching point depends on the nature of the
`transfer mechanisms. At higher user densities, point-to-multipoint transmission decreases the
`total amount of data transmitted in the downlink, and it may also reduce the control overhead
`especially in the uplink.
`The difference between broadcast and multicast modes manifests itself predominantly in
`the upper layers (i.e. the Non-Access Stratum (NAS) – see Section 3.1). Multicast includes
`additional procedures for subscription, authorization and charging, to ensure that the services
`are available only to specific users. Multicast also includes joining and leaving procedures,
`which aim to provide control information only in areas in which there are users who are
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`subscribed to the MBMS multicast service. For UTRAN, both modes were developed;
`however, for simplicity, LTE Release 9 MBMS includes only a broadcast mode.
`A point-to-multipoint transfer can involve either single or multicell transmission. In the
`latter case, multiple cells transmit exactly the same data in a synchronized manner so as
`to appear as one transmission to the UE. Whereas UTRAN MBMS supports point-to-point,
`single cell point-to-multipoint and multicell point-to-multipoint modes, LTE MBMS supports
`a single transfer mode: multicell point-to-multipoint, using a ‘single frequency network’
`transmission as described in Section 13.4.1. The number of cells participating in the multicell
`transmission can, however, be limited to one.
`UTRAN MBMS includes counting procedures that enable UTRAN to count the number
`of UEs interested in receiving a service and hence to select the optimal transfer mode (point-
`to-point or point-to-multipoint), as well as to avoid transmission of MBMS user data in
`cells in which there are no users interested in receiving the session. LTE MBMS did not
`include counting procedures in Release 9, so both the transfer mode and the transmission
`area were fixed (i.e. semi-statically configured). However, a counting procedure is introduced
`in Release 10 of LTE, as explained in Section 13.6.5.
`
`13.3 Overall MBMS Architecture
`13.3.1 Reference Architecture
`Figure 13.2 shows the reference architecture for MBMS broadcast mode in the Evolved
`Packet System (EPS – see Section 2.1), with both E-UTRAN and UTRAN. E-UTRAN
`includes a Multicell Coordination Entity (MCE), which is a logical entity – in other words,
`its functionality need not be placed in a separate physical node but could, for example, be
`integrated with an eNodeB.
`The main nodes and interfaces of the MBMS reference architecture are described in the
`following sections.
`
`13.3.2 Content Provision
`Sources of MBMS content vary, but are normally assumed to be external to the Core Network
`(CN); one example of external content providers would be television broadcasters for mobile
`television. The MBMS content is generally assumed to be IP based, and by design the MBMS
`system is integrated with the IP Multimedia Subsystem (IMS) [2] service infrastructure based
`on the Internet Engineering Task Force (IETF) Session Initiation Protocol (SIP) [3].
`The Broadcast-Multicast Service Centre (BM-SC) is the interface between external
`content providers and the CN. The main functions provided by the BM-SC are:
`
`• Reception of MBMS content from external content providers;
`• Providing application and media servers for the mobile network operator;
`• Announcement and scheduling of MBMS services and delivery of MBMS content into
`the core network (including traffic shaping and content synchronization).
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`Figure 13.2: EPS reference architecture for MBMSbroadcast mode [1].
`Reproduced by permission of © 3GPP.
`
`13.3.3 Core Network
`
`In LTE/SAE, the MBMS Gateway (MBMS GW)is the CN entry point for MBMScontrol
`signalling and traffic. The MBMS GW distributes session control signalling via the Mobility
`ManagementEntity (MME—see Section 2.2.1) to E-UTRANand handles the establishment
`and release of user plane bearers using IP multicasttraffic.
`The MME is involved with MBMSsession control (start/modification/stop) and reliable
`transmission of control messages to the E-UTRANnodesin the ‘MBMSservice area’.
`
`13.3.4 Radio Access Network — E-UTRAN/UTRAN/GERAN and UE
`
`The Radio Access Network (RAN), whether E-UTRAN, UTRANor GERAN,is responsible
`for delivering MBMSdata efficiently within the designated MBMSservice area.
`The MCE deals with session control and manages the subframe allocation and radio
`resource configuration to ensure that all eNodeBsparticipating in an MBMStransmission
`within a semi-statically configured area (see Section 13.4.2) use exactly the same configura-
`tion. The MCE also handles admission control.
`The eNodeBhandlesthe transfer of MBMScontrol, sessionstart notifications and transfer
`of MBMSdata.
`The final element in the MBMScontentand service chain is the MBMSreceiveritself:
`the UE.
`
`3§ystem Architecture Evolution.
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`13.3.5 MBMS Interfaces
`For MBMS in LTE, two new control plane interfaces have been defined (M3 and M2), as
`well as one new user plane interface (M1), as shown in Figure 13.2.
`
`297
`
`13.3.5.1 M3 Interface (MCE – MME)
`An Application Part (M3AP) is defined for this interface between the MME and MCE. The
`M3AP primarily specifies procedures for starting, stopping and updating MBMS sessions.
`Upon start or modification of an MBMS session, the MME provides the details of the MBMS
`bearer while the MCE verifies if the MBMS service (or modification of it) can be supported.
`Point-to-point signalling transport is applied, using the Stream Control Transmission Protocol
`(SCTP) [4].
`
`13.3.5.2 M2 Interface (MCE – eNodeB)
`Like the M3 interface, an application part (M2AP) is defined for this interface between the
`MCE and eNodeB, again primarily specifying procedures for starting, stopping and updating
`MBMS sessions. Upon start or modification of an MBMS session, the MCE provides
`the details of the radio resource configuration that all participating eNodeBs shall apply.
`In particular, the MCE provides the updated control information to be broadcast by the
`eNodeBs. SCTP is again used for the signalling transport.
`
`13.3.5.3 M1 Interface (MBMS GW – eNodeB)
`Similarly to the S1 and X2 interfaces (see Sections 2.5 and 2.6 respectively), the GTP-U4
`protocol over UDP5 over IP is used to transport MBMS data streams over the M1 interface.
`IP multicast is used for point-to-multipoint delivery.
`
`13.4 MBMS Single Frequency Network Transmission
`One of the targets for MBMS in LTE is to support a cell edge spectral efficiency in an
`urban or suburban environment of 1 bps/Hz – equivalent to the support of at least 16 mobile
`TV channels at around 300 kbps per channel in a 5 MHz carrier. This is only achievable
`by exploiting the special features of the LTE OFDM6 air interface to transmit multicast or
`broadcast data as a multicell transmission over a synchronized ‘single frequency network’:
`this is known as Multimedia Broadcast Single Frequency Network (MBSFN) operation.
`
`13.4.1 Physical Layer Aspects
`In MBSFN operation, MBMS data is transmitted simultaneously over the air from multiple
`tightly time-synchronized cells. A UE receiver will therefore observe multiple versions of the
`signal with different delays due to the multicell transmission. Provided that the transmissions
`from the multiple cells are sufficiently tightly synchronized for each to arrive at the UE within
`
`4GPRS Tunnelling Protocol – User Plane [5].
`5User Datagram Protocol.
`6Orthogonal Frequency Division Multiplexing.
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`the Cyclic Prefix (CP) at the start of the symbol, there will be no Inter—Symbol Interference
`(ISI). In effect, this makes the MBSFN transmission appear to a UE as a transmission from
`a single large cell, and the UE receiver may treat the multicell transmissions in the same
`way as multipath components of a single-cell transmission without incurring any additional
`complexity. This is illustrated in Figure 13.3. The UE does not even need to know how many
`cells are transmitting the signal.
`The method of achieving the required tight synchronization between MBSFN transmis-
`sions from different eNodeBs is not defined in the LTE specifications; this is left to the
`implementation of the eNodeBs. Typical implementations are likely to use satellite-based
`solutions (e.g. GPS7) or possibly synchronized backhaul protocols (e.g. [6]).
`
`Syndirorized eNodeBs
`transnitting MBMS data
`
`E
`
`Signals fromdifferenteNodeBs
`arrive with‘n cycicprefix at UE
`
`I
`
`I
`
`OFDM syn'bot duration
`+—————>
`
`cyan /:l:l
`Prefix I::I
`
`I:I:l
`Ef—J
`ISI free window
`
`Figure 13.3: ISI-free operation with MBSFN transmission.
`
`This single frequency network reception leads to significant improvements in spectral
`efficiency compared to UMTS Release 6 MBMS, as the MBSFN transmission greatly
`enhances the Signal-to—Interference-plus—Noise Ratio (SlNR). This is especially true at the
`cell edge, where transmissions which would otherwise have constituted inter-cell interference
`are translated into useful signal energy — hence the received signal power is increased at the
`same time as the interference power being largely removed.
`An example of the improvement in performance achievable using MBSFN transmission
`compared to single-cell point-to-multipoint transmission is shown in Figure 13.4. In this
`
`7Global Positioning System
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`example, the probability of a randomly located UE not being in outage (defined as MBMS
`packet loss rate < 1%) is plotted against spectral efficiency of the MBMS data transmissions
`(a measure of MBMS data rate in a given bandwidth). A hexagonal cell—layout is assumed,
`with the MBSFN area comprising one, two or three rings around a central cell for which the
`performance is evaluated. It can be seen that the achievable data rates increase markedly as
`the size of the MBSFN area is increased and hence the surrounding inter-cell interference
`is reduced. A 1 km cell radius is assumed, with 46 dBm eNodeB transmission power, 15 m
`eNodeB antenna height and 2 GHz carrier frequency.
`
`+Cemdcellwmmedw3nmsotm$flcdls
`
`
`+Cerlrdcsllsimomdedby2nngsotMBSFch|s
`
`,
`+Centrdcdlsunundadty1dmolmsmodts
`._
`
`,
`a +Cemdcellsunomdedbyhbflm
` '--i--r------r---
`
`
`1 km cel realm
`‘-+-i‘--I--i---
`
`
`46dBm eNB pour
`15 mananna helgtl
`
`
`
`,
`:3:
`":23 2 35.; :2,
`
`:3??? 3" é;
`' "
`* 5__I§§z_;s_.;_;._.;_22.31;:
`f
`-
`>
`i
`"
`
`
`
`on
`0.5
`1.0
`1.5
`2.0
`2.5
`3.0
`
`Spectral Efficiency (bps/Hz)
`
`
`
`
`
`Coverage(ProbabtyofPacketErrorRate<1%)
`
`Figure 13.4: Reduction in total downlink resource usage achievable using MBSFN
`transmission [7]. Reproduced by permission of © 2007 Motorola.
`
`MBSFN data transmission takes place via the Multicast CHannel (MCH) transport
`channel, which is mapped to the Physical Multicast Cl-lannel (PMCH) introduced in
`Section 9.2.3.
`
`The basic structure of the Physical Multicast Channel (PMCH) is very similar to the
`Physical Downlink Shared Channel (PDSCH). However, as the channel in MBSFN operation
`is in efiect a composite channel from multiple cells, it is necessary for the UE to perform
`a separate channel estimate for MBSFN reception from that performed for reception of
`data from a single cell. Therefore, in order to avoid the need to mix normal Reference
`Signals (RSs) and RSs for MBSFN in the same subframe, frequency-division multiplexing
`of the PMCH and PDSCH is not permitted within a given subframe; instead, time-division
`multiplexing of unicast and MBSFN data is used - certain subframes are specifically
`designated as MBSFN subframes, and it is in these subframes that the PMCH would be
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`transmitted.8 Certain subframes are not allowed to be used for MBSFN transmission: in a
`Frequency Division Duplex (FDD) system, subframes 0, 4, 5 and 9 in each 10 ms radio frame
`are reserved for unicast transmission in order to avoid disrupting the synchronization signals
`or paging occasions, and to ensure that sufficient cell-specific RSs are available for decoding
`the broadcast system information; in a Time Division Duplex (TDD) system, subframes 0, 1,
`2, 5 and 6 cannot be MBSFN subframes.
`The key differences from PDSCH in respect of the PMCH are as follows:
`• The dynamic control signalling (PDCCH and PHICH9 – see Section 9.3) cannot
`occupy more than two OFDM symbols in an MBSFN subframe. The scheduling of
`MBSFN data on the PMCH is carried out by higher-layer signalling, so the PDCCH is
`used only for uplink resource grants and not for the PMCH.
`• The PMCH always uses the first redundancy version (see Section 10.3.2.4) and does
`not use Hybrid ARQ.
`• The extended CP is used (∼17 μs instead of ∼5 μs) (see Section 5.4.1). As the
`differences in propagation delay from multiple cells will typically be considerably
`greater than the delay spread in a single cell, the longer CP helps to ensure that the
`signals remain within the CP at the UE receivers, thereby reducing the likelihood of
`ISI. This avoids introducing the complexity of an equalizer in the UE receiver, at the
`expense of a small loss in peak data rate due to the additional overhead of the longer CP.
`Note, however, that if the non-MBSFN subframes use the normal CP, then the normal
`CP is used in the OFDM symbols used for the control signalling at the start of each
`MBSFN subframe. This results in there being some spare time samples whose usage is
`unspecified between the end of the last control signalling symbol and the first PMCH
`symbol, the PMCH remaining aligned with the end of the subframe; the eNodeB may
`transmit an undefined signal (e.g. a cyclic extension) during this time or it may switch
`off its transmitter – the UE cannot assume anything about the transmitted signal during
`these samples.
`• The Reference Signal (RS) pattern embedded in the PMCH (designated ‘antenna port
`4’) is different from non-MBSFN data transmission, as shown in Figure 13.5. The RSs
`are spaced more closely in the frequency domain than for non-MBSFN transmission,
`reducing the separation to every other subcarrier instead of every sixth subcarrier. This
`improves the accuracy of the channel estimate that can be achieved for the longer delay
`spreads. The channel estimate obtained by the UE from the MBSFN RS is a composite
`channel estimate, representing the composite channel from the set of cells transmitting
`the MBSFN data. (Note, however, that the cell-specific RS pattern embedded in the
`OFDM symbols carrying control signalling at the start of each MBSFN subframe
`remains the same as in the non-MBSFN subframes.)
`
`The latter two features are designed so that a UE making measurements of a neighbouring
`cell does not need to know in advance the allocation of MBSFN and non-MBSFN subframes.
`The UE can take advantage of the fact that the first two OFDM symbols in all subframes use
`the same CP duration and RS pattern.
`
`8LTE does not currently support dedicated MBMS carriers in which all subframes would be used for MBSFN
`transmission.
`9Physical Downlink Control Channel and Physical HARQ Indicator Channel.
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`0%6)15HIHUHQFH
`6\PERO
`
`IUHTXHQF\
`
`PV
`
`WLPH
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`Figure 13.5: MBSFN RS pattern for 15 kHz subcarrier spacing.
`Reproduced by permission of © 3GPP.
`
`In addition to these enhancements for MBSFN transmission, a second OFDM parame-
`terization is provided in the LTE physical layer specifications, designed for downlink-only
`multicast/broadcast transmissions. However, this parameterization is not supported in current
`releases of LTE. As discussed in Section 5.4.1, this parameterization has an even longer CP,
`double the length of the extended CP, resulting in approximately 33 μs. This is designed
`to cater in the future for deployments with very large differences in propagation delay
`between the signals from different cells (e.g. 10 km). This would be most likely to occur for
`deployments at low carrier frequencies and large inter-site distances. In order to avoid further
`increasing the overhead arising from the CP in this case, the number of subcarriers per unit
`bandwidth is also doubled, giving a subcarrier spacing of 7.5 kHz. The cost of this is an
`increase in Inter-Carrier Interference (ICI), especially in high-mobility scenarios with a large
`Doppler spread. There is therefore a trade-off between support for wide-area coverage and
`support for high mobile velocities. It should be noted, however, that the maximum Doppler
`shift is lower at the low carrier frequencies that would be likely to be used for a deployment
`with a 7.5 kHz subcarrier spacing. The absolute frequency spacing of the reference symbols
`for the 7.5 kHz parameterization is the same as for the 15 kHz subcarrier spacing MBSFN
`pattern, resulting in a reference symbols on every fourth subcarrier.
`
`13.4.2 MBSFN Areas
`A geographical area of the network where MBMS can be transmitted is called an MBMS
`Service Area.
`A geographical area where all eNodeBs can be synchronized and can perform MBSFN
`transmissions is called an MBSFN Synchronization Area.
`The area within an MBSFN Synchronization Area, covered by a group of cells participat-
`ing in an MBSFN transmission, is called an MBSFN Area. An MBSFN Synchronization Area
`may support several MBSFN Areas. Moreover, a cell within an MBSFN Synchronization
`Area may form part of multiple MBSFN Areas, each characterized by different transmitted
`content and a different set of participating cells.
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`LTE – THE UMTS LONG TERM EVOLUTION
`
`Figure 13.6 illustrates an example of the usage of MBSFN areas in a network providing
`two nationwide MBMS services (N1 and N2) as well as two regional MBMS services, R1
`and R2 (i.e. with different regional content), across three different regions.
`The most natural way to deploy this would be to use four different MBSFN areas, as
`shown in the figure. It should be noted that the control information related to a service is
`transmitted by the same set of cells that transmit the data – e.g. MBSFN are