Heterogeneous network deployments in LTE

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Heterogeneous network deployments in LTE

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Heterogeneous network deployments in LTE

  1. 1. The soft-cell approachHeterogeneous networkd­ eployments in LTEComplementing high-power macro nodes with lower-power ones is an attractive meansof meeting the predicted requirements for higher data rates and additional capacity. S T E FA N PA R K VA L L , E R I K DA H L M A N, GE ORGE JÖNGR E N, S A R A L A N D S T RÖM A N D L A R S L I N DB OMLTE is rapidly emerging as In this article, the discussion focuses on mechanism. In practice, some addi-the world’s most dominant radio-interface solutions, standardized tional factors such as backhaul capac-4G technology, taking mobile by 3GPP, to enhance the performance of ity should also be included in the cellbroadband to unprecedented heterogeneous-network deployments selection process. Increasing the uptakeperformance levels. To meet (or heterogeneous deployments) with all area of a node is sometimes referred toexpectations and predictions nodes operating on the same frequency. as range expansion.for even higher data rates and Traditionally, a terminal connectstraffic capacity – beyond what is to the node from which the down- The advantages of this technique are:available in current LTE networks link signal strength is the strongest. enhanced uplink data rates – by at least– a densified infrastructure is In Figure  , the solid orange areas are 1 partially taking uplink path loss intoneeded 1. In scenarios where those in which the signal from the cor- account when associating terminals with a responding pico node is the strongest. low-power node;users are highly clustered, using Users in these zones connect to the increased capacity – receiving downlinkmultiple low-output power sites appropriate low-power node. traffic from the low-power node even if theto complement a macro cell Due to the difference in transmission received signal strength from the macro isproviding basic coverage is an power between the pico nodes and the higher allows for the reuse of transmissionattractive solution – as illustrated overlying macro node, this strategy does resources across low-power nodes; andin Figure 1. not necessarily result in the terminal improved robustness – enlarging theThis strategy results in a heterogeneous- connecting to the node to which it has coverage area of a low-power node cannetwork deployment with two cell lay- the lowest path loss – as illustrated in reduce its sensitivity to ideal placement iners. The principle can be extended to Figure 2. It is, therefore, not the best a traffic hotspot.more than two layers and the concept node-selection strategy for achievingof multiple layers, is in itself not new; high uplink data rates. A heterogeneous deployment, with ahierarchical cell structures have been The uptake area of a low-power node modest range expansion somewhere inconsidered since the mid-1990s – but, can be expanded without increasing the region of 3-4dB, is already possible inat that time, the discussion applied to the output power of the node by add- the first release of LTE, Rel-8. The bene-mobile technologies primarily offering ing an offset to the received downlink fits gained from range expansion arelow-rate voice services. signal strength in the cell-selection highly dependent on the individual sce- nario and, in many cases, modest range expansion is best. Nevertheless, 3GPP has recently discussed the applicability BOX A Terms and abbreviations of excessive range expansion with cell- selection offsets up to 9dB. Such deploy- ments are particularly problematic, as 3GPP 3rd Generation Partnership Project PDCCH physical downlink control channel 4G 4th Generation mobile PDSCH physical downlink shared channel a terminal in the range-expansion zone wireless standards PSS primary synchronization signal (the striped area shown in Figure 2) may ABS almost blank subframe RE range expansion experience very low downlink signal- BCH broadcast channel RRC Radio Resource Control to-interference ratio due to the signifi- CA carrier aggregation RRU remote radio unit cant difference in output power of the CRS cell-specific reference signal Rx radio receiver nodes. Specifically, downlink control DM-RS demodulation-specific SSS secondary synchronization signal signaling in the range expansion zone reference signals UL CoMP uplink coordinated – which is essential for the low-power ICIC inter-cell interference coordination multipoint reception node to ­ ontrol transmission activity c LTE Long Term Evolution – poses a problem. Transmission of theE R I C S S O N R E V I E W • 2 2011
  2. 2. data part is less challenging as Rel-8 sup-ports methods for ensuring non-over- FIGURE 1 Heterogeneous deploymentlapping transmissions in the frequencydomain from the macro and the low-power node using inter-cell interferencecoordination (ICIC)2. This article discusses two differentapproaches to heterogeneous deploy-ment – resource partitioning and soft-cell schemes – both of which providesupport for excessive range expansion.Resource partitioningBy restricting macro-cell transmissionsfrom using the same time-frequencyresources as the low-power node, con-trol signaling from the low-powernode to the terminal can be protect-ed. Resource partitioning can be imple-mented in either the frequency domain,by using support for carrier aggregation(Rel-10), or in the time domain, by rely- FIGURE 2 Range expansioning on almost blank subframes (ABSs),a feature that will be fully supported in Low-power nodeLTE Rel-11 (see Figure 3). Range Rx p expansionFrequency-domain partitioning owe r zoneThis method protects downlink control- (pat h los s) -1signaling from the low-power node inthe range-expansion zone by placing Macro cellcontrol signaling from the macro andlow-power nodes on separate carriers– as illustrated in Figure 3. Assumingtransmissions from low-power nodesare time synchronized with the over-lying macro, the control signaling on Downlink-signal-strength cell bordercarrier f 2 in the range-expansion zonewill not be subject to major interfer- Path-loss-based cell borderence from the macro node. At the sametime, through the use of carrier aggrega-tion, data transmissions can still benefitfrom the full bandwidth of both carri-ers. The Rel-8 ICIC mechanism can be FIGURE 3 Frequency-domain and time-domain partitioningused to coordinate use of data resources. Regardless of the extent of rangeexpansion, frequency-domain parti-tioning is a natural choice to supportheterogeneous deployments for opera-tors who already rely on carrier aggre-gation (CA) to exploit fragmentedspectrum; and who have a reasonablenumber of subscribers using CA-capable f1terminals in their networks. Carrier aggregation (CA) f2Time-domain partitioning Almost blank fThis method protects the downlink subframes (ABSs)c­ ontrol-signaling from the ­ow-power lnode by reducing macro transmis-sion activity in certain subframes E R I C S S O N R E V I E W • 2 2011
  3. 3. The soft-cell approach – which is illustrated in the bottom FIGURE 4A Independent cells part of Figure 3. The low-power node is provided with data about the set of PSSA, /SSSA, BCHA, CRSA protected subframes over the X2 inter- face and can use this information when scheduling users who are in the range- expansion zone. For backward compatibility, the mac- ro node must transmit certain signals, PSSB, /SSSB, BCHB, CRSB PSSC, /SSSC, BCHC, CRSC most notably cell-specific reference sig- nals (CRSs) and synchronization signals (PSSs/SSSs), in downlink subframe in the same way as in Rel-8. The protected subframes are, as a result, not complete- ly blank – but they are almost blank. Terminals need to apply interference suppression to receive control signaling Cell B Cell A Cell C from the low-power node. Time-domain partitioning can thus be viewed as a ter- minal-centric approach to achieving FIGURE 4B Soft cell excessive range expansion. Support for time-domain partition- PSSA, /SSSA, BCHA, CRSA ing for excessive range expansion is incomplete in Rel-10; X2 and RRC sig- naling are included, whereas inter- ference-suppression receivers are still under discussion for Rel-11. The main argument for implementing time- domain partitioning is to enable sup- port for excessive range expansion for those operators that do not want to rely on carrier aggregation. Soft-cell schemes In both frequency-domain and time- domain partitioning schemes, the low- power nodes create separate cells, each Cell A of which has an individual cell identi- ty that differs from that of the macro cell. As a consequence, each pico node transmits unique system information FIGURE 5 Heterogeneous deployment using a soft-cell scheme and synchronization signals, includ- ing reference signals – as illustrated in Figure 4A. In an alternative approach known as shared cell or soft cell, low- power nodes can be part of the macro Same PSS/SSS, cell without creating independent cells BCH, CRS – as illustrated in Figure 4B. The distinction between cell and transmission points is an important aspect of the soft-cell approach. Each cell has a unique cell identity from which the CRS is derived. With the 3 1 2 cell-identity information, a terminal Data (PDSCH) can derive the CRS structure of the cell Control (PDCCH) and obtain the system information it needs to access the network. A transmis- sion point, on the other hand, is simply one or more collocated antennae fromE R I C S S O N R E V I E W • 2 2011
  4. 4. which a terminal can receive data trans-missions. Note that the sectors of a site FIGURE 6 Comparison of different approaches to heterogeneous deploymentsconstitute separate points. Traditionally, each cell has one trans- Any backhaulmission point from which the CRS, as Medium RE Rel 8well as all data transmissions, are sent. Almost blank Separate cell subframesIn Rel-10, however, demodulation-spe-cific reference signals (DM-RSs) were Excessive RE Resource partitioningintroduced. Unlike CRSs, these signals Carrier Low-latency backhaulare subject to the same pre-coding as (allows for UL CoMP) aggregationthe associated data and are transmit-ted only when a corresponding data Soft cell Any REtransmission is detected. The termi- CRS-based DM-RS-based datanal can deduce the channel needed for data and control CRS-based control DM-RS-baseddemodulation based on the fact that data and controlboth the DM-RS and data are transmit-ted in a similar manner. This implies Rel-8 Rel-10 Rel-10+that DM-RS-based data transmission toa terminal does not have to be sent fromthe transmission point used for CRS-based information, and that time-fre-quency resources for data can be reused as a single composite node. This method by connecting one or several RRUs andat ­ ifferent transmission points. d results in an improved signal-to-noise the macro site to the same main unit. In In Figure 5, data is transmitted to ratio for control signaling through an practice, this link should use high-speedterminal 1 from the low-power node over-the-air combination of transmis- microwave or optical fiber, as it requiresfarthest to the left. Since the associat- sions from both the macro and the low- low-latency and a fairly high-capacityed DM-RS is transmitted from the same power nodes. connection for tight coupling betweentransmission point as the data, the ter- For power-control purposes, LTE ter- the macro and low-power nodes – whereminal does not need to know which minals estimate the uplink path loss control and data signaling originatepoint is used for data transmission to from the strength of the received CRS from different transmission points.achieve area-splitting gains – the reuse signal. Consequently, the case illustrat- However, with the availability of DM-RS-of time-frequency resources for data ed by terminal 2 can sometimes result based control signaling, backhaultransmission across multiple low-pow- in more accurate uplink power con- requirements will be relaxed as bother nodes within the same macro cell. trol – at least for Rel-8/9/10 terminals. A the data and control signaling can orig- The control information required in minor update to a future release of the inate from the same transmission point.Rel-10 is based on CRS and needs to be LTE standard is currently under discus- The centralization of processing pro-transmitted from, at least, the macro sion in 3GPP that will provide the same vides benefits in uplink performance,site. In many cases, this results in data uplink power-control accuracy while which is often significant enough to jus-and associated control signaling orig- allowing for greater energy efficiency tify using RRUs with centralized pro-inating from different transmission in network operations. cessing irrespective of range expansion.points. This is transparent to the termi- To further improve the performance Any combination of points – not neces-nal; it needs to match the reference sig- of the soft-cell scheme, enhancements sarily those used for downlink trans-nal with the corresponding data signal. to support DM-RS-based control signal- mission to a terminal – can be used toThe identity of the transmission point, ing are likely to be included in LTE Rel- receive transmissions from a terminal.on the other hand, is irrelevant. 11. This will provide area-splitting gains By combining the signals from differ- Figure 5 shows different ways to for control signaling, in contrast to the ent antennae in a constructive mannertransmit control information. The case CRS-based control signaling in Rel-10 at the central processing node – a meth-for terminal 1 – where control signal- and previous releases. od known as uplink softer handover – aing originates only from the macro site Terminals from previous releases significant improvement in uplink data– has already been described. This meth- that do not support DM-RS-based trans- rates can be achieved.od results in reduced network energy mission can still operate in a soft-cell In addition to avoiding much of inter-consumption, because the low-power scheme – without any area-splitting fering CRS transmissions heteroge-node is active only when there is data gains. Data transmission to such termi- neous deployments that use soft cellsto transmit. nals is CRS-based and is handled in the can provide greater mobility robust- For terminal 2, identical CRS and con- same way as control signaling. These ness than deployments with separatetrol signals can be transmitted from the terminals will benefit from low-power cells. This is important, especially whenmacro and the low-power node. As the nodes because of the improved signal- moving from a low-power node to thesame signal is transmitted from both to-noise ratio. macro. In separate cell deployment, anodes, the terminal will interpret them A soft-cell scheme can be deployed handover procedure is required to E R I C S S O N R E V I E W • 2 2011
  5. 5. The soft-cell approach switch serving cells. If, during thetime it takes to perform the handover Stefan Parkvall George Jöngrenprocedure, the terminal has moved too is currently a principal joined Ericssonfar into the macro area, it may drop the researcher at Ericsson Research in 2005 and is adownlink connection from the low- Research, with a focus on master researcher in thepower node before handover is ­ omplete c future radio access. area of radio-access– leading to a radio-link failure. In soft- Parkvall has been heavily involved in technologies. His current focus is oncell deployment, the transmission point the development of HSPA, LTE and research and standardization of multi-that should be used for downlink trans- LTE-Advanced radio access. He is a antenna and coordinated multi pointmission can be changed rapidly without senior member of IEEE and co-author (CoMP) techniques for LTE. During hisa handover procedure – thus reducing of the book 3G Evolution – HSPA and early years at Ericsson he was part ofthe probability of dropped connections. LTE for Mobile Broadband and 4G – the development of the MIMO HSDPA LTE/LTE-Advanced for Mobile test-bed. He holds an M.Sc. (1998 ) andConclusion Broadband. In 2009, he was a co- Ph.D. (2003) in electrical engineering recipient of Stora Teknikpriset from the Royal Institute of TechnologyA heterogeneous-network deployment (Sweden’s major technology award) (KTH), Stockholm, Sweden. In 1997, heis a favorable means of meeting future for his work on HSPA. In 1996, he was elected Teacher of the Year indata-rate and capacity demands. In received a Ph.D. in electrical electrical engineering at KTH.many cases, the support provided in Rel- engineering from the Royal Institute of8 is sufficient. This article has provided Technology (KTH) Stockholm. He wasan overview of various schemes, sum- previously an assistant professor in Sara Landströmmarized in Figure 6, including carrier communication theory at KTH and aaggregation, almost blank subframes is a senior researcher at visiting researcher at University ofand soft cell – with a focus on the latter. Ericsson Research in California, San Diego, in the US.The choice of scheme depends on the Luleå, Sweden. Herscenario, although the network-centric research area is Wirelesssoft-cell approach provides many bene- Access Networks and her current Erik Dahlman focus is heterogeneous networks.fits without the requirement for not-yet-standardized terminal functionality. joined Ericsson She has also been involved in Research in 1993 and is a evaluating IMT-Advanced candidate senior expert in the area technologies and service-oriented of radio access research. She joined Ericsson in 2008 technologies. He has been deeply after receiving her Ph.D. in computer involved in the development and networking from Luleå University of standardization of 3G radio access Technology, Sweden. technologies (WCDMA/HSPA) as well as LTE and its evolution. He is part of the Ericsson Research management Lars Lindbom team working on long-term radio access strategies. He is also co-author currently holds a of the book 3G Evolution – HSPA and position as systems LTE for Mobile Broadband and 4G – manager at Ericsson LTE/LTE-Advanced for Mobile Business Unit Networks, Broadband. In 2009, together with where he works on concepts and Stefan Parkvall, he received the Stora standards for future radio access, Teknikpriset award in 2009 for including standardization related to contributions to the standardization heterogeneous networks for 3GPP. He of HSPA. He holds a Ph.D. from KTH has a Ph.D. in signal processing from in Stockholm. Uppsala University. eferences R 1. S. Landström, A. Furuskär, K. Johansson, L. Falconetti, and F. Kronestedt, Heterogeneous networks (hetnets) – an approach to increasing cellular capacity and coverage, Ericsson Review, No 1, 2011. 2. E. Dahlman, S. Parkvall, and J. Sköld, 4G: LTE/LTE-Advanced for Mobile Broadband, Elsevier, 2011.E R I C S S O N R E V I E W • 2 2011

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