Heterogeneous network deployments in LTE Complementing high-power macro nodes with lower-power ones is an attractive means of meeting the predicted requirements for higher data rates and additional capacity.

mechanism. In practice, some addi-tional factors such as backhaul capac-ity should also be included in the cell selection process. Increasing the uptake area of a node is sometimes referred to as range expansion.

The advantages of this technique are: enhanced uplink data rates – by at least

partially taking uplink path loss into account when associating terminals with a low-power node;

increased capacity – receiving downlink traffic from the low-power node even if the received signal strength from the macro is higher allows for the reuse of transmission resources across low-power nodes; and

improved robustness – enlarging the coverage area of a low-power node can reduce its sensitivity to ideal placement in a traffic hotspot.

A heterogeneous deployment, with a modest range expansion somewhere in the region of 3-4dB, is already possible in the first release of LTE, Rel-8. The bene-fits gained from range expansion are highly dependent on the individual sce-nario and, in many cases, modest range expansion is best. Nevertheless, 3GPP has recently discussed the applicability of excessive range expansion with cell-selection offsets up to 9dB. Such deploy-ments are particularly problematic, as a terminal in the range-expansion zone (the striped area shown in Figure 2) may experience very low downlink signal-to-interference ratio due to the signifi-cant difference in output power of the nodes. Specifically, downlink control signaling in the range expansion zone – which is essential for the low-power node to control transmission activity – poses a problem. Transmission of the

LTE is rapidly emerging as the world’s most dominant 4G technology, taking mobile broadband to unprecedented performance levels. To meet expectations and predictions for even higher data rates and traffic capacity – beyond what is available in current LTE networks – a densified infrastructure is needed 1. In scenarios where users are highly clustered, using multiple low-output power sites to complement a macro cell providing basic coverage is an attractive solution – as illustrated in Figure 1.

This strategy results in a heterogeneous-network deployment with two cell lay-ers. The principle can be extended to more than two layers and the concept of multiple layers, is in itself not new; hierarchical cell structures have been considered since the mid-1990s – but, at that time, the discussion applied to mobile technologies primarily offering low-rate voice services.

In this article, the discussion focuses on radio-interface solutions, standardized by 3GPP, to enhance the performance of heterogeneous-network deployments (or heterogeneous deployments) with all nodes operating on the same frequency.

Traditionally, a terminal connects to the node from which the down-link signal strength is the strongest. In Figure 1, the solid orange areas are those in which the signal from the cor-responding pico node is the strongest. Users in these zones connect to the appropriate low-power node.

Due to the difference in transmission power between the pico nodes and the overlying macro node, this strategy does not necessarily result in the terminal connecting to the node to which it has the lowest path loss – as illustrated in Figure 2. It is, therefore, not the best node-selection strategy for achieving high uplink data rates.

The uptake area of a low-power node can be expanded without increasing the output power of the node by add-ing an offset to the received downlink signal strength in the cell-selection

ST E FA N PA R KVA L L , E R I K DA H L M A N, GEORGE JÖNGR E N, SA R A L A N DST RÖM A N D L A R S L I N DBOM

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The soft-cell approach

BOX A Terms and abbreviations

3GPP 3rd Generation Partnership Project4G 4th Generation mobile wireless standardsABS almost blank subframeBCH broadcast channelCA carrier aggregationCRS cell-specific reference signalDM-RS demodulation-specific reference signalsICIC inter-cell interference coordinationLTE Long Term Evolution

PDCCH physical downlink control channelPDSCH physical downlink shared channelPSS primary synchronization signalRE range expansionRRC Radio Resource ControlRRU remote radio unitRx radio receiverSSS secondary synchronization signalUL CoMP uplink coordinated multipoint reception

FIGURE 1 Heterogeneous deployment

Rx powerRx power

Macro cellMacro cell

Downlink-signal-strengthcell borderDownlink-signal-strengthcell border

Path-loss-based cell borderPath-loss-based cell border

(path loss)-1(path loss)-1

Low-power nodeLow-power node

Rangeexpansionzone

Rangeexpansionzone

FIGURE 2 Range expansion

Carrier aggregation(CA)Carrier aggregation(CA)

Almost blanksubframes (ABSs)Almost blanksubframes (ABSs)

f1f1

f2f2

ff

FIGURE 3 Frequency-domain and time-domain partitioning

data part is less challenging as Rel-8 sup-ports methods for ensuring non-over-lapping transmissions in the frequency domain from the macro and the low-power node using inter-cell interference coordination (ICIC)2.

This article discusses two different approaches to heterogeneous deploy-ment – resource partitioning and soft-cell schemes – both of which provide support for excessive range expansion.

Resource partitioningBy restricting macro-cell transmissions from using the same time-frequency resources as the low-power node, con-trol signaling from the low-power node 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-ing on almost blank subframes (ABSs), a feature that will be fully supported in LTE Rel-11 (see Figure 3).

Frequency-domain partitioningThis method protects downlink control-signaling from the low-power node in the range-expansion zone by placing control signaling from the macro and low-power nodes on separate carriers – as illustrated in Figure 3. Assuming transmissions from low-power nodes are time synchronized with the over-lying macro, the control signaling on carrier f2 in the range-expansion zone will not be subject to major interfer-ence from the macro node. At the same time, through the use of carrier aggrega-tion, data transmissions can still benefit from the full bandwidth of both carri-ers. The Rel-8 ICIC mechanism can be used to coordinate use of data resources.

Regardless of the extent of range expansion, frequency-domain parti-tioning is a natural choice to support heterogeneous deployments for opera-tors who already rely on carrier aggre-gation (CA) to exploit fragmented spectrum; and who have a reasonable number of subscribers using CA-capable terminals in their networks.

Time-domain partitioningThis method protects the downlink control-signaling from the low-power node 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

PSSA, /SSSA, BCHA, CRSAPSSA, /SSSA, BCHA, CRSA

PSSB, /SSSB, BCHB, CRSBPSSB, /SSSB, BCHB, CRSB

Cell ACell ACell BCell B Cell CCell C

PSSC, /SSSC, BCHC, CRSCPSSC, /SSSC, BCHC, CRSC

FIGURE 4A Independent cells

Cell ACell A

PSSA, /SSSA, BCHA, CRSAPSSA, /SSSA, BCHA, CRSA

FIGURE 4B Soft cell

221133

Same PSS/SSS,BCH, CRSSame PSS/SSS,BCH, CRS

Data (PDSCH)Control (PDCCH)Data (PDSCH)Control (PDCCH)

FIGURE 5 Heterogeneous deployment using a soft-cell scheme

– which is illustrated in the bottom part of Figure 3. The low-power node is provided with data about the set of 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, 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 from the low-power node. Time-domain partitioning can thus be viewed as a ter-minal-centric approach to achieving excessive range expansion.

Support for time-domain partition-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 schemesIn both frequency-domain and time-domain partitioning schemes, the low-power nodes create separate cells, each 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 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 cell without creating independent cells – 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 cell-identity information, a terminal can derive the CRS structure of the cell 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 from

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The soft-cell approach

which a terminal can receive data trans-missions. Note that the sectors of a site constitute separate points.

Traditionally, each cell has one trans-mission point from which the CRS, as well as all data transmissions, are sent. In Rel-10, however, demodulation-spe-cific reference signals (DM-RSs) were introduced. Unlike CRSs, these signals are subject to the same pre-coding as the associated data and are transmit-ted only when a corresponding data transmission is detected. The termi-nal can deduce the channel needed for demodulation based on the fact that both the DM-RS and data are transmit-ted in a similar manner. This implies that DM-RS-based data transmission to a terminal does not have to be sent from the transmission point used for CRS-based information, and that time-fre-quency resources for data can be reused at different transmission points.

In Figure 5, data is transmitted to terminal 1 from the low-power node farthest to the left. Since the associat-ed DM-RS is transmitted from the same transmission point as the data, the ter-minal does not need to know which point is used for data transmission to achieve area-splitting gains – the reuse of time-frequency resources for data transmission across multiple low-pow-er nodes within the same macro cell.

The control information required in Rel-10 is based on CRS and needs to be transmitted from, at least, the macro site. In many cases, this results in data and associated control signaling orig-inating from different transmission points. This is transparent to the termi-nal; it needs to match the reference sig-nal with the corresponding data signal. The identity of the transmission point, on the other hand, is irrelevant.

Figure 5 shows different ways to transmit control information. The case for terminal 1 – where control signal-ing originates only from the macro site – has already been described. This meth-od results in reduced network energy consumption, because the low-power node is active only when there is data to transmit.

For terminal 2, identical CRS and con-trol signals can be transmitted from the macro and the low-power node. As the same signal is transmitted from both nodes, the terminal will interpret them

as a single composite node. This method results in an improved signal-to-noise ratio for control signaling through an over-the-air combination of transmis-sions from both the macro and the low-power nodes.

For power-control purposes, LTE ter-minals estimate the uplink path loss from the strength of the received CRS signal. Consequently, the case illustrat-ed by terminal 2 can sometimes result in more accurate uplink power con-trol – at least for Rel-8/9/10 terminals. A minor update to a future release of the LTE standard is currently under discus-sion in 3GPP that will provide the same uplink power-control accuracy while allowing for greater energy efficiency in network operations.

To further improve the performance of the soft-cell scheme, enhancements to support DM-RS-based control signal-ing are likely to be included in LTE Rel-11. This will provide area-splitting gains for control signaling, in contrast to the CRS-based control signaling in Rel-10 and previous releases.

Terminals from previous releases that do not support DM-RS-based trans-mission can still operate in a soft-cell scheme – without any area-splitting gains. Data transmission to such termi-nals is CRS-based and is handled in the same way as control signaling. These terminals will benefit from low-power nodes because of the improved signal-to-noise ratio.

A soft-cell scheme can be deployed

by connecting one or several RRUs and the macro site to the same main unit. In practice, this link should use high-speed microwave or optical fiber, as it requires low-latency and a fairly high-capacity connection for tight coupling between the macro and low-power nodes – where control and data signaling originate from different transmission points. However, with the availability of DM-RS-based control signaling, backhaul requirements will be relaxed as both the data and control signaling can orig-inate from the same transmission point.

The centralization of processing pro-vides benefits in uplink performance, which is often significant enough to jus-tify using RRUs with centralized pro-cessing irrespective of range expansion. Any combination of points – not neces-sarily those used for downlink trans-mission to a terminal – can be used to receive transmissions from a terminal. By combining the signals from differ-ent antennae in a constructive manner at the central processing node – a meth-od known as uplink softer handover – a significant improvement in uplink data rates can be achieved.

In addition to avoiding much of inter-fering CRS transmissions heteroge-neous deployments that use soft cells can provide greater mobility robust-ness than deployments with separate cells. This is important, especially when moving from a low-power node to the macro. In separate cell deployment, a handover procedure is required to

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Rel-8Rel-8 Rel-10Rel-10 Rel-10+Rel-10+

Separate cellSeparate cell

Soft cellSoft cell

Medium REMedium RE Rel 8Rel 8

Excessive REExcessive RE Resource partitioningResource partitioning

Any backhaulAny backhaul

Low-latency backhaul(allows for UL CoMP)Low-latency backhaul(allows for UL CoMP)

Almost blanksubframesAlmost blanksubframes

CarrieraggregationCarrieraggregation

DM-RS-baseddata and controlDM-RS-baseddata and control

DM-RS-based data CRS-based controlDM-RS-based data CRS-based control

CRS-baseddata and controlCRS-baseddata and control

Any REAny RE

FIGURE 6 Comparison of different approaches to heterogeneous deployments

switch serving cells. If, during the time it takes to perform the handover procedure, the terminal has moved too far into the macro area, it may drop the downlink connection from the low-power node before handover is complete – leading to a radio-link failure. In soft-cell deployment, the transmission point that should be used for downlink trans-mission can be changed rapidly without a handover procedure – thus reducing the probability of dropped connections.

ConclusionA heterogeneous-network deployment is a favorable means of meeting future data-rate and capacity demands. In many cases, the support provided in Rel-8 is sufficient. This article has provided an overview of various schemes, sum-marized in Figure 6, including carrier aggregation, almost blank subframes and soft cell – with a focus on the latter. The choice of scheme depends on the scenario, although the network-centric soft-cell approach provides many bene-fits without the requirement for not-yet-standardized terminal functionality.

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The soft-cell approach

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.

References

Stefan Parkvall

is currently a principal researcher at Ericsson Research, with a focus on future radio access.

Parkvall has been heavily involved in the development of HSPA, LTE and LTE-Advanced radio access. He is a senior member of IEEE and co-author of the book 3G Evolution – HSPA and LTE for Mobile Broadband and 4G – LTE/LTE-Advanced for Mobile Broadband. In 2009, he was a co-recipient of Stora Teknikpriset (Sweden’s major technology award) for his work on HSPA. In 1996, he received a Ph.D. in electrical engineering from the Royal Institute of Technology (KTH) Stockholm. He was previously an assistant professor in communication theory at KTH and a visiting researcher at University of California, San Diego, in the US.

Sara Landström

is a senior researcher at Ericsson Research in Luleå, Sweden. Her research area is Wireless

Access Networks and her current focus is heterogeneous networks. She has also been involved in evaluating IMT-Advanced candidate technologies and service-oriented research. She joined Ericsson in 2008 after receiving her Ph.D. in computer networking from Luleå University of Technology, Sweden.

George Jöngren

joined Ericsson Research in 2005 and is a master researcher in the area of radio-access

technologies. His current focus is on research and standardization of multi-antenna and coordinated multi point (CoMP) techniques for LTE. During his early years at Ericsson he was part of the development of the MIMO HSDPA test-bed. He holds an M.Sc. (1998 ) and Ph.D. (2003) in electrical engineering from the Royal Institute of Technology (KTH), Stockholm, Sweden. In 1997, he was elected Teacher of the Year in electrical engineering at KTH.

Erik Dahlman

joined Ericsson Research in 1993 and is a senior expert in the area of radio access

technologies. He has been deeply involved in the development and standardization of 3G radio access technologies (WCDMA/HSPA) as well as LTE and its evolution. He is part of the Ericsson Research management team working on long-term radio access strategies. He is also co-author of the book 3G Evolution – HSPA and LTE for Mobile Broadband and 4G – LTE/LTE-Advanced for Mobile Broadband. In 2009, together with Stefan Parkvall, he received the Stora Teknikpriset award in 2009 for contributions to the standardization of HSPA. He holds a Ph.D. from KTH in Stockholm.

Lars Lindbom

currently holds a position as systems manager at Ericsson Business Unit Networks,

where he works on concepts and standards for future radio access, including standardization related to heterogeneous networks for 3GPP. He has a Ph.D. in signal processing from Uppsala University.