Abstract Due to the explosive growth of mobile data traffic,

operators are increasingly facing the pressure of providing high data capacity to consumers while maintaining an efficient and low-cost network. In 3GPP LTE-A evolution, it has become clear that cell deployment is migrating towards a hybrid of macro-cell and small cells, where the macro-cell provides coverage and the small cells provide high capacity for hotspot and indoor users. This paper discusses the issues and solutions related to enhancing performance for such hybrid deployment in LTE-A, including interference management, mobility, carrier aggregation, and energy efficiency.

Index TermsLTE-Advanced, pico-cell, femto-cell, small cell enhancement, WLAN interworking

I. INTRODUCTION urrently, the point-to-point radio transmission is approaching physical limit with the adoption of

techniques such as turbo coding and MIMO. It is unavoidable that the trend of shrinking cell size continues so as to meet the challenge of providing exponentially increasing data throughput to satisfy user demand: While deployment cost and complexity is a concern, bringing the base station closer to the terminal is a relatively easy way to improve the signal to noise and interference ratio. This implies that the cellular architecture is increasingly moving towards a tiered structure, with higher-power macro-cell nodes providing overall coverage while lower-power small-cell nodes provide high data rate at selected hotspots or coverage holes. Conversely, for already deployed macro-cell network, it is also useful to strategically insert small-cell nodes under the coverage of overlaid macro-cell layer so as to boost the capacity at hotspots.

For data throughput purposes, the most efficient way to use small cells is to strategically put them in locations where there is a high demand for data transmission. This includes dense urban areas with high concentration of wireless users such as shopping malls and public transportation hubs, and areas where users spend hours using the wireless medium

such as homes, office complex, coffee shops. To maximize the efficiency of the LTE-A network with

small cells, several technical areas need to be carefully studied. These topics are explored in Section II, including spectrum, inter-site carrier aggregation, inter-cell interference coordination, mobility management, and energy efficiency. In Section III, several types of small cells are discussed, including pico-cell, femto-cell, relay, and WLAN. In Section IV, problems for future work are presented. Section V concludes the paper.

II. TECHNICAL AREAS

A. Spectrum When the macro-cells are overlaid with small cells, they

can share the same carrier frequency, or reside over different carrier frequencies.

Due to the scarcity of spectrum, small cell study in LTE-A has focused on co-channel deployment of low-power nodes including pico-cell, femto-cell, or relay. In particular, the so-called heterogeneous network topic studies inter-cell interference issues arise from such deployment, including between macro-cell and small-cell, as well as between small cells.

However, if more spectrum can be made available to an operator, technically it is desirable to deploy the small cells at higher carrier frequencies, e.g., 3.5 GHz or higher, especially to support the lower mobility, high-data rate users. This is because at higher frequency the radio signal experiences larger propagation loss and the small cell would naturally have a smaller footprint. Moreover there is relatively more spectrum, and larger contiguous blocks of spectrum, available at higher bands. For WLAN small cells, this is naturally the case. For small cells deployed over licensed spectrum, extra spectrum needs to be budgeted. With low-power high-frequency small cells, a cluster of small cells can be deployed at the same carrier frequency to cover, for example, a large shopping mall, without any interference between the macro-cell and the small cell cluster.

To save spectrum, FDD/TDD modes can be used differently for small cell vs macro-cell. For example, the

Achieving High Capacity with Small Cells in LTE-A

Yufei W. Blankenship, Member, IEEE Research In Motion - Advanced Technology

Rolling Meadows, IL 60008 Email: yblankenship@rim.com

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Fiftieth Annual Allerton ConferenceAllerton House, UIUC, Illinois, USAOctober 1 - 5, 2012

978-1-4673-4539-2/12/$31.00 2012 IEEE

small cell can be deployed with TDD where the same carrier frequency is used for both uplink and downlink, while the macro cell uses FDD where one carrier frequency is used for downlink and a different carrier frequency is used for uplink. Furthermore, in a small cell deployment, the number of users per small cell node is typically not large due to the small coverage. User distribution and user traffic can fluctuate widely. With the expected high fluctuation of traffic in the small cells, it is beneficial that the TDD configuration varies with the changing proportion of downlink traffic vs. the uplink traffic. For example, when the traffic is uplink heavy (e.g., small cell users are uploading multimedia files), an uplink heavy TDD configuration (i.e., more frequent uplink subframes than downlink subframes) is used. On the other hand, when the traffic is downlink heavy (e.g., small cell users are downloading website contents), a corresponding downlink heavy TDD configuration (i.e., more frequent downlink subframes than uplink subframes) is used. Since each small cell adjusts the TDD configuration to its local traffic condition, this may cause neighbor small cells to use TDD with different DL/UL configurations. Thus uplink and downlink interference needs to be carefully managed between the small cells.

B. Inter-site Carrier Aggregation When the macro-cells and the small cells are deployed

over two carrier frequencies and the small cell is subservient to the macro-cell, the coupling of the macro-cell and the small cells can be interpreted as inter-site carrier aggregation. The macro cell is always configured as a primary cell (PCell), while the associated small cell is always configured as a secondary cell (SCell). A carrier at lower frequency f1 is used by an eNB to provide macro coverage while a carrier at higher frequency f2 is used by the small cells to improve throughput at hot spots. A typical scenario is that f1 and f2 are of different bands, e.g., f1 {800 MHz, 2 GHz} and f2 {3.5GHz, 3.8 GHz}. This is illustrated in Fig. 1. Casting the cell relationship in the inter-site carrier aggregation framework, handover can be based on macro-cells f1 coverage. Since the eNB acts as a primary cell, it can provide the majority of the control plane information for both the macro-cell and the small cells.

With inter-site aggregation of macro-cell and small cell,

the UE has the potential to have simultaneous transmission to, and reception from, the macro and small cell layers. The

resulting protocol stack is illustrated in Fig. 2. However, such dual connection will likely increase UEs RF complexity and power consumption. Tradeoffs need to be made to allow lower-complexity UE implementation.

Fig. 2. User plane protocol stack between the UE, the macro-cell, and the small cell.

Moreover, when the small cells are viewed as secondary

cells associated with the macro-cell, the macro-cell can communicate with the UEs and the small cell to dynamically or semi-dynamically adjust the configuration of the small cells. For example, the spectrum occupied by each small cell can be adjusted for load balancing and interference considerations. For load balancing purpose, a heavily loaded small cell may be allocated wider bandwidth, while a lightly loaded small cell may be allocated narrower bandwidth. For interference coordination purpose, the carrier frequency occupied by a small cell is determined such that the interference between the small cell and a neighbor cell is minimized. If the small cells are deployed in TDD mode, the macro-cell can coordinate each small cells TDD configuration (which defines the proportion of uplink subframes vs the downlink subframes) so that the small cells can be dynamically or semi-dynamically reconfigured based on the traffic situation in individual small cells and the interference conditions between the small cells. In terms of signaling, the reconfiguration information is sent from the macro-cell to the UEs in the small cell.

With the coupling of small cell to macro-cell, it also becomes possible to remove certain overhead in the small cell subframes, which are otherwise always sent periodically. For example, synchronization signal can be removed if the small cell can rely on the macro-cell to provide synchronization. If the small cells only carry data payload, LTE control channels such as PCFICH/PHICH/PDCCH, and signals such as common reference signals (CRS) can be removed. By minimizing overhead, the efficiency of the small cell can be further improved.

C. Inter-cell Interference Coordination When the macro cell and pico cells operate over the same

carrier frequency, features designed for Heterogeneous Network (HetNet) apply such as cell expansion, time-domain inter-cell interference coordination (ICIC) [1]. In HetNet deployments, ICIC is an important design issue due to the strong interference that a victim cell can experience from the aggressor cell. For example, cell edge UEs of the

Fig. 1. Inter-site carrier aggregation of macro-cells and small cells [1].

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pico-cell (i.e., victim cell) can experience strong interference from the co-channel macro-cell (i.e., aggressor cell) due to a much higher transmit power from the macro-cell. Since physical downlink control channel (PDCCH) and common reference signals (CRS) spans the entire bandwidth in an LTE subframe, frequency domain interference avoidance is not attainable. Consequently, time domain based resource sharing or coordination has been adopted as enhanced ICIC (eICIC).

For the time domain ICIC, subframe utilization across different cells are coordinated in time through so called Almost Blank Subframe (ABS) patterns. The ABSs in an aggressor cell are used to protect resources in subframes in the victim cell receiving strong inter-cell interference from the aggressor cell. ABS are subframes with reduced transmit power (including no transmission) on some physical channels and/or reduced activity. The eNB ensures backwards compatibility towards UEs by transmitting necessary control channels and physical signals as well as System Information. The ABS pattern coordination can be determined via backhaul signaling or OAM configuration.

An example of an ABS pattern is shown in Fig. 3 to illustrate the time-domain interference coordination between macro-cell and the cochannel pico-cell. In this example, the macro-eNB (i.e., the aggressor cell) configures and transfers the ABS patterns to the pico-eNB (i.e., the victim cell). The macro-eNB does not schedule data transmissions in ABS subframes (set B subframes in Fig. 3) to protect the UEs served by the pico-eNB at the edge of the pico cell. The pico-eNB may schedule transmissions to and from the UEs in its cell center regardless of the ABS patterns because there the SINR is sufficiently high (set A subframes in Fig. 3). In contrast, the pico-eNB may schedule transmission to and from the UEs at the cell edge only in ABS (set B subframes in Fig. 3).

Fig. 3. Time-domain ICIC using Almost Blank Subframes.

For the scenario where macro-cell and small small use

different carrier frequencies, there is no concern over interference between macro-cell and small cell. On the other hand, as the small cells become more densely packed to increase the system capacity, the interference between the small cells need be carefully managed.

When there is no interface between two neighbor small cells to carry interference coordination signals, interference mitigation can be achieved with assistance from UEs. The UE located between two small cells is able to

transmit and receive signals from both cells. The eNB of the neighbor cell can broadcast or unicast its resource assignment preference. Upon receiving such message from the neighbor cell, the UE then passes this information onto its serving cell. The scheduler of the serving cell is thus able to take the neighbor cells resource assignment into account.

Alternatively, the cell edge UE can passively scan its radio environment and accumulate statistics about the neighbor cells. The statistics are then reported to the serving eNB, either periodically or triggered. Aggregating statistics from multiple UE, the serving eNB is then able to obtain resource allocation information of its neighbor cells as a function of location. The scheduler of the serving eNB can then take such statistics into consideration to adaptively reduce interference.

D. Mobility Management Due to the small footprint, small-cell enhancement mainly

focuses on the low mobility users, e.g., UEs with speed of 0 15 km/h. Mobility can be intra-EUTRAN, i.e., between LTE/LTE-A cells, or inter-RAT, when the UE switches from one radio access technology to another. Intra-EUTRAN

With a large number of small cells deployed over macro-cells, it is important to minimize the core network impact caused by mobility among a large number of small-cell nodes. Mobility related procedures need to be handled carefully for both UE in connected state (e.g., RRC_CONNECTED), and UEs in idle state (e.g., RRC_IDLE). The key procedures include handover, paging, tracking area updates, and measurement reporting in connected state.

A simple strategy is not to let small cells be logical-peer entities to macro-cells. Thus handover is based on coverage of macro-cell where possible. UEs always search for the macro-cell for network entry and handover. A full handover procedure (i.e. with security key update and RACH procedure) is performed only when macro-cell changes. A UE in RRC_IDLE behaves the same as a macro UE that only connects to the macro-cell. When UE moves between small cells of the same macro-cell, radio resource control (RRC) signaling can add, remove, or reconfigure small cells for usage with the target macro-cell. From the perspective of the NAS, the UE is connected to the macro-cell, which provides the security keys at handover and the tracking area for Tracking Area Updates (TAUs). The small cells are simply considered as additional transmission resources.

A UE has only two states from the viewpoint of the access stratum protocols and the E-UTRAN, i.e. RRC_CONNECTED and RRC_IDLE, depending on whether the RRC connection has been established or not. For the small-cell scenario, it is expected that the same two states would be maintained with the macro-cell, without adding new states. When a large number of small cells are

Almost Blank Macro-cell

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-

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Normal Subframe

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deployed, mobility management issues arise for both idle mode and connected mode. In the RRC_IDLE state, the UE monitors a paging

channel from the macro-cell to detect incoming calls, acquires system information and performs neighbor cell measurement and cell (re)selection.

In the RRC_CONNECTED state, the UE transfers/receives data to/from the network via the eNB. The UE performs neighbor cell measurement and measurement reporting based on the configuration provided by the eNB. Mobility is controlled by the network via handover procedures. When the UE moves, seamless mobility without packet loss needs to be achieved for bearers with RLC acknowledged mode. This is true during mobility between small cell nodes, as well as mobility between macro-cell nodes and small cell nodes.

Considering that handover is based on macro-cell, while

the data payload relies on the small cell, it is preferable that UEs of different states connects to different cells. That is, idle mode UE camps on the macro-cell to receive paging message. When downlink or uplink data arrives, the UE performs RACH towards the small cell for data transmission. This requires that two sets of biases to be defined, one for camping on the macro-cell, the other for initiating RACH connection. For scenarios where large timing differences exist between macro-cell and pico-cell, RACH also needs to be supported on the pico-cell for timing adjustments and power measurements.

Inter-RAT

While in general inter-RAT refers to mobility procedure between any two different radio access technologies, here the inter-RAT handover specifically refers to the handover procedure between the LTE access network and the WLAN access network. Since WLAN is defined by a different standards body (IEEE 802.11), 3GPP network has to assume that WLAN access cannot be modified when interconnect WLAN with 3GPP network.

When LTE/LTE-A cells co-exist with WLAN, an important step is to determine which network a UE should attach to at a given time and location. To provide assistance to the UEs, centralized offload policies can be defined based on the Access Network Discovery and Selection Function (ANDSF). When the UE decides to discover neighbor access networks, assistance from the network can be provided via ANDSF. Network discovery and selection information is provided by the network, in order to control the UE's inter-system handover decisions and in order to reduce the battery consumption for inter-system mobility. The UE selects the most preferable available access network for inter-system mobility based on the inter-system mobility policies and user preferences. If the UE selects a preferable access network for handover, then the UE initiates handover to the selected access network.

E. Energy Efficiency With the green radio movement, operators and vendors

are working on pushing network energy efficiency (bit/s/Hz/W or Bit/Hz/J/m2) to be as high as possible, given a reasonable system complexity and cost.

One option is to power up the only small cells on an as-need basis, while the macro-cell is always-on as a backhup. With this option, significant power savings can be achieved, for example, during non-business hours in the office building. This is illustrated with an example in Fig. 4 and Fig. 5.

Macro eNB

P9

P3

UE1

P1

P8P7

P6

P5

P4

P2

UE2

Fig. 4. Power saving example: only one small cell P9 is powered up when the number of UE is small.

Fig. 5. Power saving example: four small cells are powered up when the number of UE increases.

In Fig. 4, it is illustrated that only a small number of UEs

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need to be served, for example, in the early morning hours. Thus only the macro-cell and one small cell are in normal operation. Other small cells in the area are powered off either partially or completely. In Fig. 5, it is illustrated that as the number of users increases, more small cells resume normal operation to provide service.

For UEs, power saving and battery life have always been a priority. When designing for small-cell deployment, UE energy efficiency again needs to be prioritized, for example, in procedures such as mobility measurements, channel quality reporting, small cell discovery, etc.

III. TYPES OF SMALL CELLS Several varieties of small cells are available in the

LTE/LTE-A system. Fig. 6 shows an example of an LTE network with four types of small cell eNBs: pico-eNB, home eNB (HeNB), relay nodes (RN), and trusted WLAN nodes. In the following, characteristics of each type of small cell are discussed.

A. Pico-cell Pico-eNBs are smaller, lighter LTE base stations that plug

directly into an operators core network. Logically the eNB of the pico-cells are fully independent, and perform the same functions as the macro-cell. Carriers such as Sprint Nextel are deploying indoor pico-cells to boost coverage and capacity for their nascent LTE networks [9]. It is a cost-effective way to expand the network and make it possible to continue offering unlimited data plan for smart phones. Mostly they are aimed at indoor applications. Outdoor pico deployment is more difficult due to issues such as

interference, backhaul, and property rights. In the Hetnet study, it has been considered that many

small cells such as pico-cells could be deployed with the overlaid macro cells. The pico cells can share the same carrier frequency with the macro cell or use a different carrier frequency.

Typically, Physical Downlink Control Channel (PDCCH) is severely interfered with by downlink transmissions from the macro cell. In addition, other downlink control channels and reference signals that may be used for cell measurements and radio link monitoring are also interfered with by the downlink transmissions from the macro cell. Additionally, the interference can come from neighbour pico cells.

Time domain ICIC may be utilized for pico cell users who are served in the edge of the serving pico cell, e.g. for traffic off-loading from a macro cell to a pico cell. Time domain ICIC may be utilized to allow such UEs to remain served by the pico cell at an extended range on the same frequency layer. Such interference may be mitigated by the macro cell(s) utilizing Almost Blank Subframes (ABS) to protect the corresponding pico cells subframes from the interference. A UE served by a pico cell uses the protected resources during macro-cell ABS for radio resource management (RRM) measurements, radio link monitoring (RLM) and channel state information (CSI) measurements for the serving pico cell and possible neighboring pico cell(s). This is illustrated in Fig. 7, where a pico-UE at the cell expansion area is able to receive signal from the pico-eNB via the time-domain ABS pattern.

X2 S1

S1

S1

S1

Fig. 6. An example of LTE network with four types of small cell eNBs: pico-eNB, home eNB (HeNB), relay nodes (RN), and trusted WLAN nodes.

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Fig. 7. Time-domain interference coordination to protect cell edge UE of the pico-cell [1].

B. Femto-cells Femto-cell is also gaining momentum in operator

deployment. Sprint Nextel revealed in July 2012 that it has 950,000 femtocells operating on its network, which is a substantial increase from 600,000 femtocells in May 2012 and 250,000 femtocells as of March of 2011 [8].

In 3GPP terms, eNB of an LTE femtocell is called a Home eNode B (HeNB). HeNBs can be closed for residential deployment, where the HeNB is called the Closed Subscriber Group (CSG) cell and access control is located in the Gateway (GW). For enterprise deployment, the femto-cell can be made to be open access type. In terms of interference management, CSG cells are aggressor cells that cause interference to users that are not members of the closed subscriber group.

In the CSG scenario, the UE keeps a so-called CSG whitelist, which is a list of the CSG cells that the UE is allowed to connect to. The whitelist is provided by the non-access stratum (NAS). The CSG whitelist can be either the allowed CSG list or the operator CSG list contained in the so-called UE context. Each CSG list has the form of a list of CSG IDs and the associated PLMNs.

The CSG ID of a cell belongs to the SystemInformationBlockType1 message [2], which is part of the system information broadcast over PDSCH. Each CSG ID identifies uniquely a closed subscriber group (CSG), which contains one or more cells. Thus, before the UE can determine whether it is allowed to connect to a CSG cell, it must acquire the CSG ID of the cell and check whether the CSG ID is in its whitelist.

When a UE is connected to a macro cell, the UE may be configured to send a ProximityIndication message to the macro-cell eNB [2] when the UE has detected that it has entered or left the proximity of a CSG cell that is in its whitelist. After receiving the proximity report, the macro cell may then decide to request the UE to read the system information of a CSG cell that seems to be a suitable

handover candidate for the UE, with the intention of initiating handover to the CSG cell. If the UE does not send a proximity report, the CSG cell may be treated as a closed cell, i.e. not accessible to that UE.

Dominant interference conditions may happen when non-member users are in close proximity of a CSG cell. Typically, Physical Downlink Control Channel (PDCCH) reception at the non-member UE is severely interfered with by downlink transmissions from the CSG cell to member UEs. Interference to the PDCCH reception of the macro-cell UE (i.e., non-member to the CSG) has a detrimental impact on both uplink and downlink data transfer between the UE and the macro-cell. In addition, other downlink control channels and reference signals, from both the macro-cell and neighbor femto-cells, that may be used for cell measurements and radio link monitoring are also interfered by the downlink transmission from the CSG cell to member UEs.

Depending on network deployment and strategy, it may not be possible to divert users suffering from inter-cell interference to another carrier frequency or another RAT. As another option, time domain ICIC may be used to allow such non-member UEs to remain served by the macro-cell on the same frequency layer. Unlike the pico-cell scenario where the macro-cell is the aggressor cell and the small cell is the victim cell, in the femto-cell scenario, the macro-cell is the victim cell and the small cell is the aggressor cell. Using time-domain ICIC, interference to non-member UEs may be mitigated by the CSG cell utilizing Almost Blank Subframes to protect the corresponding macro cells subframes from the interference. A non-member UE may be signalled to utilize the protected resources for radio resource management (RRM) measurements, radio link monitoring (RLM) and Channel State Information (CSI) measurements for the serving macro-cell, allowing the UE to continue to be served by the macro-cell under otherwise strong interference from the CSG cell. This is illustrated in Fig. 8.

Fig. 8. Time-domain interference coordination to protect UE of the macro-cell against the interference from a CSG cell [1].

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C. Relay In Release 10/11, relay nodes have been defined primarily

as a means to extend coverage of a macro-cell or fill a coverage hole. E-UTRAN supports relaying by having a Relay Node (RN) wirelessly connect to an eNB serving the RN, called Donor eNB (DeNB), via a modified version of the E-UTRA radio interface called the Un interface. As shown in Fig. 6, the relay node has a wireless backhaul over the Un interface to the DeNB.

The RN has the dual functionalities of eNB and UE. Over the Uu interface between RN and its served UE, the RN supports the eNB functionality where it terminates the radio protocols of the E-UTRA radio interface, and the S1 and X2 interfaces. Over the Un interface, the RN supports a subset of the UE functionality, e.g., physical layer, layer-2, RRC, and NAS functionality, in order to wirelessly connect to the DeNB.

Relay backhaul link can be outband where different carrier frequency is used over Un and Uu interaces, or inband where the same carrier frequency is used over Un and Uu interaces. For inband wireless backhaul, time-division multiplexing has been applied between the Un interface and the Uu interface between the UE and the RN. The data throughput of the UEs in the RN cell is significantly affected by the capacity of the backhaul link.

D. WLAN With wide-spread deployment of WiFi deployment,

WLAN is a type of small cell that provides a cost-effective way of offloading large amounts of mobile data traffic that benefits both operators and mobile users. WiFi access has been increasingly integrated into mobile devices, making it a readily available option to many users.

The 3GPP standard supports two types of WiFi access: untrusted and trusted non-3GPP access. Untrusted WiFi access refers to WiFi access points that do not provide sufficient security, typically APs not deployed by the operator, such as public open hotspot or private home WLAN. Trusted WiFi access refers to WiFi access points where the communication between the user equipment and the EPC is secure, typically APs deployed by the operator where the network can run the 3GPP-based network access authentication procedure (EAP-AKA) to secure the communication between the user and the core network.

Here we focus on the trusted WiFi access, which can be better leveraged by operators to provide data offloading for indoor users. Trusted WLAN Access Network (TWAN) is natively integrated into LTEs evolved packet core (EPC), which ensures a high level of interoperability between WLAN and E-UTRAN. EAP authentication provides transparent and easy access for the operators own subscribers with SIM cards or certificates. The only interaction required from the user is the initial configuration of the service set ID (SSID) when the device detects the WiFi network for the first time.

Fig. 9 illustrates the network architecture where the trusted

WLAN is integrated into the LTE system. This architecture terminates Wi-Fi sessions at the packet data network gateway (PDN-GW). This gives PDN-GW visibility into the user traffic for policy and charging control, so that similar or identical policy enforcement and charging rules can be applied regardless of the RAN the user connects to.

The TWAN includes these functions:

- A WLAN Access Network (WLAN AN). WLAN AN includes a collection of one or more WLAN access points.

- A Trusted WLAN Access Gateway (TWAG). This function terminates S2a. It also acts as the default router for the UE on its access link, and as a DHCP server for the UE. When the TWAN provides access to EPC for an UE, the packets are sent over the UE-TWAG point-to-point link and the S2a tunnel for that UE.

- A Trusted WLAN AAA Proxy (TWAP). This function terminates STa. It relays the AAA information between the WLAN Access Network and the 3GPP AAA Server or Proxy in case of roaming. It establishes the binding of UE subscription data (including IMSI) with UE MAC address on the WLAN Access Network.

SGi

PCRF

Gx

HSS

Operator's IP Services (e.g. IMS, PSS, etc.)

SWx

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3GPP AAA Server

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STa

Gxc

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UE Fig. 9. Non-roaming architecture for Trusted WLAN access to EPC [5].

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IP

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GTP-U

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UDP

IPv4/IPv6 IPv4/IPv6

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IP

L2/L1 L2/L1

Fig. 10. User plane protocol stack when the UE connects via the trusted WLAN [5].

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For Trusted WLAN access to the EPC, the PDN

connectivity service is provided by the point-to-point connectivity between the UE and the TWAG concatenated with S2a bearer(s) between the TWAG and the PDN GW. The S2a reference point connects the TWAN to the PDN GW. The SWw reference point connects the WLAN UE to the WLAN Access Network per IEEE Std 802.11-2007.

IV. FUTURE WORK This paper has provided an overview of small-cell

development in 3GPP LTE-A network. The design challenge of small cell is to utilize multiple types of nodes, achieve low cost, make deployment and maintenance easy, and increase energy efficiency, while providing high spectrum efficiency and peak data rate. This is challenging since high peak data rate implies increase in the number of small-cell nodes, as a consequence cost for backhaul, network planning, configuration, optimization, operation, and maintenance also increases.

There are many problems to be solved to further enhance performance of LTE-A for hotspot area in indoor and outdoor scenarios using low-power nodes. Several are listed below: 1) Non-ideal backhaul. Typical backhaul widely used in

the market, such as xDSL and NLOS microwave, do not have very high throughput and negligible latency.

2) Self-configuration and self-optimization. ID management and neighbour relation configuration for small cell nodes as well as network optimization efforts including mobility robustness, load balancing and energy savings for small-cell nodes should be automated by means of self-organization network (SON) mechansims.

3) Low operation and maintenance cost. This is important as the operator may deploy a large number of small-cell nodes, and they cannot be manually maintained.

4) Relay backhaul enhancement. High capacity relay backhaul is necessary to alleviate the bottleneck of relay performance. For example, MU-MIMO and carrier aggregation can be applied to the relay backhaul link.

5) Radio level coordination between 3GPP and WLAN. Due to the non-3GPP nature of WLAN, currently the integration of WLAN with LTE/LTE-A is achieved via the EPC. Studies are underway to achieve even tighter interconnection between LTE and WLAN at the radio level.

6) ANDSF enhancement. ANDSF is under study to be further improved to provide network/access point selection for seamless mobility.

7) RF requirements on UE. In some small cell scenarios, the UE is required to simultaneously maintain two connections over two RF chains, one towards the macro-cell, the other towards the small cell. This would increase the implementation complexity and power

consumption of the UE. Studies need to be carried out to balance the need for high data throughput and the need for low complexity and low power consumption.

V. CONCLUSION In 3GPP cellular network evolution, small cells are

increasingly important to offload traffic from macro base stations and to extend coverage. This paper discussed the technical areas that are important for improving small cell performance. Several types of small cells defined in 3GPP LTE-A system are presented, including pico-cell, femto-cell, relay, and WLAN. Intensive research continues to further enhance the small cell performance for the 3GPP LTE-A system.

ACKNOWLEDGMENT The author thanks RIM Advanced Technology colleagues

Jim Womack and Shiwei Gao for their great insight and feedback.

REFERENCES [1] 3GPP TS 36.300 V10.5.0 (2012-03), Evolved Universal Terrestrial

Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 10)

[2] 3GPP TS 36.331 V10.5.0 (2012-03), Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 10)

[3] 3GPP TS 36.211 V10.5.0 (2012-03), Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10).

[4] 3GPP TS 36.213 V10.5.0 (2012-03), Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures; (Release 10).

[5] 3GPP TS23.402 V11.3.0 (2012-06), "Architecture Enhancements for non-3GPP accesses".

[6] 3GPP TS 23.234 v10.0.0, 3GPP system to Wireless Local Area Network (WLAN) interworking; System description.

[7] 3GPP TS 23.327 v11.0.0, (2012-03) Mobility between 3GPP-Wireless Local Area Network (WLAN) interworking and 3GPP systems.

[8] Phil Goldstein (July 26, 2012), Sprint boosts femtocell count to 950,000 [Online]. Available: http://www.fiercewireless.com

[9] Tammy Parker (September 23, 2012), Sprint, Ericsson hashing out small cell contract details [Online]. Available: http://www.fiercewireless.com

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