of the small cell related topics has been studied in 3GPP (The 3rd Generation. Partnership Projecta) LTE(Long Term Evolution) and LTE-Advanced. The fol-.
1
Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks Qi Jiang1 , Jinsong Wu1 , Lu Zhang1 , Shengjie Zhao2 1 Alcatel-Lucent,
1.1
China, 2 Tongji University, China
Introduction The general definition of small cell is the low-powered radio access node operating in licensed and unlicensed spectrum with the smaller coverage of 10 meters to 1 or 2 kilometers, compared to a mobile macrocell with a range of a few tens of kilometers. With the introduction of this new concept, the heterogeneous network (HetNet) constructed with different layers of small cells and large cells can deliver the increased bandwidths, reduced latencies and higher uplink (UL) and downlink (DL) throughput to end users. Since 2009, the standard evolution of the small cell related topics has been studied in 3GPP (The 3rd Generation Partnership Projecta) LTE(Long Term Evolution) and LTE-Advanced. The following sections in this chapter will introduce the standardization progresses of LTE and LTE-Advanced in small cells.
1.1.1
Definition of Small Cells in 3GPP LTE-A In 3GPP LTE and LTE-Advanced, small cells can generally be characterized as either relay nodes, or picocells (also referred to as hotzone cells), controlled by a pico eNodeB, or femtocells, controlled by a Home evolved NodeBb(HeNB)(?). The common feature among the relays, picocells and femtocells is with low transmission power node and independent eNB functionality, while the typical different features can be summarized as follows: 1. Relay Node(Loa, Wu, Sheu, Yuan, Chion, Huo & Xu August 2010, Hoymann, Chen, Montojo, Golitschek, Koutsimanis & Shen February 2012): A Relay Node (RN) is a network node connected wirelessly to a source eNodeB, called the donor eNodeB. According to the different implementation types of the relay node into wireless network, the roles of the relay node played are also different. 2. Picocell: A picocell usually controls multiple small cells which are planned by a
b
The 3rd Generation Partnership Project (3GPP) unites six telecommunications standard development organizations (ARIB, ATIS, CCSA, ETSI, TTA, TTC), known as organizational partners and provides their members with a stable environment to produce the highly successful Reports and Specifications that define 3GPP technologies The evolved Node B could be abbreviated as eNodeB or eNB
4
Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks
the network operator in a similar way as the macrocells(Okino, Nakayama, Yamazaki, Sato & Kusano June 2011). The picocell is usually open to all users (Open Subscriber Group (OSG))(Mukherjee June 2011). 3. Femtocell(J.G.Andrews, H.Claussen, M.Dohler, S.Rangan & M.C.Reed Apr. 2012): The transmission power of the femtocell is usually even lower compared to that of relay node and picocells. Femtocell is typically designed to cover a house or apartment and accessible only to a limited group of users, which is named Closed Subscriber Group (CSG) cell. Currently a new emerging neighborhood femtocell network to deploy the outdoor femtocells as an open access model in malls and neighborhoods has also been designed and applied.
1.1.2
Deployment Scenarios for Small Cell in 3GPP LTE-A From the perspective of the 3GPP LTE-A, there is no restriction about the deployed spectrum band for the small cells. The small cells can be deployed on the same spectrum band as the macrocells or a dedicated spectrum band. For the later scenario, it is preferred to assign a higher frequency to deliver high data rates within a limited area without causing excessive interferences. Figure 1.1 demonstrates an deployment of the small cells in a current cellular network. According to the characteristics for different types of small cells, the following features are preferred. Small Cell Deployment Hotzone UE Picocell UE
UE Picocell UE Picocell UE
Femto
UE
UE
Picocell
Node B
Indoor coverage Femto
UE
UE
UE
Outdoor coverage Relay
UE
Figure 1.1 Illustration of Small Cell Deployment
1. Relay Node: Due to the wireless backhaul connection with the macrocell, the relay node is preferred to be deployed where the fixed wired connection is unavailable or not necessary all the time. One standard scenario for relay is the temporary coverage hole filling up or coverage extension of the macrocell coverage, such as wireless communication enablement in disaster area or rural area. Another standard scenario is the enhanced hotspot coverage, such as stadium during the game time or office building during the peak hour. According
1.2 Relay eNodeB in LTE-Advanced
5
to the wireless connection to the network, more flexible deployment can be obtained via relays compared to the other small cells from the perspective of the operator. However, due to the single hop or multi-hop relay connection between the user equipment (UE) and the core network, the latency brought by the involved relays should also be considered. 2. Picocell: Without the differentiation of the transmission power levels, the picocell can be treated as a macrocell as well. Generally, the use cases of the picocells are extensive for variety of scenarios. Picocell can improve coverage extension and capacity improvement in areas traditional macrocells cannot easily provide, such as in-building offices, shopping malls, outdoor stadium and rural area. A traditional wired backhaul interface used between macrocells is also applied between macrocell and picocell. Both the Ethernet cabling and the fiber can be used for the backhaul connection, which is based on the requirements of the operators. The picocell also may have the advantages of low cost deployment, simplified radio frequency (RF) units, and low operating expenses (OPEX). 3. Femtocell: Unlike relay and picocells, the initial objective scenarios of femtocells are the home or small business use, especially for indoor cases. In 3GPP terminology, a Home Node B (HNB) can be treated as a 3G femtocell and a Home eNode B (HeNB) can be treated as an LTE femtocell. Unlike the deployment manner of the relay and picocells controlled by the operators, the deployment of the femtocells is more inclined to an owner controlled manner. The CSG can be configured by the owner to restrict the accessed users. After the completion of payment of the Internet connection to femtocell traffic route or the authorization fee on the assigned carrier, the owner can operate its own “small network” according to its own requirement without any control from the operator, theoretically. Since there is no network control for the placement and carrier selection of the femtocells, the interference between the femtocell and the wider network will be a problem.
1.2
Relay eNodeB in LTE-Advanced As a key new feature of LTE-Advanced, relay Node has been introduced in Release 10 of the LTE specifications(3GPP1 Sept. 2012). The different types of relays have been studied widely based on their specific characteristics. In this section, the relevant standard evolutions of the relays will be introduced.
1.2.1
Relay Definition During the period of UMTS (Universal Mobile Telecommunications System) and Release 8 of LTE(3GPP2 Jan. 2013, 3GPP3 Mar. 2013), the initially defined relays were in the form of repeaters in legacy radio interface technologies, which can be treated as simple devices to just amplify and forward the transmission
6
Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks
signals from macro base stations to UEs without baseband processing, backhaul network installations or subscription fees for accesses to the fixed public networks. Although the repeater-only relays are useful for coverage improvement and easily implemented, they have some significant drawbacks, such as the potential amplified interference and the un-controlled operation separated from the Operation and Maintenance (O&M) functionality. Currently, there is a kind of developed relay Node (RN) under the full control of the radio access network presents for LTE wireless networks. The controllable RN is a network node wirelessly connected to and controlled by a source eNodeB (Donor eNodeB), while with similar monitoring and scheduling capabilities as an eNodeB to the connected UE. In contrast to a repeater, a RN processes the received signal before forwarding it. Depending on the process level, the RN can be classified into Layer 1, Layer 2 and Layer 3 relays, which ranges from an enhanced repeater to a fully fledged eNodeB with a wireless backhaul connection. 1. For Layer 1 RN, the pure radio frequency (RF) processing or RF processing with some extra baseband processing. 2. For Layer 2 RN, Medium Access Control (MAC) functions such as scheduling are enabled. On the other hand, some Layer 3 functions, such as Radio Resource Control (RRC), can also be located within the eNodeB rather than the RN. 3. For Layer 3 RN, it has its own PCI and can support the RRC functionalities, such as mobility between RNs and. All Layer 1 and Layer 2 functions are supported within such a RN. This RN can be classified as Type 1/1a/1b RN, which will be described as follows. Figure 1.2 shows the simplified diagram of the protocol stacks for Layer 1, Layer 2 and Layer 3 RNs (3GPP4 June 2008, 3GPP5 June 2008). In order to keep the aligned understandings of the 3GPP, the following terminology is used as follows: 1. Donor eNodeB/cell: The source eNodeB/cell from which the RN receives its signal. 2. Relay cell: The coverage area of the RN. 3. Backhaul link: The link between the donor eNodeB and the RN. 4. Access link: The link between the RN and a UE. 5. Direct link: The link between the donor eNodeB and a UE. 6. Inband/outband: An inband RN uses the same carrier frequency for the backhaul link as for the access link; otherwise, the RN is named outband RN. 7. Half/full duplex: A half-duplex RN cannot receive on the backhaul link at the same time as transmitting on the access link, and vice versa, whereas a full-duplex RN has sufficient antenna isolation to be able to operate without this restriction. This distinction applies to inband RNs only, since outband RNs are always full-duplex.
1.2 Relay eNodeB in LTE-Advanced
UE
RN
Donor eNodeB
Layer 1
RRC
RRC
Layer 2
MAC/RLC
MAC/RLC
Layer 3
PHY
Layer 1
RRC
Layer 2
MAC/RLC
MAC/RLC
MAC/RLC
Layer 3
PHY
PHY
PHY
Layer 1
RRC
RRC
RRC
Layer 2
MAC/RLC
MAC/RLC
MAC/RLC
Layer 3
PHY
PHY
PHY
PHY
7
Layer 1 RN
PHY
RRC Layer 2 RN
Layer 3 RN
Figure 1.2 Protocol Stacks for Layer 1/2/3 RNs
From the perspective of the 3GPP standardization view, the key point of the RN definition is whether it can be identified by the UE and the backward compatibility to legacy UEs. In the relevant 3GPP LTE-Advanced discussions, two main types of RNs have been proposed, which are Type 1 (1/1a/1b) and Type 2 RNs(3GPP1 Sept. 2012). 1. Type 1, Type 1a and Type 1b RNs: All these types of RNs can be classified as Layer 3 RNs. They can be differentiated from the normal eNodeB with their own independent Physical Cell Identifiers (PCIs) and perform like a eNodeB. Type 1 RNs are inband half-duplex RNs, while Type 1a RNs are outband. Type 1b RNs are inband full-duplex RNs with sufficient isolation between the received and transmitted signals. 2. Type 2 RNs: Unlike Type 1 RNs, Type 2 RNs do not have their independent PCIs and control channels, which can be classified as Layer 2 RNs. Type 2 RNs are usually used as the complementary or enhanced Nodes for the data transmission, where the donor coverage is not good enough. Figure 1.3 shows an example about these transmission functionalities for Type 2 RNs. Although two kinds of RNs have been proposed in 3GPP, LTE-Advanced only supports Type 1 and Type 1a RNs in Release 10 of the specifications. No specific support has been provided for Type 1b RNs, although such RNs may be able to be deployed via implementation-dependent means. In the following part, our discussion will be focused on the specifications on Type 1 and Type 1a RNs.
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Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks
Step 1
Step 2
Data
Relay
Donor eNodeB
Donor eNodeB
Control
Relay
Non-Cooperative Data Tx
Relay
Cooperative Data Tx
Data UE
UE
Data Co-Data
Relay
Donor eNodeB
Donor Control eNodeB
Co-Data UE
UE
Data Control Donor eNodeB
Relay
Data
Relay
Donor Control eNodeB
UE
Data Retransmission
Re-Tx UE
Figure 1.3 Transmission Functionalities for Type 2 RNs
1.2.2
Backhaul and Access Resource Allocation According to the definition of both the Type 1 and Type 1a RNs, they need to receive and transmit the signals from and to the donor eNodeB via the backhaul link, while transmit and receive signals to and from the UE on the access link. Some coordination or separation is needed to avoid the self-interference between the transmission and receiving. Both the frequency domain and the time domain separation have been studied in 3GPP LTE-A(3GPP6 Mar. 2010): 1. Frequency Domain Separation: A typical frequency domain separation is the design of Type 1a RNs where two independent carriers are applied on the backhaul and access link, respectively. If the separation of these two carriers is large enough, there will be no self-interference generated. For this kind of separation, no extra changes will occur when the RNs are implemented into the macrocell dominating cellular network. The key issue is whether there are extra significant benefits to allocate two independent carriers on RNs, since an independent carrier is always a luxury resource for operator. 2. Time Domain Separation: The key progress during 3GPP LTE-A study for relay is the time domain separation between the backhaul and the access link. The time domain multiplexing (TDM) mechanism is applied for Type 1 RNs. For the timeline flow, part of time window will be allocated to backhaul link and the remaining ones will be allocated to access link. Since Type 1 RN is a half-duplex RN, the UE associated on the RN will lose the connection to the eNodeB due to the lack of reference signals (RS), synchronization signal, broadcasting channel and control channels such as Physical Downlink
1.2 Relay eNodeB in LTE-Advanced
9
Control Channel (PDCCH). Further, in order to keep the accurate network measurement for the legacy UEs (Rel-8/9 UE), some specific indication about the separation of the backhaul and access link is also required.
In LTE-A, the concept of the ’Multimedia Broadcast Single Frequency Network’ (MBSFN) subframe is browsed here to realize the notification of the separation to legacy UEs. During the operation of the RNs, a radio resource control (RRC) signaling will be used to indicate UE that certain subframes are assigned to MBSFN transmission. In these subframes, the UE will only receive the control signals and RS in the first two OFDM (Orthogonal Frequency Division Multiplexing) symbols, and ignore the rest part of the subframe for both data reception and measurement purposes. Due to the existence of the control channel for legacy UE in the beginning of both the access and backhaul link subframe, a transition gap for Tx(transmit)-Rx(receive) turnover will be implemented. Since of the first two OFDM symbols for control channel, as well as the transition gap, the available OFDM symbols for a RN to receiving the backhaul signal in an MBSFN subframe is less than the full subframe length. According to the 3GPP RAN1 (radio access network layer 1) specification(3GPP6 Mar. 2010), the starting symbol of the RN’s backhaul downlink reception windows is configured by RRC (Radio Resource Control) signaling as the start of the second, third or fourth OFDM symbol. The largest gap at the end of the backhaul reception is one whole OFDM symbol. Based on these requirements, the available OFDM symbol for backhaul link reception will be ranged from 10 (from 4th to 13th) to 13 (from 2nd to 14th without gaps) in the normal cyclic prefix (CP) case. Figure 1.4 shows the MBSFN subframe configuration for Type 1 RNs.
Access Link
Backhaul Link eNodeB to RN transmission Gap
1 subframe (1ms)
Control
Data
1 subframe (1ms) MBSFN Subframe Control
Backhaul Reception
Tx-Rx Gap
RN to UE transmission
Rx-Tx and Delay Gap
Start position of backhaul reception: End position of backhaul reception: from 2nd, 3rd and 4th OFDM symbol up to 13th and 14th OFDM symbol
Figure 1.4 MBSFN Subframe Configuration for Type 1 RNs
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Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks
1.2.3
Relay Architecture Currently, the Type 1 and Type 1a relay included in Rel-10 can support the full eNodeB functionality, including termination of the radio protocols and the S1/X2 interfaces. The corresponding supported architecture for RNs in 3GPP LTE-A can be found in Figure 1.5, which is referred by 3GPP specification TS 36.300 (3GPP7 Sept. 2013). A new network interface called Un between RN and donor eNodeB is defined (3GPP7 Sept. 2013). From the architecture, it can be found that the donor eNodeB provides S1 and X2 proxy functionality between the RN and other eNodeBs, which includes Mobility Management Entities (MMEs) and Serving Gateways (S-GWs).
MME / S-GW
S11 S1
1 S1 S1
S1
S1
MME / S-GW
X2
E-UTRAN DeNB S1 X2 Un
eNB
RN
Figure 1.5 Overall E-UTRAN Architecture Supporting RNs
Figure 1.6 and Figure 1.7 provide the user plane and control stack protocol for RN, which is referred by (3GPP7 Sept. 2013): 1. For the user plane, the donor eNodeB provides the S-GW and Packet Data Network (PDN) Gateway (P-GW) functionality for the RN, which includes the management of the Evolved Packet System (EPS) bearers. 2. For the control plane, there is one S1 interface relation between the RN and the donor eNodeB, and one S1 interface between the donor eNodeB and each MME. The donor eNodeB processes and forwards all S1 messages between the RN and the MMEs for all UE dedicated procedures.
1.2 Relay eNodeB in LTE-Advanced
GTP
11
GTP
GTP
GTP
UDP
UDP
UDP
UDP
IP
IP
IP
IP
PDCP RLC MAC PHY
PDCP RLC MAC PHY
L2
L2
L1
L1
S1/X2-U
S1/X2-U
RN
DeNB
S-GW / eNB
Figure 1.6 RN User Plane Protocol Stack
S1-AP
S1-AP
S1-AP
S1-AP
SCTP
SCTP
SCTP
SCTP
IP
IP
IP
IP
PDCP RLC MAC PHY
PDCP RLC MAC PHY
L2
L2
L1
L1
S1-MME / X2-CP RN
S1-MME / X2-CP DeNB
MME / eNB
Figure 1.7 RN Control Plane Protocol Stack
According to the stack protocols of the RN, the RN can be treated as a UE when performing the initialization and the backhaul connection with the donor eNodeB. During the procedure of the initialization, the configuration parameters will be transmitted to the RN, including a list of donor eNodeBs to which it is allowed to attach, from a RN O&M server. After that, the RN will detach from the network as a UE and perform like a eNodeB to receive the signals from the real mobile users.
1.2.4
Backhaul Link Design Compared to the legacy UE connection to eNodeB, the appearance and the corresponding two links sustained by RN provide some specific designs of the backhaul link on current cellular network. In this section, these specific designs will be provided below.
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Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks
Backhaul Reference Signals Both Rel-8/9 defined cell-specific reference signals (CRS) and Rel-10 defined demodulation reference signals (DM-RS), can be reused on backhaul link(3GPP6 Mar. 2010). Since of the tail gap configured for Rx-Tx transition and propagation delay (see Figure 1.4), the DM-RS located in the second slots of one subframe cannot be used (antenna port 7 to 10, located in the last two OFDM symbol of one subframe). Besides, a further restriction is that the backhaul link in Release 10 is limited to a maximum of four spatial layers, and DM-RS antenna ports 11 to 14 have never been used for backhaul link transmission.
Backhaul Control Channels in Downlink Duo to the one or two OFDM symbols PDCCH transmission to UE on MBSFN subframe, as well as the timing synchronization between the access and backhaul link, the RN cannot receive the PDCCH from the donor eNodeB. In order to resolve this, a new backhaul link control channel named as Relay PDCCH (RPDCCH) is provided. It is located in the legacy PDSCH region and multiplexed with data channel in frequency domain multiplexing (FDM) manner. The example multiplexing among PDCCH, R-PDCCH and PDSCH is shown in Figure 1.8. For access link UE from RN or for direct link UE from Donor eNodeB
R-PDCCH for Downlink
P D C C H
G a p
PDSCH for Backhaul link
PDSCH for direct link
First Slot
PRB Pairs assigned for RPDCCH
R-PDCCH for Uplink G a p
PRB Pairs assigned for backhaul link PDSCH
PRB Pairs assigned for direct link PDSCH
Backhaul Reception Window Second Slot
Figure 1.8 Backhaul Control Channel Design for RN
Backhaul Control Channel for Uplink The physical uplink control channel (PUCCH) transmission by RN will be generally in the same way as the transmission between UE and donor eNodeB. There
1.2 Relay eNodeB in LTE-Advanced
13
are only two slight differences. The first one is that the resource used for backhaul PUCCH will be reserved for high layer signaling. Consequently, in contrast to the PDCCH, there is no relationship between the resources used for R-PDCCH and PUCCH transmission.
Backhaul Data Channels The data transmission for both downlink and uplink on the backhaul link applies the same physical channels defined (PDSCH and PUSCH) for the access link. For the multiplexing of the PDSCH and R-PDCCH for backhaul link, three alternatives are applied as follows (shown in Figure 1.9, for simplicity, the gap between the Tx-Rx turnover has not been included): 1. Alternative 1: A downlink grant in the first slot and an uplink grant in the second slot, 2. Alternative 2: Only one downlink grant in the first slot, 3. Alternative 3: Only one uplink grant in the second slot.
Alternative 1
PDCCH
R-PDCCH: Downlink Grant
R-PDCCH: Uplink Grant
Alternative 2
PDCCH
R-PDCCH: Downlink Grant
No R-PDCCH (Possibly backhaul link PDSCH)
Alternative 3
PDCCH
No R-PDCCH No PDSCH
R-PDCCH: Uplink Grant
First Slot
Second Slot
Figure 1.9 Multiplexing of R-PDCCH and PDSCH
Backhaul Data Channels For outband RN, there is no special restriction on the scheduling or timeline requirement between the donor eNodeB and RN. For inband RN, due to the additional link for backhaul link transmission, the MBSFN subframe for downlink transmission, as well as the available transmission resources for the corresponding feedback to or from RN with a proper timeline. Some specific subframe configuration for backhaul link transmission should be designed. For FDD operation, the subframe configured for backhaul link transmission follow a periodicity of 8 ms(microseconds), which keep aligned with the UE to eNodeB transmission. An 8-bit bitmap is therefore sufficient to configure the downlink backhaul subframes. The timing association between the uplink grant and uplink transmission, as well as timing association between the backhaul link
14
Long Term Evolution (LTE) and LTE-Advanced Activities in Small Cell Networks
PDSCH transmission and corresponding ACK/NACK on uplink, maintain the same as that using legacy principles. For TDD operation, the design of the backhaul and access link subframe configurations is more complicated than that for FDD (Frequency Division Duplexing) mode. Among seven different types of uplink-downlink configuration, the Configuration 0 and 5 cannot support the backhaul transmission due to quite limited downlink or uplink subframes within one radio frame (two DL subframes for Config. 0 and one UL subframe for Config. 5). Different kinds of the configurations specified for backhaul links, as well as the timeline association criterion, have been specified in (3GPP1 Sept. 2012).
1.3
Pico eNodeB in LTE-Advanced The standardization discussion for pico eNodeB has been studied in Rel-10 and Rel-11 version inter-cell interference coordination(3GPP6 Mar. 2010). The main focus of LTE-A for pico eNodeB is on the support of heterogeneous network deployment where the macrocell and picocells share the same frequency. The picocell can improve the user experience. However, as a two-blade sword, the overlaying deployment also results in co-channel interference between the macrocell and picocells. This section will focus on the enhanced solution and corresponding specification effort to resolve this issue restricted in Macro-Pico scenario. The Macro-Femto scenario will be discussed in next section.
1.3.1
Inter-cell Interference for LTE-A In the homogeneous macro-cellular network, the UE is basically served by the strongest cell BS with the strongest receiving power. Due to the restriction of the transmission power, as well as less antenna gain compared with that in macrocell, the difference of the receiving power from macrocell and picocell is almost 25dBm in 3GPP specification if the same pathloss is assumed. According to this, the number of UEs served by picocell will be quite limited, which would result in quite limited gains of the picocell deployment. In order to fully explore the cell-splitting gain, the serving eNodeB can intentionally ’bias’ the handover offset values of some UEs in RRC CONNECTED mode to transfer them to the picocells, which is called as Cell Range Expansion (CRE)(3GPP9 2009, 3GPP8 2010). If a UE is transferred in this way, the desired signal from the pico eNodeB would be even lower compared to the interference received from the macro eNodeB. Figure 1.10 shows the heterogeneous network interference scenarios in downlink and uplink. Due to the CRE implementation, the major interference to border pico UE in downlink is generated from macrocell BSs while the major interference to picocell BS in uplink is generated from macro UEs. Figure 1.11 shows the SINR(signal to interference-noise ratio) performance of
1.3 Pico eNodeB in LTE-Advanced
15
CRE Range DL Signal DL Interference Pico UE
Picocell
UL Signal UL Interference
Macro Cell Macro UE
Figure 1.10 Interference Scenario for Heterogeneous Network CDF of SINR in Serving Sector 1 0.9
CDF = P(SINR
