Self Back Haul

WWRF21-WG4-07

C Hoymann, A Rácz, N Johansson, J Lundsjö - WWRF 21-WG4 Stockholm, Sweden, 2008 wireless-world-research.org

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A Self-backhauling Solution for LTE-Advanced Christian Hoymann, András Rácz, Niklas Johansson, Johan Lundsjö Ericsson Research

Abstract— After years of research, multi-hop technology seems to be ready for introduction in cellular technology standards. In 3GPP various multi-hop solutions are being discussed for potential inclusion in Long-Term Evolution (LTE) Advanced, a coming release of the LTE standard to meet the requirements of IMT-Advanced as defined by the International Telecommunications Union (ITU). In this paper we discuss various options for inclusion of multi-hop capabilities in LTEAdvanced with particular focus on self-backhauling, a layer 3 multi-hop solution for cost efficient backhauling of base stations.

I. INTRODUCTION Multi-hop communication claims to offer cost efficient coverage extension and capacity increase. Hence it has been a hot research topic over decades and multi-hop functionality has been integrated in all kinds of wireless systems ranging from system concepts and prototypes (WINNER) [12][13], over standards for local area networks (IEEE 802.11s)[8][10] and metropolitan area networks (IEEE 802.16j) [7][9]. Apart from simple repeaters, multi-hop communication has not yet been part of commercial cellular networks from the 3GPP or 3GPP2 technology families. However, 3GPP is discussing multi-hop as one technical component of a coming release of Long-Term Evolution (LTE) [1][4][5] called LTE-Advanced [3] the 3GPP proposal for IMTAdvanced. This paper presents how a multi-hop enabled LTEAdvanced system might look like, leveraging on the large amount of research on multi-hop concepts that has been conducted. First, it provides a brief overview of relevant technology components that have been proposed for LTEAdvanced, in particular different options for how to extend the presented LTE functionality with multi-hop capabilities. Second, it details on one of these options, namely layer 3 self-backhauling. II. LTE-ADVANCED In March 2008 3GPP started to discuss the evolution of LTE, named LTE-Advanced. LTE-Advanced targets to meet the requirements of the International Telecommunications Union (ITU) for next generation mobile systems, named IMT-Advanced. Several technology components have been discussed [2][6], including: Wider bandwidth: The widest carrier bandwidth specified for LTE is 20 MHz. The extension to wider bandwidths is part of the LTE evolution towards LTE Advanced since spectrum allocated might have carriers larger than 20 MHz. Considering spectrum compatibility with legacy LTE, carrier aggregation is the natural choice. Such approach should

support combinations of aggregated carriers with various bandwidths. Spectrum aggregation: Spectrum aggregation is seen as a generalization of carrier aggregation such that non-adjacent carriers are aggregated there. This technique allows leveraging multiple carriers, such as carriers already allocated, carriers newly allocated and even re-farming of spectrum used for other technologies. These bands are, in general, scattered in the frequency domain. Multi antennas: Multi antenna techniques are a key component of LTE and they are further enhanced in LTE Advanced. Spatial multiplexing increases the peak spectral efficiency. In uplink, spatial multiplexing with up to four simultaneous data streams is a strong candidate for LTE Advanced. In downlink, higher-order spatial multiplexing is considered as well. Since beamforming can increase cell edge user throughput, a combined beamforming / spatial multiplexing approach is recommended to be introduced, too. Coordinated multipoint transmission: Coordinated multipoint transmission is a technology where transmission and/or reception is coordinated across several geographically separated points. In uplink the support for joint processing of signals received at multiple geographically separated points is relatively straight forward and the impact on the LTE radio interface specification is expected to be marginal. In downlink, coordinated multipoint transmission can have different flavors. It ranges from dynamically coordinated scheduling across separate points to joint transmissions from multiple geographically separate points. The impact on the radio interface specification depends on the selected approach, but is expected to be higher than in uplink. Multihop functionality: Multihop functionality is claimed to offer cost efficient coverage extension and/or capacity increase [11]. Different approaches, e.g., Layer 1 repeater, Layer 2 relay, and Layer 3 self-backhauling will be outlined in section III. As main topic of this paper, Layer 3 selfbackhauling is presented in detail in sections IV and V. III. CLASSIFICATION OF MULTI-HOP SCHEMES Numerous multi-hop schemes have been proposed in the literature. One way of classifying them is according to the protocol layer at which user plane data forwarding takes place. We then arrive at three main categories of multi-hop schemes of relevance for LTE-Advanced, namely Layer 1 repeater, Layer 2 relay, and L3 wireless router. The selfbackhauling solution described in the following chapter belongs to the third category, L3 wireless router.

WWRF21-WG4-07 A. Layer 1 Repeater A repeater receives a certain signal, amplifies it and transmits it again (aka amplify and forward). User plane forwarding to and from the User Equipments (UEs) is performed on Layer 1, i.e., the physical layer. However, control signaling may possibly occur up to Layer 3, so the term Layer 1 repeater really only refer to the user plane. A repeater typically introduces very little delay compared to other multi-hop solutions operating on higher layers. At the same time, a repeater can not differentiate between received desired signals and received noise and interference since no decoding operation is performed in the repeater. Both noise and desired signal are amplified and forwarded and therefore the repeater can not improve the SINR from input to output. A repeater can either be frequency translating, where the repeated signal is transmitted on a different carrier frequency relative to the received signal, or on-frequency operating, where received and transmitted signal are on the same carrier. In case of an on-frequency repeater, the repeated signal and any direct signal will add like channel multipath in the receiver. On-frequency repeaters typically need some form of self-interference cancellation functionality. Conceptually a simple repeater can be thought of as an analog Power Amplier (PA). However, a repeater could also be more advanced and e.g. contain a controllable bank of band-pass filters, perform measurements, transmit reference signals etc. An even more advanced repeater could consist of several receive and transmit antennas enabling multi-stream signal repetition. B. Layer 2 Relay A layer 2 relay forwards user plane traffic on Layer 2. As for the Layer 1 repeater, control signaling may still occur on higher layers. As the relay node decodes, re-encodes, and forwards received data blocks a delay is introduced. However, no noise is forwarded by the relay node and rate adaptation may be performed individually for each link. The direct and the relayed signal interfere so that relay and eNodeB transmissions have to be separated, e.g., by time or frequency multiplexing. Many different technology options exist for a layer 2 relay solution. Layer 2 protocols, such as MAC, RLC, PDCP can operate either end-to-end or on a per-hop basis. Control functionality, such as resource allocation and system broadcast, can be performed centrally by the eNodeB or it can be distributed amongst relays. Control signaling may be placed on layer 2 and/or layer 3. Common to all layer 2 technology options is that they will require rather large standard changes. For example, not terminating PDCP (the “highest” L2 sub-layer of LTE) in the relay node would mean that the in-order delivery service of PDCP function would have to be replaced by something new, including the associated control signaling between relay nodes and eNodeBs at handover. The European research project Wireless World Initiative New Radio (WINNER) investigated Layer 2 relaying as a key technology that can be applied to enhance coverage and capacity of a base station [12]. Performance evaluation showed the potential to improve coverage, especially to

2 overcome uplink power limitations. However, capacity improvement in terms of spectral efficiency could only be realized by intelligent and dynamic resource partitioning and reuse schemes. An additional cost evaluation showed that the introduction of relays has a positive impact on capital and operational expenditures provided that the cost of a relay is less than approximately 1/3 that of a base station [13]. C. Layer 3 Wireless Router A wireless router (a.k.a. Layer 3 relay) forwards IP packets on Layer 3, i.e., the network layer. A wireless router has similar capabilities and characteristics as a Layer 2 relay, such that it does not amplify noise and interference but that it introduces processing delays. In contrast to a Layer 2 relay, radio protocols (layer 1 and 2) are not affected. In fact, since a layer 3 wireless router will include full eNodeB functionality it can be seen as an “ordinary” eNodeB being backhauled via the same RAT that is used over the access link towards the UE. The wireless router provides advantages of decode & forward (Layer 2) relaying without requiring new network nodes or modified radio protocols. The characteristics of a wireless router motivate its usage for self-backhauled eNodeBs as it will be discussed in the following sections. IV. SELF-BACKHAULED ENODEB The network infrastructure that is used to connect eNodeBs to the core network is an IP-based transport network, which can comprise of different Layer 1 / Layer 2 technologies, e.g., leased telephone lines, bre optic cables, Ethernet or microwave links. The type of transport network and Layer 2 technologies employed is a deployment issue, depending on the availability, cost, ownership, operator preferences, etc., of such networks in the particular deployment scenario. However, in some deployment scenarios it may be technically difficult, or not cost efficient, to employ existing transport network technologies. The costs of the transport network often play a significant part of the overall operation costs of the network. The need for alternative solutions might be accelerated when moving towards extremely dense deployments of eNBs, a target scenario for LTE-Advanced. For instance, microwave solutions require line-of-sight deployment, and it may not be economically viable to deploy optical fibers everywhere. This is the reason why it could be interesting to use the LTE radio interface as a backhaul link to connect an eNodeB (named Self-backhauled eNodeB (sNB)) via another eNodeB (named Anchor eNodeB (aNB)) to the core network. We call this method self-backhauling. UL

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Fig. 1. Transmissions on access and backhaul link in UL band

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Fig. 2. Self-interference at a self-backhauled eNodeB

The purpose of self-backhauling is to reduce the cost for the transport network and in turn to provide cost-efcient backhauling as a complement to other backhaul technology options. LTE uses different transmission schemes for Downlink (DL) and Uplink (UL). In general, an eNodeB transmits DL signals using OFDMA and it receives UL signals using SCFDMA. An sNB whose backhaul trafc is served like "regular UE trafc" by the aNB requires additional functionality: such an sNB additionally transmits SC-FDMA UL signals, see Fig. 1 and it receives OFDMA DL signals, see Fig. 2. Consequently, an sNB requires UE-like transceiver capabilities. A. In-band self-backhauling In the concept of in-band self-backhauling physical resources are (dynamically) shared between self-backhauling and UE trafc, i.e. backhauling is performed inside the regular spectrum band. Since an sNB transmits and receives in the same band its transmitted signal interferes with the received signal, see Fig. 2. So called self-interference occurs at the sNB in DL as well as in UL. Note that the aNB does not generate self-interference. At the sNB, the receive power of its own transit signal is orders of magnitude higher than the receive power of the desired UE signal so that the SINR is reduced drastically. Without additional effort the desired signal cannot be decoded successfully. frequency

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Fig 3. Desired DL transmissions (solid line) and interfering signals (dotted lines) of access and backhaul link

There are at least two potential technical means to mitigate self-interference. The first one, coordinated transmission and reception, means that self-interference at the sNB is mitigated by time multiplexing the backhaul into the sNB’s access link, where scheduling of the sNB and aNB is coordinated. Figure 3 shows the involved DL transmissions on the access links (aNB-UE and sNB-UE) as well as the DL transmissions on the backhaul link (aNBsNB). Figure 4 shows 4 TTIs of the aNB’s DL schedule (top) and the corresponding DL schedule of the sNB (bottom). In the aNB, resource allocations for the backhaul link (marked with horizontal lines) are embedded into allocations for regular UEs (marked with vertical lines). All transmissions are scheduled on orthogonal resources by the aNB scheduler. In order to mitigate self-interference, the sNB’s DL schedule is subdivided into subsequent transmit (Tx) and receive (Rx) phases. During Tx phases, e.g., 1st and 3rd TTI, the sNB allocates resources to its own UEs, i.e., the sNB transmits user data. In the 2nd and 4th TTI the sNB switches to Rx so that it can receive transmissions on the backhaul link. During TTIs dedicated to Rx, no UE can be served by the sNB. An analog scheme needs to be applied in the UL. Rx phases need to be negotiated on a long-term basis so that the aNB can allocate resources and the sNB can switch to Rx mode. They should occur periodically. The length of Rx (and Tx) phases can be one or more TTIs. Within the negotiated Rx phase, the actual resource can be allocated (channel dependent) anywhere in the time-frequency grid. The mechanism to negotiate Rx phases can be quite similar to the Discontinuous Reception (DRX) operation of LTE, a UE power saving feature. The second method to reduce self-interference is to employ a dedicated antenna for self-backhauling. Such an antenna could be highly directive, e.g. a parabolic antenna. It could be pointed directly towards the peer eNodeB without sacricing the downtilt of the eNodeB antenna used for the access link. At the aNB a dedicated antenna allows to allocate backhaul resources independently from UE resources. The dedicated antenna could be seen as a separate sector that serves only one UE, which is the sNB. Since physical resources are no longer shared between backhaul and user traffic, UE performance is not affected by self-backhauling. If the separation of backhaul and access antenna at the sNB is sufcient to suppress self-interference, access and backhaul are independent at the sNB as well. No Tx and Rx phases are required and the sNB can continuously serve UEs

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while doing self-backhauling. If the antenna separation at the sNB is not sufficient, the sNB need to coordinate transmission and reception by negotiating Rx phases as shown above. The drawback of an extra antenna is the extra cost for equipment and deployment, the advantage is a higher capacity for both access and backhaul link. B. Out-band self-backhauling In contrast to in-band self-backhauling where access and backhaul link share the same spectrum band, selfbackhauling could also be performed out-band, i.e., access and backhaul link operate on separate spectrum bands. At the aNB the out-band solution leads to independent resource allocation of both links. With separate carriers, access and backhaul link do not interfere each other at the sNB. The sNB can continuously serve its UEs on one carrier while performing self-backhauling on the other carrier. Selfinterference does not occur. The drawback of the out-band solution is the extra transceiver costs. The advantage is a better performance of access and backhaul link. Out-band self-backhauling could use spectrum bands which are high up in the radio spectrum (frequencies above 3 GHz) and which are therefore not very useful for Non Line-of-Sight transmission. However, such bands could be used for self-backhauling under Line-ofSight propagation conditions. Self-backhauling could even utilize unused TDD spectrum. To do so the self-backhauling would be based on LTE TDD while the access would remain FDD. Spectrum (and carrier) aggregation functionality is proposed as potential technical component of LTE Advanced, refer to section II. Such functionality would allow for a more spectrally efficient performance of outband self-backhauling. Figure 5 shows an example DL resource allocation of an aNB that aggregates three carriers. In the lower two carriers the aNB allocates resources for UE trafc only (vertical lines). On the third subcarrier the aNB allocates resources for backhaul trafc (horizontal lines) as well. The sNB uses the carriers differently. The lowest carrier is used for the access link only, i.e., the sNB transmits user trafc. The highest carrier is the backhaul carrier, there the sNB receives backhaul data from the aNB. The middle carrier serves as guard band to avoid self-interference; it is not used by the sNB. With carrier aggregation functionality, the aNB can efficiently utilize resources from all three carriers. Beside the restriction of allocating backhaul traffic to certain carriers only, resources can be App. TCP/UDP dynamically shared between access and IP backhaul.

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V. PROTOCOL ARCHITECTURE A. User plane Figure 6 shows the user plane protocol stack including the E-UTRAN and the S1 interface of a conventional, i.e., nonself-backhauled system. The radio access uses the protocols MAC, RLC and PDCP [5]. The user plane part of the S1 interface is based on the GPRS Tunneling Protocol (GTP) protocol, which uses a tunneling mechanism ensuring that IP packets destined to a given UE are delivered to the eNodeB where the UE is currently located. GTP encapsulates the original IP packet into an outer IP packet which is addressed to the proper eNodeB. The S1 interface can be operated over various Layer 1 / Layer 2 technologies, e.g., fiber optic cables, leased (copper) lines, or microwave links.

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Fig 6. User plane protocol stack (E-UTRAN and S1 interface) Transmissions on access and backhaul link in UL band

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Fig 5. Example resource allocation of an aNB aggregating three carriers

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Fig 7. Potential user plane protocol stack for self-backhauled eNodeB access and backhaul link in UL band

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rules in the aNB such that control plane traffic is mapped on high priority radio bearers. This needs to be configured when the S1 interface between sNB and MME is established, i.e., at sNB setup/configuration. VI. CONCLUSIONS

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Fig 8. Potential control plane protocol stack for self-backhauled eNodeBs

Figure 6 also shows an example TCP/IP based application, such as web browsing. The corresponding peer entities operate in the UE and at the server hosting the web application. For simplicity, peer protocol entities of the server are drawn in the Serving Gateway (S-GW), however, in general they are located somewhere in the Internet. One potential approach to integrate an sNB into the user plane protocol architecture is shown in Fig. 7. There, the aNB is seen as part of the transport network, acting like a wireless IP router in-between the sNB and the core network. In principle IP packets addressed to the sNB are routed via the aNB based on the sNB IP address. Regular IP routing mechanisms could be used. That approach imposes that the aNB has to handle (conventional) IP packets destined for a UE associated to that aNB differently than (self-backhauling) IP packets destined for the sNB (actually destined for UEs associated to the sNB). The former IP packets are de-capsulated and plain user IP packets are transmitted over the air. The mapping of IP packets to radio bearers at the aNB is done based on the GTP tunnel endpoint. The latter IP packets are directly routed towards the sNB resulting in GTP-encapsulated IP packets being transmitted over the air. Here the mapping to radio bearers cannot rely on GTP tunnel endpoints but, e.g., on IP Quality of Service (QoS) mechanisms such as IP Differentiated Services. As a result the sNB cannot be controlled like a regular UE by the Mobility Management Entity (MME). A modified QoS management would have to be introduced. B. Control plane The control plane of the S1 interface connects the eNodeB and the MME. The corresponding protocol is called S1-AP and operates on top of the Stream Control Transmission Protocol (SCTP). The MME uses the S1-AP protocol to establish, configure and tear down bearers, to authenticate UEs, to control ciphering of NAS signaling, and to support UE mobility. The approach to introduce self-backhauling in the control plane could look similar to the one on the user plane: IP packets carrying control plane data for an sNB are routed via the aNB, which is acting like an IP router in the transport network. IP packets carrying control plane data for the aNB are de-capsulated at the aNB. Regular routing protocols can be applied. The corresponding control plane protocol stack is depicted in Fig. 8. Although packet forwarding and routing for the control plane would be similar to the user plane, the control plane traffic should be mapped to bearers with higher QoS. The MME acting as the serving MME for the sNB will have to configure the radio bearers and set the packet classification

Multi-hop capability is likely to become part of the 3GPP LTE-Advanced standards. We have described different options of multi-hop forwarding on Layer 1, 2 or 3, detailing on Layer 3 self-backhauling, a cost-efficient solution for backhauling of eNodeBs. Self-backhauling provides major benefits of conventional Layer 2 relaying such as signal regeneration due to decode and forward operation, and per-link optimization of the radio transmissions. Furthermore, self-backhauling avoids major disadvantages of layer 2 relays: it does not introduce new network nodes and it requires only little standardization effort. These characteristics are very important when considering smooth integration of a new feature into an existing standard and fast adoption in the commercial market. Thus we conclude that self-backhauling is a promising technology component of LTE-Advanced, for which further research on various optimizations is highly motivated. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

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