What Is a Relay Rack?

Relay (Relay) technology is to add one or more relay nodes between the base station and the mobile station, which is responsible for forwarding the wireless signal one or more times, that is, the wireless signal must reach the mobile station after multiple hops. Taking the simpler two-hop relay as an example, it is to divide a base station-terminal link into two links: base station-relay station and relay station-terminal, so there is a chance to replace a link with a poor quality with two links with a lower quality. Good links for higher link capacity and better coverage.

Chinese name: Relay technology

Foreign name: Relay
Applied discipline: Communication

Introduction to Relay Technology

Compared with previous mobile communication systems, LTE-Advanced may use high-frequency carriers with poor coverage and the need to support high data rate services, so more sites may need to be deployed. If the backhaul link between all base stations and the core network still uses the traditional wired connection method, it will bring greater deployment difficulty and cost to the operator, and the site deployment flexibility will also be limited. Therefore, 3GPP initiated research on relay technology in LTE-Advanced to solve the above problems and provide wireless backhaul link solutions. From a wider perspective, as shown in Figure 10-8, the relay technology can not only solve the problems of deployment flexibility and cost, but also has a very wide range of application prospects, which has attracted the interest of many operators and manufacturers.

The relay node (RN, Relay Node) is wirelessly connected to the eNode B cell (Donor Cell) to which it belongs, as shown in Figure 10-9. There are three air links:

The interface between the RN and its home cell is the Un interface, or Backhaul Link;

The interface between the R-UE (the UE that belongs to the RN) and the RN is a Uu interface, or an access link;

The interface between the UE and the eNode B is a Uu interface, or a direct link.

According to the different functions implemented by relay nodes in the network, relays can be divided into the following types.

(1) Classification according to the way RN accesses the Donor cell

Inband (RN): The backhaul link and the access link reuse the same carrier frequency resources.

Outband RN: The backhaul link and the access link use different carrier frequency resources.

(2) Classified according to the working mode of RN

Transparent RN: The R-UE cannot feel that it is communicating through the transparent RN.

Non-transparent RN: R-UE can feel that it is communicating through non-transparent RN.

(3) According to the functions of RN

RN that does not independently manage the cell: This type of RN has no independent cell ID and no independent radio resource management function (at least part of the radio resource management function is mainly performed by the eNode B where the Donor Cell is located). Smart repeaters, decode and forward relays, layer 2 relays, and type 2 relays described below belong to this type of RN.

Figure 10-8 Application scenarios of several relay technologies

Figure 10-9 Air interface after the introduction of relay nodes

Independently managing the RN of the cell: This type of RN has an independent cell ID and independent radio resource management function, and the cell it manages can access the LTE R8 terminal. Both layer 3 relays and type I relays described below belong to this type of RN.

Relay technology

1 Type I Relay Relay technology type 1 relay (Type I Relay)

In the LTE-Advanced research, 3GPP RAN mainly studies and standardizes "type I relay", and its characteristics are as follows.

Type I relays are inband relays.

Type I relay manages independent cells and has an independent physical layer cell ID, sending independent synchronization signals, reference symbols, etc.

The R-UE belonging to the type I relay directly receives scheduling signaling and HARQ feedback signaling from the relay node, and directly sends uplink control and feedback information to the relay node.

Type I relay allows LTE R8 terminal access.

For LTE-A terminals, the type I relay allows to provide enhanced features different from ordinary LTE R8 eNode B to improve system performance.

It can be seen that the type I relay belongs to the aforementioned in-band, non-transparent, independent management cell RN, and the type I relay has similar functions to ordinary eNode B.

According to the previous definition, the in-band type RN multiplexes the same carrier frequency resources on the access link and the backhaul link. If the signals of the two links are transmitted and received at the same time, because There is not always good signal isolation, so there will be situations where the RN's transmitted signal interferes with its own received signal, as shown in Figure 10-10. In order to avoid the occurrence of such self-interference, the type I relay works on the access link and the backhaul link in a time division manner. In particular, the type I relay for the TDD mode:

l Donor eNode B RN transmission is completed in the downlink subframes of eNode B and RN;

l RN Donor eNode B transmission is completed in the uplink subframes of eNode B and RN.

Figure 10-10 Schematic diagram of in-band relay self-interference

In LTE R8, the terminal detects and measures the control area in each downlink subframe in a non-DRX state. In order to ensure that the reception of the backhaul link of the type I relay does not affect the LTE R8 terminal, the R8 protocol has been adopted. The working mode of the defined MBSFN subframe is shown in Figure 10-11. In the non-control area of an MBSFN subframe, the RN receives downlink backhaul data from Donor eNode B, and does not send any signals to the R-UE. The base station informs the RN of the downlink subframe transmitted as the backhaul through high-level signaling. At the same time, the base station needs to inform the RN in advance as uplink subframes for backhaul transmission, and the RN avoids scheduling the R-UE in these uplink subframes.

Figure 10-11 MBSFN subframe receiving Donor eNode B data

For TD-LTE-Advanced systems with RN deployments, in order to support uplink and downlink symmetrical and asymmetric services, the access link can be configured with uplink and downlink symmetrical and asymmetrical subframe configurations. Therefore, the backhaul link The actual service situation supports flexible subframe allocation methods, as shown in Figure 10-12. This part of the content is still in the process of 3GPP RAN1 research and discussion.

Figure 10-12 Two different RN frame configuration methods

The RN needs to send control signaling to the R-UE in the control area of the MBSFN subframe. Due to the limitation of self-interference, it cannot receive signals sent by Donor eNode B at the same time. Therefore, 3GPP RAN1 is studying and discussing the downlink control signaling design specifically for RN , Called R-PDCCH (Relay-PDCCH). There are two main types of current two P-PDCCH design schemes.

(1) Conventional R-PDCCH: Donor eNode B allocates the same R-PDCCH region to multiple RNs belonging to it. Each RN obtains its own control information in the common region by using a blind detection method similar to LTE R8 UE. make.

(2) RN specific R-PDCCH: Donor eNode B allocates dedicated R-PDCCH resources to each RN, and each RN obtains control signaling in its own resources.

At the same time, 3GPP RAN1 is also studying the multiplexing design of R-PDCCH and R-PDSCH (backhaul downlink data transmission channel) in the backhaul downlink subframe. Currently, the following three multiplexing design schemes are still under discussion. For the sake of simplicity, the impact of RN transceiver switching on backhaul transmission is not described here.

(1) TDM multiplexing mode: In the non-control area of the MBSFN subframe, the relationship between R-PDCCH and R-PDSCH is pure time division multiplexing, as shown in Figure 10-13. The R-PDCCH frequency occupies the entire system bandwidth, and the number of OFDM symbols occupied in time can be configured by the base station.

Figure 10-13 R-PDCCH and R-PDSCH time division multiplexing

(2) FDM multiplexing mode: In the non-control area of the MBSFN subframe, the R-PDCCH and R-PDSCH have a simple frequency division multiplexing relationship, as shown in Figure 10-14. The R-PDCCH occupies all OFDM symbols in the non-control region in the MBSFN subframe in time, and the number of PRBs occupied by the frequency can be configured by the base station.

Figure 10-14 R-PDCCH and R-PDSCH frequency division multiplexing

(3) TDM + FDM hybrid mode: In the non-control area of the MBSFN subframe, the R-PDCCH and the R-PDSCH resources occupying the same frequency domain location are TDM multiplexed, and the other part of the R-PDSCH resources is FDM multiplexed. Mode, as shown in Figure 10-15. The R-PDCCH occupies PRB and the number of symbols can be configured by the base station.

Figure 10-15 R-PDCCH and R-PDSCH time / frequency division mixed multiplexing

Type II Relay Relay Technology Type II Relay

While discussing "Type I Relay", 3GPP RAN1 also studied other types of relays. A "Type II Relay" scheme has attracted some companies' research interest. Type II Relays have the following characteristics :

Type II relay is an in-band relay node;

It does not have an independent physical layer cell identifier, and cannot create a new cell;

It is transparent to LTE R8 terminals, that is, such terminals are unaware of the existence of Type II relay nodes;

It can transmit PDSCH;

It does not transmit at least CRS and PDCCH.

It can be seen that the type II relay belongs to the "non-independently managed cell" and "transparent" relay type, which is mainly used to enhance the PDSCH receiving performance of the terminal, thereby achieving the purpose of improving the overall throughput of the cell. Since type II relays do not send common signals such as CRS and PDCCH, they cannot be used as a solution for extending cell coverage. The working scheme of the Type II relay mainly includes the following three types, as shown in Figures 10-16 to 10-18.

Downlink non-cooperative transmission, that is, the base station sends (retransmission) scheduling information and downlink data packets to the relay node. The initial transmission and retransmission of downlink data are performed between the relay node and the user terminal. The base station does not participate in transmitting data to the user. Downlink data transmission of the terminal;

Coordinated initial transmission and retransmission of downlink, that is, the base station sends (retransmission) scheduling information and downlink data packets to the relay node, and the initial transmission and retransmission of downlink data are completed by the cooperation of the base station and the relay node;

Cooperative downlink retransmission, that is, the initial transmission of downlink data is performed between the base station and the user terminal. When retransmission is required, the base station sends retransmission scheduling information to the relay node, and the base station and the relay node cooperate to send downlink data to the user terminal. package.

Figure 10-16 Downlink non-cooperative transmission

Figure 10-17 Downlink cooperative initial transmission and retransmission steps

Figure 10-18 Downlink cooperative retransmission steps

Generally speaking, 3GPP RAN1's research on Type II relay is still in the preliminary feasibility discussion stage, and there is no consensus on the specific work plan.

Relay technology

The primary problem of relay research is the choice of type I relay architecture. According to the different protocol stack structures, the relay architecture is mainly divided into architectures A and B. Architecture A contains three architecture options, called Alt1, Alt2, and Alt3; architecture B contains only one architecture option, called Alt4.

A Relay Technology Relay Architecture A

Architecture A is characterized in that both the user plane and the control plane of the S1 interface are terminated in the RN. In architecture A, Alt1 is the most basic architecture option, and Alt2 and Alt3 are obtained by optimizing Alt1.

As shown in Figure 10-19, the RN consists of two logical functions: the eNode B function and the UE function (also called Relay-UE). Among them, the eNode B function is used to provide access services for User-UE (UE working under RN); the Relay-uE function is used to send and receive data on the backhaul connection. In order to make the UE function of the RN work normally, the MME of the Relay-UE and the SGW / PGW function of the Relay-UE are introduced in the LTE-Advanced system.

Figure 10-19 RN network architecture diagramArchitecture A

As can be seen from Figure 10-19, the differences in the architecture options Alt1, Alt2, and Alt3 are transparent to the RN. They belong to the same architecture system, and the difference is reflected in the integration of different functional entities into the DeNode B. In Alt1, the DeNode B function and the SGW / PGW function corresponding to the Relay-UE of the RN are respectively located in different physical nodes; while in Alt2 and Alt3, the DeNode B function and the SGW / PGW function corresponding to the Relay-UE of the RN are integrated Into DeNode B.

It should be noted that the relay GW function in Figure 10-19 is optional and it only exists in the architecture option Alt2. The relay gateway is used to complete the functions of the Home eNode B GW, and is integrated in the DeNode B entity, which enables the DeNode B to view and relay the S1 interface and X2 interface messages transmitted through it in a proxy manner. The relay gateway function is not visible to the RN, the core network node of the UE, and other eNode Bs.

For the above three architecture options, there is no need to change the existing S1 interface protocol. Under Alt1 and Alt3, DeNode B only maps the S1 interface messages encapsulated in the tunnel to an Un interface bearer for transmission. DeNode B cannot learn the specific content of the S1 interface messages that are relayed. In Alt2, DeNode B can learn the relayed S1 interface messages. Another advantage brought by the relay GW function of DeNode B in Alt2 is that it reduces the impact of the expansion of the number of RNs under DeNode B on the UE core network node. DeNode B shields the RN serving the UE from the core network node of the UE. From the perspective of the core network node of the UE, the cell controlled by RN is the cell controlled by DeNode B. At the same time, DeNode B shields the core network node of the UE from the RN. From the perspective of the RN, DeNode B is the core network node of the UE.

Similar to the S1 interface protocol, there is no need to change the X2 interface protocol for the above three architecture options. Under Alt2, DeNode B can learn the X2 interface messages relayed through it. Under Alt2, DeNode B shields the neighboring RN from serving RN. From the perspective of neighboring eNode B, the cell controlled by RN is the cell controlled by DeNode B. DeNode B shields the RN from neighboring eNode B. Neighboring cells are all cells controlled by DeNode B.

1. Data transfer process in Alt1 / 3

For Alt1 and Alt3, the UE and RN bearer and downlink data packet transmission processes are shown in Figure 10-20.

(1) The data packet sent to the UE is determined by the UE's PGW according to the corresponding packet filtering rules (usually classified according to the QoS of the service to which the data packet belongs), and passes through the corresponding GTP tunnel (located at the UE's SGW / PGW and RN).

(2) For the above data packets, the UE SGW / PGW classifies the RN EPS bearer type to which it belongs according to the packet filtering rule classification (usually based on the QoS of the service to which the data packet belongs), and indicates in the DS field in the IP packet header.

(3) The PGW of the RN receives the GTP tunnel data packet whose destination address is the RN, classifies it into different RN bearers according to the packet filtering rules (based on the DS field in the IP packet header), and passes the data packet according to the classification The second layer GTP tunnel (located between the RN's SGW / PGW and DeNode B) is used for transmission. For multiple UEs served by the same RN, multiple UE EPS bearers with similar QoS requirements are mapped to the same RN EPS bearer.

(4) DeNode B maintains a one-to-one mapping relationship between the RN GTP tunnel and the RN radio bearer, determines the corresponding RN radio bearer according to the RN GTP tunnel to which the received data packet belongs, and sends the data packet to the RN on the Un interface.

Figure 10-20 User data transmission processAlt1 / 3

(5) The RN maintains a one-to-one mapping relationship between the UE GTP tunnel and the UE radio bearer, determines the corresponding UE radio bearer according to the UE GTP tunnel to which the received data packet belongs, and sends the data packet to the UE on the Uu interface.

In the uplink, the RN completes the mapping from the UE bearer to the RN bearer based on the QCI carried by the UE.

2. Data transfer process in Alt2

For Alt2, between the UE's SGW / PGW and DeNode B, each UE bears a corresponding GTP tunnel. This tunnel is converted into another GTP tunnel at DeNode B for transmission from DeNode B to RN. Two The GTP tunnels are mapped one by one. The transmission process of downlink data packets is shown in Figure 10-21. The differences between Alt2 and Alt1 / 3 are mainly reflected in the following two aspects.

Figure 10-21 User data transmission processAlt2

(1) DeNode B can know the QoS information carried by the EPS of each UE by parsing the S1 message. Therefore, DeNode B is based on the QCI of the UE EPS carried by the received data packet. Filtering) to determine the RN radio bearer to which the data packet belongs.

(2) DeNode B converts the GTP tunnel carried by the UE from the SGW / PGW into another GTP tunnel carried by the UE directed to the RN, and the two are one-to-one mapping. This truncation of the GTP tunnel carried by the UE makes the RN and the core network invisible to each other, which improves the scalability of the network.

For uplink, the RN completes the mapping from the UE bearer to the RN bearer based on the QCI carried by the UE.

Comparing Figure 10-20 and Figure 10-21, we can see a significant difference: under Alt1 / 3, the GTP tunnel carried by the UE is invisible to DeNode B; while under Alt2, the GTP tunnel carried by the UE is not visible to DeNode B. Is visible.

It should be noted that the EPS bearer is composed of a GTP tunnel and a corresponding radio bearer. In Alt2 and Alt3, because the SGW / PGW function of the RN is integrated in DeNode B, the GTP tunnel carried by the RN still exists logically, so the EPS bearer of the RN also exists objectively. This forms another feature of Architecture A: there is a nested relationship between the UE EPS bearer and the RN EPS bearer.

Each of the three options in Architecture A has the following advantages: Alt1 has the best compatibility with the existing network; Alt3 integrates the RN's SGW / PGW function into DeNode B, which reduces the number of nodes in the data transmission path, compared to Alt1 Reduced data transmission delay; under Alt2, the information of the UE served by the RN is visible to the DeNode B, which provides the possibility for further process optimization.

The three seed options of Architecture A use the same Un interface. This means that the same RN can be applied to all sub-options of Architecture A, and there is no need to distinguish the three types of RN architectures under Architecture A in the standard, which also brings flexibility in implementation for network deployment. Operators can choose Alt1 to quickly deploy RN after a simple upgrade of the existing network; or they can choose Alt2 and Alt3 to deploy RN after a complex upgrade of the existing network to obtain better network performance.

B Relay Technology Relay Architecture B

The architecture B is characterized in that the user plane of the S1 interface terminates at DeNode B, and the control plane terminates at RN. Under this architecture, DeNode B integrates the function of the relay GW and can parse the S1 and X2 interface messages passed through it. Similar to Alt2, from the perspective of the core network node and neighboring eNode B, the RN appears as a cell managed by DeNode B.

The difference from Architecture A is that the user plane bearer structure of the Un port in Architecture B no longer adopts the structure of nesting RN EPS bearers on the outer layer of the UE EPS bearers, but directly maps the UE EPS bearers to the Un interface RN radio bearers one by one. . This mapping method avoids the problem of excessive protocol overhead of the Un interface caused by GTP tunnel nesting. At the same time, since the RN EPS bearer is no longer required during the user-UE data transmission process, functions related to the RN EPS bearer, such as the UE function of the relay, the MME and the SGW / PGW function of the Relay-UE, etc. -The UE no longer plays a role in data transmission, as shown in Figure 10-22.

Figure 10-22 RN network architecture diagramArchitecture B

Under Architecture B, although the RN PGW / SGW does not contribute to the user-UE data transmission process, it is still indispensable because control information of the RN, such as OAM information, needs to be sent to the RN via the RN PGW / SGW.

The GTP tunnel does not extend to the Un interface. It also brings certain disadvantages. For example, DeNode B needs to perform protocol conversion on the user plane data that needs to be forwarded to the RN, that is, it is first taken out of the GTP tunnel and then sent to RN, the introduction of this process will undoubtedly increase the complexity of the protocol.

For Alt4, the UE and RN bearer and downlink data packet transmission processes are shown in Figure 10-23.

For a UE connected to the RN, each GTP tunnel will be mapped one-to-one with the RN radio bearer of the Un interface, that is, each GTP tunnel will exclusively occupy one Un interface RN radio bearer. It should be noted that the exclusive UE RN bearer does not contain the identification information of the GTP tunnel, so it is not a GTP tunnel. Unlike architecture A, which uses GTP tunnel identification information to distinguish between different UE bearers, architecture B requires PDCP on the Un interface, and the RLC or MAC protocol layer needs to add UE identification, that is, the MAC / RLC / PDCP protocol of the Uu interface needs to be implemented. Makeover.

Figure 10-23 User data transmission stepsAlt4

Relay Technology Architecture Selection

The advantage of architecture A is that it has a small impact on the protocol and flexible deployment, but the disadvantage is that the Un interface is inefficient. The advantage of architecture B is that the Un interface is highly efficient, but the disadvantage is that the existing protocol process is greatly changed. Both architectures have advantages and disadvantages compared to each other and need further evaluation.