WO2006023771A2

WO2006023771A2 - Method and apparatus for transparent relaying

Info

Publication number: WO2006023771A2
Application number: PCT/US2005/029624
Authority: WO
Inventors: Philippe J. Sartori; Kevin L. Baum; Brian K. Classon; Mark C. Cudak
Original assignee: Motorola, Inc.
Priority date: 2004-08-18
Filing date: 2005-08-18
Publication date: 2006-03-02
Also published as: JP4615566B2; EP1784931A2; JP2008511210A; WO2006023771A3; CN101006660A; KR100884699B1; EP1784931A4; KR20070034123A

Abstract

The present invention provides a method for enabling transparent relaying of data in order to improve the performance of a cellular system. Particularly, relaying on the uplink is performed by fixed entities called Transparent Relays (TR). The base station may allocate one or more TRs to relay the data transmissions associated with a particular CID (e.g., a particular subscriber station) on the uplink.

Description

METHOD AND APPARATUS FOR TRANSPARENT RELAYING

Field of the Invention

The present invention relates generally to relaying information and in particular, to a method and apparatus for relaying information within a communication system.

Background of the Invention

Wireless communication systems are known in the art. In many such systems remote communication units (at least some of which may be mobile) communicate with one another and/or with others via system infrastructure such as fixed-location transmitters and receivers. In general, wireless communication systems are characterized by a corresponding communication range (typically characterized by either or both of a transmission range and a reception range) beyond which the wireless communications capability of the system infrastructure cannot usefully extend.

Repeaters are also known in the art. Such devices typically serve to extend the communication range of a given communication system (by extending the transmission and/or reception range). Via this mechanism, for example, a relatively low power remote communication unit can effectively communicate with a relatively distant system receiver notwithstanding that the remote communication unit is otherwise out- of-range of the distant system receiver. Such repeaters often operate in an autonomous automatic mode and repeat whatever transmissions they successfully receive.

Unfortunately, despite various improvements to both systems and remote communication units, there remain times and circumstances when the transmissions of a communication unit that is within the communication range of a given communication system are nevertheless not received reliably at a given desired level of quality of service. Various causes exist for this result, including but not limited to shadow fading and other propagation issues. Performance requirements can also pose an impact. For example, as data transmission rates demands continue to increase (leading frequently to a corresponding increase in bandwidth), the ability of an otherwise in-range remote communication unit to successfully effect a desired level of service without a concurrent significant increase in transmission power usually becomes impaired. Therefore, a need exists for a method and apparatus for relaying information within a communication system that addresses the above-mentioned issues. Brief Description of the Drawings

FIG. 1 is a block diagram of a base station and subscriber stations.

FIG. 2 depicts the possible communication pathways between base stations, subscriber stations, and relays.

FIG. 3 is a call-flow diagram showing allocation of one or more relays. FIG. 4 illustrates resource allocation for relays.

Detailed Description of the Drawings

The present invention provides a method for enabling transparent relaying of data in order to improve the performance of a cellular system. In one embodiment, one or more relays are deployed in a sector of a cell. These relays will be referred to as transparent relays (TRs) because the present invention enables them to operate in the system in a nearly "transparent" manner from the point of view of the subscriber stations in the cell. In one example, three TRs can be deployed in a sector in locations that are selected to provide significant link budget improvements to subscriber stations (SSs) located far from the base station (BS), as shown in FIG. 1.

In one embodiment of the invention, multiple TRs (e.g., more than one but possibly only a subset of the total number of TRs in a sector) may be selected and instructed to perform relaying (or be activated) for a particular SS. In this case, the selected TRs monitor (receive and demodulate/decode) the uplink data transmissions made by the SS. Then the selected TRs re-encode and re-transmit the data on a different channel resource than was used by the SS for the original transmission (e.g., a different time slot, a different subchannel, a different spreading code, etc.). In order to provide a macro-diversity benefit, the transmissions from each of the selected TRs can be made on the same channel resource (in this case the TRs are preferably synchronized to the BS timing and frequency so that the transmissions from the selected TRs arrive at the BS approximately synchronously).

A method is also provided to eliminate transmissions from any of the selected TRs whenever a selected TR fails to correctly receive/decode the data transmission from the SS. This method involves leaving the channel resource assigned to the TR for the transmission empty. With this method, all TRs in a sector can be made into a single group, assigned a group ID such as a multicast group ID, and then the SS monitoring and TR resource assignments can be made very efficiently to the group as compared to sending separate commands or assignments to each TR. When all TRs in a sector are activated for a particular SS transmission, it is expected that some of the TRs will correctly decode the SS transmission (e.g., close to the SS) and others will not (e.g., far from the SS). Even in this case, effective macro-diversity can still be provided since the TRs that correctly decode the SS transmission will transmit together while the TRs that did not successfully decode the SS transmission will remain silent on the channel resource that was assigned to the TR group for transmission of the SS data. A significant advantage of this embodiment is that there is no need for a SS to "handoff ' from one TR to another as is moves across the cell: instead, the present invention provides an effective method for automatically using the best TRs in the sector.

In an additional embodiment, one or more TRs are instructed to monitor the data transmissions of multiple SSs. For example, the selected TRs can be given a list of connection IDs (CIDs), and this means that the selected TRs are being instructed to monitor the uplink data transmissions on all of the uplink channel resources that are allocated to those CIDs. The selected TRs are also given an uplink channel resource allocation that will be used by the TRs to transmit (e.g., retransmit the data from the SSs) the data received from the SSs. An efficient assignment method is provided whereby a block resource assignment can be provided to the TRs and the TRs use a predetermined data ordering rule to insure that transmissions from all TRs send the same data portions (e.g., data segments from different SSs) on exactly the same corresponding resources to insure that the transmissions from multiple TRs provide macro-diversity combining at the BS rather than interfering with each other. An additional aspect of the invention is that the data transmission rates can be different from different SSs, and the data transmission rate used by the TRs can be different from the SSs. In this case, the BS can calculate the total necessary channel resource allocation needed for the TRs by taking into account the modulation and coding rates being used by the SSs being relayed and the modulation and coding rate being used by the TRs (see example later in the document). In this case^ each of the selected TRs preferably uses the same modulation and coding rate to preserve the macro-diversity benefit.

In an additional embodiment, a method is provided for selecting and adjusting the modulation and coding rate used for the transmissions.

In an additional embodiment, the BS will store the received signal from an SS that was received on one set of channel resources and will combine the stored signal with a received signal from a TR on a different set of channel resources. This provides an additional form of macro-diversity since signals from the SS and the TR are being combined within the BS as a signal processing step.

The above embodiments can be combined to provide the greatest overall functionality. Various aspects of theses embodiments as well as aspects of additional embodiments are described in the following.

The detailed embodiments presented below are described within the context of the IEEE 802.16e standardization effort that is ongoing. The detailed solution is presented for the OFDMA mode of 802.16e, but the invention is also applicable to other systems. Table 1 lists the acronyms used in this document.

Table 1. List of Acronyms

The ULJVlAP is the message that specifies the resource assignment on the uplink. This message can be extended to convey other information than the resource assignment, such as e.g., information relative to adaptive antenna processing or uplink relaying.

REG-REQ message is the 'registration request' message. It is a message that the SS sends after performing the initial ranging. This message is a pre-requisite before any data transmission, and is acknowledged by the BS by sending a REG-RSP, or 'registration response' message.

The above-relaying technique can be applied to the uplink for IEEE 802.16e OFDMA. There is a serious link budget problem on the uplink that drastically reduces the uplink data rates and the system throughput, even for reasonable cell radii (2 km). The solution presented here enables the seamless introduction of simple one-hop relaying on the uplink to deal with this issue. Downlink transmissions are not relayed at all, thereby drastically reducing the complexity of the relay.

The solution presented here has the following advantages:

• The transparent relay (T-relay) is a simplified unit that only needs to perform a few layer-one operations and a minimal set of layer-two tasks. Moreover, the relay does not need to wired to the network.

• The relaying process is a transparent process that requires no changes in the SS and very minimal signaling changes to accommodate the relay enabling process.

• One or more T-relays can be deployed in each sector. A particular T-relay does not need to be aware of other T-relays.

• The BS always remains in control of the transmission, thereby resulting in increased transmission reliability. • The architecture still permits hybrid- ARQ (HARQ) to be performed on the uplink.

The disparity in PA power between SS and BS suggests that a solution can be tailored to provide the necessary assistance to the uplink while not being involved in the downlink. As a result, cost efficiencies can be achieved by creating a subordinate relationship between relays and the BS allowing the relays to be low cost while ensuring robust reliable transmission supervised by a central authority. The cost efficiencies may be realized by reducing relay complexity such that it only focuses on layer-one operations and a minimal set of layer-two tasks. In addition, control messages do not have to be relayed - only bearer data. Moreover, the transparent relay (T-relay) configuration simplifies system deployment. These T-relays can be deployed in existing cells to address uplink coverage issues without having to re-address cell planning as might be required when adding microcells to achieve the same end.

FIG. 2 depicts the possible communication pathways between BSs, SS, and T- relays. FIG. 2a shows the typical communication paths in a cellular system with the T-relay disabled. A BS coordinates the resources in the cell by distributing control information and arbitrating access requests. In addition, the BS transmits bearer data directly to the SS and receives bearer data directly from an SS. FIG. 2b shows the communication paths with the T-relay enabled. In this case, the BS still coordinates resources in the cell by distributing control information and arbitrating access requests. Additionally, the BS continues to transmit bearer data directly to the SS. However, the uplink bearer data from the SS follows a triangular path first being received and detected by the T-relay then re-encoded and transmitted to the BS by the T-relay. FIG. 2c and FIG. 2d show two variations on the T-relay configuration. FIG. 2c shows multiple active s-relays simultaneously repeating the SS bearer data to the BS. FIG. 2d shows the simultaneous co-existence of a relayed and non-relayed uplink communication. A key and highly beneficial aspect of the T-relay configuration is that the SS may be completely unaware of the existence of a relay within in the system. Other control-related uplink functions, such as ranging and bandwidth request, are still handled by the direct SS-to-BS path in order to simplify the relaying scheme.

In the broadest sense, the above procedure is the minimum required to increase uplink bearer data transmissions rates. If the T-relay cannot be employed, the data rate cannot take into account the relay. If the T-relay can be employed, the data rate can take into account the T-relay and be such that the data rate is much higher. As each data transmission is adaptive to the channel conditions, conditions may be such that even if a T-relay can be employed, it may not be employed if the direct link to the BS is of sufficient quality. Typically, the determination of whether a T-relay is to be employed is made for each transmission, but may also be made on a longer term average channel quality basis (e.g., taking shadowing but not fast fading into account). The T-relay (more generally, one or more T-relays) is a subordinate relay because the resource allocation for the SS to T-relay link is provided by the BS.

Operations to be Performed

As mentioned earlier, the relaying process is completely transparent to the SS, thereby requiring no additional operations to be performed. However, the link between the T-relay and the BS needs to be established and maintained. An implementation of the various tasks that need to be performed is detailed below.

Network Entry and Initialization

The network entry and initialization process is the same as for a conventional

SS, except that at one point (the registration process), the unit must identify itself as a T-relay.

Relay Assignment to a Connection (listening)

The relay assignment process is done on a frame-by-frame basis. Each T-relay (or group of T-relays) is assigned the CIDs whose transmissions it needs to monitor in the uplink portion of the current frame. Therefore, by decoding the UL_MAP, each T- relay knows every resource it needs to listen to and attempt to detect. Note that a T- relay may monitor one or more connections (e.g., one or more SSs) and may be part of one or more multicast groups. For example, a T-relay may be assigned to monitor two different connections and may be addressed by different CIDs (e.g., a special CID if the T-relay being activated, or even a multicast CID if it is part of a group of relays being activated).

Alternatively, instead of being done on a per-frame basis, the assignment could remain valid until a future assignment or a de-assignment is received. Or, the assignment could be valid for a pre-determined amount of time (e.g., 10 frames).

Communication of Relay Assignments (transmitting)

The assignment of resources for the transmission from the T-relay(s) to the BS is also done on a frame-by-frame basis. Each T-relay (or group of T-relays) is assigned resources via the existing UL-MAP-IE message. Each T-relay only relays connections for which it has successfully decoded the data.

Modulation/Coding Scheme Selection

Since the T-relays only relay back connections for which it has successfully decoded the data, the BS can initially start assigning a high MCS to the SS. The T-relay monitors the link. If it cannot successfully decode the data, it does not send anything to the BS. The BS, not receiving anything, knows that the chosen MCS is too high, and then assigns a lower MCS to the SS until it receives something from one or more T- relays. The process would allow an initial MCS selection. After this initial MCS selection, the HARQ process can fine-tune the MCS selection.

Alternatively, a blind AMC selection could be made by the BS based on some open-loop approximation. The simplest method of MCS selection would be to use an aggressive default value with the hope that the SS is close to either one of several relays or the BS. The system may rely on Hybrid ARQ and retransmission to mitigate all poorly chosen AMC levels.

Alternatively, the relay may eavesdrop on the ranging channel, then report information to the base. The relay could use the ranging channel to report this information. Or, dedicated resources (Say 1 RE) could be reserved by the BS for this process. To avoid collision between all the relays, each relay is assigned a Walsh code and spreads the report with the assigned code. When there is only one relay, the relay can piggyback the MCS information with the relayed data. When multiple relays and RF combining, this solution is not applicable. The relays could send an MCS increase request on the ranging channel. Alternatively, the CDMA embodiment mentioned above is applicable as well. Also, it might be possible to use some type of analog feedback (e.g., by modifying the transmit power) to notify the base that a higher MCS could be supported.

The relays can also transmit beacons at given times, the SS can measure received power, and makes its own MCS request.

Hybrid ARO Process

This architecture can accommodate HARQ on the uplink. The T-relay (or group of T-relays) buffer the data received from the SS. When a T-relay successfully decodes the packet, it sends the data to the BS on the resources explicitly assigned on the uplink. If the decoding is not successful, the T-relay (or group of T-relays) does not transmit anything. Not receiving anything, the BS knows that the transmission was not successful, and sends the control for the next HARQ transmission.

Text Changes to be made in the IEEE 802.16 Standard

[In Section 6.1, After third paragraph, add the following paragraph]

Also, in order to improve the uplink data rates, transparent relaying on the uplink may be enabled. Transparent relays (TR) are deployed in a cell in order to break the SS to BS link into a SS to TR link and a TR to BS link on an "as-beneficial" basis in order to provide higher data rates and/or capacity on the uplink, or when the SS to BS link is of unacceptable quality. This process is completely transparent for the SS: it is not aware that it is being relayed.

[In Section 6.3.2.3.7 Registration request (REG-REQ) message , Just before

6.3.2.3.8 insert the following bolded text]

The REG-REQ may contain the following TLVs: (...) Transparent Relay Capabilities (11.7.19) [In Section 6.3.2.3.8 Registration response (REG-RSP) message, just before 6.3.2.3.9, insert the following bolded text]

The REG-RSP may contain the following TLVs: (...)

Transparent Relay Capabilities (11.7.19) [Add a new section 6.3.2.3.43.7.8]

Section 6.3.2.3.43.7.8 Compact_UL_MAP Transparent Relay Monitor

The Transparent Relay Monitor Information Element provides the list of SS CIDs whose transmissions are to be monitored (detected) during the UL part of the current frame and relayed in the next frame..

[Add a new section 6.3.2.3.57]

Section 6.3.2.3.57 Transparent Relay CID Assign (TR-CID) Message

The base will assign one or more secondary relay CIDs to a transparent relay for the purpose of sending relay monitor command and allocating resource for a retransmission of the monitored SS(s) data. The relay CID may be assigned to only one transparent relay or multiple transparent relays. Upon reception, a SS will delete all previously assigned relay CIDs and adopt those newly assigned.

Parameters shall be as follows: CID (in the generic MAC header) SS's Primary Management CID. Transaction ID

Unique identifier for this transaction assigned by the sender. All other parameters are coded as TLV tuples. Relay CID (see 11.16)

Section 6.3.2.3.58 Transparent Relay CID Assign ACK (TR-ACK) Message

This message is sent in response to a TR-CID assignment message.

Unique identifier for this transaction assigned by the sender. Confirmation Code Zero indicates the request was successful. Non-zero indicates failure. [Add a new section 11.7.19]

Section 11.7.19 Transparent Relay Capability

This field indicates whether the unit is a re ular SS or a trans arent rela .

[Add a new section 6.3.18]

Section 6.3.18 Transparent Relay Operation

In order to improve data rates on the uplink, relaying some uplink transmission may be advantageous. Relaying on the uplink is performed by fixed entities called Transparent Relays (TR). The BS may allocate one or more TR to relay the data transmissions associated with a particular CID (e.g., a particular SS) on the uplink. The process is shown in FIG. 3. In the ULJvIAP, the uplink resource allocations are sent to the transmitting SSs and an UL TR MONITOR IE is included to instruct a TR or a group of TRs to monitor the uplink transmissions associated with one or more SS CIDs. In the uplink, the SSs transmit, and the TRs monitor the transmissions they were assigned to monitor. The monitored transmissions are demodulated and decoded by the TRs so that they can be re-encoded and re-transmitted (relayed) in the next frame. In the next frame, in a second UL_MAP message, each active TR receives its resource assignment for the relaying transmission. This resource assignment is made per TR CID, or for a multicast group TR CID. When the assignment is made per a multicast group TR CID, these are T-relays are expected to simultaneously relay the monitored transmission providing a macro diversity gain.

The MAC PDUs are transmitted from a TR in exactly the same relative order as they were received and are modulated and coded with the Modulation Coding Scheme (MCS, based on the UIUC) specified in the UL_MAP_IE addressed to the TR. The MAC PDUs of each respective SS are encoded separately as if the BS had sent separate allocations for each SS that is being relayed. If the TR did not correctly decode the data from a particular user (CID), it does not relay that data and leaves this portion of the assignment empty.

An example of TR resource allocation is given in FIG. 4. In the example described, the TR has to relay three transmissions: MAC PDU Sl, MAC PDU S2, MAC PDU S3, whose assignments appeared in this same relative order in the previous frame. The TR receives its resource allocation (in an UL_MAP_IE) for relaying the three transmissions with 64-QAM R= 1/2 and the necessary amount of subchannels to perform the relaying operation. The first PDU to be relayed, MAC PDU Sl is modulated and encoded with the new MCS and mapped onto the first resources. The TR was not able to receive correctly MAC PDU S2, therefore it does not transmit anything for S2, but leaves blank the portion of the assignment resources where it should have relayed this PDU. Resources for relaying MAC PDU S3 are assigned after the resources that were provisioned for MAC PDU S2. This TR resource assignment process enables the BS to know where to find the relayed data for each MAC PDU even though the relay uses a different MCS than the SSs. Macro-diversity is also provided when multiple TRs are assigned to relay the same CIDs.

[Add a new section 8.4.5.4.15]

Section 8.4.5.4.15 UL_MAP Transparent Relay Monitor

The Transparent Relay Monitor Information Element provides the list of SS CIDs whose transmissions are to be monitored (detected) during the UL part of the current frame and relayed in the next frame.

Table XYZ-OFDMA UL-MAP Transparent Relay Monitor IE format

[Add a new section 11.17]

Section 11.16 Transparent Relay CID Assign

The value of this field specifies the CID assigned by the BS to a particular transparent relay. This field shall be present in the TR-CID assignment message. The BS shall use the assigned value in the ULJvIAP Transparent Relay Monitor IE and Compact UL MAP Transparent Relay Monitor IE to instruct the relay to monitor particular uplink allocations. The BS shall use the assigned value in ULJVIAP IE to allocate resources for the retransmission of monitored SS data.

While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. It is intended that such changes come within the scope of the following claims.

Claims

1. A method for a relay device to relay information within a communication system, the method comprising the steps of: receiving a message indicating subscriber stations io monitor; receiving an assignment message indicating a relay-to-base resource; receiving transmissions from the subscriber stations; demodulating and decoding the subscriber stations¹ transmissions; re-raoduladng and re-encoding rhe subscriber stations' transmissions; and relaying the re-modulated and re-encoded transmissions over the relay-to-base resource, wherein the transmissions are relayed in a predetermined order.

2. The method of claim 1 wherein the step of relaying the transmissions in the predetermined order comprises the step of relaying the transmissions in the order they where received.

3. The method of claim 1 wherein the step of relaying the transmissions in the predetermined order comprises the step of relaying the transmissions in a same relative order as the assignments appeared in a previous frame.

4. The method of claim 1 wherein the slep of relaying the transmissions comprises the step of relaying only transmissions that were demodulated and decoded correctly by leaving a portion of the assigned relay-to-base resource, associated with a transmission that was not decoded correctly, empty.

5. The method of claim 1 wherein the step of relaying the re-modulaied and re- encoded transmissions over the relay-to-base resource comprises the step of relaying the re-modulated and re-encoded transmissions over a relay-to-base resource utilized substantially simultaneously by another relay device.

6. The method of claim 1 further comprising determining a modulation and coding rate, to utilize when re-modulating and re-encoding the one or more subscriber stations' transmissions.

7. The method of claim 1 further comprising the step of sending a message indicating relaying capability prior to relaying transmissions from the one or more subscriber stations.

SUBSTITUTE SHEET (RIlE 26)