WO2009050628A1

WO2009050628A1 - Parallel transmission for enhanced spatial reuse efficiency

Info

Publication number: WO2009050628A1
Application number: PCT/IB2008/054190
Authority: WO
Inventors: Xiangyu Wang
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-10-19
Filing date: 2008-10-13
Publication date: 2009-04-23
Also published as: TW200935933A

Abstract

The present invention relates to apparatuses, a system and a method of enabling a parallel second transmission for enhanced spatial reuse efficiency in a wireless network, wherein a hidden node status is determined at a first network node (R3) in relation to a first transmission between other two modes (S1 and R1), and a request for reception that solicits the parallel second transmission to the network node (R3) is issued if the hidden node status has been determined. The issued request for reception is received at a second network node (S3) which transmits a data message or a confirmation message in order to initiate a desired parallel transmission.

Description

Parallel transmission for enhanced spatial reuse efficiency

FIELD OF THE INVENTION

The present invention generally relates to apparatuses, a system and methods of enabling parallel transmission in a wireless network such as - but not restricted to - a wireless local area network (WLAN).

BACKGROUND OF THE INVENTION

Wireless local area networks (WLANs) as defined e.g. in the IEEE 802.11 specifications are almost omnipresent today. The 802.11 standard makes it mandatory that all stations implement a distributed coordination function (DCF) which is a form of carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CA is a contention-based protocol making certain that all stations first sense the medium before transmitting. The main goal is to avoid having stations transmit at the same time, which results in collisions and corresponding retransmissions. If a station wanting to send a frame senses energy above a specific threshold on the medium (which could mean the transmission of another station), the station wanting access will wait until the medium is idle before transmitting the frame.

Although somewhat more complex, this process of accessing the medium can be seen as a meeting where everyone is polite and each person only speaks when no one else is talking. In addition, participants who understand what the person is saying nod their head in agreement. The collision avoidance aspects refers to the random delay that a station must perform before actual transmissions. Typically the random delay is in the form of backoff time slots. The collision avoidance by random backoff is widely used in wireless communication systems, especially in WLAN and WPAN systems like IEEE 802.11 and IEEE 802.15 as well as wired communication systems, such as IEEE 802.3 (Ethernet). The backoff process is used in communication systems for random access to a channel among a number of multiple contending stations, as indicated for example in D. Bertsekas and R.

Gallager, "Data Networks", Chaper 4, Prentice Hall, 1992. Usually it is combined with the CSMA protocol. The backoff process has been introduced to allow fair access to a channel by all participating stations and to adjust transmissions according to network congestion level in a distributed way. Before a station starts a transmission, it selects a random backoff number, BO, within a certain contention windows (CW), say [0, CW]. A station selects the random number, BO, uniformly across the contention window so as to achieve fair access to the channel. If a station discovers after sending a packet that the transmission resulted in a collision because other stations were transmitting, the station will double its contention window to 2*CW and repeat the process. To discover a collision, a station either uses collision detection circuitry or relies on the receiver to inform it.

Many wireless systems of unlicensed operations have overlapping transmission regions that are difficult to control. These include ad-hoc wireless networks, in which nodes may move freely and overlapping of transmission regions is a nature consequence. These also include infrastructure-based networks, such as enterprise wireless networks, hot-spots, wireless networks in neighbourhoods, etc, where overlapping of transmission areas are inevitable due to unlicensed operations that are spontaneous by nature and due to propagation effects that are difficult to measure, predict and control.

Overlapping transmission areas is a major issue in MAC (Medium Access Control) design. The problems with MAC, in particular the IEEE 802.11 MAC, in ad-hoc multi-hop networks have been identified. There are a few misbehaviours of ad-hoc multi-hop networks when MAC is not designed properly. These include, broken link, fairness, reduced throughput or long delay. All these problems are also relevant for infrastructure-based wireless networks with overlapping areas. A common design in many of the proposals is to use a Request-To -Transmit /

Clear-To-Transmit (RTS/CTS) message handshake, to mitigate possible hidden nodes. Fig. 1 shows a schematic signaling diagram of an RTS/CTS handshake procedure. Based on a determined BO number, a sender S determines a backoff period after which it starts transmitting an RTS request if the channel is idle. The receiver R responds with a CTS response from which the sender S deduces that the channel is free for data transmission. Successful receipt of the transmitted data is acknowledged by the sender S with an ACK response. The effect of RTS/CTS handshake is to clear the surrounding area of an intended receiver such that the receiver is safe in receiving the following (longer) DATA transmission and to clear the surrounding area of a sender such that the sender is safe in receiving the following ACK transmission. The spatial effect of this RTS/CTS exchange is shown graphically in and is analyzed for example in C. K. Ho and J. P. Linnartz, "Calculation of the Spatial Reservation Area for the RTS/CTS Multiple Access Scheme", 27th Symp. on Information Theory in the Benelux, 2006. Fig. 2 shows a schematic plane view of a surrounding area of a first sender or sender node Sl and a first receiver or receiver node Rl, where the black dots indicate nodes (or terminals) of a wireless network. The grey circular area around the first sender node Sl covers those nodes which receive an RTS message sent from the first sender node S 1 (arrow (I)). These nodes will be refrained from transmitting. The CTS response (arrow (2)) sent back from the first receiver node Rl clears the circular white area around the first receiver node Rl, and those nodes which fall within this area will be refrained from transmitting, as otherwise the reception at the first receiver node Rl will be destroyed. In this way, the RTS/CTS exchange, if established successfully, can substantially guarantee successful transmission and reception of a following longer data packet (arrow (3)) sent without collisions. Successful receipt of the data packet is then acknowledged (ACK) by the first receiver node Rl (arrow (4)).

However, the RTS/CTS exchange together with the carrier sensing mechanism in the IEEE 802.11 MAC protocol result in too conservative utilization of the spatial resource in a network. Within the area cleared by the RTS and CTS messages, as indicated by the grey and white circular areas, respectively, neither reception nor transmission is possible. Parallel transmissions can thus only be established outside these two circular areas, such as the one indicated by the circular areas around a second sender node S2 and a second receiver node R2 in Fig. 2 (respective arrows (1) and (2)). The grey and white circular areas have special roles in improving spatial reuse efficiency. For the grey circular area, the nodes inside it are termed as "exposed nodes". They cannot receive because they are in the transmission area of the respective sender. Hence the term "exposed nodes" indicates they are exposed to a sender. However, they can send as long as their transmissions do not interfere with the transmission from the sender at the receiver. In the IEEE 802.11 MAC protocol, these nodes are, however, refrained from transmissions due to the RTS message and/or due to the carrier sensing mechanism. In various proposed protocol designs, this problem is tackled in various degrees, as exposed nodes may have a chance to establish parallel transmissions.

The nodes in the white circular areas are called "hidden nodes" as they are hidden from the sender. They are blocked by the clearing CTS message but do not receive any RTS message. These hidden nodes should be refrained from any transmitting, as such a transmission would otherwise damage the established transmission from the sender to the receiver. SUMMARY OF THE INVENTION

It is an object of the present invention to improve spatial efficiency in multi- hop networks.

This object is achieved by apparatuses as claimed in claim 1 and 8 and by methods as claimed in claim 11 and 12. In a software-based implementation, the above object may be achieved by a computer program product which comprises code means for producing the steps of the above methods when run on a computer device.

Accordingly, the hidden nodes are enabled to receive parallel transmissions as they are outside the grey circular area of the sender that indicates the sender's transmission area. As more parallel transmissions are thereby possible, spatial reuse efficiency can be improved.

The hidden node or hidden receiver may be adapted to determine its own hidden node status if it detects a clearing response but no transmission request of the first transmission. Thus, by simply determining whether the above two messages are detectable or not, a network node can derive its hidden node status and can ask for or solicit a parallel transmission to itself if it has determined an own hidden node status. As a example for the case of a WLAN network environment, the clearing response may be a clear-to-send (CTS) message and the transmission request may be a request-to-send (RTS) message.

The request for reception may be a control message or a part of any type of message or a busy tone, adapted to solicit transmission of information in the next available transmission time period. More specifically, request for reception may be a broadcast message, a multicast message or a singlecast message. Hence, a high flexibility is given for implementing the proposed solution. In case of using a multicast message, a transmission order for an addressed group of senders can be indicated in the request for reception, so that a received response can be allocated to the sender based on the timing of receipt.

At the receiving end of a broadcast request for reception, an apparatus may be adapted to content by a backoff process and to transmit its data message or its confirmation message if it wins the contention. As an alternative, e.g. in case of a singlecast request for reception, the apparatus may be adapted to transmit its data message or its confirmation message if it learns that it is the requested sender by monitoring transmission history and predicting or by deriving information from upper protocol layers. As another option, e.g. in case of a multicast request for reception , the apparatus may be adapted to transmit its data message or its confirmation message at a timing derived from a transmission order indicated in the request for reception. Hence, the proposed solution can be advantageously applied for all kind of messages to achieve the desired increase in spatial effϊcieny.

The above apparatuses may be implemented as whole network nodes, devices, or terminals, or as signaling control modules, chips, or chipsets, to be incorporated in network nodes, devices, or terminals.

Further advantageous developments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described based on various embodiments with reference to the accompanying drawings in which:

Fig. 1 shows a schematic signaling diagram of an RTS/CTS handshaking process in a wireless network;

Fig. 2 shows a diagram indicating clearance of the surrounding areas of sender and a receiver; Fig. 3 shows a diagram indicating clearance of the surrounding areas of sender and a receiver and enabled parallel transmissions according to various embodiments;

Fig. 4 shows a schematic signaling diagram of an exemplary parallel transmission for a hidden node according to a first embodiment;

Fig. 5 shows a schematic signaling diagram of an exemplary parallel transmission for a hidden node according to a second embodiment;

Fig. 6 shows a schematic signaling diagram of an exemplary parallel transmission for a hidden node according to a third embodiment; and

Fig. 7 shows a schematic block diagram of a software-based implementation of various embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, various embodiments are described on the basis of a wireless ad-hoc network environment, e.g. a WLAN involving an RTS/CTS handshaking procedure.

In the following embodiments, spatial reuse efficiency is improved by enabling hidden nodes to receive parallel transmissions. Provision of additional parallel transmissions improves spatial reuse efficiency further. The idea is illustrated in the following embodiments by taking an example protocol design, such as defined for example in "CCF", Section 9.14, "IEEE P802.1 Is: ESS Mesh Networking", Draft version 0.03, Aug 2006. The original CCF operation is for multi-channel operation. Here, for reasons of simplicity, only a single-channel operation is used to describe the embodiments.

Fig. 3 shows a schematic plane view of a network environment according to various embodiments where hidden nodes are enabled to receive parallel transmissions. In Fig. 3, one transmission is effected between a first sender node Sl and a first receiver node Rl. As in Fig. 2, arrow (1) indicates transmission of an RTS message, arrow (2) indicates transmission of a CTS message, arrow (3) indicates data transmission, and arrow (4) indicates transmission of an acknowledgement. This applies to all transmission indicated by continuous and dashed arrows in Fig. 4. The initial transmission of the RTS message by the first sender node Sl and the responsive CTS message by the first receiver node Rl causes respective circular clearance areas around the first sender node Sl and the first receiver node Rl, similar to Fig. 2.

Additionally, a second sender node S2 is shown in Fig. 3, which cannot receive because it is an exposed node located in the grey transmission area of the first sender node Sl. However, as already mentioned initially, it can send to a second receiver node R2 as long as its transmission does not interfere with the transmission from the first sender node Sl at the first receiver node Rl, which is the case in the scenario of Fig. 3.

However, according to the embodiments, nodes (such as a third receiver node R3 or a fourth receiver node R4) that fall into the white circular area around the first receiver node Rl, but outside the grey circle of the first sender node Sl, will be allowed to send a solicitation message (or request for reception) indicated as RTR (Request-To-Receive), as indicated by an arrow (5) in Fig. 3.

In the first embodiment, this RTR solicitation message (arrow (5)) is a broadcast message issued by the third receiver node R3 or the fourth receiver node R4 to invite potential sender nodes or transmitters (such as a third sender node S3 or a fourth sender node S4, respectively) to transmit. When surrounding transmitters receive the RTR message, they may content e.g. by a usual back off process or the like and the first one which has the smallest back off counter value wins the contention and transmits to the third receiver node R3 a data message DATA or a confirmation message ATS (Acknowledge-to -S end) depending on the protocol used, as indicated by an arrow (6). The parallel transmissions in turn generate their own clearing areas around the sender nodes S3, S4 and the receiver nodes R3, R4. Fig. 4 shows a schematic signaling diagram of an exemplary message exchange for parallel transmission for a hidden node according to the first embodiment, where hidden nodes use a broadcast mode to solicit transmissions to them.

According to Fig. 4, a data transmission from Sl to Rl is initiated by an RTS/CTS handshaking. Then, a first (conventional) parallel transmission is initiated by an RTS/CTS handshaking between S2 and R2. Additionally, a second parallel transmission is initiated by an RTR broadcast message according to the first embodiment, by which R3 solicits the second parallel transmission from surrounding sender nodes. S3 wins the content procedure and responds with an ATS message after a predetermined backoff period. In the second embodiment, the hidden receiver node R3 does not send out an

RTR message in a broadcasting form. Rather, it sends a singlecast RTR message to a particular sender node S3. A hidden receiver may determine the particular sender by monitoring transmission history and may try to predicate this. As an alternative, the hidden receiver may learn about the particular sender by taking knowledge from upper layers. For example an open receiving UDP port at the transport layer that receives an A/V stream may indicate some further data packets to be received from the particular sender.

Fig. 5 shows a schematic signaling diagram of an exemplary message exchange for parallel transmission for a hidden node according to the second embodiment, which uses a singlecast mode to solicit the parallel transmission. According to Fig. 5, a data transmission from Sl to Rl is initiated by an

RTS/CTS handshaking. Then, a first (conventional) parallel transmission is initiated by an RTS/CTS handshaking between S2 and R2. Additionally, a second parallel transmission is initiated by an RTR singlecast message according to the second embodiment, by which R3 solicits the second parallel transmission specifically from S3 which directly responds with an ATS message.

In the third embodiment, the hidden receiver node R3 sends out a RTR (multicast) message to a specific group of senders which may have data packets for it. Here, in the RTR message, a multicast address corresponding to a group of senders is indicated or simply addresses of a group of potential senders are indicated. In turn, a group of senders may content to send to R3 as in the first embodiment or a group of senders may follow a predetermined order, which may simply be the order indicated in the RTR message, and the one with precedence and which also has data to send will send out data to R3. Fig. 6 shows a schematic signaling diagram of an exemplary parallel transmission for a hidden node according to the third embodiment, which uses a multicast mode to solicit the parallel transmission.

According to Fig. 6, a data transmission from Sl to Rl is initiated by an RTS/CTS handshaking. Then, a first (conventional) parallel transmission is initiated by an RTS/CTS handshaking between S2 and R2. Additionally, a second parallel transmission is initiated by an RTR multicast message according to the third embodiment, by which R3 solicits the second parallel transmission from a group of sender nodes and indicates a predetermined transmission order in the RTR message, e.g., by listing addresses of the addressed sender nodes. In the present example, potential sender nodes S5, S6, and S3 have been addressed in this order. Due to the specified transmission order, subsequent time slots following the RTR message can be allocated to the addressed sender nodes. In Fig. 6, the first slot after the RTR message is allocated to the first sender node S5 in the list, which should have sent an ATS here if it had data available for transmission. The following slot is allocated to the second sender node S6 in the list, which should have sent an ATS here if it had data available for transmission. Finally, the next or third slot is allocated to the third sender node S3 in the list, which indeed sends an ATS in its allocated slot, as it has data available for transmission as a second parallel transmission.

Fig. 7 shows a schematic block diagram of a software-based implementation of the proposed signaling mechanism. Here, the sender nodes and the receiver nodes each comprise a processing unit 210, which may be any processor or computer device with a control unit which performs control based on software routines of a control program stored in a memory 212. Program code instructions are fetched from the memory 212 and are loaded to the control unit of the processing unit 210 in order to perform the processing steps of the above functionalities described in connection with the respective Figs. 3 to 6. These processing steps may be performed on the basis of input data DI and may generate output data DO, wherein at the sender node the input data DI may correspond to received RTR message and the output data DO may correspond to or initiate the confirmation (e.g. ATS) or the parallel data transmission (DATA). On the other hand, at the receiver nodes, the input data may correspond to a received CTS request from the other transmission and the output data may correspond to or initiate a soliciting RTR message.

At this point, it is noted that the above functionalities described in connection with Figs. 3 to 6 may as well be implemented as discrete hardware or signal processing units. In summary, apparatuses, a system and a method of enabling a parallel second transmission in a wireless network have been described, wherein a hidden node status is determined at a first network node R3 in relation to a first transmission, and a request for reception that solicits the parallel second transmission to the network node R3 is issued if the hidden node status has been determined. The issued request for reception is received at a second network node S3 which transmits a data message or a confirmation message in order to initiate a desired parallel transmission.

It is noted that the present invention is not restricted to the above embodiments and can be applied as an enhancement to a broad range of protocol design proposals, including those that are being standardized in the IEEE 802.11 working group, or for any other network types, such as virtual cellular networks, where one-hop communication takes place between mobile stations and access points, or multi-hop ad hoc networks, including sensor networks. The proposed mechanism for enabling parallel transmissions can be applied widely to many proposed protocol solutions for multi-hop networks and for a VCN network concept as described for example in Paul Vranken, "Design and Evaluation of Cooperative Access

Points", TU/e Master's Thesis, Jan 2006, or in Luca Zappaterra, "Performance Evaluations in Wireless LAN Virtual Cellular Networks", Univ. of Ferrara, Oct 2006. Hence it is usefully for WLAN, WPAN (including ZigBee) that are mainly based on contention-based protocols. Moreover, the present invention is applicable to ongoing IEEE 802.11 standards, e.g., IEEE 802.15.5 standardization and their future extensions.

Finally but yet importantly, it is noted that the term "comprises" or

"comprising" when used in the specification including the claims is intended to specify the presence of stated features, means, steps or components, but does not exclude the presence or addition of one or more other features, means, steps, components or group thereof. Further, the word "a" or "an" preceding an element in a claim does not exclude the presence of a plurality of such elements. Moreover, any reference sign does not limit the scope of the claims.

Claims

CLAIMS:

1. An apparatus for enabling a parallel second transmission in a wireless network, said apparatus (R3) being adapted to determine an own hidden node status in relation to a first transmission, and to send a request for reception that solicits said parallel second transmission to itself if it has determined said own hidden node status.

2. An apparatus according to claim 1, wherein said apparatus (R3) is adapted to determine said own hidden node status if it detects a clearing response but no transmission request of said first transmission.

3. An apparatus according to claim 2, wherein said clearing response is a clear- to-send message and said transmission request is a request-to-send message.

4. An apparatus according to any one of the preceding claims, wherein said request for reception is a control message or a part of a message or a busy tone, adapted to solicit transmission of information in the next available transmission time period.

5. An apparatus according to any one of the preceding claims, wherein said request for reception is a broadcast message, a multicast message or a singlecast message.

6. An apparatus according to claim 5, wherein said apparatus (R3) is adapted to learn about a particular sender to be addressed by said singlecast or multicast message, by monitoring transmission history and predicting or by deriving information from upper protocol layers.

7. An apparatus according to claim 5, wherein said apparatus (R3) is adapted to indicate in said multicast message a transmission order for an addressed group of senders.

8. An apparatus for enabling a parallel transmission in a wireless network, said apparatus (S3) being adapted to receive a request for reception and to transmit a data message or a confirmation message in order to initiate a desired parallel transmission.

9. An apparatus according to claim 7, wherein said apparatus (S3) is adapted to content by a backoff process and to transmit said data message or said confirmation message if it wins the contention.

10. An apparatus according to claim 7, wherein said apparatus (S3) is adapted to transmit said data message or said confirmation message at a timing derived from a transmission order indicated in said request for reception.

11. A method of enabling a parallel second transmission in a wireless network, said method comprising determining a hidden node status at a network node (R3) in relation to a first transmission, and sending a request for reception that solicits said parallel second transmission to said network node (R3) if said hidden node status has been determined.

12. An method of enabling a parallel transmission in a wireless network, said method comprising receiving a request for reception at a network node (S3) and transmitting a data message or a confirmation message in order to initiate a desired parallel transmission.

13. A computer program product comprising code means for producing the steps of method claim 11 or 12 when run on a computer device.

14. A system for enabling parallel transmission in a wireless network, said system comprising at least one apparatus according to claim 1 and at least one apparatus according to claim 7.