Improving the performance of a wireless CSMA-based MAC communication system using a spatially selective antenna Number:7,522,552 from the United States Patent and Trademark Office (PTO) owispatent

Title: Improving the performance of a wireless CSMA-based MAC communication system using a spatially selective antenna

Abstract: Described herein is a device to be used in a wireless communication system with CSMA-based MAC, comprising a mechanism for transmitting at least a first and second Request-to-Send (RTS) messages and at least first and second data packets on a transmission medium, and a spatially selective antenna. In one aspect, the device uses a distributed antenna that combines the antenna elements of several devices and the device observes the transmissions of other devices, analyzes the observed transmissions for transmission patterns and adapts its own transmissions to the detected transmission patterns. Also described is a corresponding system, method and computer program to be used in a wireless communication system.

Patent Number: 7,522,552 Issued on 04/21/2009 to Fein,   et al.

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Inventors:	Fein; Yaron (Rehovot, IL), Peleg; Yaron (Tel Aviv, IL)
Assignee:	Patents - Professional Solutions (PRO-PATS) Ltd (Herzeliya, IL)
Appl. No.:	10/985,837
Filed:	November 9, 2004

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Foreign Application Priority Data

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Nov 10, 2003 [EP]			03025719

Current U.S. Class:	370/328 ; 342/368; 370/455; 455/450; 455/63.4
Current International Class:	H04B 1/00 (20060101); H04L 12/413 (20060101); H04Q 3/00 (20060101)
Field of Search:	370/328,338,445,449,450 455/63.4,450,455 342/368,378

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References Cited [Referenced By]

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U.S. Patent Documents

5231634	July 1993	Giles et al.
6188913	February 2001	Fukagawa et al.
6363062	March 2002	Aaronson et al.
6404756	June 2002	Whitehill et al.
6611231	August 2003	Crilly, Jr. et al.
6970682	November 2005	Crilly et al.
7075902	July 2006	El Batt
7170873	January 2007	Cisar et al.
7224685	May 2007	Proctor, Jr.
2002/0158801	October 2002	Crilly et al.
2002/0172186	November 2002	Larsson
2002/0181426	December 2002	Sherman
2003/0120809	June 2003	Sastry et al.
2003/0227934	December 2003	White et al.
2004/0004951	January 2004	Zuniga et al.
2005/0063340	March 2005	Hoffmann et al.
2005/0078707	April 2005	Maltsev et al.
2005/0138199	June 2005	Li et al.

Other References

European Search Report, EP03025719, Aug. 31, 2004, 10 pages. cited by other . "IEEE Std 802.11, Part 11; Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", ANSI, IEEE Std 802.11, 1999, XP002282917. cited by other.

Primary Examiner: Backer; Firmin
Assistant Examiner: Juntima; Nittaya
Attorney, Agent or Firm: Sherman; Vladimir Professional Patent Solutions

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Claims

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The invention claimed is:

1. A device to be used in a wireless communication system with CSMA-based MAC, comprising: means for transmitting at least a first and second Request-to-Send (RTS) messages and at least a first and second data packets on a transmission medium, and a spatially selective antenna, wherein said second RTS message is transmitted after said first RTS message and before said transmission of said first data packet is finished, wherein said at least first and second data packet are transmitted on said transmission medium at least partially in parallel, and wherein said device uses said spatially selective antenna to direct a transmission null towards a first station for the transmission of said second RTS message, to direct a transmission peak towards said first station for the transmission of said first data packet, and to direct a transmission peak towards a second station for the transmission of said second data packet.

2. The device according to claim 1, wherein said device further uses said spatially selective antenna to direct a transmission null towards said second station for the transmission of said first data packet, and to direct a transmission null towards said first station for the transmission of said second data packet.

3. A device according to claim 1, wherein said first RTS message is transmitted with an omnidirectional antenna characteristic.

4. A device according to claim 1, further comprising means for receiving Clear-to-Send (CTS) messages that are transmitted by said stations in reply to said RTS messages and Acknowledgement (ACK) messages that are transmitted by said stations in reply to said data packets, wherein said device uses said spatially selective antenna to direct according reception peaks and reception nulls towards said transmitting stations so that at least two messages that are transmitted by at least two of said stations at least partially in parallel on said transmission medium, respectively, can be properly received by said device.

5. The device according to claim 4, further comprising means for estimating transmission parameters that are required to direct transmission and/or reception peaks or nulls towards said stations, wherein said transmission parameters are at least partially estimated from receive signals at said spatially selective antenna that originate at least partially from said CTS and/or ACK messages that are transmitted by said stations, or from RTS messages and data packets that have been transmitted by said stations before.

6. The device according to claim 1, further comprising: means for transmitting and/or receiving signals to and/or from stations that use spatially selective antennas.

7. The device according to claim 1, further comprising: means for determining the duration of an idle period of said transmission medium, wherein said device is allowed to start a transmission on said transmission medium only if the duration of said idle period is larger than a first interframe space (IFS), which is chosen smaller than a second IFS that has to be awaited in said wireless communication system by default in order to prioritize medium access of said device.

8. The device according to claim 1, wherein said spatially selective antenna is a sectored antenna with dynamically activated sectors, or a switched beam antenna, or an adaptive antenna array with controllable weights.

9. The device according to claim 1, wherein said spatially selective antenna is a distributed antenna consisting of the antenna elements of at least two devices of said wireless communication system, wherein said devices are access points or stations of said wireless communication system that are connected by means of a wired or wireless link so that signals transmitted from and/or received at the respective antenna elements can be jointly processed.

10. The device according to claim 1, further comprising: means for controlling a transmission power that is emitted by said device and/or by said stations, wherein said power control is performed in order to reduce an overall interference power while providing a Signal-to-Noise-and-Interference Ratio that is required for correct signal reception at both said device and said stations.

11. The device according to claim 1, further comprising: means for observing the transmissions of other devices within and/or outside said communication system, means for analyzing the observed transmissions in order to detect transmission patterns therein, and means for at least partially adapting the transmissions initiated by said device to said detected transmission patterns in order to reduce interference between said device and said other devices.

12. The device according to claim 1, wherein said device represents an access point, a station or a relay in a wireless communication system.

13. The device according to claim 1, wherein said wireless communication system is operated according to the IEEE 802.11 standard or a derivative thereof, including the IEEE 802.11 g standard.

14. The device according to claim 1, wherein said wireless communication system is a point-to-point or point-to-multipoint directional radio link system that replaces the transmission lines of an xDSL system.

15. A device to be used in a wireless communication system with CSMA-based MAC, comprising: means for transmitting one Request-to-Send (RTS) message and at least a first and second data packets on a transmission medium, and a spatially selective antenna, wherein said one RTS message contains information on at least a first and a second stations to which said at least first and second data packet are to be transmitted, respectively, wherein said device uses said spatially selective antenna to direct a transmission peak towards said first station for the transmission of said first data packet and a transmission peak towards said second station for the transmission of said second data packet, and wherein said transmission of said first data packet takes place at least partially in parallel to the transmission of said second data packet.

16. A device to be used in a wireless communication system with CSMA-based MAC, comprising: means for transmitting a first Request-to-Send (RTS) message and a data packet on a transmission medium, means for transmitting a NULL message, which indicates that said transmission medium is idle, and a spatially selective antenna, wherein said NULL message is transmitted after the transmission of said first RTS message and before the transmission of said data packet is finished, and wherein said device uses said spatially selective antenna to direct a transmission null towards a first station for said transmission of said NULL message and to direct a transmission peak towards said first station for said transmission of said data packet.

17. A method to be used in a wireless communication system with CSMA-based MAC, comprising the steps of: transmitting a first Request-to-Send (RTS) message on a transmission medium; transmitting a second RTS message on the transmission medium; and transmitting at least a first and second data packets on the transmission medium, wherein said at least first and second data packets are transmitted on said transmission medium at least partially in parallel, and wherein a spatially selective antenna is used to direct a transmission null towards a first station for the transmission of said second RTS message, to direct a transmission peak towards said first station for the transmission of said first data packet, and to direct a transmission peak towards a second station for the transmission of said second data packet.

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Description

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CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC .sctn. 119 from European Patent Application No. EP 03 025 719.0, filed Nov. 10, 2003, which is incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a device, a system, a method and a computer program to be used in wireless communication systems.

BACKGROUND OF THE INVENTION

Since the introduction of lightweight portable computers (laptops, notebooks), a great deal of attention has been focused on the development of wireless computer networks (Wireless Local Area Network, WLAN). Thanks to standardization in the field of LANs, it is comparatively easy to find systems that will still be upgradable even in a few years' time. Around 70% of all computers connected to networks are compliant with the Institute of Electrical and Electronics Engineers (IEEE) 802.3 (Ethernet) and IEEE 802.5 (Token Ring) standards. Connection is normally over a permanent wireline link. The problems that can occur are the surfacing of mechanical defects (corrosion) after a few years and violations of rules on radiated interference. It is difficult to adapt these networks to cope with changing office conditions. Mobile network nodes are not possible.

The obvious approach is to leave out the cable entirely. This idea is almost as old as the concept of the so-called ALOHA system, which used radio to connect terminals to their processing computers. The newer WLANs work with the most up-to-date radio technology. Data is encrypted and extensive error-protection mechanisms are available. Integrity of data is also guaranteed. Just like wireline LANs, WLANs can be divided into different architectures and performance categories. Many companies offer products for wireless point-to-point connections, but only very few build LANs for multipoint communication. Today wireless networks use spread-spectrum, narrowband microwave or infrared signals for transmission. Because of legal regulations, networks using spread-spectrum and narrowband microwave cannot be operated in most countries unless special authorization has been given. The only exemption of this is operation in a license exempt band, e.g., Industrial, Scientific and Medical (ISM) band, where under a set of given rules for channelization and emitted power operation of radio equipment is allowed, generally. An example of this is the 2.4 GHz band, where the first WLANs have been positioned.

A WLAN need not to be organized centrally, and instead may have a completely distributed architecture with a dynamic allocation of network and network node identifiers. In contrast to a wireline network, WLANs using the same radio channel cannot be separated from one another. Overlapping can occur. Another problem with radio channels is the range restriction. Mobile WLAN nodes and unfavourable propagation characteristics can cause the fragmentation of a network.

As a result of the channel characteristics and the applications for which WLANs are advantageous, WLANs have to be frequently operated in an overlapped fashion. If the radio range of some of the stations in network A should overlap with some of the stations in network B then these members share the transmission medium and its transmission capacity in the area where the overlapping occurs. An overlapping of networks produces two effects: First, the senders in the different WLANs use the same frequency band, thereby increasing the occurrences of interference. As a result, optimal use of the frequency band is no longer possible because not all the stations are able to receive from each other (hidden stations) and therefore can cause interference to each other. Second, a station receives data packets from several WLANs with different WLAN identifiers (LIDs). All received data packets are evaluated, and only those with its own LIDs are accepted. As a result, there is a decrease in the maximum possible data transmission capacity and consequently also the data transmission rate in this area.

In view of this problem, it is thus an object of the invention to improve the performance of wireless communication systems by enhancing the coexistence with other wireless communication systems.

As an example for a WLAN standard, IEEE 802.11 has been designed for use in Industrial, Scientific and Medical (ISM) bands. The Federal Communications Commission (FCC) in the USA prescribed maximum power levels only, band edge interference, and the requirement to use spread spectrum in order to minimize interference with already existing communication systems. It designated the frequency bands 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz to these ISM bands.

IEEE 802.11 has defined two Physical Layer (PHY) standards for the 2.4 GHz ISM band: one using the Frequency Hopping Spread Spectrum (FHSS) technique, and one using the Direct Sequence Spread Spectrum (DSSS) technique. An alternative is the specification of an Infrared (IR) physical layer. IEEE 802.11 stations using any-of the three technologies operate at a data rate of 1 Mbit/s (optionally 2 Mbit/s) and at 11 Mbit/s according to standard IEEE 802.11 b. Recently work has been finished on the specification of a 20 Mbit/s PHY at 5 GHz. Target frequencies are those opened by the FCC in 1997 for Unlicensed Information Infrastructure Networks (U-NII).

The protocols of IEEE 802.11 are specified for slowly moving stations, usually indoors but not limited to this, communicating among each other (Ad Hoc Mode) or with stations beyond their direct communication range with the support of an infrastructure (Infrastructure Mode). The communication is packet-oriented.

The IEEE 802.11 MAC protocol provides two types of service: asynchronous and contention-free. The asynchronous type of service is provided by the Distributed Coordination Function (DCF), which implements as the basic access method the Carrier Sense Multiple Access (CSMA) with Collision Avoidance (CA) protocol. The contention-free type of service is provided by the Point Coordination Function (PCF), which basically implements a polling access method. Unlike the DCF, the implementation of the PCF is not mandatory. Furthermore, the PCF itself relies on the asynchronous service provided by the DCF.

The time between two frames is called Interframe Space (IFS). In order to determine whether the medium is free, a station has to use the carrier sense function for a specified IFS. The standard specifies four different IFSs, which represent three different priority levels for the channel-access. The shorter the IFS, the higher the priority. The IFSs are specified as time gaps on the medium and are independent of the channel data rate. Owing to the different characteristics of the different PHY specifications, the IFS time durations are specific for each PHY.

According to the DCF, a station must sense the medium before initiating the transmission of a packet. This mechanism is schematically depicted in FIG. 1.

FIG. 1 schematically depicts the transmissions of frames 101a. . . 101e of five stations 102a . . . 102e, where time proceeds in each row of FIG. 1 from left to right. Because the frames 101a . . . 101e have to be transmitted on the same shared transmission medium, a CSMA/CA protocol is obeyed by each of said stations 102a . . . 102e. The first row of FIG. 1 shows the transmission of a frame 101a by station 102a. As indicated by the vertical arrows 103b . . . 103d in the second, third and fourth row of FIG. 1, data packets arrive at stations 102b, 102c and 102d, so that these stations require to access said shared transmission medium to transmit the arriving data packets. Said three stations 102b, 102c and 102d now start sensing the medium. If the transmission medium is sensed as being busy, the transmission of stations 102b, 102c and 102d is deferred and a backoff process is started for each station, wherein said backoff process is only started after a DCF IFS (DIFS) period 104-1 during which the medium is determined to be idle for the duration of the DIFS. Specifically, each station computes a random number uniformly distributed between zero and a maximum called Contention Window (CW). The random number is multiplied by the slot time, resulting in the backoff interval used to set the backoff timer. In FIG. 1, said backoff intervals 105b . . . 105e are schematically depicted, wherein already elapsed backoff time slots are depicted as white boxes and remaining backoff time slots are depicted as gray boxes. For instance, in the third row of FIG. 1, the backoff interval of station 102c only consists of four time slots, and after the duration of said time slots, station 102c starts the transmission of a frame 101c.

The backoff timers 105b . . . 105e are decremented only when the medium is idle, whereas they are frozen when another station is transmitting. This can be spotted in the second row of FIG. 1. Station 102b has computed a backoff interval 105b consisting of nine time slots. After four elapsed time slots, the backoff timer is frozen due to the transmission of said frame 101c by said station 102c.

Each time the medium becomes idle, the station waits for a DIFS and then periodically decrements the backoff timer. The backoff timer of station 102b thus is only decremented again after the next DIFS period 104-2, where two time slots of the backoff interval 105b of station 102b elapse before the backoff timer is frozen again due to a frame 101d transmitted by station 102d.

As indicated by the last row of FIG. 1, a data packet 103e arrives at station 102e during the transmission of frame 101c, so that station 102e has to start a backoff process as well. As can be seen by comparing the backoff intervals 105b of station 102b and 105e of station 102e after the next DIFS period 104-2, both backoff intervals 105b and 105e have the same length, so that, after the next DIFS period 104-3, three time slots elapse until both stations 102b and 102e concurrently start the transmission of frames 101b and 101e, respectively. If two or more stations start transmission simultaneously, a collision 106 occurs.

Unlike wired networks (e.g. with CSMA/CD in IEEE 802.3), in a wireless environment Collision Detection (CD) is not possible. Hence, a positive acknowledgment ACK 207 (see FIG. 2) is used to notify the sending station 202 that the transmitted frame 205 has been successfully received. The transmission of the ACK 207 is initiated at a time interval equal to the Short IFS (SIFS) 206-3 after the end of the reception of the previous frame 205.

If the acknowledgment is not received in a specified time interval, the station assumes that the transmitted frame was not successfully received, and hence schedules a retransmission and enters the backoff process again. However, to reduce the probability of collisions, after each unsuccessful transmission attempt the Contention Window is doubled until a predefined maximum (CWmax) is reached. After a successful transmission, the Contention Window is reset to CW.sub.mi.sub.n.

After each frame transmission, a station must execute a new backoff process. Therefore at least one backoff is in between two transmissions of the same station.

In view of the above-mentioned problems, it is thus a further object of the present invention to improve wireless communications systems by reducing the number of collisions.

In radio systems based on medium sensing, a phenomenon known as the hidden-station problem may occur. This problem arises when a station is able to successfully receive frames from two different stations but the two stations cannot receive signals from each other. In this case a station may sense the medium as being idle even if the other one is transmitting. This results-in a collision at the receiving station.

To deal with the hidden-station problem, the IEEE 802.11 MAC protocol includes a mechanism based on the exchange of two short control frames, as depicted in FIG. 2: a Request-to-Send (RTS) frame 201 that is sent by a potential transmitter 202 to the receiver 203 and a Clear-to-Send (CTS) frame 204 that is sent by the receiver 203 in response to the received RTS frame 201. Said CTS frame 204 can be sent by the receiver 203 after waiting for a SIFS 206-1. If the CTS frame 204 is not received within a predefined time interval, the RTS frame 201 is retransmitted by executing the backoff algorithm described above. After a successful exchange of the RTS and CTS frames, the data frame 205 can be sent by the transmitter 202 after waiting for a SIFS 206-2. The implementation of the RTS packet 201 is optional, whereas all stations must be able to answer to a RTS frame 201 with the belonging CTS frame 204.

The RTS 201 and CTS 204 frames include a duration field that specifies the time interval necessary to completely transmit the data frame and the related acknowledgment (ACK) 207. This information is used by stations 208, 209 that can hear either the transmitter 202 or the receiver 203 to update their Network Allocation Vector (NAV) 210, 211, a timer that, unlike the backoff timer, is continuously decremented irrespective of the status of the medium. Since stations 208, 209 that can hear either the transmitter 202 or the receiver 203 refrain from transmitting until their NAV 210, 211 has expired, the probability of a collision occurring because of a hidden station 208, 209 is reduced. Of course, the drawback of using the RTS/CTS mechanism is an increased overhead, which may be significant for short data frames.

Furthermore, the RTS/CTS mechanism can be regarded as a way to improve the MAC protocol performance. In fact, when the mechanism is enabled, collisions can obviously occur only during the transmission of the RTS frame 201. Since the RTS frame 201 is usually much shorter than the data frame 205, the waste of bandwidth and time due to the collision is reduced.

However, when using the RTS/CTS mechanism, not only the hidden stations, but all stations in the coverage area of the transmitter 202 and the receiver 203 receiving said RTS frame 201 or CTS frame 204 update their NAVs 210, 211 and refrain from initiating further data transfers. This may result in a waste of bandwidth in particular if stations with spatially selective antennas are deployed in the WLAN system, because the spatially selective transmission and reception of frames naturally requires much less stations to be calmed down when trying to mitigate the hidden station problem.

Prior art document U.S. Pat. No. 6,611,231 B2 discloses methods, apparatuses and systems for use in a wireless routing network. One apparatus, for example, includes an adaptive antenna that is configurable to receive a transmission signal from a transmitter and in response, transmit corresponding multi-beam electromagnetic signals exhibiting a plurality of selectively placed transmission peaks and transmission nulls within a far field region of a coverage area. U.S. Pat. No. 6,611,231 B2 discloses to determine if there is a potential for interference with neighboring nodes prior to transmitting a CTS message, to transmit said CTS message to a targeted node using a narrow beam, if there is no significant potential for interfering with said neighboring nodes, and to transmit said CTS message to said targeted node and one or more of said neighboring modes using one or more beams if there is significant potential for interfering with said neighboring nodes.

This approach requires knowledge on the spatial propagation channels towards said neighboring nodes prior to transmission. Furthermore, said neighboring stations thus are intentionally calmed down by said CTS message to reduce interference, which may result in a waste of bandwidth. U.S. Pat. No. 6,611,231 B2 further discloses to have a pair or more of spatially separated wireless routing devices on a location or node. For example, a separation of about 20 wavelengths may be provided between the antenna arrays. The routing devices can allow a higher percentage of receive time using one of the antenna arrays, and also provide the potential of simultaneous transmit streams from the same approximate site.

In view of the above-mentioned problems, it is thus a further object of the present invention to improve the performance of wireless communication systems by enhancing the use of the available transmission bandwidth.

SUMMARY OF THE INVENTION

It is thus a general object of the present invention to provide a device, a system, a method and a computer program that improves the performance of a wireless communication system.

It is proposed that a device to be used in a wireless communication system with CSMA-based MAC comprises means for transmitting at least a first and a second RTS message and at least a first and a second data packet on a transmission medium, and a spatially selective antenna, wherein said second RTS message is transmitted after said first RTS message and before said transmission of said first data packet is finished, wherein said at least first and second data packet are transmitted on said transmission medium at least partially in parallel, and wherein said device is adapted to use said spatially selective antenna to direct a transmission null towards a first station for the transmission of said second RTS message, to direct a transmission peak towards said first station for the transmission of said first data packet, and to direct a transmission peak towards a second station for the transmission of said second data packet.

Said device may for instance be an access point of a wireless communication system serving a plurality of stations, wherein Medium Access Control (MAC) for said device and said stations that compete for the jointly used transmission medium is performed by a Carrier Sense Multiple Access (CSMA) technique, wherein as well Collision Avoidance (CA) may be performed. In a packet-oriented CSMA system, said device and said stations are only allowed to transmit messages and data packets if the shared transmission medium is sensed to be idle, wherein said transmission medium is understood to be defined by the time, carrier frequency (or sub-carrier frequency in an Orthogonal Frequency Division Multiplex (OFDM) system), spreading code and polarization state of the transmission.

If a first data packet bound for a first station arrives at said device, for instance via the core network said device is connected to, the device transmits a Request to Send (RTS) message which contains an identifier of said first station and a variable that indicates the duration a transfer of said first data packet on said transmission medium would require. This duration may include the time required by said first station to acknowledge the receipt of said first data packet. Said RTS message may be transmitted by said device with an omnidirectional antenna pattern, for instance by a single omnidirectional antenna, or by controlling the spatially selective antenna to create an approximately omnidirectional pattern. The RTS message may then be received by several stations or devices within the coverage area of said access point. With said identifier of said first station being contained in said RTS message, said first station may recognize said identifier and may transmit a Clear to Send (CTS) message. The procedure of transmitting a RTS message in order to receive a CTS message as answer is denoted as "polling". The transmission of the CTS message may take place with a spatially selective antenna or with an omnidirectional antenna. Said CTS message may contain said identifier of said first station and said variable that indicates the duration of said transfer, as well. Other stations or devices receiving said RTS message may recognize said identifier contained in said RTS message and refrain from transmitting messages or data packets for a period of time that corresponds to the duration as indicated by said variable that may be contained in said RTS message. These devices and stations are thus "calmed down" for the transmission of said first data packet and an optional Acknowledgment (ACK) message that may be transmitted after the transmission of said first data packet by said first station. In this way, collisions during the data exchange between said device and said first station with messages or data packets transmitted by other stations on said transmission medium during the transmission of said CTS message (transmitted by said first station on said transmission medium) or first said data packet (transmitted by said device on said transmission medium) may be avoided. When receiving the CTS message, said device may recognize the identifier that may be contained in said CTS message. Said device may deploy its spatially selective antenna to estimate transmission parameters of said first station, for instance the spatial signature or the Direction of Arrival (DOA) of said first station. Said estimated transmission parameters, together with said identification of the first station, may be stored in a routing table for later use. The CTS message may also be received by other stations or devices in the coverage area of said first station, which may recognize the identifier and variable indicating the duration of said transfer and refrain from transmitting messages or data packets for a period of time that corresponds to the duration as indicated by said variable that may be contained in said CTS message, thus avoiding collisions during the data exchange between said device and said first station with stations or devices in the coverage area of said first device.

Upon reception of said CTS message, said device may start with the transmission of said first data packet, wherein said device uses its spatially selective antenna, for instance an adaptive antenna array consisting of several antenna elements that may be controlled in base band depending on the desired antenna array characteristic, to direct a transmission peak towards said first station. Directing a transmission peak towards a station is to be understood as a technique of distributing as much transmission power as possible on one or more propagation paths of a spatial channel between said device with said spatially selective antenna and said first station. Thus the antenna characteristic does not necessarily have to take its maximum in the direction of said first station, as seen from said spatially selective antenna at said device, in particular if the Line-of-Sight (LOS) between said device and said first station is blocked or otherwise heavily attenuated. Distributing power on the respective propagation paths is generally considered as forming "beams" towards the angles under which said propagation paths arrive or depart from said spatially selective antenna (in azimuth and/or elevation). In the sequel, the term "beam" will also be used in a more general sense to describe the antenna characteristic of said spatially selective antenna used to transmit a data packet to a station, wherein said antenna characteristic may of course comprise a plurality of beams, each for power distribution on a single propagation path in the spatial channel between said device and a station.

In directing a transmission peak towards a station, the spatial channels between said device and further stations, to which signals shall be transmitted by said spatially selective antenna at least partially in parallel to the transmission of said first data packet bound for said first station, might as well be taken into account. In particular if several data packets have to be transmitted at least partially in parallel, i.e. if spatial multiplexing takes place, it is not always possible to distribute transmission power on all the propagation paths between said device and said first station, because propagation paths between further stations and said device may overlap with the propagation paths of said first device. Overlapping paths then should not be used at all, or a different set of stations the data packets of which are to be spatially multiplexed should be chosen by applying spatial scheduling techniques, which may be based on the information contained in said routing table.

Directing a transmission peak towards said first station with a spatially selective antenna when transmitting said first data packet ensures that a substantial part of the transmission energy is concentrated towards said first station, so that, for a constant overall transmission power, the Signal-to-Noise-and-Interference Ratio (SNIR) of the signals received at said first station is considerably increased as compared to the transmission with an omnidirectional antenna. Furthermore, less interference is caused at devices or stations which are not positioned in the elongation of the beams that may be formed by the spatially selective antenna to illuminate the propagation paths in the spatial channel between said device and said first station. Increasing the SNIR at said first station and reducing the interference caused at other stations of the wireless communication system (and also neighboring communication systems operating in the same frequency range) is only one aspect of the device according to the present invention.

The increased SNIR may equally well be exploited in a way that less error protection may be admitted to the transmitted signals, thus increasing the data rate of the signals transmitted in said first data packet, or higher modulation alphabets may be used, which, for constant symbol rate, as well increases the data rate. For instance, instead of a PHY mode with Binary Phase Shift Keying (BPSK), a PHY mode with Quaternary Phase Shift Keying (QPSK) could be applied as modulation technique. When QPSK is used instead of BPSK, the duration of a transmission of a data packet is effectively halved, i.e., the same amount of data can be transmitted in half of the time. When no further data is sent in the remaining half of the original BPSK transmission time, it is easily seen that the interference power that is imposed on other stations in the elongation of said beams formed by said spatially selective antenna has been reduced in the time domain. Alternatively, the remaining half of the original BPSK transmission time can be used to transmit a further QPSK-modulated data packet, thus doubling the throughput. The spatially selective antenna thus can be deployed to either decrease the interference or to increase the throughput in the wireless communication system, so that the use of the available transmission bandwidth is enhanced.

In state-of-the-art wireless communication systems, said device uses a timer that indicates when said transmission medium will no longer be busy, i.e. occupied by the transmission of said first data packet and an optional ACK message transmitted by said first station. In an IEEE 802.11 system, this timer may be identified as the Network Allocation Vector (NAV) of an access point. According to the present invention, said device includes information on said NAV in said RTS messages to calm down the stations that receive said RTS message and the CTS message transmitted by said first station that contains the same information, but does not observe the NAV himself, so that further data packet transmissions can take place concurrently to an already established data packet transmission between said device and said first station. The prerequisite for this concurrent transmission of data packets, which is controlled and performed by said device, is a spatially selective antenna, that allows for spatial multiplexing of several data packets on the same transmission medium. It is advantageous that the stations to which data packets are transmitted to in parallel are spatially separable, i.e. the respective spatial channels between said device and said stations have to be approximately orthogonal to an extent that allows proper signal reception at each station without receiving too much of the data packets that are intended to be received at the remaining stations.

By transmitting a second RTS message, which contains an identifier of a second station and a variable indicating the duration a transfer of a second data packet on said transmission medium would require, said device prepares the transmission of a second data packet, so that the transmission of said first data packet and said second data packet may take place at least partially in parallel on the same transmission medium. Both data packets thus can be transmitted at the same time, on the same carrier frequency (or frequency sub-carrier in an Orthogonal Frequency Division Multiplex (OFDM) system), with the same spreading code, and with the same polarization. In order not to disturb the already set up transmission of said first data packet between said device and said first station, said second RTS message is transmitted by said spatially selective antenna with a transmission null directed towards said first station. Directing a transmission null towards a station is understood here in a way that as few transmission power as possible is to be distributed on the propagation paths in the spatial channel between said device and said station. When several transmission peaks or nulls have to be formed in parallel due to spatial multiplexing of more than one station, it might not always be possible to force the power that is received at a station that is to be nulled exactly to zero. It often is sufficient to reduce the amount of transmission power that is received at a station that is to be nulled below the noise or interference level.

Whereas the first station now does not receive said second RTS message, further stations such as a second station may do. Based on said identifier and said variable indicating the duration of a transfer of said second data packet, said second station may notice that it is no longer required to refrain from transmitting, because it has been directly polled by said device via said second RTS message. The second station may thus respond to said second RTS message with a CTS message, into which said identification and said variable may have been copied. Upon reception of said CTS message, said device may start the transmission of a second data packet, wherein said spatially selective antenna is used to direct a transmission peak towards said second station.

This procedure may be repeated with a third RTS and a third data packet being transmitted to a third station, respectively. The polling of stations by sending RTS messages and waiting for CTS messages before transmission of data packets by said device is optional. If the transmission parameters for the stations to which data packets shall be transmitted are known, possibly from a routing table, said device may directly start with the transmission of data packets to stations.

When transmitting said second data packet to said second station concurrently to the transmission of said first data packet to said first station, said device may advantageously direct a transmission null towards said first station to reduce interference. Alternatively, spatial scheduling techniques based on the transmission parameters of said first and second station, e.g. the DOAs of both stations, are applied to decide if concurrent data transmission to both stations is possible without explicitly having to form transmission nulls towards the respective other station.

In the above-described scenario with two data packets being transmitted at least partially in parallel, it may as well be possible to wait with the transmission of the first data packet until the CTS from the second station is received, and to start the transmission of the first and second, data packet jointly. This has the advantage that, if the transmission parameters of the second station are estimated from the received CTS message that is transmitted by said second station, the transmission parameters of said first and second station can be considered when transmitting said first and second data packets, i.e. a peak is directed towards said first station and a null towards said second station for the beam that is formed for the transmission of said first data packet, and a peak is directed towards said second station and a null is directed to said first station for the beam that is formed for the transmission of said second data packet.

According to this approach, by sending said first RTS message and receiving said first CTS message, said access point has successfully reserved the transmission medium that is shared among a plurality of stations and devices for the duration that is specified in the first RTS message and may have been copied into the first CTS message by said first station. In effect, said access point now uses the reserved transmission period to poll a further station with a second RTS message and to await the reception of a second CTS message originating from said polled second station. The remainder of the reserved transmission period then is used for the transmission of data packets to both said first station and said second station under the use of spatial multiplex. Performing a second polling procedure for the second station is possible because the RTS/CTS messages are much shorter than the data packets, and because the deployment of a spatially selective antenna allows to increase the SNIR at each receiving station, so that a higher PHY mode can be used (for instance, QPSK-modulation instead of BPSK-modulation) and, correspondingly, less time for the transmission of the same amount of data is required.

However, if the transmission of said first data packet has already begun when the CTS message transmitted from said second station is received at said device, it might as well be possible to re-shape the beam that is used for the transmission of said first data packet so that the transmission parameters estimated from said received CTS message of said second station can be considered in said beam as well.

In transmitting said CTS message, said second station may advantageously use a spatially selective antenna to direct a transmission peak towards said device as well, so that an already set up data exchange between said device and said first station is disturbed only in a minimum way. The transmission parameters required for directing a transmission peak towards said device by said second station may be estimated from said received second RTS message.

It may be possible that there exist stations with a spatially selective antenna and stations with an omnidirectional antenna in the same wireless communication system. It may further be possible that for the initiation of data transfer, said device uses RTS/CTS-polling for said first station, and no RTS/CTS polling for the second station, or vice versa.

This approach thus represents an effective way of enhancing the use of the available transmission bandwidth in a wireless communication system.

It is further proposed that a device to be used in a wireless communication system with CSMA-based MAC, comprises means for transmitting one RTS message and at least a first and a second data packet on a transmission medium, and a spatially selective antenna, wherein said one RTS message contains information on at least a first and a second station to which said at least first and second data packet are to be transmitted, respectively,

wherein said device is adapted to use said spatially selective antenna to direct a transmission peak towards said first station and a transmission null towards said second station for the transmission of said first data packet and a transmission peak towards said second station and a transmission null towards said first station for the transmission of said second data packet, and

wherein said transmission of said first data packet takes place at least partially in parallel to the transmission of said second data packet.

Said device, for instance an access point of said wireless communication system that serves a plurality of stations, polls said first and second station with one single RTS message that contains identifiers of the first and second station and variables that indicate how long the transmission of said first and second data packet (optionally including the duration of an ACK message transmitted by said first and second stations in reply to said first and second data packets, respectively) will occupy the jointly used transmission medium. In an IEEE 802.11 system, this may for instance be achieved by modifying the single-cast/multi-cast addressing. Said polling is advantageously performed with an omnidirectional antenna characteristic, so that all stations within the coverage area of said device are informed of the future data transmission and refrain from transmitting messages and data packets by themselves. The first and second station, upon reception of said one RTS message, may recognize said identifiers, and copy their according identifier and variable into first and second CTS messages, which are transmitted by said first and second station, respectively. Upon reception of said CTS messages, said device may start the transmission of a first data packet to said first station, i.e. by directing a transmission peak towards said first station, and the transmission of a second data packet to said second station, i.e. by directing a transmission peak towards said second station, so that both data packets are spatially multiplexed on the jointly used transmission medium. It is advantageous to further transmit transmission nulls towards the respective other station during the transmission of said data packets. The transmission parameters of said first and second station that are required to direct the transmission peaks may be estimated by said device during the reception of said respective CTS messages, or be known in advance. Other devices and stations receiving said CTS messages notice that data transmission will take place and do not transmit packets or messages at least during the time that is indicated by said variable in said CTS messages. The above-described technique can be performed for three or more data packets that are at least partially transmitted in parallel on said jointly used transmission medium to respective stations, as well, by including further identifiers and variables into said one RTS message.

Apparently, this approach allows for the concurrent transmission of several data packets on the shared transmission medium and thus allows for an enhanced use of the available transmission bandwidth.

According to an embodiment of a device according to the present invention, the device is further adapted to use said spatially selective antenna to direct a transmission null towards said second station for the transmission of said first data packet, and to direct a transmission null towards said first station for the transmission of said second data packet.

Directing a transmission peak towards the station a data packet is to be transmitted to and transmission nulls towards all other known stations to which data packets are transmitted concurrently vastly reduces the interference between the concurrent data transmissions and further increases the SNIR at each station that is receiving a data packet.

It is further proposed that a device to be used in a wireless communication system with CSMA-based MAC, comprises means for transmitting a first RTS message and a data packet on a transmission medium, means for transmitting a NULL message, which indicates that said transmission medium is idle, and a spatially selective antenna, wherein said NULL message is transmitted after the transmission of said first RTS message and before the transmission of said data packet is finished, and

wherein said device is adapted to use said spatially selective antenna to direct a transmission null towards a first station for said transmission of said NULL message and to direct a transmission peak towards said first station for said transmission of said data packet.

If said device is an access point of a wireless communication system, said device uses said first RTS message, which includes an identifier of a first station and a variable indicating the duration of the transmission of a data packet, in order to poll a first station. Said RTS message is advantageously transmitted with an omnidirectional antenna characteristic. Upon reception, said first station may copy said identifier and said variable into a CTS message and transmit said CTS message. In this way, other stations and devices in the coverage area of said device and said first station recognize how long the jointly used transmission medium will be busy with the transmission of said data packet between said device and said first station and refrain from transmitting messages and data packets during the time indicated by said variable. Upon reception of said CTS message, in a state-of-the-art system said device would start the transmission of said data packet, and the medium would be blocked by said transmission. However, by transmitting a NULL message, wherein said spatially selective antenna is used to direct a transmission null towards said first station for the transmission of said NULL message, the stations and devices that receive said NULL message are informed that the transmission medium is idle. Said first station is, of course, excluded from said resetting operation by directing a transmission null towards said first station for the transmission of said NULL message. For the transmission of said data packet to said first station, said device directs a transmission peak towards said first station, in order to increase the SNIR at said first station and in order to cause as few interference power at other devices and stations in said wireless communication system (and neighboring communication systems).

By the spatially selective transmission of said NULL message, excluding said first station, all devices and stations that received said RTS or CTS messages and said NULL message are now enabled to transmit messages and data packets on their own, so that parallel transmission of data packets can take place and the transmission medium is more effectively used. Collisions between said additional transmissions and said transmission of said data packet between said device and said first data packet are mitigated by the transmission peak directed towards said first station by said device to enhance its SNIR, and may be further mitigated if the devices or stations that start parallel data or message transmissions use spatially selective antennas as well and direct transmission nulls towards said first station for their transmissions. The transmission parameters of said first station may be estimated by said devices and stations during the reception of said CTS message that is transmitted by said first station, or may be known in advance.

Said NULL message may be transmitted before the transmission of said data packet, or in parallel to the transmission of said data packet, so that a longer time duration for the transmission of the data packet is available, which may be used to increase the packet length or data rate of the data packet.

This approach thus represents an effective way of reducing the duration of periods during which devices and stations are calmed down by a spatially selective antenna and thus enhances the use of the available transmission bandwidth in a wireless communication system.

It is further proposed that a device to be used in a wireless communication system with CSMA-based MAC comprises means for transmitting a first RTS message and/or data packet on a transmission medium, means for setting a timer that indicates when said transmission medium will no longer be busy, and a spatially selective antenna, wherein said timer is set to its minimum value,

wherein said device is adapted to use said spatially selective antenna to direct a transmission peak towards a first station for said transmission of said data packet, and wherein said first data packet contains information on the duration of further data packet transmissions for said first station.

Said timer is set to its minimum value, so that only a short data packet can be transmitted. By sending the RTS message, advantageously with an omnidirectional antenna characteristic, all devices and stations in the coverage area of said device are calmed down, but only for a minimum duration. After the reception of a CTS message from the station that has been polled by said device with said first RTS message, said device may start the transmission of said first data packet, wherein a spatially selective antenna is used to direct a transmission peak towards said first station. The SNIR is thus increased at said first station, and less interference power is caused at other stations and devices that are not positioned in the elongation of the beam that is radiated by said spatially selective antenna. Said first data packet, which is received by said first station, may advantageously contain an identifier of said first station and a variable indicating the duration of transfers of further data packets that said device intends to transmit to said first station. Only the devices and stations, including said first station, that are positioned in the elongation of the beam that is radiated by said spatially selective antenna during the transmission of said first packet receive this information, and only said devices and stations excluding said first station are calmed down by this information, i.e. refrain from further message or data packet transmission during the period as indicated in said first data packet. Said first station, in contrast, recognizes that said device uses said first data packet to poll said first station, and may respond to said polling with an ACK or CTS message, so that said device may transmit said further data packets to said first station, wherein advantageously a transmission peak is directed towards said first station to keep the overall interference power low.

Choosing the minimum possible timer value in said first RTS message thus calms the devices and stations in the coverage area of said device only down for the minimum possible time, so that further data transmission can be initiated and performed during the actual data transmission between said device and said first station. Said data transmission between said device and said first station can be extended by information contained in said first data packet, which calms down only those stations that are positioned in the elongation of the beam that is radiated by said device for the transmission of said first data packet.

It may be advantageous that the transmission of an acknowledgment can be postponed in the present approach, so that no acknowledgment has to be transmitted by the first station at least after the reception of the first data packet. The time interval after which said device awaits an acknowledgment from the first station may be increased. It thus can be avoided that the first station transmits an acknowledgment in omnidirectional mode and causes interference to other stations or devices.

This approach thus represents an effective way of reducing both the number of stations and devices that are calmed down and the duration of the respective calming down periods.

According to an embodiment of a device according to the present invention, said first RTS message is transmitted with an omnidirectional antenna characteristic.

Said first RTS message is intended to calm down the stations and devices that are in the coverage area of said first device, which is of particular importance for the collision-free exchange of further messages and data packets between said device and said first station. If this data