GB2390778A

GB2390778A - Data transmission using rotating antenna

Info

Publication number: GB2390778A
Application number: GB0216008A
Authority: GB
Inventors: Matthew Murray Williamson; James Thomas Edward Mcdonnell; John Deryk Waters; Robert John Castle
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 2002-07-11
Filing date: 2002-07-11
Publication date: 2004-01-14
Also published as: GB0216008D0

Abstract

Apparatus 11 for transmitting data comprising a transmitter 12, a rotatable antenna 14, a device for rotating the antenna 15, 16, and a controller 13 for controlling the transmitter 12, the antenna 14 and the rotation of the antenna 14. Antenna 14 may be a directional antenna and the controller 13 should control the speed of rotation of the antenna 14 in dependence upon the amount of data to be transmitted, such that the time taken for the major lobe of the antenna 14 to pass over a spot should be greater than the time taken to transmit a chunk of data. Preferably, data receiving apparatus 19 should be provided, comprising of an antenna 21, a receiver 20, a memory 24 and display 25. The antenna rotating device 15, 16 may rotate the device backwards and forwards through a given arc, to communicate with each receiving apparatus 19. The controller 13 of the transmitter 12 should break large amounts of data to be transmitted into smaller chunks that can be received during a single burst, and add a check number to each of the chunks, the receiver 19 having a controller 23 to record and re-order and concatenate the data chunks. The transmitters 11 and receivers 19 may both actually be transceivers and communicate using wireless protocol 802.11b.. The controller 13 may control the MAC layer of a communications stack modifying it so that communication takes place only when the beam is pointing at the receiver 19, perhaps using a mapping algorithm based on antenna angle and user communication.

Description

( 1 Data Communications This invention relates to a method of, and

apparatus for, data communications, and in particular to a method of, and apparatus for, data communications having an increased 5 hot spot range.

In a communications system, a hot spot is an area of high bandwith connectivity, that is to say an area in which high bandwidth connections can be made. The signal strength at a given distance from a transmitter depends upon both the power of the transmitter 10 and the gain of its antenna. Increasing the gain of the antenna focusses the beam of radiation emitted by the transmitter into a narrower beam, thereby increasing the range of the transmitter. However, because of the narrower beam of a high gain antenna, the overall coverage of the transmitter does not increase. This is illustrated by comparing the signal areas (see Figures 1A, IB and 1C) for, respectively, an omni-directional 15 antenna, a directional antenna of lower gain, and a directional antenna of high gain.

Thus, the signal area of the omni-directional antenna la of Figure lA is the area of the circle 2a whose centre is that antenna, and whose radius depends upon the power of the associated transmitter. The signal area of the low power, directional antenna lb of Figure IB is the area of its major lobe 2b. Similarly, the signal area of the antenna Ic 20 of Figure IC is the area of its major lobe 2c. Clearly, the hot spot diameter (that is to say the diameter of the largest circle that will fit within the lobe) at any given range is smaller for the antenna lc than for the antenna lb, even though the antenna to has a greater range.

25 Of course, the range and coverage of a directional antenna such as the antennas lb and lc could be increased by increasing the power of the associated transmitter, but this is not always a practical solution to increasing range, for example because of cost considerations or because of interference with neighbouring transmitters. Moreover, government regulations restrict transmitter power.

The aim of the invention is to provide a communications transmitting apparatus having greater coverage for the same transmitter power.

The present invention provides apparatus for transmitting data, the apparatus 5 comprising a transmitter, a rotatable antenna, means for rotating the antenna, and control means for controlling the transmitter, the antenna, and the rotation of the antenna This apparatus is particularly suitable for transmitting bursty data.

Advantageously, the antenna is not omnidirectional, but is a directional antenna.

Preferably, the control means controls the speed of rotation of the antenna in dependence upon the amount of data to be transmitted by the transmitter.

In a preferred embodiment, the control means is such that the speed of rotation of the antenna is substantially less than BIGHT, where is the apex angle of the major lobe of the antenna, and T is the time taken to transmit a single chunk of data.

20 The invention also provides a system comprising data transmitting apparatus as defined above, and data receiving apparatus, the data receiving apparatus, comprising an antenna and a receiver.

This system may further comprise one or more other data receiving apparatus.

25 Preferably, the means for rotating the antenna are such that the antenna rotates backwards and forwards through a given arc. Where the system includes a plurality of data receiving apparatus, each such apparatus may be positioned within said arc.

This system is a broadcast system, in which case the control means is such that, when 30 the data to be transmitted cannot be transmitted in a single burst that can be received by the receiver, the data is split into chunks, each of which can be received by the receiver in a single burst, the control means further comprising means for adding a check

( 3 number to each chunk of data transmitted. In this case, the or each receiver may further comprise control means for recording all the chunks of received data, ordering the received chunks of data in the order of the check numbers, and concatenating the data.

5 Advantageously, the or each data or each receiving apparatus further comprises display means for displaying received data, and a memory for storing received data.

In a preferred embodiment, the transmitter forms part of a first transceiver, and the or each receiver forms part of a respective second transceiver. In this case, the system is a 10 data communications system.

In a preferred embodiment, the control means may be such as to control the media access control (MAC) layer of the communications stack. Advantageously, the first and second transceivers communicate using the wireless protocol 802.11b. Preferably, the 15 control means includes software which is such as to modify the MAC layer so that the first transceiver only attempts to send data to a given second transceiver when the beam sent by the rotatable antenna is pointing towards that second transceiver. More preferably, the control means includes an algorithm for carrying out said software control, the algorithm mapping the antenna angle and user communication, so that the 20 first transceiver only attempts to send data to said second transceiver when the antenna is pointing towards that second transceiver.

The invention further provides a method of transmitting data using a transmitter and a rotatable antenna, the method comprising controlling the speed of rotation of the 25 antenna in dependence upon the amount of data to be transmitted.

Preferably, the speed of rotation of the antenna is substantially less than 0/2T, where 0 is the apex angle of the major lobe of the antenna, and T is the time taken to transmit a single chunk of data.

The invention also provides a method of communicating data between a first transceiver and a second transceiver, a rotatable antenna being associated with the first

( 4 transceiver, the method comprising controlling the underlying layers of the network architecture to ensure that transmission is only attempted when the beam of the rotating antenna is pointing towards the second transceiver.

5 The invention will now be described in greater detail, by way of example, with reference to the Figures 2 to 4 of the drawings, in which: Figure 2 is a schematic representation of a broadcast system constructed in accordance with the invention; Figure 3 illustrates the coverage area of the antenna of the system shown in Figure 10 2; Figure 4 is a schematic representation of a communications system constructed in accordance with the invention; and Figure 5 is a diagram illustrating control of the system of Figure 4.

15 Referring to the drawings, Figure 2 shows an access point I I of a data communications system, the access point including a transmitter 12 and a control module 13. A directional antenna 14 associated with the access point 11 is mounted on a rotatable platform 15, the platform being in drivable engagement with an electric motor 16 which is controlled by the control module 13. The access point 11 is associated with a service 2 20 provider 17, the service provider being, for example, for supplying video data via a video streamer 18.

A user, indicated generally by the reference numeral 19, has a receiver 20 and an! associated antenna 21. The user 19 is provided with hardware 22 associated with the 25 receiver 20, the hardware including a control module 23, a memory 24 and a display 25.

The display 25 would be a visual display unit such as a computer monitor where the downloaded data is visual, or an audio sound reproduction apparatus where the downloaded data is audio.

30 The system of Figure 2 could also be used for broadcasting small chunks of data, in particular chunks of data that can be completely downloaded in one sweep. A typical example of data that can be so downloaded is an advert. The system of Figure 2 could

( 5 also be used for longer broadcasts, that is to say broadcasts of data which cannot be downloaded in a single sweep. This would involve broadcasting, along with each chunk of data that can be downloaded in a given sweep, a check number. The user would then record all the chunks of data, ordering them in the order of the check 5 numbers, and concatenating the data to get the entire download.

As shown in Figure 3, the major lobe of the antenna 14 has an angle at its apex, this angle determining the coverage area of the antenna. Clearly, as the antenna 14 rotates, the major lobe rotates with the antenna so that the receiver 20 has intermittent 10 reception. Typically, if the angle is r/3 (60 ), the receiver 20 will receive data from the access point I 1 for 1/6th of the time, this assuming that the antenna 14 is rotated at a constant speed. The video streamer would broadcast the data in chunks, each chunk of data being accompanied by a check number. The user 19 would record all the received chunks of data, and would order and concatenate the chunks as described 15 above. This download would be expected to take six times the download rate possible with continuous reception. To obtain real time video streaming, therefore, the video data would have to be transmitted at six times the normal rate.

Figure 4 shows a modification of the system of Figure 2, and so like reference numerals 20 will be used for like parts, and only the modifications will be described in detail. The basic difference between the two systems is that the system of Figure 4 is a communications system rather than a broadcast system. Thus, the access point I I has a transceiver 12' instead of the transmitter 12, and the user 19 has a transceiver 20' instead of the receiver 20. This communications system is used for the transmission of packet 25 switched data such as web pages. The user's transceiver 20' will be used to transmit a request for, say, a web page, and the access point I I will return the result of that request to the user. The transceivers 12' and 20' may use the wireless protocol 802.1 lb but other systems such as HiperLAN/2 could be used. Alternatively, for short range communications Bluetooth could be used.

The system of Figure 4 could be used for transmission of other non-timecritical bursty data such as messaging, audio data, web pages or any form of packet switched data. In

( 6 the latter case, the access point 11 would be connected directly to the Internet, rather than to a specific service provider. Alternatively, the access point 11 could be connected to a walled garden (that is to say a pre-determined part of the Internet to which the access point permits access to users - for example the web pages of a given 5 web site). In each case, received data is stored in the memory 24 under control of the control module 23, so that a complete downloaded stream of data can be stored for use later. Obviously, where the data relates to a web page, this will be displayed immediately on the display 25. Alternatively, for audio or video data, the control may be such that the received audio or video data is outputted in real time.

If the transceivers 12' and 20' use the wireless protocol 802.1 lb, any network protocol can be layered on top. The most common networking stack is the TCPAP suite. This layered architecture is used to provide guaranteed packet delivery and ordering (all the data will be communicated, and it will arrive in the correct order). Different layers in 15 the stack perform different functions. The physical layer is the physical medium used to transmit the data. The IP layer offers transmission of packets between machines with no guarantees of packet delivery. The TCP layer, on top of the IP layer adds guaranteed packet delivery and ordering.

20 The TCP/IP protocol stack is designed to work on reliable lower layers, and is highly sophisticated at controlling transmission in the presence of delays or congestion. The throughput is, however, greatly reduced when packets are dropped. A dropped packet is one that is not received.

25 The communications system as described above would be very vulnerable to dropped packets. The user 19 has intermittent reception, so packets would be dropped whenever the user is out of reception. In addition, when using the IEEE 801.1 lb protocol, when one user is transmitting, all the others cannot, and must wait to transmit. Moreover, if a user is moving, the onset of the on signal (that is to say the time when that user first 30 receives a signal within a given sweep) will change due to the movement of that user since reception of the previous sweep.

Consequently, if the communications system described above were to be used, the TCP/IP connection would probably be sustained, but the throughput would be very poor because of the number of packets dropped between sweeps.

S In order to overcome this difficulty, the access point 11 is provided with control software to alter the MAC layer of the protocol stack so that it only attempts to send data to the user 19 when the beam sent by the antenna 14 is pointing towards that user, thereby ensuring that packets are delayed rather than dropped. The algorithm for carrying out this software control would essentially keep a map of antenna angle (a) and 10 user communication, and only attempt to send data to the user l9 when the angle is correct. The antenna angle a is the angle the line joining the user 19 and the access point 11 makes with a fixed reference line.

The operation of this algorithm will now be described with reference to Figure 5. Thus, 15 the algorithm is such as to measure the angle of the antenna when the user l9 no longer lies within the antenna beam. The angle of the centre line of the beam transmitted by the transceiver 12' is, therefore, - B/2, where is the apex angle of the beam. The algorithm then controls transmission by the transceiver 12' to the transceiver 2Q' of the user 19, so that transmission begins when the angle a = ( - B/2) (# + 8/2), 20 where,B is a parameter which depends upon the expected mobility of the user 19' any non-linearity in the conical shape of the antenna beam, and the accuracy of the measurements of the angles and B. There are two obvious ways to measure when the user 19 no longer lies within the antenna beam, namely: 1) To observe the rate of dropped packets at the MAC layer, as this will rise sharply 25 when the user l9 no longa lies within the antenna beam.

2) To transmit an "are you there" packet to the user 19 regularly, and to listen for replies. A well known protocol, called ICMP, is available to do this. The transmitted packet is a "ping" packet. Usually these packets are constructed at the IP layer, and then passed as normal to the MAC layer for transmission. Since our system requires 30 modifications to the MAC layer to handle packets from higher layers, if these ping packets were treated as normal traffic, then it would not be possible to use them to detect when the user goes out of reception. The solution is to generate ping packets at

the MAC layer, and to transmit them to the user. The user's stack would process these packets as normal, and reply. The MAC layer of the access device can then intercept the reply packet, and use it to determine that the user is still in reception.

5 Transmission control by measuring the angle can be on a one-off basis. Alternatively, could be measured for each revolution of the antenna 14, or only occasionally (say once every ten revolutions). A running average of the value of US could also be used.

The access point 11 would be provided with buffers (not shown) to hold data for each 10 of the users such as the user 19. Buffered data for each user (say the user 19) would then be sent when that user is next in "alignment" untie the beam of the antenna 14.

Further buffering will occur until all the data for that user has been sent. The effective bandwidth for the user 19 will then be B/2'r of the maximum link bandwidth where the maximum link bandwidth is the bandwidth which is received by the user 19 when it is I S within the beam.

Although the software control transfers some of the wireless compensation functions normally carried out by the TCP layer to the MAC layer, the TCP layer still maintains overall control of packet transmission. Accordingly, care must be taken to ensure that 20 the delay in transmitting is not long enough for the TCP layer to time out. For a web page, for example, this time out period is about thirty seconds. If there was no delay in the system, there would be maximum throughput, and the throughput rate would drop towards zero as the delay increased to thirty seconds. Thus, although in theory successful transmission of a web page could be carried out if the delay is less than thirty 25 seconds, in practice substantial degradation will occur as the delay approaches thirty seconds. In order to ensure a good transfer, therefore, the delay should be much less than the theoretical thirty second limit (e.g delays of three to five seconds). In order to achieve this, the software control should be such as to vary the speed of rotation of the antenna 14, so that the gaps between being in reception are less than this.

The conununications system of Figure 4 could be used for any packet switched data. If it was used for video streaming, then unlike the broadcast case, there would be no need

( 9 to transmit check numbers, as control of the ordering and concatenating of received data chunks would be carried out by the TCP layer.

It will be apparent that modifications could be made to the systems described above.

5 For example, if the access point I 1 is to service a number of users in the same general locality, the antenna 14 could be programmed to reciprocate, in an arc centred on the antenna, the arc defining a sector within which all the users are positioned.

It would also be possible to modify the system described above for other applications.

I O Thus, the system could be used for time critical applications such as voice communication, by providing the access point with a plurality of wide angle antennas, that is to say antennas whose major lobes have large apex angles. Alternatively, or in addition, a high rotational speed could be used to decrease gaps between sweeps of the antenna or antennas.

Claims

1. Apparatus for transmitting data, the apparatus comprising a transmitter, a rotatable antenna, means for rotating the antenna, and control means for controlling the 5 transmitter, the antenna, and the rotation of the antenna.

2. Apparatus as claimed in claim 1, wherein the antenna is a directional antenna.

3. Apparatus as claimed in claim I or claim 2, wherein the control means controls 10 the speed of rotation of the antenna in dependence upon the amount of data to be transmitted by the transmitter.

4. Apparatus as claimed in claim 2 or claim 3, wherein the control means is such that the speed of rotation of the antenna is substantially less than 812nT, where is the 15 apex angle of the major lobe of the antenna, and T is the time taken to transmit a single chunk of data.

5. A system comprising data transmitting apparatus as claimed in any one of claims I to 4, and data receiving apparatus, the data receiving apparatus comprising an 20 antenna and a receiver.

6. A system as claimed in claim 5, further comprising one or more other data receiving apparatus.

25

7. A system as claimed in claim 5 or claim 6, wherein the means for rotating the antenna are such that the antenna rotates backwards and forwards through a given arc.

8. A system as claimed in claim 7 when appendant to claim 6, wherein each data receiving apparatus is positioned within said arc.

9. A system as claimed in any one of claims 5 to 8, wherein the control means is such that, when the data to be transmitted cannot be transmitted in a single burst that

( can be received by a given receiver, the data is split into chunks, each of which can be received by that receiver in a single burst, the control means further comprising means for adding a check number to each chunk of data transmitted.

5

1 O. A system as claimed in claim 9, wherein the or each receiver further comprises control means for recording all the chunks of received data, ordering the received chunks of data in the order of the check numbers, and concatenating the data.

11. A system as claimed in any one of claims 5 to 10, wherein the or each data 10 receiving apparatus further comprises display means for displaying received data.

12. A system as claimed in any one of claims 5 to 11, wherein the or each data receiving apparatus further comprises a memory for storing received data.

IS

13. A system as claimed in any one of claims 5 to 12, wherein the transmitter forms part of a first transceiver, and the or each receiver forms part of a respective second transceiver.

14. A system as claimed in claim 13, wherein the control means is such as to 20 control the MAC layer of the communications stack.

15. A system as claimed in claim 13 or claim 14, wherein the first and second transceivers communicate using the wireless protocol 802.1 l b.

25

16. A system as claimed in claim 14, wherein the control means includes software which is such as to modify the MAC layer so that the first transceiver only attempts to send data to a given second transceiver when the beam sent by the rotatable antenna is pointing towards that second transceiver.

30

17. A system as claimed in claim 16, wherein the control means includes an algorithm for carrying out said software control, the algorithm mapping the antenna

angle and user communication, so that the first transceiver only attempts to send data to said second transceiver when the antenna is pointing towards that second transceiver

18. A method of broadcasting data using a transmitter and a rotatable antenna, the 5 method comprising controlling the speed of rotation of the antenna in dependence upon the amount of data to be transmitted.

19. A method as claimed in claim 18, wherein the speed of rotation of the antenna is substantially less than 8/2,rT, where is the apex angle of the major lobe of the 10 antenna, and T is the time taken to transmit a single chunk of data.

20. A method of communicating data between a first transceiver and a second transceiver, a rotatable antenna being associated with the first transceiver, the method comprising controlling the underlying layers of the network architecture to ensure that 15 transmission is only attempted when the beam of the rotating antenna is pointing towards the second transceiver.