GB2445390A

GB2445390A - Frequency Hopping Communication Systems

Info

Publication number: GB2445390A
Application number: GB0621152A
Authority: GB
Inventors: Graham Tricker
Original assignee: Hypertag Ltd
Current assignee: Hypertag Ltd
Priority date: 2006-10-24
Filing date: 2006-10-24
Publication date: 2008-07-09
Anticipated expiration: 2026-10-24
Also published as: GB2445390B; GB0621152D0

Abstract

A beacon has a wireless transmitter for sending data to mobile receivers in its vicinity. The transmitter establishes a frequency hopping communication channel with a mobile receiver that has requested data. Sometimes, data transmitted in a time slot is corrupted by ambient interference but this corruption is detected so that the corrupted data can be re-transmitted. To serve a large number of mobile units it is proposed to include multiple transmitters (2) within the same beacon (1) but it has been found that this creates an unexpected problem because the number of corruptions caused by two of the transmitters (2) transmitting at the same frequency at the same time increases more than proportionally with the number of transmitters. The problem is solved by assigning different frequencies to respective transmitters (2).

Description

2445390

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Improvements to Frequency Hopping Communication Systems

This invention relates to improvements in a communication system of the type in which a beacon transmits data to a mobile receiver over a frequency hopping communication channel.

5 This invention is thought to be particularly relevant, though not limited to proximal information delivery systems using a short range radio system in which the beacon continually or intermittently broadcasts a search signal to which a mobile unit within receiving range of the beacon (normally around 10 metres) may respond so as to set up a communication channel allowing the beacon to download information stored. 10 Such systems may use the communication protocol known by the trade mark "Bluetooth" of Bluetooth SIG, Inc..

In a typical system of the aforementioned type, the search signal is broadcast at a frequency that hops in a random-like way between frequencies within a broad range of frequencies. The frequency hopping takes place at a frequency that is typically 15 1600IIz. Each mobile unit has a tuneable receiver which, when switched to a 'discover' mode, is caused to hop between the same frequencies as the beacon, also in a random-like way but at a significantly slower switching rate than that of the beacon. The beacon, which acts as a "master" will at some point broadcast its search signal on the same frequency as that to which the mobile, which acts as a "slave" is currently 20 tuned. Receipt of the broadcast by the mobile causes the latter to respond giving, inter alia, its identity which is stored by the beacon. The beacon thus accumulates a list of the IDs of all the mobiles which are in range and for which the short range communication facility is switched on. The beacon then enters a connection mode during which it hops between channels, broadcasting the fact that it is looking for the 25 mobiles identified by the stored IDs and inviting them to receive data. The user of any of these mobiles, wishing to receive the data may accept the invitation, whereupon a data communication channel called a "piconet" is established by the beacon. For this purpose, the beacon defines a pseudo-random frequency hopping

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sequcnce using the ID and clock of the bcacon. This sequence is transmitted to the mobile and data can then be transferred between the two devices over the established frequency hopping connection. Data is transferred in packets between each frequency hop.

5 The advantage of using frequency hopping is that it allows connections to be made in environments where there are likely to be a great deal of background noise. If the connection between the mobile and the beacon hops to a frequency at which noise from an external source exists, resulting corruption of the data is detected and the lost data is re-sent in a subsequent time slot.

10 The communication protocol described briefly above works well in scenarios where Bluetooth is conventionally used e.g. in environments where there are a number of continuous connections, such as between electronic peripheral devices in an office environment.

However, in other scenarios, where a large number of mobile devices arc likely to 15 accept the invitation to receive data from a beacon within a small period of time, the protocol does not work well. For example, if the data to be transferred is a thirty second video, this might typically take 30 seconds to transfer at a typical download speed of 150 Kilobytes per second, depending on the mobile device. If there are say 6 mobile devices connected, one of these would have to wait 2.5 minutes before a 20 connection is made or all six devices would face a download lasting 3 minutes. Mobile users arc unlikely to tolerate a wait of this length of time.

An initial approach considered by the inventors to solve this problem was to use two or more transceivers in the beacon to accommodate this additional workload. However this proved not to work for reasons which will now be described with 25 reference to Figure 1.

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Figurc 1 illustrates schematically the situation that arises if three Bluetooth or similar transmitters arc included in the beacon, showing which transmitter is active at different time slots T1 to T4 and at different frequencies fl to f9. In a standard Bluetooth system the there are 79 frequencies to which the connection may hop 5 spaced at lMhz intervals but just nine are considered here for the purposes of illustration.

All three transceivers Txl, Tx2 and Tx3 communicate to their respective mobile devices on a different pseudo-random hopping sequence, each transmitting a package of data during each contiguous time slot of which four arc shown at T1 to T4. At time 10 slot Tl, the pseudo random sequences established during the formation of the relevant connection, are assumed in the diagram to have caused the three transmitters to operate at different frequencies fl, £2 and f3. At time slot T2 they operate at frequencies f2, f4 and f8. At time slot T3, transmitters Tx2 and Tx3 are shown as both operating at frequency £5 with transmitter Txl operating at frequency fl; and at time 15 slot T4, transmitters Txl and Tx3 both operate at frequency f3 with transmitter Tx2 operating at frequency 19. These collisions cause corruption of the data. The corruption is detected so that the data can be re-transmitted. However, as the number of transmitters increases, the frequency of collision increases more than proportionally and as a consequence, the proposed duplication of transmitters proves, 20 in practice, not to be a practicable way of providing the required increase in transmission speed.

The invention provides a wireless communication system comprising a beacon for transmitting data to a number of mobile receivers; the beacon having a source of data, a transmitter for transmitting the data and means for establishing a frequency hopping 25 communication channel between the beacon and mobile receiver, characterised in that the beacon has at least a second transmitter for communicating data, means for establishing a second frequency hopping channel between the second transmitter and a second mobile receiver and means for allocating different frequencies to respective different channels.

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Bccausc the use of the invention reduces the number of frequencies available for each transceiver, it might be expected that the channels would be more vulnerable to collisions caused by the possible existence of background noise from sources external to the communication system of the invention. However, in the scenarios where this 5 invention was conceived and is thought to be of greatest use the potential interference caused by other master transceivers related to the same device is far higher than any potential background noise. Example such situations include public places such as a bus stop or a festival event where it is likely that all or most mobile receivers within range and having an activated short range wireless facility will be so activated for the 10 particular purpose of receiving information from the base station.

It is preferable that each channels should hop between frequencies which, though different from those of other channels, are distributed over the same frequency range. In this way, if there is a portion of the spectrum that has high background noise, each communication channel will still be able to hop to a part of the spectrum where there 15 is less or no noise

One embodiment of the invention will now be described by example with reference to the following figures in which:

Figure 1 is a graphical representation showing which of three frequency hopping transmitters in a beacon, not employing the technique of the present 20 invention, would be active at different frequencies fl to f9 at different times

Tl, T2, T3 and T4.

Figure 2 is a schematic view of a beacon in accordance with the invention; and

Figure 3 is a graphical representation similar to that of Fig 1 but showing 25 transmissions from the beacon of Fig 2.

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Rcfening to Fig 2 there is shown a bcacon 1 which may be incorporated into an electronic tag located at a location which is accessible to the public. An example, described here to help in understanding the invention, is a bus stop.

The beacon 1 contains three identical short range "Bluetooth" communication 5 microchips represented by the blocks 2 and a data storage device 3 containing information relating to the times of buses cxpcctcd to arrive at and depart from the bus stop. In the simple form of the invention described, this storage device is a simple timetable but in a more advanced variation means could be included to filter or process information read from a memory within the device 3 so that only information 10 relevant at a particular time or to a particular user is read out.

Each Bluetooth chip 2 includes a short range radio transceiver 2A and a hop sequence generator indicated schematically at 2B. The hop sequence generator 2B, changes the tuning of the transceiver during operation so as to switch, according to a pseudorandom sequence, between different permitted frequencies within a specified 15 frequency range. In practice, as previously mentioned, there are 29 of these frequencies but just nine, fl to f9, are considered here for the purposes of simplifying this description.

The Bluetooth specification specifies an IICI (host controller interface) command set which includes the command 'AFH' (adaptive frequency hopping). By means of this 20 command, bit masks are used to program the chips 2 so that each one is able to use only a proportion of the available frequencies, with the frequencies used by any one chip being made unavailable for use by the other chips. The frequencies made available for use by each of the different chips are spread over the whole or a substantial part of the available spectrum so that they are interposed between 25 frequencies assigned to the other chips. In this way the effect of external interference on any one chip in a particular area of the spectrum is minimised.

The wireless chips are given notional consecutive identity numbers ni equal to 1,2,3.... n2 where n2 is the number of wireless chips. Each chip is assigned

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frequencics fh, where n - m+xn2 and x is any integer including zero. Thus, in the illustrated thrcc-chip system, chip 1 is assigned frequencies fi, ft, f?; chip 2 is assigned frequencies f2,fs,f*; and chip 3 is assigned frequencies f„ f(„ f». Thus, the frequencies assigned to any one chip are evenly distributed over the available 5 spectrum and are interposed between the frequencies assigned to the other chips.

During operation of this system, a mobile unit 4 in the vicinity of the bcacon 1 establishes connection with one of the transceivers 2 which is currently in its search mode (whichever one whose frequency first coincides with that of the mobile). A communication channel is then be set up by that transceiver in the manner previously 10 described and as defined in the Bluetooth specification but with the difference that only certain frequencies are permitted. Data is then downloaded from the data store 3 to the mobile with no possibility of any corruption from data being downloaded on other frequencies by other transceivers 2.

It is emphasised that the particular embodiment of the invention that has been 15 described is just one example of how the invention, as defined by the following Claims, may be implemented. In alternative embodiments just two or more than three transceivers may be included in the beacon, the benefits of the invention increasing with a greater number of such transceivers. The system, which we have described as "wireless" can operate in a frequency spectrum of between 2.402 GHz and 2.480 20 GHz, as in Bluetooth, or in any other frequency of the electromagnetic spectrum including infrared and microwaves, or it could be based on acoustic or any other radiated signals. Although it is envisaged that the "beacon" will often be at a fixed location, this is not necessary for the functioning of the invention and the beacon could be located on a moving platform or vehicle or could be hand held. Furthermore, 25 the data store, indicated at 3 on Fig 2, instead of storing data in a semi-permanent form, could be linked to the internet, a sensor system or to other communication devices so that the data is continuously or regularly updated. Another design variation would be to include a data store 3 for each chip, these separate stores all containing identical information. This would eliminate the need for connections between the 30 separate chips in environments where such connection might be inconvenient.

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Claims

1 A communication system comprising a bcacon for radiating data to a number of mobile receivers; the beacon having a source of data, a transmitter for transmitting the data and means for establishing a frequency hopping communication channel 5 between the beacon and mobile receiver, characterised in that the beacon has a second transmitter for communicating data, means for establishing a second frequency hopping channel between the second transmitter and a second mobile receiver and means for allocating different frequencies to respective different channels.

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2. A communication system according to Claim 1 characterised in that the frequency hopping channels hop between frequencies within the same frequency range.

3. A communication system according to Claim 2 characterised in that frequencies of any one channel are interposed between frequencies of the or each 15 other channel.

3. A wireless communication system constructed in accordance with Claim 1, 2 or 3 and having a range of about 10 metre.

4. A communication system according to any preceding Claim characterised in that the beacon is located at a place available to the public and in that the data is

20 associated with that location.

5. A communication system according to any preceding claim characterised in that each transmitter is a radio transmitter.

6. A communication system according to Claim 5 characterised in that the transmitters operate in a spectrum between 2.402 GHz and 2.480 GHz.