GB2293295A

GB2293295A - Wireless local area network synchronisation arrangement

Info

Publication number: GB2293295A
Application number: GB9418527A
Authority: GB
Inventors: Peter John Minett
Original assignee: Plessey Semiconductors Ltd
Current assignee: Plessey Semiconductors Ltd
Priority date: 1994-09-14
Filing date: 1994-09-14
Publication date: 1996-03-20
Also published as: GB9418527D0

Abstract

A WLAN synchronisation arrangement utilises an absolute timing reference signal to which both base station and remote terminal refer periodically. Both base station and remote terminal are configured to activate their communication circuitry at predetermined values of the timing reference signal, their respective data clocks being set to a known phase at the detection of those precise values. The timing signal is preferably a time-of-day signal and is transmitted on a link separate from the main communication link of the network. The timing reference may either be transmitted from the base station to the remote terminal, or be received by both base station and remote terminal from an external source. The timing signal receiver is separate from the main receiver and is a low-power, narrow-band and low-data-rate receiver. The advantage of the synchronisation arrangement is that the respective transceivers are active for a shorter time and battery life in the remote terminals is lengthened.

Description

WIRELESS LOCAL AREA NETWORK SYNCHRONISATION ARRANGEMENT The invention relates to a synchronisation arrangement in a wireless local area network (WLAN) and in particular, but not exclusively, a synchronisation arrangement in a frequency-hopping WLAN system utilising the ISM (Industrial, Scientific & BR< Medical applications) bands, i.e. principally 2.4-2.5 GHz.

WLAN arrangements exist in which a transceiver, e.g. a remote terminal, which is due to receive data from another transceiver, e.g. a base station, has its own internal clock timing source brought into synchronism with that of the transmitting transceiver by means of a preamble transmitted to it by the transmitting transceiver. The preamble is a digital code which allows the receiving transceiver to align the phase of its own clock source to that of the transmitting transceiver, after which transmission of the communication data can take place on the same transmission link.

A drawback of.this known arrangement is that the use of a preamble, which in itself can be of appreciable length, increases the time during which the receiving transceiver's circuitry is active and this, in the case of remote battery-operated terminals. leads to reduced battery life. It also has the effect of reducing the amount of traffic that is possible on the network.

It would be desirable to provide a WLAN synchronisation arrangement which mitigates or overcomes the above drawbacks.

In accordance with a first aspect of the invention there is provided a wireless local area network comprising a first transceiver arrangement and a second transceiver arrangement. the first and second transceiver arrangements each comprising a data clock signal generating means for generating a data clock signal for the transfer of communication data into or out of the respective transceiver arrangement, wherein the data clock signals of the first and second transceiver arrangements are synchronisable with each other by reference to a common timing signal which is supplied to at least one of the transceiver arrangements by way of a timing transmission link which is independent of a communication link existing between the two transceivers.

The advantage of this arrangement is that, by eliminating the need to synchronise one of the transceiver arrangements by a preamble signal transmitted by the other transceiver arrangement on the main communication link, but utilising instead an absolute timing reference, the time taken up by the transmission of the preamble is eliminated, or substantially reduced, and therefore the demand on the communication band employed is lessened. Although a separate receiver is required to receive the timing signal, that receiver may be made low-power and narrow-bandwidth, thereby reducing the power demand in the transceiver arrangement or arrangements concerned.

Each of the transceiver arrangements may comprise a wake-up means responsive to the timing signal for initiating the transmission or receipt of a communication signal to or from the other transceiver arrangement. The timing signal may be a digital timing signal and the wake-up means may be arranged to respond to the timing signal at one or more predetermined digital values of that timing signal.

By waking up, i.e. activating, the communication circuitry in the two transceiver arrangements at precisely those predetermined timing values only. the communication circuitry power demand is reduced, which increases the life of any batteries used in the transceiver arrangements.

The wake-up means may be arranged to respond to the timing signal at substantially equally spaced digital values of that timing signal. This ensures that there is a regular communication route open between the transceiver arrangements for those occasions when data is required to be transmitted or received.

The timing signal may be transmitted to one of the transceiver arrangements from the other transceiver arrangement, or it may alternatively originate from a source external to the network and be received by both transceiver arrangements.

The timing signal may be an accurate time-of-day signal and may also supply date information. An advantage of this is that the separate receiver necessary in one or both of the transceiver arrangements for picking up the time-of-day reference can be put to use in other areas, e.g. to switch on other, ancillary equipment at predetermined times on predetermined days. Even where the base station is responsible for generating the absolute time reference, additional usage can still be made of this reference at the base station. Also, in the case of an external reference signal, such a signal is already commonly available in many countries as a continuously transmitted time clock reference and a suitable receiver for picking this up may well be readily available.

In accordance with a second aspect of the invention, there is provided a transceiver arrangement for use in a wireless local area network as described above, the transceiver arrangement comprising a communication transmitting and receiving means for transmitting and receiving communication data. a data clock generating means for generating a data clock signal for the transfer of the communication data into or out of the communication transmitting and receiving means, and a timing signal receiving means for receiving a timing signal for the control of the phase of the generated data clock signal, the timing signal receiving means being arranged to receive the timing signal on a transmission link which is separate from that used to receive the communication data and to use the timing signal to control the phase of the data clock signal.

In accordance with a third aspect of the invention, there is provided a transceiver arrangement for use in a wireless local area network as described above, the transceiver arrangement comprising a communication transmitting and receiving means for transmitting and receiving communication data, a data clock generating means for generating a data clock signal for the transfer of the communication data into or out of the communication transmitting and receiving means, a timing signal generating means for generating a timing signal for the control of the phase of the generated data clock signal, and a timing signal transmitting means for transmitting the timing signal to a remote transceiver arrangement, the timing signal transmitting means being arranged to transmit the timing signal on a transmission link which is separate from that used to transmit the communication data.

In a first embodiment of the invention, a network consisting of a base station and a number of remote terminals are in contact with each other via a communication link, but in addition the base station transmits to each of the remote terminals a timing signal on a separate transmission link. The timing signal takes the form of an accurate time of-day digital clock signal and it may be transmitted on the same frequency as the communication link or on a different frequency. Each of the remote terminals contains, in addition to its main, broadband communication receiver, a low-power, narrow-band, low-data-rate receiver specifically for picking up the time-of-day signal from the base station.This separate receiver operates continuously as an absolute time reference for the terminal and each terminal is configured to activate its main, broadband receiver at specific regular times of day, different times for different terminals, so that the terminals can be individually addressed. The base station is also configured to switch its transmission circuits into active mode at those exact same times of day, when communication data are required to be sent from the base station to the remote terminals. The same applies in reverse when data are to be transmitted from a remote terminal to the base station. Thus, each terminal is periodically in an active state with regard to the base station, data being passed to or from these two at precisely those times at which the respective transceiver circuits become active.

To this end, the base station and each remote terminal are equipped with a wakeup means which is responsive to the timing signal and which detects those absolute times of day for which the respective base station or terminal is configured and sets an internal data clock to a known reference phase when an appropriate time is detected.

Since all terminals and the base station use the same absolute timing signal as a reference. mutual svnchronism is assured.

Detection may be achieved by any known correlation means, e.g. by one or more software- or hardware-programmed digital words representing absolute times-of-day and which are compared with the actual times of day received on the separate, dedicated low-power receiver. The base station, of course, needs no low-power receiver as such, since the absolute time reference originates from the base station, but its own wake-up means will nevertheless correlate its own digital wake-up words with the digital absolute clock signal, which may be hardwired to the base transceiver circuitry.

In a second embodiment of the invention, the absolute time reference takes the form of an external time-of-day reference source. This may be, for example in the UK, the clock transmitter situated at Daventry which operates on long-wave, or a GPS (Global Positioning by Satellite) time reference, or any other suitable reference. In this case, the base station as well as the remote terminals employs a low-power, narrowband receiver for receiving this external time-of-day signal.

This second embodiment is advantageous in that it reduces the hardware overhead at the base station and allows accurate synchronisation to occur even in remote situations, so long as the base station and the terminals can pick up the external reference. One such area of application is in the monitoring of consumer-type data, for example gas or electric power metering. Also, in such applications the remote terminals used may be compact battery-driven units where battery life is critical and the reduced usage of the main, high-power communication transceiver offered by the present invention can be highly beneficial.

A further spin-off of the invention is that the absolute time reference used for synchronisation can be employed at both base station and remote terminal for purposcs other than WLAN synchronisation, e.g. for controlling the switching of external electrical equipment and even simply as an additional clock for telling the time.

While it has been assumed that base station and remote terminal will establish communication links at regular times each day, there will be applications in which communication will need to take place only infrequently on certain days, perhaps only in certain months and even in specified years. To this end, it will be necessary for the timing signal used for the network contain date information as well as accurate time-ofday information, One such application has already been mentioned, namely remote metering, where reading of the quantities concerned may only need to take place, say, four times a year.

Claims

1. A wireless local area network comprising a first transceiver arrangement and a second transceiver arrangement, the first and second transceiver arrangements each comprising a data clock signal generating means for generating a data clock signal for the transfer of communication data into or out of the respective transceiver arrangement, wherein the data clock signals of the first and second transceiver arrangements are synchronisable with each other by reference to a common timing signal which is supplied to at least one of the transceiver arrangements by way of a timing transmission link which is independent of a communication link existing between the two transceivers.

2. A wireless local area network as claimed in Claim 1, in which each of the transceiver arrangements comprises a wake-up means responsive to the timing signal for initiating the transmission or receipt of a communication signal to or from the other transceiver arrangement.

3. A wireless local area network as claimed in Claim 2, in which the timing signal is a digital timing signal and the wake-up means is arranged to respond to the timing signal at one or more predetermined digital values of that timing signal.

A. A wireless local area network as claimed in Claim 3, in which the wakeup means is arranged to respond to the timing signal at substantially equally spaced digital values of that timing signal.

5. A wireless local area network as claimed in any one of the preceding claims, in which one of the transceiver arrangements compriscs a timing signal generating means for generating the common timing signal, and a timing signal transmission means for transmitting the generated timing signal to the other transceiver arrangement.

6. A wireless local area network as claimed in any one of Claims 1 to 4, in which both transceiver arrangements are arranged to receive a timing signal from a timing source external to the network.

7. A wireless local area network as claimed in any one of the preceding claims, in which the data clock signals of the first and second transceiver arrangements are arranged to be synchronisable to each other at one or more predetermined times of day.

8. A wireless local area network as claimed in Claim 7, in which the data clock signals of the first and second transceiver arrangements are arranged to be synchronisable to each other. on one or more predetermined dates.

9. A wireless local area network substantially as hereinbefore described.

10. A transceiver arrangement for use in a wireless local area network as claimed in any one of the preceding claims, the transceiver arrangement comprising a communication transmitting and receiving means for transmitting and receiving communication data, a data clock generating means for generating a data clock signal for the transfer of the communication data into or out of the communication transmitting and receiving means, and a timing signal receiving means for receiving a timing signal for the control of the phase of the generated data clock signal, the timing signal receiving means being arranged to receive the timing signal on a transmission link which is separate from that used to receive the communication data and to use the timing signal to control the phase of the data clock signal.

11. A transceiver arrangement for use in a wireless local area network as claimed in any one of Claims 5, 7 or 8, the transceiver arrangement comprising a communication transmitting and receiving means for transmitting and receiving communication data, a data clock generating means for generating a data clock signal for the transfer of the communication data into or out of the communication transmitting and receiving means, a timing signal generating means for generating a timing signal for the control of the phase of the generated data clock signal, and a timing signal transmitting means for transmitting the timing signal to a remote transceiver arrangement, the timing signal transmitting means being arranged to transmit the timing signal on a transmission link which is separate from that used to transmit the communication data.

12. A transceiver arrangement substantially as hereinbefore described.