WO2009074935A1

WO2009074935A1 - Synchronisation method

Info

Publication number: WO2009074935A1
Application number: PCT/IB2008/055121
Authority: WO
Inventors: Brian John Mayhew
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2007-12-10
Filing date: 2008-12-05
Publication date: 2009-06-18
Anticipated expiration: 2010-06-10

Abstract

The present invention relates to a method of receiving data packets for synchronising a first communication device with a second communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication. The method comprises the following steps performed by the first communication device (300): (a) receiving (701) at any time slot a data packet from the second communication device, the data packet containing information for deriving the next free time slot; (b) decoding (703) at least part of the contents of the received data packet; (c) based on the decoding result, determining (705) the occurrence of the next free time slot; and (d) based on the determination switching off (711) a radio part (305) of the first communication device (300) until the next free time slot is expected to be received.

Description

Synchronisation method

TECHNICAL FIELD

The present invention relates to a method of synchronising communication devices in a time division multiple access (TDMA) network. The invention also relates to corresponding computer program products and communication devices.

BACKGROUND OF THE INVENTION

A TDMA network uses a notional sequence of time slots that are designated for particular wireless nodes to gain exclusive access to a radio channel, thereby avoiding the corruption of transmissions from other nodes. Before the time slots can be used, all cooperating nodes must synchronise their slot timers so that each node knows when each time slot begins and ends. Synchronisation is typically achieved by receiving a special beacon packet that bounds the set of time slots. Fig.l illustrates an exemplary situation where a master node is communicating with a slave node in a TDMA network.

In Fig.1 the slave node has already claimed the second time slot in the frame, and the master node uses that slot to transmit a data packet to the slave. It is to be noted that the master node also transmits the beacon packets. No transmissions occur in unused slots, allowing nodes to switch off their radios to save power. When the slave needs to transmit a data packet to the master, it waits for a polling packet from the master in the same time slot and transmits the data packet in response.

The slave node must receive a beacon packet in order to synchronise its slot timer with the master frame. However, the slave may have to switch on its radio for any time up to a whole frame before it can receive a beacon packet. This is illustrated in Fig.2, where the slave node starts listening to the beacon packets after the first beacon shown in Fig.2 and detects a first beacon approximately one frame later.

This long period of idle listening wastes power, and may not even be possible if the receiver node uses a battery that can only supply power intermittently. Other schemes whereby the slave receiver is switched on and off while waiting for the beacon packet run the risk of missing the beacon, which can extend the synchronisation process over several frames before a beacon is received. Thus, there is a need for an improved method of synchronising a power constrained device in a TDMA network.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a method of receiving data packets for synchronising a first communication device with a second communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the method comprises the following steps performed by the first communication device:

- receiving at any time slot a data packet from the second communication device, the data packet containing information for deriving the next free time slot; decoding at least part of the contents of the received data packet;

- based on the decoding result, determining the occurrence of the next free time slot; and

- based on the determination switching off a radio part of the first communication device until the next free time slot is expected to be received.

Thus, the present invention provides an efficient way of reducing power consumption of communication devices, such as slave nodes. More specifically, the first communication device, which can be a power restricted slave node, only has to switch on its receiver for up to one time slot period in order to receive the next packet from the second communication device, which can be a master node, and set its slot timer. If the packet indicates that the time slot is free, the first communication device can reply with a data packet to claim the slot; if the slot is not free, the first communication device can switch off its radio until a time calculated from the packet data that the next free slot is due. This can result in a significant reduction in power consumption of the first communication device, particularly if the frame has many time slots. For applications where the first communication device is mobile and has to move in and out of range of the second communication device, the power saving can be made each time it has claimed a new time slot.

According to a second aspect of the invention there is provided a computer program product comprising instructions for implementing the method according to the first aspect of the invention when loaded and run on computer means of the first communication device.

According to a third aspect of the invention there is provided a communication device for receiving data packets and arranged to be synchronised with another communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the communication device comprises: a receiving unit for receiving at any time slot a data packet from the other communication device, the data packet containing information for deriving the occurrence of the next free time slot; a decoder for decoding at least part of the contents of the received data packet; and a data processor for determining the occurrence of the next free time slot based on the decoding result; and further arranged, based on the determination, for switching off a radio part of the communication device until the next free time slot is expected to be received.

According to a fourth aspect of the invention there is provided a method of transmitting data packets for synchronising a first communication device with a second communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the method comprises: the second communication device transmitting at every time slot a data packet, each data packet containing information for deriving by the first communication device the occurrence of the next free time slot.

According to a fifth aspect of the invention there is provided a computer program product comprising instructions for implementing the method according to the fourth aspect of the invention when loaded and run on computer means of the second communication device.

According to a sixth aspect of the invention there is provided a communication device for transmitting data packets and arranged to be synchronised with another communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the communication device comprises: a transmitter unit for transmitting at any time slot a data packet to the other communication device, the data packet containing information for deriving by the other communication device the occurrence of the next free time slot; and a data processor for choosing the type of the data packets based on the state of reservation of the current time slot, wherein the different data packet types are distinguishable by their headers and parameter fields. BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which:

- Fig.l illustrates different notations used when a slave node communicates with a master node in a communication network;

- Fig.2 illustrates the situation where a slave node is waiting to detect a beacon packet during one frame period;

- Fig.3 is a simplified block diagram of a slave node;

- Fig.4 shows several time slots along a time line when all time slots are free;

- Figs. 5 and 6 show several time slots along a time line when not all time slots are free; and

- Fig.7 is a flow chart illustrating a method of synchronising a slave node with a master node in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The teachings of the present invention are applicable in TDMA networks where a first communication device, in the following description referred to as a slave node, is arranged to communicate with a second communication node, also referred to as a master node in the following description. The communication between the nodes may either take place over a wired medium or over a radio interface in a wireless manner. The different nodes can for instance be wireless sensors arranged to operate in accordance with a TDMA based communication system. The master nodes can be defined to be devices that can handle active connections with several other nodes, such as slaves, and typically master devices are not power constrained. The slave nodes on the other hand are devices that can generally handle only one active connection at a time with another device, i.e. a master node, and the slave devices are typically power constrained.

Fig.3 shows a simplified block diagram of a slave node 300 in accordance with the present invention. It is to be noted that the block diagram shown in Fig.3 can equally illustrate the master node. The slave node 300 contains a receiver unit 301, which further consists of an antenna 303 and radio part 305, or radio frequency (RF) section as shown in Fig.3, of which purpose is to choose the incoming signal from the antenna 303 and amplify the received signal. The slave node 300 further comprises an analogue baseband block 307 which further contains a decoder 309. No further blocks of the analogue baseband block are shown in Fig.3 since these other blocks are not really essential for understanding the principles of the present invention. There is also shown a conventional digital baseband block 311, which is connected to a central processing unit (CPU) 313.

In accordance with some embodiments of the present invention, to reduce the time that the power-constrained slave node 300 has to be switched on, the master node beacon packets are replaced by numbered packets that the master (which is not power-constrained) transmits at every time slot. Each packet contains data to indicate the time slot number, whether the time slot is free, the number of slots in the frame, the duration of a time slot, and the number of the next free slot. It is to be noted that depending on the implementation details, not all that information needs to be included in each packet.

An exemplary situation where all time slots are free is explained next in more detail with reference to Fig.4. A frame of SYNC packets is transmitted by the master, where each packet contains data fields to support the slave time synchronisation process. In this example each packet transmitted by the master node contains at least the fields shown in Table 1.

Table 1 : Explanation of different fields in SYNC packet.

Slave A wishes to claim a time slot, so it switches on its radio 305 at arbitrary time (1). Since the moment (1) happens to be a moment while a SYNC packet is received, the slave A detects that the channel is busy, so it switches off for half the default slot duration. If a SYNC packet has been received before, then the actual slot duration D can be used instead. It switches on again at (2) as shown in Fig.4 and waits for the next SYNC packet (3)-(4); it can then reply with a data packet to claim the slot. The maximum time that its radio is on is less than one slot duration.

Referring again to Fig.4, slave B wishes to claim a time slot, but when it switches on at (5) the channel is free, so it remains on until it receives the next SYNC packet (6)-(7). The maximum time that its radio is on is equal to one slot duration.

Next let us consider a situation where not all time slots are free. This situation is explained with reference to Fig.5. When a slot is claimed by a slave, the master changes the slot's packet type from SYNC to POLL, starting from the next frame, to indicate that the slot is in use and to elicit a response from the slot's slave node. A frame with some time slots allocated and some not allocated contains a mixture of SYNC and POLL packets, with a POLL packet at every allocated slot. Each packet transmitted by the master node contains at least the fields shown in Table 2.

Table 2: Explanation of different fields in POLL packet.

Slave C receives the first complete packet (8)-(9) after switching on its receiver. The packet is not a SYNC packet and therefore the slave cannot respond to claim the slot; instead, it sleeps with its radio switched off until the next free slot is due (10), indicated by field F in the POLL packet:

; and T_sleep = D - {F - S)- T_poll .

In the above formulas, T_pon indicates the length of the POLL packet and T_sι_eep indicates the time to the next unused time slot starting from the end of the last received packet, in this case POLL packet.

Once an available slot is allocated to a slave node, it has to decide when it can communicate next time with the master node. Thus, once a slave, in this case node D, has responded to a SYNC packet (12)-(13) to claim a time slot, it needs to know how long to sleep with its radio switched off before the same time slot (14) is due again. The fields N and D in the SYNC packet received by the slave allow it to calculate the sleep duration, with no power wastage. The same calculation can be made later using the same data fields in the POLL packet. This situation is illustrated in Fig.6. Now T_sι_eep is calculated in the following way:

¹ sleep - ^ ^{i V 1} SyKC

Where T_sync indicates the length of the SYNC packet and is obtained by

T — T — T Fig.7 shows a flow chart illustrating the synchronisation method in accordance with an embodiment of the present invention. In step 701, the slave node 300 receives at any time slot a data packet from the master node. Then in step 703 at least part of the contents of the data packet is decoded so that in step 705 the occurrence of the next free time slot can be determined. In case the current time slot is free, it suffices to decode the header, which indicates the type of the received packet, i.e. POLL or SYNC packet. On the other hand, if the current time slot is not free, then other fields need to be decoded as well as explained above. The determination whether the current time slot is free is done in step 707.

If it is determined that the current time slot is free, then in step 709 the current time slot is reserved for transmission between the slave node 300 and the master node. Then in step 710 the radio part 305 is switched off until the next free time slot is expected to be received. If on the other hand it was determined in step 707 that the current time slot is not free, then in step 711 the radio part 305 is switched off until the next free time slot is expected to be received. Then finally in step 713 the radio part 305 is switched on again when the next free time slot is expected to be received and that time slot is then reserved for transmission. The procedure then comes to an end.

Above some embodiments of the present invention were described. Above it was explained a process whereby a periodic frame of data packets transmitted by the master node in a wireless TDMA network is used to synchronise the slave node 300 to any packet in the frame with minimal power expenditure. The present invention thus contributes to extending battery life in power-constrained slave nodes in master-slave wireless networks. Such networks where energy efficiency is highly critical include Body Area Networks in medical applications, and intelligent inventory tags in supply chain management systems.

The invention equally relates to a computer program product that is able to implement any of the method steps of the embodiments of the invention when loaded and run on computer means of the nodes of the communication network. The computer program may be stored/distributed on a suitable medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The invention equally relates to an integrated circuit that is arranged to perform any of the method steps in accordance with the embodiments of the invention.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not restricted to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

1. A method of receiving data packets for synchronising a first communication device (300) with a second communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the method comprises the following steps performed by the first communication device (300):

- receiving (701) at any time slot a data packet from the second communication device, the data packet containing information for deriving the next free time slot; decoding (703) at least part of the contents of the received data packet;

- based on the decoding result, determining (705) the occurrence of the next free time slot; and

- based on the determination switching off (71 1) a radio part (305) of the first communication device (300) until the next free time slot is expected to be received.

2. A method according to claim 1, wherein the information comprises at least one of the following: indication whether the current time slot is free, the number of the current time slot, the number of time slots in the current frame, the duration of the current frame and the time slot number of the next free time slot.

3. A method according to any of the preceding claims wherein the method further comprises reserving (709) the time slot for transmission once it is detected free and then switching off the radio part (305) until the next free time slot is expected to be received.

4. The method according to any one of claims 1 or 2, wherein the method further comprises directly after it has been determined that the current time slot is not free switching off (711) the radio part (305) until the next free time slot is expected to be received.

5. The method according to any of the preceding claims, wherein the method further comprises switching on (713) the radio part (305) before receiving the data packet.

Claims

6. The method according to any of the preceding claims, wherein the data packets are divided into two types distinguishable by their header and parameter fields, wherein a specific header indicates whether the current time slot is free.

7. A computer program product comprising instructions for implementing the steps of the method according to any one of claims 1 through 6 when loaded and run on computer means of the first communication device (300).

8. A communication device (300) for receiving data packets and arranged to be synchronised with another communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the communication device (300) comprises: a receiving unit (301) for receiving at any time slot a data packet from the other communication device, the data packet containing information for deriving the occurrence of the next free time slot; a decoder (309) for decoding at least part of the contents of the received data packet; and a data processor (313) for determining the occurrence of the next free time slot based on the decoding result; and further arranged, based on the determination, for switching off a radio part (305) of the communication device (300) until the next free time slot is expected to be received.

9. A method of transmitting data packets for synchronising a first communication device (300) with a second communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the method comprises:

- the second communication device transmitting at every time slot a data packet, each data packet containing information for deriving by the first communication device (300) the occurrence of the next free time slot.

10. A computer program product comprising instructions for implementing the steps of the method according to claim 9 when loaded and run on computer means of the second communication device.

11. A communication device for transmitting data packets and arranged to be synchronised with another communication device in a radio communication system operating in accordance with a time division multiple access scheme, and where the communication takes place over a radio interface in time slots allocated for the communication, the communication device comprises: a transmitter unit for transmitting at any time slot a data packet to the other communication device, the data packet containing information for deriving by the other communication device the occurrence of the next free time slot; and a data processor for choosing the type of the data packets based on the state of reservation of the current time slot, wherein the different data packet types are distinguishable by their headers and parameter fields.