WO2010068180A1

WO2010068180A1 - Communication device, method for scheduling a message exchange and computer program product

Info

Publication number: WO2010068180A1
Application number: PCT/SG2009/000460
Authority: WO
Inventors: Ananth Subramanian; Xiaoming Peng; Po Shin Francois Chin
Original assignee: Agency For Science, Technology And Research
Priority date: 2008-12-09
Filing date: 2009-12-01
Publication date: 2010-06-17
Also published as: TW201025944A

Abstract

A Communication device is described comprising an allocation circuit configured to allocate a first presence information exchange time period for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices; and a determining circuit configured to determine, based on the number of other communication devices in the group of communication devices, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining which communication devices are part of the group of communication devices; wherein the allocation circuit is further configured to allocate the second presence information exchange time period, such that the second presence information exchange time period starts at the determined starting time.

Description

COMMUNICATION DEVICE, METHOD FOR SCHEDULING A MESSAGE EXCHANGE AND COMPUTER PROGRAM PRODUCT

Embodiments of the invention generally relate to a communication device, a method for scheduling a message exchange and a computer program product.

The ECMA standard, see [1] , specifies the Ultra Wide Band (UWB) Physical layer (PHY) and Medium Access Control (MAC) layer for a high-speed, short-range wireless network utilizing all or part of the spectrum between 3100 and 10600 MHz supporting data rates of up to 480 Mbps.

According to the communication protocol 'of the ECMA standard as described in [1] , a superframe structure is used consisting of a periodically repeated time interval, the superframe, comprising a Beacon Period (BP) and a data period. According to the ECMA standard as described in [1] , the superframe is of fixed duration and the beacon period can accommodate the beacons of up to 96 devices. Every device in a beacon group is supposed to send a beacon in the beacon period of every superframe and no transfer of useful data (other than beacons) is allowed in the beacon period. Therefore, the beacon period introduces overhead, ^• particularly when the number of devices is large. As an example, if there are 46 devices in a beacon group, then the beacon period has a duration of 16 Medium Access Slots (MASs) in a superframe duration of 256 MASs. Hence in the above case the overhead due to beaconing is 6.25%. If there are 94 devices in the beacon group, then the overhead due to beaconing is 12.5%. Moreover, the maximum number of devices ^•possible in a beacon group is 96 (including those allowed by two signaling slots) . Thus, there is a strict upper bound on the density of nodes. In Personal Area Networks (PANs) unlike - in Local Area Networks (LANs) or Regional Area Networks (RAN), the density of nodes may be large and significant. Moreover, using the communication protocol of the ECMA standard as described in [1] inter-beacon group interferences may be introduced when two simultaneously operating beacon groups operating in different but overlapping channels come in close proximity with each other. Though devices in a beacon group are able to communicate with each other, devices in different beacon groups may not be able to communicate with each other.

In documents [2] and [3] , synchronization schemes are described that enable devices in a beacon group to be synchronized to clock period level. ^'

Embodiments may be seen to be based on the problem to reduce the overhead caused by time periods for exchanging messages between communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices, e.g. the overhead arising due to beaconing in a communication system according to the ECMA standard.

In one embodiment, a communication device is provided including an allocation circuit configured to allocate a first presence information exchange time period for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices and a determining circuit configured to determine, based on the number of other communication devices in. the group of communication devices, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining which communication devices are part of the group of communication devices wherein the allocation circuit is further configured to allocate the second presence information exchange time period, such that the second presence information exchange time period starts at the determined starting time.

According to other embodiments, a method for scheduling a message exchange and a computer program product in accordance with the communication device described above are provided.

Illustrative embodiments of the invention are explained below with reference to the drawings.

FIG. 1 shows a communication system according to an embodiment .

FIG. 2 shows a superframe structure according to an embodiment .

FIG. 3 shows a communication device according to an embodiment.

FIG. 4 shows a flow diagram according to an embodiment.

FIG. 5 shows a frequency spectrum diagram.

It should be noted that embodiments described in the following that are described in the context with the communication device are analogously valid for the method for scheduling a message exchange and the computer readable medium.

FIG. 1 shows a communication system 100 according to an embodiment .

The communication system 100 includes a first communication device 101, a second communication device 102, a third communication device 103, and a fourth communication device 104. The communication devices 101, 102, 103, 104 are for example communication devices of an ad-hoc communication system 100, i.e. ad-hoc communication devices. The communication devices 101, 102, 103, 104 are for example radio communication devices.

The communication devices 101, 102, 103, 104 form a group of communication devices 105, for example a beacon group according to the ECMA standard, for example the ECMA standard as described in [1] .

The communication devices 101, 102, 103, 104 communicate according to a communication protocol. In one embodiment, the communication devices 101, 102, 103, 104 communicate according to the communication protocol of the ECMA standard, e.g. as described in [1], based on a superframe structure as described in the following with reference to figure 2.

FIG. 2 shows a superframe structure 200 according to an embodiment .

The superframe structure 200 comprises a plurality of successive superframes. In this example, an nth superframe 201 and an n+lth superframe 202 are illustrated. Each superframe 201, 202 comprises a beacon period of variable ■ length which serves for exchanging beacons among the communication devices 101, 102, 103, 104 among the communication group 105. A beacon period 203, 204 may be seen as a- presence information exchange time period for exchanging messages between a plurality of communication devices 101,

102, 103, 104 of the group of communication devices 105 for determining which communication devices are part of the group of communication devices 105. Each superframe 201, 202 has a superframe start timing and each beacon period 203, 204 has a Beacon Period Start Time (BPST) 207, 208 which is for example equal to the start timing of the respective superframe.

The superframes 201, 202 are sub-divided into Medium Access Slots (MAS) each having for example a length of 256μs.

The length of the beacon period 203, 204 of a superframe 201, 202 depends on the number of communication devices 101, 102,

103, 104 that send beacons in the beacon period 203, 204. Thus, if a lot of communication devices 101, 102, 103, 104 send beacons in the beacon period 203, 204, the overhead due to the beacon transmission in the respective superframe 201, 202 may be relatively high such that the data rate of other useful data transmission is undesirably low.

Furthermore, the length of the beacon pe.riod 203, 204 may be limited such that the number of communication devices 101, 102, 103, 104 that may transmit beacons in a beacon period 203, 204 may be limited.

According to one embodiment, a beacon group is enabled to have a larger number of communication devices (larger than 96) in spite of the limitation of the beacon period length as in [1] . Moreover, according to one embodiment, the overhead introduced by beaconing is limited irrespective of the number of communication devices in the beacon group.

A communication device according to an embodiment is described in the following with reference to figure 3/

FIG. 3 shows a communication device 300 according to an embodiment .

The communication device 300 comprises an allocation circuit configured to allocate a first presence information exchange time period. for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication, devices .

The communication device 300 further comprises a determining circuit 302 configured to determine, based on the number of other communication devices in the group of communication devices, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining which communication devices are part of the group of communication devices.

The allocation circuit 301 is further configured to allocate the second presence information exchange time period, such that the second presence information exchange time period starts at the determined starting time. For example, the communication devices 101, 102, 103, 104 of the communication group 105 have the structure and functionality of the communication device 300.

Illustratively, in one embodiment, it is estimated how many communication devices are present in the group of communication devices (e.g. based on the presence information exchange of the first or more presence information exchange time periods) and the length of the time intervals between presence information exchange time periods that should be used are determined based on the number of communication devices in the group of communication devices. For example,- by increasing the length of the time intervals between presence information exchange time periods when the number of communication devices grows, the increase of signaling overhead with increasing number of communication devices in the group of communication devices can be avoided or at least constrained. The length of the time intervals between presence information exchange time periods may for example be adjusted by adjusting the superframe duration in case of the superframe structure as described with reference to figure 2 or by having the communication device skip the transmission and/or reception of presence information exchange messages for one or more presence information exchange time periods, e.g. having the device skip the transmission of a beacon for one or more beacon periods .

The communication device 300 may comprise a memory which is for example used in the processing carried out^' by. the determining circuit.

In an embodiment, a "circuit" may be understood as any kind of a logic implementing entity, which may be special purpose circuitry or a processor executing software stored in a memory, firmware, or any combination thereof. Thus, in an embodiment, a "circuit" may be a hard-wired logic circuit or a programmable logic circuit such as a programmable processor, e.g. a microprocessor (e.g. a Complex Instruction Set Computer (CISC) processor or a Reduced Instruction Set Computer (RISC) processor) . A "circuit" may also be a processor executing software, e.g. any kind of computer program. Any other kind of implementation of the respective functions which will be described in more detail below may also be understood as a "circuit" in accordance with an alternative embodiment.

A computer program product is for example a computer readable medium on which instructions are recorded which may be executed by a computer, for example including a processor, a memory, input/output devices etc.

In one embodiment, the determining circuit is configured to determine a time period that should lie between two subsequent presence information exchange time periods and to determine the starting time of second presence information exchange time period based on the time period that should lie between two subsequent presence information exchange time periods.

For example, the time period that should lie between two subsequent presence information exchange time periods is a useful data exchange time period for exchanging useful data between a plurality of the group of communication devices.

In one embodiment, the allocation circuit allocates the first presence information exchange time period as a presence information exchange time period in which the communication device transmits a message for indicating that it is part of the communication device group.

In one embodiment, the allocation circuit allocates the second presence information exchange time period as a presence information exchange time period in which the communication device transmits a message for indicating that it is part of the communication device group.

The communication device may further comprise a receiving circuit configured to receive during one or both of the first presence information exchange time period and the second presence information exchange time period, from at least, one other communication device of the communication group, a message indicating that the other communication device is part of the communication group.

In one embodiment, the allocation circuit allocates a third presence information exchange time period as a presence information exchange time period and allocates the third presence information exchange time period as a presence information exchange time period in which the communication device does not transmit a message for indicating that it is part of the communication device group if the third presence information exchange time period starts earlier than the second presence information exchange time period.

The communication device may for example further comprise a receiving circuit configured to receive, during the third presence information exchange time period, a message indicating that the other communication device is part of the communication group from at least one other communication device of the communication group.

The communication device may further comprise a transmitting circuit configured to send a message for indicating that it is part of the communication device group during at least one of the first presence information exchange time period and the second presence information exchange time period.

In one embodiment, the communication device further comprises a transmitting circuit configured to send a message specifying the starting time of the second presence information exchange time period.

The communication device may further comprise a transmitting circuit configured to send a message specifying a candidate starting time of the second presence information exchange time period.

In one embodiment, the communication device further comprises a receiving circuit configured to receive a message specifying a_. candidate starting time of the second presence information exchange time period.

The determining circuit may for example be configured to determine the starting time of the second presence information exchange time period based on the number of other communication devices in the group of communication devices and the received candidate starting time.

For example, the determining circuit is configured to determine the starting time of the second presence information exchange time period as the maximum of the received candidate starting time and a calculated candidate starting time determined by the determining circuit based on the number of other communication devices in the group of communication devices .

In one embodiment, the determining circuit is configured to determine the starting time of the second presence information exchange time period based on the number of other communication devices in the group of communication devices and the starting time of the first presence information exchange time period.

The group of communication devices is for example a beacon group.

The first presence information exchange time period and the second presence information exchange time period are for example beacon periods.

In one embodiment, the communication device and the other communication devices of the group of communication devices are communication devices according to the ECMA standard. In this embodiment, the determining circuit may for example determine the starting time of the second presence information exchange time period based on the number of occupied beacon slots which depends on the number of communication devices in the group of communication devices.

The determining circuit is for example configured to determine the starting time of the second presence information exchange time period such that the starting time of the second presence information exchange time period is the later the more communication devices are in the group of communication devices.

The communication device 300 for example carries out a method for scheduling a message exchange as illustrated in figure 4.

FIG. 4 shows a flow diagram 400 according to an embodiment.

The flow diagram 400 illustrates a method for scheduling a message exchange.

In 401, a first presence information exchange time period for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices is allocated.

In 402, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining which communication devices are part of the group of communication devices is determined based on the number of other communication devices in the group of communication devices.

In 403, the second presence information exchange time period is allocated such that the second presence information exchange time period starts at the determined starting time.

In one embodiment, a variable superframe duration is used which is determined, depending on the number of occupied beacon slots in a beacon period. In one embodiment, when the number of occupied beacon slots as seen by a communication device (or reported to a communication device from another - communication device) grows or shrinks, the superframe duration used or intended to be used by the communication device also grows or shrinks respectively.

In one embodiment, the duration of a beacon slot during which a beacon may be sent by a communication device is 64μs. With a MAS (Medium Access Slot) duration of 25βμs, there are four beacon slots per MAS.

Let the number of occupied beacon slots as seen by a communication device (e.g. inclusive of the communication device's own beacon slot, i.e. including the beacon slot used by the communication device itself for sending its beacon) in a previous superframe be n, for example. Let L be defined as for example L =■ 256. In the following, an embodiment is described in which a methodology is used by which the communication devices in a communication network, e.g. the communication system 100 described above with reference to figure 1, may maintain a stable and variable superframe duration (with all communication devices able to know the duration of the next superframe) .

In one embodiment, each communication device of a communication group 105 includes in its beacon a Superframe

Occupancy Information Element (SOIE).. This information element allows the communication devices of the group of communication devices to synchronize the superframe length.

The format of the SOIE is for example as illustrated in table 1.

Table 1: SOIE format

^%λDev. Addr. i" (i from 1 to N) denotes the device address of the ith communication device as reported by the communication device sending the beacon containing the SOIE.

The value of the Length field is given by

2 or 3 or 4 + (1 or 2 or 3)M + K + 2 N depending on whether the current superframe duration is included in the SOIE and depending on the length of the BP (Beacon Period) Length field.

The SOIE provides information on the beacon period observed by the communication device sending the SOIE. The Beacon Period Length field is set to the length of the beacon period, measured in beacon slots. The Beacon Slot Info is for example as described in the ECMA standard as in e.g. [I]. The λ^λDev. Addr. i" fields correspond to beacon slots encoded as occupied in the Beacon Slot Info Bitmap. They are included in ascending beacon slot order. The Superframe Duration Change Countdown field indicates the number of superframes (with current superframe duration) after which the device will change its superframe duration, i.e. the superframe duration which it uses for a superframe. The countdown value in the Superframe Duration Change Countdown field is decremented by one every superframe (even when the beacon is skipped by the communication device, i.e. if the communication device does not transmit a beacon in a superframe) . The New Superframe Duration field gives the duration in multiples of L Medium Access Slots. The value in this field is the superframe duration to which the communication device will change its superframe duration after a superframe in which it sends a SOIE with the Superframe Duration Change Countdown value set as zero.

As an example, if the value in the New Superframe Duration field is 2 then the new superframe duration is 2 times L Medium Access Slots. The Current Superframe Duration field is optional and denotes the current superframe duration in multiples of L Medium Access Slots.

In one embodiment, the communication device sets the Superframe Duration Change Countdown field to a value equal to 32 by default (for example when the communication device is booting up and it is the only device) . The communication device sets the New Superframe Duration to be of value 1 by default (for example when the device is booting up and it is the only device) . As communication devices keep joining the communication group, the SOIE sent every superframe shall be updated every superframe according to rules given as follows .

In one embodiment, in every superframe, a device shall follow the following rules if the device does not relocate its BPST to an alien BPST (i.e., the BPST of an alien device not synchronized to the device where the alien device is not part of the device's communication group) :

A) If the device transmitted a beacon with Superframe Duration Change Countdown field set as a non-zero value in the previous superframe then 1. The device shall update its New Superframe Duration field in the current superframe by the maximum of (a) Ceil {n/(32c(n)) } where c(n) = Ceili^Ceil {n / 96} } and ^Λn' is the number of occupied beacon slots as reported in its previous SOIE plus number of its own beacon slots in the previous superframe

(b) Largest reported value of New Superframe Duration field from the set containing received beacons from neighbors in the previous superframe and its own transmitted beacon in the previous superframe if its Superframe Duration Change Countdown field in the current superframe is more than 8. Else, the device shall update its New Superframe Duration field in the current superframe by the largest reported value of New Superframe Duration field from the set containing received beacons from neighbors in the previous superframe and its own transmitted beacon in the previous superframe.

2. The device shall set its Superframe Duration Change Countdown field in the current superframe as the largest reported Sup^'erframe Duration Change Countdown field from the set containing received beacons in the previous superframe and its own transmitted beacon in the previous superframe minus one.

3. The device shall keep its Current Superframe Duration field unchanged from the previous superframe if it did not receive any beacon in the previous superframe reporting a larger Current Superframe Duration.

In the above, Ceil{x} ^'denotes the nearest integer to x greater than x. B) If the device transmitted a beacon in the previous superframe in which the Superframe Duration Change Countdown field had a zero value then

1. The device shall change its superframe duration for the current superframe to the maximum of the value included in the previous New Superframe Duration field transmitted by- it in the previous superframe times L Medium Access Slots and the maximum New Superframe Duration field reported in the received beacons in the previous superframe times L Medium Access Slots. The device shall update the Current Superframe Duration field accordingly in the current superframe.

2. The device shall set its Superframe Duration Change Countdown field in the current superframe as 32.

3. The device shall set its New Superframe Duration field in the current superframe as Ceil {n/ (32c (n) ) } .

If a device receives a beacon whose BPST is aligned to its own and with a SOIE reporting a larger Current Superframe Duration than its own in the current superframe, the device shall change its superframe duration in the current superframe to that larger Current Superframe Duration field value reported in the received beacon times L Medium Access Slots and

1. The device shall update its Superframe Duration Change Countdown field in the next superframe as 32.

2. The device shall update its New Superframe Duration field in the next superframe' s beacon as maximum of ( a ) Ceil { n/ ( 32 c ( n ) ) }

(b) largest reported value of New Superframe Duration in the set containing the received beacons from neighbors in the previous superframe and its own transmitted beacon in the previous superframe.

If a device skips its beacon .transmission in the current superframe, it shall still update the Superframe Duration Change Countdown, New Superframe Duration and Current Superframe Duration fields according to rules given above and in the next superframe, it shall update the above fields according to rules given above as though its beacon had been transmitted in the current superframe.

In the above, L may for example be chosen to be 256 or 32 or any other value.- In general, the above rules may be extended to the case where the superframe duration is given by Ceil{ (n+2) /4c (n) } times h where c(n) is an integer greater than or equal to one which may depend on n and h is an appropriate integer. An example of c(n) is Ceil{N₀/96} or Ceil{n/384}, where N₀ is the maximum number of neighbor devices reported by any device. Another example of c(n) is Ceil{log₂{2n} } .

In one embodiment, a communication device is allowed to send beacons in multiple beacon slots commensurate to the number of devices it needs to report in its beacons. A device shall have one primary beacon slot and possibly multiple extension beacon slots.

A bit in the Device Control field of the Beacon frame (see [1] ) can for example be set to ONE to inform other communication devices that it is an extended beacon. If the above bit is set to zero, then the beacon is a primary beacon.

For example, according to the current ECMA standard specification, with 53.3 Mbps data rate, the number of octets that can be accommodated in the payload of one Beacon frame is. limited and thus the number- of Dev. Addrs. that can be reported is limited.

As an example, according to one embodiment, if a communication device needs to report 384 communication devices, it may choose to include a few information elements and 96 Dev. Addrs. in a primary beacon and 96 Dev. Addrs. each in three extended beacons .

The way a device chooses an available beacon slot to send either a primary beacon or an extended beacon is for example the same as given in the ECMA specification. Communication groups with different BPSTs may be merged according to predetermined rules. However, when a communication device moves its BPST to the BPST of an alien beacon (i.e. of a beacon of a communication device of a different communication group) , the device shall also change to the superframe duration reported in the alien beacon. The communication device shall also set its Superframe Duration Change Countdown field in the superframe immediately after it moves its BPST to 32. The communication device shall also set its New Superframe Duration field based on the alien beacon it received. In one embodiment, when two communication groups come in proximity with each other, the devices in the two communication groups upon merger or synchronization may change their superframe duration to a duration equaling the sum of the previous two superframe durations of the two respective communication groups subject to a maximum superframe duration value. In one embodiment, beacon slot contraction and beacon slot collision related rules are used as specified by the ECMA standard, e.g. as in [1] .

In one embodiment, before an entering communication device actually joins a beacon group, it shall scan the channel for two superframes (with maximum superframe duration allowed in a communication group) and set its superframe duration to the value reported in a received beacon (e.g. the beacon received last) or set its superframe duration by measurement from two received beacons.

A communication device may report all the other communication devices in the communication device group in as few a number of beacons or beacon slots as possible. According to the embodiment described above, the superframe duration changes commensurate to the number of occupied beacon slots.

In one embodiment, the rules for interpreting an extended beacon are the same as that of interpreting a primary beacon except that the extended beacon reports additional Dev. Addrs. (communication device addresses) in addition to those reported in the primary beacon.

Synchronization schemes (see [2], [3]) exist that enable devices in a beacon group to be synchronized to clock period level and that can be used with the proposed embodiments. However, in such a synchronization scheme, the devices may need to resynchronize, for example, every fixed number of superframes in order to maintain clock period level synchronization. (It should be noted that according to the ECMA specification as in [1] , the communication devices need to realign or re-synchronize their BPSTs every superframe but still synchronization to clock period level is not achieved) . In one embodiment, a synchronization scheme is used that enables devices in a beacon group to be synchronized to clock period level and which requires resynchronization after a certain number of superframes along with the use of above embodiments.

It is possible to use slotted offset TFCs (Time Frequency Codes; see [2] , [3] ) wherein an OFDM (Orthogonal Frequency

Division Multiplexing) symbol is transmitted only during an OFDM Symbol Transmission Duration (OSTD) and all the nth numbered OSTDs of all devices counted from the start of a Medium Access Slot are aligned to clock ^'period level. In. one embodiment, there is a guard interval between every two

OSTDs. That is, in one embodiment, every Medium Access Slot has a set of OSTDs but there is a guard time embedded between every two OSTDs. The first OSTD of a Medium Access Slot begins at the start of the Medium Access Slot. The second OSTD in a Medium Access Slot follows the first OSTD after a guard time. This guard time (which is provided, in the general case., between two data transmission time intervals of a data transmission period) may be variable and adaptive commensurate to the superframe duration (or, for the general case, the duration of the data transmission period) . For example, if the superframe duration is less than M Medium Access Slots (say M = 4 times 256) , the guard time between two OSTDs is x ns (e.g. x=40) . Thereafter, for every additional M Medium Access Slots added to the superframe duration, an additional y ns (e.g. y = 10, assuming a minimum clock rate of 66MHz) is added to the guard time. The guard time in next superframe is for example the same as the guard time in the current superframe, if the superframe duration has not changed from the current to the next superframe.

In the following, an embodiment is described that may be seen as a modification of the embodiment described above. According to the embodiment described in the following, there is a bound on the maximum superframe duration while the beacon period length is variable and the superframe duration is variable as in the embodiment described above. In this embodiment, the structure of the SOIE is as shown in table 2.

Table 2: Alternate SOIE format

The value of the length field is for example given by (3 or 4 or 5) + K + 2 N in this embodiment, depending on whether the current superframe duration is included in the SOIE and on the length of the BP Length field.

Comparing tables 1 and 2, it can be seen that according to table 2, the New Superframe Duration field is fixed to 1 octet in comparison with possible variable M octets according to table 1. However, as in the embodiment described above, ■the superframe duration is adapted according to the number of occupied beacon slots. The New Superframe Duration and the Current Superframe Duration fields give the duration in terms of multiples of L Medium Access Slots. An alternate option of beaconing according to one embodiment is that as the number of neighbors of a communication device

(i.e. the number of other communication devices in the same beacon group) increases, the device may reduce the frequency with which it sends its primary and extended beacons. In particular, if the number of devices reported in its SOIE is less than a particular value S. (e.g. S = 384), the device shall send a beacon every superframe irrespective of the duration of the superframe. If the number of devices reported in its SOIE is greater than or equal to S times m and less than S times (m+1) where m is an integer greater than or equal to one, then it suffices for the device to send its beacon at least once in every m superframes. In this case, multiple devices may alternate using a 'same beacon slot, (and appropriate beacon collision and beacon contraction rules may be defined if a .device advertizes the next time slot it will use a particular beacon slot in its beacon) . The value of m may be upper bounded by a fixed value U, e.g. 4. According to the above, while beaconing, a^' communication device shall report in its SOIE all the- devices it has heard beacons from in the past U superframes and not just in the previous superframe. In one embodiment, the superframe duration grows or shrinks commensurate to the average number of occupied beacon slots, wherein the average is for example taken from the previous U superframes.

In one embodiment, even if a device is allowed to transmit a beacon only once every m superframes, the device shall update the New Superframe Duration field and the Superframe Duration Change Countdown field every superframe locally. Whenever a device transmits a beacon, it shall include in its beacon its updated New Superframe Duration and its updated Superframe Duration Change Countdown fields that were updated in the current superframe.

In the embodiment described above with reference to table 1, it was remarked that the superframe duration may be given by- Ceil { (n+2) /4c (n) }times h. If c(n) is chosen as a constant then the superframe duration increases linearly with n. If c(n) is chosen as Ceil {1+ Iog₂(n) } then the superframe duration grows only sub-linearly. The values of Ceil {1+ Iog₂(n)} can be efficiently stored in every device in the manner as given in table 3 below (requiring only a few bytes of memory) .

Table 3: Values of Ceil{l+ log₂ (n) } for different values of n.

Optionally, an upper • limit on the superframe duration may be introduced. In such a case, the choice of c(n) may for example be c(n) = Ceil{ ^{Ceil{n / 96}} } and the maximum number of communication devices may be optionally restricted to 382 communication devices. (The ECMA specification as described in [1] currently allows for a maximum of 94 devices in a beacon group; the above is valid discounting the use of signaling slots) . For the above embodiment, table 4 gives a comparison between the current superframe structure according to the ECMA specification and the superframe structure according to the embodiment. It has been assumed that a maximum of 96 Dev. Addrs. shall be reported in a beacon slot.

Table 4: Comparison between the ECMA specification superframe structure and the superframe structure according to an embodiment . In the following, modifications to Information Elements (IEs) in [1] are proposed in order to cater to the above embodiments assuming a maximum possible superframe duration.

In one embodiment, the Distributed Reservation Protocol (DRP) IE is used to negotiate or advertize a reservation for certain Medium Access Slots using a particular TFC offset. The format of the DRP IE is illustrated in table 5.

Table 5: DRP IE

The format of a DRP Allocation field is illustrated in table

Table 6: DRP Allocation field format

The Block Bitmap field identifies the blocks that contain reserved MASs. If a bit in the field is set to ONE, the corresponding block contains reserved MASs, where bit zero corresponds to block zero. There can be up to a total of eight blocks in a superframe. Each block has 16 zones and each zone has 16 MASs. The first MAS in a superframe is in block zero. In one embodiment, the DRP Availability IE is used by a device to indicate its view of the current utilization of MASs in the current superframe. The DRP Availability IE is illustrated in table 7.

Table 7: DRP Availability IE

The format of DRP MAS Availability field is given in table ^'8.

Table 8: DRP MAS Availability field format

The Block Bitmap field identifies the blocks that contain available MASs. If a bit in the field is set to ONE, the corresponding block contains available MASs, where bit zero corresponds to block zero. There can be up to a total of eight blocks in a superframe. Each block has 16 zones and each zone has 16 MASs. The first MAS in a superframe is in block zero.

The Zone Bitmap identifies the zones that contain available MASs. Bit zero corresponds to zone zero.

The MAS Bitmap specifies which Medium Access Slots in the zones identified by the Zone Bitmap field are available. If a bit in the field is set to ONE, the corresponding Medium Access Slots within the zones identified by the Zone Bitmap field are available, where bit zero corresponds to MAS zero within each zone.

Interpretation field can be as in [2], [3] .

Recently, China' s Wireless Personal Area Network (C-WPAN) working group within NITS (National Information Technology Standardization Technical Committee) worked on a Dual Carrier Time Frequency Codes (DC-TFCs) based scheme towards standardization. This scheme uses two non-adjacent bands with 264MHz bandwidth each to transmit OFDM symbols simultaneously with specific hopping patterns. Considering China's spectrum availability for UWB technology, a band plan that divides the entire available band in to 10 smaller^' bands with bandwidth of 264MHz each was proposed as part of the standardization activities in China. The available Chinese spectrum with the above proposed band plan for UWB transmission is shown in figure 5.

FIG. 5 shows a frequency spectrum diagram 500.

The frequency spectrum diagram 500 illustrates the available spectrum for UWB transmission in china.

Frequencies are indicated in the direction of a first axis (x-axis) 501 and availability is indicated in the direction of a second axis (y-axis) 502.

In the following, options for channelization for embodiments used in accordance with the Chinese spectrum allocation (as shown in figure 5) pertaining to UWB transmission with DC- TFCs are given. According to one embodiment, 25 channels are used. All the channels that hop across the same number of dual bands are uniform in the sense that the dual bands have just one band gap between the two bands of the dual bands. The listing of the above 25 channels is given in table 9 below. In each of the channels that use hopping across dual bands slotted offset TFCs [2] , [3] can be used. In this first option, channel 1 may be optionally dedicated for use as a control channel for the purpose of inter beacon group communication. In such a case, a communication device that transmits in its data channel (2-25) shall send at-least one control frame in the control channel (if given an opportunity) every fixed number of superframes.

The above control frame shall be transmitted in the control channel and shall be addressed to broadcast address. The proposed format of the control frame that should be transmitted in the control channel according to one embodiment is given in table 10. The control frame shall carry information about the channel number (included as part of the channel information octet; see table 9) the device's beacon group, is operating in. In an alternate option, instead of a control frame, a beacon frame with Channel Information IE may be used. An entering device or a start up device shall always scan all the channels and shall either join the operating channel or choose an orthogonal channel. The channel scan is for example made simple by the use of control channel. If an entering device for a fixed number of superframes does not find any control frame or beacon frame in the control channel that reports the use of a particular channel, then that particular channel may be inferred as vacant . In one embodiment, CSMA (sensing may be based on preamble) with back off is used as channel access scheme to access the control channel. In one embodiment, every device (the device that may transmit OFDM data or beacon in a data channel) has to contend for the control channel to transmit at least one control frame or beacon frame in the control channel in every superframe or in every fixed number of superframes. If the control channel is sensed to be busy (which may be based on preamble detection) a random back off is invoked in the control channel. An active operating device in a beacon group that detects a simultaneously operating beacon group in another channel (based on the reception of a control frame ^'or beacon frame in the control channel) shall do one of the following if the other channel number ^' (see table 9) has a lower value than its current operating channel number:

1. Issue a channel change IE to change to the channel number of the other channel, or

2. Change to an orthogonal channel orthogonal to other channel.

In one embodiment, an active operating device in a beacon group that detects a simultaneously operating beacon group in another channel (based on the reception of a control frame or beacon frame in the control channel) shall not issue a channel change IE to change to the channel number of the other beacon group if the other channel number has a higher value than its current operating channel number and the other channel number is not orthogonal to the current operating channel. Hopping across four dual bands

Hopping across three dual bands

Hopping ^• across two dual bands

Table 9: Channelization for UWB transmission in Chinese allocated spectrum.

Table 10: Format of a control frame transmitted in the control channel.

According to a second option pertaining to channelization, there are only two channels - channel 1 and channel 2. However, channel 2 is proposed to be used using slotted offset TFCs (see [2], [3]) . Channel 1 may either be used as a data channel or a control channel. If channel 1 is used as a control channel, then beacons for the beacon group that may operate in channel 2 may be sent in channel 1. Channel 1 may also be used for other reserved control purposes for example device discovery or other low duty cycle control signals or such (keeping in view the power spectral requirements) . It should be noted that in this second option since only one beacon group may exist in the bandwidth provided by bands 3- 10, the problem of overlapping channel interference does not arise. It should also be noted that though only one beacon group may exist in the bands 3-10, the entire bandwidth of bands 3~10 may be used effectively using slotted offset TFCs and much more than 94 (current limit on the number of devices in a beacon group according to ECMA specification) devices (even up to 1000 devices) can be catered to using the embodiments described above.

The channelization options according to embodiments (e.g. with regard to the Chinese UWB spectral allocation) aid communication devices operating in simultaneously operating beacon groups that are in proximity with each other to avoid interfering with each other.

The following documents are cited in the specification:

[1] Standard ECMA-368, High Rate Ultra Wideband PHY and MAC Standard, Dec. 2007 [2] WO 2009/038545 Al [3] WO 2009/054812 Al

Claims

1. Communication device comprising an allocation circuit configured to allocate a first presence information exchange time period for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices; and a determining circuit configured to determine, based on the number of other communication devices in the group of communication devices, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining- which communication devices are part of the group of communication devices; wherein the allocation circuit is further configured to allocate the second presence information exchange time period, such that the second presence information exchange time period starts at the determined starting time.

2. The communication device according to claim 1, wherein the determining circuit is configured to determine a time period that should lie between two subsequent presence information exchange time periods and to determine the starting time of second presence information exchange time period based on the time period that should lie between two subsequent presence information exchange time periods.

3. The communication device according to claim 2, wherein the time period that should lie between two subsequent presence information exchange time periods is a useful data exchange time period for exchanging useful data between a plurality of the group of communication devices.

4. The communication device according to any one of claims 1 to 3, wherein the allocation circuit allocates the first presence information exchange time period as a presence information exchange time period in which the communication device transmits a message for indicating that it is part of the communication device group.

5. The communication device according to any one of claims 1 to 4, wherein the allocation circuit allocates the second presence information exchange time period as a presence information exchange time period in which the communication device transmits a message for indicating that it is part of the communication device group.

6. The communication device according to any one of claims 1 to 5, further comprising a receiving circuit configured to receive during one or both of the first presence information exchange time period and the second presence information exchange time period, from at least one other communication device of the communication group, a message indicating that the other communication device is part of the communication group.

7. The communication device according to any one of claims

1 to 6, wherein the allocation circuit allocates a third presence information exchange time period as a presence information exchange time period and allocates the third presence information exchange time period as a presence information exchange time period in which the communication device does not transmit a message for indicating that it is part of the communication device group if the third presence information exchange ^'time period starts earlier than the second presence information exchange time period.

8. The communication device according to claim 7, further comprising a receiving circuit configured to receive, during the third presence information exchange time period, a message indicating that the other communication device is part of the communication group from at least one other communication device of the communication group.

9. The communication device according to any one of claims 1 to 8, further comprising a transmitting circuit configured to send a message for indicating that it is part of the communication device group during at least one of the first presence information exchange time period and the second presence information exchange time period.

10. The communication device according to any one of claims 1 to 9, further comprising a transmitting circuit configured to send a message specifying the starting time of the second presence information exchange time period.

11. The communication device according to any one of claims 1 to 10, further comprising a transmitting circuit configured to send a message specifying a candidate starting time of the second presence information exchange time period.

12. The communication device according to any one of claims 1 to 11, further comprising a receiving circuit configured to receive a message specifying a candidate starting time of the second presence information exchange time period.

13. The communication device according to claim 12, wherein the determining circuit is configured to determine the^' starting time of the second presence information exchange time period based on the number of other communication devices in the group of communication devices and- the received candidate starting time.

14. The communication device according to claim 13, wherein the determining circuit is configured to determine the starting time of the second presence information exchange time period as the maximum of the received candidate starting time and a calculated candidate starting time determined by the determining circuit based on the number of other communication devices in the group of communication devices.

15. The communication device according to any one of claims 1 to 14, wherein the determining circuit is configured to determine the starting time of the second presence information exchange time period based on the number of other communication devices in the group of communication devices and the starting time of the first presence information exchange time period.

16. The communication device according to any one of claims

1 to 15, wherein the group of communication devices is a beacon group.

17. The communication device according to claim 16, wherein the first presence information exchange time period and the second presence information exchange time period are beacon periods .

18. The communication device according to any one of claims 1 to 17, wherein the communication device and the other communication devices of the group of communication devices are communication devices according to the ECMA-368 standard.

19. The communication device according to any one of claims 1 to 18, wherein the determining circuit is configured to determine the starting time of the second presence information exchange time period such that the starting time of the second presence information exchange time period is the later the more communication devices are in the group of communication devices.

20. Method for scheduling a message exchange comprising allocating a first presence information exchange time period for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices; determining, based on the number of other communication devices in the group of communication devices, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining which communication devices are part of the group of communication devices; allocating the second presence information exchange time period, such that the second presence information exchange time period starts at the determined starting time .

21. Computer program product comprising instructions which, when executed by a computer, make the computer perform a method for scheduling a message exchange comprising allocating a first presence information exchange time period for exchanging messages between a plurality of communication devices of a group of communication devices for determining which communication devices are part of the group of communication devices; determining, based on the number of other communication devices in the group of communication devices, a starting time of a second presence information exchange time period for exchanging messages between a plurality of the communication devices of the group of communication devices for determining which communication devices are part of the group of communication devices; allocating the second presence information exchange time period, such that the second presence information exchange time period starts at the determined starting time .