WO2009044327A1

WO2009044327A1 - Method of gaining access to a network

Info

Publication number: WO2009044327A1
Application number: PCT/IB2008/053953
Authority: WO
Inventors: Matthew P. J. Baker; Timothy J. Moulsley; Olivier J-M. Hus; Paul Bucknell
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-10-01
Filing date: 2008-09-29
Publication date: 2009-04-09
Also published as: TW200935811A

Abstract

The present invention relates to a method for gaining access to a network by a secondary station, said network comprising at least one primary station, said method comprising transmitting at a secondary station a message to the primary station using a retransmission protocol, wherein the number of allowed retransmissions of the message from the secondary station has a dependency on a current parameter of the secondary station.

Description

METHOD OF GAINING ACCESS TO A NETWORK

FIELD OF THE INVENTION

The present invention relates to a method of gaining access to a network transmission resource, and to a radio station operated in accordance with this method.

This invention is, for example, relevant for mobile communication systems as UMTS (Universal Mobile Telecommunication System) or for other communication systems with for instance a random access channel.

BACKGROUND OF THE INVENTION

In a conventional telecommunication system in a network, a Random Access Channel (RACH) is used for gaining access to the network. In a random access procedure, a base station or a primary station allocates a physical resource to mobile stations or secondary stations. This resource can be a time slot, and/or a frequency subcarrier, and/or a code. Conventionally, this random access procedure, as illustrated on Fig.l, begins with the request of a secondary station UEl sent to the primary station. On Fig.l, data transmitted by the secondary station UEl is depicted by time chart 1, and data transmitted by the primary station is depicted by time chart 3. The request sent by the secondary station UEl is named a Random Access preamble 10 in a UMTS system, and contains a randomly selected signature. This request can be sent in a particular block of the RACH, i.e. in a particular time- frequency block, randomly selected out of a set of time- frequency blocks.

Then, the primary station may respond to this preamble in a Random Access Response 20 by granting transmission resources for transmission of the next message, and providing the secondary station UEl with a temporary identifier, called in UMTS a temporary Cell-specific Radio Network Temporary Identifier (denoted C-RNTI). The secondary station UEl then transmits its message 30 containing DATA 1 in the allocated physical resource.

However, in such a random access procedure, collisions occur when a plurality of secondary stations send their respective random access preamble in the same time- frequency block with the same preamble signature. For instance in the previous example, a second secondary station UE2, whose transmission is illustrated on time chart 2, transmits a Random Access Preamble 10 in the same time-frequency block and with the same preamble signature as the secondary station UEl. In such a case, the primary station is not able to determine that a plurality of secondary stations UEl and UE2 are requesting resources, and sees the preamble request as originated by only one secondary station. Then, the primary station will reply by means of the response 20 by providing them with a temporary C-RNTI in this example, and a resource grant. When the plurality of secondary stations UEl and UE2 send their messages 30 and 40, which contain respectively DATAl and DATA2, in the allocated resource block, it is likely that the primary station will not be able to decode the received superimposed messages, as each message, which corresponds to each secondary station of the plurality of stations, is different.

Therefore, the primary station will indicate that the message was not received correctly by means of a non-acknowledgement message (denoted NACK) 50. This can be done for instance with an ARQ protocol (Automatic Repeat-reQuest protocol), such as an Hybrid-ARQ protocol. When receiving the NACK 50, the secondary stations UEl and UE2 will retransmit simultaneously DATAl and DATA2 respectively in corresponding messages 60 and 70. However, it is likely that no retransmissions will succeed, since each time the messages will be superimposed. Consequently, the secondary stations UEl and UE2 have to start the Random Access procedure again, which leads to delays, inefficient use of radio communication resources and wasted power.

SUMMARY OF THE INVENTION

It is then an object of the invention to alleviate the above-identified problems.

Another object of the invention is to propose a method for gaining access to a network, which permits to limit the consequences of a collision.

Still another object of the invention is to propose a method for gaining access, which reduces the delays due to collisions for some category of secondary stations.

To this end, the invention proposes a method for gaining access to a network by a secondary station, said network comprising at least one primary station, said method comprising the secondary station transmitting a message to the primary station using a retransmission protocol, wherein the number of allowed retransmissions of the message from the secondary station has a dependency on a current parameter of the secondary station.

As a consequence, if a collision occurs between two secondary stations, the secondary station having the highest number of allowed retransmissions will have a chance to retransmit its data alone, since the other secondary station will have reached its respective number of allowed retransmissions earlier. Thus, at least one of the secondary stations will have access to the network, which thus limits the consequences of a collision. Moreover, in some variants of the invention, it is possible to provide a higher number of allowed retransmissions for secondary stations of a predetermined category, like being of a particular subscription level or having for instance high priority data to transmit (for example safety-critical data or emergency calls).

The present invention also relates to a secondary station comprising means for carrying out the method in accordance with the invention.

In another aspect of the invention, a primary station is proposed comprising means for carrying out the method in accordance with the invention.

These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein:

Fig.l, already described, is a set of time charts representing a conventional random access procedure between a plurality of secondary stations and a primary station;

Fig.2 is a block diagram representing a network comprising a primary station and a secondary station in accordance with the invention;

Fig.3 is a set of time charts representing a random access procedure implemented in accordance with the invention; and - Fig.4 is a flow chart representing a method in accordance with the invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system of communication 300 as depicted in Fig.2, comprising a primary station 100, like a base station or an evolved Node B (eNodeB), and at least one secondary station 200 like a mobile station or a User Equipment (also denoted UE).

The radio system 300 may comprise a plurality of the primary stations 100 and/or a plurality of secondary stations 200. The primary station 100 comprises a transmitter means 110 and a receiving means 120. An output of the transmitter means 110 and an input of the receiving means 120 are coupled to an antenna 130 by a coupling means 140, which may be for example a circulator or a changeover switch. Coupled to the transmitter means 110 and receiving means 120 is a control means 150, which may be for example a processor. The secondary station 200 comprises a transmitter means 210 and a receiving means 220. An output of the transmitter means 210 and an input of the receiving means 220 are coupled to an antenna 230 by a coupling means 240, which may be for example a circulator or a changeover switch. Coupled to the transmitter means 210 and receiving means 220 is a control means 250, which may be for example a processor. Transmission from the primary radio station 100 to the secondary station 200 takes place on a first channel 160 and transmission from the secondary radio station 200 to the first radio station 100 takes place on a second channel 260.

As explained before, when a secondary station 200 wishes to gain access to the network, it must make a request to the primary station 100 on the Random Access Channel (RACH) for instance. Then, the primary station 100 replies and indicates an allocated resource for the transmission of data. The secondary station uses the allocated resource for transmitting its data.

However, in a conventional telecommunication system, when a collision occurs between a plurality of stations, it may result in none of the colliding stations gaining access to the network, which causes delays, inefficient use of radio communication resources and wasted power. A system in accordance with a first embodiment of the invention permits access more easily for the secondary stations 200 and reduces the delay. This embodiment will be illustrated with the help of both Figs.3 and 4. Fig.3 comprises a time chart 401 corresponding to the transmissions of a first secondary station UEl, a time chart 402 corresponding to the transmissions of second secondary station UE2, and a time chart 403 corresponding to the transmissions from the primary station 100. In accordance with this embodiment, if the two secondary stations UEl and UE2 request access with respective Random Access Preambles 410 in the same resource block, and with the same signature, at step S400 of Fig.4, the primary station 100 has the impression of being contacted by only one secondary station. At step S401, the primary station transmits a reply 420, which is received by the two secondary stations UEl and UE2. In this reply 420, the primary station indicates an allocated resource that is to be used by the secondary stations for transmitting their data. Furthermore, the primary station 100 may transmit a temporary Identifier, for instance a C-RNTI as explained above. This temporary identifier is used for the connection of the secondary stations, and expires some time after generation. Some secondary stations that have already had access to the network may already possess a still-valid temporary identifier. This is the case for "non initial access" UEs. At step S402, both secondary stations UEl and UE2 will transmit their respective data DATAl and DATA2 in corresponding messages 430 and 440. These messages may further include the Non- Access Stratum UE IDs (denoted NAS UE ID) of the corresponding secondary stations, e.g. the International Mobile Subscriber Identity (denoted IMSI). Because DATAl and DATA2 are different, it is likely that none of the messages 430 and 440 will be correctly received by the primary station, at step S403. If the primary station 100 is able to decode the received message -which is for instance the case if there is no collision, or if one of the two colliding messages was received very weakly- it will signal an ACK to the secondary stations at step S404, in accordance with a predetermined retransmission protocol, such as ARQ, or HARQ or similar. The ACK signal indicating that the message was correctly received can be conveyed for instance by an absence of reply. In such a case, at step S404, the primary station will transmit nothing to the secondary station.

If the primary station is not able to decode the message, it will signal a NACK 450 at step S405. When the secondary stations UEl and UE2 receive this Non- Acknowledgement message 450, they will check how many times they are allowed to retransmit the message (S406). In accordance with the invention, the number of allowed retransmissions, meaning how many times each secondary station is allowed to try again to transmit its message, may differ from one secondary station to another. In the first embodiment, this number depends on whether each secondary station already possessed a temporary identifier, such as the C-RNTI, before requesting access. This means that it depends on whether a secondary station is performing a non- initial access, and whether the temporary identifier is still valid. If this is the case, the number of allowed retransmissions will be a maximum or at least greater than the number of allowed retransmissions corresponding to initial-access secondary stations.

In this example, the secondary station UEl is performing a non- initial access, and the secondary station UE2 is performing an initial access. Then, the number of allowed retransmissions of UE2, for instance 1, is lower than the number of allowed retransmissions of UE 1, for instance 3. Thus, as can be seen clearly on figure 3, the secondary stations UEl and UE2 retransmit their data once in respective messages 460 and 470. Then, the number of retransmissions for the secondary station UE2 reaches the number of allowed retransmissions, and the secondary station UE2 aborts its access attempt at step S407. Consequently, for the second try 490, the secondary station UEl is the only transmitting station, which is then well received by the primary station 100, and acknowledged by an ACK 500 at step S404.

This makes it possible to give an advantage to secondary stations that already had had access to the network. This is advantageous as such secondary stations may be already well-adapted to the considered network. The invention prevents the collision causing the rejection of all the colliding secondary stations, but results eventually in a successful access. Consequently, the number of secondary stations experiencing delay is reduced.

This is also useful as secondary stations with a C-RNTI are likely to be accessing the network regularly, and repeated delays would be undesirable for the user's experience, while the longer delay would occur only for the initial access.

In a variant of this embodiment, the secondary stations that have not had access to the network already, i.e. secondary stations not possessing a temporary access identifier, could be given an advantage, by making the number of allowed retransmissions corresponding to this case greater than the number of allowed retransmissions corresponding to the "non-initial access" stations. This would make it possible to provide easy access for stations that have not had the chance to have access to the resources. Thus, all stations may have the possibility to have access to the network. In this first embodiment, the number of allowed retransmissions corresponding to each category (non-initial access/initial access) is signalled to the secondary stations on a broadcasting channel (BCH). However, in a variant of this embodiment, if there are two categories for instance (non-initial access/initial access), only the number of allowed retransmissions corresponding to the initial access category for example is signalled to the secondary stations. Thus, when the secondary stations enter the retransmission protocol, the "initial access secondary stations" will stop retransmitting their data after reaching the signalled number of allowed retransmissions. The "non-initial secondary stations" will continue retransmitting their data until the primary station receives the message correctly or until the primary station stops sending NACKs.

Thus, this avoids signalling the number for this category, reducing the overhead, and allowing greater flexibility in the selection of the numbers of allowed retransmissions. This would also allow the system to operate with no defined limit on the number of retransmissions i.e. the maximum number of retransmissions could be infinity.

In another variant of the first embodiment, the categories may be chosen differently, or further categories may be added. Thus, it is possible to select the numbers of allowed retransmissions out of a set of predetermined values on the basis of the priority of the secondary stations for example. It means that a secondary station having safety messages or emergency messages to send is privileged so its access to the resource is facilitated. The priority could be estimated on the basis of the data to be transmitted, or on a characteristic of the secondary station, for instance properties of the subscription. For example, some users may have restricted access to a service due to their subscription. In such a case, the number of allowed retransmissions could be low for this category.

Another possibility would be to use the number of failed attempts of gaining access to the network to estimate the priority of the considered secondary station. Thus, the more a secondary station fails gaining access to the network, the higher the number of allowed retransmissions is, and the more likely the next attempt will be successful.

In a second embodiment of this invention, the number of allowed retransmissions could be selected for each secondary station by a specific algorithm. For instance, the number of allowed retransmissions may take three different values, e.g. 1, 3, and 5. Each secondary station selects with an algorithm one of these values. For instance, each secondary station may use a characteristic, such as a permanent identifier, like the serial number of its processor, or the IMSI, and applies it in a predetermined function to select one of the three possible values (for example secondary stations with an odd- valued NAS ID could select one maximum number of retransmissions while secondary stations with an even- valued NAS ID would select a different maximum number of retransmissions). Another possibility is to select the number of allowed retransmissions at random (or derive the number of retransmissions as a predetermined function of a randomly generated number), before each access request for example. In practice, generation of sufficiently random numbers may be achieved by a pseudo-random process, so in this description "random" may also mean "pseudo-random". In some embodiments, the predetermined function may be the identity function, in which the output number is the same as the input number.

This second embodiment would address the fact that the first embodiment of the invention described above does not help in the case of a collision between a plurality of secondary stations which possess a C-RNTI, or between a plurality of secondary stations which do not possess a C-RNTI. From the second embodiment, it is to be understood that the number of allowed retransmissions for a given secondary station may change from one access attempt to another.

The maximum numbers of retransmissions may be selected from a set of numbers known to the primary station. After reaching each maximum number of retransmissions in the set, the primary station would flush its buffer and start the reception process again; at each such occasion, the secondary stations which had selected the lower maximum number of retransmissions would stop retransmitting, leaving a reduced number of the originally-colliding secondary stations to continue retransmitting and improving the chance for the primary station to decode a data message.

It is to be noted that the first and the second embodiments could be combined. For example, for "initial-access secondary stations", the number of allowed retransmissions can be selected at random out of a first subset of possible values, e.g. 1 and 2. Similarly, the number of allowed retransmissions for "non-initial access secondary stations" can be selected at random out of a second subset of possible values, e.g. 3 and 4. Thus, if a collision occurs with for instance three secondary stations, including one "initial access secondary station", and two "non-initial access secondary stations", it is possible that the respective numbers of allowed retransmissions of the non- initial access secondary stations are different, leading to the access of one secondary station at the end of the retransmission protocol. In another example, when the colliding secondary stations are only "initial access secondary stations", it is possible that the retransmission protocol ends with one secondary station gaining access to the network.

In either embodiments of the invention, the preconfiguration of the maximum numbers of retransmissions may comprise predetermination, or higher-layer signalling (for example on a broadcast channel).

It is to be noted that this invention applies not only to mobile communication systems using ARQ or HARQ as part of the random access procedure, such as the LTE of UMTS, but is possibly also applicable to other standards such as evolutions of WiMAX or cdma2000.

Indeed, the present invention is not limited to the UMTS or mobile telecommunications systems described herein as an example, but could be extended to any other communication systems allocating resources to terminals requesting access, including for example ad-hoc networks where peer entities may compete for access to the transmission resources.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features, which are already known in the art of radio communication and the art of transmitter power control and which may be used instead of or in addition to features already described herein.

Claims

1. A method for gaining access to a network by a secondary station, said network comprising at least one primary station, said method comprising the secondary station transmitting a message to the primary station using a retransmission protocol, wherein the number of allowed retransmissions of the message from the secondary station has a dependency on a current parameter of the secondary station.

2. The method of claim 1, wherein the current parameter comprises a number selected using a predetermined selection algorithm, and wherein the dependency comprises setting the number of allowed retransmissions as a function of the number selected using the predetermined selection algorithm.

3. The method of claim 2, wherein the predetermined selection algorithm makes a random selection from a set of predetermined numbers.

4. The method of claim 2, wherein the predetermined selection algorithm comprises deriving a function of a characteristic of the secondary station.

5. The method of claim 4 wherein the characteristic of the secondary station is a permanent identifier of the secondary station or one of its component parts.

6. The method of any of claims 2 to 5, wherein the function is the identity function.

7. The method of claim 2, wherein the current parameter comprises whether the secondary station already possesses an access identifier, and wherein the dependency comprises selecting the number of allowed retransmissions to a first value if the secondary station already possesses an access identifier and to a second value if the secondary station does not already possess an access identifier.

8. The method of claim 7, further comprising the network providing the access identifier to the secondary station.

9. The method of claim 7 or 8, wherein the first value is greater than the second value.

10. The method of claim 7 or 8, wherein the first value is less than second value.

11. The method of any of the preceding claims, wherein if the secondary station already possesses an identifier, the secondary station will cease retransmission of the message if the secondary station stops receiving negative acknowledgement signals.

12. The method of any of the preceding claims, wherein if the secondary station does not already possesses an identifier, the secondary station will cease retransmission of the message if the secondary station stops receiving negative acknowledgement signals.

13. The method of any one of claims 5 to 11, wherein one of the first and second numbers is predetermined.

14. The method of any of the preceding claims, wherein the number of allowed retransmissions is signalled by the network.

15. The method of any of the preceding claims, wherein the number of allowed retransmissions is selected at least partly on the basis of a priority level of the secondary station.

16. The method of claim 15, wherein the priority level of the secondary station depends on a property of data to be transmitted.

17. The method of claim 15 or 16, wherein the priority level of the secondary station depends on a number of previous failed attempts to access the network.

18. A primary station comprising means for carrying out the method of the preceding claims.

19. A secondary station comprising means for gaining access to a network, said network comprising at least one primary station, wherein the secondary station comprises means for transmitting a message to the primary station using a retransmission protocol, wherein the number of allowed retransmissions of the message from the secondary station depends on a current parameter of the secondary station.