1. A communication device for use in a communication system in which a base station operates a cell that is selectable by the communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the communication device comprising: means for selecting said cell when said communication device is in said region where connection attempts by the communication device to said base station operating that cell are at risk of failure; means for attempting, within said region, to initiate a connection with said base station that operates said cell when said cell has been selected; means for identifying consecutive failures in respective attempts to initiate said connection; means for logging information identifying that consecutive failures in said respective attempts to initiate said connection have occurred; and means for reporting said information identifying that consecutive failures in said respective attempts to initiate said connection have occurred to said communication system at a reporting opportunity.

2. The communication device of claim 1 further comprising means for receiving, from a base station, configuration data for configuring the communication device to log information identifying that consecutive failures in respective attempts to initiate said connection have occurred.

3. The communication device of claim 2 wherein said receiving means is operable to receive said configuration data using at least one signalling message.

4. The communication device of claim 3 wherein said at least one signalling message comprises at least one ‘UE Information Request’ message.

5. The communication device of any of claims 1 to 4 wherein said selecting means is operable to consider said at least one selected cell to be barred from at least one subsequent (re)selection when said identifying means identifies consecutive failures in respective attempts to initiate said connection.

6. The communication device of any of claims 1 to 5 wherein said region where connection attempts by the communication device to the base station operating that cell are at risk of failure comprises a region in which downlink signals from said base station are stronger than downlink signals from other base stations but in which uplink signals from said 37 communication device cannot be successfully received by said base station.

7. A base station for use in a communication system in which the or another base station operates a cell that is selectable by a communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the base station comprising: means for establishing a connection with a communication device via a cell operated by said base station; means for sending information to said communication device, using said established connection, for configuring said communication device for logging information identifying that consecutive failures in respective attempts to initiate a connection with the or another base station of said communication system have occurred.

8. The base station of claim 7 further comprising means for receiving a report from said communication device comprising said information identifying that consecutive failures in respective attempts to initiate a connection have occurred.

9. The base station of claim 8 wherein said receiving means is operable to receive said information identifying that consecutive failures in said respective attempts to initiate said connection have occurred using at least one signalling message.

10. The base station of claim 9 wherein said at least one signalling message comprises at least one ‘UE Information Response’ message.

11. The base station of claim 9 or 10 wherein said at least one signalling message comprises at least one information element (IE).

12. The base station of claim 11 wherein said at least one information element comprises at least one of: an information element for indicating that a consecutive connection establishment failure has occurred (e.g. a ‘Consecutive Conn Est Fail’ IE), an information element for indicating the number of consecutive connection establishment failures that has occurred, (e.g. a ‘Num of Consecutive Conn Est Fail’ IE); and an information element for indicating that a random access response failure has occurred (e.g. a ‘RAR failure’ IE).

13. The base station of any of claims 7 to 12 wherein said information identifying that consecutive failures in said respective attempts to initiate said connection have occurred comprises information identifying at least one of: information identifying said selected cell, information identifying a geographical location of said communication device; information identifying a time when said consecutive failures in said respective attempts to initiate said connection have occurred; information identifying a signal strength of said 38 selected cell; information identifying a signal strength of at least one neighbour cell; and information identifying that a random access response has been detected.

14. The base station of any of claims 7 to 13 wherein said sending means is operable to send information to said communication device for configuring said communication device for logging information identifying that consecutive failures in attempting to establish a Radio Resource Control (RRC), and/or a Media Access Control (MAC), connection has occurred.

15. A communication system comprising a communication device according to any of claims 1 2 to 4, 5 and 6 and a base station according to any of claims7 to 14.

16. A communication device for use in a communication system in which a base station operates a cell that is selectable by the communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the communication device comprising: a processor configured to: select said cell when said communication device is in said region where connection attempts by the communication device to said base station operating that cell are at risk of failure; a transceiver configured to: attempt, within said region, to initiate a connection with said base station that operates said cell when said cell has been selected; said processor configured to: identify consecutive failures in respective attempts to initiate said connection; and log information identifying that consecutive failures in said respective attempts to initiate said connection have occurred; and said transceiver configured to: report said information identifying that consecutive failures in said respective attempts to initiate said connection have occurred to said communication system at a reporting opportunity.

17. A base station for use in a communication system in which the or another base station operates a cell that is selectable by a communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the base station comprising: a processor; and a transceiver configured to 39 establish a connection with a communication device via a cell operated by said base station; and send information to said communication device, using said established connection, for configuring said communication device for logging information identifying that consecutive failures in respective attempts to initiate a connection with the or another base station of said communication system have occurred.

18. A method performed by a communication device in a communication system in which a base station operates a cell that is selectable by the communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the method comprising: selecting said cell when said communication device is in said region where connection attempts by the communication device to said base station operating that cell are at risk of failure; attempting, within said region, to initiate a connection with said base station that operates said cell when said cell has been selected; identifying consecutive failures in respective attempts to initiate said connection; logging information identifying that consecutive failures in said respective attempts to initiate said connection have occurred; and reporting said information identifying that consecutive failures in said respective attempts to initiate said connection have occurred to said communication system at a reporting opportunity.

19. A method performed by a communication device in a communication system in which a base station operates a cell that is selectable by the communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the method comprising: selecting said cell when said communication device is in said region where connection attempts by the communication device to the base station operating that cell are at risk of failure; attempting, within said region, to initiate a connection with said base station that operates said cell when said cell has been selected; receiving control data authorising said communication device to (re)select a different cell when it is identified that at least one failure in an attempt to initiate said connection has occurred; identifying that at least one failure in an attempt to initiate said connection has 40 occurred; and (re)selecting a different cell responsive to receipt of said control data and identification of at least one failure in an attempt to initiate said connection.

20. A method performed by a base station in a communication system in which the or another base station operates a cell that is selectable by a communication device in a region where connection attempts by the communication device to the base station operating that cell are at risk of failure, the method comprising: establishing a connection with a communication device via a cell operated by said base station; and sending information to said communication device, using said established connection, for configuring said communication device for logging information identifying that consecutive failures in respective attempts to initiate a connection with the or another base station of said communication system have occurred.

21. A method performed by a base station for operating, in a communication system, a cell that is selectable by any of a plurality of communication devices in a region where connection attempts by a communication device to said base station are at risk of failure, the method comprising: obtaining information for use in identifying that said cell is a cell that covers a region in which the cell is selectable by a communication device and in which connection attempts by that communication device to said base station are at risk of failure; and providing, to a communication device attempting to initiate connection to said base station, control data authorising said communication device to (re)select a different cell when said communication device identifies that at least one failure in an attempt to initiate said connection has occurred.

22. A computer implementable instructions product comprising computer implementable instructions for causing a programmable communications device to perform the method of any of claims 18 to 21.

DESCRIPTION
Title of the Invention
COMMUNICATION SYSTEM
5
Technical Field
The present invention relates to a communication system. The invention has particular,
but not exclusive, relevance to establishing a radio connection in Long Term Evolution (LTE)
systems as currently defined in associated 3rd Generation Partnership Project (3GPP) standards
10 documentation.
Background Art
A communication system operating according to the LTE standards typically comprises
an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet
15 Core (EPC) network. A base station (eNB) of the E-UTRAN provides User Equipment (UE),
such as mobile telephones, access to the core network (and hence to other user equipment or
other network nodes) via one or more of the base station’s cells.
Communication between the mobile telephones and the base station is controlled using a
Radio Resource Control (RRC) protocol as defined in 3GPP TS 36.331 V11.4.0. RRC handles
20 the control plane signalling of Layer 3 between mobile telephones and the radio access network,
and includes, amongst other things, functions for broadcasting system information, paging,
connection establishment and release, radio bearer establishment, reconfiguration and release,
mobility procedures, and power control.
At any given time, mobile telephones may operate either in an ‘RRC idle mode’ (in
25 which no data communication takes place) or an ‘RRC connected mode’ (in which data
communication may take place between the mobile telephone and its serving base station).
As mobile telephones operating in the RRC connected mode move around in the area
covered by the communication system, they are handed over from one cell (i.e. operated by a
base station) to another cell (operated by the same or a different base station), depending on
30 signal conditions and other requirements, such as requested quality of service, the type of service
used, overall system load, and the like. A trigger for handing over a mobile telephone to a new
cell may be based on measurements of the cells performed by the particular mobile telephone.
The type of triggers and the related measurements to be performed by mobile telephones are
detailed in section 5.5.4 of the 3GPP TS 36.331 standard. In summary, such triggers may
3
generally relate to an event when the mobile telephone’s serving cell (or a neighbouring cell)
becomes better (or becomes worse) than either a pre-defined threshold or a pre-determined offset
value. Further details of the overall mobility sequence are described in section 10.1.2 of the
3GPP TS 36.300 V11.6.0 standard, which describes the configuration of measurements by the
base station and the subsequent triggering of handover5 .
On the other hand, whilst in the RRC idle mode, mobile telephones are programmed to
select a ‘serving’ cell, having a good quality signal, to camp on so that when new data is to be
transmitted to/from these mobile telephones, they can benefit from favourable signal conditions.
In the event that an idle mobile telephone detects a new cell with better signal quality than the
10 current serving cell, e.g. due to the mobile telephone changing its location, the mobile telephone
can perform a so-called cell reselection procedure (i.e. a sort of handover for idle user
equipment). However, an idle mode mobile telephone does not inform the network about the
selected new cell as long as this cell is within the same ‘tracking area’ (i.e. a larger geographic
area comprising a pre-defined set of cells), because the radio network transmits system
15 information and UE specific paging messages within the whole tracking area thus making it
possible to initiate communication to/from the mobile telephone regardless of the current cell it
camps on. Further details of the cell reselection procedure are disclosed in the 3GPP TS 36.304
V11.4.0 standard, the contents of which are incorporated herein by reference.
In order to benefit from the lowest energy consumption and to free up valuable system
20 resources, the mobile telephones return to the RRC idle mode whenever possible. The base
station controls the transition between the various operating modes for each mobile telephone
within its cell(s). Since the setting up and termination of an RRC connection between the base
station and the mobile telephone requires exchanging of signalling messages and hence utilises
valuable system resources, and also takes some time to complete, the transition from connected
25 to idle mode is allowed under specific circumstances as defined in the 3GPP TS 36.331 standard,
the contents of which are incorporated herein by reference. For example, the serving base station
might instruct a mobile telephone to enter the RRC idle mode only after it has confirmed that
there is no more data to be transmitted to/from the particular mobile telephone (e.g. both uplink
and downlink buffers are empty).
30 When an idle mobile telephone has data to send again (or when an active mobile
telephone enters a cell of the base station for the first time), in order to be allocated
communication resources it initiates a so called RRC connection establishment procedure by
sending an appropriately formatted RRC connection request message to the base station.
However, before communicating RRC messages (or any other data), the mobile telephone needs
19
telephone 3 has previously established an RRC connection configures the mobile telephone 3
(e.g. via appropriate signalling messages over the established RRC connection or via other
means, as schematically indicated by arrow B in Figure 1) to log and report an occurrence of
consecutive communication failures with respect to a cell of the communication network 1.
Such consecutive communication failures may include, for example, a consecutive rando5 m
access failure and/or a consecutive RRC connection request failure (which is, in itself, indicative
of consecutive random access failures). The mobile telephone 3 is also beneficially configured
to log other related information including, for example: information identifying the cell and/or
location in which the consecutive random access failures have been experienced; and/or the
10 results of signal quality/strength measurements for the cell in which the consecutive random
access failures have been experienced and/or for neighbouring cells.
Therefore, when the mobile telephone 3 is located within the overlapping coverage area
of the phantom cell Cell 1’and Cell 2, it will collect information indicative of consecutive
random access failures resulting from geographic/environmentally induced communication
15 imbalance issues and related information. This information can then be reported later when the
mobile telephone 3 successfully establishes an RRC connection with any suitably configured
base station (e.g. base station 5-2).
The base station 5-2 receiving the report may then forward it to the base station 5-1
operating the affected cell (Cell 1) via the X2 interface provided between them, or the base
20 station 5-2 receiving the report may forward it to any other network node, e.g. an MDT server 11
in the core network 7, for alleviation of the problem in the affected cell.
Advantageously, the information relating to consecutive communication failure is
collected from the mobile telephone 3 using a request-response procedure similar to the
procedures specified in the 3GPP TS 37.320 V11.3.0 standard titled “Radio measurement
25 collection for Minimization of Drive Tests (MDT)”. In this example however, instead of
reporting only the last successfully completed random access procedure or the last connection
establishment failure (from which it is not possible to determine the existence of the
geographic/environmentally induced communication imbalance issues), the mobile telephone 3
advantageously reports, to the network, that a consecutive communication failure has occurred.
30 Advantageously, the network node receiving the mobile telephone’s 3 report, e.g. the
base station 5-1 operating the adversely affected cell (Cell 1), can determine from the
information included in the report and any additional information available to this network node
(e.g. information on congestion within the cell to which the report relates) whether the reported
consecutive connection establishment failure is related to an unusual propagation of the base
27
share the report with a further network node via its network interface 55 (e.g. with a
neighbouring base station operating the affected cell using the X2 interface or with the MDT
server 11 via the S1 interface).
Advantageously, the mobile telephone 3 can thus inform its serving base station 5 (or any
other network node) about the occurrence of a consecutive connection request failure situatio5 n
experienced in the selected cell. Using the report and any further information available to the
network, the base station 5 operating the affected cell is able to take appropriate corrective
actions.
Random Access Procedure
10 Figure 7 shows an example timing diagram illustrating a method performed by
components of the communication system 1 when performing signalling relating to a random
access procedure initiated by a mobile telephone 3.
A random access procedure may be initiated by the MAC module 47, although in this
case it is initiated by a higher layer. More specifically, in this case the RRC module 45 requests
15 the MAC module 47 to transmit the ‘RRC Connection Request’ message (step S601 of Figure 6)
to the base station 5, in order to establish an RRC connection via the selected cell of this base
station 5. This request of the RRC module 45 in turn triggers a random access procedure to be
performed between the mobile telephone 3 and the base station 5, in accordance with section 5.1
of 3GPP TS 36.321 V11.3.0, the contents of which are incorporated herein by reference.
20 As can be seen, the random access procedure starts at step S701, in which the MAC
module 47 generates and sends an appropriately formatted Random Access Preamble to the
corresponding MAC module 67 of the base station 5 operating the selected cell. Next, in step
S702, the MAC module 47 increments its ‘preamble counter’ by 1 (i.e. from the initial value of
‘0’) to keep track of the number of consecutive random access attempts performed for this cell.
25 If the Random Access Preamble is successfully received by the MAC module 67 of the
base station 5, it returns a Random Access Response to the MAC module 47 at step S703. If
both steps S701 and S703 have been completed successfully, the MAC module 47 proceeds to
step S705, in which it sends any scheduled transmission (in this case the RRC Connection
Request message from the RRC module 45) to the base station 5. Finally, in step S707, receipt
30 of the scheduled transmission is followed by contention resolution signalling between the
respective MAC modules 47, 67 to help ensure that any potential contention between different
devices 3 is resolved.
However, if step S701 and/or step S703 cannot be performed successfully, steps S701
and S702 are repeated until the ‘preamble counter’ reaches a predefined value, when the MAC

We Claim:
1. A communication device for use in a communication system in which a base station
operates a cell that is selectable by the communication device in a region where connection
attempts by the communication device to the base station operating that cell are at risk of
failure, the communication device comprising:
means for selecting said cell when said communication device is in said region where
connection attempts by the communication device to said base station operating that cell
are at risk of failure;
means for attempting, within said region, to initiate a connection with said base station that
operates said cell when said cell has been selected;
means for identifying consecutive failures in respective attempts to initiate said
connection;
means for logging information identifying that consecutive failures in said respective
attempts to initiate said connection have occurred; and
means for reporting said information identifying that consecutive failures in said respective
attempts to initiate said connection have occurred to said communication system at a
reporting opportunity.
2. The communication device of claim 1 further comprising means for receiving, from a base
station, configuration data for configuring the communication device to log information
identifying that consecutive failures in respective attempts to initiate said connection have
occurred.
3. The communication device of claim 2 wherein said receiving means is operable to receive
said configuration data using at least one signalling message.
4. The communication device of claim 3 wherein said at least one signalling message
comprises at least one ‘UE Information Request’ message.
5. The communication device of any of claims 1 to 4 wherein said selecting means is
operable to consider said at least one selected cell to be barred from at least one subsequent
(re)selection when said identifying means identifies consecutive failures in respective
attempts to initiate said connection.
6. The communication device of any of claims 1 to 5 wherein said region where connection
attempts by the communication device to the base station operating that cell are at risk of
failure comprises a region in which downlink signals from said base station are stronger
than downlink signals from other base stations but in which uplink signals from said
37
communication device cannot be successfully received by said base station.
7. A base station for use in a communication system in which the or another base station
operates a cell that is selectable by a communication device in a region where connection
attempts by the communication device to the base station operating that cell are at risk of
failure, the base station comprising:
means for establishing a connection with a communication device via a cell operated by
said base station;
means for sending information to said communication device, using said established
connection, for configuring said communication device for logging information identifying
that consecutive failures in respective attempts to initiate a connection with the or another
base station of said communication system have occurred.
8. The base station of claim 7 further comprising means for receiving a report from said
communication device comprising said information identifying that consecutive failures in
respective attempts to initiate a connection have occurred.
9. The base station of claim 8 wherein said receiving means is operable to receive said
information identifying that consecutive failures in said respective attempts to initiate said
connection have occurred using at least one signalling message.
10. The base station of claim 9 wherein said at least one signalling message comprises at least
one ‘UE Information Response’ message.
11. The base station of claim 9 or 10 wherein said at least one signalling message comprises at
least one information element (IE).
12. The base station of claim 11 wherein said at least one information element comprises at
least one of: an information element for indicating that a consecutive connection
establishment failure has occurred (e.g. a ‘Consecutive Conn Est Fail’ IE), an information
element for indicating the number of consecutive connection establishment failures that
has occurred, (e.g. a ‘Num of Consecutive Conn Est Fail’ IE); and an information element
for indicating that a random access response failure has occurred (e.g. a ‘RAR failure’ IE).
13. The base station of any of claims 7 to 12 wherein said information identifying that
consecutive failures in said respective attempts to initiate said connection have occurred
comprises information identifying at least one of: information identifying said selected
cell, information identifying a geographical location of said communication device;
information identifying a time when said consecutive failures in said respective attempts to
initiate said connection have occurred; information identifying a signal strength of said
38
selected cell; information identifying a signal strength of at least one neighbour cell; and
information identifying that a random access response has been detected.
14. The base station of any of claims 7 to 13 wherein said sending means is operable to send
information to said communication device for configuring said communication device for
logging information identifying that consecutive failures in attempting to establish a Radio
Resource Control (RRC), and/or a Media Access Control (MAC), connection has occurred.
15. A communication system comprising a communication device according to any of claims 1
2 to 4, 5 and 6 and a base station according to any of claims7 to 14.
16. A communication device for use in a communication system in which a base station
operates a cell that is selectable by the communication device in a region where connection
attempts by the communication device to the base station operating that cell are at risk of
failure, the communication device comprising:
a processor configured to:
select said cell when said communication device is in said region where connection
attempts by the communication device to said base station operating that cell are at
risk of failure;
a transceiver configured to:
attempt, within said region, to initiate a connection with said base station that
operates said cell when said cell has been selected;
said processor configured to:
identify consecutive failures in respective attempts to initiate said connection; and
log information identifying that consecutive failures in said respective attempts to
initiate said connection have occurred; and
said transceiver configured to:
report said information identifying that consecutive failures in said respective
attempts to initiate said connection have occurred to said communication system at a
reporting opportunity.
17. A base station for use in a communication system in which the or another base station
operates a cell that is selectable by a communication device in a region where connection
attempts by the communication device to the base station operating that cell are at risk of
failure, the base station comprising:
a processor; and
a transceiver configured to
39
establish a connection with a communication device via a cell operated by said base
station; and
send information to said communication device, using said established connection,
for configuring said communication device for logging information identifying that
consecutive failures in respective attempts to initiate a connection with the or another
base station of said communication system have occurred.
18. A method performed by a communication device in a communication system in which a
base station operates a cell that is selectable by the communication device in a region
where connection attempts by the communication device to the base station operating that
cell are at risk of failure, the method comprising:
selecting said cell when said communication device is in said region where
connection attempts by the communication device to said base station operating that
cell are at risk of failure;
attempting, within said region, to initiate a connection with said base station that
operates said cell when said cell has been selected;
identifying consecutive failures in respective attempts to initiate said connection;
logging information identifying that consecutive failures in said respective attempts
to initiate said connection have occurred; and
reporting said information identifying that consecutive failures in said respective
attempts to initiate said connection have occurred to said communication system at a
reporting opportunity.
19. A method performed by a communication device in a communication system in which a
base station operates a cell that is selectable by the communication device in a region
where connection attempts by the communication device to the base station operating that
cell are at risk of failure, the method comprising:
selecting said cell when said communication device is in said region where
connection attempts by the communication device to the base station operating that
cell are at risk of failure;
attempting, within said region, to initiate a connection with said base station that
operates said cell when said cell has been selected;
receiving control data authorising said communication device to (re)select a different
cell when it is identified that at least one failure in an attempt to initiate said
connection has occurred;
identifying that at least one failure in an attempt to initiate said connection has
40
occurred; and
(re)selecting a different cell responsive to receipt of said control data and
identification of at least one failure in an attempt to initiate said connection.
20. A method performed by a base station in a communication system in which the or another
base station operates a cell that is selectable by a communication device in a region where
connection attempts by the communication device to the base station operating that cell are
at risk of failure, the method comprising:
establishing a connection with a communication device via a cell operated by said
base station; and
sending information to said communication device, using said established
connection, for configuring said communication device for logging information
identifying that consecutive failures in respective attempts to initiate a connection
with the or another base station of said communication system have occurred.
21. A method performed by a base station for operating, in a communication system, a cell that
is selectable by any of a plurality of communication devices in a region where connection
attempts by a communication device to said base station are at risk of failure, the method
comprising:
obtaining information for use in identifying that said cell is a cell that covers a region in
which the cell is selectable by a communication device and in which connection attempts
by that communication device to said base station are at risk of failure; and
providing, to a communication device attempting to initiate connection to said base station,
control data authorising said communication device to (re)select a different cell when said
communication device identifies that at least one failure in an attempt to initiate said
connection has occurred.
22. A computer implementable instructions product comprising computer implementable
instructions for causing a programmable communications device to perform the method of
any of claims 18 to 21.