TRANSMISSION METHOD AND MOBILE STATION TO CARRY OUT THE
METHOD
BACKGROUND OF THE INVENTION
The present invention relates to the field of wireless cellular
communication networks, and more particularly to a method for transmitting in
transmit diversity mode in a mobile station of a wireless cellular communication
network.
The term "wireless cellular communication network" as used herein
refers to any type of radio (or wireless) cellular network, in particular, GSM
("Global System for Mobile communications"), UMTS (« Universal Mobile
Telecommunications System »), CDMA, CDMA 2000 (3GPP2), FDD
("Frequency Division Duplex"), TDD ("Time Division Duplex"), WiMAX, evolved-
UTRAN (also known as "Long Term Evolution" or LTE).
The term "mobile station" as used herein refers to any type of fixed or
mobile (or portable) communication terminal (such as, for example, portable,
pocket, hand-held, computer-included or car-mounted mobile devices) capable
of exchangi ng data with a rad io-communication network on a radiocommunication
link. Consequently, it may be, among other things, a mobile
telephone apparatus (also referred to as "cellular phone" or "smartphone"), a
laptop computer or personal digital assistant (PDA) equipped with a radio
communication interface, a server or local router equipped with a radio
communication interface, a high-frequency radio receiver, or a terrestrial or
satellite television receiver. In UMTS and LTE systems, the mobile station is
referred to as "User Equipment" (UE).
In a typical cellular wireless communication system, mobile stations
communicate via a radio access network (RAN) with one or more core
networks. The radio access network provides radio coverage spanning a
geographical area which is divided into cell areas, with each cell area being
served by a radio access node, such as a base station. A cell is a geographical
area where radio coverage is provided by a base station equipment at a base
station site. The radio access nodes and mobiles stations are adapted for
communicating over the air interface when a mobile station is within radio
coverage of a cell served by a radio access node or base stations site.
The radio access network infrastructure of a cellular wireless
communication system typically comprises base stations distributed over the
covered territory for communicating with mobile stations located in the zones,
or cells, that they serve. The macrodiversity technique consists in providing for
a mobile station the ability to communicate simultaneously with separate base
stations in such a way that, in the downlink direction (from the base stations to
the mobile stations), the mobile stations receive the same information several
times and, in the uplink direction, the signal transmitted by the mobile station is
picked up by the base stations in order to form different estimates that can then
be combined in the network infrastructure.
Macrodiversity procures increased reception that improves the
performance of the system due to the combination of different observations of a
same information item. It also makes it possible to carry out soft intercellular
transfer ("soft handoff", or SHO) when the mobile station is moving.
Macrodiversity techniques are provided in the UMTS system, in the context of
wideband CDMA (W-CDMA) for frequency duplex communications (FDD). For
example, the fact that a radio signal value transmitted by a UE is received by
several Node-Bs is referred to as macrodiversity on the uplink, and such
macrodisveristy results from the reception of an estimate of radio signal
transmitted from the UE, through a so-called "active set" of Node-Bs.
Moreover, in a cellular wireless communication system using spatial
transmit diversity, data are transmitted over the air interface by a transmitter
using n transmit antennas where n > 2 , to a receiver using m receive antennas
where m > 1. Several transmit diversity scheme have been defined, such as for
example the space-time transmit diversity scheme (STTD) specified for by the
3GPP for cellular wireless communication systems of the UMTS type. Other
transmit diversity schemes, known as closed loop transmit diversity schemes,
have been specified for UMTS, in which the same spread spectrum signal is
transmitted on each of the n transmit antennas, with transmit parameters
determined based on feedback data provided by the receiver which reflects the
quality of the received signal. Usually, such feedback data is calculated based
on impulse response estimates corresponding to propagation channels for
each of the antennas. The transmitter station applies parameters (reflecting a
phase shift or a phase shift and a gain) on each transmit antenna so that the
signals transmitted from the different antennas reach the receiver without any
significant phase difference. Such a transmit diversity scheme in which the
transmitter station transmits on each transmit antenna with weights calculated
from the feedback data is also referred to as "beamforming transmit diversity".
In the present description, the invention will be described more
particularly in its application, non limiting, to third generation communication
networks of the UMTS type. In this system, the invention finds application
within the framework of the so-called "High Speed Downlink Packet Access"
(HSDPA) feature, currently being specified by the 3GPP (3rd Generation
Partnership Project), an overall description of which may be found in the 3GPP
25.308 technical specification "UTRA High Speed Downlink Packet Access
(HSDPA); Overall Description; Stage 2 (Release 10)", version 10.4.0, published
in March 201 1 by the 3GPP. HSDPA allows high rate downlink transmission,
i.e. from a base station to a mobile station, of data to a set of mobile stations
(UEs) located in the coverage area of the base station.
FIG. 1 shows the architecture of such a UMTS network. The switches
of the mobile service 10 , belonging to a core network (CN), are linked on the
one hand to one or more fixed networks 11 and on the other hand, by means of
a so-called lu interface, to command equipment 12 or RNCs ("Radio Network
Controllers"). Each RNC 12 is linked to one or more base stations 13 by means
of a so-called lub interface. The base stations 13 , distributed over the territory
covered by the network, are capable of communicating by radio with the mobile
terminals 14, 14a, 14b called UE ("User Equipment"). The base stations can be
grouped together to form nodes called "node B". Certain RNCs 12 may
furthermore communicate with one another by means of a so-called lur
interface. The RNCs and the base stations form an access network called
UTRAN ("UMTS Terrestrial Radio Access Network").
The UTRAN comprises elements of layers 1 and 2 of the ISO model
with a view to providing the links required on the radio interface (called Uu),
and a stage 15A for controlling the radio resources (RRC, "Radio Resource
Control") belonging to layer 3 , as is described in the 3GPP TS 25.301 technical
specification "Radio Interface Protocol Architecture", version 6.0.0, Release 6 ,
published in January 2004 by the 3GPP. In view of the higher layers, the
UTRAN acts simply as a relay between the UE and the CN.
In a closed loop uplink transmit diversity system of the UMTS type, the
UE transmits radio signals to the Node-B(s) using more than one antenna, and
uses feedback information provided by the Node-B to determine the
transmission parameters. The Node-B determines a set of transmit diversity
(TxDiv) weights based on estimations of the uplink propagation channel, and
signals these determined weights to the UE, which in turn uses the received
TxDiv weights on its susbsequent transmission. In beamforming transmit
diversity schemes, the weights are selected to give the maximum possible gain
with the given propagation channel so as to focus a transmission beam from
the UE to the serving Node-B. Focusing most of the transmission energy to a
specific Node-B will also result in attenuated transmission to other Node-Bs as
shown in Figure 2 . Typically this gives a gain to the Node-B of interest whilst
reducing interferences to the other Node-Bs.
However, in soft handover, a UE will be in communication with more
than one Node-B and thus beamforming the transmission to one Node-B (e.g.
the serving Node-B) would result in attenuated transmission to the other Node-
Bs (e.g. the non-serving Node-Bs). Since soft handover provides gain to the UE
at the cell boundaries, the effect of beamforming transmit diversity reduces this
soft handover gain. While it would be possible that the non-serving Node-Bs
also provide transmit diversity weights to the UE so that the UE can determine
the best weights to be used that would maximize gain for all Node-Bs, this is
subject to availability of non-serving Node-Bs providing transmit diversity
weights. Therefore there is a need to maximize the soft handover gain for a UE
performing beamforming, and not receiving transmit diversity weight feedback
from the non-serving Node-Bs.
SUMMARY OF THE INVENTION
According to one aspect, the invention proposes a method, in a
cellular radio-communication system comprising a core network and an access
network, the access network comprising base stations for providing wireless
links to at least one mobile station, the mobile station being capable of
operating in a first transmission mode in which beamforming transmit diversity
is not used and in a second transmission mode in which beamforming transmit
diversity is used, the method comprising switching from the second
transmission mode to the first transmission mode upon determining that at least
one predetermined criterion related to the mobile station operation in soft
handover is satisfied.
The criterion or one of the criteria based on which the decision to
switch from the second transmission mode to the first transmission mode may
be defined so that it is satisfied if the number of base stations in the active set
of the mobile station when operating in soft handover exceeds a predetermined
threshold.
An additional criterion might be defined so that it is satisfied if one
considers only, for application of the above-mentioned criterion, those base
stations in the active set of the mobile station when operating in soft handover
for which the radio link with the mobile station exceeds a predetermined quality
threshold.
The criterion or one of the criteria based on which the decision to
switch from the second transmission mode to the first transmission mode may
be defined so that it is satisfied if a request message for switching from the
second transmission mode to the first transmission mode is received from the
access network.
Another broad aspect provides a mobile station, in a cellular radiocommunication
system comprising a core network and an access network, the
access network comprising base stations for providing wireless links to at least
one mobile station, comprising an antenna system comprising at least two
antennas for transmission in beamforming transmit diversity mode, a radio
module adapted for operating in a first transmission mode in which
beamforming transmit diversity is not used and in a second transmission mode
in which beamforming transmit diversity is used, and a control module adapted
for determining that at least one predetermined criterion related to the mobile
station operation in soft handover is satisfied and for, responsive to said
determination, switching the operation of the radio module from the second
transmission mode to the first transmission mode upon.
The control module may be further adapted for determining that at least
one predetermined criterion is satisfied if the number of base stations in the
active set of the mobile station when operating in soft handover exceeds a
predetermined threshold.
The control module may also be further adapted for determining that at
least one predetermined criterion is satisfied if the number of base stations in
the active set of the mobile station when operating in soft handover, and for
which the radio link with the mobile station exceeds a predetermined quality
threshold, exceeds a predetermined threshold.
The control module may be further adapted for determining that at least
one predetermined criterion is satisfied if a request message for switching from
the second transmission mode to the first transmission mode is received from
the access network.
The methods proposed herein may advantageously be embodied in a
Universal Mobile Telecommunications System (UMTS) wireless communication
system, the mobile station being a UMTS capable User Equipment.
Another broad aspect provides a computer readable medium having
processor executable instructions thereon for implementation by a processor,
the instructions executing the above-mentioned methods.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will become apparent
from the following description of non-limiting exemplary embodiments, with
reference to the appended drawings, in which:
- FIG. 1 is a diagram of a UMTS network to which the invention may
be applied;
- FIG. 2 is a diagram showing a UE beamforming to a serving Node-B
and attenuating transmission to non-serving Node-Bs;
DESCRIPTION OF PREFERRED EMBODIMENTS
The invention will now be described in further details in the framework
of the UMTS HSDPA feature.
It is proposed to turn off (or disable) the transmit diversity function at
the UE when one or several criteria related to the UE's operation in soft
handover are satisfied. The following embodiments provide example of those
criteria and the way they can be combined with each other. The decision to turn
transmit diversity back on may also further be linked to part or all of those
additional criteria. The overall gain achieved by using soft handoff without
transmit diversity may indeed be greater than that when transmit diversity is
used. Switching off transmit diversity (and therefore transmitting from only one
antenna at a time) results in a more uniform transmission radiation pattern
which is less sensitive to the Node-B location(s).
The terms "turn off", "switch off", "disable" as used herein for the
transmit diversity function of the UE refer to the transition from one state in
which the UE transmits using a transmit diversity scheme by means of a
plurality of antennas, to another state in which the UE does not transmit using a
transmit diversity scheme.
In a first embodiment, the UE switches off transmit diversity if the
number of cells in its active set becomes greater than a predetermined
threshold. In this embodiment, if such predetermined threshold is set equal to
one, then the transmit diversity will be switched off whenever the UE is in soft
handoff. This predetermined threshold may be specified in the initial UE
configuration, or set (and also possibly updated at a later stage) and signalled
to the UE by the infrastructure network.
In a second embodiment, the UE switches off transmit diversity if the
number of cells in its active set becomes greater than a predetermined
threshold and a signal quality criterion is satisfied for one or more of the cells in
the active set. This signal quality criterion might comprise the signal quality of
the second cell being within a predetermined distance (e.g. X dB) from the
signal quality of the strongest cell (or of the serving cell - corresponding to the
serving Node-B) for a predetermined period of time (e.g. T seconds). The
signal quality might comprise the received SINR or another similar metric.
It should be noted that the UE may notify its serving Node-B that it has
switched to a transmission mode in which transmit diversity is no longer used.
This autonomous disabling of transmit diversity at the UE followed by a
notification to the UTRAN allows for a faster response, and utilizes the SHO
gain as soon as it is higher than that of beamforming transmit diversity.
The UE may also turn beamforming transmit diversity back on and
inform its serving Node-B if none of the non-serving Node-Bs shows a signal
quality which is within a predetermined distance (e.g. X' dB) from that of the
serving Node-B for a predetermined time period (e.g. seconds). In the
examples provided above, the parameters X and X', and T and , respectively,
may be chosen equal.
In a third embodiment, instead of having the UE autonomously
disabling transmit diversity when certain predefined criteria are satisfied, the
UE triggers an indication, e.g. in the form of a layer 1 signal, MAC message or
a RRC measurement report to the UTRAN, when the criteria are satisfied.
Upon receipt of such indication, the Node-B sends to the UE a request
message (preferably carried by a "fast" signal such as an HS-SCCH order, or
alternatively an RRC reconfiguration message) to disable beamforming
transmit diversity at the UE.
In a fourth embodiment, receipt of the above-described request
message to disable beamforming transmit diversity at the UE from the Node-B
constitutes one criterion among others on which the decision to disable
beamforming transmit diversity is based.
The combination of the third and the fourth embodiments avoids the
UTRAN having continuously to compare and evaluate the relative gains of
beamforming transmit diversity and soft handover. The indication from the UE
provided in the third embodiment enables the UTRAN to compare and evaluate
the relative gains of beamforming transmit diversity and soft handover only
upon receipt of this indication. In this regard it should be noted that the UTRAN
has knowledge of the soft handover gain, and the Node-B (in the UTRAN) has
knowledge of the beamforming transmit diversity gain since it knows the
propagation channel and the transmit diversity weights used. Determining an
estimate of the gain of each scheme is therefore possible at the network. This
saves significant processing at the UTRAN side since a comparison would be
required for each UE if the indication was not available. If, based on this
comparison, it is determined that beamforming transmit diversity is not worth
maintaining for a given UE, the UTRAN can send a message to such UE
requesting that beamforming transmit diversity be disabled.
The UE may also trigger an indication to the UTRAN if none of the nonserving
Node-Bs shows a signal quality which is within a predetermined
distance (e.g. X' dB) from that of the serving Node-B for a predetermined time
period (e.g. seconds). In the examples provided above, the parameters X
and X', and T and , respectively, may be chosen equal. The UTRAN can then
send a signal to the UE requesting that the beamforming transmit diversity be
turned back on, and the UTRAN can once again stop comparing the gains
between beamforming transmit diversity and soft handover.
In a fifth embodiment, the beamforming transmit diversity function is
turned off when the UE is changing serving cell (or serving Node-B). A change
in serving cell usually occurs when one of the non-serving cells has a better
signal quality than that of the serving cell. Hence, during this period, it is
beneficial that the transmission to the target serving cell (i.e. the cell that will
become the new serving cell of the UE) is not attenuated. Once the UE has
successfully moved over to the new serving cell, the new serving cell can
restart beamforming transmit diversity for this UE.
On FIG. 2 is shown a UE (20) connected to a serving Node-B (22), and
two non-serving Node-Bs (21 , 23). The UE 20 transmits to the Node-Bs using
beamforming transmit diversity, and the transmission beam, focused on the
serving Node-B (22) for one part (24) and attenuating through its shape
transmission to the non-serving Node-Bs (21 , 23) in other portions of the beam
(25) is also illustrated on the figure. As part of the existing UE soft handover
procedure, the UE (20) will measure the CPICH quality of its serving Node-B
(22) and non-serving Node-Bs (21 , 23). The UE (20) is capable of operating in
a first transmission mode in which beamforming transmit diversity is not used
and in a second transmission mode in which beamforming transmit diversity is
used. It comprises an antenna system comprising at least two antennas for
transmission in beamforming transmit diversity mode, connected to a radio
module adapted for operating in a first transmission mode in which
beamforming transmit diversity is not used and in a second transmission mode
in which beamforming transmit diversity is used. The radio module is in turn
connected to a control module which includes a processor and memory means.
An event is set at the control module to trigger if any of the non-serving Node-
Bs (21 , 23) CPICH quality is within 2 dB of that of the serving Node-B (22) for
more than 1 second. Note that an existing event such as Event 1d (currently
defined as one of the events for intra-frequency measurement reporting, for
change of best cell) could be used for this purpose if its parameters are
adapted. In this example we assume that the first event trigger results in the
following measurements: the CPICH measured signal quality (signal strength in
dB) is, for Node-B 2 1, -90dB, for Node-B 22, -91 .5dB and for Node-B 23, -
94dB. The UTRAN has knowledge of the gains of the soft handover. These
gains altogether with the measurements from the event trigger are provided to
the Node-B 22. The Node-B 22 evaluates the gains from several TTI obtained
using beamforming transmit diversity and we assume that the gain from
beamforming transmit diversity is higher than that of the soft handover. In such
case no action is required. Upon occurrence of another event trigger, the
following measurements are obtained: the CPICH measured signal quality
(signal strength in dB) is, for Node-B 2 1, -90dB, for Node-B 22, -91 dB and for
Node-B 23, -91 ,5dB. At this point, the gains achieved from soft handover via
the non-serving Node-Bs, in our example Node-B 2 1 and Node-B 23, re
estimated to be higher than that of beamforming transmit diversity. The serving
Node-B 22 then sends a HS-SCCH order to the UE 20 to turn off beamforming
transmit diversity, i.e. to switch to a transmission mode in which transmit
diversity is not used.
A person of skill in the art would readily recognize that steps of various
above-described methods can be performed by programmed computers.
Herein, some embodiments are also intended to cover program storage
devices, e.g., digital data storage media, which are machine or computer
readable and encode machine-executable or computer-executable programs of
instructions, wherein said instructions perform some or all of the steps of said
above-described methods. The program storage devices may be, e.g., digital
memories, magnetic storage media such as a magnetic disks and magnetic
tapes, hard drives, or optically readable digital data storage media. The
embodiments are also intended to cover computers programmed to perform
said steps of the above-described methods.
The description and drawings merely illustrate the principles of the
invention. It will thus be appreciated that those skilled in the art will be able to
devise various arrangements that, although not explicitly described or shown
herein, embody the principles of the invention and are included within its spirit
and scope. Furthermore, all examples recited herein are principally intended
expressly to be only for pedagogical purposes to aid the reader in
understanding the principles of the invention and the concepts contributed by
the inventor(s) to furthering the art, and are to be construed as being without
limitation to such specifically recited examples and conditions. Moreover, all
statements herein reciting principles, aspects, and embodiments of the
invention, as well as specific examples thereof, are intended to encompass
equivalents thereof.
The functions of the various elements shown in the FIGs., including any
functional blocks referred to or labeled as "processor", "controller" or "control
module", may be provided through the use of dedicated hardware as well as
hardware capable of executing software in association with appropriate
software. When provided by a processor, the functions may be provided by a
single dedicated processor, by a single shared processor, or by a plurality of
individual processors, some of which may be shared. Moreover, explicit use of
the term "processor", "control module" or "controller" should not be construed to
refer exclusively to hardware capable of executing software, and may implicitly
include, without limitation, digital signal processor (DSP) hardware, network
processor, application specific integrated circuit (ASIC), field programmable
gate array (FPGA), read only memory (ROM) for storing software, random
access memory (RAM), and non volatile storage. Other hardware, conventional
and/or custom, may also be included.
CLAIMS
1. A method, in a cellular radio-communication system comprising a core
network and an access network, the access network comprising base stations
for providing wireless links to at least one mobile station, the mobile station
being capable of operating in a first transmission mode in which beamforming
transmit diversity is not used and in a second transmission mode in which
beamforming transmit diversity is used, the method comprising switching from
the second transmission mode to the first transmission mode upon determining
that at least one predetermined criterion related to the mobile station operation
in soft handover is satisfied.
2 . A method according to claim 1 wherein at least one predetermined criterion
is satisfied if the number of base stations in the active set of the mobile station
when operating in soft handover exceeds a predetermined threshold.
3 . A method according to claim 2 wherein at least one predetermined criterion
is satisfied if the number of base stations in the active set of the mobile station
when operating in soft handover, and for which the radio link with the mobile
station exceeds a predetermined quality threshold, exceeds a predetermined
threshold.
4 . A method according to any of claims 1 to 3 , wherein at least one
predetermined criterion is satisfied if a request message for switching from the
second transmission mode to the first transmission mode is received from the
access network.
5 . A mobile station, in a cellular radio-communication system comprising a core
network and an access network, the access network comprising base stations
for providing wireless links to at least one mobile station, comprising:
- an antenna system comprising at least two antennas for transmission in
beamforming transmit diversity mode;
- a radio module adapted for operating in a first transmission mode in which
beamforming transmit diversity is not used and in a second transmission mode
in which beamforming transmit diversity is used;
- a control module adapted for determining that at least one predetermined
criterion related to the mobile station operation in soft handover is satisfied and
for, responsive to said determination, switching the operation of the radio
module from the second transmission mode to the first transmission mode
upon.
6 . A mobile station according to claim 5 wherein the control module is further
adapted for determining that at least one predetermined criterion is satisfied if
the number of base stations in the active set of the mobile station when
operating in soft handover exceeds a predetermined threshold.
7 . A mobile station according to claim 6 wherein the control module is further
adapted for determining that at least one predetermined criterion is satisfied if
the number of base stations in the active set of the mobile station when
operating in soft handover, and for which the radio link with the mobile station
exceeds a predetermined quality threshold, exceeds a predetermined
threshold.
8 . A mobile station according to any of claims 5 to 7 , wherein the control
module is further adapted for determining that at least one predetermined
criterion is satisfied if a request message for switching from the second
transmission mode to the first transmission mode is received from the access
network.
9 . A computer readable medium having processor executable instructions
thereon for implementation by a processor, the instructions executing a method
according to claims 1 to 4 .