CHANNEL STATE INFORMATION FEEDBACK
FIELD OFTHE INVENTION
The present invention relates to a n apparatus and method for feeding back channel
state information.
BACKGROUND
Base stations in wireless communications systems provide wireless connectivity to user
equipment within the geographic areas, or cells, associated with the base station. The
wireless communications links between the base station and each of the user
equipment typically include one or more downlink (or forward) channels for transmitting
information from the base station to the user equipment and one or more uplink (or
reverse) channels for transmitting information from the user equipment to the base
station. Multiple-input-multiple-output (MIMO) techniques may be employed when the
base station and, optionally, the user equipment include multiple antennas. For
example, a base station that includes multiple antennas can transmit multiple
independent and distinct signals to multiple user equipment concurrently on the same
frequency band.
For example, consider a cellular system with M antennas at the base station and N
antennas at the user equipment. In such communication systems, the radio channel
between the base station and the user equipment can be described in terms of N x M
links (sub-channels). Each link typically has a time-varying complex gain (i . amplitude
and phase). If the radio channel is wide band .e. the signal rate is greater than the
delay spread of the channel), the complex gain varies across the bandwidth of the
transmitted signal. The overall state of the radio channel can therefore be described as
a series of complex weights. This channel state information is measured by the user
equipment and fed back to the base station in order to allow the base station to adapt
characteristics of the signals transmitted to the user equipment to match in the most
appropriate way to the prevailing channel state in order to improve, for example,
throughput.
Although techniques exist to provide channel state information feedback, they each
have their own shortcomings. Accordingly, it is desired to provide a n improved
technique for providing channel state information.
SUMMARY
According †o a first aspect there is provided a method of providing channel state
information for a wireless communications channel provided between a first network
node having at least one transmission antenna and a second network node having at
least one reception antenna, the method comprising the steps of: estimating a current
value of at least a first type of channel state information for at least one sub-channel
within the channel from signals received by the at least one reception antenna over
the channel from the at least one transmission antenna; determining whether the
current value of the first type of channel state information varies from previouslytransmitted
value for the first type of channel state information by at least a
predetermined amount; and if so, transmitting a n indicator indicative of the current
value for the first type of channel state information to the first network node.
The first aspect recognises that a problem with existing techniques is that a given type
of channel state information is transmitted even when that type of channel state
information provides little improvement in, for example, throughput. This is because
although channel state information is beneficial for improving the throughput of
transmissions between the first and second network node, relatively large periods of
time may exist when the degree of change or variation in a particular aspect of the
channel state described by a given type of channel state information is relatively low
and relatively little benefit is derived from providing these statically arranged, periodic
updates in that type of channel state information since only minor changes to the
characteristics of the signals transmitted will be made which may have little effect on
throughput.
Accordingly, a n estimation or determination of the current channel state for each type
of channel state information is made for each channel or sub-channel. The variation or
deviation of the current channel state from the most recently transmitted channel state
information for that type is then determined or calculated. Should the difference
between the current channel state and the most recently transmitted channel state
information be greater than or equal to a predetermined threshold amount, then an
indication of the current channel state is transmitted to the first network node. In this
way, it can be seen that only when a suitably large variation in the channel state for
that type of channel state information occurs are resources allocated for the
transmission of the current channel state to the first network node. This means that
resources need not be regularly allocated for the transmission of the current channel
state information, but these resources may be dynamically utilised as and when the
need arises. Hence, should there be little benefit in updating the channel state, then
no update need occur for that type of channel state information. However, when a
significant change in channel state does occur, then an update may be transmitted.
Such an approach helps to minimise the routine allocation of resources, thus freeing
these resources for, for example, transmission of another type of channel state
information. Accordingly, rather than routinely providing channel state updates for
each type of channel state information, these updates only occur on demand, when
providing such an update makes a measured improvement to throughput. For
example, a situation can occur where the geographical relationship between the
network nodes is relatively static, as are any major sources of interference or reflection.
Therefore, for relatively large periods of time, the channel state environment is
reasonably static. In those situations, there is a reduced need to provide channel state
updates since those updates are likely to only provide a minimal benefit to throughput.
However, when the geographical relationship between the two network nodes is
changing rapidly or where major sources of interference or reflection are also
changing, the throughput may be significantly affected due to a comparatively rapidly
changing channel state environment which may be ameliorated through providing
channel state updates.
In one embodiment, the step of transmitting comprises the step of: transmitting an
indicator indicative of a current value of a second type of current channel state
information when the step of determining indicates that the current value of the first
type of channel state information does not vary from the previously-transmitted value
of the first type of channel state information by at least the predetermined amount.
Hence, the channel state information may comprise different types or components of
channel state, each of which may be transmitted. Theses different types may relate to
different characteristics of the channel state. Some types of channel state information
may represent characteristics of the channel state which vary quickly, whilst others may
vary more slowly. Likewise, some types of channel state information may represent
characteristics of the channel state which vary more quickly at some times, whilst others
may vary more quickly at other times. When no update for one type of channel state
information is necessary, the resources that would have otherwise been routinely
allocated to enable transmission of that unnecessary update may instead be allocated
to support the transmission of updates of other types of channel state information. It will
be appreciated that this enables the available resources to be dynamically allocated
to enable the transmission of updates of that type of channel state information for
each channel or sub-channel which best improves the performance of the channel or
sub-channel.
In one embodiment the step of transmission comprises the step of: avoiding
transmission of the indicator indicative of the current value of the first type of channel
state information when the step of determining indicates that the current value of the
first type of channel state information does not vary from the previously-transmitted
value of the first type of channel state information by at least the predetermined
amount. This embodiment recognises that a problem with existing techniques is that
the amount of resources allocated for the transmission of channel state information is
generally constant which prevents those resources being utilised for other purposes, in
particular for being redistributed between the different types of channel state
information. For example, channel state information may be regularly transmitted at
the same periodic interval from the second network node to the first network node. If
these resources cannot be dynamically redistributed between different types of
channel state information, it means that more resources than necessary will need to be
periodically allocated to enable all types of channel state information to be
transmitted sufficiently frequently, which provides a constant drain of these finite
resources. Accordingly, should one aspect of the current channel state have not
varied from the most recently transmitted channel state information for that channel or
by greater than the predetermined amount, then no update to that aspect of the
channel state need be provided and so the resources for the transmission for such an
update can be reallocated. This helps to ensure that resources are only allocated
when required and frees up these resources for other uses at other times. This means
that resources need not be regularly allocated for the transmission of every aspect of
the current channel state, but these resources may be dynamically utilised asand
when the need arises.
In one embodiment, the method comprises the step of: receiving a n indicator
indicative of the predetermined amount from the first network node. Accordingly, the
first network node which may be, for example, a base station, may set the
predetermined amount based on, for example, quality or other measurements made
by the base station. In this way, the first network node is able to control the extent of
any deviation of the channel state which is acceptable without triggering the
occurrence of a channel state update.
In one embodiment, a Frobenius matrix may be utilised to determine whether the
current value of the at least a first type of channel state information varies from a
previously-transmitted value of the at least a first type of channel state information by
more than the predetermined amount. In such an embodiment, the previously
transmitted channel state information may be stored asa matrix and compared
against a matrix storing the current channel state information.
In one embodiment the distance between vectors used to quantise the channel state
information may be used to determine whether the current value of the at least a first
type of channel state information varies from a previously-transmitted value of the at
least a first type of channel state information by more than the predetermined amount.
If the change in the channel state information is greater than the distance between a
predetermined number of the vectors the predetermined amount can be determined
to have been exceeded. In one such embodiment, if the change in the channel state
information is greater than the complete span of the space described by the set of
vectors, then the predetermined amount can be determined to have been exceeded.
In one embodiment, the indicator indicative of the current value of at least the first
type of channel state information comprises an indicator indicative of a difference
between the current value of at least the first type of channel state information and the
previously-transmitted value of at least the first type of channel state information.
Accordingly, rather than transmitting the complete channel state information to the first
network node, only the difference needs be transmitted. It will be appreciated that
transmitting the difference will often further reduce the amount of information that
needs to be transmitted, thereby also reducing the resources needed to be allocated.
In one embodiment, the step of transmitting comprises the step of: transmitting one of
an indicator indicative of the current value of the first type of channel state information
when the step of determining indicates that the current value of the first type of
channel state information varies from the previously-transmitted value of the first type of
channel state information by at least a first predetermined amount and an indicator
indicative of the current value of the second type of channel state information when
the step of determining indicates that the current value of the second type of channel
state information varies from the previously-transmitted value of the second type of
channel state information by at least a second predetermined amount in preference to
the other. Accordingly, different threshold levels of change may be set which are
appropriate for each of these different types of channel state information. Also, this
embodiment recognises that a problem with the static update approach mentioned
above is that the updates occur irrespective of whether such updates provide a
beneficial effect on the throughput. Furthermore, the static update approach provides
updates for one type of channel state typically more frequently than updates for other
types of channel state. In contrast, enabling the transmission of either one type of
channel state information in preference to the other type of channel state information,
or vice versa, provides flexibility to prioritise the transmission of one component or type
of the channel state information over the other. In one embodiment, the second
predetermined amount may be zero to cause updates to the second type of channel
state information to be regularly provided.
In one embodiment, the step of transmitting comprises the step of: increasing a number
of bits utilised for transmitting the one of the indicator indicative of the current value of
the first type of channel state information and the indicator indicative of the current
value of the second type of channel state information in preference to the other.
Accordingly for the channel state information given priority, the number of bits
available for that preferential type of channel state information is increased. This helps
to ensure that the preferential type of channel state information is allocated more
resources at that time than other types of channel state information.
In one embodiment, the step of transmitting comprises the step of: increasing a number
of time slots utilised for transmitting the one of the indicator indicative of the current
value of the first type of channel state information and the indicator indicative of the
current value of the second type of channel state information in preference to the
other.
In one embodiment, the first type of channel state information comprises long-term
channel state information, the second type of channel state information comprises
short-term channel state information and the step of transmitting comprises the step of:
transmitting the indicator indicative of the current long-term channel state information
when the step of determining indicates that the current value of the long-term channel
state information varies from the previously-transmitted value of the long-term channel
state information by at least the first predetermined amount. Hence, the channel state
information may relate to long-term and short-term channel state characteristics. Socalled
long-term and short-term channel state information is well known in the art and
indicates different components or characteristics of the channel state information. For
example, the long-term channel state information may capture the spatial correlation
properties of the channel which usually changes slowly over time and over frequency.
Such long-term channel state information may typically represent the correlation
between antennas, a n averaged signal to noise plus interference ratio (SNIR), a
pathloss, and the like. As mentioned above, these characteristics vary comparatively
slowly. The other component or type is the so-called short-term channel state
information which represents instantaneous properties of the channel which are
generally rapidly varying in time and frequency. Typically, such short-term channel
state information may be the phase and amplitude of signals being received, the
instantaneous SNI , and the like. Known techniques provide both the short-term and
long-term channel state information for each channel or sub-channel which, when
combined, provides an indication of the overall channel state. These known
techniques typically provide channel state updates at regular periodic intervals with
many short-term updates being provided for each long-term update. However, in this
embodiment, the long-term channel state information is only updated when a
sufficiently large change in this channel state information occurs. In embodiments,
changes to the short-term channel state information may otherwise be transmitted
since this channel state is likely to vary most quickly.
In one embodiment, the step of transmitting comprises the step of: transmitting a n
indicator indicative of the current value of the long-term channel state information in
preference to transmitting the indicator indicative of the current value of the short-term
channel state information when the step of determining indicates that the current
value of the long-term channel state information varies from the previously-transmitted
value of the long-term channel state information by at least the first predetermined
amount. Accordingly, should a significant change in the long-term channel state
occur, then this may be transmitted in preference to changes in the short-term channel
state.
In one embodiment, the step of transmitting comprises the step of: increasing a n
allocation of bits for transmitting the current value of the long-term channel state
information. Accordingly, more bits may be allocated to support the transmission of the
update to the long-term channel state information. It will be appreciated that such an
approach was not possible in the known techniques which only enable a
predetermined fixed number of bits to be utilised at fixed periodic intervals.
In one embodiment, the transmissions occur within time-slots and the step of
transmitting comprises the step of: increasing allocation of time slots for transmitting the
current value of the long-term channel state information. By increasing the number of
time slots allocated for the long-term channel state updates, the speed with which such
updates can occur is increased, thereby improving the throughput. It will be
appreciated that this approach was also not possible with the known techniques which
required that the long-term channel state updates could only occur at fixed periodic
intervals thereby delaying the speed at which such updates could be made.
In one embodiment, the method comprises the steps of: arranging the current value of
the at least a first type of channel state information into at least one vector; and
quantising the at least one vector by selecting one of a plurality of codebook vectors
at a first level of a hierarchical codebook of vectors, the indicator indicative of the
current value of the at least a first type of channel state information comprising an
indication of an index to the one of a plurality of codebook vectors. It will be
appreciated that such hierarchical code books of vectors provide a number of code
book vectors at each level of the hierarchical code book, each of which is selected
based on predetermined criteria such as, for example, that code book vector being,
for example, the closest match to the vector to be quantised, the best match to that
vector or offering the minimum error, although it will also be appreciated that other
selection criteria may be applied since using a hierarchical code book enables
subsequent refinement to further code book vectors which may better represent the
vector being quantised. An index to the selected code book vector may then be
provided to the first network node. Hence, rather than transmitting the selected vector
itself as the current channel state information, only the index to that vector need be
provided, which it will be appreciated will be typically representable with a smaller
number of bits. The first network node, which also has a copy of the code books of
vectors, can then identify the selected vector and utilise that vector to determine the
current channel state information and adapt its transmission accordingly. Hence, it can
be seen that the amount of channel state feedback information provided can be
drastically reduced.
In one embodiment, the method comprises the steps of: requantising the at least one
vector by selecting one of a plurality of hierarchically-related codebook vectors from
hierarchically-related levels of the hierarchical codebook of vectors, the indicator
indicative of the current value of the at least a first type of channel state information
comprising an indication of an index to the one of a plurality of hierarchically-related
codebook vectors. Accordingly, the quantisation of the vector may be successively
refined by selecting code book vectors which are related to the vector whose index
has been previously advised to the first network node. This successive refinement or
requantisation enables an improved indication of the current channel state information
to be provided back to the first network node over time.
In one embodiment, the one of a plurality of hierarchically-related codebook vectors
from hierarchically-related levels of the hierarchical codebook of vectors comprises
one of a plurality of child codebook vectors from child levels of the hierarchical
codebook. Accordingly, for vectors which change slowly over time, subsequent
refinements in their quantisations may be possible by selecting child code book vectors
of the code book vector previously indicated to the first network node. Such child
code book vectors are typically closer refinements of the previously advised parent
code book vector.
In one embodiment, the one of a plurality of hierarchically-related codebook vectors
from hierarchically-related levels of the hierarchical codebook of vectors comprises
one of a plurality of parent codebook vectors from parent levels of the hierarchical
codebook. Hence, for more rapidly changing channel state, it may be necessary to
traverse to parent levels of the hierarchical code book to select a more appropriate
code book vector which quantises the vector.
In one embodiment, differing hierarchical codebooks of vectors are provided for the
first type of channel state information and the second type of channel state
information, at least one of the codebooks being hierarchical. Accordingly, different
code books or sets of code books may be provided for each of the short-term and
long-term channel state information. It will be appreciated that each of those code
books or sets of code book may be optimised for those different types of channel state
information.
In one embodiment, the indicator indicative of the current value of the at least a first
type of channel state information encodes an identifier indicating which of the
hierarchical codebooks of vectors was used to quantise or requantise the at least one
vector. Accordingly, the indicator may encode an identifier which informs the recipient
which hierarchical code book of vectors was used to encode the vector. It will be
appreciated that this indication may explicitly indicate which particular hierarchical
code book of vectors was used or provide a relative indication of the code book of
vectors used. For example, if two code books of vectors may be used the indicator
may be a single bit; a "0" bit may indicate that one of the code books was used,
whereas a may indicate that the other code book was used. Alternatively, a "0"
may indicate that the received indication contains a vector from the same code book
as that used previously, whereas a may indicate that a different code book to that
used previously was used to create that vector. It will be appreciated that there are
many ways to implement such absolute and relative indications.
In one embodiment, the indicator indicative of the current value of at least one type of
channel state information further comprises a relative or absolute indicator of the level
of the codebook of vectors from which the at least one of a plurality of codebook
vectors was selected, wherein if the relative or absolute indicator indicates a level not
present in the previously-indicated codebook of vectors it indicates the type of channel
state information indicated by the indicator indicative of the current value of at least
one type of channel state information.
According to a second aspect, there is provided a network node having at least one
reception antenna and operable to provide channel state information for a wireless
communications channel provided between another network node having at least
one transmission antenna and the network node, the network node comprising:
estimating logic operable to estimate a current value of at least a first type of channel
state information for at least one sub-channel within the channel from signals received
by the at least one reception antenna over the channel from the at least one
transmission antenna; determination logic operable to determine whether the current
value of the first type of channel state information varies from a previously-transmitted
value of the first type of channel state information by at least a predetermined amount;
and transmission logic operable, if the determination logic indicates that the current
value of the first type of channel state information varies from the previously-transmitted
value of first type of the channel state information by at least the predetermined
amount, to transmit of an indicator indicative of the current value of the first type of
current channel state information to the first network node.
In one embodiment, the transmission logic is operable to transmit an indicator
indicative of a current value of a second type of current channel state information
when the determination logic indicates that the current value of the first type of
channel state information does not vary from the previously-transmitted value of the first
type of channel state information by at least the predetermined amount.
In one embodiment, the transmission logic is operable to avoid transmission of the
indicator indicative of the current value of the first type of channel state information
when the determination logic indicates that the current value of the first type of
channel state information does not vary from the previously-transmitted value of the first
type of channel state information by at least the predetermined amount.
In one embodiment, the network node comprises: reception logic operable to receive
an indicator indicative of the predetermined amount from the first network node.
In one embodiment, a Frobenius matrix may be utilised to determine whether the
current value of the at least a first type of channel state information varies from a
previously-transmitted value of the at least a first type of channel state information by
more than the predetermined amount.
In one embodiment, the distance between vectors used to quantise the channel state
information may be used to determine whether the current value of the at least a first
type of channel state information varies from a previously-transmitted value of the at
least a first type of channel state information by more than the predetermined amount.
If the change in the channel state information is greater than the distance between a
predetermined number of the vectors the predetermined amount can be determined
to have been exceeded. In one such embodiment if the change in the channel state
information is greater than the complete span of the space described by the set of
vectors, then the predetermined amount can be determined to have been exceeded.
In one embodiment, the indicator indicative of the current value of at least the first
type of channel state information comprises an indicator indicative of a difference
between the current value of at least the first type of channel state information and the
previously-transmitted value of at least the first type of channel state information.
In one embodiment, the transmission logic is operable to transmit one of an indicator
indicative of the current value of first type of channel state information when the
determination logic indicates that the current value of the first type of channel state
information varies from the previously-transmitted value of the first type of channel state
information by at least a first predetermined amount and an indicator indicative of the
current value of the second type of channel state information when the determination
logic indicates that the current value of the second type of channel state information
varies from the previously-transmitted value of the second type of channel state
information by at least a second predetermined amount in preference to the other.
In one embodiment, the transmission logic is operable to increase a number of bits
utilised for transmitting the one of the indicator indicative of the current value of the first
type of channel state information and the indicator indicative of the current value of
the second type of channel state information in preference to the other.
In one embodiment, the transmission logic is operable to increase a number of time
slots utilised for transmitting the one of the indicator indicative of the current value of
the first type of channel state information and the indicator indicative of the current
value of the second type of channel state information in preference to the other.
In one embodiment, the first type of channel state information comprises long-term
channel state information, the second type of channel state information comprises
short-term channel state information and the transmission logic is operable to transmit
the indicator indicative of the current long-term channel state information when the
determination logic indicates that the current value of the long-term channel state
information varies from the previously-transmitted value of the long-term channel state
information by at least the first predetermined amount.
In one embodiment, the transmission logic is operable to transmit an indicator
indicative of the current value of the long-term channel state information in preference
to transmitting the indicator indicative of the current value of the short-term channel
state information when the determination logic indicates that the current value of the
long-term channel state information varies from the previously-transmitted value of the
long-term channel state information by at least the first predetermined amount.
In one embodiment, the transmission logic is operable to increase an allocation of bits
for transmitting the current value of the long-term channel state information.
In one embodiment, transmissions occur within time-slots and the transmission logic is
operable to increase an allocation of time slots for transmitting the current value of the
long-term channel state information.
In one embodiment, the network node comprises: arranging logic operable to arrange
the current value of the at least a first type of channel state information into at least
one vector; and quantisation logic operable to quantise the at least one vector by
selecting one of a plurality of codebook vectors at a first level of a hierarchical
codebook of vectors, the indicator indicative of the current value of the at least a first
type of channel state information comprising an indication of an index to the one of a
plurality of codebook vectors.
In one embodiment, the quantisation logic is operable to requantise the at least one
vector by selecting one of a plurality of hierarchically-related codebook vectors from
hierarchically-related levels of the hierarchical codebook of vectors, the indicator
indicative of the current value of the at least a first type of channel state information
comprising an indication of an index to the one of a plurality of hierarchically-related
codebook vectors.
In one embodiment the one of a plurality of hierarchically-related codebook vectors
from hierarchically-related levels of the hierarchical codebook of vectors comprises
one of a plurality of child codebook vectors from child levels of the hierarchical
codebook.
In one embodiment, the one of a plurality of hierarchically-related codebook vectors
from hierarchically-related levels of the hierarchical codebook of vectors comprises
one of a plurality of parent codebook vectors from parent levels of the hierarchical
codebook.
In one embodiment, differing hierarchical codebooks of vectors are provided for the
first type of channel state information and the second type of channel state
information, at least one of the codebooks being hierarchical.
In one embodiment, the indicator indicative of the current value of the at least a first
type of channel state information encodes a n identifier indicating which of the
hierarchical codebooks of vectors was used to quantise or requantise the at least one
vector.
In one embodiment, the indicator indicative of the current value of at least one type of
channel state information further comprises a relative or absolute indicator of the level
of the codebook of vectors from which the at least one of a plurality of codebook
vectors was selected, wherein if the relative or absolute indicator indicates a level not
present in the previously-indicated codebook of vectors it indicates the type of channel
state information indicated by the indicator indicative of the current value of at least
one type of channel state information.
According to a third aspect, there is provided a computer program product operable,
when executed on a computer, to perform the method steps of the first aspect.
Further particular and preferred aspects are set out in the accompanying independent
and dependent claims. Features of the dependent claims may be combined with
features of the independent claims as appropriate, and in combinations other than
those explicitly set out in the claims.
BRIEF DESCRIPTION OFTHE DRAWINGS
Embodiments of the present invention will now be described further, with reference to
the accompanying drawings, in which:
Figure 1 illustrates the main components of a wireless telecommunications network
according to one embodiment;
Figure 2 illustrates scheduling of channel state update messages;
Figure 3 illustrates a n example hierarchical codebook structure; and
Figures 4 and 5 illustrate example utilisations of a hierarchical codebook structure.
DESCRIPTION OFTHE EMBODIMENTS
OVERVIEW
Figure 1 illustrates a n arrangement of a base station 20 and user equipment 30 of a
wireless telecommunications network, generally 10, according to one embodiment.
The base station 20 and the user equipment 30 are examples of the first and second
network nodes although it will be appreciated that other examples of network nodes to
which the present technique may be applied exist and indeed the functionality of the
base station and user equipment may be reversed. Although just one base station and
user equipment is shown for sake of clarity, it will be appreciated that many such base
stations and user equipment may be deployed in such a wireless telecommunications
network. Each base station 20 is provided with M antennas, whilst each user equipment
30 is provided with N antennas. Typically, at least one of M and N is a n integer value
greater than 1. A MIMO radio channel is established between the base station 20 and
the user equipment and sub-channels are provided between each antenna of the
base station 20 and user equipment 30. It will be appreciated that this arrangement is
applicable to both single-user MIMO as well as multi-use MIMO.
The state of the each sub-channel is assessed by the user equipment 30 and fed back
to the base station 20 to enable the base station 20 to adapt its transmissions based on
the channel state information. The channel state information may comprise many
different types of channel state information, each of which shares similar characteristics
which together provide a n overall indication of the state of the channel. It will be
appreciated that these different types of channel state information vary at different
times and their significance changes. As will be explained in more detail below, the
channel state of sub-channels is fed back from the user equipment 30 to the base
station 20 in a dynamic way. That is to say that dynamic scheduling of the channel
state feedback occurs, with the most significant changes to channel state being fed
back in preference to other changes. The significance of the changes is assessed
based on whether the channel state has changed by more than a predetermined
amount when compared with a previously provided indication of the channel state.
This enables updates relating to non-significant changes in channel state to be
avoided, thereby freeing resources which would otherwise have been utilised for those
less significant changes. For example, the freed up resources may be utilised to
transmit another type of channel state information or indeed to avoid the transmission
of any channel state information whatsoever for periods of time.
Each base station 20 comprises at least one processing means adapted to receive a n
indication of the channel state and means to adjust its transmissions based on this
channel state information. Each base station 20 also comprises at least one processing
means adapted to at least one predetermined threshold for transmission to the user
equipment 30. These threshold may be varied dynamically to adjust the extent of any
changes in channel state which may be tolerated prior to a n update in channel state
needing to be sent from the user equipment 30.
Each user equipment 30 comprises the functionality of a mobile terminal for transmission
and reception of signals in a network using radio transmission. Furthermore, the user
equipment 30 comprises at least one processing means adapted to determine the at
least one type of channel state, assess whether the difference between this channel
state and the most recently transmitted channel state for that type exceeds a threshold
and, if so, to allocate resources for the transmission of updated channel state
information.
CHANNEL STATEINFORMATION UPDATESCHEDULING
As mentioned above, each user equipment 30 receives a predetermined threshold for
each type of channel state information from the base station 20. These thresholds are
stored locally by the user equipment 30 and are used to assess whether a n update to
that type of channel state information is required or not. In this example, two types of
channel state information are described. The first is long-term channel state
information, which attempts to describe the spatial-correlation properties of the sub
channel which change slowly over time and over frequency. Examples of this type of
channel state information include the correlation between antennas, the average
SNIR, as well as pathloss. The other type of channel state information is short-term
channel state information, which tries to match the instantaneous properties of the sub¬
channel, which are varying in time and frequency. Examples of such short-term
channel state information include instantaneous SNIR signal phase and amplitude.
On initialisation, the user equipment will determine the channel state for each of its sub
channels and store this information as one or more matrices within the user equipment
30. The user equipment 30 will determine that no channel state information has been
provided to the base station 20 and so will begin allocating resources for transmitting
this channel state information to the base station 20. In this example, the channel state
information is transmitted over a control sub-channel between the user equipment 30
and the base station 20. However, it will be appreciated that other mechanisms for
transmitting the channel state information may be utilised. In this example, the
transmission of changes to long-term channel state information is given priority over
changes in short-term channel state information. However, other types of channel
state information may be given preference in other arrangements. Accordingly, the
initial long-term channel state information is fed back to the base station 20, followed
by the initial short-term channel state information. Details of this channel state
information for each type fed back to the base station 20 are stored, typically as a
matrix, within the user equipment 30.
Thereafter, as the user equipment 30 measures the current channel state being
experienced by that user equipment 30, the measured current state is stored in one or
more corresponding matrices. The previously-transmitted channel state and the current
channel state may be compared using a Frobenius matrix to estimate the variation
between the matrices. When a variation for that type of channel state information is
greater than a corresponding threshold provided by the base station 20, then the user
equipment will allocate resources for the transmission of the updated channel state
information for that type.
For example, asshown in Figure 2, at time interval Ti, the current long-term channel
state is judged to have changed by an amount which is greater than the threshold,
which triggers the allocation of resources for the transmission of the current long-term
channel state information to the base station 20 in preference to the transmission of the
current short-term channel state information. Hence, the current long-term channel
state information is transmitted to the base station 20 from the user equipment 30 over a
number of consecutive time slots. In this example, the change in long-term channel
state information was relatively high and required a number of time slots to enable
sufficient information about these changes to be conveyed to the base station 20. In
order to minimise the amount of information that needs to be transmitted to the base
station 20, rather than transmitting an absolute indicator of the channel state
information each time a change occurs, an indication of the difference between the
current channel state and the previously transmitted channel state may instead be
provided. Also, as will be explained in more detail below, a hierarchical feedback
system may be used to successively refine the information being fed back. At time T2 .
sufficient information regarding the current long-term channel state has been
transmitted to the base station 20 and so no further resources are required to be
allocated for the transmission of long-term channel state information.
However, at time T3, another change to the long-term channel state which is greater
than the predetermined amount is detected, but in this example only one time slot is
required to convey that change and so, at time T , no further resources for the
transmission of long term channel state information need to be allocated.
Likewise, changes to short term channel state information which are greater than a
corresponding threshold for that type of channel state information may be transmitted
from the user equipment 30 to the base station 20. In this example, a n update to the
short-term channel state information is made at every opportunity in every time slot
when these resources are not being used to update the long-term channel state
information. However, it will be appreciated that once sufficient information on the
current short-term channel state has been provided by the user equipment 30 to the
base station 20, then no further updates need be provided, thereby freeing these
resources for use for other types of channel state information.
In the event that saturation of these updates occurs, which could be detected either
by the user equipment 30 or the base station 20, then rather than continuing to transmit
a difference, the user equipment may transmit another absolute indication of the
channel state information to the base station 20. Likewise, both the user equipment 30
and the base station 20 may instruct a complete update to the channel state
information in a similar manner to that performed on initialisation.
HIERARCHICAL FEEDBACK
In this example, a hierarchical feedback system is used to encode the updates to
channel state information. This enables the updates to be refined from one reporting
instant to the next. This approach works particularly well if the channel state is
changing sufficiently slowly so that the feedback can be aggregated over multiple
feedback intervals so that the aggregated bits index a larger codebook. In other
words, a first message provides a n initial approximation of the channel state, with
subsequent messages further refining the approximation of the channel state.
As it can be seen in Figure 3, each quantisation codebook can be organized using a
binary tree structure, in a way that all the codewords at the j level have the same j-1
significant bits. Such a structure can be used as an enabler for hierarchical
quantisation.
The hierarchical quantisation method can be explained referring to two messages, the
basic feedback message and the refined feedback message. The basic feedback
message is obtained by sending back to the base station 20 b, bits specifying the best
quantisation codeword at level b of the binary tree, typically by specifying its index.
The refined feedback message is obtained as follows. Assuming that at the time
interval f- 7, in one embodiment, both the base station 20 and user equipment 30 share
a codeword h,. (. - l ) belonging to the / ( —l ) > , level. It will be appreciated that
the base station 20 and user equipment 30 need to save the sequence of selected
codewords, starting from the level bi up to level /(i -l) ) . The case /(i - l ) = , can be
obtained as a particular case, where an "UP' move corresponds to a new codeword in
the b level.
If c= h,. ( -l) , then a new candidate is chosen in the / ( - l ) +6, - 1level, b, - \ bits are
sent back to the base station 20 indicating the position in the subtree starting from
h, ( -l) , while one bit is used to signal a 'DOWN' move within the tree. If c h,. (i - l ) ,
then a new candidate is chosen in the / ( -1) - ( , -1) level, 6, - 1 bits are sent back to
the base station 20 indicating the position in the subtree starting from , t -l) , while
one bit is used to signal a 'UP' path within the tree.
Consider the following two examples shown in Figures 4 and 5.
In Figure 4, at the time interval, a refinement (OOWN') of the quantisation codeword
is done starting from /(i -l) = 3 . A new vector in level 5 is sent back to the base station
20. At the + 7 time interval, a second refinement ('DOWN') is done starting from
l(t) = 5 . A new vector in level 7 is sent back to the base station.
In Figure 5, at the 1 time interval, a refinement ('DOWN') of the quantisation codeword
is done starting from l t - l ) = 3 . A new vector in level 5 is sent back to the base station
20. At the †+ 7t time interval, due to a channel variation, an 'UP' move is signalled to the
base station 20, in the same subtree ίϊ ,. t ) belongs to.
The previously described embodiment using bi-1 bits for signalling the codeword in a
given tree level, and 1 bit for signalling an "UP" or "DOWN" move in the tree, can be
generalized to the case where xi bits are used for signalling a given codeword in a
given tree level, whereas yi bits are used to signal a move in the tree. And these xi,yi
can be a function of the level in the tree, user, etc.
Both the base station 20 and the user equipment 30 share the same codebooks. In one
embodiment, the base station 20 is operable to transmit codebooks to the user
equipment 30 for storage therein.
Accordingly, as can be seen in Figure 2, each of the feedback updates may be
hierarchical feedback updates, providing hierarchical feedback reports, each
providing a refined updates aggregated over multiple feedback intervals to index
different levels within the corresponding code book. For example, the long-term and
short-term channel state information may have different codebooks. Typically, each
type of channel state information may have more than one code book, the most
appropriate of which may be selected based on the current channel state information.
In the example shown in Figure 2, each of the short-term feed back updates prior to
time period T may successively refine the short term channel state information by
traversing through levels of the hierarchical tree.
The updates may encode an indicator which indicates to the base station 20 that each
of these updates relate to the same codebook. The updates to the long-term channel
state information occurring between times Ti and T2 may encode a n indicator
indicating to the base station 20 that these updates relate to long-term channel state
information and provide an indication of the corresponding codebook. Hence, each
update may encode an indicator of the particular codebook to which that update
relates or may instead simply indicate when a change in codebook occurs. For
example, if two sets of codebooks are being used then the first update sent after time
Ti may simply indicate that a switch from the current short-term codebook to the
current long-term codebook needs to be made. Likewise, the first update following
time T2 may indicate that a switch from the current long-term codebook to the current
short-term codebook needs be made. Likewise, where each type of channel state
information utilises more than one codebook then an indicator may be provide within
the updates indicating that a change in the codebook for that type of channel state
information needs be made. In this way, it can be seen that the appropriate
codebook utilise to encode the updated channel state information may be indicated
to the base station 20.
In one embodiment, the short-term channel state information feedback reports use
levels p to p+q of a hierarchically-structured set of codebooks, and the long-term
channel state information feedback is treated as if it corresponds to a level p-1 of the
hierarchical structure, even though the long-term feedback typically uses a codebook
which is completely unrelated to the codebooks of levels p to p+q which are
hierarchically related to each other. Thus when an "up" move is signalled that would
effect a transition above level p in the tree, it effectively redefines the meaning of the
other bits of the feedback report, signifying that they represent the quantisation of the
long-term component of the feedback not the short-term component of the feedback.
Similarly, a "down" move from the level p-1 would switch the meaning of the other bits
of the feedback report back to being the quantisation of the short-term component of
the feedback. In a general embodiment, a single feedback report structure exists, that
may be used for either long-term or short-term reporting. It comprises 1 bit that enables
the reporting to switch between long-term and short-term components (using different
quantisation codebooks), and a further plurality of bits to represent the corresponding
quantised long-term or short-term feedback. If the short-term component of the
feedback is hierarchically structured, the switch bit enables stepping up or down
through the levels of the hierarchy. In some embodiments an additional bit may be
provided to provide a "reset" function - i.e. enabling the feedback to jump up to level
p of the hierarchy in a single step.
In one embodiment, there is provided a method for reporting two types of channel
state information from a secondary station to a primary station, wherein the reports
comprise a plurality of bits indicating a value of one or other of the types of channel
state information, and at least one bit indicating the type of the channel state
information.
In one embodiment, in a first report, a first value of the bit indicating the type of the
channel state information indicates a switch from the first type of channel state
information to the second type of channel state information, and in the next report the
first value of the bit indicating the type of the channel state information indicates a
switch from a coarser quantisation level to a finer quantisation level of the second type
of channel state information.
In one embodiment, in a first report, a second value of the bit indicating the type of the
channel state information indicates a switch from a coarser quantisation level to a finer
quantisation level of the second type of channel state information, and in the next
report the second value of the bit indicating the type of the channel state information
indicates a switch from the second type of channel state information to the first type of
channel state information.
In one embodiment the reports further comprise a bit indicating a reset from the
previously-used quantisation level of the second type of channel state information to
the coarsest quantisation level of the second type of channel state information.
A person of skill in the art would readily recognise 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 functions of the various elements shown in the Figures, including any functional
blocks labelled as "processors" or "logic", may be provided through the use of
dedicated hardware aswell 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" or "controller" or "logic" 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.
Similarly, any switches shown in the Figures are conceptual only. Their function may be
carried out through the operation of program logic, through dedicated logic, through
the interaction of program control and dedicated logic, or even manually, the
particular technique being selectable by the implementer as more specifically
understood from the context.
It should be appreciated by those skilled in the art that any block diagrams herein
represent conceptual views of illustrative circuitry embodying the principles of the
invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state
transition diagrams, pseudo code, and the like represent various processes which may
be substantially represented in computer readable medium and so executed by a
computer or processor, whether or not such computer or processor is explicitly shown.
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 inven†or(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.
CLAIMS
. A method of providing channel state information for a wireless communications
channel provided between a first network node having at least one transmission
antenna and a second network node having at least one reception antenna, said
method comprising the steps of:
estimating a current value of at least a first type of channel state information for
at least one sub-channel within said channel from signals received by said at least one
reception antenna over said channel from said at least one transmission antenna;
determining whether said current value of said first type of channel state
information varies from a previously-transmitted value for said first type of channel state
information by at least a predetermined amount; and
if so, transmitting an indicator indicative of said current value for said first type of
channel state information to said first network node.
2. The method of claim 1, wherein said step of transmitting comprises the step of:
transmitting an indicator indicative of a current value of a second type of
current channel state information when said step of determining indicates that said
current value of said first type of channel state information does not vary from said
previously-transmitted value of said first type of channel state information by at least
said predetermined amount.
3. The method of claim 1 or 2, wherein said step of transmission comprises the step
of:
avoiding transmission of said indicator indicative of said current value of said first
type of channel state information when said step of determining indicates that said
current value of said first type of channel state information does not vary from said
previously-transmitted value of said first type of channel state information by at least
said predetermined amount.
4. The method of any preceding claim, comprising the step of:
receiving an indicator indicative of said predetermined amount from said first
network node.
5. The method of any preceding claim, wherein said indicator indicative of said
current value of at least said first type of channel state information comprises an
indicator indicative of a difference between said current value of at least said first type
of channel state information and said previously-transmitted value of at least said first
type of channel state information.
6 . The method of any preceding claim, wherein said step of transmitting comprises
the step of:
transmitting one of
an indicator indicative of said current value of said first type of channel
state information when said step of determining indicates that said current
value of said first type of channel state information varies from said previouslytransmitted
value of said first type of channel state information by at least a first
predetermined amount and
an indicator indicative of said current value of said second type of
channel state information when said step of determining indicates that said
current value of said second type of channel state information varies from said
previously-transmitted value of said second type of channel state information by
at least a second predetermined amount in preference to the other.
7. The method of claim 6, wherein said step of transmitting comprises the step of:
increasing a number of bits utilised for transmitting said one of said indicator
indicative of said current value of said first type of channel state information and said
indicator indicative of said current value of said second type of channel state
information in preference to the other.
8 . The method of claim 6 or 7, wherein said step of transmitting comprises the step
of:
increasing a number of time slots utilised for transmitting said one of said
indicator indicative of said current value of said first type of channel state information
and said indicator indicative of said current value of said second type of channel state
information in preference to the other.
9. The method of any one of claims 5 to 8, wherein said first type of channel state
information comprises long-term channel state information, said second type of
channel state information comprises short-term channel state information and said step
of transmitting comprises the step of:
transmitting said indicator indicative of said current long-term channel state
information when said step of determining indicates that said current value of said
long-term channel state information varies from said previously-transmitted value of
said long-term channel state information by at least said first predetermined amount.
10. The method of any one of claims 5 to 9, wherein said step of transmitting
comprises the step of:
transmitting an indicator indicative of said current value of said long-term
channel state information in preference to transmitting said indicator indicative of said
current value of said short-term channel state information when said step of
determining indicates that said current value of said long-term channel state
information varies from said previously-transmitted value of said long-term channel
state information by at least said first predetermined amount.
11. The method of any preceding claim, comprising the steps of:
arranging said current value of said at least a first type of channel state
information into at least one vector; and
quantising said at least one vector by selecting one of a plurality of codebook
vectors at a first level of a hierarchical codebook of vectors, said indicator indicative of
said current value of said at least a first type of channel state information comprising an
indication of an index to said one of a plurality of codebook vectors.
12. The method of claim 11, comprising the step of:
requantising said at least one vector by selecting one of a plurality of
hierarchically-related codebook vectors from hierarchically-related levels of said
hierarchical codebook of vectors, said indicator indicative of said current value of said
at least a first type of channel state information comprising an indication of an index to
said one of a plurality of hierarchically-related codebook vectors.
13. The method of claim 11 or 12, wherein differing hierarchical codebooks of
vectors are provided for said first type of channel state information and said second
type of channel state information, at least one of said codebooks being hierarchical,
and said indicator indicative of said current value of said at least a first type of channel
state information encodes an identifier indicating which of said hierarchical codebooks
of vectors was used to quantise or requantise said at least one vector.
14. The method of claim 13, wherein said indicator indicative of said current value
of at least one type of channel state information further comprises a relative or
absolute indicator of the level of said codebook of vectors from which said at least one
of a plurality of codebook vectors was selected, wherein if said relative or absolute
indicator indicates a level not present in the previously-indicated codebook of vectors
i† indicates the type of channel state information indicated by said indicator indicative
of said current value of at least one type of channel state information.
15. A network node having at least one reception antenna and operable to
provide channel state information for a wireless communications channel provided
between another network node having at least one transmission antenna and said
network node, said network node comprising:
estimating logic operable to estimate a current value of at least a first type of
channel state information for at least one sub-channel within said channel from signals
received by said at least one reception antenna over said channel from said at least
one transmission antenna;
determination logic operable to determine whether said current value of said
first type of channel state information varies from a previously-transmitted value of said
first type of channel state information by at least a predetermined amount; and
transmission logic operable, if said determination logic indicates that said
current value of said first type of channel state information varies from said previouslytransmitted
value of first type of said channel state information by at least said
predetermined amount to transmit of an indicator indicative of said current value of
said first type of current channel state information to said first network node.